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Sci.chem FAQ - Part 5 of 7

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Archive-name: sci/chem-faq/part5
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Last-modified: 22 October 1999
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See reader questions & answers on this topic! - Help others by sharing your knowledge
18.6  What is the most bitter compound? 

Denatonium Benzoate = Bitrex, or even in some strange chemistry circles,
N-[(2-[2,6-Dimethylphenyl)amino]-2-oxoethyl]-N,N-diethylbenzenemethan-
aminium benzoate [3734-33-6]. It is added to toxic chemicals ( such as 
methylated spirits ) as a deterrent to accidental ingestion.

18.7  What is the sweetest compound? 

Most scales use sucrose as a sweetness of 1, and compare the relative
sweetness of other sweeteners to sucrose.

Name              Relative Sweetness           Category 
D-Glucose                 0.46            Natural Food Product
Lactose                   0.68               "      "     "
D-Fructose                0.84               "      "     "
Sucrose                   1                  "      "     "
Cyclamate                30               EC Permitted, USA Prohibited
Aspartame               200               EC, USA Permitted.
Saccharin               300               EC Permitted, USA Prohibited
Sucralose               650               Au, Ca Permitted, trials elsewhere
Alitame               2,000               Undergoing trials
Thaumatin             3,000               EC permitted, US chewing gum only.
Carrelame           160,000               Guanidine sweetener
Bernardame          200,000                   "        "
Sucrononate         200,000                   "        "
Lugduname           220,000                   "        "

The guanidine sweeteners are not expected to be approved for food use.
There are several other important attributes of sweeteners, such as
low toxicity, no after-taste, whether metabolised or excreted, etc.,
that must also be considered.

The potency scale is fairly flexible, and differing publications can
assign different values. The August 1995 copy of the Journal of Chemical
Education contained several papers from a symposium on sweeteners [3,4],
and an article in Chemistry and Industry also discusses sweeteners from
both natural and artificial sources [5], and Kirk Othmer has a monograph
on sweeteners.    

The sweetener used in "diet" beverages is usually Aspartame, and they
are usually required to display a warning for phenylketurics that the
product contains a source of phenylalanine. As Aspartame slowly degrades
in acid solutions, such products also have a "use-by" date. 

Although banned by the FDA in 1970 ( because a mixture of saccharin and 
cyclamate caused tumours in test animals ), saccharin has been still 
marketed under extensions of approval,  Ironically, subsequent work 
implicated the saccharin, and the cyclamate was found not to be the 
tumour-causing agent, but it is still banned. 

18.8  What salts change the colour of flames?.

Both Vogel ( qualitative inorganic ) and the Rubber Handbook list details of 
flame tests for elements. The spectra of the alkaline earth compounds are 
relatively complex, so using filters to view the flame can change the colour 
observed as dominant lines are filtered out. In general, except for copper, 
any compound of an element can be used, however toxic salts ( such as 
cyanides ) should not be used. Halogen salts are usually readily available, 
and are reasonably volatile. In all cases, perform experiments in a 
well-ventilated area - preferably a fume hood. The emission spectra in the 
visible region is the sum of several emission lines, with dominant lines
masking others. The visible spectrum is approximately :-
Red              800 - 620 nm
Orange           620 - 600 nm
Yellow           600 - 585 nm
Green            585 - 505 nm
Blue             505 - 445 nm
Violet           445 - 400 nm           

There are also the various bead tests employing borax ( sodium tetraborate 
Na2B4O7.10H2O ), Microcosmic salt ( NaNH4HPO4 ), or sodium carbonate 
(Na2CO3), using both oxidising and reducing flames. The bead test procedures 
are detailed in Vogel ( qualitative inorganic ), and similar texts.
 
Element         Colour          Some of the contributing lines, and comments.

Arsenic         Light Blue      449.4 nm, 450.7 nm. 
  ( Arsenic is highly toxic - only perform in fume hood under supervision )
Barium          Green-Yellow    553.6 nm, 539.1 nm, 536.1nm, 614.2 nm.
                Blue (faint)    455.4 nm, 493.4 nm.
Cesium          Red-Violet      852.1 nm.
Calcium         Orange          618.2 nm, 620.3 nm.
                Yellow-Green    530.7 nm, 559.5 nm.
                Violet (faint)  422.7 nm.
                                Greenish with blue glass. 
Copper          Emerald Green   521.8 nm, 529.2 nm, 515.3 nm.
                                Not chloride, or in presence of HCl 
                Azure Blue      465.1 nm.
                                Copper chloride, or HCl present
Lead            Light Blue      500.5 nm.
  ( Lead is highly toxic - only perform in fume hood under supervision )
Lithium         Carmine Red     670.78 nm, 670.79 nm.
                Orange (faint)  610.1 nm.  
                                Violet with blue glass
Potassium       Red             766.5 nm, 769.9 nm. 
                Violet          404.4 nm, 404.7 nm.
                                Purple-red with blue glass
Rubidium        Violet          780.0 nm, 794.8 nm.  
Sodium          Yellow          589.0 nm, 589.6 nm. 
                                Invisible when viewed with blue glass
Strontium       Scarlet Red     640.8 nm, 650.4 nm, 687.8 nm, 707.0 nm.
                Violet          460.7 nm, 421.5 nm, 407.8 nm.
                                Violet with blue glass
Tellurium       Green           557.6 nm, 564.9 nm, 566.6 nm, 570.8 nm.
  ( Tellurium is highly toxic - only perform in fume hood under supervision )
Thallium        Green           535.0 nm.
  ( Thallium is highly toxic - only perform in fume hood under supervision )   
Zinc            Whitish Green   Large number of peaks between 468.0-775.8 nm. 
  ( Zinc fumes are toxic - only perform in a fume hood under supervision )

Impressive coloured flames have been obtained using chlorides and a methanol 
flame in a petri dish [6]. Even more spectacular results have been obtained
by nitrating cellulose filter paper, and impregnating it with salts prior
to ignition [7].

18.9  What chemicals change colour with heat, light, or pressure?.
 
Compounds that visibly and reversibly change colour when subjected to a 
change in their environment are known as chromogenic materials. There are 
four major categories - electrochromic, photochromic, piezochromic, and
thermochromic, all of which are extensively discussed in a recent, well 
referenced, monograph in Kirk Othmer [8]. 

Electrochromic materials exhibit a change in light transmittance or 
reflectance induced by direct current at potentials of approximately one 
volt. The change usually is an oxidation-reduction reaction, using either
inorganic or organic compounds, and the colour change can occur at either 
the anode or the cathode - which are usually thin films. There are two major
classes, the ion-insertion/extraction type - such as tungsten trioxide, and
the noninsertion group - such as the viologens, a family of halides of
quaternary bases derived from 4,4'-bipyridinium. One viologen example is 
1,1'-diheptyl-4,4'-bipyridinium bromide [6159-05-3], which changes from
clear to bluish purple. The most common application of viologens has been
the electrochromic interior rearview mirrors available for cars since 1988.
These utilise a substituted viologen as the cathode colouring material, with 
a compound like phenylene diamine as the anode colouring electrochromic
material. The mechanism details, along with a description of the ingenious 
control system, are described in a recent comprehensive review of 
electrochromic materials [9].  

Photochromic materials undergo a reversible change in light absorption that
is induced by electromagnetic radiation, however most common applications
involve reversible changes in colour or transparency on exposure to visible
or ultraviolet light. This is often seen as a change in the visible spectrum 
( 400 - 700 nm ), and can be rapid or very slow. There are two major classes
of photochromic materials, inorganic and organic. 

Examples of the inorganic type are the silver halides, which are suspensions 
of fine ( 10-20 nm ) silver halide crystals dispersed throughout a glass that
has been slowly cooled. An alternative technique involves diffusion of the 
silver halide into the surface of the glass. The cuprous ion can catalyse 
both the photochromic darking and thermal fading reactions, and the colour 
can be shifted from grey to brown by the addition of gold or palladium - 
which may be added to the glass in trace amounts. The most popular current
application for glass containing silver halide is for prescription eyewear.
  
The organic photochromic systems can be subdivided according to the type of
reaction. Geometric isomerism can result in different optical properties,
eg azobenzene ( C12H10N2 [103-33-3] ) undergoes photoisomerization, and the 
cis form [1080-16-6] has higher absorbance than the trans form [17082-12-1].
Cycloaddition can produce photochromism, such as the reversible formation of 
the colourless 4b,12b,endoperoxide ( C28H14O4 [74292-77-6] ) from the red 
parent compound dibenzo(a,j)perylene-8,16-dione ( C28H14O2 [5737-94-0] ).
Dissociation, either heterolytic ( photolysis of triphenylmethyl chloride 
[76-83-5] ), or homolytic ( photolysis of bis(2,4,5-triphenylimidazole 
[63245-02-3] to form a red-purple free radical ), may also produce 
photochromism. 

UV can excite polycyclic aromatics, such as 1,2,5,6-dibenzacridene ( C21H13N 
[226-36-8] ), to their triplet state, which has a different absorption 
spectrum. Viologens may undergo redox reactions and exhibit photochromic 
behaviour when crystalline and subjected to UV. The most popular photochromic
materials utilise reversible electrocyclic reactions, and are often indolino
spiropyrans and indolino spiroxazines, however the mechanism also covers
fulgide, stilbene, and dihydroindolizine examples. Details and structures
are provided in the Kirk Othmer monograph [8], and the Journal of Chemical
Education has published descriptions and preparation techniques for both 
inorganic [10] and organic photochromic compounds and sunglasses [11].

Piezochromic materials change colour as they are compressed. There are three
common types:- organic molecules ( such as N-salicylidene-2-chloroaniline 
[3172-42-7] ), metal cluster compounds ( such as the octahalodirhenates, 
(Re2X8)2-, where X=Cl,Br,I ), and copper (II) organic complexes with
compounds like ethylene diamine. They are still being researched, and 
interested readers should investigate the references in the Kirk Othmer 
monograph [8].

Thermochromic materials reversibly change colour as their temperature is 
changed. There are a very large number of systems, but one common example 
of thermochromic transitions in metal complexes is the transition between 
the blue tetrahedral and pink octahedral coordinations of cobalt (II) when 
cobalt chloride is added to anhydrous ethanol and the temperature changed. 
Examples of thermochromic transitions in inorganic compounds include 
Ag2HgI4 [12344-40-0] and VO2, and several inorganic sulfides also have large 
changes occurring in the infra-red range, and are being considered for IR 
imaging applications. 

There are thousands of organic thermochromic compounds, with well known 
examples including di-beta-naphthospiropyran [178-10-9] ( thermally-induced 
heterolytic bond cleavage resulting in ring opening), poly(xylylviologen  
dibromide [38815-69-9] ( charge transfer interactions resulting in hydration- 
dehydration changes ), and ETCD polydiacetylene [63809-82-5] ( thermally-
induced transitions in the unsaturated backbone resulting in rearranged side 
groups ). Information on photochromism in organic and polymeric compounds is 
available in published reviews [12,13].
    

Subject: 19. Physical properties of chemicals 19.1 Rheological properties and terminology Contributed by Jim Oliver RHEOLOGY What is RHEOLOGY ? RHEOLOGY describes the deformation of a material under the influence of stresses. Materials in this context can be solids, liquids or gases. In this FAQ we will be concerned only with the rheological properties of liquids.[1] Perry discusses the some aspects of the behaviour of gases, and Ullmann discusses elastic solids. When liquids are subjected to stress they will deform irreversibly and flow. The measurement of this flow is the measurement of VISCOSITY. IDEAL liquids are very few, whereas non-ideal examples abound. Ideal liquids are : water and pure paraffin oil. Non-ideal examples would be toothpaste or cornflour mixed with a little water. [2] What is VISCOSITY ? VISCOSITY is expressed in Pascal seconds (Pa.s) and to be correct the conditions used to measure the VISCOSITY must be given. This is due to the fact that non-ideal liquids have different values of VISCOSITY for different test conditions of SHEAR RATE, SHEAR STRESS and temperature. [3,4] A graph describing a liquid subjected to a SHEAR STRESS (y axis) at a particular SHEAR RATE (x axis) is called a FLOW CURVE. The shape of this curve reveals the particular type of VISCOSITY for the liquid being studied. [3] What is a NEWTONIAN LIQUID ? NEWTONIAN LIQUIDS are those liquids which show a straight line drawn from the origin at 45 degrees, when graphed in this way. Examples of NEWTONIAN liquids are mineral oil, water and molasses. (Isaac NEWTON first described the laws of viscosity) [1] All the other types are NON NEWTONIAN. What does NON NEWTONIAN mean ? a. PSEUDOPLASTIC liquids are very common. These display a curve starting at the origin again and curving up and along but falling under the straight line of the NEWTONIAN liquid. In other words increasing SHEAR RATE results in a gradual decreasing SHEAR STRESS, or a thinning of viscosity with increasing shear. Examples are toothpaste and whipped cream. b. DILATANT liquids give a curve which curves under then upward and higher than the straight line NEWTONIAN curve. (Like a square law curve) Such liquids display increasing viscosity with increasing shear. Examples are wet sand, and mixtures of starch powder with small amounts of water. A car may be driven at speed over wet sand, but don't park on it, as the car may sink out of sight due to the lower shear forces (compared to driving over) the wet sand. There are other terms used which include : THIXOTROPY - this describes special types of PSEUDOPLASTIC liquids. In this case the liquid shows a YIELD or PLASTIC POINT before starting to thin out. What this means is the curve runs straight up the y axis for a short way then curves over following ( but higher and parallel to ) the PSEUDOPLASTIC curve. This YIELD POINT is time dependant. Some water based paints left overnight develop a FALSE BODY which only breaks down to become useable after rapid stirring. Also: the curve describing a THIXOTROPIC liquid will be different on the way up (increasing shear rate) to the way down (decreasing shear rate). The area inside these two lines is a measure of it's degree of THIXOTROPY. This property is extremely important in industrial products, e.g to prevent settling of dispersed solids on storage. [3] A RHEOPECTIC liquid is a special case of a DILATANT liquid showing increasing viscosity with a constant shear rate over time. Again, time dependant but in this case _increasing_ viscosity. Why do some liquids become solid ? A few special liquids (dispersions usually) display extraordinary DILATANT properties. A stiff paste slurry of maize or cornflour in water can appear to be quite liquid when swirled around in a cup. However on pouring some out onto a hard surface and applying extreme shear forces (hitting with a hammer) can cause a sudden increase in VISCOSITY due to it's DILATANCY. The VISCOSITY can become so high as to make it appear solid. The "liquid" then becomes very stiff for an instant and can shatter just like a solid material. It should be noted that the study of viscosity and flow behaviour is extremely complex. Some liquids can display more than one of the above properties dependant on temperature, time and heat history. What are Electrorheological Fluids? ( added by Bruce Hamilton ) Electrorheological (ER) fluids change their flow properties when an electric field is applied, and are usually dispersions of polarizable particles in an insulating base fluid [5]. Their apparent viscosity can change by orders of magnitude in milliseconds when a fews watts of electrical power are applied. The shear stress versus shear rate properties of ER fluids vary as a function of the applied electric field, When an electric field is applied, the fluid switches from a liquid to semisolid. The particles are usually irregularly- shaped 0.5-100um and present at concentrations of 10-40% by mass. ER fluids are dielectric particles in an insulating medium ( such as silicone oil ), along with additives ( such as surfactants, dispersants, and possibly a polar activator ). ER fluid effectively function as leaky capacitors. The electric field can be either AC, pulsed DC, or DC, with AC producing less electrophoresis of particles to electrodes. There are two categories of ER particulate materials, extrinsically polarizable materials ( which require a polar activator ), and intrinsically polarizable materials. Extrinsically polarizable materials can be polar nonionic compounds ( such as silica, alumina, or polysaccharides ), or polar ionic materials ( such as the lithium salt of polymethacrylic acid ), Intrinsically polarizable materials provide simpler systems - because a polar activator is not required, and they have a lower thermal coefficient of conductance. The most common examples are the ferroelectrics like barium titanate (BaTiO3 ) and polyvinylidene difluoride, however their performance has been poor, as has been that of metal powders ( such as iron and aluminium - even when coated with an insulating layer ), and research is concentrating on conducting polymers ( such as polyanilines and pyrolysed hydrocarbons ) [5,6]. The ability to utilise computer-based electrical switching to control ER fluid properties has resulted in vehicle suspension and industrial vibration control as major target applications for ER fluids. Demonstration systems have been built, and they match performance predictions, however cost and durability issues still have to be solved [7]. 19.2 Flammability properties and terminology There are several properties of flammable materials that are frequently reported. It should be remembered that most discussions concerning flammable liquids usually consider air as the oxidant, but oxygen and fluorine can also be used as oxidants for combustion, and they will result in very different values. The flammability limits in air are usually reported as the upper and lower limits ( in volume percent at a certain temperature, usually 25C ), and represent the concentration region that the vapour ( liquid HCs can not burn ) must be within to support combustion. Hydrocarbons have a fairly narrow range, ( n-hexane = 1.2 to 7.4 ), whereas hydrogen has a wide range ( 4.0 to 75 ). The minimum ignition energy is the amount of energy ( usually electrical ) required to ignite the flammable mixture. Some mixtures only require a very small amount of energy (eg hydrogen = 0.017mJ, acetylene = 0.017mJ ), whereas others require more (eg methanol = 0.14mJ, n-hexane = 0.29mJ, diethyl ether = 0.20mJ, acetone = 1.15mJ, dichloromethane = 133mJ @ 88C ), and some require significant amounts, (eg ammonia = >1000mJ ). The flash point is the most common measure of flammability today, especially in transportation of chemicals, mainly because most regulations use the flash point to define different classes of flammable liquids. The flash point of a liquid is the temperature at which the liquid will emit sufficient vapours to ignite when a flame is applied. The test consists of placing the liquid in a cup and warming it at a prescribed rate, and every few degrees applying a small flame to the air above the liquid until a "flash" is seen as the vapours burn. Note that the flame is not applied continuously, but is provided at prescribed intervals - thus allowing the vapour to accumulate. There are a range of procedures outlined in the standard methods for measuring flash point ( ASTM, ISO, IP ) and they have differing cup dimensions, liquid quantity, headspace volume, rate of heating, stirring speed, etc., but the most significant distinction is whether the space above the liquid is enclosed or open. If the space is enclosed, the vapours will be contained, and so the flash point is several degrees lower than if it is open. Most regulations specify closed-cup methods, either Pensky-Martens Closed Cup or Abel Closed Cup. It is important to remember that these methods are only intended for pure chemicals, if there is water or any other volatile non-flammable compounds present, their vapours can extinguish or mask the flash. For used lubricants, this may be partially overcome by using the TAG open cup procedure - which is slightly more tolerant of non-flammable vapours. A material can be flammable, but may not have a flash point if other non-flammable volatile compounds are present. For alkane hydrocarbons, flash point increases with molecular weight. There is an older measure, called the fire point, which is the temperature at which the liquid emits sufficient vapours to sustain combustion. The fire point is usually several degrees above the flash point for hydrocarbons. The minimum autoignition temperature is the temperature at which a material will autoignite when it contacts a surface at that temperature. The procedure consists of heating a glass flask and squirting small quantities of sample into it at various temperatures until the vapours autoignite. The only source of ignition is the heat of the surface. For the smaller hydrocarbons the autoignition temperature is inversely related to molecular weight, but it also increases with carbon chain branching. Autoignition temperature also correlates with gasoline octane ratings ( refer to Gasoline FAQ available in rec.autos.tech, which lists octane ratings and autoignition temperatures for a range of hydrocarbons.) Flash Point Autoignition Flammable Limits Temperature Lower Upper ( C ) ( C ) ( vol % at 25C) methane -188 630 5.0 15.0 ethane -135 515 3.0 12.4 propane -104 450 2.1 9.5 n-butane -74 370 1.8 8.4 n-pentane -49 260 1.4 7.8 n-hexane -23 225 1.2 7.4 n-heptane -3 225 1.1 6.7 n-octane 14 220 0.95 6.5 n-nonane 31 205 0.85 - n-decane 46 210 0.75 5.6 n-dodecane 74 204 0.60 - n-tetradecane 99 200 0.50 - 19.3 Supercritical properties and terminology? Supercritical fluids have some very unusual properties. When a compound is subjected to conditions around the critical point ( which is defined as the temperature at which the gas will not revert to a liquid regardless how much pressure is applied ), the properties of the supercritical fluid become very different to the liquid or the gas phases. In particular, the solubility behaviour changes. The behaviour is neither that of the liquid or that of the gas. The transition between liquid and gas can be completely smooth. The pressure-dependant densities and corresponding Hildebrand solubility parameters show no break on continuity as the supercritical boundary is crossed. Physical properties fall between those of a liquid and a gas. Diffusivities are approximately an order of magnitude higher than the corresponding liquid, while viscosities are an order of magnitude lower. These properties ( along with low surface tension ) allow SCFs to have liquid-like solvating power with the mass transport characteristics of a gas. Potential Supercritical Fluids Compound Critical Critical Density Temperature Pressure ( C ) ( bar ) (g cm^-3) Ammonia 132.4 112.8 0.235 Carbon dioxide 30.99 73.75 0.468 CFC-12 111.8 41.25 0.558 Dimethyl ether 126.9 52.7 0.271 Ethane 32.4 49.1 0.212 HCFC-22 96.15 49.90 0.524 HCFC-123 183.68 36.62 0.550 HFC-116 19.7 29.8 0.608 HFC-134a 101.03 40.57 0.508 Methanol 240.1 83.1 Nitrous oxide 36.4 72.54 0.453 Propane 96.8 42.66 0.225 Water 374.4 227.1 Xenon 16.6 58.38 1.105 Nitrous oxide is seldom used because early researchers reported explosions. Note that using liquid CO2 at pressure ( as for the commercial extraction of hops ) is still just liquid CO2 extraction, not supercritical CO2 extraction. There are several good general introductions to supercritical fluids [8,9,10] 19.4 Formation of gaseous bubbles in liquids Discussions about the behaviour of dissolved gases in liquids, especially when discussing carbonated beverages, are usually more appropriate in sci.physics and/or sci.mech.fluids, and there is a good text available [11]. Section 23.9 of this FAQ lists the change in solubility with temperature for common atmospheric gases in water at near-ambient pressure. As the temperature increases, the solubility decreases, creating a supersaturated solution that can result in bubble formation. A similar effect occurs if the pressure is reduced. The formation of bubbles can be understood in thermodynamic terms using the Gibbs free energy of the bubble. Gibbs free energy = -n * R * T ln(C/Cs) + gamma * A A = Surface area of the bubble. C = Concentration of gas in the liquid, Cs = Concentration of gas in the liquid at saturation, gamma = Interfacial tension between the gas and the liquid n = Number of moles of gas in the bubble = (P*V)/(R*T), where P = pressure, and V = volume of a sphere. R = Gas Constant T = Temperature After inserting the expressions for the surface area of a sphere (r = radius) and number of moles, and differentiating, then we obtain:- r(mininum) = 2 * gamma / ( P * ln(C/Cs)) This describes the size of a bubble that would continue to grow under the existing conditions, rather than redissolve. Of course, the expression assumes homogeneous precipitation of the bubble, and in real life most bubbles are created heterogeneously. Statistics and kinetics are also required to determine the rate of formation of bubbles, and predict the effect of changing parameters such as temperature. As the liquid is warmed, bubbles may be created faster, as the higher temperatures overcome the activation barrier - which is the difference between the Gibbs free energy when r is less than r(minimum), and the Gibbs free energy at r(minimum). The formation of a bubble also dramatically perturbs the system, even causing secondary bubbles to form. Secondary bubble formation may be implicated in the production of copious quantities of froth from shaken, quickly-opened, carbonated drink containers. The sites for gaseous bubble formation in supersaturated drinks are typically small particles, or minor flaws on the smooth surface of the container. 19.5 Why is Mercury a liquid at room temperature?. First, let's look at the melting points of some of the elements surrounding mercury in the periodic table ( in degrees C ) :- Period IB IIB IIIA 4s3d4p Cu 1083 Zn 419.5 Ga 29.8 5s4d5p Ag 960.8 Cd 320.9 In 157 6s(4f)5d6p Au 1063 Hg -38.4 Tl 304 The interesting comparison is between Hg and Au, as their properties differ dramatically, although their electron structures are similar:- 14 10 1 Au(g) : Xe | 4f , 5d , 6s 79 54 14 10 2 Hg(g) : Xe | 4f , 5d , 6s 80 54 Very few chemistry textbooks discuss relativistic effects on chemical properties, despite the availability of a comprehensive review by P.Pyykko [12]. There several good introductory articles on the derivation and calculation of various relativistic effects in molecules and atoms, so I'm not going to include details [13,14,15]. Suffice to say, that whilst smaller elements can treated simply, larger elements need treatment based on the Dirac equation, which shows that the s electrons are approaching the speed of light, consequently relativistic effects are important. If we take the relativistic mass of mercury (m);- Mo where m = -------------------- c (speed of light) = ~137 atomic units _____________ v = Z = 80 / ( v ) 2 Mo = rest mass / 1 - ( - ) \/ ( c ) The masses of the 1s electrons are increased by approximately 20% over their rest masses, which means that the radius is decreased by 20% - since mass appears in the denominator of Bohr radius calculations. All the other s shells also contract, with the 6s contracting ~14%, because their electron speeds near the nucleus are comparable, and the contraction of the inner part of the wave function also pulls in the outer tails. The p orbitals also contract a similar amount, and these contractions also results in increases the screening for d and f orbitals, which may then expand - about 3% for the 5d orbital of mercury. In mercury, the relativistically-contracted 6s2 orbital is full, thus the the two electrons do not contribute much to the metal-metal bond, which is not the situation for gold. The bonding in mercury is believed to be mainly van der Waals forces with a contribution from 6p orbital interaction. The relativistic contraction of the filled 6s2 orbital, when added to the contraction across the sixth row of the periodic table, results in relatively weak Hg-Hg bonds that are responsible for mercury being a liquid at room temperature. For those curious to know more, a recent article in J.Chem.Ed. provides much more detail and several good references [16]. Relativistic effects are also responsible for the colour of gold ( partially explained by the 5d -> 6s transition in gold requiring less energy than the 4d -> 5s transition in silver, resulting in a smaller d-s gap ) [12,14,16].
Subject: 20. Optical properties of chemicals 20.1 Refractive Index properties and terminology When light passes between media of different density, the direction of the beam is changed as it passes through the surface, and this is called refraction. In the first medium, the angle between the light ray and the perpendicular is called the angle of incidence (i), and the corresponding angle in the second medium is called the angle of refraction (r). The ratio sine i / sine r is called the index of refraction, and usually the assumption is that the light is travelling from the less dense (air) to more dense, giving an index of refraction that is greater than 1. Although the theoretical reference is a vacuum, air ( 0.03% different ) is usually used. The refractive index of a compound decreases with increasing wavelength ( dispersion ), except where absorption occurs, thus the wavelength should be reported. The D lines of sodium are commonly used. The refractive index of a liquid varies with temperature and pressure, but the specific refraction ( Lorentz and Lorentz equation ) does not. The molar refraction is the specific refraction multiplied by the molecular weight, and is approximately an additive property of the groups or elements comprising the compound. Tables of atomic refractions are available in the literature, as are descriptions of the common types of refractometers [1]. 20.2 Polarimetry properties and terminology Supplied by: Vince Hamner <vinny@vt.edu> Polarimetry is a method of chemical analysis that is concerned with the extent to which a beam of linearly polarised light is rotated during its transmission through a medium containing an optically active species.[2] Helpful discussions regarding polarised light may be found elsewhere.[3,4] In general, a compound is optically active if it has no plane of symmetry and is not superimposable on its mirror image. Such compounds are referred to as being "chiral". Sucrose, nicotine, and the amino acids are only a few of these substances that exhibit an optical rotary power. A simple polarimeter instrument would consist of: 1). a light source -- typically set to 589 nm (the sodium "D" line) 2). a primary fixed linear polarising lens (customarily called the "polariser") 3). a glass sample cell (in the form of a long tube) 4). a secondary linear polarising lens (customarily called the "analyser") and 5). a photodetector.[5] Biot is credited with the determination of the basic equation of polarimetry.[6,7] The specific rotation of a substance (at a given wavelength and temperature) is equivalent to the observed rotation (in degrees) divided by the path length of the sample cell (in decimeters) multiplied by the concentration of the sample (for a pure liquid, -density- replaces concentration). Influences of temperature, concentration, and wavelength must always be taken into consideration. If necessary, it is possible to apply corrections for each of these variables.[8] A few early contributors to our understanding of optical activity and polarimetry include: Malus, Arago, Biot, Drude, Herschel, Fresnel, and Pasteur.
Subject: 21. Molecular and Structural Modelling Supplied by: Dave Young (young@slater.cem.msu.edu) 21.1 What hardware do I need to run modelling programs? There are two types of programs that are referred to as molecular modeling programs. This first is a program which graphically displays molecular structures as Lewis structures, ball & stick, etc. The second is a program which does a calculation to tell you something about the molecule, such as it's energy, dipole moment, spectra, etc. For an introductory description of various types of computations, see http://www.cem.msu.edu/~young/topics/contents.html There are many programs of both sorts available for a large range of machines. The speed, memory, graphics and disk space on the machine will determine the size of molecules that can be modelled, how accurate the model is, and how good the images will look. There are a few programs that will run on a 286 PC with Windows. There are some fairly nice things that can be done on a 386 with about 8 MB of RAM and Windows. The professional computational chemists are generally using work stations and larger machines. Currently many computational chemists are using machines made by Silicon Graphics (SGI) ranging from the $5,000 Indy to the $1,000,000 power challenge machines. These are all running Irix, which is SGI's adaptation of Unix. SGI is popular for two reasons; first that the power is very good for the price, second that SGIs run the largest range of chemical software. However, you will find some computational chemistry software that can run on almost any machine. As far as graphics quality, the SGI Onyx (about $250,000) is about the top of the line. Even if you find a machine that claims to have better graphics than this, chances are you won't find and chemistry software that can utilise it. For chemical calculations there is no limit to the computing power necessary. There are some calculations that can only be done on the biggest Crays or massively-parallel machines in the world. There are also many calculations which are too difficult for any existing machine and will just have to wait a few years or a few centuries. 21.2 Where can I find a free modelling program? The single best place for public domain modelling software is probably the anonymous FTP server at ccl.osc.edu in the directory pub/chemistry/software. "ccl" stands for "computational chemistry list server" and is a list frequented mostly by professional computational chemistry researchers. This machine contains their archives with quite a bit of information as well as software. For work stations and larger, the program GAMESS (General Atomic and Molecular Electronic Structure System) can be obtained as source code from Mike Schmidt at mike@si.fi.ameslab.gov GAMESS is a quantum mechanics, ab initio and semi-empirical program. It is powerful. but not trivial to learn how to use. The COLUMBUS program for work stations and larger can be obtained by anonymous FTP from ftp.itc.univie.ac.at It is a HF, MCSCF and multi-reference CI program. This is probably the most difficult program to use that is in use today since it requires the user to input EVERY detail manually. However, because you control everything there are some calculations that can only be done with COLUMBUS. CACAO is an extended Huckel program available by anonymous FTP at cacao.issecc.fi.cnr.it 21.3 Where can I find structural databanks? 21.4 Where can I find ChemDraw or ChemWindows For ChemDraw (Macintosh, Windows, UNIX) CambridgeSoft Corporation 875 Massachusetts Avenue Cambridge, MA 02139 Phone: (800) 315-7300 or (617) 491-2200 Fax: (617) 491-7203 Internet: info@camsci.com http://www.camsci.com For ChemIntosh or ChemWindows SoftShell 1600 Ute Avenue Grand Junction, CO 81501 Phone: (970) 242-7502 Fax: (970) 242-6469 Internet: info@softshell.com http://www.softshell.com
Subject: 22. Spectroscopic Techniques All of these are covered in texts on instrumental Analysis [1-4], and I will eventually include a paragraph about each. 22.1 Ultra-Violet/Visible properties and terminology 22.3 Nuclear Magnetic Resonance properties and terminology 22.4 Mass Spectrometry properties and terminology 22.5 X-Ray Fluorescence properties and terminology 22.6 X-Ray Diffraction properties and terminology 22.7 Fluorescence/Phosphorescence properties and terminology
Subject: 23. Chromatographic Techniques There are chromatography mailing lists and WWW sites available that provide comprehensive introductions and access to chromatography experts. The following are simple introductions to popular techniques. 23.1 What is Paper Chromatography? Paper chromatography was the first analytical chromatographic technique developed, allegedly using papyrus (Pliny). It was first published by Runge in 1855, and consists of a solvent moving along filter or blotting paper. The interaction between the components of the sample, the solvent, and the paper, results in separation of the components. Most modern paper chromatography is partition chromatography, where the cellulose of the paper is the inert support, and the water adsorbed ( hydrogen bonded ) from air onto the hydroxyl groups of the cellulose becomes the stationary phase. If the mobile phase is not saturated with water, then some of the stationary phase water may be removed from the cellulose - resulting in a separation that is a mixture of partition and adsorption. Paper chromatography remains the method of choice for a wide range of coloured compounds, and is used extensively in both natural and artificial pigment research. The technique is suitable for any molecules that are significantly less volatile than the solvent, and many examples and references are provided in Heftmann [1]. 23.2 What is Thin Layer Chromatography? Thin layer chromatography involves the use of a particulate sorbant on an inert sheet of glass, plastic, or metal. The solvent is allowed to travel up the plate with the sample spotted on the sorbant just above the solvent. Depending on the sorbant, the separation can be either partition or adsorption chromatography ( cellulose, silica gel and alumina are commonly used ). The technique came to prominence during the late 1930s, however it did not become popular until Merck and Desaga developed commercial plates that provided reproducible separations. The major advantage of TLC is the disposable nature of the plates. Samples do not have to undergo the extensive clean-up steps required for HPLC. The other major advantage is the ability to detect a wide range of compounds cheaply, using very reactive reagents ( iodine vapours, sulfuric acid ) or indicators. Non-destructive detection ( fluorescent indicators in the plates, examination under a UV lamp ) also means that purified samples can be scraped off the plate and be analysed by other techniques. There are special plates for such preparative separations, and there are also high-performance plates that can approach HPLC resolution. The technique is described in detail in Stahl [2] and Kirchner [3]. 23.3 What is Gas Chromatography? Gas chromatography is the use of a carrier gas to convey the sample ( as a vapour ) through a column consisting of an inert support and a stationary phase that interacts with sample components, thus it is usually partition chromatography. There are also a range of materials, especially for permanent gas and light hydrocarbon analysis that utilise adsorption. The simplest partition systems consisted of a steel tube filled with crushed brick that had been coated with a hydrocarbon that had a high boiling point, eg squalane. Today, the technique uses very narrow fused silica tubes ( 0.1 to 0.3mm ID ) that have sophisticated stationary phase films ( 0.1 to 5um ) bonded to the surface and also cross-linked to increase thermal stability. The ability of the film to retard specific compounds is used to ascertain the "polarity" of the column. If benzene elutes between normal alkanes where it is expected by boiling point ( midway between n-hexane and n-heptane ), then the column is "non-polar" eg squalane and methyl silicones. If the benzene is retarded until it elutes after n-dodecane, then the column is "polar" eg OV-275 ( dicyanoallyl silicone ) and 1,2,3-tris (2-cyanoethoxy) propane. In general, polar columns are less tolerant of oxygen and reactive sample components, but the ability to select different polarity columns to obtain satisfactory peak resolution is what has made GC so popular. The column is placed in an oven that has exceptional temperature control, and the column can be slowly heated up to 350-450C ( sometimes starting at -50C to enhance resolution of volatile compounds ) to provide separation of wide-boiling range compounds. The carrier gas is usually hydrogen or helium, and the eluting compounds can be detected several ways, including flames ( flame ionisation detector ), by changes in properties of the carrier ( thermal conductivity detector ), or by mass spectrometry. The availability of "universal" detectors such as the FID and MS, makes GC a popular tool in laboratories handling organic compounds. There are also columns that have a layer of 5-10 um porous particulate material (such as molecular sieve or alumina ) bonded to the inner walls ( PLOT = Porous layer open tubular ), and these are used for the separation of permanent gases and light hydrocarbons. GC is restricted to molecules ( or derivatives ) that are sufficiently stable and volatile to pass through the GC intact at the temperatures required for the separation. Specialist books on the production of derivatives for GC are available [4,5]. There are several manufacturers of GC instruments whose catalogues and brochures provide good introduction to the technique. (eg Hewlett Packard, Perkin Elmer, Carlo Erba ). The catalogues of suppliers of chromatography consumables also contain explanations of the criteria for selection of the correct columns and conditions for analyses, and they provide an excellent indication of the range of applications available. Well-known suppliers include Alltech Associates, Supelco, Chrompack, J&W, and Restek. They also sell most of the standard GC texts, as do the instrument manufacturers. Popular GC texts include "Basic Gas Chromatography" [6], "High-Resolution Gas Chromatography" [7], and "Open Tubular Column Gas Chromatography" [8]. There are Standard Retention Index Libraries available [9], however they really only complement unambiguous identification by mass spec. or dual-column analysis. 23.4 What is Column Chromatography? Column chromatography consists of a column of particulate material such as silica or alumina that has a solvent passed through it at atmospheric or low pressure. The separation can be liquid/solid (adsorption) or liquid/liquid (partition). The columns are usually glass or plastic with sinter frits to hold the packing. Most systems rely on gravity to push the solvent through. The sample is dissolved in solvent and applied to the front of the column. The solvent elutes the sample though the column, allowing the components to separate based on adsorption ( alumina, hydroxyapatite) or partition ( cellulose, diatomaceous earth ). The mechanism for silica depends on the hydration. Traditionally, the solvent was non-polar and the surface polar, although today there are a wide range of packings including bonded phase systems. Bonded phase systems usually utilise partition mechanisms rather than adsorption. The solvent is usually changed stepwise, and fractions are collected according to the separation required, with the eluted solvent usually monitored by TLC. The technique is not efficient, with relatively large volumes of solvent being used, and particle size is constrained by the need to have a flow of several mls/min. The major advantage is that no pumps or expensive equipment are required, and the technique can be scaled up to handle sample sizes approaching a gram in the laboratory. The technique is discussed in detail in Heftmann [1]. 23.5 What is High Pressure Liquid Chromatography? HPLC is a development of column chromatography. it was long realised that using particles with a small particle size ( 3, 5, 10um ) with a very narrow size distribution would greatly improve resolution, especially if the flow rate and column dimensions could be adjusted to minimise band-broadening. Pumps were developed that could handle both the chemicals and pressures required. Traditional column chromatography ( nonpolar solvent and polar surface ) is described as "normal" and, as well as silica, there are columns with amino, diol, and cyano groups. If the system uses a polar solvent ( water, methanol, acetonitrile etc. ) and a non-polar surface it is described as "reverse-phase". Common surface treatments of silica include octadecylsilane ( aka ODS or C18), and it has been the development of reverse-phase HPLC that has experienced explosive growth. Reverse-phase HPLC is the method of choice for larger non-volatile biomolecules, however it is only recently that a replacement "universal" detector ( evaporative light-scattering ) has emerged. The most popular detector (UV), places constraints on the solvents that can be used, and the refractive index detector can not easily be used with solvent gradients. There are several excellent books introducing HPLC, including the classic "Introduction to Modern Liquid Chromatography" [10]. HPLCs can be a pain to operate, and novices should borrow "Troubleshooting LC Systems" by Dolan and Snyder [11]. There is also a handy basic primer on developing HPLC methods by Snyder and Kirkland [12], however, unlike GC, you also need to search the journals ( Journal of Chromatography, Journal of Liquid Chromatography ) to find relevant examples to assist with method development. 23.6 What is Ion Chromatography? Ion chromatography has become the method of choice for measuring anions ( eg Cl-, SO4=, NO3- ) in aqueous solutions. It is effectively a development from ion-exchange systems ( which were extensively developed to deionise water and soften aqueous process streams ), and brings them down to HPLC size. IC uses pellicular polymeric resins that are compatible with a wide pH range. The sample is eluted through an ion-exchange column using a dilute sodium hydroxide solution. The eluant is passed through self-regenerating suppressors that neutralise eluant conductance, ensuring electrochemical detectors ( conductivity or pulsed amperometric ) can detect the ions down to sub-ppm concentrations. The major manufacturer of such systems is Dionex, who hold several patents on column, suppression, and detection technology. There are several books covering various aspects of the technique [13,14]. 23.7 What is Gel Permeation Chromatography? Gel Permeation chromatography ( aka Size Exclusion chromatography ) is based on the ability of molecules to move through a column of gel that has pores of clearly-defined sizes. The larger molecules can not enter the pores, thus they pass quickly through the column and elute first. Slightly smaller molecules can enter some pores, and so take longer to elute, and small molecules can be delayed further. The great advantage of the technique is simplicity, it is isocratic ( single solvent - no gradient programming ), and large molecules rapidly elute. The technique can be used to determine the molecular weight of large biomolecules and polymers, as well as separating them from salts and small molecules. The columns are very expensive and sensitive to contamination, consequently they are mainly used in applications where alternative separation techniques are not available, and sample are fairly clean. The best known columns are the Shodex cross-linked polystyrene-divinylbenzene columns for use with organic solvents, and polyhydroxymethacrylate gel filtration columns for use with aqueous solvents. "Modern Size Exclusion Chromatography" [15], and Heftmann [1], provide good overviews, and there are some good introductory booklets from Pharmacia. 23.8 What is Capillary Electrophoresis? Capillary electrophoresis uses a small fused silica capillary that has been coated with a hydrophilic or hydrophobic phase to separate biomolecules, pharmaceuticals and small inorganic ions. A voltage is applied and the analytes migrate and separate according to their charge under the specific pH conditions, as also happens for electrophoresis. The capillary can also be used for isoelectric focusing of proteins. The use of salt or vacuum mobilisation is no longer required. 23.9 How do I degas chromatographic solvents? One major problem with pressurising chromatography systems using liquid solvents is that pressure reductions can cause dissolved gases to come out of solution. The two locations where this occurs are the suction side of the pump ( which is not self-priming, consequently a gas bubble can sit in the pump and flow is reduced ), and at the column outlet ( where the bubbles then pass through the detector causing spurious signals). Note that the problem is usually restricted to solvents that have relatively high gas solubilities - usually involving an aqueous component, especially if a gradient is involved where the water/organic solvent ratio is changing. As water usually has a higher dissolved gas content, then a gradient programme may cause the gases to come out of solution as the mobile phase components mix. There are three traditional strategies used to remove problem dissolved gases from chromatographic eluants. Often they are used in combination to lower the dissolved gases. a. Subject the solvent to vacuum for 5-10 mins. to remove the gases. b. Subject the solvent to ultrasonics for 10-15 mins. to remove the gases. c. Sparge the solvent with a gas that has a very low solubility compared to the oxygen and nitrogen from the atmosphere. Helium is the preferred choice - 5 minutes of gentle bubbling from a 7um sinter is usually sufficient, although maintaining a positive He pressure is even better. Note that most aqueous-based solvents usually have to be degassed every 24 hours. Also remember that solubility of gases increases as temperature decreases, so ensure eluants are at instrument temperature prior to degassing. Helium is preferred as the degassing solvent because it has relatively low solubility in water, and the solubility is less affected by temperature. The following data is from Kaye and Laby, 13th edition, and the units are the number of cm3 of gas at 0C and 760 mmHg which dissolve in 1 cm3 of water at the temperature stated ( when the gas is at 760 mmHg pressure and in equilibrium with the water ). Temp.(C) 0 10 20 30 40 50 60 Helium 0.0098 0.0091 0.0086 0.0084 0.0084 0.0086 0.0090 Hydrogen 0.0214 0.0195 0.0182 0.0170 0.0164 0.0161 0.0160 Nitrogen 0.0230 0.0185 0.0152 0.0133 0.0119 0.0108 0.0100 Oxygen 0.047 0.037 0.030 0.026 0.022 0.020 0.019 Argon 0.054 0.041 0.032 0.028 0.025 0.024 0.023 CO2 1.676 1.163 0.848 0.652 0.518 0.424 0.360 I've no explanation for the aberrant trend for helium at higher temperatures, but I assume it's real - but it's irrelevant for HPLC solvents that are usually stored at ambient temperature. Points to note - the lower solubility of helium over the range of concern, *and* the lower rate of change of decreasing solubility with increasing temperature. There is heat generated in the compression of the solvent, along with friction in HPLC pump heads and, more importantly, HPLC columns are often heated - thus the solvent could outgas and form bubbles in UV detector cells that are at ambient. By using helium, there is less chance of that happening. For example, if the temperature increased from 10C to 40C, the undissolved gas volume would be 0.0007 cm3 for helium, and 0.0066 cm3 for nitrogen. Modern HPLCs are sold with a "solvent degassing module" that removes undissolved gases in the solvent automatically. These usually consist of a tube made from gas-permeable membrane that passes through a vacuum chamber. 23.10 What is chromatographic solvent "polarity"? There are four major intermolecular interactions between sample and solvent molecules in liquid chromatography, dispersion, dipole, hydrogen-bonding, and dielectric. Dispersion interactions are the attraction between each pair of adjacent molecules, and are stronger for sample and solvent molecules with large refractive indices. Strong dipole interactions occur when both sample and solvent have permanent dipole moments that are aligned. Strong hydrogen-bonding interactions occur between proton donors and proton acceptors. Dielectric interactions favour the dissolution of ionic molecules in polar solvents. The total interaction of the solvent and sample is the sum of the four interactions. The total interaction for a sample or solvent molecule in all four ways is known as the "polarity" of the molecule. Polar solvents dissolve polar molecules and, for normal phase partition chromatography, solvent strength increases with solvent polarity, whereas solvent strength decreases with increasing polarity in reverse-phase systems. The subject is discussed in detail in Snyder and Kirkland [10]. ------------------------------ Subject: 24. Extraction Techniques 24.1 What is Solvent Extraction? Solvent extraction is usually used to recover a component from either a solid or liquid. The sample is contacted with a solvent that will dissolve the solutes of interest. Solvent extraction is of major commercial importance to the chemical and biochemical industries, as it is often the most efficient method of separation of valuable products from complex feedstocks or reaction products. Some extraction techniques in involve partition between two immiscible liquids, others involve either continuous extractions or batch extractions. Because of environmental concerns, many common liquid/liquid processes have been modified to either utilise benign solvents, or move to more frugal processes such as solid phase extraction. The solvent can be a vapour, supercritical fluid, or liquid, and the sample can be a gas, liquid or solid. There are a wide range of techniques used, and details can be found in Organic Vogel, Perry, and most textbooks on unit operations. 24.2 What is Solid Phase Extraction? Solid Phase Extraction (SPE) is an alternative to liquid/liquid extraction, and has become the method of choice for the separation and purification of a wide range of samples in the laboratory. The sample is usually dissolved in an appropriate solvent and passed through a small bed of adsorbent of very consistent particle size and shape to maximise separation efficiency. The compounds are eluted with step changes of small volumes of solvents. The major advantage is that solvent volumes are greatly reduced. There is a newer, modified technique that is used in analytical laboratories, called Solid Phase Micro Extraction. This immerses a fused silica fibre coated with a stationary phase into the sample solution for several minutes, The analytes adsorb onto the stationary phase, which is subsequently pushed into a hot GC injector to rapidly desorb the sample for analysis. 24.3 What is Supercritical Fluid Extraction? Refer to Section 19.3 for some critical data on common supercritical fluids. Supercritical fluids have been investigated since last century, with the strongest commercial interest initially focusing on the use of supercritical toluene in petroleum and shale oil refining during the 1970s. Supercritical water is also being investigated as a means of destroying toxic wastes, and as an unusual synthesis medium [1]. The biggest interest for the last decade has been the applications of supercritical carbon dioxide, because it has a near-ambient critical temperature (31C), thus biological materials can be processed at temperatures around 35C. The density of the supercritical CO2 at around 200bar pressure is close to that of hexane, and the solvation characteristics are also similar to hexane, thus it acts as a non-polar solvent. Around the supercritical region CO2 can dissolve triglycerides at concentrations up to 1% mass. The major advantage is that a small reduction in temperature, or a slightly larger reduction in pressure, will result in almost all of the solute precipitating out as the supercritical conditions are changed or made subcritical. Supercritical fluids can produce a product with no solvent residues. Examples of pilot and production scale products include decaffeinated coffee, cholesterol-free butter, low-fat meat, evening primrose oil, squalene from shark liver oil. The solvation characteristics of supercritical CO2 can be modified by the addition of an entrainer, such as ethanol, however some entrainer remains as a solvent residue in the product, negating some of the advantages of the "residue-free" extraction. There are other near-ambient temperature supercritical fluids, including nitrous oxide and propane, however there are safety issues with some of them. There are several introductory texts on supercritical fluid extraction, including some the ACS Symposium series [2-4]. There are also a large number of articles on applications of the technique, including processing [5], extraction of natural products [6], and chemical synthesis [7]. The major concentration of information occurs in the various proceedings of the International Symposium on Supercritical Fluids [8]. There is also a Journal of Supercritical Fluids. 24.4 What traditional process extracted perfume from flower petals? The traditional cold-fat extraction process is known as " enfleurage". It is a very interesting, historical process used to obtain the essential oils and perfume components from rose, jasmine, and other flowers. The rose and jasmine flowers continue to produce perfume during the long process. Thus the technique can obtain more perfume from those flowers than if they were just macerated and extracted by hot fat, solvent or steam when they were picked - as happens to many other plant perfume sources. The process uses a fat comprised of 40 parts of beef tallow and 60 parts of lard. The two fats are melted together, and repeatedly beaten under cold water and alum solutions to purify them. Benzoin is added to the fat mixture to prevent biological degradation. The fat is spread about 4mm thick on both sides of 0.5 x 0.5 metre glass plates in wooden frames. Flowers are pressed into the fat on one side of the frame only, and the frames stacked vertically so that the flowers are very close to the layer of fat on the frame above. After 1-3 days, the flowers are stripped off and fresh flowers added to the other layer of fat that had not been used, and the frame are again stacked. The cycle is repeated about 30 - 35 times, or until the fat is saturated with perfume. The saturated fat is known as "pomade". The fat is removed from the frames and extracted with alcohol to collect the perfume. the alcohol is cooled and filtered to remove most of the dissolved fat. The alcohol solution is called the "extract", and the residue after evaporation of the solvent is known as the "enfleurage absolute".
Subject: 25. Radiochemical Techniques 25.1 What is radiochemistry? ------------------------------

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