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Sci.chem FAQ - Part 7 of 7

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31.11 What is Sarin nerve gas?.

Sarin is a nerve gas that was used in 1988 by Iraq against its Kurdish 
population, and in 1995 by Japanese terrorists against Tokyo subway users.
Sarin and its companion nerve gases ( Tabun and Soman ) were discovered 
in the late 1930s by Gerhard Schrader at I.G.Farben during research into 
pesticides. The lethal dose for humans may be as low as 0.01mg/kg [18], 
unless treated immediately. Sarin inhibits acetylcholinesterase, an enzyme 
that breaks down acetylcholine. Acetylcholine carries signals between nerves 
and muscles, and build-up causes over-stimulation of muscles ( including 
the involuntary ones controlling  eye, lungs, bowel ), which then go into 
spasms. Treatment involves atropine ( shuts down the overstimulated nerves ), 
or oxime drugs ( can prise Sarin off the enzyme ), and must be immediate.  
More details and references can be found in the Merck Index.

There are many different methods of manufacture, but the Tokyo product
appears to have been prepared using a procedure involving phosphorus 
trichloride and methyl iodide. The product was impure and diluted with 
acetonitrile to improve volatility. To stockpile Sarin, the product has to 
be pure ( 90-99% of the Iraqi Sarin degraded in < 2 years, whereas US Sarin 
only degraded a few % over 30 years ). The standard US government procedure 
( aka "di-di" ) starts with dimethyl methylphosphonate (DMMP), and ends
with a distillation to remove impurities [19].

    O                  O            O                  O     CH3
    ||     thionyl     ||   HF      ||  isopropyl      ||   /
CH3-P-OCH3 ------> CH3-P-Cl --> CH3-P-F ---------> CH3-P-O-CH   
    |     chloride     |            |    alcohol       |    \
    OCH3               Cl           F                  F     CH3

   DMPP              Dichlor     Difluor            Sarin (GB)

31.12 What are Dioxins?.

"Dioxins" are a group of closely-related compounds which are known as
polychlorinated dibenzo-p-dioxins (PCDDs). "dioxins" also commonly includes 
polychlorinated dibenzofurans (PCDFs). All organic molecules that contain 
chlorine are also members of the "organochlorine" family.

               1       9                    1       9
             2/ \ _o_ / \8                2/ \ ___ / \8  
             | O |_o_| O |                | O |_o_| O |
             3\ /     \ /7                3\ /     \ /7
               4       6                    4       6

            Dibenzo-p-dioxin              Dibenzofuran

As dioxins are fat soluble, they will accumulate in fatty tissue. In general, 
it is only the biologically active ( molecules containing the 2,3,7,8 
substitution ) congeners that accumulate, with levels of the higher 
homologues predominating [20]. It is important to remember that of all the 
dioxins and furans, only those containing 4 to 8 chlorine atoms, _and_ with 
chlorine atoms in the 2,3,7,8 positions are currently considered toxic. 
The compounds only containing 0 to 3 chlorine atoms are currently not 
considered toxic, however once all four of the 2,3,7,8 positions are filled, 
the most toxic congener is created ( 2,3,7,8 TCDD = "dioxin" ). As additional 
chlorines are added, the toxicity decreases, except that 2,3,4,7,8 
pentachlorodibenzofuran is more toxic than 2,3,7,8 tetrachlorodibenzofuran.

There is evidence that suggests concentrations of dioxins and furans in 
human adipose tissue are falling [21]. The analysis for dioxin can reliably 
detect ppq ( parts per quadrillion = picograms/kilogram ) levels, but some
evidence suggests dioxins may still have toxic effects at such low levels. 
The toxicity of dioxins is currently being carefully assessed by the US EPA 
- who are due to present a comprehensive report in the next few months. The 
draft of the report, and various reviews, have been available for public 
comment and external peer review. A good discussion of current perceptions
is available in a special report published in the January 1995 Environmental 
Science and Technology [22], where both sides of the debate are presented. 

Dioxins can arise naturally from forest fires, but the major sources are
from incineration and the manufacture and use of organic chemicals. The most 
well-known source is perhaps as an impurity in the defoliant Agent Orange
which was widely used in Vietnam. Agent Orange was a 50:50 mixture of the
n-butyl esters of the herbicides 2,4-D ( 2,4-dichlorophenoxyacetic acid ) and
2,4,5-T ( 2,4,5-trichlorophenoxyacetic acid ), and dioxin was present as
an impurity in the 2,4,5-T. Another well known incident was an industrial
accident at a 2,4,5-trichlorophenol manufacturing plant in Seveso, Italy
on 10 July 1976 which resulted in the release of 1-5 kg of dioxin.  
Other dioxin sources involve combustion ( leaded gasoline, coal combustion, 
metallurgical processes )[20].   

As the various congeners have differing toxicity, dioxins are usually 
reported using Toxic Equivalents systems. These assign to each congener a 
toxicity factor relative to 2,3,7,8-TCDD, and these factors are used to 
calculate the 2,3,7,8-TCDD Toxic Equivalent. The International Toxic
Equivalent Factor (I-TEF) system, proposed by the Challenges to Modern 
Society Committee of the North Atlantic Treaty Organisation is widely used.  
  
Food is the major source of dioxins for humans, and typical dietary intakes 
in the US for a 65kg adult were estimated to be between 18-192 pgTEQ/day [23],
and UK intakes were estimated to be 125 pgTEQ/day [20]. The Regional Office 
for Europe of the World Health Organisation suggests 10 pg/kg body weight/day
for 2,3,7,8-TCDD ( 600 pgTEQ/day for 60kg person ), as a Tolerable Daily 
Intake, whereas the US-EPA suggests an intake of 0.006 pg/kg/day over a 70 
year life will lead to one excess cancer in one million people. 
Sources of Dioxins in the UK diet                  pgTEQ/day
Meat, meat products, poultry, and offals            38
Cow's milk                                          23 
Fats and oils                                       19
Milk products                                       12
Fish                                                 7.7
Eggs, cereal products, fruit, and vegetables        25.3

31.13 What is Red Mercury?.

Red mercury is supposed to be a very powerful explosive that is being made 
in Russian nuclear reactors. According to one report, it is a cherry red and 
semi-liquid compound of pure mercury and mercury antimony oxide that is 
irradiated for up to 20 days in a nuclear reactor [24]. It is claimed that 
when incorporated in a fusion bomb, it can yield sufficient chemical energy 
to fuse tritium atoms. Experts are sceptical that such an energetic compound 
could be sufficiently stable to be used as an explosive, and no compelling
evidence for the existence of red mercury has yet been produced.  

31.14 How do I remove stains and deposits?

- Test any planned treatment on an unimportant part of the material first,
  (spots and holes aren't currently fashionable ).
- Chemicals for removing stains are often toxic and corrosive, handle with
  care, and follow any provided safety instructions.
- Often stains are a diverse mix of chemicals, and the best solution is
  to solubilise as much as possible, remove insoluble material through
  washing, and then carefully bleach. Chemists should not assume they can
  perform this process better than housewives. 
- Some stains are more easily removed by physical means - such as using
  abrasives ( household cleaning pastes, steel wool, metal polishes, etc ),
  or freezing solid and scraping ( chewing gum ).
- The fresher the stain, the easier to remove. Avoid using hot water or soap 
  on unknown stains, and use solvents (eg glycerine) to help keep the stain
  fresh. In cases when the stain is known to be water-soluble (eg bird 
  droppings ), it is often preferable to allow the stain to dry and carefully 
  scrape most away, before additional treatment.
- When using solvents, apply around the outer edge of the stain and work 
  towards the centre to prevent a stain ring forming. 
- Many stains result from pigments, and they are seldom soluble, so once the
  other components are removed, use physical agitation to remove the 
  insoluble material.
- Sometimes a dried stain can be "freshened" by using more of the stain 
  before treating - but use this approach very cautiously.

There are books on stain removal [25,26,27], and many of the common recipes 
are also often found in some home-care and cookery books. Home-care magazines 
also have question and answer sections that frequently include advice on 
how to remove specific stains. Very few chemical books cover chemical 
cleaning and stain removal, and smart chemists avoid offering to remove 
stains :-). Common stains are usually attacked with the household chemical 
arsenal that may include:-
* Absorbents - Cornflour, French chalk, fuller's earth, starch, talcum powder.
* Acids - Inorganic = hydrochloric ( galvanising remover, concrete cleaner ). 
        - Organic = acetic ( white vinegar ), citric ( lemon juice ), 
                    tartaric ( cream of tartar ) 
* Alkalis - Sodium hydroxide ( drain cleaner ), ammonia solution, 
* Bleaches - Sodium hypochlorite solution ( household bleach ), calcium 
             hypochlorite ( bleaching powder ), hydrogen peroxide 
* Drycleaning Fluids - 1,1,1-trichloroethane, perchloroethylene.
* Enzymes - Pepsin
* Petroleum Fractions - mineral turpentine, kerosine, gasoline, white spirits.
* Sodium carbonate ( washing soda ), sodium bicarbonate ( baking soda ),
  sodium tetraborate ( borax ).
* Solvents - Acetone ( nail polish remover ), amyl acetate, methanol, ethanol 
             ( methylated spirits ), glycerine, toluene, xylenes, iso propyl 
             alcohol.
* Terpenes - Eucalyptus oil, citrus oil, camphor 

Specific Stain Strategies.

Ballpoint - methylated spirits, fullers earth, glycerine.
Blood - cold salty water, cornflour paste, or dilute bleach.
Copper deposits on sink or bathtub - ammonia (1 hr) then detergent.
Chewing Gum - freeze, or sponge with eucalyptus oil.
Chocolate - methylated spirits, or soak in 5% borax solution
Lipstick - glycerine, eucalyptus oil, dry-cleaning fluids.
Rust - oxalic acid, citric acid, tartaric acid
Tar - toluene, xylenes, eucalyptus oil.
Tea or Coffee - glycerine, warm borax solution
Wine - glycerine, borax solution, lemon juice

31.15 How do I remove rust?. 

It depends on the sample and amount of rust. If the material is heavily 
rusted, then physical techniques ( sand blasting ) may be appropriate. 
Chemical techniques on steel usually involve phosphoric acid, and the 
concentration depends whether the treatment can be washed off. An 
excellent discussion is available in  Product Finishing [28], along with
simple formulations. For removing light rust without subsequent removal
of the solution, 15% phosphoric acid + 4% n-butanol + <0.1% sulfuric acid 
is used, but if the solution can be washed away, then a faster acting
33% phosphoric acid + 2% n-butanol mixture is preferred.

31.16 How do I remove silver tarnish?.

Several commercial silver polishes use a fine abrasive ( such as calcium 
carbonate ) to physically remove the tarnish, and also include in their 
formulation components ( such as benzotriazole ), that form complexes or 
films that slow the reappearance of tarnish. There are also a variety of
chemical tarnish removers that contain sulfuric acid, sodium thiosulfate, 
or hot sodium phosphate, a chemical formulary should contain recipes.

A very popular technique for removing tarnish ( silver sulfide ), involves
an electrochemical cell that utilises the electrochemical series. In an
electrochemical cell, oxidation occurs at one electrode, whilst reduction
occurs at the other.  Electrical contact between the two metals provides the 
pathway for electron migration to occur from the anode to the cathode, and
when immersed in an electrolyte, charge will be balanced by transfer of
the sulfide ions. 

The standard electrode potential at 25C of Ag+ + e- -> Ag is +0.799V, and if 
the other metal is anodic relative to silver, the silver in the sulfide at 
the cathode will gain electrons and revert to metallic silver. The metal 
at the anode will be oxidized by the removal of electrons. The sulfide ions
can travel to the anode via the electrolyte. The electrochemical series 
indicates metals that will work, and some readily-available and cheap metals 
are iron, zinc, aluminium and magnesium. The standard electrode potential at 
25C of Al3+ + 3e- -> Al is -1.66V, and aluminium foil is cheap. 

When aluminium is the anode, and water is the electrolyte, the aluminium
surface will eventually be coated with an insulating film of aluminium 
sulfide - which will gradually decrease the cleaning speed. Adding sodium 
bicarbonate ( baking soda ) or sodium carbonate ( washing soda ) to the 
electrolyte will assist in the evolution of hydrogen as the silver is 
converted, and the hydrogen will combine with the sulfide ions to produce 
some hydrogen sulfide gas ( rotten eggs smell ). The aluminium at the anode 
will be converted to aluminium oxide. 

Aluminium oxidises on contact with air, forming an insulating layer of oxide
that is present on the surface of foil, disposable alumimium dishes, or 
pots that are commonly used as the anode when cleaning silver at home. 
Such a layer is brittle, and can micro-crack in hot water, and the higher 
temperatures can also increase the redox reaction rate. Cleaning formulations
may include sodium chloride ( table salt ), boric acid, or borax. It should 
be noted that the process is not converting the silver back to the original 
hard, lustrous surface, but into a soft, white powder that can be removed by 
rubbing. The sulfide has actually dislocated some silver atoms from the metal
surface, and the cleaning treatment will not relocate them, but it will 
eliminate the unsightly, dark, tarnish.

A typical procedure is to line a pyrex glass dish with alumimium foil so the 
bright side of the foil will contact the solution. Add one litre of water, 
and heat until near boiling. Add one tablespoon of sodium chloride and one 
tablespoon of sodium bicarbonate, and gently stir to dissolve. Ensure that
the silverware has been washed in warm dishwashing detergents, and well
rinsed in warm water, to remove dirt and grease. Carefully add the silverware 
to the dish, ensuring that each item is in contact with the aluminium foil, 
and leave until the tarnish has disapppeared. Because no protective film
has been provided, the tarnish may reappear rapidly.

31.17 How do I electroplate or anodise materials?.

There are several excellent books and journals on metal treatments in the
hobbies and metalworking sections of public libraries. For the serious
plater, the journals Surface Finishing and Product Finishing discuss all the
the chemical and electrical aspects - including disposal and destruction of
wastes. Their Annual Handbooks, along with the Canning Handbook of
Electroplating, have many recipes and details for the serious electroplater.
In any metal finishing process, the preparation of the substrate is of
great importance, and the recommended sequence of cleaning, pickling,
plating, and especially passivating should be carefully followed. 
Failure to correctly passivate newly-deposited protective surfaces is the
main cause of the rapid formation of unsightly corrosion products. 

31.18 How fast do solvents pass through human skin?

It obviously depends on the solvents, and traditional measurements have
been made using dead skin, but some recent work has provided a simple
comparison of individual solvents. It must be emphasised that mixtures of
solvents may have significantly different rates [29]. 

                                Permeability Constants in g/m2h 
 Solvent                        Average          Standard Deviation
 Dimethylsulfoxide                176                   42
 N-Methyl-2-pyrrolidone           171                   59
 Dimethylacetimide                107                   19
 Dimethylformamide                 98                    1.1
 Methyl ethyl ketone               53                   29
 Methylene chloride                24                    8.4
 Water  [^3H radiolabeled]         14.8                  0.1
 Ethanol                           11.3                  0.5
 Butyl acetate                      1.6                  0.1
 gamma butyrolactone                1.1                  0.1
 Toluene                            0.8                  0.7
 Propylene carbonate                0.7                  0.4
 Sulfolane                          0.2

31.19 What is the pH of Coca-cola?.

Phosphoric acid is an approved food-grade acid that is added to Coca-cola 
to provide some of the taste. When CO2 is added to "carbonate" aqueous
solutions, carbonic acid is formed. A tin of coca-cola was cooled in a
refrigerator to 7C and the pH of the sample measured over time. The pH
electrode was not temperature-compensated, so a correction was applied.

  Time              pH                pH
               (when opened)     (corrected)            
Initial          2.75 @ 7C           2.6
30 seconds       2.78 @ 7C           2.6
60 seconds       2.80 @ 7C           2.6
2 minutes        2.82 @ 7C           2.7
4 minutes        2.80 @ 7C           2.6

A sample was degassed in an ultrasonic bath for several minutes and the
pH measured, and compared to a control sample maintained at the same 
temperature and time profile.
                    pH
Degassed pH      2.52 @ 21c
RT sample        2.50 @ 20C

So the conclusion is that the pH of Coca-Cola is approximately 2.5 - 2.7.
Not surprisingly, the relatively weak carbonic acid from the dissolved CO2 
did not significantly affect the pH of the beverage. The aluminium beverage 
containers are lined with a thin polymer or lacquer coating to prevent the 
aluminium metal dissolving into the beverage.

Note that the differing flotation characteristics of cans of the normal  
versions of carbonated beverages like Coca-Cola ( they may sink ), and 
the diet versions ( they may float ), has nothing to do with the amount
of CO2 present. It is due to the increase in solution density from the 
few percent of sugar added to the normal version, while the diet version
has only a few hundred ppm of an artificial sweetener that is much sweeter
than sugar.

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