Top Document: Sci.chem FAQ - Part 4 of 7 Previous Document: News Headers Next Document: 17. Preparation of chemicals See reader questions & answers on this topic! - Help others by sharing your knowledge 16.1 What are the best drying agents for liquids and gases? The Rubber Handbook lists the traditional information on drying agents that involve on chemical action. This lists phosphorus pentoxide and magnesium perchlorate as the most effective desiccants. However, later work by Burfield [1-9] has demonstrated that much of the traditional information is misleading. He found that the efficiency of the desiccant is strongly dependent upon the solvent. He also found that Drierite ( anhydrous calcium sulphate ) is only a moderately efficient desiccant for organic solvents [9], and that correctly prepared molecular sieves are often the preferred desiccant [2]. His publications are highly recommended. 16.2 What is the effect of oven drying on volumetric glassware? Many older laboratory texts insist that volumetric glassware should not be oven dried because of the danger of irreversible and unpredictable volume changes. However most modern laboratory glassware is now made of Pyrex, and work by D.R.Burfield has demonstrated that low temperature drying does not significantly affect the calibration of volumetric glassware [10]. He demonstrated that exposing volumetric flasks and pipettes to 320C, either continuously or thermally cycled, resulted in no significant detectable change to the calibration. He concluded that "oven temperatures in the range of 110-150C should provide efficient drying of glassware with no risk of discernible volume changes, even after prolonged use, providing that Pyrex glass is the material of construction". 16.3 What does the Karl Fischer titration measure? In 1935 Karl Fischer used the reaction between iodine, sulfur dioxide, and water to produce a technique for quantifying water [11]. In aqueous solution, the reaction can be presented as I2 + SO2 + 2H2O <=> 2HI + H2SO4. He used anhydrous methanol to dissolve the I2 and SO2, and added pyridine to move the equilibrium to the right by reacting the acidic products. Fischer assumed his modifications did not change the reaction and one mole of iodine was equivalent to two moles of water. Smith et al.[12], demonstrated that both the methanol and pyridine participate in the reaction and one mole of iodine is equivalent to one mole of water. They suggested two steps:- (1) SO2 + I2 + H2O + 3RN -> 2RN.HI + RN(SO2)O (2) RN(SO2)0 + CH3OH -> RN(SO4CH3)H (where R = base = C5H5 for pyridine) This was further investigated by E.Scholz [13], who proposed: (1) CH3OH + SO2 + RN -> (RNH)SO3CH3 (2) H20 + I2 + (RNH)SO3CH3 + 2RN -> (RNH)SO4CH3 + 2(RNH)I (where R = Base) The advantage of the Karl Fischer titration is that it has few interferences and can quantify water from < 1ppm to 100% in diverse samples, ranging from gases to polymers. It will measure all water that is made available to the reagent. the endpoint is usually ascertained using a dead-stop endpoint, and for low water levels coulometric techniques are used to quantitatively produce the iodine by anodic oxidation of iodide. The procedures are described in detail in ASTM, AOAC etc. 16.4 What does the Dean and Stark distillation measure? The Dean and Stark procedure can be used to measure the water content of a diverse range of samples, and has been extensively used in industrial laboratories to measure water in petroleum oils. The technique can measure % levels of water, but is not as accurate as the Karl Fischer titration, and is not applicable to samples where the water is not liberated by the solvent. The sample is mixed with a solvent ( usually a toluene/xylene mix ) and refluxed under a condenser using a special receiver. There are two common designs of receivers, one for solvents that are heavier than water, and the more common one for solvents that are lighter than water - illustrations will be shown in most laboratory glassware supplier catalogues. The water and solvent are refluxed, and as they condense the two phases separate as they run into the receiver. The water remains in the receiver while the solvent returns to the flask. The Dean and Stark technique is also useful for removing unwanted water from reactions, eg the synthesis of dibutyl ether by the elimination of water from two molecules of n-butanol using acidic conditions. An example of this is provided in the preparation of dibutyl ether described in Vogel, and detailed procedures for the determination of water using Dean and Stark are provided in ASTM and AOAC. 16.5 What does Kjeldahl nitrogen measure? The Kjeldahl procedure is routinely used to measure the protein nitrogen content of organic compounds, especially natural foodstuffs. Contrary to popular belief, the procedure does not determine total nitrogen on all organic compounds, as it is not applicable to materials containing N-O or N-N linkages without modifications to the method. This discrepancy is becoming of more significance as automated nitrogen analysers using other techniques are producing different results because they measure the total nitrogen present. The method usually involves high temperature ( 390C ) digestion of the sample using concentrated sulfuric acid, a catalyst ( Cu, Hg, or Se ), and a salt to elevate the acid boiling point. In some cases 30% hydrogen peroxide is also used, making the digestion effectively a high-temperature piranha solution attack on the organic matter. After digestion, the sample is made strongly alkaline and the ammonia is steam distilled into a boric acid solution, and aliquots are titrated against a standard acid using an indicator solution endpoint. Some organics compounds require aggressive digestion conditions to make all the organic nitrogen available, consequently Kjeldahl procedures should not normally be used on samples that may have N-O or N-N bonds. Details of procedures for foods are in the AOAC handbooks, and general Kjeldahl procedures are detailed in the ASTM volumes. 16.6 What does a Soxhlet extractor do? The soxhlet extractor enables solids to be extracted with fresh warm solvent that does not contain the extract. This can dramatically increase the extraction rate, as the sample is contacting fresh warm solvent. The sample is placed inside a cellulose or ceramic thimble and placed in the extractor. The extractor is connected to a flask containing the extraction solvent, and a condenser is connected above the extractor. The solvent is boiled, and the standard extractor has a bypass arm that the vapour passes through to reach the condenser, where it condenses and drips onto the sample in the thimble. Once the solvent reaches the top of the siphon arm, the solvent and extract are siphoned back into the lower flask. The solvent reboils, and the cycle is repeated until the sample is completely extracted, and the extract is in the lower flask. There is an alternative design where the hot solvent vapour passes around the thimble, thus boiling the solvent in the thimble - this can be a problem if low-boiling azeotropes form. Procedures for using soxhlet extractors are described ( along with illustrations which might make the above description comprehensible :-) ), in Vogel and many other introductory organic laboratory texts. 16.7 What is the best method for cleaning glassware?. As scientific glassware can be used for a variety of purposes, from the ultra-trace determination of sub-ppq levels of dioxin, to measuring % concentrations of inorganic elements, there is no single cleaning method that is "best" for all circumstances. Difficult and intractable deposits often involve the use of hazardous and corrosive chemicals, and details of the necessary safety precautions for each cleaning solution should obtained before attempting to clean glassware. The use of heat and/or ultrasonic agitation can greatly improve the removal rate of many deposits, especially inorganic and crystalline deposits. Whilst the semiconductor industry use piranha solution ( refer Section 12.9 ), and several other reactive and toxic chemicals for cleaning, those reagents can react dangerously with the residues found in laboratories, and their use is prohibited in some institutions. Such chemicals should only be used after extensive prior consultation with laboratory management and safety staff - to either identify safer alternatives, or to ensure that appropriate protective and safety systems are in place. If the probable composition of material deposited on the glassware is known, then the most appropriate cleaning agent can be readily selected. There are several safe aqueous cleaning solutions that are routinely used. If possible, glassware should be washed or soaked immediately with an appropriate solvent for the residue. This will make subsequent cleaning easier, but all traces organic solvents must be removed before using any cleaning solution. The most common aqueous-based soaking solutions are commercial formulations that usually contain alkalis, chelating agents, and/or surfactants, and can be used either at ambient temperature, or temperatures up to boiling ( with ventilation - caustic fumes are noxious ). These are very effective for general grime, most labels, pyrogens, and many common chemical residues, and well known examples include RBS-35, Decon, Alconox, and Pyroneg. Their main advantages are low toxicity and ease of disposal. The next common strategy involves physical abrasion to remove deposits inside flasks, usually with a bottle brush and an aqueous cleaning solvent ( like those above ) or a suitable organic solvent. A refinement is to add sand, pumice, glass spheres, or walnut shell chips, along with some water or solvent, and shake vigorously. It's important that the sand should not have sharp edges - as it can scratch the glass. It has been suggested that table salt in solvent ( eg petroleum spirit, methylene chloride, acetone ) is superior, as it doesn't scratch the glass, can be easily removed by washing with water, and has minimal disposal problems [14]. The traditional glassware cleaning solution is "chromic acid", and many analytical chemistry texts detail the preparation [15,16]. Chromium (VI) is highly toxic ( mutagenic, carcinogenic ), and disposal is expensive, as all solutions containing more than 5 mg/l of chromium are considered hazardous waste in the USA. Disposal of chromic acid requires a two-stage process, involving bisulfite addition to reduce Cr(VI) to Cr(III), followed by neutralisation of the acid. There have also been several reports of spontaneous explosions of chromic acid cleaning solutions [17,18,19], consequently the use of chromic acid for cleaning glassware is declining, and several alternative glassware cleaners have recently been evaluated [20]. Sodium dichromate dihydrate is usually used to prepare chromic acid, as potassium dichromate is less soluble in sulfuric acid. One technique is to dissolve 140g of technical grade sodium dichromate dihydrate in approximately 100 ml of water. Add two litres of technical grade 98% sulfuric acid to a 4-5 litre glass beaker that is sitting in a cold water bath in a fume cupboard. Carefully stir the acid gently and pour a few mls of the dichromate solution slowly into the acid. Keep repeating the addition every few seconds - after the previous dose has been dispersed. As long as the stirring is gentle and continuous, little or no splattering should occur, but the solution will become quite warm. Allow to cool before storing in a glass-stoppered reagent bottle. Always ensure that the stopper is sufficiently loose to release any gas pressure. Never use a screw-capped or similar types of sealed containers. If made correctly, the chromic acid solution should have no precipitate, will be a deep red colour, and will last for years in a glass-stoppered bottle. Ensure the glassware to be cleaned does not have any residual organic solvents. Chromic acid is very effective at around 80C, but an overnight soak at ambient temperature is commonly used. If the solution develops a green hue, it is exhausted and should be disposed of, or regenerated, using appropriate procedures. Slowly pouring used acid down a drain with the cold water tap fully open is no longer considered appropriate. There is a recent report of a technique to regenerate chromic acid cleaning solution ( by distillation of water and oleum ) that reduces disposal quantities [21]. The major problems with chromic acid are the multiple rinses, and perhaps even alkaline EDTA treatment [16], that are necessary to remove all the chromium from glassware - especially if it is required for cell culture or trace analysis, and the increasing problems of safe and legal disposal of spent solutions. An alternative to chromic acid is "Nochromix", which is commercial solid formulation that contains 90-95% of ammonium persulfate ( ((NH4)2)S208 ) along with surfactants and other additives. The powder is dissolved in water and mixed with 98% sulfuric acid. The solution is clear, but turns orange as the oxidizer is consumed, and further additions of solid are routinely required. It is available from Godax Laboratories, New York. A similar bath that is reported to be very effective can be made by the addition of 19 grams of reagent grade ammonium persulfate to two litres of reagent grade 98% sulfuric acid [22]. Add more ammonium persulfate and acid every few weeks, as necessary. One popular replacement for chromic acid in organic laboratories has been alcoholic sodium hydroxide or potassium hydroxide solutions. These remove most deposits, with metals and hydrocarbons greases ( Apiezon ), as notable exceptions. One advantage they have is that they will remove silicone grease deposits from joints and stopcocks, especially if warmed to 65C, and the glassware immersed for up to 10 minutes [23]. Prolonged immersion, even at ambient temperature, will damage ground-glass joints, dissolve glass sinters, and will leave glass surfaces translucent or opaque. The solution can be prepared by either adding two litres of 95% ethanol to 120 mls of water containing 120 grams of sodium hydroxide [16], or by dissolving 100 grams of potassium hydroxide in 50 ml of water and, after cooling, make up to one litre [15]. Solutions based on hydrofluoric acid, usually containing 1-5% of HF, also rapidly attack glass, and destroy sinters, but are very effective for removal of carbonaceous and fine silica deposits. They also remove silicone greases, but alcoholic caustic solutions are preferred [23,24,25]. Hydrofluoric acid is corrosive and extremely nasty if it comes in contact with humans. It requires extensive safety precautions before use. For most deposits, only a few minutes are required, and ultrasonic agitation often assists the removal of deposits. Cleaned glassware usually remains transparent. Cleaners containing HF should not be used on volumetric glassware. Another acidic solution, comprising of a 3:1 mixture of concentrated sulfuric acid and fuming nitric acid, is also extremely effective for removing grease and dirt, but also requires extensive safety precautions. The grease and dirt can often be removed more safely using hot aqueous-based cleaners. If you have intractable organic-based deposits in flasks without standard ground glass ( or clear glass ) joints, then some deposits can be carefully burned off in a glass annealing furnace. The glass needs to carefully follow a slow heating and cooling schedule to minimise thermal stresses and distortion. My experience has been that standard joints do tend to freeze more often after such treatment. Also note that glassblowers may not want to coat their annealing furnace with your rubbish, so they may prohibit the use of their furnace for such activity. User Contributions:Top Document: Sci.chem FAQ - Part 4 of 7 Previous Document: News Headers Next Document: 17. Preparation of chemicals Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: B.Hamilton@irl.cri.nz
Last Update March 27 2014 @ 02:12 PM
|
Comment about this article, ask questions, or add new information about this topic: