Hot plate digestion

ASTM. 1998b. ASTM E 1644. Standard practice for hot plate digestion of dust wipe samples for the determination of lead by atomic spectrometry. American Society for Testing and Materials. 

Table 14.9 Element recovery for closed vessel microwave digestion versus open vessel hot plate digestion of EPA sludge sample (all values are pg g-1)... 

Adequate digestion methods are important in the determination of all metals, including aluminum. Que Hee and Boyle (1988) showed that Parr bomb digestions were always superior to hot plate digestions for many elements, including aluminum, in feces, liver, and testes. Microwaving in closed vessels produced lower aluminum recoveries in liver than Parr bomb digestions. The Parr bomb values for citrus leaves were within 5% of the NBS certified values. 

Until recently, most laboratories were still using hot plate as their only method of sample preparation, a technique that, except for its sources of energy, dates back to the age of alchemy. Because of the indirect nature of heating in hot plate digestions, attainment of the final chemical conditions is relatively slow (Kingston Walter 1992 Kingston Haswell 1997). 

Acid digestion is a well-suited technique and recently Parr bomb mineralization has been preferred to microwave or hot-plate digestion [39]. 

Mix 42 5 g. of acetone cyanohydrin (Section 111,75) and 75 g. of freshly powdered ammonium carbonate in a small beaker, warm the mixture on a water bath FUME CUPBOARD) and stir with a thermometer. Gentle action commences at 50° and continues during about 3 hours at 70-80°. To complete the reaction, raise the temperature to 90° and maintain it at this point until the mixture is quiescent (ca. 30 minutes). The colourless (or pale yellow) residue solidifies on coohng. Dissolve it in 60 ml. of hot water, digest with a little decolourising carbon, and filter rapidly through a pre-heated Buchner funnel. Evaporate the filtrate on a hot plate until crystals appear on the surface of the liquid, and then cool in ice. Filter off the white crystals with suction, drain well, and then wash twice with 4 ml. portions of ether this crop of crystals of dimethylhydantoin is almost pure and melts at 176°. Concentrate the mother liquor to the crj staUisation point, cool in ice, and collect the... 

With samples that are difficult to dissolve, the first approach is usually to try digesting the sample with an acid or base. Table 7.2 lists the most commonly used acids and bases and summarizes their use. Digestion is commonly carried out in an open container, such as a beaker, using a hot plate as a source of heat. The chief advantage of this approach is its low cost as it requires no special equipment. Volatile reaction products, however, are lost, leading to a determinate error if analyte is included among the volatile substances. 

Because of the risk of lead poisoning, the exposure of children to lead-based paint is a significant public health concern. The first step in the quantitative analysis of lead in dried paint chips is to dissolve the sample. Corl evaluated several dissolution techniques. " In this study, samples of paint were collected and pulverized with a Pyrex mortar and pestle. Replicate portions of the powdered paint were then taken for analysis. Results for an unknown paint sample and for a standard reference material, in which dissolution was accomplished by a 4-6-h digestion with HNO3 on a hot plate, are shown in the following table. 

Kocher, m. boiler, cooker, digester hot plate, -ausbeute,/. cooker yield, jdeld on cooking or boiling,... 

Most average homes contain a microwave oven. Researchers have taken that same technology and applied it to sample extraction. Using microwave technology with acidic solutions has become a commonplace replacement for traditional acid digests. The sample is placed in a closed, chemical-resistant vessel and heated in a microwave. This process is much faster than hot-plate techniques and has become widely accepted by such agencies as the US EPA. 

In this method lg dried soil in a silica crucible is mixed with 5ml 0.2N sodium hydroxide and 10ml absolute ethanol and left to evaporate on a hot plate for 12h. The residue is digested with 0.6N sulphuric acid. After treatment with ethylene oxide and acetonitrile the acetonitrile layer is recovered. 

After either procedure, filter off the calcium sulfate on a Buchner funnel. It should be washed until it is pure white, air dried, and preserved as a specimen, but the wash water should not be added to the main filtrate. It will be found that it will filter better if it is digested overnight on the hot plate, for a precipitated powder always tends to grow into somewhat coarser crystals on long warming in the mother liquor. (Why is this )... 

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