Open vessels decomposition

The most common reagents for open-vessel decomposition of inorganic analytical samples are the mineral acids. Much less frequently, ammonia and aqueous solutions of the alkali metal hydroxides are used. Ordinarily, a suspension of the sample in the acid is heated by flame or a hot plate until the dissolution is judged to be complete by the total disappearance of a solid phase. The temperature of the decomposition is the boiling (or decomposition) point of the acid reagent. 

Wet decomposition in open vessels allows large sample sizes and achieves low detection limits, but may give rise to systematic errors due to ... 

Microwave-mediated reactions can also be easily carried out without solvents (see Section 4.1). The requirements for these dry media reactions are different to those for reactions in solution. As no solvent is involved, the pressure built-up is rather low, and in most instances such reactions are performed under open-vessel conditions. On the other hand, these mixtures can easily be locally overheated, even though the overall bulk temperature may be comparatively low (macroscopic hotspot formation). Stirring and accurate temperature measurement can prove rather difficult within such a matrix, impeding the investigation of certain reaction conditions. Thus, degradation or decomposition of reagents can be a severe problem for these kind of reactions. 

Wet decomposition in open vessels, however, does not reflect the latest achievements in this field. A relatively large amount of reagent leads to elevated blanks, so that tackling samples in the ng and suh-ng range is rarely possible. 

Two arguments can he put forward in favour of wet chemical decomposition methods in open vessels, firstly, price, and, secondly, sample throughput. A wet decomposition can, if necessary, be performed with a beaker and a heating plate, whereas apparatus for modem high performance decompositions can cost as much as 25 000. With respect to sample throughput, the automatic wet digestion device can be used for up to 1 SO samples per hour for some materials. Such throughput cannot be realized by any other method. Some special cases exist that necessitate decomposition in open vessels (for instance, when the sample needs to be treated with perchloric acid). 

CANDLE.—Read, in connection with the processes described at pages 430,431 —The operations of saponification of the fat, and decomposition of the lime-soap by sulphuric acid, have been hitherto effected in open vessels, under the ordinary pressure of the atmosphere. An improvement on this plan has been recently proposed by M, Delapchiek, of Besangoo, who substitutes dose vessels in Order to saponify the fat. Wash and decompose the lime-soap under a pressure of about twenty-two pounds to the square inch, by which the operations are considerably facilitated, and their duration shortened. The products by this process are said to be superior to those obtained in open vessels, and a considerable economy of flaid is effected. The apparatus consists essentially of a horizontal boiler, with a man-hole and two safety valves at the top, The heat is furnished by a large steam-pipe, running in a depression along the bottom of the boiler. The... 

Recently transalkylation of pyrazoles was demonstrated by Grandberg and Kost.626 On heating with benzyl chloride in an open vessel at 150°, 1-methyl-, 1-ethyl-, and l-propyl-3,5-dimethyl-pyrazole replaced the 1-alkyl by a benzyl group. Both decompositions... 

Open vessel acid digestion, one of the oldest techniques, is undoubtedly the most common method of sample decomposition or dissolution of organic and inorganic sample material used in chemical laboratories. This very inexpensive technique is of inestimable value for routine analysis because it can easily be automated all the relevant parameters (time, temperature, introduction of decomposition reagents) lend themselves to straightforward control. 

Recently Kishorc and laye 1116] examined thermal decomposition of cyclonite by differential scanning calorimetry. The curves of the decomposition arc of an S shape (similar to those of tetryl — Rg. 53). Isothermal curves are similar to Figs 55 and 56. Tlie author calculated the value of decomposition of cyclonite in an open vessel as being 41 2 kcal/mol. They also reported the values of obtained by other authors. In addition to those given in Vol. HI, p. 83 (by Robertson), they are those of ... 

The use of microwave ovens for the decomposition of both inorganic and organic samples, first proposed in the mid-1970s, is now an important method of sample preparation. Microwave digestions can be carried out in either closed or open vessels, but closed vessels are more popular because of the higher pressures and higher temperatures that can be achieved. 

When Ar = />-CH3OC6H4 yields of a,G)-diiodides are very good up to = 40. If the cleavage is carried out by boiling in open vessels, the solid material is filtered off after some hours and heated again with fresh hydriodic acid. The crude diiodides are washed with water, dried, and freed from humin-like decomposition products of hydroquinone by extraction in a Soxhlet with light petroleum. 

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