D. M. Servant The Polytechnic Wolverhampton, WVI ILY England
Radiochemical Investigation of Some Sample Destruction Procedures
In chemical analysis an element must usually be freed from its mineral or organic matrix and he brought into aqueous solution before it can he determined. It is ohviously vital that the method used give complete recovery. Several authors, notably Gorsuchl and G i l l i ~ have , ~ described the use of radioactive tracers to estimate recoveries of elements in the ashing of organic materials. Among the advantages of the radiochemical method are the absence of contamination errors from the reagents and apparatus used, and the possibility of measuring the tracer a t various stages of the procedure, so that losses may he ascribed to different processes such as volatilization, adsorption on apparatus, formation of insoluble compounds, etc. The simple undergraduate experiment described here assesses a number of methods for the recovery of silver from organic matrices. The material employed is stripping film as used in autoradiography. A single plate, scored into one centimeter squares, provides sufficient samples for many experiments. The radioactive silver is used as bromide in order that it will be in a similar chemical form to that in the sample material. The sample destruction procedures used are dry ashing a t 1000°C and a t 450°C,and wet ashing with sulfuric acid and hydrogen peroxide, and with a mixture of perchloric and nitric acids. The experimental work is simplified by carrying out the ashing procedures in the containers used for the determinations of radioactivitv. hut the results show that to ohtain the highest accuracy in a n actual determmation of silver it would he necessarv to modify the apparatus to - avoid loss of spray. Radioactive Tracer and Counting Apparatus S i l v e r - l l h (Radiochemical Center code S E S 1) was used in the form of aqueous silver nitrate a t specific activity 1.4 Ci/g. . - The solution was diluted to contain 0.1 &i/ ml. Samples were counted in a Panax well type scintillation counter usine 7-ml elass s a m d e bottles (Panax type E), or in silica crucyhles &ced on t i p of the crystal well.^
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Experimental Procedure Dry Ashing a t 1000DC A 1 ml tracer solution and 1ml of 5% aqueous HBr were evaporated to dryness in the presence of 1 cm2 stripping film in a silica crucible. The crucible was counted, and then heated at 1000°C far 1 hr (Safety note: Although the amount of radioactivity used is small, it is essential to ensure that no contamination hazard arises from its volatilization.) After ewling, the crucible was counted again. Dry Ashing a t 450°C A 1 ml tracer solution and 1ml of 5% aqueous HBr were evaporated to dryness in the presence of 1 cm2 stripping film in a silica crucible. The crucible was counted. Fifty milligrams of magnesium nitrate was added as ashing aid and the crucible was heated at 450°C for 3 hr. After cooling, the contents of the crucible were washed with dilute nitric acid through a filter (4.5 cm diameter Whstman No. 1 paper) and the filtrate made up to 50 ml. The crucible, the filter paper (crushed into the bottom of a counting bottle), and 2 ml of the filtrate were counted. Wet Ashing with Sulfuric Acid and Hydrogen Peroxide A 1 rnl tracer solution and 1 ml of water were counted in a counting bottle. One milliliter of 5% HBr and 1 cm2 of stripping film were added and the solution evaporated almost to dryness. A 2 ml portion of 2 M HzSOI was added and the solution heated at -80'C for 1 hr. A 0.5 ml portion of 30% hydrogen peroxide solution was now added and the solution heated for a further hr. After cooling, the contents of the battle were diluted with dilute nitric acid and filtered. The radioactivities of the bottle, the filter, and the filtrate were measured as in the case of the dry ashing at 450-C. Wet Ashing with Perchloric-Nitric Acid Mixture This was carried out as in the previous method except that in place of the sulfuric acid 2 ml of a mixture of three parts perchloric to two parts nitric acid was used. After heating for 1hr the solution was diluted, filtered, and the measurments of radioactivity made as before. Results The results of three runs by each of the methods described are given below. They are expressed as percentages of the original silver obtained in the final solution,
retained on the filter, and retained on the reaction vessel, together with the overall recovery. In the case of the dry ashing a t 1000°C only the overall recovery is given. Dry ashing a t 1000°C I n 111 Overall recovery 9.6 11.3 5.6 Dry ashing a t 450°C Retained on crucible Retained on filter Recovered in filtrate Overall recovery
I
I1
III
19.9 64.0 7.8 91.7
13.9 72.2 6.7 92.8
6.0 80.0
6.4 92.4
Wet ashing with sulphuric acid and hydrogen peroxide Retained on reaction vessel Retained on filter Recovered in filtrate Overall recovery
I
I1
n1
10.2 74.1 1.3 85.6
6.8 91.7 0.5 99.0
4.4 89.6 0.7 94.7
The results obtained from the various procedures are largely as expected. Visual observation showed that all the procedures destroyed the backing material of the film. However, in the cases of dry ashing at 450°C and wet asbing with sulfuric acid and hydrogen peroxide most of the silver remained as insoluble bromide and was retained on the filter. Only the oxidation with the mixture of perchloric and nitric acids brought the silver into solution. In no case was the overall recovery of silver complete. For the dry ashing the loss may be attributed to the volatility of the silver halide and amounted to approximately 90% in the case of the 1000°C experiment. In the wet ashing procedures there were presumably mechanical losses as spray, though the solutions were never allowed to boil. The losses were rather lower in the sulfuric and hydrogen peroxide procedure where the silver did not come into solution. The losses could be minimized in an actual analysis by the use of apparatus designed to prevent the loss of spray. Acknowledgment
The author is grateful to John Bailey for technical assistance.
Wet ashing with perchloric-nitric acid mixture Retained on reaction vessel Retained on filter Recovered in filtrate Overall recovery
Discussion
I
I1
111
0.1 5.1 85.4 90.6
0.0 4.2 84.8 89.0
0.0 3.3 81.9 85.2
'Garsuch, T.T.,Analyst, 84,135 (1959). 2Pijck, J., Hoste, J., and Gillis, J., Pmc. Intern. Symposium Micmcbem., Birmingham Univ. 1958,48 (Pub. 1959).
Volume 51, Number 8,August 1974
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