Determination of arsenic in foodstuffs contaminated by arsenical war gases.

Date:
1941
Catalogue details

Licence: Public Domain Mark

Credit: Determination of arsenic in foodstuffs contaminated by arsenical war gases. Source: Wellcome Collection.

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    Method for the Determination of Arsenic in Foodstuffs contaminated by Arsenical War Gases. PRINCIPLE. The sample of contaminated foodstuff is oxidised with nitric and sulphuric acids and excess of nitric acid removed; the solution is then reduced with sulphur dioxide and the tervalent arsenic estimated by the Gutzeit method. 1. OXIDATION. About 10 g. of the sample are introduced into a 250 ml. Kjeldahl flask, 30 ml. of conc. nitric acid (AsT.) are added and the contents of the flask are mixed by rotation. The flask is then gently warmed, with periodic rotation or mechanical shaking, until all frothing has ceased. More nitric acid is added (about 10 ml.) followed by 5 ml. of conc. sulphuric acid (AsT.) and the heating is continued carefully, precaution being taken against local overheating and charring. If the initial digestion is too vigorous a sudden evolution of nitric oxides occurs, followed by local charring, production of sulphur dioxide and loss of arsenic. This is particularly the case with foods containing chlorides and those contaminated with lewisite. Presumably a local reduction of AsY to As'” occurs, with distillation of arsenic as trichloride from the medium. The attainment of accurate results is, therefore, dependent upon the exercise of extreme care during the initial stages of the digestion. When practically all the organic matter has been destroyed, more vigorous heat- ing is allowed, with addition of further quantities of conc. sulphuric acid and drop additions of nitric acid until oxidation is complete and a clear sulphuric acid solution results. The time of oxidation varies with the material, fatty substances requiring much longer periods and larger quantities of acids than others. The contents of the Kjeldahl flask are then cooled, diluted with about 50 ml. of distilled water and, after being boiled for a few minutes, evaporated down to sulphuric acid fuming. This is to ensure the complete removal of nitric compounds which inhibit the evolution of arsine in Stage III. It is essential, therefore, that the sulphuric acid shall boil quite vigorously to sweep out all residual nitric vapours from the flask. II. REDUCTION. The contents of the flask, after cooling, are diluted to 25 ml. with distilled water and a steady stream of sulphur dioxide is passed into the solution for 10 minutes to reduce the arsenic to the tervalent condition. Excess of sulphur dioxide is then removed by boiling. At this stage the solution is very prone to bump when heated and precautions must be taken to avoid loss of liquid in this way. The most satisfactory procedure is to heat on a mechanical shaker fitted with electric hot-plates. | All sulphur dioxide must be removed, otherwise hydrogen sulphide will be generated in the Gutzeit vessel and modify the resulting arsine stain.
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