Premium
The determination of arsenic in gelatin by means of the molybdenum‐blue reaction
Author(s) -
Eastoe J. E.,
Eastoe B.
Publication year - 1953
Publication title -
journal of the science of food and agriculture
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.782
H-Index - 142
eISSN - 1097-0010
pISSN - 0022-5142
DOI - 10.1002/jsfa.2740040702
Subject(s) - chemistry , ammonium molybdate , hydrochloric acid , arsenic , molybdenum blue , reagent , gelatin , inorganic chemistry , nitric acid , iodide , zinc , molybdenum , nuclear chemistry , organic chemistry , phosphate
An absorptiometric method, based on the molybdenum‐blue reaction, for the determination of arsenic in gelatin is described. The method is suitable for the simultaneous treatment of a number of samples and is more accurate than existing methods based on the Gutzeit test. Gelatin is digested with dilute hydrochloric acid and the arsenic, after reduction, is separated as the hydride by the action of nascent hydrogen liberated by zinc. The arsine is absorbed in iodine solution, which, after the addition of ammonium molybdate and 1‐amino‐2‐naphthol‐4‐sulphonic acid reagents, is then heated at 100°. The optical density of the resulting blue solution, measured photoelectrically, is proportional to the concentration of arsenic over the range 0‐25 μg. The use of the aminonaphtholsulphonic acid as reducing agent has been shown to eliminate a source of error that is encountered if hydrazine is used, when the test solutions are matched visually. In addition the aminonaphtholsulphonic acid confers on the colour improved stability to light. The procedure has been checked against a second method in which the gelatin is completely oxidized with nitric acid. This shows that arsenic, in the forms normally present in commercial gelatins, is made available for separation by digestion with dilute hydrochloric acid on a steam bath. Claims in the literature 1 that tervalent arsenic can be completely recovered from hydrochloric acid solution containing only stannous chloride and potassium iodide by the action of zinc have not been confirmed. Only 50% of the arsenic was recovered from such solutions, but the addition of certain organic materials increased the percentage recovery. Reproducible recoveries, ranging from 96% for 5 μg. of arsenic to 88% for 25μg., were obtained in the presence of gelatin. Error due to incomplete recovery of arsenic can be avoided by carrying out an actual separation of arsenic for the preparation of a calibration curve. The overall accuracy of the method is ± 2% when a spectrophotometer is used for a range of 0‐5 p.p.m. of arsenic in gelatin. The accuracy varies from ± 3‐6%, with selenium‐photocell absorptiometers, to ± 5‐10%, when the solutions are matched visually.