The application of thiosalts in analysis : Complete precipitation of sulphides

1. A new method of precipitating completely and quickly molybdenum, platinum and arsenic (in arsenates) as sulphide', has been described. 2. The new method is based upon the immediate formation of thiosalts of these metals by adding concentrated ammonium sulphide and the subsequent decomposition of the thiosalt by adding excess of hydrochloric acid. 3. The great usefulness of the method in qualitative analysis lies not only in the completeness of precipitation of the metals but also in the remarkable saving in the time taken to precipitate the metals. 4. It has been shown that the method is applicable also to till the other metals of the arsenic group. 5. Selenium which is precipitated by decomposing the thiosalt is easily soluble in ammonium sulphide while the selenium precipitate from a hot acid solution is generally in a form which is insoluble. 6. Vanadium can be precipitated completely with the arsenic group by this method if it is previously treated with hydrogen peroxide. 7. The use of the hydrogen sulphide gas has been replaced by a far more convenient reagent-concentrated ammonium sulphide.