UH. PENNY ON V.-Note on the Valuation of Protorhloride of Tin. BY DR. P ENN Y GIASG~VY. In a paper on the Valuation of Red Prussiate of Potash published in the ‘‘Quarterly Journal of the Chemical Society,”* Mr. Francis Leishing states an objection to the process for estimating the amount of tin in double muriate of tin by bichromate of potash. His words are cc In this instance the liquid double muriate of tin might contain a very considerable admixture of protochloride of iron without even changing its external appearance; and this in practice highly obnoxious admixture would by the sole application of the bichromate of potash test not only escape detection but the iron would actually be calculated as tin.” As this statement may lead some toreject the process without trial or perhaps induce some manufacturer of these tin-solutions to infer that protochloride of iron could be added to chloride of tin without risk of detection it may be as well to show that Mr.Leishing’s objection is really without foundation. It is quite obvious that if acetate of lead be exclusively relied upon as the agent for deterniining when a sufficient quantity of the bichro- mate of potash has been added the process will be vitiated by the presence of iron. In the description of the process however,? it is particularly men- tioned that the (( sulphocyanide test,” consisting of a mixture of pro-tosulphate of iron and sulphocyanide of potassium is much more delicate and satisfactory than acetate of lead. This test completely removes the objection arising from the presence of iron.Sesquichloride of iron cannot exist in the same solution with pro-tochloride of tin ;and accordingly no sesqui-compound of iron can be * Vol. VI 31. -f Quarterly Journal IV 239. THE VALUATION OF PROTOCHLORIDE OF TIN. formed during the addition of the bichromate until the protochloride of tin is wholly converted into bichloride and then the further addi- tion of bichromate instantly produces sesquiehloride of iron the smallest trace of which is immediately revealed by the sulphocyanide test. In point of fact instead of protochloride of iron “escaping detection,” and being calculated as tin,” as Mr. Francis Leisbing affirnis its presence would actually assist the operator in catching the precise moment when the conversion of protochloride of tin into bichloride is perfected.It might indeed be an advantage to add a small quantity of a proto-compound of iron to the tin solution in place of mixing sulphate of iron with the sulphocyanide of potassium as directed in the paper referred to. The addition of protochloride of iron to double rnuriate of tin would in practice be a truly obnoxious admixture but its presence could in no way interfere with the bichromate process for the valua- tion of tin crystals double muriate of tin &c. provided the proper test be employed. It certainly seems almost unneeessary to notice the objection as every one acqiiainted with the applications of these tin preparations must be aware that adulteration with iron would very soon manifest itself in the course of the usual operations. The justness of Mr. Leishing’s remark upon the importance of alkalimetric methods of valuation being preceded by qualitative test- ing of the substance under examination will readily be admitted and especially as the same observation may be extended to nearly all our ordinary processes of analysis.