Phosphorous, Ilypophosphoric and Phosphoric Acids. 93 



longed heating at a fairly high temperature is essential, 

 acidity accelerating the reaction. 8 Success was finally 

 attained by adding an excess of mercuric chloride, then 

 concentrated hydrochloric acid in quantity sufficient to 

 make up about two per cent of the total volume, and 

 finally heating the liquid at 80°-85° for seven hours. 

 The precipitates were collected on asbestos and dried at 

 100°-105°. 



From two 45 cm 3 portions of the solution 0.9331 and 

 0.9302 grm. Hg 2 Cl 2 were obtained, equivalent to 0.003604 

 and 0.003593 grm. H 3 P0 3 per cm 3 . 



(c) Oxidation by Permanganate. — The method em- 

 ployed was a modification of that of Kuhling. 9 Portions 

 of the phosphorous acid solution were measured into 

 conical flasks, and to each was added 40 cm 3 of 10% 

 solution of crystallized zinc sulphate, and an excess of 

 a potassium permanganate solution containing about 

 3.16 grams of that salt per liter. The zinc sulphate 

 serves to prevent the solution from becoming alkaline. 

 The flasks were heated on the steam bath for 90 minutes, 

 cooled and treated with an excess of acidified potassium 

 iodide solution, and the iodine thus liberated was titrated 

 with thiosulphate in the presence of starch. Kuhling 

 filtered off the manganese dioxide and estimated the lat- 

 ter by treatment with potassium iodide, but the modifica- 

 tion described above is more rapid and convenient. "We 

 would also emphasize the importance of a precaution not 

 mentioned by Kuhling, that of determining the standard 

 of the permanganate, or, rather, the ratio of the per- 

 manganate to the thiosulphate, by means of blank experi- 

 ments carried out under precisely the same conditions as 

 the determinations themselves. This procedure tends 

 to cancel out a possible error resulting from decomposi- 

 tion of the permanganate during the heating. The 

 results obtained by this method are recorded in full in 

 Table I. Their average gives for the standard of the 

 solution the value 0.003594 grm. H 3 P0 3 per liter, as com- 

 pared with 0.003591 for the average of the results of 

 (a) and (b). 



Although the volumetric method is probably equally 

 accurate, we shall give the benefit of the doubt to the 

 older methods and accept the value 0.003591 as the 



"Linhart, this Journal, 35, 353, 1913. 

 Ber. ehem. Ges., 33, 2914, 1900. 



