108 Van Name and Huff — Hydrolysis and Electrical 



and tend to give high results. We have already stated 

 thai the error so produced is negligible, but we are now 

 in a position to support the statement with figures. 

 Let us assume that the hydrogen-ion concentration pro- 

 duced by acidifying is equal to that in Experiment 6. 

 It need be no greater and will usually be less. Assume 

 further that the titration requires ten minutes, and that 

 the temperature is 25° (the temperature will usually be 

 less). Then, if the concentration of hypophosphoric acid 

 is C, dC/dt = O0000023C, 3 and when dt is ten minutes 

 dC/C is 0-000023, that is, 1/400 of one per cent of the 

 hypophosphoric acid is decomposed. This would be 

 wholly negligible except where the concentration of the 

 hypophosphoric acid was many times greater than that 

 of the phosphorous acid, and in such cases it would be 

 easy, by employing lower temperature and acidity, to 

 restrict the decomposition to a small fraction of that 

 calculated above. Moreover, the oxidation of phosphor- 

 ous acid by iodine is so slow in acid solution that only 

 a partial oxidation would take place under the conditions 

 of the titration, so that the actual error would be smaller 

 than that estimated from the amount of hydrolysis. It 

 is evident, therefore, that if this method is used with 

 proper attention to the conditions of analysis, the 

 hydrolysis error will be insignificant. 



This, however, is very far from being true for the 

 iodometric method proposed by Rosenheim and Pinsker 4 

 for the estimation of phosphorous and hypophosphorous 

 acids in the presence of hypophosphoric acid, which is 

 based on the assumption that the hypophosphoric acid is 

 not appreciably hydrolyzed when the solution, made acid 

 by the addition of ''5-10 cm 3 of 10 per cent hydrochloric 

 acid," is heated for several hours in a container 

 immersed in boiling water. In support of this assump- 

 tion the authors quote two blank experiments in which 

 only a trifling loss of iodine was shown on heating with 

 hypophosphoric and hydrochloric acid under the above 

 conditions. Since, however, the iodine was titrated with 

 thiosulphate in the presence of bicarbonate, a thoroughly 

 discredited procedure known to give erroneous results, 5 



3 Comparison of Exps. 4 and 5 shows that the reaction is about 34 times 

 more rapid at 60° than at 25°. Hence if K (;o o — 0-000076, K, 5 o = 00000023. 

 4 Zs. anorg. Chem., 64, 333 ff., 1909. 

 r ' See the preceding article, p. 91, third foot note. 



