Conductivity of Hypophosphoric Acid Solutions. 107 



At constant temperature, K increases rapidly with the 

 concentration of the hydrogen ion, and much more 

 rapidly than the latter, being apparently roughly pro- 

 portional to its square. The exact relation can hardly 

 be determined from our values on account of the high 

 concentration of the hydrochloric acid in most experi- 

 ments. Any considerable change in the hydrogen-ion 

 concentration during the course of the experiment would 

 produce a marked variation in K. That no such effect 

 is visible is due to the fact that the change in the hydro- 

 gen-ion concentration resulting from the hydrolysis is 

 in any case rather small, and here, where hydrochloric 

 acid is present in large excess, the relative change is too 

 small to affect the results appreciably. 



Temperature Coefficient. — From the mean values of 

 K in experiments 4 and 5, 0-000186 for 25° and 0-00631 

 for 60°, the temperature coefficient of the reaction for a 

 10° temperature rise may be calculated by means of the 

 relation 



log 10 K 2 - lofcJK, = A (t.-t,), whence log 10 -%t^ = 10A . 

 The value of £^° so found is 2-7. 



Practical Applications. — The above results show that 

 in dealing with free hypophosphoric acid the hydrolytic 

 decomposition can seldom be ignored, though this may 

 occasionally be permissible in cases where one or more 

 of the factors, temperature, acidity, concentration, or 

 time, have exceptionally low values. It is plain that acid 

 solutions of hypophosphates can not be boiled without 

 loss, nor can they safely be concentrated by heat, and 

 even at room temperature there will be serious danger of 

 loss (a) in solutions containing a large excess of other 

 mineral acids, and (b) when the hypophosphate itself 

 is present in high concentration, as is the case, for 

 example, in strong solutions of pure hypophosphoric 

 acid. 



An objection might be made to the method of analysis 

 which we have used in the measurement of the reaction 

 velocity on the ground that the solution is made acid 

 before the final titration of the iodine. Any hydrolysis 

 of the hypophosphoric acid during this period of acidity 

 would increase the original amount of phosphorous acid 



