TECHNIQUE OF ELECTROCHEMICAL MEASUREMENTS 463 



stirrer with the same rapidity as the fluid which lies below it, 

 consequently, if the acid is poured upon the surface, casein is 

 precipitated within the foam and is only with great difficulty 

 redissolved, hence solutions prepared in this manner yielded very 

 irregular results both in the gas-chain and in the conductivity de- 

 terminations. In delivering the acid into the solution it is very 

 necessary to avoid holding the opening of the pipette too close to 

 the side of the beaker, as in that case a film of casein is precipitated 

 on the glass and this film redissolves with great difficulty. 



Since the conductivities of solutions of the caseinates must be 

 determined as soon as possible after the introduction of casein 

 into the solution employed to dissolve it, for otherwise hydrolysis 

 introduces a considerable error, it is evident that as little time as 

 possible must be consumed in bringing the temperature of the 

 solution to that at which its conductivity is to be determined. 

 This could be achieved in either of two ways: either a small 

 volume of fluid could be employed, so disposed as to take up the 

 temperature of the water-bath very quickly; or the water-bath 

 may be maintained at a somewhat higher temperature than that 

 actually desired, and the conductivity of the fluid can be measured 

 at the moment when it reaches the desired temperature. For 

 reasons which will be sufficiently obvious the latter procedure is 

 found to be more convenient. The water-bath is kept at a tem- 

 perature lying between 31.5 and 32.5 degrees, a preliminary 

 measurement of the conductivity of the solution is made at a 

 temperature between 29 and 29.5 degrees and this preliminary 

 determination is corrected at precisely 30 degrees.* 



Solution of 1 per cent casein in 0.03 N KOH : 



X X 10 5 determined immediately 348. 8 



X X 10 5 determined after allowing solution to stand at about 



30 degrees for 20 minutes 356. 3 



It is obvious, however, that in this procedure the gas-chain 

 measurements are made at a temperature some 2 degrees higher 

 than the conductivity measurements, and it may be inquired to 



* For maintaining constant temperatures, W. M. Clark employs an air- 

 bath; Cf. Journ. Amer. Chem. Soc., 35 (1913), p. 1889; Journ. Biol. Chem., 

 23 (1915), p. 475. C. L. A. Schmidt, Univ. of Calif. Publ. Pathol., 2 (1916), 

 p. 157, employs an oil-bath similar in construction to that described by 

 G. A. Hulett, Physical Review, 32 (1911), p. 257. 



