462 



APPENDIX 



determined. The effect of the presence of KCl upon the dissocia- 

 tion of the KOH is negHgible provided the KOH and KCl are 

 always sufficiently dilute to be practically completely dissociated. 

 The extent of the error which is introduced into the determina- 

 tion of X by dissolving the casein in the first instance in a solution 

 of too high alkalinity may be gauged from the following results : 



FINAL SOLUTION 3 PER CENT CASEIN IN 0.015 N KOH 



I have mentioned that the desired concentration of the KOH 

 unneutralized by HCl in the solutions containing casein is attained 

 by the addition, to the solutions of the casein in excess of KOH, of 

 HCl while stirring. This is a matter of some importance. If acid 

 be poured into a solution of a caseinate which is imperfectly mixed, 

 the casein which is precipitated in the acid portions of the fluid 

 forms bulky coagula and is only with difficulty redissolved, even 

 if the quantity of alkali still unneutralized by the acid is more than 

 sufficient to hold in solution all of the casein that may be present. 

 Consequently, the solution of the caseinate must be rapidly stirred 

 while the acid is being added. The same procedure, of course, 

 considerably enhances the rapidity with which the casein dissolves 

 in the alkali employed for its solution. I place the ffiiid in which 

 the casein is to be dissolved, together with the casein,* in a beaker 

 of squat form and 400 cc. capacity. The mixture is then agitated 

 by a flattened glass rod bent at right angles so that the horizontal 

 arm is about 2^ cm. long and as near as possible to the bottom of 

 the beaker; this is rotated at the rate of about 1600 revolutions 

 per minute by a small motor. As soon as the casein is completely 

 dissolved the acid is delivered into solution, a few drops at a time, 

 from a pipette, the opening of which is held at some depth below the 

 surface of the liquid. In some of the earlier experiments the acid 

 was poured upon the surface of the solution, but all of these solu- 

 tions foam to a certain extent and the foam is not agitated by the 



* Regarding the procedure to be followed in adding the casein to the solvent 

 Cf. Chap. XII, 1. 



