NON-DEPENDENCE OF COMPOSITION ON DILUTION 207 



excess of these reagents admits of a very different explanation, 

 and the data cited above also enable us to conclude that water 

 is not able to appreciably decompose the potassium salt of casein. 

 We have seen that whether the percentage of casein in solution 

 is 0.5 or 3.0 or intermediate between these, the quantity of potas- 

 sium hydrate which is bound by a given weight of casein at 

 absolute neutrality is the same. The dilution (relative mass of 

 water to salt) may vary 600 per cent and yet the salt remains 

 unaltered; it is not decomposed by the water in any preceptible 

 degree. This result has also been obtained for other bases, 

 including comparatively weak bases such as Ca(OH) 2 , by the 

 observers, cited in the previous section, who have employed the 

 method of direct titration and these observers have also shown 

 that at neutrality to phenolphthalein the composition of the 

 casein salt (= 80 X 10~ 5 equivalents of base per gram) is always 

 the same no matter what the dilution of the system. Moreover, 

 as the curves in the figure on p. 200 clearly reveal, the compo- 

 sition of the casein salt, when the casein is exerting its maximum 

 combining-capacity, is the same for very different concentrations 

 of this salt. That this should be so in alkaline solution is not 

 surprising, since the excess of alkali might be expected to drive 

 the reaction: 



R.COOH + KOH <=> RCOOK + H.OH 



over towards the right; but that it is so in absolutely neutral 

 solution, when the concentration of free KOH is evanescently 

 small, is an extremely striking fact. 



But, not only is the composition of the caseinate of potassium 

 independent of its dilution in neutral solution, it is also inde- 

 pendent of its dilution in a solution which is pronouncedly acid 

 in reaction. It will be shown in the succeeding chapter that the 

 amount of a base which is bound by casein at "saturation" of the 

 base with casein, that is, when there is just sufficient base to hold 

 the casein in solution is also independent of the dilution of the 

 system. Now in this solution the acidity is about 10~ 5 H + 

 (cf. the table of reactions to indicators on p. 91). A glance at 

 the formula 



R.COOH + KOH<=R.COOK + H.OH 



suffices to show that in acid solutions, when the KOH concen- 

 tration is excessively small and the H+ concentration large the 



