486 AMPHOTEmC COLLOIDS. IV 



wards washing away the excess of alkali. In each case sodium gelatin- 

 ate was formed but the amount formed varied with the concentration 

 of the alkali solution used; and the pH varied correspondingly. It 

 was of importance to measure the amount of Na or Ca in combina- 

 tion with the gelatin, to make sure that we were dealing with phenom- 

 ena of a stoichiometrical character; it was especially necessary to 

 make sure whether or not Na and Ca combine with gelatin in equiva- 

 lent proportions. The metal gelatinate used for this purpose was not 

 only washed as indicated, but was also dialyzed over night through 

 collodion bags against 400 cc. of distilled water. 



Our former experiments^ allow us to measure the amount of metal 

 contained in a given mass of gelatin when the pH is known. Metal 

 gelatinates can only exist on the alkahne side from the isoelectric 

 point of gelatin, this point being defined by a pH = 4.7. The Na or K 

 or Ca in combination with the gelatin at each pH can be calculated in 

 the following way. We determine the cc. of 0.01 n NaOH required to 

 bring 25 cc. of 1 per cent isoelectric gelatin of different pH (lying be- 

 tween 4.7 and 7.0) to the point of neutraHty (pH = 7.0). By deduct- 

 ing this value from the quantity required to bring 25 cc. of 1 per cent 

 gelatin solution from the isoelectric point to pH = 7.0 — which was al- 

 ways found to be about 4.5 cc. of 0.01 n NaOH^ — we obtain the amount 

 of 0.01 N Na or of | Ba or f Ca in combination with the 25 cc. of 1 

 per cent gelatin at any pH between 4.7 and 7.0. When pH is > 

 7.0 we ascertain the amount of 0.01 n HCl required to bring the 25 

 cc. of gelatin topH = 7.0, and add this to the value 4.5. In Fig. 1 the 

 abscissae are the pH, the ordinates the cc. of 0.01 N Na, K, or | Ba 

 found in combination with 25 cc. of gelatin for each pH. The results 

 of three different experiments with the. three different alkaUes named 

 are plotted (Fig. 1), showing the degree of agreement of the results. 

 The curves are exactly the same whether gelatin has been treated 

 with KOH, NaOH, LiOH, Ba(0H)2, or Ca(0H)2. It is also obvious 

 from the curves that we are deaUng with a simple salt formation of 



6 Loeb, J., /. Gen. Physiol., 1918-19, i, 363. 



' In our preceding paper we determined the cc. of 0.01 N NaOH required to 

 bring 25 cc. of 1 per cent gelatin from the isoelectric point to the turning point of 

 phenolphthalein (pH = 9.0) and found 5.5, which agrees with our present result 

 where we neutralize to pH = 7.0 instead of to 9.0. 



