GELATIN 



411 



melted at 40° C, diluted to 100 c.c and portions of each put 

 into test-tubes and exposed to light, the previous manipulations 

 having been carried out in a dark room. The pH's of the 

 solutions remaining are also determined, and also the quantities 

 of silver remaining in them. In 20 minutes all the gelatin 

 solutions with a pH > 47, that is, from pH 4-8 and above, 

 become opaque, and then black, while all the solutions of 

 pH < 4-7 remain transparent, even when exposed to the light 

 for months or years. The solutions of pH 4-7 become opaque, 

 but remain white, no matter how long they are exposed to 

 light ; at this pH — the isoelectric point — gelatin is not in 

 combination with silver, but it is insoluble. Hence the cation, 

 silver, is only in combination with gelatin when the pH is > 4-7. 

 The accompanying figure (Fig. i) is a photograph of a set of 

 test-tubes thus prepared. It should be noticed that in all 

 cases the pH of the various solutions is on the acid side of 

 the neutral point, pH = 7. 



The analytical figures are given in the following table, and 

 illustrated in Fig. 2, which at the same time shows the varia- 

 tion in swelhng of gelatin at different pH's. 



c.c. o'oi N-Ag in combination with 0*25 gm gelatin at different 



pH's 



pH 

 c.c. 



3-6 

 0-5 



37 

 0-3 



3-9 

 0-3 



4-1 



0'2 



4-3 



0*2 



4-6 



0-2 



47 

 0-55 



5-0 

 1-25 



5-3 

 3-2 



57 

 4-0 



6-1 

 4-85 



6-4 

 4-9 



Results similar to those with silver nitrate are obtained when 

 a nickel or copper salt is used. With potassium ferrocyanide 

 the gelatin retains the ferrocyanide, as gelatin ferrocyanide, 

 on the acid side of the isoelectric point, the gelatin turning 

 blue after a few days, owing to the formation of ferric salt. 



Another deduction which can be made in connection with 

 the pH of gelatin solutions is that if the effects on swelling, 

 viscosity, etc., of various monobasic acids (HCl, HBr, HNO3, 

 CCI3. COOH, etc.). and acids such as succinic, tartaric, citric, 

 and phosphoric acids, which dissociate only into two ions 

 at ordinary dilutions, are compared at the same pH, they 

 should be the same ; dibasic acids which dissociate into three 

 ions at ordinary dilutions, such as sulphuric acid, should give 

 different effects. The accompanying figure (Fig. 3) shows that 

 within the error of experiment this is the case for swelhng, and 

 it has been shown to hold for other physical properties. If the 

 curves are compared with those which would be expected on 

 the basis of the Hofmeister series, those for phosphates, 



