190 



SCIENCE 



[N. S. Vol. XLVI. No. 1182 



In low concentrations of disodium phosphate, 

 gelatin swells decidedly more than in pure 

 water, but as these lower concentrations give 

 way to higher ones, the gelatin swells less and 

 less imtil, when sirfficiently high concentrations 

 are attained, the gelatin swells decidedly less 

 than in piare water. 



These same general truths may be stated for 

 trisodium phosphate, except that the absolute 

 amounts of water absorbed in solutions of this 

 salt are, at the same molar concentration, 

 decidedly higher than in the case of the di- 

 sodium salt. Low concentrations of trisodium 

 phosphate bring about much greater swelling 

 than higher ones. With progressive increase 

 in the concentration of the trisodium salt, 

 there is a progressive decrease in the amount 

 of swelling until a concentration is finally 

 reached in which the swelling is decidedly less 

 than in pure water. 



Having studied in this fashion the relation 

 of swelling to type of salt and its concentra- 

 tion, we investigated next the amount of water 

 absorbed by gelatin in phosphate mixtures of 

 com]3ositions varying from the extreme of 

 pure phosphoric acid on the one hand through 

 mono-, di- and trisodium phosphate to pure 

 sodimn hydroxid on the other. These mix- 

 tures were made in different ways. Beginning 

 with pure phosphoric acid, we added succes- 

 sively greater quantities of sodium hydroxid, 

 or beginning with sodium hydroxid, we added 

 successively greater amounts of acid until the 

 theoretical neutralization had been accom- 

 plished; or we began with pure acid and re- 

 placed this with more and more of the mono- 

 di-, or trisodium phosphate until the opposite 

 extreme of a pure alkali was reached; or we 

 began with a definite concentration of any one 

 of the phosphates and added progressively 

 greater amounts of either acid or alkali. The 

 results in all these expriments were practically 

 the same. In 24 to 48 hours the gelatin at- 

 tained its maximal swelling (practically). 

 When the amount of swelling is plotted on the 

 vertical and the changes in the composition of 

 the solutions from acid tlurough the mixtures 

 of the mono-, di- and trisodium salts to pure 

 alkali on the horizontal, a curve, roughly Y- 



shaped, is obtained. Greatest swelling is ob- 

 served in the pure acid solution and least in a 

 solution consisting essentially of monosodium 

 phosphate. From this point on, there is a 

 gradual increase in the swelling of the gelatin 

 until the disodium salt is passed, when there 

 occurs a more abrupt rise until the trisodium 

 salt is reached, beyond which the curve rises 

 still more steeply until the sodium hydroxid 

 end of the series is attained. 



The swelling of gelatin in monosodium, di- 

 sodiiun and trisodium citrate follows the same 

 general laws as its swelling in the correspond- 

 ing salts of phosphoric acid. Monosodium 

 citrate in all concentrations increases some- 

 what the swelling of gelatin over the amount 

 of swelling in pure water. The same is true 

 of low concentrations of disodium citrate. 

 But the higher concentrations of this salt de- 

 press the swelling to below that attained in 

 pure water. These statements also hold for the 

 trisodium salt. As we succeed in getting 

 more base into the citrate, there appears a 

 distinctly greater tendency to depress the 

 amount of water absorption. 



In studying the amoimts of water absorbed 

 in citrate raixtuxes varying between the ex- 

 treme, on the one hand, of pure citric acid, 

 through mono-, di- and trisodium citrate to 

 pure sodium hydroxid, we observed that the re- 

 sults (when amount of swelling is plotted on 

 the vertical and progressive change in compo- 

 sition of solution on the horizontal) yield a U- 

 shaped curve. Greatest swelling is obtained 

 in the pure acid, the amoimt of this swelling 

 decreasing progressively as we approach the 

 monosodium salt. From the monosodium to 

 the disodium salt the curve falls more gently, 

 until a minimal point is reached in a mixture 

 of about equal parts of monosodium citrate 

 and disodium citrate. From here on, the 

 curve rises gradually to the trisodium salt, 

 after which it ascends steeply as we pass 

 toward the extreme of the pure alkali. 



We have also studied in this fashion the 

 effects of carbonate mixtures. As the sodium 

 bicarbonate in a pure solution of this salt is 

 gradually displaced by a molecularly equiv- 

 alent amount of sodium carbonate, and this 



