702 DONNAN EQUILIBRIUM AND OSMOTIC PRESSURE 



the beginning of the experiment to 3.5 through the addition of H3PO4. 

 When the bags containing gelatin phosphate solutions are put into 

 water the latter diffuses rapidly into the gelatin solution thereby 

 lowering the concentration of the gelatin solution. To avoid this 

 error so much gelatin phosphate solution was poured into each bag 

 and glass tube that at the beginning of the experiment the liquid 

 reached already to about that level which from preceding experiments 

 we knew the gelatin solution would reach in the manometer at the 

 point of osmotic equilibrium. All experiments were made in dupli- 

 cate. In addition to the osmotic pressure we measured the pH 

 inside and outside after equilibrium was reached. From these latter 

 data the osmotic pressure due to the H and PO4 ions could be cal- 

 culated, being equal to 



(2y + z- 2x) X 2.5 mm HjO 



By deducting this value from the observed osmotic pressure in each 

 case it was hoped to obtain a rational value for the share of the pro- 

 tein particles in the observed osmotic pressure. Table IV gives the 

 results. 



The reader's attention is called to the last two rows of figures 

 (Table IV) giving the difference between the observed and the cal- 

 culated osmotic pressures, since if this difference actually represents 

 the osmotic pressure due to the gelatin particles, the figures should be 

 in direct proportion to the concentration of the gelatin. The experi- 

 ments were all made in duplicate to give some idea of the magnitude 

 of error and it is obvious that the error may be considerable, 25 per 

 cent or more, because the errors in the observed and the calculated 

 values are additive. Thus the ''difference" is for f per cent solution 

 in one case 92, in the other 61, a variation of 50 per cent! If we take 

 this into consideration we may conclude that the differences between 

 the observed and the calculated osmotic pressures are compatible 

 with the idea that the difference is the value for the osmotic pressure 

 due to the gelatin particles in solution. 



This would lead us to the conclusion that the osmotic pressure due 

 to the gelatin particles in a 1 per cent solution (of originally isoelectric 

 gelatin) of gelatin phosphate of pH 3.60 is about 100 mm. H2O. 

 Since the osmotic pressure of 1 grammolecule is about 250,000 mm. 



