OSMOTIC PRESSURE OF COLLOIDS 



533 



anion. The latter forms a h]no feiric salt on standinji for a few days. The 

 other tubes in this series are coh)urlcss. Similarly, the second series shows a 

 blue ranfie from pK 4-8-5-5 due to the combination of the gelatin with the 

 (■()|)|)ci- to give a coj)j)er gelatinatc. Fnuii these experinieiits one infers that 

 at a /;H between l-f) and 1-8 gelatin acts neither as a cation nor as an anion. 

 4-7 is, therefore, approximately the isoelectric point of gelatin. 



(c) Detennination of the isoelectric point of gelatin by the alcohol precipitation 

 method (Pauli). Prepare a series of buffer solutions of the sodium acetate- 

 acetic acid series, having a range about the isoelectric point of gelatin, e.g. 

 4-1 to 5-3 as follows : 



Take five boiling tubes and put 8 c.c. of distilled water in the first and 

 5 c.c. of distilled water in each of the others. To tube 1 

 add 2 c.c. of N acetic acid and mix. Transfer 5 c.c. of 

 this N/5 acid to tube 2 and mix, and so on, rejecting the 

 5 c.c. of N/80 acid removed from tube 5. That is, a 

 series of tubes having 5 c.c. of fluid of the following pH 

 values has been prepared : 



Tube No. 12 3 4 5 



^H 



4-1 



44 4-7 



5-3 



To each tube add 2 c.c. of N/10 sodium acetate. Mix 

 and add 2 c.c. of a 1 per cent, gelatin sol. Shake. Now 

 slowly add methylated spirit to tube 3 till a cloudiness is 

 just visible. This will require about 8 c.c. Add this 

 amount to each of the other tubes and leave for half an 

 hour. In which tubes is turbidity most pronounced ? 



33. Osmotic Pressure of Gelatin. 



(1) Fit up an osmometer with a collodion membrane 

 (as described on p. 557). Fill the collodion sac with 

 a 1 per cent, gelatin sol and place the osmometer in 

 water up to the level of the rubber stopper. It is 

 necessary to keep the apparatus and its contents at 

 about 16-20° C. to prevent gelatinisation. In about 

 20-30 hours the level of the fluid in the vertical tube 

 will be steady and there ought to be about 40 mm. of 

 solution above the level of the fluid outside. 



(2) A simple comparative experiment to illustrate 

 the low osmotic pressure of gelatin at its isoelectric 

 point may be carried out by means of three thistle 

 funnels (Fig. 56). Over the mouth of each funnel is placed a collodion 

 membrane B (in the same way as is described in the preparation of a collodion 

 dialyser). Into the first osmometer put enough acid gelatin to come above 

 the wide part into the tube. Charge the second one similarly with isoelectric 

 gelatin and the third with alkaline gelatin, and place all three vertically 

 in the same water. After a day one can see that tube 2 has practically no 

 osmotic pressure, while the other two exhibit a pressure of about 70 mm. of 

 solution. The results are not quite accurate because the unprotected collo- 

 dion forming the floor of this simple osmometer is distensible and yields more 

 with the greater hydrostatic pressure. Another disturbing factor is the 

 electrostatic attraction of the gelatin for the CI ions and the Na ions, whereby 

 both CI and Na are prevented from diffusing freely through the membrane 

 (.see Donnan Equilibrium, p. I'^S). They thus cl.^.o exert an osmotic effect and 

 exaggerate the rise in the first and third tubes. 



Fig. 109.— Osmometer 

 with collodion or parch- 

 ment membrane (see 



text). 



