COAGULATION AT ISOELFX'TRIC POINT 535 



electrode, is filled with and placed in a solution of Ko'SU, ((JU5 per cent.). 

 A current of 2— t volts is passed so that the electrode inside the pot is the 

 cathode. Note the rise in level of tlic lliiid inside the pot. Note also the 

 increase in the alkalinity of the fluid outside the ])()t. 



(c) Make a collodion tube to fit one linih of the U-tube (Fi^^ 19). 



(1) Fill both limbs with dilute K.2SO, solution. Mark the level of the 

 fluid in both limbs and, using non-pohvrisable electrodes, pass a current of 

 4 volts for some time throup;h the sohition. Note that water passes towards 

 the cathode and that the cathodal fiuid becomes acid. 



(2) Repeat, using tartaric acid in the collodion sac and pure water outside. 

 Test for tartaric acid. 



(3) Fill the sac with gelatin sol and leave overnight. Wash out the 

 sol and repeat the experiments (r) 1 and 2. 



30. Coagulation of Sols at the Isoelectric Point (see also Isoelectric Point). 



(a) By heat. Prepare some egg albumin sol. Divide it equally into thres 

 lots. To lot 1 add 1 c.c of N . HCl for every 20 c.c. of sol. Leave lot 2 alone. 

 Add 1 c.c. of N . NaOH to lot 3 for every 20 c.c. of sol. Place in water bath and 

 gently w^arni to 60° C. Cool and examine. Filter if necessary. 



Heat 5 c.c. of each lot to the boiling-point. Only No. 2 coagulates. Care- 

 fully neutralise No. 1 with N/10 NaOH and No. 3 with N/10 HCl. A precipi- 

 tate is formed. Add more alkali or acid as the case may be. The precipitate 

 dissolves. This precipitation by neutralisation is reversible. Repeat the 

 neutralisation experiment on the remainder of 1 and 3. Filter oft' the precipi- 

 tates and suspend each in about half a tube of water. Now heat to boiling. 

 A coagulation forms. Coagulation is irreversible. 



(b) Effect of electrolytes on colloids. MetJiod. Take three test tubes (cZfa^i') 

 for each experiment. To tube 1 add 10 c.c. of the salt solution. To tube 2 

 add 10 c.c. of water + 1 c.c. of salt solution, mix and transfer 1 c.c. to tube 3. 

 To tube 3 add 10 c.c. of water -\- 1 c.c. from tube 2. Mix, withdraw 1 c.c. and 

 reject it. To each tube add 1 c.c. of colloid. Mix and leave alone for one 

 hour. 



Colloids to use. (l)Suspensoid. Dialysed iron (B.D.H.) (1 m 10). (2) Emul- 

 •soid. Gum mastic {q.v.). 



Salts to use. (i.) To show the effect of varying the anion : 2 N solutions of 

 the nitrate, chloride, acetate and sulphate of potassium. 



(ii.) To show the effect of varying the cation : 2 N solutions of the chlorides 

 of sodium, calcium, magnesium and ammonium. 



(c) Mutual precipitation of colloids, (i.) Add gum mastic to colloidal iron 

 till precipitation occurs. Filter and test filtrate for iron. 



(ii.) Add colloidal iron to diluted blood ssrum. Filter through cotton-wool 

 and test filtrate for iron and for protein. 



(iii.) Add tannic acid drop by drop to diluted blood serum till precipitation 

 is complete. Now add more tannic acid. The precipitated colloid again 

 resumes the sol form. 



(iv.) Capillary analysis {q.v.). 



37. Protection of (Hydrophobic) Suspensoids by (Hydrophilic) Emulsoids. 



(a) Five cubic centimetres of colloidal iron (B.D.H. diluted 1 in 0) is jilaced 

 in each of two test tubes. To one add 5 c.c. of 0-1 per cent, gelatin (slightlv 

 acidified) and to the other 5 c.c. of water (also slightly acidified). To each add 

 enough of any of the salts (given in Experiment 36) to precipitate the iron. 

 Precipitation occurs rapidly only in the tube free from gelatin. Determine 

 how much more of the salt solution has to be added to precipitate the 

 gelatinised iron. 



