GELATION 539 



40. Gelation. 



[a] Heat a .suialJ (|uant it y of I per cent, solution i)[ (J) gelatin, (2) scruiu, 

 (3) dextrine. Cool. What is the result ? Has reheating any effect ? 



[b] Effects of soluleti on gelation. Into four boiling tul)es put the same 

 quantity of 2h per cent, gelatin. In (uie tube dissolve about 5 7 per cent, 

 magnesium sulphate crystals. Potassium iodide crystals are added to the 

 second tube, while a few drops of 1<) per cent, formalin are mixed with the 

 gelatin in the third tube. The fourth tube is left as a control. Allow all 

 tubes to stand overnight and examine by tilting and shaking. Flow do 

 you explain the varied viscosity '( 



Sulphates, citrates and phosphates increase tlie viscosity of aqueous 

 emulsoid gels. Iodides, bromides, cyanides and some chlorides similarly 

 decrease viscosity. Alcohol, formaldehyde, etc.. in small amounts increase 

 viscosity. 



(c) Effect of non -electrolytes on the setting of gelatin. A 6 per cent, solution 

 of gelatin is prepared and divided into three exactly similar tubes. To one 

 tube is added enough cane sugar to make a 10 per cent, solution. To another 

 sample sufficient urea to make a 5 per cent, solution. The third tube will 

 contain gelatin alone. Warm all tubes in a water batli to ensure complete 

 liquidity and uniformity. Put 1 c.c. of fluid from each tube on to three 

 separate watch-glasses and add a small lead shot to each, cover wuth a second 

 w-atcli-glass to prevent evaporation and gently rock the glasses from time to 

 time and note how^ long the fluid in each takes to set (lack of mobility of 

 shot). 



Examine the firmness of the gelatin w^hen set. 



When the gelatin left in the tubes has set, turn out the little cylinders of 

 gel. Weigh, and measure each diameter. 



Place them in a relatively large volume of water oveniight. Dry with 

 blotting paper and again weigh and measure. 



Sugar causes the gel to set more rapidly and swell less than either of the 

 other two. Urea retards gelation and causes the largest imbibition of water. 



Similar experiments may be made with other non-electolytes,. e.g. : 



1. Control . . 6 c.c. of 6 per cent, gelatin + 1 c.c. of water. 



2. Urea . . ,, ,, + 1 gram of urea. 



3. Aldehyde . „ ,, -f- 1 c.c. 40 per cent, for- 



malin. 



4. Alcohol . ,, ,, +1 c.c. methylated spirit. 



5. Cane Sugar . ,, ,, +1 gram sucrose. 



(d) Effect of electrolytes o)t gelation, (i.) Eft'ect of various anions. 



1. Control tube . 3 c.c. 6 per cent, gelation -f 3 c.c. water (very faintly 



acid) . 



2. Sulphate . ,, ,, -|- 3 c.c. normal K.2SO4. 



3. Sulphocyanide „ „ -f 3c.c. N. K.CNS". 



4. Chloride . „ „ + 3 c.c. N. K.Cl. 



5. Salicylate . „ ,, -|- 3 c.c. N. K.C7H5O3. 



(ii.) Effect of various cations. 



1. Control tube . 3 c.c. 6 per cent, gelatin + 3 c.c. faintly acid water. 



2. Sodium . . „ „ +3 c.c. N. NaoSO^. 



3. Calcium . ^^ ^^ ^ 3 c.c. 0-03" N. CaSO.,. 



(sat. sol.) 



4. Magnesium . ,., „ -f 3 c.c. N. MgS04. 



