EMULSIONS 



415 



2. To determine the optimum concentration of colloid for the stabilisation 

 of an emulsion. Into each of three mortars introduce 20 c.c. of wa,ter, 

 (a) containing 1-25 per cent, of commercial soft soap, (6) 1-875 per cent, 

 and (c) 2-5 per cent. To each slowly add 120 c.c. of, say, cottonseed oil, 

 stirring regularly but not too vigorously meanwhile. If possible, put on 

 a mechanical shaker for | hour. Pour into tall cylinders and allow to stand 

 for some days. 



3. Effect of concentration of oil on rigidity. Stir into four lots of 25 c.c. 

 of 25 per cent, soft soap in mortars, (a) 50 c.c., (6) 100 c.c., (c) 200 c.c., 

 (d) 400-500 c.c. of cottonseed oil. Place in shallow dishes. Note rigidity. 

 What happens when the optimum concentration of oil has been passed. 



4. Divide an emulsion of oil in water (soap), (i.e. 120 c.c. oil in 20 c.c. 

 7 per cent, soft soap) into nine portions. No. (1) will serve as control. 

 To the others add a few drops of one of the following N solutions, (ii) HC1, 

 (iii) NaOH, (iv) Ca(OH) 2 , (v) CaCl 2 , (vi) NaCl, (vii) Alcohol, (viii) CHC1 3 , 

 (ix) Ether. 



5. Instead of soap any hydrophilic colloid may be used, e.g. albumin, 

 casein, acacia, dextrin, starch. The carbohydrate-stabilised emulsions 

 are the hardest to break. 



6. To 5 grams of dry casein in each of three mortars add slowly (a) 50 c.c. 

 N/20NaOH, (6) 50 c.c. water and to (c) 50 c.c. N/20HC1. Allow to stand 

 overnight and then slowly stir 75 c.c. of cottonseed oil into each. Pour 

 into clear jars and allow to stand. Why does (6) separate out ? 



38. Model of Mucoid Secretion. (Fischer.) Grind up in a mortar a 

 small quantity of gum acacia and one or two c.c. of cottonseed oil. Put 

 a drop of this mixture on a microscope slide with cover glass and examine. 

 Place a drop of water at the edge of the cover slip and note what happens 

 when it touches the oil layer. 



39. Comparison of the Electrical Conductivity of Equimolecular Solutions 

 of Mineral and Organic Acids. Conductivity being the reciprocal of resist- 

 ance, obviously can be measured by a resistance method, e.g. by the 



FIG. 78. Diagram of Conductivity Apparatus. 



Wheatstone bridge (Fig. 78). The current from 6 is divided between two 

 circuits (1) by R } and R 2 and (2) by R 3 and R (where R = resistance in 

 ohms). The amount of current travelling by these circuits is such that the 

 drop of potential in both is the same. If then a lead be taken from the 

 junction between R : and R 2 and connection made with the R 3 , R route 



