OSMOTIC PRESSURE 511 



of diffusion. A narrow strip of translucent squared paper pasted over the 

 length of the test tube will aid in the determination. 



(c) Temperature. Use two sam])les of cone. CuSO^ — one at 0° C. and the 

 other at 40° C. 



Note that the water added should be at the same temperature as the solu- 

 tion and the tubes should be kept at constant temperature during the duration 

 of the experiment. 



4. Partial Separation of two Solutions by Diffusion. 



(i.) Add sufficient dilute alkaline aqueous eosiji to a solution of night blue 

 to give a dark violet mixture. Allow this mixture to stand in contact with 

 water (as above) for a day. The supernatant fluid will be stained red and the 

 underlying fluid will be a bluish violet. 



(ii.) Make some congo red just blue by the addition of a few c.c. of N/10 

 sulphuric acid and allow this blue liquid to lie in contact with water tinged 

 with phenol red for twenty-four hours. The acid diffuses from the congo 

 red into the water. The result is a yellow fluid lying over a red one. 



(iii.) Other pairs of rapidly diffusible and slowly diffusible substances are : 

 picric acid -j- alkali blue, picric acid -|- alizarin red, alkali blue -f acid 

 fuchsin. 



5. Osmotic Pressure of Crystalloids. 



(a) By Osmometer. Preparation of Semipermeable Membrane. Take a 

 clean porous pot such as is sold for Leclanche units. Allow it to soak for a 

 day in distilled water. Fill it with a 0-25 per cent, solution of copper sulphate 

 and immerse it in a 0-21 per cent, solution of potassium ferrocyanide for a 

 day or two. Wash thoroughly in distilled water. The copper sulphate and 

 potassium ferrocyanide meet in the porous pot and a membrane of copper 

 ferrocyanide is there formed (see Expt. 50). The prepared pot may keep for 

 years and be used many times. 



A rubber stopper with two holes should be permanently fixed in its mouth 

 with wax. Through one hole should be passed a long glass tube or a U-shaped 

 glass manometer. The other hole carries a tap funnel for filling the pot. 

 The solution to be tested should be coloured with methylene blue or other 

 dye which is easily seen. 



(i.) What happens after 24 hours or so when a 10 per cent, cane sugar 

 solution is placed in the pot and the pot immersed in water ? 



(ii.) Now add sugar to the fluid outside the pot till its concentration is 

 the same as that inside the pot and leave for the same period, as before. 



(iii.) Increase the concentration of sugar outside and note the effect on 

 the level of fluid in the manometer. 



(iv.) Clean out the pot and fill it, in turn, with the following solutions : — 



ilf/64 cane sugar. 

 ilf/128 sodium chloride. 

 M/192 calcium chloride. 

 ilf/192 sodium sulphate. 

 They should all rise to the same height in the same time, i.e. they are isotonic 

 solutions. 



(v.) Prepare an osmometer with a collodion membrane (as in Expt. 33) 

 and again determine the relative osmotic pressures of the above four solu- 

 tions. The rise in hydrostatic pressure, in the case of a collodion membrane, 

 is not equal for solutions of equal osmotic pressure. The cane sugar in half 

 an hour shows scarcely any osmotic pressure, the CaClg solution gives the 

 greatest rise, about 50-60 mm., next comes the NaCl with about 15 mm., 

 and the NagSO^ at from 4^ 5 to — 3 mm. Why tlo these values differ 



