652 APPENDICES 



To convert one system of notation to the other is a simple matter of 

 logarithms. 



For example, to convert pH T'G to other notation — 



pH 7-6=- 10-7-6 = 10-7x10-0-6 



(the antilogarithm of -0-6 is 0-25) . • . =0-25 x 10-7. 

 Conversely, Ch 5x 10-6 = log 5 + log 10—6. 



= 0-6990 + (-60000). 



= 5-3, 

 or 5 X 10-6 = 10-699 X 10-6 = 10-5-3 = pH 5-3. 



(The student will remember that in dealing with logs, multiplication is 

 done by addition division by subtraction, and squaring by multiplication 

 by 2, etc.) 



Semipermeable Membrane. 



A membrane is simply a thin film of substance. It may be composed 

 of almost any material, and therefore may have all degrees of permeability. 

 When a membrane allows water to pass through, but prevents the passage 

 of a substance in solution in the water, it is said to be semipermeable to 

 the solution. Thus water passes through a membrane of copper-ferro- 

 cyanide, but cane sugar in solution is kept back. Copper-ferrocyanide is 

 therefore said to be semipermeable to a solution of cane sugar in water. 



Osmosis and Dialysis. 



The passage of water through a semipermeable membrane is termed 

 Osmosis. The passage of substance in solution through such a membrane 

 is termed Dialysis. 



Osmotic Pressure. 



If a solution of a substance in water be placed in a bag which is semi- 

 permeable to the solution, and the bag with its contents be immersed in 

 pure water, the molecules of water will have free passage through the walls 

 of the bag, but the molecules of the solute will be held back. No pressure 

 will be exercised by the water, since it has free passage. The molecules 

 of the solute, however, will imj^inge on the inside surface of the bag with 

 a certain amount of force, which is not balanced by molecules of solute 

 impinging on the outside. There will thus be a force exerted tending to 

 distend the bag. This force is called Osmotic Pressure. 



It has been found that the osmotic pressure of a solution is the same 

 as the pressure exerted by a gas having the same number of molecules per 

 unit of volume as the solute. The solute thus behaves as if it were a "as 

 and the solvent absent. As in the case of a gas, the pressure is projjortional 

 to the temperature and to the concentration. This is easily understood, 



