36 MEDICAL MYCOLOGY 



Since HOH is water and since the quantity formed in a given solution by reassociation 

 of ions is negligible in comparison with the sum total of molecules present, we may for all 

 useful purposes write the equation: 



Kw ■= [H-] [0H-] 



The above is a relationship that holds in all aqueous solutions. However concentrated 

 the hydrogen ions, there still must always be enough hydroxyl ions present to satisfy the 

 constant ; however concentrated the hydroxyl, there must still be enough hydrogen ions. Thus, 

 for instance, we might express the acidity of solutions in terms of the hydroxyl ion concentra- 

 tion. As a matter of convention, but not necessity, we express acidity and alkalinity in terms 

 of hydrogen ion concentration. 



Kw has been determined with considerable care to be practically or lO-i*. Thus 



1014 



[H+] [0H-] = 10-14 always holds, whatever the solute or solutes. In chemically pure, 

 distilled water, which is of course entirely neutral, [H+] c= [0H-] = 10-7. A neutral solu- 

 tion, then, is one in which this equation holds. For higher concentrations of hydrogen ions, 

 those from [H+] = IQi up to [H+] = 10"^ (remember the exponent is negative) solutions 

 give acid reactions. For lower concentrations, between [H+] = 10"^ and [H+] = lO'i*, solu- 

 tions are alkaline. 



As a convenience in manipulation, the reciprocal of the hydrogen ion concentration is 

 generally used. This obviates the use of the negative exponent. Thus, in alkaline solutions, 



varies between 10^ and IQi*, another way of expressing the facts expressed above, but 



somewhat simpler. It is still simpler, and quite permissible, to use the logarithm of the 



reciprocal _ _ = 10" logj, = x logjJO. Since tlie logarithm of ten is unity, 



[H+J H+ 



log = X. This X, the logarithm of the reciprocal of the hydrogen ion concentration, is 



H* 



what is referred to as the pH of a solution. It follows, then, that from pH 1 to pH 7 a solu- 

 tion is acid and from 7 to 14, a solution is alkaline. 



There are two common methods of measuring hydrogen ion concentration. 

 The most direct method is by measuring the potential of the so-called hydro- 

 gen electrode in a portion of the solution to be determined. It has been found 

 that with this quantity measured, the concentration may be deduced very 

 simply according to the following formula : 



Potential , 1 



= log 



Numerical factor [H"^] 



With the factor once determined and the potential directly measured, there 

 should be no further difficulty. The apparatus, however, is expensive and 

 while apparently simple to manipulate is actually unreliable except in the 

 hands of an expert who appreciates the various possible sources of error and 

 is ever on the alert for them. There are many types of potentiometer on the 

 market, but it is safe to say that none of them is fool-proof. 



The indirect method, the colorimetric, depends upon the fact that certain 

 series of organic compounds exhibit great changes in color with varying pH. 

 Phenolphthalein, for instance, is colorless in acid solution, but becomes ma- 

 genta at pH 9. Litmus is violet in alkaline solutions, red in acid, changing at 

 approximately pH 7. Suitable indicators may be found for almost any desired 



