H and its control

The acidity or alkalinity of a reaction mixture is most important. It can control the rate of reaction, the nature of the species present and even subjective properties such as taste. The original definition of pH (Sorensen, 1909) related it to the concentration of hydrogen ions. Two facts should be recognized. First, like many ions in solution, the hydrogen ion does not exist in aqueous solutions as a hare H species, but as a variety of hydrated ions, such as HjO. Second, the determination of pH is often carried out by methods that measure the activity of the hydrogen ions, ufHsCT) 

At low concentrations ( 10 molar), y is close to 1, and the difference between concentration and activity is small for uni-univalent electrolytes. 

The practical (or operational) definition of pH recognizes that it is determined using electrochemical cells having an electrode selective to hydrogen ions. This has been discussed in Topics C2 and C3, but a typical cell is  

The constant E depends on the exact nature of the reference and glass electrodes, and is best eliminated by calibration with a standard solution S which has a pH that is accurately known. 

Subtracting these and converting the logarithms gives a practical definition of pH  

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