The pH value, buffering and neutralization capacity of water

The pH value considerably influences the course of chemical and biochemical processes in waters. It causes the differentiation of the speciflc forms of occurrence of some elements, it is one of the aspects for the evaluation of the aggressivity of water and it influences the eflUciency of many technological processes employed for the treatment and puriflcation of waters. 

In clean natural water the pH can be calculated from the content of free CO2 and hydrogen carbonates using the expression for the first dissociation constant of carbonic acid. Dissociation to the 2nd degree can be neglected as its effect becomes significant only as pH 8.3. Due to the inaccurate determination of free CO2 the calculation provides only rough results. On the contrary, from a known value of pH and the content of HCO3 the content of free CO2 can be calculated. 

Distilled water which is in equilibrium with CO2 in clean air (0.03% by volume) contains about 0.55 mg 1 CO2 at 20°C. By calculation it results that its pH is 5.65. 

The pH value can be markedly influenced by its biological processes. For example, during biological nitrification hydrogen ions are released which react with the hydrogen carbonates present to release free CO2 and the pH of the water decreases. Conversely the increase of pH of water can be observed during biological denitrification or reduction of sulphates. 

The buffer value of water (BV) expresses the ability to resist, or buffer, pH changes after addition of acids and bases. It can be defined as a ratio of an infinitesimal addition of a strong base Cj, or strong acid in mol 1 to the corresponding pH change induced in the following way  

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