Box 11-1 Alkalinity and Acidity

Alkalinity is defined as the capacity of natural water to react with H+ to reach pH 4.5, which is the second equivalence point in the titration of carbonate (CO5 ) with H. To a good approximation, alkalinity is determined by OH-, CO j, and HCOf  

When water whose pH is greater than 4.5 is titrated with acid to pH 4.5 (measured with a pH meter), all OH-, CO , and HCO3 will have reacted. Other basic species also react, but OH-, CO , and HCOf account for most of the alkalinity in most water samples. Alkalinity is normally expressed as millimoles of H+ needed to bring I L of water to pH 4.5. 

Alkalinity and hardness (dissolved Ca24 and Mg2+, Box 12-3) are important characteristics of irrigation water. Alkalinity in excess of the Ca2+ + Mg2+ content is called residual sodium carbonate. Water with a residual sodium carbonate content equivalent to 2.5 mmol H+/L is not suitable for irrigation. Residual sodium carbonate between 1.25 and 2.5 mmol H+/L is marginal, whereas si.25 mmol H+/L is suitable for irrigation. 

Acidity of natural waters refers to the total acid content that can be titrated to pH 8.3 with NaOH. This pH is the second equivalence point for titration of carbonic acid (H2C03) with OH-. Almost all weak acids in the water also will be titrated in this procedure. Acidity is expressed as millimoles of OH- needed to bring 1 L of water to pH 8.3. 

The first two and last two equivalence points are unrecognizable, because they occur at too low or too high a pH. 

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