Interrelationships of Acidity and Alkalinity in Water

Interrelationships of Acidity and Alkalinity in Water. <<<...return

The major buffering system in natural waters is the carbonate system.

This system neutralizes acids and bases so as to reduce fluctuations in pH. It also forms an indispensable reservoir of carbon for photosynthesis because there is a limit on the rate at which carbon dioxide can be obtained from the atmosphere to replace the carbon dioxide in water which has been used by the plants. Thus, the productivities of waters are closely correlated with the carbonate buffering systems.

The addition of mineral acids preempts the carbonate buffering capacity, the original biological productivity is reduced in proportion to the degree that such capacity is exhausted. Therefore, it is necessary to maintain the minimum essential buffering capacity as it is to confine the pH of the water within tolerable limits.

A solution is said to be buffered if its pH is not greatly changed by the addition of moderate quantities of acids or bases. Buffering effects occur in systems where equilibria involving hydrogen ions occur and the range of ph over which buffering is effective depends upon the nature of the solute species.

Most natural waters are buffered to some extent by reactions which involve dissolved carbon dioxide species. The most effective buffering action of these species is within the ph range of mostnatural waters from near 6.0 to about 8.5.

The major equilibria among dissolved carbon dioxide species hydrogen and hydroxide ions have been given in the discussion on pH. The equilibria are presented graphically by species distribution diagram from Hem, Figure 4.2.2.5.4.

This diagram shows the percentages of undisassociated carbonic acid, bicarbonate and carbonate ions as a function of pH.

For example, at a pH of 7.0, about 18% of the total is H2C03 and 82% is HCO3-. The species distribution diagram aids in the interpretation of the chemistry of the alkalinity determination. When bicarbonate activity and pH are known, the activities of the undisassociated carbonic acid and of the carbonate ions can be readily estimated. The diagram shows why the concentrations of carbonate cannot be determined very accurately by titration.

The pH at which carbonate constitutes 1% of the total dissolved carbon dioxide species is about 8.3. This is where the titration end point for carbonate would generally be placed. This is a low enough pH that about 1% of the total carbonate is in the form of H2CO3. If a water contains much bicarbonate and only a little carbonate, the overlapping of the two steps in the vicinity of pH 8.3 may make it difficult to determine the carbonate even to the nearest miliequilivalent per liter. Usually if the carbonate concentration is small compared to the bicarbonate concentration, a value for carbonate can be calculated from the equilibrium equations more accurately than it can be measured by titration.

Chemical Water Quality Parameters  <<<...return
Primary Anions and Cations in Water
Dissolved Gasses in Water
Primary Nutrients in Water and Eutrophication
Toxic Constituents in Water
Pesticides in Water
Oil in Water

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Physical Water Quality
Chemical Water Quality
Biological Water Quality
Water Basics 101



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