Adjustment of pH
On some sites the freshwater pH is too low to achieve optimal growth for fish or shellfish. At other sites the buffering capacity of the water is low, and it is difficult to avoid pH fluctuation in the water. This again results in negative effects on growth. Sites where acid rain is a problem are particularly exposed to this. Further, in re-use systems biological filters are used to remove ammonia and this causes a drop in pH that must be corrected to maintain optimal growing conditions.
pH is the measure of acidity or alkalinity in a solu-tion. It is presented on a number scale between 1 and 14, where 7 represents neutrality and the lower numbers indicate increasing acidity and higher numbers increasing alkalinity. Each unit of change represents a tenfold change in acidity or alkalinity. What is measured is the negative logarithm of the effective hydrogen-ion concentration or hydrogen-ion activity in gram equivalents per litre of the solution.
If substances are added to the water they may act as acids, bases or be neutral. Acids give free hydrogen ions (H+) and bases free (hydroxyl ions) (OH−).
The alkalinity of the water is a measure of its capacity to neutralize acids, meaning its ability to keep the pH constant. If the alkalinity of the water is low fluctuation in pH occurs easily. The carbon-ate system normally represents the major part ofthe alkalinity in aquaculture systems, together with hydroxides (OH−). In the carbonate system com-pounds are related to each other via different equilibria:
CO2+ H2O ↔ H2CO3
H2CO3↔ HCO−3+ H+
HCO−3↔ CO23−+ H+
H2CO3= carbonic acid; HCO−3= bicarbonate ion; CO23−= carbonate ion; CO2= carbon dioxide. Water with free bicarbonate will take up H+ ions.
The amount of each ion in the water is pH-related: in water of low pH there is excess carbon dioxide/carbonic acid; in water of pH 7 there is excess bicarbonate, and in water of high pH excess carbonate ion. The units for alkalinity are either mg/l as CaCO3 or milliequivalents per litre (meq/l) where 1 meq/l is equal to 50 mg/l CaCO3.
A buffer is defined as a substance capable of neutralizing both acids and bases in a solution and thereby maintaining the original acidity or alkalinity of the solution and the resistance to pH changes when adding moderate amounts of base or acids.
Hardness is sometimes confused with alkalinity, mainly because it can be expressed using the same units. Hardness is, however, a term for the sum of all metal ions in the water. This is dominated by the bivalent cations of calcium (Ca2+) and magnesium (Mg2+), but manganese (Mn2+), iron (Fe2+), sodium (Na+) and potassium (K+) may also be important. Because calcium and magnesium are included, hard water may also have high alkalinity, but this is not necessarily always so. If, for instance, sodium andpotassium are responsible for the alkalinity, the hardness can be low. The following water classifica-tion system can be used: soft water, hardness less than 50 mg CaCO3/l; moderately hard water 50– 150 mg CaCO3/l; hard water 150–300 mg CaCO3/l; very hard water, above 300 mg CaCO3/l.1Conduc-tivity may be used as a unit to determine hardness, measured as microsiemens per cm (μS/cm).