Interpretation of pH, Acidity, and Alkalinity in Aquaculture and Fisheries

Abstract

Measurements of pH, acidity, and alkalinity are commonly used to describe water quality. The three variables are interrelated and can sometimes be confused. The pH of water is an intensity factor, while the acidity and alkalinity of water are capacity factors. More precisely, acidity and alkalinity are defined as a water's capacity to neutralize strong bases or acids, respectively. The term “acidic” for pH values below 7 does not imply that the water has no alkalinity; likewise, the term “alkaline” for pH values above 7 does not imply that the water has no acidity. Water with a pH value between 4.5 and 8.3 has both total acidity and total alkalinity. The definition of pH, which is based on logarithmic transformation of the hydrogen ion concentration ([H⁺]), has caused considerable disagreement regarding the appropriate method of describing average pH. The opinion that pH values must be transformed to [H⁺] values before averaging appears to be based on the concept of mixing solutions of different pH. In practice, however, the averaging of [H⁺] values will not provide the correct average pH because buffers present in natural waters have a greater effect on final pH than does dilution alone. For nearly all uses of pH in fisheries and aquaculture, pH values may be averaged directly. When pH data sets are transformed to [H⁺] to estimate average pH, extreme pH values will distort the average pH. Values of pH conform more closely to a normal distribution than do values of [H⁺], making the pH values more acceptable for use in statistical analysis. Moreover, electrochemical measurements of pH and many biological responses to [H⁺] are described by the Nernst equation, which states that the measured or observed response is linearly related to 10-fold changes in [H⁺]. Based on these considerations, pH rather than [H⁺] is usually the most appropriate variable for use in statistical analysis.