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United States Department of Agriculture

Agricultural Research Service


Location: Warmwater Aquaculture Research Unit

Title: Interpretation of pH, acidity, and alkalinity in fisheries and aquaculture

item Boyd, Claude -
item Tucker, Craig -
item Viriyatum, Rawee -

Submitted to: North American Journal of Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 7, 2011
Publication Date: September 27, 2011
Repository URL:
Citation: Boyd, C.E., Tucker, C.S., Viriyatum, R. 2011. Interpretation of pH, acidity, and alkalinity in fisheries and aquaculture. North American Journal of Aquaculture. 73(4):403-408.

Interpretive Summary: The concepts of pH, acidity, and alkalinity are fundamental variables that define water quality but they are often misinterpreted in aquaculture and fisheries studies. This paper explains the concepts of pH, acidity, and alkalinity, and discusses practical relationships among those variables. The concept of pH averaging as an expression of the central tendency of pH measurements is also explained. This concept is poorly understood because it is commonly believed that pH values, which are log-transformed numbers, cannot be averaged directly. Evidence is presented showing that direct averaging of pH values is the simplest and most logical approach for most uses, and that direct averaging is based on sound practical and statistical principles.

Technical Abstract: Measurements of pH, acidity, and alkalinity are commonly used to describe water quality. The three variables are interrelated and are sometimes confused. The pH of water is an intensity factor, while the acidity and alkalinity of waters are capacity factors. More precisely, acidity and alkalinity are defined as the capacity of a water to neutralize strong bases or acids, respectively. The terms “acidic” for pHs below 7 and “alkaline” for pHs above 7 do not imply that water with pH below 7 has no alkalinity and water with pH above 7 has no acidity. Waters between pH 4.5 and 8.3 have 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+] before averaging appears to be based on the concept of mixing solutions of different pH. In practice, however, [H+] averaging will not provide the correct “average pH” because buffers present in natural waters have a greater affect on final pH than does dilution alone. For nearly all uses of pH in fisheries and aquaculture, pHs may be averaged directly. When pH datasets are transformed to [H+] to estimate average pH, extreme pH values distort average pH. Values for pH appear are more normally distributed than [H+], making them more acceptable for use in statistical analysis. Moreover, electrochemical measurements of pH and many biological responses to hydrogen ion are described by the Nernst equation, which states that the measured or observed response is linearly related to ten-fold changes in [H+]. Based on these considerations, pH, rather than [H+], is usually the most appropriate variable for statistical analysis.

Last Modified: 7/23/2016