Models such as the Anaerobic Digestion Model No. 1 (ADM1) assume that pH can be calculated directly from the concentration of hydrogen ions. However because pH is, by definition, the negative logarithm of the hydrogen ion activity, and thus pH measurements represent hydrogen ion activities, this approach may lead to a bias between measured and predicted pH values. Implementing ionic strength effects into the charge balance equation and the calculation of pH is a theoretical improvement to this. In this study a model, implementing a procedure for calculating pH, was developed to analyse the effect of ionic strength on pH in a pig manure. By adding KCl to samples of pig manure, experimental results could be analysed with help from the model. A modified form of the Davies equation was found to give the most accurate prediction of pH in the pig manure studied in this paper with changing ionic strength.

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