The linear relation between the pH control reagent addition rate, the net conversion rates of metabolites, the carbon dioxide mass transfer rate and the feed rates is developed and shown to have the same form for batch, fed-batch, and continuous reactors, regardless of the number of feeds. The magnitudes of various biological and solution chemistry effects on the pH control reagent addition rate are estimated to be negligible, facilitating the use of the pH control reagent addition rate as a straightforward indicator of primary metabolism. Finally, application of the linear relation, termed the abiotic proton balance, is discussed for a number of fermentation processes.