# Which Value for the First Dissociation Constant of Carbonic Acid Should Be Used in Biological Work?

## Document Type

Article

## Publication Date

5-1991

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## Abstract

The apparent first dissociation constant of carbonic acid has been defined in different ways in the literature. Harned and co-workers (8-10) have defined it in terms of molalities of the participating species, including H ions: K_{s} = m_{H}mH_{CO3}/m_{CO2}. In contrast, Hastings and Sendroy have defined an apparent constant in which acidity is expressed as H ion activity: K'_{1} = a_{H}m_{HCO3}/m_{CO2}. These constants differ by a factor γ_{H}, the activity coefficient of H ions at the prevailing ionic strength. Therefore, pK'_{1} is greater than pK_{s} by an amount equal to -log γ_{H}, which, at µ = 0.16 M, is approximately 0.1. It is important that the correct value for the apparent dissociation constant or its logarithmic form be entered in the mass action expression or in the Henderson-Hasselbalch equation in order to prevent significant errors in the computation by means of these equations of quantities that cannot be directly measured. Specifically, for the derivation of bicarbonate concentration from P_{CO2} and pH (-log a_{H}), pK'_{1} is to be used and not an uncorrected pK_{s}.

## Repository Citation

Putnam, R. W.,
& Roos, A.
(1991). Which Value for the First Dissociation Constant of Carbonic Acid Should Be Used in Biological Work?. *American Journal of Physiology - Cell Physiology, 260* (5), C1113-C1116.

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