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

• Main Authors: Putnam, Robert W., Roos, Albert
• Format: Text
• Language: unknown
• Published: CORE Scholar 1991
• Subjects: Medical Cell Biology; Medical Neurobiology; Medical Physiology; Medical Sciences; Medicine and Health Sciences; Neurosciences; Physiological Processes; Hastings; Carbonic acid
• Online Access: https://corescholar.libraries.wright.edu/ncbp/752; http://ajpcell.physiology.org/content/260/5/C1113.reprint

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: Ks = mHmHCO3/mCO2. In contrast, Hastings and Sendroy have defined an apparent constant in which acidity is expressed as H ion activity: K'1 = aHmHCO3/mCO2. These constants differ by a factor γH, the activity coefficient of H ions at the prevailing ionic strength. Therefore, pK'1 is greater than pKs by an amount equal to -log γH, 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 PCO2 and pH (-log aH), pK'1 is to be used and not an uncorrected pKs.