A new operational approach to PCO2 determination in crustacean hemolymph

A new method of calculating PCO2 based on mathematical expressions derived from a measured 'Davenport diagram' is described. The measured 'Davenport diagram' is constructed from in vitro buffer curves relating total CO2 to pH at PCO2 levels which adequately encompass the in vivo range of the acid-base status for the species in question. The 'Davenport diagram' is described by three linear equations such that PCO2 can be accurately calculated from in vivo measured CCO2 and pH. The equations are specific for a given species at a given temperature and hemolymph ionic strength, as are the constants in the more classical Henderson-Hasselbalch equation. The accuracy of the method is equal to calculations using the Henderson-Hasselbalch equation with corrected values for pK1' and alpha CO2 and in vivo PCO2 measured directly. This procedure is equally applicable to fluids with dissolved pigments such as hemolymph from freshwater and marine crustaceans and to human blood. The major benefit of this method of calculating PCO2 is that correction nomograms for the constants in the Henderson-Hasselbalch equation are not required.