The values of pK 1 + pK 2 for the dissociation of carbonic acid in seawater

Abstract

The values of pK 1 + pK 2 for the dissociation of carbonic acid have been determined in seawater as a function of temperature (0 to 45°C) and salinity (5 to 42). They were determined by the addition of NaHCO 3 to seawater stripped of CO 2 until the pH 0 = 1/2(pK 1 + pK 2) was constant. The pH 0 was measured using potentiometric and spectrophotometric techniques. The values of pH 0 determined by the two methods are in good agreement (± 0.002). Our values of 1/2(pK 1 + pK 2) are in good agreement (0.005) with the results of Mehrbach et al. (1973) and the combined data have been fitted to the equation 1/2( pK 1+ pK 2)=−247.8958+6.564628 S-3.322×10 −4 S 2+12074.50/ T+37.764148 ln T+( −290.721 S+0.12980 S 2)/ T−0.983517 S ln T with a σ = 0.0052. Both studies indicate that the measurements of pK 2 in artificial seawater are lower than the values in real seawater. Values of the pK 1 in seawater were also determined from potentiometric titrations of seawater at a few temperatures (15 to 45°C). The results are in better agreement (0.01) with the results of Mehrbach et al. between 20 to 30°C than other workers. Our results and those of Mehrbach et al. have been combined to yield (σ = 0.0056) pK 1=−43.6977−0.0129037 S+1.364×10 −4 S 2+2885.378/ T+7.045159 ln T and (σ = 0.010) pK 2=−452.0940+13.142162 S-8.101×10 −4 S 2+21263.61/ T+68.483143 ln T+( −581.4428 S+0.259601 S 2)/ T−1.967035 S ln T These studies indicate that the values of K 1 (SW) > K 1 (ASW) by ∼0.01 and K 2 (SW) < K 2 (ASW) by ∼0.04 near 25°C. Measurements of pK 1 + pK 2 and pK 1 in artificial seawater with and without boric acid show the same trends, indicating that the effect is due to interactions of boric acid with HCO 3 − and CO 3 2−. Further studies are needed to elucidate these interactions.