76 



77?^ Sea-water and its Physical and Chemical Properties 



in proportion to the total amount present: according to the theory of interionic forces 

 developed by Milner, Bjerrum, Debye and Huckel, amongst others, only a small frac- 

 tion is involved. This fraction of the total concentration is termed the "activity"; the 

 equilibrium thus involves not the total concentrations of the different ions, for instance 

 [H+] but the activities, in this case/JH+J, where/is the "activity coefficient" and the 

 above equations are replaced by others where the factors on the left-hand side are 

 multiplied by the activity coefficients /i, /a, /g and/4. The constants Ki and K2 remain 

 unchanged; they are termed "activity constants". However, instead of taking the 

 effect of the neutral salts directly into consideration it can be allowed for by its effect 

 on the dissociation constant; the apparent dissociation constants K[ and Kl are termed 

 the "concentration constants". At the suggestion of the International Council for 

 Oceanography Research they have been determined by Buch and co-workers (1932) 

 Wattenberg, (1936). Table 29 gives numerical values for —log K[ and —log K2 for 

 different temperatures and salinities (see also Buch, 1951). 



The calculation of the concentration of the individual forms of carbon dioxide in 

 sea-water (free carbon dioxide, carbon dioxide pressure, carbonate and bicarbonate 



Table 29. Values of the first and second dissociation constants of carbonic acid in sea- 

 water at different temperatures and salinities 



ions and total carbon dioxide) is now a simple calculation if the hydrogen-ion con- 

 centration, the pH, is measured directly and the titration alkalinity is found from the 

 salinity using the relationship given on page 74. This calculation can be shortened 

 considerably if the carbon dioxide pressure and the total carbon dioxide are tabulated 

 or plotted graphically for the most frequently occurring values of salinity, temperature 

 and pH. 



The relationship between pH and the concentrations of free carbon dioxide, 

 carbonate and bicarbonate can be shown clearly in a diagram (Fig. 40), where the 

 percentage of each form is given as a function of the hydrogen-ion concentration. The 

 S-shaped curves separate these factors in such a way that for any value of the pH 

 the composition of the total carbon dioxide present is given along the ordinate. The 

 curves for sea-water are drawn with full lines, the curves for pure water with dashed 

 lines; the first is displaced towards lower pH-values. The presence of neutral salts in 

 sea-water displaces the equilibrium towards the acid side because the apparent dissocia- 

 tion constant increases. It can be seen that at very low pH-values there is almost only 

 free carbon dioxide present, as the pH rises the concentration of bicarbonate increases 

 and reaches a maximum at pH = 7-5; the carbonate ion becomes important only at 

 higher pH-values, The two vertical lines in Fig. 40 show the normal range of the pH 



