The carbonic acid-carbonate equilibrium in sea water.

Abstract

Ca, Mg, and CO₂ in sea water are the common constituents which are most variable in quantity. The condition of these substances is sensitive to the slight changes of the ocean, in contrast to the stability of soluble neutral salt forming substances. They are, furthermore, concerned in a direct way with vital reactions. CO₂ is also one of the most important indicators of vital activity and the suitability of the solution for organisms because it is a product of destructive metabolism and a means for the fixation of energy. If the carbonic acid-carbonate system is considered distinctly, it is conspicuous first that it requires time to equilibrate sea water with a gas phase containing CO₂. If acid or alkali is added to sea water, a number of hours aeration will be required before a constant pH is attained with any tension of CO₂.

Bubbling and shaking sea water with pure hydrogen slowly increases the pH to about 9.2. This is about the point found by Atkins and others as the limit for photosynthetic activity of several marine algae. After acidification and bubbling to remove CO₂, an alkali titration curve lags below the curve for a base of the same concentration as the excess base of sea water (0.0025 N). Such a curve shows the buffer effect of non-volatile buffers alone. A direct acid titration shows the combined buffer effect of both volatile and non-volatile weak acids.

Titration curves at various CO₂ tensions differ from those a - pure carbonate solutions in requiring more acid. Taking this difference as a measure of the buffer effect of other weak acids than carbonic, their apparent dissociation constant is calculated as k = n × 10^-8 L. J. Henderson's artificial sea water was 0.0015 M in boric acid, but boric acid is not reported so abundant in sea water. Silica is reported in a concentration sufficiently large to have this effect if it lxhaves like an acid as weak as boric. This point is not revealed with the existing data, hut there is suggested an important topic because of the presence of silicon in many organisms antl geological processes.