The H-ion concentration of aqueous solutions is determined by the dissociation of water as affected by the temperature and the presence of acids or bases. There are about 55 mols of in a liter of water, that is, its concentration is 55 m in respect to the formula. Of these 55 mols, only about m is dissociated into II' and OH' if the water is pure. In other words, the concentration of X OH' is If we add acid we increase the H ions and if we add alkali we increase the OH ions, but the concentration of 11•XOH' remains constant This dissociation constant of water is abbreviated K„, and hence —log Kp = 14. Rise in temperature increases the dissociation of water; hence —log K„ decreases, being 14.07 at 20° and 13.73 at 30°, or a fall of 0.34; hence the P. falls 0.17. We might expect the P. of sea-water to fall the same amount as that of pure water with rise in temperature, but such is not the case. Rise in temperature causes increased hydrolysis of bicarbonates, and hence increases the OH-ion concentration, and these OH ions combine with the newly formed H ions to form water again. Any change in the H-ion concentration of sea-water due solely to change in temperature (i. e., provided the temperature change is not allowed to cause a loss or gain of is within the limits of error of our measurements. All measurements were made with the hydrogen electrode, which under proper conditions is affected only by the hydrogen ions. The indicators that were calibrated are weak acids and hence should be affected by H ions and not by OH ions. The indicators are affected by Na ions to a slight extent, as given in the calibration table. Temperature changes affect these indicators, but the ordinary changes in temperature affect them so slightly that it has not yet been possible to estimate the temperature correction. Furthermore, the temperature coefficient in sea-water and
standard borax solution is probably the same.
The tension of a solution is the fugacity or escaping tendency of It is measured by estimating the partial pressure of in an atmosphere in equilibrium with the solution—i. e., the pressure of required to prevent the escape of from the solution. The air is not quite in equilibrium with the surface-water of the sea, so that it is not sufficient to determine the partial pressure in the air unless it is shaken with sea-water. Since the average in air is 3 parts in 10,000, the CO2 pressure is given in parts per 10,000 of a standard atmosphere of 760 mm. Hg. The total content of sea-water is the total amount of that may be obtained from it by boiling or evacuating or aeration with indifferent gas after adding an excess of acid to decompose the carbonates. It is not in the form of a gas in sea-water, but exists in several molecular species: CO2, H2CO3, HCO,', and carbonates and bicarbonates of all of the bases present. The CO2 is estimated as cubic centimeters of the dry gas at 0° and 760 mm., and may be reduced to milligrams by multiplying by 1.965.
The H• concentration of sea-water is maintained fairly constant by the presence of salts of weak acids called buffers. These are, first CO2 and second H21303, HsPO4, Si02, and which are not lost by volatilization under conditions in the sea, and are collectively abbrevi ated to non-volatile buffers. The P. of sea-water remains constant so long as these buffers (especially CO2) are not increased or diminished in proportion with the bases combined with them.