Calcium in Sea Water by Electrode Measurement

Abstract

Ionized calcium in standard International Association of Physical Oceanography sea water (chlorinity, 19.373 parts per thousand) was measured with a recently developed, highly selective, calcium electrode. We found that 84 percent of the calcium in sea water exists as Ca++, in fair agreement with the 91 percent calculated for the chemical model of sea water of Garrels and Thompson. If one used the activity coefficient for calcium ion of Garrels and Thompson, 0.28, calcium-ion activity in this sea water would be 0.00249. Another activity coefficient for calcium ion in sea water, 0.24, derived from Berner''s measured total-activity coefficient and our measurement of the fraction of ionized calcium, yields a slightly lower activity, 0.00214.