ON THE CHEMICAL PRECIPITATION OF CaCO, IN SEAWATER * The question of the solubility of calcite and aragonite in sea-water is a matter of interest in relation to the geology of limestone and dolo mite. Murray and Hjort maintain that sea-water is so complicated a mixture that the solubility of can not be calculated with cer tainty (from the law of mass action), but that the experiments of An derson and of Cohen and Raben show that sea-water is saturated with calcite. They add (p. 181) that dolomite is less soluble than calcite in carbonated water. Their book summarizes observations showing that calcium carbonate is precipitated in shallow tropical waters, but that even shells are dissolved in the red-clay bottoms of the depths.
Mayer (1916) placed pieces of Cassis shell in sea-water for more than a year and found that they maintained their weight within about 0.1 per cent. The precipitation of at Tortugas was studied by T. Wayland Vaughan, R. B. Dole, and G. H. Drew. Drew observed that a denitrifying bacillus, Pseudomonas calcis, obtained from the sea-water was capable of changing calcium nitrate to calcium carbonate in cul ture media and supposed a similar process to occur in sea-water. Since Vaughan has observed that calcium carbonate is constantly precipi tating at Tortugas, Drew's hypothesis necessitates the presence of an appreciable amount of nitrates or nitrites, and I have attempted to determine them.
A half liter of sea-water was boiled in an all-glass still and the distil late collected in a series of 25 c.c. Nessler's tubes; another series of Nessler's tubes were filled with a graded series of concentrations of ammonium chloride; 1 c.c. of Nessler's reagent was added to each tube and agitated; after 15 minutes the tubes were compared colorimetri cally and the ammonia recovered from the sea-water was estimated; after no more ammonia could be distilled from the sea-water, amal gamated aluminium shavings were introduced into the still and the distillation process was repeated. The ammonia recovered was formed by reduction of nitrates and nitrites. Duplicate analyses gave less than 0.01 mg. of nitrogen per liter as ammonia and less than 0.01 mg. nitrogen per liter as nitrates and nitrites. Raben found more than 10 times these quantities in North Sea water (Murray and Hjort, p. 368). Evidently Pseudomonas calcis and other organisms have almost completely removed the fixed nitrogen from Tortugas sea water. The effect of this probably explains the scarcity of life in the vicinity of Tortugas as compared with colder seas (law of mini mum). There is, however, a constant renewal of fixed nitrogen from the atmosphere, from the decay of organisms, and probably from water rising from the depths of the ocean. If Pseudomonas calcis is an im
portant agent in the precipitation of its action is evidently more intense in places where calcium salts, nitrates, and nitrites are carried from the land into the sea.
That calcium carbonate is withdrawn from surface waters of the sea is shown by chemical analyses. Dittmar found an average of 0.44 per cent less calcium in surface waters than in deeper waters. This is true, notwithstanding the fact that calcium carbonate is constantly being added to the surface waters. The drainage of the land contains an excess of calcium carbonate and flows out on the surface of the sea, where the water evaporates, leaving the excess of in the sea water. The action of organisms in building calcareous structures may account for a large part of the depletion of surface waters, but the pre cipitation of calcareous mud at Tortugas has been observed by Vaughan.
The analysis of the calcium content of sea-water requires double precipitation and filtration for separation from magnesium, and hence large samples and great care are required for accuracy. Theoretically, however, we may detect differences in calcium content by titration. Dittmar showed that sea-water is remarkably constant in composition, except for calcium, and gases. The water content is inversely proportional to Cl and the gases may be eliminated by boiling after the addition of enough acid to decompose the carbonates. If we dis regard carbonic acid, there is an excess of bases in sea-water—i. e., the sum of the base equivalents is greater than the sum of acid equiva lents. Since calcium is added to or taken from sea-water in the form of any change in the calcium content causes an equivalent change in the excess base or alkaline reserve, as it is called by chemists. The alkaline reserve may be titrated while boiling the sea-water to eliminate The exact value of the titration depends on the indi cator used and the exact color of the indicator that is taken as the end point; hence only those titrations done in the same manner can be strictly compared (see method described in first part of this paper). The alkaline reserve per liter was recorded and some titrations were made at 20° and others at 30°, but the errors due to change in volume of the sea-water is within the limits of accuracy of the method. If the sea-water is diluted with rain-water, the alkaline reserve will be low ered, but this error may be compensated by dividing by Cl. In other words, a change in the quotient of the alkaline reserve by Cl indicates a gain or loss of CaCO3.