254 Papers from the Department of Marine Biology. 



Table 24 indicates that some CaC0 3 has been removed from Tor- 

 tugas sea-water, as compared with other sea-water, and to a greater 

 extent from Key West sea-water. In other words, the precipitation 

 observed by Vaughan is not due to a greater amount of calcium in 

 Tortugas or Key West sea-water, but to local conditions which cause 

 the precipitate to form. 



According to the law of mass-action, in a saturated solution of 

 CaCOs, in sea-water at constant temperature, salinity, etc., 



[Ca' ] X [CO 3 "1 = a constant 



Not all of the calcium is, however, in the form of CaCO 3 and Ca ' ', 

 for some is undissociated CaCl 2 , CaS0 4 , Ca(OH) 2 , and CaHC0 3 . 

 The chlorides and sulphates are constant, but [CaHCO 3 ] and [Ca(OH) 2 ] 

 change with the total C0 2 content of the sea-water. But I have shown 

 (McClendon, 19176) that if the alkaline reserve remains constant the 

 total C0 2 of the sea-water (within limits found in nature) varies 

 inversely with the pH ( = log. H* concentration) . Hence the de- 

 termination of the pH may be substituted for that of the total CO 2 . 

 The determinations I have made of the water of the Pacific and 

 North Atlantic showed the pH to vary from about 8.1 to 8.25 and those 

 of Dr. A. G. Ma>er in the Pacific showed only a little wider range 

 (table 11). Earlier observations at Tortugas gave the same range, but 

 my more extended observations in this summer of 1917 show the 

 inadequacy of a few determinations. The pH is influenced by plant 

 and animal life and rises at Tortugas to 8.35 during the day over well- 

 lighted bottoms rich in vegetation, and falls to 8.18 during the night. 

 It may be said, therefore, that conditions in shallow water over eel- 

 grass or other seaweed or corals (with symbiotic algae) favor the pre- 

 cipitation of CaCOs. 1 



The question arises whether the occasional high pH of Tortugas sea- 

 water is sufficient to explain the precipitation of CaCO 3 , or whether 

 nuclei for the separation of the solid phase are necessary. A large 

 amount of CaCl 2 may be added to sea-water without causing a pre- 

 cipitation. If the pH is increased by the addition of NaOH, the result 

 depends on the speed at which the alkali is added. If the NaOH is 

 added suddenly in the form of a strong solution, colloidal precipitation 

 membranes form about the drops and if the membranes are broken by 

 shaking or stirring, a great mass of Mg(OH) 2 is included in the pre- 

 cipitate. If a very dilute solution of NaOH is added very slowly, 

 CaCO 3 possibly contaminated with Mg(OH) 2 is precipitated. The 

 exact pH at which precipitation first occurs can not be determined by 

 this method, as the first precipitation occurs before the solutions are 

 mixed and the crystals thus formed serve as nuclei for further precipita- 

 nt would be interesting to know whether corals and calcareous algffi deposit as much CaCOa in 

 the dark as in the light. Corals from deep water are smaller, more fragile, and deposit less 

 CaCOs than those of shallow water, but the same is true of animals without symbiotic algse. The 

 deposition is, however, related to the pH, since Palitzsch has shown that the pH decreases with 

 depth. 



