4. Rate of reaction for "redox" elements: The rates of reactions 

 that are normally slow may be substantially increased, e.g., at the 

 surface of algae during photosynthesis and at the surface of CaCOa 

 crystals. 



5. Determination of reaction products: pH may control products 

 where alternate or sequential pH-dependent reactions take place. 



6. Sorption: Sorption of an element by suspended particulate matter 

 may be influenced by hydrogen ions, which compete for or modify 

 sorption sites. The influence of pH on the degree of protolysis of the 

 sorbing solute should also be considered. 



7. Flocculation: The degree of incorporation of solutes may depend 

 on the pH at which flocculation takes place. 



There is no general agreement on how pH should be measured nor 

 on the precision or accuracy of the results. There is further complica- 

 tion because pH varies if the water sample is brought to a different 

 pressure and temperature during measurement. Careful studies of 

 the technique of pH measurement and of the eff^ect of temperature and 

 pressure are needed. The development of in-situ pH measurements 

 should give new understanding of this important nonconservative 

 property of seawater. 



Alkalinity 



The major anions in seawater are chloride, sulfate, bromide, bicar- 

 bonate, carbonate, and borate. Only the latter three have any appreciable 

 power as bases. Phosphate and siHcates (i.e. H2POI, HP04"^, POr^, 

 H2Si04"^ and HaSiOr) are moderately eff'ective as bases, but are gen- 

 erally present in minor concentrations (<1 ppm) except in anoxic 

 waters where sulfides (i.e. HS" and S^^) and ammonia may become 

 appreciable. The total concentration of these "Lewis Bases," in addi- 

 tion to the small excess of hydroxide ions, are referred to as the "alka- 

 linity" of seawater — the capacity of seawater to neutralize the addition 

 of a strong acid without an extreme change in pH. It is a property of 

 seawater that prevents large disturbances to life processes in the ocean. 

 Alkalinity may be at times a main variable in the chemical composition 

 of seawater. The secretion of calcium carbonate shells by organisms 

 and their subsequent dissolution seem to be the main factors influencing 

 the alkalinity. Changes can arise from the complexing of carbonate 

 species, boric acid volatility, biological production of organic matter 

 and ammonia, and others. The ratio of alkalinity to the chlorinity (the 

 specific alkalinity) varies significant with depth, in anoxic basins, near 

 coasts, and in shallow tropical seas where calcium carbonate precipitates. 

 A better understanding of the species contributing to the total alkalinity 

 in diff'erent regions is needed. Moreover, since calcium carbonate is 



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