Complexes affecting the solubility of calcium carbonate in water

The equilibrium or saturation pH for calcium carbonate is an important criteria in the treatment of our water supplies. In lime softened waters, particularly, which contain somewhat more magnesium than calcium on an equivalent basis, the true value of the pHs frequently found to be higher than the theoretical value by 0.3 to 0.4 units. The existence of complexes of magnesium and calcium with carbonate, bicarbonate, hydroxide and sulfate would explain this phenomenon. Calculation of the true pH of saturation, and adjustment of the water to that pH, is essential to avoid corrosion and incrustation in our water distribution systems. The objectives of this project were to evaluate the dissociation constants of the calcium and magnesium complexes with the hydroxide, carbonate, and bicarbonate anions at temperatures of 5, 15, and 25°C. This work was conducted at ionic strengths in the range from .002 to .02, corresponding roughly to waters with total dissolved solids contents of 100 to 1,000 mg/l. The dissociation constants have been determined in terms of activities, so that they are valid at least over the range of ionic strengths at which the tests were made. A titration method was used to measure the effects of complex formation on the pH of reaction mixtures and appropriate computer programs were developed to calculate the dissociation constants. Dissociation constants have been developed for the complexes of calcium and magnesium with the hydroxide and carbonate anions at 5, 15, and 25°. The bicarbonate complex of magnesium was studied at 25°C, but no reliable and valid dissociation constant for this complex was obtained from the study. It appears probable that the constant for the calcium bicarbonate complex may be equally difficult to evaluate. The development of reliable dissociation constants of the complexes studied in this project, and a method to utilize them in calculating pHs would be an aid in producing water of high quality and preventing deterioration of water distribution systems, which accounts for about 60% of the total investment in water utilities.U.S. Department of the InteriorU.S. Geological SurveyOpe