Annual Fluctuations in Sulphate and Bicarbonate Hardness in Ponds

In four ponds studied, sulphato in solution was high in spring, low in autumn, while bicarbonate was low in spring, high in autumn. The fall in sulphate was probably due to reduction to sulphide which became locked up in the mud as ferrous sulphide. The fluctuation in bicarbonate was partly due to the removal of CO2 during photosynthesis and partly to the action of sulphuric acid when the sulphide was oxidized in winter. UVTRODUCTION AND METIlODS In the course of a study of the annual fluctuations of the chemical composition of the water in a number of ponds, Tucker (1.958) showed that there was a tendency for the calcium content to fall in summer, while the titration against standard acid (alkalinity) tended to rise. This tendency was more marked in some ponds than others (Fig. l), and Tucker concluded that the fluctuations were greatest in productive ponds, normally characterized by a high content of dissolved organic matter. At first sight it seems likely that an important factor in these fluctuations is the higher level of photosynthetic activity in summer. According to Rabinovitch (194556) the probable action of algae during photosynthesis is the removal of COZ from solution in the water, and the equilibria which arc likely to involve the tit&able base arc : CO 2 * H,CO 3 ;“, HI+ + HCO3+ 2r-I+ + co3-(I) CaCO 3 * Ca+* + CO,(2) co,+ co2 + II,0 e 2HCO3(3) The effect on (1) of the removal of CO* from the system is a fall in the hydrogen ion concentration, and in the concentration of carbonate and bicarbonate ions. The effect on (3) is the splitting off of carbon dioxide from the bicarbonate ions, and the precipitat’ion of calcium carbonate according to (2). There will therefore be a fall in the conccntrntion of bicarbonat,c and calcium. The total effect is that the pH rises and the amount of fitratable base decreases. Thus the fall in calcium concentration can be explained by the photosynthesis of the plants, but the rise in concentration of bicarbonate is not in accordance with cxpectation. The present paper is the record of an attempt to verify an alternative hypothesis, arrived at after helpful discussions with Dr. F. J. II. Mackereth of the Ferry House Laboratory of the Freshwater Riological Association. This is, that sulphide ions derived from the bottom deposits under anaerobic conditions in summer are oxidized to sulphuric acid in winter, converting a proportion of the calcium bicarbonate to sulphate. This would lead to a fall in the titer to standard acid, and if the sulphate were reduced to sulphide when the pond became anaerobic again in summer, the calcium ions could take up bicarbonate again. Tucker’s observations were therefore repeated on certain ponds, and in addition estimates were made of sulphate and oxygen concentration in the water. The oxygen concentration was measured in samples taken from a few cm above the mud surface, but other determinations were made on samples from t,he surface of the water at the edge of the pond. Calcium was determined by versenate titration, using ammonium purpuratc as indicator (Hetz and X011 1950), and total alkalinity by titration to the end point of xylene-cyanole methyl orange with standard acid. Sulphate was determined by precipitat,ion with bcnzidinc hydrochloride