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The adsorption of phosphate on manganese dioxide (δMnO 2 ) was determined in seawater as a function of pH, temperature, and salinity. The adsorption in a simple electrolyte solution (0.7 M NaCl) was adequately fitted using the triple-layer surface complexation model, assuming that phosphate formed outer-sphere complexes on the surface of δMnO 2 . SO 4 2and humic acid were found to suppress the adsorption at low pH, but had no effect in the pH range of seawater. The addition of Ca 2+ and Mg 2+ in 0.7 M NaCl was found to enhance the adsorption of phosphate on δMnO 2 at pH > 4. This increased adsorption might be due to the changes in both the surface charge of δMnO 2 and the solution speciation of phosphate in the presence of Ca 2+ and Mg 2+ . The results suggest that manganese oxides can act as important adsorbents of phosphate in natural waters, as well as in surface sediments, when they are present at higher concentrations than other reactive constituents, such as iron and aluminum oxides.

The dissociation constants (KI = [H+][I2-]/[HI-]) of two sulfonephthalein indicators (bromocresol purple and phenol red) were determined as function of temperature (10-30 degrees C) at zero ionic strength. Freshwater pH, on the free hydrogen ion concentration scale (molal units), can be precisely calculated from measurements of indicator absorbance ratios (lambda2A/lambda1A) using the following equations: pH = pKI + log((R - e1)/(e2 - Re3)) and pKI = pKI(degrees) - AdeltaZ2(mu1/2 /(1 + mu1/2) - 0.3 mu), where R = lambda2A/lambda1A, pKI = -log KI, mu is the ionic strength, deltaZ2 = 4, and values of A for 283 < or = T < or = 303 can be estimated from the equation: A = 0.5092 + (T-298.15) x 8.5 x 10(-4). For bromocresol purple (lambda1 = 432 nm, lambda2 = 589 nm), pKI(degrees) = 5.226 + 378.1/T, e1 = 0.00387, e2 = 2.858, and e3 = 0.0181. For phenol red (lambda1 = 433 nm, lambda2 = 558 nm), pKI(degrees) = 5.798 + 666.7/T, e1 = 0.00244, e2= 2.734, and e3 = 0.1075. These two indicators can be used to make accurate pH measurements of freshwaters (river water, lake water, groundwater, rainwater, etc) within the range 4.5 < or =pH < or =8.5. The precision of pH measurements using phenol red in well-buffered freshwaters is on the order of +/-0.001 or better.

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