River water copper titrations

The chemical speciation of copper in river water and model solutions was investigated by a titration technique in which cupric ion activities were measured at constant pH as the total copper concentration ([Cujoj]) was varied by incremental additions of CUSO4. pCu(-log cupric ion activity) was measured with a cupric ion-selective electrode (Orion 94-29) and pH with a glass electrode (Beckman 39301) both coupled to a single junction Ag/AgCl reference electrode (Orion 90-01) in a temperature controlled (25 + 0.5°C) water bath. Total copper concentrations in the titrated solutions were determined directly by atomic absorption spectrophotometry (Perkin Elmer 603) using a graphite furnace (Perkin Elmer 2200). Measurement of total copper concentrations is necessary because of adsorptive loss of copper from solution onto container and/or electrode surfaces. 

Titrations were performed on untreated, filtered, and UV-treated filtered river water samples at in situ and adjusted pH values. The effect of pH on copper speciation was investigated by titration of filtered Newport River water at pH 7.0 and filtered Newport and Neuse waters at pH 8.0. Newport River water was adjusted to pH 7.0 by decreasing the partial pressure of CO2 from the initial ambient value of about 10 times the atmospheric level. To adjust the pH to 8.0, sodium bicarbonate was added to bring the river water samples to a concentration of 0.5 mM with subsequent adjustment of Pc02 Titrations were also conducted at pH 7,0 in model solutions consisting of 0.01 KNO3 and 0.1 mM NaHC03 with and without the addition of 0.75 histidine to test electrode behavior in solutions of known chemistry. 

Non-ideal behavior of the cupric ion electrode occurs at pECujot] > 7 in the titrations of both filtered and unfiltered Newport River water at pH 5.95 and filtered water at pH 7.0 and 8.0. At low total copper concentrations, measured pCu values approach a constant value independent of the total copper in solution. Similar behavior was observed for filtered Neuse River water at pH 8.0, but not at pH 6.78. 

As indicated by titration data (Figures 6 and 7), binding of copper in both Neuse and Newport River water decreases with increasing total copper in a manner consistant with a stepwise titration of a number of different ligands and/or binding sites. Binding of copper increases with increasing pH consistant with reactions with protonated weak acids. 

Figure 6. Copper titrations of Neuse River water at 25°C. (%) Untreated water at in situ pH 6.78 glass-fiber filtered water at pH 6.78 (A) glass-fiber filtered water at pH 8.00 (0) UV-treated glass—fiber filtered water at 6.78 (4) twice filtered XJV-treated water at pH 6.78, first filtration by glass-fiber prior to UV-irradiation, second filtration by membrane (0.2fxm nuclepore) after irradiation. Model curves through data points were calculated according to stability constants determined in this work (Tables I and II). Dotted lines indicate limits on data used for calculation of conditional stability constants for organic binding.

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