Anglesite (PbSO 4) solubility in acetate solutions: The determination of stability constants for lead acetate complexes to 85°C
Anglesite solubilities were determined at 25, 40, 55, 70, and 85°C in solutions with equal molal concentrations of acetic acid and sodium acetate (each ranging from 0.001 to 0.25 m) and at pH values near 4.6. Experiments were conducted using 1000 ml, externally heated glass reaction vessels, and lead concentrations were measured using atomic absorption spectrophotometry. For run conditions with free acetate concentrations less than about 0.01 m, measured a anglesite solubilities are less than or equal to 100 mg/l. Above free acetate concentrations of 0.01 m, measured anglesite solubilities rise sharply to concentrations near 1000 mg/l. Extracted from these solubility data, by a numerical successive linear extrapolation method, were stoichiometries and thermodynamic stability constants (ionic strength = 0) for the complexes Pb(CH 3COO) + and Pb(CH 3COO) 2. Log K1 and log K2 values for the formation reactions Pb + + CH 3COO - = PbCH 3COO + and Pb 2+ + 2CH 3COO - = Pb(CH 3COO) 2 respectively are log K1: (2.4±0.1, 25°C), (2.4±0.1, 40°C), (2.5±0.1, 55°C), (2.6±0.1, 70°C), (2.7±0.2, 85°C) and log K2: (3.4±0.4, 25°C), (3.6±0.2, 40°C), (3.8 ±0.2, 55°C), (4.0±0.2, 70°C), (4.3±0.2, 85°C). Lead hydrolysis was found to be significant in those experiments conducted at 70 and 85°C. However, hydrolyzed lead was not detected at 25, 40, and 55°C. PbOH + as percent of total observed lead ranged from 0.2 to 24% and 2 to 86% in those experiments at 70 and 85°C, respectively. Calculations of lead speciation in acetate solutions show that the equivalence point for PbCH 3COO + and Pb(CH 3COO) 2 shifts to lower acetate concentrations from 25 to 85°C. These speciation calculations also suggest that lead acetate complexes could be significant in high acetate formation waters. These waters typically contain at least 1,000 ppm acetate (0.017 molal as acetic acid). Furthermore, if these waters are not too acid and if concentrations of competing ligands are low, lead acetate complexes should be the dominant lead species.