To measure oxygen isotopes in SO24- for geologic studies, SO24- was precipitated with excess Ba2+.24 In the presence of HNO3, BaSO4 precipitate is contaminated by NO3-. The solid can be purified by washing, redissolving in the absence of HNO3, and reprecipitating. For purification, 30 mg of BaSO4 were dissolved in 15 mL of 0.05 M DTPA (Figure 11-4) in 1 M NaOH with vigorous shaking at 7oC. BaSO4 was reprecipitated by adding 10 M HCl dropwise to obtain pH 3 - 4 and allowing the mixture to stand for 1 h. The solid was washed twice by centrifuging, removing the liquid, and resuspending in deionized water. The molar ratio NO3-/SO42- was reduced from 0.25 in the original precipitate to 0.001 after two cycles of dissolution and reprecipitation. What are the predominant species of sulfate and DTPA at pH 14 and pH 3? Explain why BaSO4 dissolves in DTPA in 1 M NaOH and then reprecipitates when the pH is lowered to 3 - 4.