A reprecipitation was employed to remove occluded NO3-from BaSO4 prior to isotopic analysis of oxygen for geologic studies.13 Approximately 30 mg of BaSO4 crystals were mixed with 15 mL of 0.05 M DTPA (Figure 11-4) in 1 M NaOH. After dissolving the solid at 70°C, it was reprecipitated by adding 10 M HCl dropwise to obtain pH 3 - 4 and allowing the mixture to stand for 1 h. The mixture was centrifuged, the mother liquor was removed, and the solid was resuspended in deionized water. Centrifugation and washing were repeated a second time to reduce the molar ratio [NO-3]/[SO2-4] from 0.28 in the original precipitate to 0.001 in the purified material. What will be the predominant species of sulfate and DTPA at pH 14 and at pH 3? Explain why BaSO4 dissolves in DTPA in 1 M NaOH and then reprecipitates when the pH is lowered to 3 -4.