A reprecipitation was employed to remove occluded nitrate from BaSO4 precipitate prior to isotopic analysis of oxygen for geologic studies.9 Approximately 30 mg of BaSO4 crystals were mixed with 15 mL of 0.05 M DTPA in 1 M NaOH. After dissolving the solid with vigorous shaking at 708C, it 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 isolated by centrifugation, removal of the mother liquor, and resuspension in deionized water. Centrifugation and washing was repeated a secondtime to reduce the molar ratio NO-3 /SO-4 2 from 0.25 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 pH 3? Explain why BaSO4 dissolves in DTPA in 1 M NaOH and then reprecipitates when the pH is lowered to 3–4.
A reprecipitation was employed to remove occluded nitrate from BaSO4 precipitate prior to isotopic analysis of oxygen for geologic studies.9 Approximately 30 mg of BaSO4 crystals were mixed with 15 mL of 0.05 M DTPA in 1 M NaOH. After dissolving the solid with vigorous shaking at 708C, it 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 isolated by centrifugation, removal of the mother liquor, and resuspension in deionized water. Centrifugation and washing was repeated a second
time to reduce the molar ratio NO-3 /SO-4 2 from 0.25 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 pH 3? Explain why BaSO4 dissolves in DTPA in 1 M NaOH and then reprecipitates when the pH is lowered to 3–4.
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