Samples Containing Thorium or Uranium

Hydrolytic Stability and Preferred Matrices

• The Th⁺⁴ ion is most resistant to hydrolysis of all the quadrivalent ions because it is the largest. Th(OH)³⁺ and Th(OH)₂²⁺ become detectable at pH 2-3 and precipitation of the hydroxide begins at pH 3.5 to 4.0. Of the common ions the presence of acetate and sulfate increase the pH at which thorium hydroxide precipitation begins thereby improving the hydrolytic stability.
• Th⁺⁴ should not be mixed with oxalate, phosphate, sulfate, or fluoride where insoluble precipitates are formed.
• U(IV) begins precipitation as the hydroxide at a pH of about 1.5. The presence of sulfate improves this pH to 2.5.
• UO₂⁺² begins to precipitate as UO₂ (OH) ₂*H₂O at a pH of 3.2. Acetate, fluoride and sulfate all improve the hydrolytic stability with fluoride giving the most striking improvement to a pH of 7.
• Both the IV and uranyl forms of U prefer a nitric acid media. If unsure of the oxidation state, then avoid fluoride, phosphate, and sulfate.
• Nitrate solutions of U and Th are stable with any of the elements that are stable in nitric acid matrices. Avoid elements containing HF and HCl in the matrix, as well as P as the orthophosphate and S as the sulfate. The pH of all mixtures should be less than 2 for long-term stability.

The elements that are stable/soluble and commonly diluted in aqueous/HNO₃ are shaded in red below:

(1)  Os should never be mixed with HNO₃ due to the formation of the very volatile OsO₄.

(2)  Cl is oxidized to molecular Cl₂ which is volatile and adsorbs on plastic.

(3)  Br and I are oxidized to molecular Br₂ and I₂ which adsorb onto plastic.

(4)  Dilutions of Hg and Au in HNO₃ below 100 ppm should be stored in borosilicate glass due to Hg⁺² adsorption on plastic.

(5)  Not soluble above concentrations of 1000 µg/mL.

(6)  Trace levels of HCl or Cl- will form AgCl, which will photoreduce to Ag⁰.

F  Denotes that the element can be diluted in HNO₃ if complexed with F^-.

Cl  Denotes that the element can be diluted in HNO3 if complexed with Cl-.

HF  Denotes that the element should have excess HF present when diluted with HNO₃.

T  Denotes that the tartaric acid complex can be diluted in HNO₃.

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