Titrimetric determination of sulfate


Courtesy of Mettler - Toledo Int. Inc

Most methods for the titrimetric determination of sulfate do not work reliably if interfering divalent cations such as calcium and magne¬sium are present in the sample. Using the methods described below calcium and magnesium will not interfere and can even be deter¬mined in the same analysis.

Several different titration methods are used to determine sulfate content:

  • Titration with barium perchlo¬rate using thorin as indicator. Indication: DP5 Phototrode™ at 555 nm (colorimetric elec¬trode)
  • Titration with barium chloride. Indication: Ba ISE (ion selective electrode)
  • Titration with barium perchlo¬rate. Indication: Pb ISE (ion se¬lective electrode)
  • Titration with lead perchlorate. Indication: Pb ISE (ion selective electrode)
  • Titration with barium chloride. Indication: InLab710 (conducto¬metric electrode)

All these methods work very reliably as long as no interfering divalent cat¬ions (e.g. calcium, magnesium) are present in the sample. These must first be removed using a strongly acid ion exchanger, which of course involves more work.

The following article describes a method for sulfate determination in which cations like calcium or magnesium do not interfere and can even be determined in the same method.

Sulfate determination with the Ca ISE

The basic principle for the determi¬nation of sulfate using the calcium ISE (Ca ISE) is really very simple. Sulfate present in the sample is pre¬cipitated with an excess of barium chloride at pH 4. The barium still present in the solution after com¬plete precipitation of the sulfate is then back titrated at pH 10 using EDTA and a Ca ISE.

If the sample also contains calcium and/or magnesium ions, the method takes advantage of the fact that the Ca ISE is sensitive to barium and calcium and to magnesium. Since all three cations form complexes at pH 10 with EDTA, they can be ana¬lyzed alongside one another.

If the sample does not contain mag¬nesium, the direct method can be used; otherwise the two-step method is necessary.

The direct method
The sulfate in the sample solution is precipitated with an excess of barium chloride at pH 4. After the solution has been adjusted to pH 10, the excess barium is back titrated using EDTA and the Ca ISE. If cal¬cium is present in the sample, the EDTA complexes preferentially with calcium and then with barium.

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