Rapid Determination of Sulfate

Titration of Ammonium, Sodium and Potassium Sulfate by Use of Sodium Rhodizonate Test Paper

Abstract

Sulfate in macro-quantities was rapidly determined by titrating its hot solution with a standard 0.3N BaCl₂ solution, using preservable Na rhodizonate test paper as an external indicator. The standard BaCl₂ solution may be standardized either by the usual gravimetric method or by the volumetric method of using (NH₄)₂SO₄ solution prepared by neutralizing a know volume of a standard 0.3N H₂SO₄ with NH₄OH. The spot test upon a Na rhodizonate test paper for Ba⁺⁺ was sensitively done by touching a very small quantity of the sample solution several times on the same spot, using a glass rod with a fine tip (diam. 1-2mm.). The procedure is carried out as follows: 0.4-0.5g sample of (NH₄)₂SO₄, Na₂SO₄ or K₂SO₄ (or their bisulfate) was taken into a 300-ml conical beaker, diluted to about 100ml with hot water (or the sample solution was neutralized with NH₄OH or CH₃ CO₂H against methyl red, if necessary), heated at 90-100°C and titrated immediately at a moderate speed with 0.3N BaCl₂ until near the expected end point, then titrated dropwise with thorough stirring until the pink spot or ring was just appeared upon the Na rhodizonate test paper. When the solution became cool, it should be reheated for the titration and the end point should be determined at above 60°C. In the back-titration method, the excess of 0.3N BaCl₂ was added and back-titrated with the standard 0.3N (NH₄)₂SO₄ until the pink spot upon the test paper was just disappeared. The effect of the presence of various substances were studied in the present procedure. For technical use, determination of sulfate such as (NH₄)₂SO₄ in fertilizer was satisfactory carried out. The present method of determination for sulfate in macro-quantities can be carried out within 10minutes without using any organic solvent, or special and expensive apparatus.