The role of barium chloride in the analysis of sodium sulfate in saline solution

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The following is the analysis method for the sodium sulfate content in primary brine and secondary brine:.8 Analysis of Sodium Sulfate This method is applicable to the determination of sodium sulfate content in water samples. 1.8.1 Principle An excess of a barium chloride-magnesium chloride mixture is added to the sample to precipitate the sulfate ions; the excess barium salts are then titrated using an EDTA standard solution, in the presence of magnesium salts, with chromotropic acid T as an indicator. 1.8.2 Reagents and instruments a) Hydrochloric acid: 1 M solution; b) 0.1 mol/L mixed solution of barium chloride and magnesium chloride; c) Eriochrome Black T: 0.5% ethanol solution; d) Ammonium-ammonium chloride buffer solution: pH = 10; e) EDTA standard solution: 0.05 mol/L; f) General laboratory instruments. 1.8.3 Analysis procedure: a) Transfer 5 mL of the sample into an Erlenmeyer flask (the amount of sample taken depends on the quantity of sodium sulfate it contains). Rinse the inner walls of the flask with a wash bottle to remove any remaining sample, add 50 mL of distilled water, add 1–2 drops of hydrochloric acid for acidification, then accurately add 10 mL of a mixture of barium chloride and magnesium chloride, as well as 10 mL of an ammonia-ammonium buffer solution (pH = 10). Add 3–5 drops of chrome black T indicator and mix well. Titrate with a 0.05 mol/L EDTA standard solution until a pure blue color is reached; a blank test should also be conducted simultaneously. b) Blank test: Take a clean Erlenmeyer flask, add 50 mL of distilled water, acidify it with 1–2 drops of hydrochloric acid, then accurately add 10 mL of the barium chloride-magnesium chloride mixed solution and 10 mL of the ammonia-ammonium chloride buffer solution (pH=10). After mixing in 3–5 drops of chromotropic acid indicator, titrate with a 0.05 mol/L EDTA standard solution until a pure blue color is reached as the endpoint. What are the functions of barium chloride and magnesium chloride?