Determination of Sulfate in Water: The Gravimetric Method

High concentrations of sulfate in water can cause undesirable effects, such as a bitter taste, laxative effects, and corrosion of pipelines. Therefore, monitoring sulfate levels is crucial for assessing water quality for drinking, industrial, and environmental purposes. Among various analytical techniques, the gravimetric method is a classic, highly accurate, and widely used procedure for determining high sulfate concentrations.

The Barium Sulfate Precipitation Method
The most common gravimetric approach for sulfate analysis is based on the precipitation of sulfate ions with barium chloride.

Principle
The method involves precipitating sulfate ions (SO₄²⁻) in a water sample with a solution of barium chloride (BaCl₂) under acidic conditions. This reaction forms insoluble barium sulfate (BaSO₄) crystals.

The chemical reaction is:SO₄²⁻ (aq) + Ba²⁺ (aq) → BaSO₄ (s)

The white precipitate of BaSO₄ is then filtered, washed, dried, and weighed with high precision. The mass of the precipitate is directly used to calculate the original sulfate concentration in the water sample.

Key Steps in the Procedure

1. Sample Preparation: The water sample is acidified and heated to near boiling. This helps to eliminate interfering carbonate ions and promotes the formation of pure, coarse BaSO₄ crystals, which are easier to filter.

2. Precipitation: A hot solution of barium chloride is added slowly and with constant stirring to the hot, acidified sample. This ensures complete and selective precipitation of sulfate.

3. Digestion: The mixture containing the precipitate is allowed to stand hot for several hours. This "digestion" process increases the crystal size of BaSO₄, making them less likely to pass through the filter.

4. Filtration and Washing: The precipitate is collected using a pre-weighed filter crucible (e.g., Gooch crucible). It is then washed thoroughly with hot distilled water to remove any trapped impurities or chlorides.

5. Drying and Weighing: The crucible containing the BaSO₄ precipitate is dried in an oven at a constant temperature (typically 105-150°C) until a constant mass is achieved. The final mass is recorded.

Calculation
The sulfate concentration is calculated using the mass of the precipitate and the initial volume of the water sample.

Formula:
SO₄²⁻ (mg/L) = [ (Mass of BaSO₄ in mg) × (Molecular weight of SO₄²⁻ / Molecular weight of BaSO₄) ] / [Volume of sample in Liters]

The gravimetric factor for converting BaSO₄ to SO₄²⁻ is 0.4116.

The gravimetric barium sulfate method remains a fundamental and reliable technique for the quantitative analysis of sulfate in water. Its high accuracy makes it a standard reference method against which other faster, instrumental techniques (like turbidimetry or ion chromatography) are often calibrated. For laboratories requiring precise results without sophisticated equipment, this method is an excellent choice.