How to detect sulfate content in water using spectrophotometry

Here's a concise explanation for detecting sulfate in water using spectrophotometry:

Spectrophotometric Determination of Sulfate in Water (Turbidimetric Method)

Principle: Sulfate ions (SO₄²⁻) react with barium chloride (BaCl₂) in an acidic medium to form fine barium sulfate (BaSO₄) crystals. The turbidity (cloudiness) caused by these suspended crystals is proportional to the sulfate concentration and can be measured by spectrophotometry.

Procedure:

1. Acidify Sample: Add hydrochloric acid (HCl) to the water sample to prevent precipitation of carbonates and phosphates.

2. Add Stabilizing Agent: Mix in a stabilizing solution (often containing glycerol, NaCl, and HCl) to promote uniform crystal formation and prevent rapid settling.

3. Initiate Precipitation: Rapidly add solid barium chloride (BaCl₂) crystals while stirring vigorously to ensure fine, uniform BaSO₄ particle formation.

4. Stabilize & Measure: Allow the mixture to stand for a controlled time (e.g., 10-15 minutes) to stabilize turbidity. Measure the absorbance (optical density) of the suspension at a wavelength between 420 nm and 450 nm using a spectrophotometer against a reagent blank.

5. Calibration: Construct a calibration curve using standard sulfate solutions treated identically to the samples. Plot absorbance vs. sulfate concentration.

6. Determine Concentration: Read the sulfate concentration of the unknown sample from the calibration curve based on its measured absorbance.

Key Considerations:

• Timing: Turbidity must be measured within a consistent time window after adding BaCl₂, as particle growth and settling affect absorbance.

• Stabilizer: Glycerol (or gelatin) helps maintain a stable suspension for reproducible readings.

• Mixing: Vigorous, consistent mixing during BaCl₂ addition is critical for uniform crystal size.

• Interferences: Silica and significant suspended solids can interfere. Sample filtration (if needed) must be done before acidification to avoid dissolving sulfate precipitates.

Application: This method is suitable for sulfate concentrations typically ranging from approximately 1 mg/L to 40 mg/L SO₄²⁻. Visual comparison is possible for higher concentrations, but spectrophotometry provides better precision.