Estimation of Sulphate as Barium Sulphate by Gravimetric Analysis

Introduction

Gravimetric analysis is a quantitative analytical technique in which the amount of a substance is determined by measuring the mass of a suitable compound formed from it.

Sulphate ions can be estimated by converting them into an insoluble white precipitate of barium sulphate (BaSO₄) using barium chloride solution.

The precipitate is filtered, washed, dried or ignited and weighed. Its mass is then used to calculate the amount of sulphate present in the original sample.

1. Principle

The method is based on the reaction between sulphate ions and barium ions to form highly insoluble barium sulphate.

When barium chloride solution is added to a sample containing sulphate ions, a white precipitate of barium sulphate is formed.

Chemical reaction:

Ba²⁺ + SO₄²⁻ → BaSO₄↓

The precipitate is separated from the solution, washed to remove soluble impurities, dried or ignited and weighed accurately.

Since the reaction occurs in a 1:1 molar ratio, the amount of sulphate can be calculated from the mass of barium sulphate obtained.

[IMG 1 — Horizontal educational infographic illustrating the principle of sulphate estimation: sulphate-containing sample solution → addition of barium chloride → formation of white BaSO₄ precipitate → filtration → drying or ignition → weighing.]

2. Reagents Required

1. Sample Solution: Contains the sulphate ions to be estimated.

2. Barium Chloride Solution (BaCl₂): Acts as the precipitating reagent and reacts with sulphate ions to form insoluble barium sulphate.

3. Dilute Hydrochloric Acid (HCl): Used to maintain an acidic medium and help reduce interference from certain other precipitating ions.

4. Distilled Water: Used for preparing solutions and washing the precipitate.

3. Procedure

The general procedure for the gravimetric estimation of sulphate as barium sulphate is as follows:

1. Accurately weigh the prescribed quantity of the sample, if a solid sample is being analysed.

2. Dissolve the sample in distilled water and prepare a clear solution.

3. Add the prescribed quantity of dilute hydrochloric acid to maintain a suitable acidic medium.

4. Heat the solution nearly to boiling.

5. Add hot barium chloride solution slowly with continuous stirring.

6. Continue adding the reagent until precipitation is complete, as confirmed by the prescribed analytical procedure.

7. Allow the precipitate to stand and digest under suitable conditions to obtain larger, more easily filterable particles.

8. Filter the precipitate using suitable ashless filter paper or an appropriate filtration crucible.

9. Wash the precipitate with a suitable washing liquid to remove soluble impurities.

10. Dry and ignite the precipitate under the prescribed conditions.

11. Cool the precipitate in a desiccator and weigh it accurately.

12. Repeat heating, cooling and weighing until a constant mass is obtained.

[IMG 2 — Horizontal laboratory infographic showing the complete experimental procedure: sample preparation, acidification, heating, slow addition of BaCl₂, white precipitate formation, digestion, filtration, washing, ignition and final weighing.]

4. Chemical Reaction and Endpoint

The main reaction is:

BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl

Here, barium chloride reacts with sodium sulphate to produce insoluble barium sulphate and soluble sodium chloride.

Observation: A white precipitate of barium sulphate is formed.

Unlike volumetric titration, this method does not have a colour-change endpoint. Completion of precipitation is checked by adding a few drops of barium chloride solution to the clear supernatant liquid. If no additional precipitate forms, precipitation is considered complete under the prescribed test conditions.

5. Calculation

The amount of sulphate is calculated from the mass of barium sulphate obtained.

Molecular weights:

• Barium sulphate (BaSO₄) = 233.39 g/mol

• Sulphate ion (SO₄²⁻) = 96.06 g/mol

The chemical reaction shows that 1 mole of BaSO₄ corresponds to 1 mole of sulphate ions.

Calculation formula:

Mass of sulphate = Mass of BaSO₄ × (Molecular weight of SO₄ ÷ Molecular weight of BaSO₄)

Substituting the values:

Mass of sulphate = Mass of BaSO₄ × (96.06 ÷ 233.39)

Mass of sulphate = Mass of BaSO₄ × 0.4116

Therefore:

1 g of BaSO₄ is equivalent to approximately 0.4116 g of sulphate.

Example

Suppose the mass of barium sulphate obtained after ignition is 0.500 g.

Step 1: Given value

Mass of BaSO₄ = 0.500 g

Step 2: Apply the formula

Mass of sulphate = 0.500 × 0.4116

Step 3: Calculate

Mass of sulphate = 0.2058 g

Answer: The original sample contains approximately 0.2058 g of sulphate ions.

Percentage of sulphate:

Percentage of sulphate = (Mass of sulphate ÷ Mass of original sample) × 100

Note: If only a portion of the prepared sample solution was analysed, the result must be multiplied by the appropriate dilution or aliquot factor.

6. Precautions

• Add barium chloride solution slowly with continuous stirring to obtain a suitable precipitate.

• Maintain the prescribed acidic conditions to minimize interference.

• Digest the precipitate under suitable conditions to improve particle size and filterability.

• Wash the precipitate properly to remove soluble impurities.

• Avoid losing precipitate during filtration and transfer.

• Use the prescribed ignition conditions to obtain a stable product.

• Cool the crucible in a desiccator before weighing to prevent moisture absorption.

• Continue heating, cooling and weighing until constant mass is obtained.

7. Applications

• Sulphate Estimation: Used for quantitative determination of sulphate ions in suitable samples.

• Water Analysis: Helps determine sulphate content in water samples.

• Chemical Analysis: Used in the analysis of inorganic substances containing sulphate.

• Quality Control: Supports the evaluation of chemical raw materials and suitable pharmaceutical substances.

• Gravimetric Analysis: Demonstrates the application of precipitation and accurate mass measurement in quantitative analysis.

Key Takeaways

• Sulphate ions are estimated by precipitating them as barium sulphate.

• Barium chloride is used as the precipitating reagent.

• Barium sulphate forms a white, highly insoluble precipitate.

• The precipitate is digested, filtered, washed, dried or ignited and weighed.

• One mole of BaSO₄ corresponds to one mole of sulphate ions.

• Each 1 g of BaSO₄ represents approximately 0.4116 g of sulphate.

• Accurate results depend on complete precipitation, proper washing and obtaining a stable final mass.