Estimation of Magnesium Sulphate and Calcium Gluconate

Estimation of Magnesium Sulphate and Calcium Gluconate: Complexometric titration is a volumetric analytical method in which a metal ion reacts with a complex-forming reagent to produce a stable coordination complex. The most commonly used complexing agent is disodium EDTA (ethylenediaminetetraacetic acid).

Both magnesium sulphate and calcium gluconate contain metal ions that can be quantitatively estimated using EDTA:

• Magnesium sulphate → Mg²⁺
• Calcium gluconate → Ca²⁺

The metal ions react with EDTA in a 1:1 molar ratio.

A. Estimation of Magnesium Sulphate

1. Principle

Magnesium sulphate dissociates in aqueous solution to produce magnesium ions:

MgSO₄ ​→ Mg²⁺ + SO₄

The magnesium ions react with disodium EDTA to form a stable, soluble magnesium–EDTA complex.

Mg²⁺ + Y⁴⁻ → MgY²⁻

Thus, the amount of magnesium sulphate present in the sample can be determined from the volume of standard EDTA solution required for complete complexation.

The titration is generally performed at approximately pH 10 using an ammonia-ammonium chloride buffer.

2. Reagents Required

1. Magnesium sulphate sample
2. Standard disodium EDTA solution
3. Ammonia-ammonium chloride buffer, pH 10
4. Eriochrome Black T (EBT) indicator
5. Distilled water

3. Role of Each Reagent

Disodium EDTA

EDTA acts as the complexing/titrating agent.

It combines with Mg²⁺ to form a stable complex:

Mg²⁺ + EDTA → Mg − EDTA

Ammonia-Ammonium Chloride Buffer

The buffer maintains the solution at approximately pH 10.

Maintaining the correct pH is important because the formation and stability of the metal–EDTA complex and the behaviour of the indicator depend on pH.

Eriochrome Black T

EBT is a metal ion indicator.

In the presence of Mg²⁺, it forms a wine-red coloured complex.

At the endpoint, EDTA removes Mg²⁺ from the indicator complex, leaving the free indicator, which is blue.

4. Indicator Reaction

Initially:

Mg²⁺ + Indicator → Mg − Indicator

The magnesium–indicator complex produces a wine-red colour.

When EDTA is added:

Mg – Indicator + EDTA →Mg – EDTA + Indicator

The free indicator produces a blue colour.

Endpoint

Wine red → Blue

5. Procedure

1. Accurately weigh the given magnesium sulphate sample.
2. Dissolve the sample in a suitable quantity of distilled water.
3. Transfer the solution quantitatively into a volumetric flask.
4. Make up the volume to the mark with distilled water.
5. Pipette a known volume of this solution into a clean conical flask.
6. Add approximately pH 10 ammonia-ammonium chloride buffer.
7. Add a few drops of Eriochrome Black T indicator.
8. The solution develops a wine-red colour.
9. Titrate the solution with standardized disodium EDTA solution.
10. Continue titration with continuous swirling.
11. Near the endpoint, add EDTA dropwise.
12. The endpoint is reached when the colour changes from wine red to clear blue.
13. Record the burette reading.
14. Repeat the titration until concordant readings are obtained.

6. Calculation

The reaction between Mg²⁺ and EDTA is:

Mg²⁺ : EDTA = 1 :

Therefore:

M_MgSO4​​ V_MgSO4 ​​= M_EDTA​V_EDTA​

The amount of magnesium sulphate can therefore be calculated from the volume and concentration of EDTA consumed.

For magnesium sulphate heptahydrate:

MgSO4​⋅7H2​O

Molecular weight ≈ 246.47 g/mol.

Therefore:

Mass = Moles × 246.47

If the pharmaceutical sample contains MgSO₄·7H₂O, the calculation should specifically use its molecular weight.

B. Estimation of Calcium Gluconate

1. Principle

Calcium gluconate is a calcium salt of gluconic acid. In solution, it provides calcium ions:

Ca(C₆​H₁₁​O₇​)₂ ​→ Ca²⁺ + 2C₆​H₁₁​O₇^−​

The calcium ions react with EDTA to form a stable calcium–EDTA complex.

Ca²⁺ + Y⁴⁻ → CaY²⁻

The reaction takes place in a 1:1 molar ratio.

The amount of calcium gluconate is calculated from the volume of standard EDTA solution required to reach the endpoint.

2. Reagents Required

1. Calcium gluconate sample
2. Standard disodium EDTA solution
3. Suitable alkaline medium/buffer
4. Murexide indicator
5. Distilled water

3. Role of Murexide Indicator

Murexide is a commonly used metal ion indicator for calcium.

It forms a coloured complex with Ca²⁺.

During titration, EDTA has a stronger affinity for calcium than the indicator. Therefore, EDTA removes Ca²⁺ from the calcium–indicator complex.

This produces a characteristic colour change at the endpoint.

Simplified reaction

Ca – Indicator + EDTA → Ca – EDTA + Indicator

Endpoint

The exact shade can depend on the medium and indicator concentration, but it is commonly described as a transition from pink/red toward purple or blue-purple.

4. Why Alkaline Medium is Used

The calcium–EDTA complex formation is favoured under suitable alkaline conditions.

The pH must be controlled because:

• EDTA has different protonation states at different pH values.
• The stability of the calcium–EDTA complex depends on pH.
• The colour behaviour of murexide also depends on pH.

Therefore, maintaining the appropriate pH gives a clearer and more reliable endpoint.

5. Procedure

1. Accurately weigh the given calcium gluconate sample.
2. Dissolve it in distilled water.
3. Transfer the solution to a volumetric flask.
4. Make up the volume to the mark with distilled water.
5. Pipette a known volume of the prepared solution into a conical flask.
6. Add the required alkaline solution/buffer.
7. Add a small quantity of murexide indicator.
8. The solution develops the characteristic calcium–indicator colour.
9. Titrate with standardized disodium EDTA solution.
10. Add EDTA slowly near the endpoint with continuous swirling.
11. Observe the characteristic colour change.
12. Note the burette reading.
13. Repeat the titration to obtain concordant values.

6. Chemical Reaction

The principal reaction is:

M_Ca​V_Ca​ = M_EDTA​V_EDTA​

Thus:

Ca²⁺ : EDTA = 1:1

One mole of EDTA reacts with one mole of calcium ions.

7. Calculation of Calcium Gluconate

The basic relationship is:

M_Ca​V_Ca ​= M_EDTA​V_EDTA​

From the amount of calcium determined, the corresponding amount of calcium gluconate can be calculated.

Calcium gluconate has the approximate molecular formula:

C₁₂​H22​CaO_14​

Molecular weight ≈ 430.37 g/mol for anhydrous calcium gluconate.

Therefore:

Mass of calcium gluconate=Moles×430.37Mass\ of\ calcium\ gluconate = Moles\times430.37

The exact calculation should use the molecular form specified in the pharmacopoeial or practical method being followed.

Difference Between the Two Estimations

Precautions

1. Use a standardized EDTA solution.
2. Accurately weigh the sample.
3. Use a calibrated volumetric flask and pipette.
4. Maintain the required pH throughout the titration.
5. Add the indicator in a small and consistent quantity.
6. Near the endpoint, add EDTA dropwise.
7. Shake/swirl the flask continuously during titration.
8. Do not overshoot the endpoint.
9. Perform at least three titrations and obtain concordant readings.
10. Use freshly prepared or properly stored reagents.

Editorial Note

This article has been carefully researched and written by Deepak Rajput with a focus on accuracy, clarity, and evidence-based healthcare information. Pharmaacademias.com