Estimation of Calcium Gluconate by Complexometric Titration
Introduction
Calcium gluconate is a calcium salt of gluconic acid used in pharmaceutical preparations as a source of calcium. Its quantitative estimation is important for determining the amount of calcium gluconate present and checking the quality of the pharmaceutical substance.
It can be analysed by complexometric titration using EDTA (Ethylenediaminetetraacetic acid) as the complexing agent. EDTA reacts with calcium ions to form a stable, soluble complex.
A suitable metal ion indicator, such as murexide, is used to identify the endpoint.
1. Principle
The estimation of calcium gluconate is based on the formation of a stable 1:1 complex between calcium ions and EDTA.
When calcium gluconate dissolves in water, it releases calcium ions. During titration, these calcium ions react with EDTA to form a calcium-EDTA complex.
Chemical reaction:
Ca²⁺ + EDTA⁴⁻ → [Ca-EDTA]²⁻
Murexide indicator initially forms a coloured complex with calcium ions. As EDTA is added, it binds calcium ions more strongly and releases the free indicator.
This produces a colour change that indicates the endpoint.
Typical colour change with murexide:
Pink → Purple or blue-violet
The exact endpoint colour depends on the pH and prescribed analytical conditions.
[IMG 1 — Horizontal educational infographic showing calcium gluconate estimation by EDTA. Illustrate calcium ions forming a pink complex with murexide, followed by EDTA binding calcium ions and releasing the free purple indicator.]
2. Reagents Required
1. Calcium Gluconate Sample: The substance being analysed.
2. Standard Disodium EDTA Solution: Used as the titrant to form a stable complex with calcium ions.
3. Murexide Indicator: Used to detect the endpoint through a colour change.
4. Sodium Hydroxide Solution: Used to maintain the alkaline pH required for the selected calcium-indicator system.
5. Distilled Water: Used for dissolving the sample and preparing solutions.
3. Procedure
The general procedure for estimating calcium gluconate by complexometric titration is:
Accurately weigh the prescribed quantity of calcium gluconate sample.
Dissolve the sample in distilled water using the prescribed analytical procedure.
Transfer a measured quantity of the prepared solution into a conical flask.
Add the prescribed quantity of sodium hydroxide solution to obtain the required alkaline pH.
Add a small quantity of murexide indicator.
Fill a burette with standardized disodium EDTA solution.
Titrate the sample slowly with EDTA while continuously mixing.
Add EDTA dropwise near the endpoint.
Observe the colour change from pink to purple or blue-violet.
Record the volume of EDTA consumed.
[IMG 2 — Horizontal laboratory infographic showing the calcium gluconate assay: sample weighing → dissolution → addition of sodium hydroxide → addition of murexide indicator → titration with standard EDTA → pink-to-purple endpoint.]
4. Endpoint and Observation
The endpoint is identified by the colour change of the murexide indicator.
Stage | Observation |
|---|---|
Before titration | Pink calcium-indicator complex |
During titration | EDTA gradually binds calcium ions |
At endpoint | Purple or blue-violet free indicator |
A clear and persistent endpoint colour indicates that the calcium ions have been complexed by EDTA.
5. Calculation
The amount of calcium gluconate is calculated from the volume and concentration of EDTA consumed.
The reaction between calcium ions and EDTA takes place in a 1:1 molar ratio.
For calcium gluconate monohydrate:
Chemical formula = Ca(C₆H₁₁O₇)₂·H₂O
Molecular weight ≈ 448.39 g/mol
For a 1:1 reaction, 1 mole of EDTA reacts with 1 mole of calcium gluconate.
If 0.05 M EDTA is used:
1 mL of 0.05 M EDTA is equivalent to:
448.39 × 0.05 ÷ 1000 = 0.0224195 g
Therefore:
1 mL of 0.05 M EDTA ≈ 22.42 mg of calcium gluconate monohydrate.
Formula:
Amount of calcium gluconate (mg) = Volume of EDTA used (mL) × 22.42
Example
Suppose 15 mL of 0.05 M EDTA is required to reach the endpoint.
Step 1: Given values
Volume of EDTA = 15 mL
Molarity of EDTA = 0.05 M
Equivalence factor = 22.42 mg/mL
Step 2: Apply the formula
Amount of calcium gluconate = 15 × 22.42
Step 3: Calculate
Amount = 336.3 mg
Answer: The titrated sample contains approximately 336.3 mg of calcium gluconate monohydrate.
Percentage purity:
Percentage purity = (Amount of calcium gluconate found ÷ Weight of sample taken) × 100
Note: This equivalence factor is calculated for calcium gluconate monohydrate. If anhydrous calcium gluconate is used, its molecular weight and equivalence factor will differ. The actual assay must follow the applicable pharmacopoeial monograph, including its specified EDTA concentration, indicator and endpoint conditions.
6. Applications of Calcium Gluconate Estimation
Purity Determination: Helps determine the purity of calcium gluconate samples.
Pharmaceutical Quality Control: Supports quality evaluation of calcium gluconate raw materials and preparations.
Raw Material Testing: Helps verify calcium content before pharmaceutical manufacturing.
Quantitative Analysis: Provides a method for determining calcium gluconate content.
Complexometric Analysis: Demonstrates the application of EDTA in calcium estimation.
Key Takeaways
Calcium gluconate can be estimated by complexometric titration using EDTA.
EDTA forms a stable 1:1 complex with calcium ions.
Murexide is a commonly used indicator for calcium estimation.
Alkaline conditions are required for the selected calcium-indicator system.
The endpoint is generally identified by a pink-to-purple or blue-violet colour change.
The calculation depends on the molecular form of calcium gluconate and the concentration of EDTA.
The applicable pharmacopoeial procedure should be followed for official assay results.