precipitation and weighing of analyte-derived products.

2. Scope

This SOP applies to the Analytical Method Development (AMD) department and is intended for gravimetric purity testing of raw materials, active pharmaceutical ingredients (APIs), and excipients where analyte isolation through precipitation is feasible and compliant with pharmacopoeial guidelines.

3. Responsibilities

• Analytical Chemist: Executes method trials, prepares reagents and samples, performs gravimetric analysis, and records all observations and calculations.
• Reviewer: Verifies gravimetric balances, method repeatability, and documentation integrity.
• QA Officer: Ensures the SOP adheres to GMP/GLP standards and regulatory expectations.
• Head – AMD: Reviews and approves the developed method for transfer to quality control or regulatory inclusion.

4. Accountability

The Head of AMD is accountable for ensuring validated gravimetric methods are scientifically justified, documented, and implemented appropriately for analytical use.

5. Procedure

5.1 Principle of Gravimetric Purity Analysis

1. Gravimetric analysis involves the conversion of an analyte to a stable, isolatable solid (usually a precipitate), followed by filtration, drying, and precise weighing.
2. Purity is determined from the mass of the final precipitate and its known chemical composition in relation to the analyte.

5.2 Selection of Precipitation Reaction

1. Select a gravimetric reaction where the analyte reacts quantitatively with a reagent to form a low-solubility, filterable, and weighable compound.
2. Examples:
   • Chloride determination via AgNO₃ → AgCl precipitate
   • Sulfate determination via BaCl₂ → BaSO₄ precipitate
   • Calcium via oxalate → CaC₂O₄, then ignition to CaO
3. Document rationale in Annexure-1: Precipitate Selection Log.

5.3 Reagent and Equipment Preparation

1. Prepare analytical-grade reagents freshly using deionized water.
2. Dry crucibles or filters at 105–110°C and cool in desiccators before use.
3. Calibrate analytical balance and maintain records.
4. Ensure clean sintered glass crucibles or ashless filter paper is used.
5. Log preparation in Annexure-2: Reagent and Equipment Log.

5.4 Sample Preparation

1. Weigh accurate amount of sample based on assay range (typically 100–500 mg).
2. Dissolve sample in appropriate medium (e.g., dilute nitric acid or water).
3. Filter if insoluble residues are present.
4. Document in Annexure-3: Sample Preparation Sheet.

5.5 Precipitation and Isolation

1. Add gravimetric reagent slowly to the stirred sample solution.
2. Maintain appropriate pH and temperature conditions to promote complete precipitation (e.g., warm solution for BaSO₄).
3. Allow precipitate to settle for 1–2 hours or age overnight if required.
4. Filter precipitate using pre-weighed sintered crucibles or filter paper.
5. Wash residue with cold distilled water or ethanol to remove excess reagents.
6. Document steps in Annexure-4: Precipitation and Isolation Log.

5.6 Drying, Ignition (if needed), and Weighing

1. Dry the precipitate at 105–110°C to constant weight OR ignite to convert it to stable residue (e.g., CaC₂O₄ to CaO).
2. Cool in desiccator and weigh to nearest 0.1 mg.
3. Repeat drying and weighing until consistent mass is recorded.
4. Record all weights in Annexure-5: Gravimetric Weighing Sheet.

5.7 Calculation of Purity

Purity (%) = (Weight of precipitate × Gravimetric Factor × 100) / Weight of Sample

Gravimetric Factor = (Molar mass of analyte / Molar mass of precipitate)

5.8 Method Optimization

1. Study:
   • Solubility of precipitate in wash medium
   • Effect of reaction time and pH
   • Mass balance and stoichiometry confirmation
2. Optimize precipitate settling time and filtration efficiency.
3. Document trials and improvements in Annexure-6: Optimization Summary.

5.9 Method Validation

1. Specificity: Test with potential interfering ions and confirm selectivity.
2. Accuracy: Spike recovery studies with known standards (98–102% acceptance).
3. Precision: RSD ≤ 2.0% across six replicates.
4. Robustness: Test variations in drying temperature and wash procedures.
5. Summarize validation in Annexure-7: Validation Report.

6. Abbreviations

• SOP: Standard Operating Procedure
• AMD: Analytical Method Development
• API: Active Pharmaceutical Ingredient
• RSD: Relative Standard Deviation

7. Documents

1. Precipitate Selection Log – Annexure-1
2. Reagent and Equipment Log – Annexure-2
3. Sample Preparation Sheet – Annexure-3
4. Precipitation and Isolation Log – Annexure-4
5. Gravimetric Weighing Sheet – Annexure-5
6. Optimization Summary – Annexure-6
7. Validation Report – Annexure-7

8. References

• USP General Chapter <541> – Gravimetric Procedures
• IP Appendix 3.5 – Determination by Gravimetry
• ICH Q2(R1) – Validation of Analytical Procedures

9. SOP Version

Version: 2.0

10. Approval Section

	Prepared By	Checked By	Approved By
Signature
Date
Name
Designation
Department

11. Annexures

Annexure-1: Precipitate Selection Log

Analyte	Precipitate	Justification	Analyst
Chloride	AgCl	Low solubility and stable	Sunita Reddy

Annexure-2: Reagent and Equipment Log

Item	ID	Calibrated On	Next Due	Checked By
Balance	AB-123	01/04/2025	01/10/2025	Ajay Mehra

Annexure-3: Sample Preparation Sheet

Sample Code	Weight Taken	Solvent Used	Prepared By
API-031	250 mg	Water	Rajesh Kumar

Annexure-4: Precipitation and Isolation Log

Reaction	pH	Settling Time	Observation
BaCl₂ + SO₄2−	4.5	2 hrs	Dense white ppt

Annexure-5: Gravimetric Weighing Sheet

Sample	Weight of Residue	Final Mass	% Purity
Sample A	0.1352 g	0.1352 g	99.4%

Annexure-6: Optimization Summary

Parameter	Change Tested	Impact	Conclusion
Drying Temp	105°C vs 110°C	No impact	105°C sufficient

Annexure-7: Validation Report

Parameter	Criteria	Result	Status
Accuracy	98–102%	99.3%	Pass
Precision	RSD ≤ 2%	0.95%	Pass
Specificity	No interference	Confirmed	Pass

Revision History:

Revision Date	Revision No.	Details	Reason	Approved By
04/05/2025	2.0	Expanded procedure and validation for precision gravimetric analysis	Annual SOP Review