The goal is to ensure accurate, reproducible, and GMP-compliant acidity measurements aligned with pharmacopeial and regulatory requirements.

2. Scope

This SOP applies to all acidity assay method development activities performed in the Analytical Method Development (AMD) laboratory for APIs, raw materials, and dosage forms including syrups, suspensions, injectables, and tablets.

3. Responsibilities

• Analytical Scientist: Performs method development and documents observations and results.
• Reviewer: Verifies method performance characteristics and confirms calculations.
• QA: Reviews final method and validation data for compliance with quality standards.
• Head – AMD: Approves the validated method and ensures proper archiving and reporting.

4. Accountability

The Head of AMD is accountable for ensuring that titrimetric methods developed for acidity determination are accurate, validated, and available for technology transfer and regulatory submission.

5. Procedure

5.1 Selection of Titration Method

1. Determine if acid strength is strong (e.g., HCl) or weak (e.g., acetic acid).
2. Select appropriate titrant:
   • NaOH (0.1 N or 0.05 N) for most organic and inorganic acids
   • Use phenolphthalein or methyl orange based on pKa
3. Reference compendial methods (USP, Ph. Eur.) where available.

5.2 Sample Preparation

1. Weigh accurately 0.2–1.0 g of API or formulation equivalent.
2. Dissolve in purified water (or ethanol/water mixture if needed).
3. Use Annexure-1: Sample Preparation Record for Acidity Assay to document solution preparation.

5.3 Titration Procedure

1. Pipette an aliquot into a clean conical flask.
2. Add 2–3 drops of selected indicator.
3. Titrate with standard 0.1 N NaOH from a calibrated burette until endpoint is reached (color change observed).
4. Repeat titration to obtain concordant values (≤ 0.1 mL difference).
5. Calculate acidity using the formula:

   Acidity (%) = (V × N × Eq. Wt × 100) / Sample Weight (mg)
   

6. Document results in Annexure-2: Titrimetric Calculation Sheet.

5.4 Method Optimization

1. Optimize titrant strength based on:
   • Expected acid concentration
   • Sample matrix and solubility
2. Evaluate impact of:
   • Solvent system
   • Sample quantity
   • Indicator performance
3. Record observations in Annexure-3: Method Development Worksheet.

5.5 Method Validation

1. Perform validation as per ICH Q2(R1):
2. Specificity: Ensure no interference from excipients or buffers.
3. Accuracy: Recovery between 98–102% of theoretical acid content.
4. Precision: %RSD ≤ 2.0% for six replicates.
5. Linearity: Five concentration levels; r² ≥ 0.999.
6. Robustness: Test against minor variations in titrant strength, volume, and temperature.
7. Compile data in Annexure-4: Validation Summary Report.

6. Abbreviations

• AMD: Analytical Method Development
• USP: United States Pharmacopeia
• RSD: Relative Standard Deviation
• QA: Quality Assurance
• SOP: Standard Operating Procedure

7. Documents

1. Sample Preparation Record for Acidity Assay – Annexure-1
2. Titrimetric Calculation Sheet – Annexure-2
3. Method Development Worksheet – Annexure-3
4. Validation Summary Report – Annexure-4

8. References

• ICH Q2(R1) – Validation of Analytical Procedures
• USP <401>, <621> – Titrimetry
• Ph. Eur. Monographs – Acid-Base Assay Techniques
• 21 CFR Part 211 – Subpart I: Laboratory Controls

9. SOP Version

Version: 2.0

10. Approval Section

	Prepared By	Checked By	Approved By
Signature
Date
Name
Designation
Department

11. Annexures

Annexure-1: Sample Preparation Record for Acidity Assay

Date	Sample Name	Weight Taken (mg)	Solvent	Prepared By
02/05/2025	API-A	500	Water	Ajay Mehra

Annexure-2: Titrimetric Calculation Sheet

Trial No.	Volume of Titrant (mL)	Normality	Sample Weight (mg)	Acidity (%)
1	12.5	0.1	500	0.91

Annexure-3: Method Development Worksheet

Parameter	Condition	Observation	Conclusion
Indicator	Phenolphthalein	Sharp pink endpoint	Suitable

Annexure-4: Validation Summary Report

Parameter	Acceptance Criteria	Result	Status
Precision	RSD ≤ 2%	1.2%	Pass
Accuracy	98–102%	99.3%	Pass

Revision History:

Revision Date	Revision No.	Details	Reason	Approved By
04/05/2025	2.0	Updated calculation logic and validation requirements	Regulatory Audit Alignment