SOP for Specific Gravity and Density Determination in AMD Laboratory

Department	Analytical Method Development
SOP No.	SOP/AMD/055/2025
Supersedes	SOP/AMD/055/2022
Page No.	Page 1 of 14
Issue Date	19/05/2025
Effective Date	20/05/2025
Review Date	19/05/2026

1. Purpose

The purpose of this SOP is to define the method for determining the specific gravity and density of liquid or semi-solid pharmaceutical substances using pycnometer, digital density meter, or hydrometer methods as per pharmacopoeial standards.

2. Scope

This procedure applies to the Analytical Method Development (AMD) department and is relevant for raw materials, in-process liquids, oral suspensions, and finished liquid drug products where specific gravity or density is a critical quality attribute.

3. Responsibilities

• Analytical Chemist: Responsible for preparation, calibration, measurement, and recording of data.
• Reviewer: Reviews raw data, calculations, and confirms reproducibility.
• QA Personnel: Ensures procedural adherence and reviews for compliance with regulatory requirements.
• Head – AMD: Accountable for final review and approval of the developed method and its validation.

4. Accountability

The Head of the AMD department is accountable for ensuring proper execution and documentation of specific gravity and density determinations according to regulatory expectations and pharmacopeial standards.

5. Procedure

5.1 Selection of Method

1. Choose the most appropriate method depending on sample type and equipment availability:
   • Pycnometer Method – for high precision (preferred for small volumes)
   • Digital Density Meter – for automated high-throughput determination
   • Hydrometer Method – for rough estimations (less preferred)
2. Refer to IP, USP <841>, or Ph. Eur. 2.2.5 for procedural standards.

5.2 Equipment Calibration

1. Ensure all instruments used (balances, thermometers, digital meters) are calibrated and within valid calibration period.
2. Record balance calibration status and instrument ID in Annexure-1: Equipment Calibration Log.

5.3 Pycnometer Method

1. Clean and dry a calibrated pycnometer (typically 10 or 25 mL capacity).
2. Weigh the empty pycnometer (W₁).
3. Fill the pycnometer with distilled water at 25°C. Weigh and record (W₂).
4. Empty and dry the pycnometer. Fill it with the test sample at the same temperature and weigh (W₃).
5. Calculate specific gravity:

   Specific Gravity = (W3 - W1) / (W2 - W1)

6. Record readings in Annexure-2: Pycnometer Method Data Sheet.

5.4 Digital Density Meter Method

1. Switch on the digital density meter and allow it to equilibrate to 25°C.
2. Rinse the U-tube with distilled water, then sample solvent, and finally the sample.
3. Inject the test sample slowly into the sample inlet until U-tube is completely filled without bubbles.
4. Record the density value displayed (in g/mL) and temperature.
5. Perform three determinations and calculate mean.
6. Record in Annexure-3: Digital Density Log.

5.5 Hydrometer Method (if applicable)

1. Fill a clean, dry cylinder with the test liquid to about 3/4th full.
2. Lower the hydrometer into the sample carefully to avoid sticking to the side.
3. Note the specific gravity value at the bottom of the meniscus at 25°C.
4. Correct for temperature if required using reference correction charts.
5. Document in Annexure-4: Hydrometer Record Sheet.

5.6 Method Optimization

1. Compare values obtained using at least two methods for the same sample.
2. Check:
   • Reproducibility (n=3)
   • Effect of temperature variation (±2°C)
   • Effect of sample viscosity on reading stability
3. Select the method with highest precision and least operator variability.
4. Document findings in Annexure-5: Method Optimization Log.

5.7 Method Validation

1. Precision: Six replicates using pycnometer or digital meter. RSD ≤ 1.0%
2. Repeatability: Conduct intra-day measurements by the same analyst.
3. Intermediate Precision: Inter-analyst, inter-day verification using same method.
4. Accuracy: Validate against certified reference standard or known density liquids.
5. Robustness: Alter temperature, sample volume, and cleaning method.
6. Record in Annexure-6: Validation Summary Report.

6. Abbreviations

• SOP: Standard Operating Procedure
• AMD: Analytical Method Development
• RSD: Relative Standard Deviation
• USP: United States Pharmacopeia
• IP: Indian Pharmacopoeia

7. Documents

1. Equipment Calibration Log – Annexure-1
2. Pycnometer Method Data Sheet – Annexure-2
3. Digital Density Log – Annexure-3
4. Hydrometer Record Sheet – Annexure-4
5. Method Optimization Log – Annexure-5
6. Validation Summary Report – Annexure-6

8. References

• USP <841> – Specific Gravity and Density
• IP Appendix – Specific Gravity Tests
• Ph. Eur. 2.2.5 – Relative Density
• 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: Equipment Calibration Log

Instrument	ID	Calibrated On	Due Date	Checked By
Analytical Balance	AB-102	01/04/2025	01/10/2025	Sunita Reddy

Annexure-2: Pycnometer Method Data Sheet

Sample	W1 (Empty)	W2 (Water)	W3 (Sample)	Specific Gravity	Analyst
Syrup-A	25.136 g	35.980 g	38.215 g	1.119	Ajay Mehra

Annexure-3: Digital Density Log

Sample	Reading 1	Reading 2	Reading 3	Mean Density (g/mL)
Syrup-B	1.122	1.124	1.123	1.123

Annexure-4: Hydrometer Record Sheet

Sample	Temperature (°C)	Hydrometer Reading	Corrected Specific Gravity
Solution-C	25	1.085	1.085

Annexure-5: Method Optimization Log

Trial	Method Used	Observation	Precision	Conclusion
1	Pycnometer	Stable and precise	RSD 0.45%	Accepted
2	Hydrometer	High variability	RSD 2.2%	Rejected

Annexure-6: Validation Summary Report

Parameter	Criteria	Result	Status
Precision	RSD ≤ 1.0%	0.48%	Pass
Accuracy	Within ±0.01 g/mL of reference	0.002 g/mL	Pass
Robustness	Stable with ±2°C temp change	No change	Pass

Revision History:

Revision Date	Revision No.	Details	Reason	Approved By
04/05/2025	2.0	Included digital meter method and validation annexures	Annual review update