Conductivity of Purified Water SOP

Learn SOP for Conductivity of Purified Water testing, apparatus, calibration, and procedure in pharmaceutical systems.

Introduction

The Conductivity of Purified Water is a critical quality parameter in pharmaceutical manufacturing. It reflects the presence of ionic impurities and ensures compliance with pharmacopeial standards. Accurate measurement helps maintain water purity required for formulations and processes.

This article provides a Standard Operating Procedure (SOP) for determining conductivity, including apparatus, calibration, and multi-stage testing methods.

Principle of Conductivity

Conductivity (K) is the reciprocal of resistivity (ρ) and indicates how well a solution conducts electricity.

Where:

• R = Resistance
• L = Distance between electrodes
• S = Surface area of electrodes

Units:

• Siemens per meter (S/m)
• Commonly used: µS/cm (microsiemens/cm)

Reference temperature: 25°C

Apparatus Required

The following equipment is used for measuring the Conductivity of Purified Water:

• Conductivity meter (with AC supply)
• Temperature probe with compensation system
• Conductivity cell with:
  • Platinum electrodes (platinum black coated)
  • Glass protection tube
• Certified reference materials (e.g., potassium chloride solution)

Cell Constants Commonly Used:

• 0.1 cm⁻¹
• 1 cm⁻¹
• 10 cm⁻¹

Calibration of Conductivity Cell

Cell Constant Determination

1. Use a certified reference solution (e.g., KCl).
2. Ensure conductivity is close to the expected test sample.
3. Rinse the cell:
   • With distilled water
   • Then with reference solution (twice)
4. Measure resistance at 25 ± 1°C

Formula:

K = Rmeasured​ × Kreference​

•  Must be within ±5% of specified value

Calibration of Conductivity Meter

Calibration ensures accuracy and reliability.

Procedure:

• Replace conductivity cell with certified resistors (±0.1% accuracy)
• Verify instrument response
• Calibrate each scale if required
• Ensure resolution of 0.1 µS/cm

Testing Method: Conductivity vs Temperature

• Rinse conductivity cell thoroughly
• Use non-temperature compensated readings initially
• Perform measurements as per pharmacopeial guidelines

Three-Stage Conductivity Testing

Stage 1

1. Measure temperature and conductivity
2. Refer to standard table (no interpolation)
3. Compare measured value with limit

Result:

• If within limit → Pass
• If above → Proceed to Stage 2

Stage 2

1. Take ≥100 ml sample
2. Maintain 25 ± 1°C
3. Stir continuously
4. Monitor conductivity until stable

Acceptance Criteria:

• ≤ 2.1 µS/cm → Pass

• 2.1 µS/cm → Proceed to Stage 3

Stage 3

1. Add saturated KCl (0.3 ml per 100 ml sample)
2. Measure pH (accurate to 0.1)
3. Determine conductivity limit using pH table

Result:

• Within limit & pH 5.0–7.0 → Pass
• Outside limit → Fail

Important Notes

• Online testing is allowed with properly calibrated systems
• Temperature compensation must be disabled during measurement
• Equipment must be correctly installed in the water system

Conclusion

The Conductivity of Purified Water is a fundamental test in pharmaceutical quality control. Following a structured SOP ensures compliance with regulatory standards and guarantees the purity of water used in manufacturing.

Accurate calibration, proper apparatus handling, and adherence to the three-stage testing method are essential for reliable results.

Frequently Asked Questions (FAQs)

1. What is the conductivity of purified water?

It is a measure of ionic impurities present in purified water, typically expressed in µS/cm.

2. Why is conductivity important in pharmaceuticals?

It ensures water purity and compliance with pharmacopeial standards.

3. What is the acceptable conductivity limit?

Typically ≤ 2.1 µS/cm at 25°C (depending on pharmacopeia).

4. What is a conductivity cell constant?

It is a factor that relates electrode geometry to measured conductivity.

5. Why is temperature important in conductivity measurement?

Conductivity varies with temperature, so measurements are standardized at 25°C.

6. What solution is used for calibration?

Potassium chloride (KCl) solution is commonly used.

7. What happens if Stage 1 fails?

Testing proceeds to Stage 2 for further evaluation.

8. Can conductivity be measured online?

Yes, with properly calibrated and installed instruments.

9. What is the role of pH in Stage 3?

pH helps determine the final acceptable conductivity limit.

10. What causes high conductivity in purified water?

Presence of dissolved ions such as salts, CO₂, or contaminants.