Water for Injection (WFI) is a cornerstone of sterile pharmaceutical manufacturing. As one of the most widely used excipients in parenteral products, its purity is critical to patient safety. Over recent years, the European Pharmacopoeia (Ph. Eur.) has continued to evolve its monographs to align with modern analytical capabilities and risk-based quality approaches. In the latest revision to the European Pharmacopoeia (Ph. Eur. 12.3), which is effective in July 2026, the monograph for WFI has been updated to no longer permit the use of the traditional oxidisable substances test and mandate that Total Organic Carbon (TOC) is the only method of analysis for the control of organic impurities in sterilised WFI.

This blog explores the scientific rationale, regulatory implications, and practical impact of this transition for pharmaceutical professionals.

Why Change the Test Method?

Limitations of the Oxidisable Substances Test

The oxidisable substances test has historically been used as a simple, wet-chemistry method to detect organic contamination. It relies on the reduction of potassium permanganate under acidic conditions, observed visually through a colour change.

While useful in the past, this method presents several limitations:

• Lack of specificity – it only detects compounds that can be oxidised under the test conditions
• Subjective endpoint – interpretation of colour can vary between analysts
• Limited sensitivity – not capable of detecting low levels of organic contamination

TOC: A Modern Analytical Approach

What is TOC?

Total Organic Carbon (TOC) is a quantitative analytical technique that measures the amount of carbon bound in organic molecules present in water. It is widely used across pharmaceutical water systems and aligns with modern analytical expectations.

Key Advantages of TOC

The adoption of TOC brings several benefits:

1. Enhanced Sensitivity and Accuracy

TOC can detect trace levels of organic contamination far below the detection capability of the oxidisable substances test.

2. Non-Specific but Comprehensive

Unlike the older test, TOC captures all organic carbon, not just oxidisable compounds, providing a more holistic assessment of water purity.

3. Automation and Data Integrity

TOC instruments:

• Are fully automated
• Provide digital records
• Integrate with laboratory data systems

This supports compliance with:

• EU GMP Annex 11 (Computerised Systems)
• Data integrity expectations (ALCOA+ principles)

The Ph. Eur. shift supports international harmonisation of testing requirements for pharmaceutical waters, particularly with the USP.

Conclusion

The removal of the oxidisable substances test for analysing organic impurities in the European Pharmacopoeia supports international harmonisation of testing requirements for pharmaceutical waters, particularly with the USP. It reflects a broader industry shift toward science-based, data-driven quality control.