Accurate and reliable pH measurements are essential for maintaining optimal process control. However, many users encounter pH drift, a phenomenon that compromises data accuracy over time. Understanding what pH drift is, its primary causes, and how to prevent it can help you optimize sensor performance and avoid unnecessary downtime.
Understanding pH Drift: Causes and Solutions
What Is pH Drift?
pH drift refers to the gradual deviation of a pH measurement from the actual expected value. This shift can result from multiple factors, leading to inaccurate readings and process inefficiencies. By identifying and addressing the root causes of pH drift, you can significantly improve the longevity and reliability of your pH sensors.
Below, we explore three primary causes of pH drift and the best practices to mitigate them.
1. Plugged Liquid Junction
Most glass pH electrodes rely on a liquid junction (diaphragm) to connect the internal reference system with the measured solution. Various junction types exist, including ceramic, PTFE, and open-hole designs. Selecting the appropriate junction type for your application is crucial for maintaining sensor performance.
However, if the junction becomes clogged with contaminants, it can block the connection between the reference system and the solution, leading to measurement drift.
Solution:
To maintain an unobstructed junction:
- Regularly clean the electrode by soaking it in a 5% diluted HCl or caustic solution for 10–15 minutes.
- After soaking, thoroughly rinse the electrode and perform a two-point calibration to restore accuracy.
2. Contaminated Reference Electrolyte (Poisoning)
The reference electrolyte inside a glass combination electrode plays a vital role in maintaining a stable potential. Most electrodes use 3 mol/L KCl (potassium chloride) in gel, liquid, or polymerized form.
Over time, the measured solution can diffuse into the reference electrolyte, contaminating it and shifting the electrode’s zero-point. Once contaminants reach the Ag/AgCl (silver/silver chloride) reference element, sensor failure is likely.
Solution:
To minimize contamination risks:
- Consider using double junction electrodes such as the patented Silamid® reference system utilized in Knick, which provides additional protection for the Ag/AgCl element.
- Unlike a bare silver wire coated in silver chloride, the Silamid system incorporates a glass tube lined with silver and packed with AgCl powder, offering enhanced resistance to contamination.
- A secondary junction filter on the end of the glass tube acts as a protective barrier against contamination, extending sensor life and reducing drift.
3. Electrical Interference and Signal Noise
Electrical interference is another common cause of pH drift, particularly in analog measurement systems. Many pH sensors transmit signals using a coaxial cable, which can be susceptible to:
- Ground loops
- Signal leakage
- External electromagnetic fields from surrounding equipment
These external factors can distort the millivolt signal, causing erratic readings and drifting pH measurements.
Solution:
To eliminate interference issues:
- Handle analog sensor cables with care and ensure they are shielded from potential interference sources.
- Opt for digital sensor technology, such as Memosens, which eliminates electrical interference.
- Memosens sensors convert the analog signal into a fully digital signal within the sensor head, transmitting it via an RS-485 protocol to the transmitter. This prevents signal distortion, ensuring highly accurate and stable pH measurements.
Conclusion
pH drift can lead to inaccurate data, process inefficiencies, and increased troubleshooting time. By understanding its primary causes—plugged junctions, electrolyte contamination, and electrical interference—you can take proactive measures to prevent it.
Selecting the right pH sensor for your application, performing regular maintenance, and leveraging advanced digital technology like Memosens will help you achieve precise, reliable, and long-lasting pH measurements.
Would you like expert advice on choosing the best sensor for your process? Contact us today!