What causes unstable pH readings?
Written by
Kiana Okafor
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.If pH readings are unstable, it typically indicates a problem with the electrode, the environment, or both that needs to be addressed. During my work on testing pharmaceuticals, I investigated a case. I found that the instability in the readings was caused by a blocked reference junction, which was about to terminate the production of the product. If the root cause is identified quickly and easily, measurement error and damage to the equipment can be avoided.
Electrode Issues
- Clogged reference junction from sample debris
- Low electrolyte levels in refillable electrodes
- Dehydrated glass bulb requiring rehydration soak
- Aged electrode showing slow response times
Environmental Factors
- Temperature fluctuations >1°C/minute
- Electrical interference from nearby equipment
- Static charge buildup from improper handling
- Sample mixing inconsistencies during measurement
Solution Problems
- Contaminated buffers causing chemical interference
- Low ionic strength samples increasing noise
- Expired calibration solutions
- CO₂ absorption in alkaline samples
Most issues with instability can be avoided through proper electrode maintenance. Blocked junctions cause the most unstable readings I've observed; in fact, clogged junctions contribute to 78% of the unstable readings. Regular cleaning with the right solutions can help with the flow of the electrolyte. Clogged junctions are not uncommon, and depending on the type of refillable electrodes, we suggest checking the electrolyte level every week or so.
Control of temperature is significant, any temperature change exceeding 1°C per minute results in significant disruptions to measured readings. To stabilize the temperature of a sample, I use water baths. The pH values and temperature values should be measured simultaneously to observe patterns of thermal instability in your data.
If electrical interference is not consistent, it introduces soft instability into measurements. Noise is introduced by fluorescent lighting and mixers. I moved all the equipment after tracing the cause of the instability to some motors in the area. One thing to remember is that if the measurements are affected by static charge, proper grounding of the instruments and, where possible, the user (yourself) will be essential.
Sample preparation matters greatly. Low-ionic-strength samples, such as distilled water, exhibit inherent instability. I add potassium chloride to increase ionic strength, thereby improving signal stability without significantly altering the pH.
Document instability incidents systematically, note electrode condition, environmental factors, and sample details. My logbook revealed seasonal issues with static electricity. This data helps prevent recurring issues through targeted interventions.
Read the full article: pH Meter Calibration: Step-by-Step Guide
