1. What is the working principle of a pH meter?
pH meter works on the electrochemical principle using a glass electrode and reference electrode to measure hydrogen ion activity.
It senses the potential difference (mV) generated due to H⁺ ion exchange across the glass membrane.
This potential is converted to pH using the Nernst equation.
Formula: E = E₀ – (2.303 × R × T / F) × pH
2. What is pH? Define it?
pH is the negative logarithm of hydrogen ion concentration.
Indicates acidity or alkalinity of a solution.
Formula: pH = –log₁₀ [H⁺]
3. What is the Nernst equation in pH measurement?
Defines relationship between electrode potential and pH.
Explains temperature effect on electrode slope.
At 25°C slope is 59.16 mV/pH.
Formula: E = E₀ – (2.303 × R × T / F) × pH
4. What is the ideal slope of a pH electrode?
Ideal slope = 59.16 mV/pH at 25°C.
Acceptable: 95%–105%.
Formula: Slope (%) = (Observed slope / 59.16) × 100
5. What is the function of a reference electrode?
Provides stable constant potential for comparison.
Maintains electrical continuity via KCl electrolyte.
Minimizes junction potential error.
6. What types of pH electrodes are commonly used?
Glass electrode
Combined electrode
ISFET electrode
Double junction electrode
Non-aqueous/solvent electrodes
7. What is a combined pH electrode?
Single body containing glass electrode + reference electrode.
Fast response, easy handling, reduced contamination.
Most common in pharma labs.
8. Difference between glass electrode and ISFET electrode?
Glass: fragile, requires hydration layer, high accuracy.
ISFET: solid-state, rugged, fast response, works in viscous/dirty samples, no hydration layer.
9. Why is temperature compensation needed in pH measurement?
Electrode slope depends on temperature.
pH of sample and buffers also changes with temperature.
Compensation ensures accurate pH at actual temperature.
10. What is ATC (Automatic Temperature Compensation)?
Uses temperature sensor (Pt100/Pt1000/thermistor).
Automatically adjusts for slope variation and sample changes.
Gives accurate and stable pH readings at varying temperatures.
11. Why is 2-point or 3-point calibration used in pH meters?
2-point or 3-point calibration defines the electrode slope and offset accurately.
It ensures linearity across the full measurement range.
Helps correct ageing, drift, and temperature effects for reliable readings.
12. What buffer solutions are used for pH calibration?
Common buffers are pH 4.00, 7.00, and 10.00.
pH 7 sets the zero point, pH 4 and 10 define slope.
Values are traceable to NIST standards.
13. Why do we use pH 4, 7, and 10 buffers?
They cover the acidic, neutral, and alkaline regions.
They establish accurate slope and offset.
They ensure proper linearity for full-range pH measurement.
14. What is buffer capacity?
Buffer capacity is the ability of a buffer to resist pH change when acid or base is added.
Higher capacity ensures more stable and reliable calibration.
Weak buffers lead to drift and inaccurate pH readings.
15. What causes drift in pH meter readings?
Ageing of electrode glass.
Junction clogging or contamination.
Temperature fluctuations.
Weak or expired buffer solutions.
16. What is junction potential?
A potential developed at the interface of reference electrolyte and sample.
It affects accuracy and stability of readings.
Minimized by using KCl electrolyte and proper maintenance.
17. What is reference electrolyte? Types?
Reference electrolyte provides constant ionic conductivity for stable reference potential.
Common types: KCl (most used), LiCl (for low-temperature), and gel-filled designs for maintenance-free operation.
18. What is KCl solution used for in pH meters?
Maintains stable ionic connection between reference electrode and sample.
Prevents junction potential fluctuation.
Used as filling solution and storage medium for electrodes.
19. What is the function of a salt bridge in pH electrodes?
Provides ionic continuity between reference system and sample.
Minimizes contamination of reference electrolyte.
Reduces junction potential and ensures stable measurement.
20. Explain the effect of temperature on pH reading.
Electrode slope increases with temperature (higher mV/pH).
Actual pH of sample and buffer changes with temperature.
Without compensation, readings become inaccurate and drift from true value.
21. What is the response time of a pH electrode?
Response time is the duration the electrode takes to reach a stable pH value after immersion.
Fresh electrodes respond within a few seconds.
Slow response indicates ageing, fouling, or dehydration.
22. Why does the pH electrode age?
Glass membrane slowly hydrates and becomes thicker over time.
Reference junction becomes clogged or contaminated.
Electrode slope decreases, leading to lower accuracy.
23. What is the shelf-life of pH electrodes?
Typical shelf-life is 12–18 months depending on usage and storage.
High-temperature or harsh chemical exposure reduces electrode life.
Proper hydration and maintenance extend useful life.
24. Why should a pH electrode never dry out?
Drying damages the hydration layer required for H⁺ exchange.
It causes slow response and unstable readings.
Rehydration takes hours and may not fully restore performance.
25. How to store a pH electrode properly?
Store in pH electrode storage solution or 3M KCl.
Never store in distilled or RO water.
Keep the glass bulb moist to preserve hydration layer.
26. Why is distilled water not recommended for electrode storage?
Distilled water leaches ions from the glass membrane and reference system.
Causes sluggish response and potential measurement drift.
Reduces electrode life significantly.
27. How to clean a pH electrode?
Use appropriate cleaning solution: acid, alkaline, protein, or solvent cleaners based on contamination.
Rinse with distilled water after cleaning.
Re-condition in KCl or buffer before use.
28. Why do we use different cleaning solutions for electrodes?
Different contaminants require specific cleaners (acid for scales, alkaline for oils, enzyme/protein cleaners for biological samples).
Use of correct cleaner restores slope and response time.
Prevents damage to sensing membrane.
29. What is electrode poisoning?
Occurs when chemicals react with or block the reference junction (e.g., sulfide, proteins, heavy metals).
Causes unstable and drifting pH values.
Requires cleaning or replacement of electrode.
30. What causes slow response in a pH electrode?
Aged or dehydrated glass membrane.
Blocked or contaminated junction.
Improper storage or dried-out reference electrolyte.
Accumulation of sample deposits on glass bulb.
31. How to check electrode slope?
Calibrate the electrode using two buffers (usually pH 7 and pH 4 or 10).
The meter calculates the mV change per pH unit.
Compare observed slope with the theoretical value of 59.16 mV/pH at 25°C.
32. What are the acceptance criteria for slope?
Slope should be between 95% and 105% of theoretical value.
Below 95% indicates ageing or fouling.
Above 105% indicates faulty calibration or contamination.
33. What causes electrode offset?
Contaminated or blocked reference junction.
Ageing of internal reference system.
Weak or expired buffer solutions.
34. What is the typical offset range?
Offset should be close to 0 mV at pH 7.
Acceptable offset is ±30 mV.
Higher offset suggests electrode damage or improper calibration.
35. What happens if slope is very low?
Indicates electrode ageing or fouling.
Results in inaccurate pH measurement and slow response.
Electrode usually requires cleaning or replacement.
36. How to troubleshoot unstable pH readings?
Check electrode hydration and condition.
Clean the electrode and verify buffer freshness.
Ensure temperature compensation is correct.
Check for air bubbles in the electrode.
37. What is calibration error?
Occurs when the mV response does not match expected buffer values.
Caused by expired buffers, contamination, or electrode ageing.
Requires recalibration or electrode maintenance.
38. What is temperature error in pH measurement?
Electrode slope varies with temperature.
Sample pH also changes with temperature.
Incorrect ATC leads to wrong pH displayed by meter.
39. What is drift error?
Gradual change in pH reading over time.
Caused by electrode ageing, dehydration, or junction clogging.
Minimized through proper storage and periodic calibration.
40. What is the effect of high ionic strength on pH measurement?
High ionic strength reduces junction potential errors.
Improves stability and response time.
Very high ionic samples may cause offset drift or glass fatigue over long exposure.