pH Meters


Q: What does pH measure?

A: pH is a measure of the degree of acidity or alkalinity of a substance. Modern instruments have made pH measurement almost as simple as temperature measurement.
pH

Q: What tips can you give for pH buffer use?

A: These solutions of known pH value allow the user to calibrate the pH meter to make accurate measurements.
For the highest accuracy:
• Standardisation should be performed with fresh buffer solutions.
• Buffers used should bracket the range of pH for the samples being tested.
• Buffers should be at the same temperature as the samples. (For example; if all your samples are at 50°C, warm your buffers to 50°C using a beaker in a water bath.)
• Buffer values are dependent upon temperature. The following table lists the NIST values of pH buffers at various temperatures.


Table of NIST Values of pH Buffers at Various Temperatures
Temperature (°C) 4.01 Buffer 6.86 Buffer 9.18 Buffer
10 4.00 pH 6.92 pH 9.33 pH
20 4.00 pH 6.88 pH 9.22 pH
25 4.01 pH 6.86 pH 9.18 pH


Q: How can I get reliable pH measurements?

A: Here are some basic steps that will help you get better, more reliable pH measurements:
• Soak new electrodes before use! A 50/50 mixture of pH 4 buffer and saturated potassium chloride is an excellent soak solution.
• Reference electrode frits must be free-flowing to allow internal solution to make contact with the sample solution. Maintain the frit in a clean condition.
• Use fresh buffers for calibration
• Temperature impacts electrode performance. Use temperature compensation, or keep all samples and standards at the same temperature.
• Calibrate frequently

Q: What is automatic temperature compensation (ATC)?

A: The Solution Temperature Effect
When there is an increase or decrease in the temperature of a solution, the pH of the solution can change. This change is not an error caused by the variation in temperature; it is the true pH of the solution at the new temperature. Since this is not an error, there is no need to correct or compensate for the solution temperature effect.

The pH Electrode Temperature Effect
There is only one major temperature effect in pH measurement that can cause errors in readings. This is the change in the electrode's response (or sensitivity) to pH which results from changes in the samples temperature. It is the only reasonably predictable error due to changes in temperature, and is the only temperature related factor that pH instruments with temperature compensation can correct for. This temperature error is very close to 0.003 pH/°C/pH unit away from pH 7. If a sample is measured without using an automatic temperature probe, the solutions temperature needs to be entered into the meter manually to allow it to account for this error.

Q: How should an electrode be stored when not in use?
A: Electrodes should be stored in electrode storage solution (025 192) between readings and for short-term storage. If storage solution is not readily available, liquid-filled electrodes can be stored in pH 4.0 buffer solution. To return an electrode to long-term storage, prepare it in the same condition in which you received it; usually, this means simply moistening and replacing the end cap of gel-filled electrodes to protect and keep the sensing membrane active. To store liquid-filled reference and combination electrodes, refill with electrolyte, cover the fill hole, and moisten and replace the protective plastic cap.

Important note: Never store an electrode in distilled or deionised water. This may lead to slow, sluggish response.

Q: How should a pH electrode be cleaned?

A: A dirty glass membrane is usually indicated by beads of water forming on the bulb when it's rinsed with distilled water. The bulb can be cleaned as follows:

• For protein layers, soak in a freshly prepared solution of 1% pepsin in 0.1N HCl for 30 minutes.
• For inorganic deposits, wash with a 1M EDTA solution, 2M ammonia, or 2M acid.
• For grease and similar films, wash with acetone, methanol, etc.

Q: How should an electrode be reconditioned?

A: Prolonged use, excessive alkaline immersion, or high-temperature operation will cause surface leaching of the membrane glass. The result is extremely noisy and/or sluggish response, which cannot be remedied simply by cleaning the electrode. If this occurs, the following procedures will often provide stability and pH sensitivity. Always consider the electrode's materials of construction before using these procedures.

1. Empty the reference chamber, rinse with deionised water, empty and refill with the specified filling solution.
2. Soak the electrode in hot (50°C – 60°C) reference electrolyte for a few minutes.
3. Soak the electrode overnight in pH 4 buffer.
4. Remove any exterior salt deposits with distilled water.
5. If the filling solution does not flow through the junction by this time (generally due to an unusually low junction porosity), use gentle suction to pull filling solution through the junction and repeat from step 2.

6. Sometimes the material clogging the junction requires more severe action. Should the above fail, proceed as follows:

a. Use a solvent specific to the solution or material plugging the junction, if possible.
b. Soak the membrane overnight in 0.1 M HCI.
c. If measurements have been made in samples containing protein, remove protein deposits by soaking the electrode bulb in 0.1 M HCl containing 1% pepsin.
d. Repeat from step 1.

If all these fail, the electrode should be discarded safely and replaced.