pH meters: how they work and how to calibrate one

A pH meter measures pH as the voltage between a glass electrode and a reference electrode, converted at about 59 mV per pH unit. It is one of the most used instruments in a lab, and one of the easiest to get wrong. The reading on the screen is only as good as the last calibration and the state of the electrode. This guide explains how the meter turns a voltage into a pH value and how to calibrate it. It also covers what the slope and offset figures mean, and how to look after the electrode so it keeps giving reliable readings.

For what pH and pKa mean, and how buffers hold pH steady, see buffers and pH. Terms are defined in the glossary.

How a pH meter works

A pH meter has two parts: the electrode (or probe) that goes into the sample, and the meter, which is a very sensitive voltmeter.

The glass electrode

The tip of the electrode is a thin bulb of special glass. Its surfaces form a hydrated layer that exchanges hydrogen ions with the solution around it. When the hydrogen ion concentration inside the bulb differs from the sample outside, a small voltage builds up across the glass. The inside of the bulb is filled with a solution of fixed pH, so the voltage depends only on the pH of the sample.

The reference electrode

A voltage can only be measured between two points. The second point is a reference electrode, which gives a steady voltage whatever the sample. Most modern probes are combination electrodes, with the glass electrode and the reference built into one body. The reference is usually a silver wire coated in silver chloride, sitting in potassium chloride solution.

The reference connects to the sample through a junction: a small porous plug, ceramic frit or sleeve that lets a tiny amount of filling solution seep out. That slow leak completes the circuit. A blocked junction is one of the commonest causes of slow or drifting readings.

From voltage to pH

The voltage follows the Nernst equation. At 25 °C, an ideal electrode changes by 59.16 mV for each pH unit. At pH 7 the voltage is close to zero. Acidic samples give a positive voltage and alkaline samples a negative one, at roughly 59 mV per unit.

The slope depends on temperature: about 54 mV per unit at 0 °C and about 62 mV at 40 °C. That is why meters measure the temperature too.

Calibration

No electrode behaves exactly as theory says. Its zero point drifts and its response weakens as it ages. Calibration measures the real behaviour against buffers of known pH, so the meter can correct for it.

Choosing buffers

• Two-point calibration uses two buffers, typically pH 4.01 and 7.00 for acidic samples, or 7.00 and 10.01 for alkaline ones.
• Three-point calibration uses all three, for accuracy across the whole range.
• Bracket your samples. The buffers should sit either side of the pH you plan to measure.

Use fresh, in-date buffer, and pour a small amount into a clean beaker rather than dipping the probe into the stock bottle. Buffer at pH 10 absorbs carbon dioxide from the air and drifts downwards, so don't reuse it.

Step by step

1. Rinse the electrode with purified water and blot it gently with a lint-free tissue. Don't wipe the bulb: rubbing can build up static charge and scratch the glass.
2. Start with the pH 7.00 buffer. Immerse the bulb and junction, stir gently or let it settle, and wait for a stable reading before confirming.
3. Rinse and blot again.
4. Move to the next buffer and repeat.
5. Check the result. The meter reports slope and offset (see below). If they are outside the acceptable range, clean or replace the electrode before using it.
6. Check against a buffer you didn't calibrate with, if accuracy matters. It should read within about 0.02–0.05 pH units of its stated value.

How often to calibrate depends on the work. Daily, or before each session, is common, and recording each calibration is part of a lab's routine quality control (see QA vs QC). Calibrate more often for exact work or when readings seem to drift. For a quick, rough check without a meter, see pH indicators.

Slope and offset

After calibrating, most meters show two numbers:

These ranges are common manufacturer guidance, not a fixed rule. Check the instructions for your electrode.

A slope that is falling over time shows the glass is ageing or coated. An offset that is drifting often points to a problem with the reference or the junction.

Temperature

Temperature affects a pH reading in two separate ways:

1. The electrode's response changes. The Nernst slope rises with temperature. Automatic temperature compensation (ATC) corrects for this using a temperature sensor in or beside the probe.
2. The sample's real pH changes. Many solutions have a pH that genuinely shifts with temperature. Tris buffer, for example, falls by about 0.03 pH units for each degree of warming. ATC does not correct for this, because it isn't an error. It is the true pH at that temperature.

So measure at the temperature you will use the solution, and record it alongside the pH. Buffer bottles list their pH at several temperatures for the same reason.

Getting a reliable reading

• Stir gently and evenly, or not at all, but do the same for calibration and samples.
• Wait for a stable reading. Most meters flag when the value has settled.
• Rinse between samples to avoid carrying one into the next.
• Low-ionic-strength samples, such as purified water, give slow and wandering readings because they conduct poorly. They need a special electrode or a small addition of salt. See lab water for why pure water's pH is hard to pin down.
• Very high pH above about 12, especially with sodium present, reads low. This is called alkaline or sodium error.
• Tris and protein samples can react with or clog a silver-chloride reference junction. A double-junction electrode keeps the sample away from the silver.

Troubleshooting

Looking after the electrode

• Store it wet in the maker's storage solution, usually 3 M potassium chloride. A dry glass bulb loses its hydrated layer and takes hours to recover.
• Never store it in purified water. Ions leach out of the glass and the reference solution, which shortens the electrode's life.
• Top up refillable electrodes with the correct filling solution, and open the refill hole while measuring so the junction flows.
• Clean it when it slows down. Mild detergent removes grease, dilute acid removes mineral deposits, and enzyme cleaners remove protein. Rinse well and recalibrate afterwards.
• Expect to replace it. Glass electrodes wear out with use, typically within one to two years, and sooner with harsh samples.

pH meters and peptide work

pH matters when making up buffers for assays and when choosing a solvent, because a peptide's charge depends on pH. See isoelectric point and net charge and peptide solubility. For calculating the acid and base forms needed for a target pH, use the buffer calculator, then check the result with a calibrated meter.

Related guides

For accuracy and precision in any measurement, see accuracy, precision and significant figures. For acetic acid as a weakly acidic solvent, see acetic acid as a peptide solvent.