The pH meter displays 6.84. Two minutes later it reads 6.97. After re-immersing the electrode, 7.10 suddenly appears.

Is the sample unstable? Is the electrode defective? Or was the meter calibrated incorrectly?

In pH measurements, even small details can influence the result: a contaminated buffer solution, an insufficiently rinsed electrode, incorrect calibration points, or temperature differences between buffer and sample. Particularly in quality control and analytical work, such deviations can quickly become relevant.

This guide shows how to measure the pH value correctly, select suitable buffer solutions for calibration, and avoid typical errors before they distort your measurement results.

Table of contents

What does a pH meter actually measure?

Simply put, the pH value describes how acidic or alkaline an aqueous solution is.

The familiar scale used in everyday laboratory work is often as follows:

• pH < 7 – acidic,
• pH = 7 – neutral,
• pH > 7 – alkaline

This classification is practical, but the actual measurement is more complex.

A digital pH meter does not determine the pH value directly the way a thermometer does. The measuring electrode generates an electrical potential that depends on the activity of hydrogen ions in the solution. The meter then converts this signal into a pH value.

For this, however, it must know how the electrode currently responds.

This is exactly where calibration comes into play.

Why must a pH meter be calibrated?

A pH electrode does not remain completely unchanged throughout its entire service life.

Over time, the following can change, among other things:

• Slope of the electrode,
• Zero point,
• Response time,
• Condition of the glass membrane,
• Condition of the diaphragm,
• Electrolyte.

The behavior can also differ slightly between two measurement days.

Calibration therefore links the currently measured electrical signal of the electrode with known pH values.

For this purpose, buffer solutions with a defined pH value are used.

Metrohm sums up the principle very aptly: The quality of a calibration depends directly on the quality of the buffer solutions used.

Anyone who calibrates with a contaminated or unsuitable buffer solution therefore introduces an error before the actual sample measurement.

Which buffer solutions are suitable for calibration?

Buffer solutions have a defined pH value and are composed in such a way that this value changes as little as possible under minor external influences.

They serve as reference points for the measuring instrument.

Which buffers are needed depends above all on the pH range in which the samples to be measured later fall.

At Biolaboratorium, for example, buffer solutions with various defined pH values are available, including pH 2.00, 4.20, 5.00, 8.50, 10.00, and 12.40.

However, the selection should not be based on the principle:

“We have pH 7 in the cabinet, so we always calibrate with it.”

What matters is the sample to be measured later.

Example

You expect a sample to have a pH value of approximately 4.5.

In that case, the calibration should sensibly cover this range.

If, on the other hand, you regularly measure strongly alkaline solutions with pH values around 10, calibrating exclusively in the acidic range is unsuitable.

The most important rule is:

The expected pH value of the sample should fall within the calibrated measuring range.

One-, two-, or three-point calibration?

Many modern pH meters support multiple calibration points.

But when do you need how many?

One-point calibration

In one-point calibration, the instrument is adjusted using only one buffer solution.

This can be sufficient for simple, exploratory measurements in a very narrow range.

The disadvantage:

The calibration essentially corrects the zero point, but provides less information about the actual slope of the electrode.

For more demanding quantitative measurements, multipoint calibration is therefore usually more appropriate.

Two-point calibration

Here, two different buffers are used.

This allows the instrument to determine both the zero point and the electrode slope much more accurately.

For reliable measurements, Mettler Toledo recommends at least a two-point calibration.

A typical example would be calibration with a neutral or near-neutral buffer and a second buffer in the acidic or alkaline range.

Which second buffer makes sense depends on the samples to be measured later.

Three-point calibration

Anyone who regularly tests a broader pH range can add a third calibration point.

This makes it possible to cover a larger measuring range.

Metrohm points out that a three-point calibration can provide higher accuracy over a wide measurement range than a two-point calibration.

More calibration points are not automatically better, however, when:

• the buffers were poorly selected,
• solutions are contaminated,
• temperatures differ greatly from one another,
• the electrode was not prepared correctly,

Three bad reference points do not make a good calibration.

Why the calibration range must match the sample

A common error arises not from the device but already during the selection of the buffers.

Suppose you calibrate at:

pH 4 and pH 7.

Then measure a sample with approximately:

pH 10.

The meter must now extrapolate outside the calibrated range.

Measurement uncertainty can increase significantly as a result.

Metrohm therefore recommends selecting the buffers so that the expected pH value of the sample lies within the calibration range.

In practice, this means:

Acidic samples

Match the calibration range to the acidic or neutral range.

Near-neutral samples

Select calibration points as close as possible to the expected range.

Alkaline samples

Also use an alkaline reference point.

Widely differing samples

Multiple calibration points or separate measurement series can be useful.

Calibration should therefore always be application-driven—not based on which bottles happen to be in the lab.

How strongly does temperature affect the pH value?

Temperature is one of the most underestimated factors in pH measurements.

It affects several things at once:

• the behavior of the electrode,
• the electrical measurement signal,
• the actual pH value of the sample,
• the pH value of the buffer solution.

This is why a professional buffer solution often lists its defined value together with a reference temperature.

For Biolaboratorium buffer solutions pH 4.20 and pH 8.50, the respective setpoint is specified, for example, at 20 °C.

Automatic temperature compensation does not solve every problem

Many modern pH meters have a temperature sensor and automatic temperature compensation.

That is helpful.

It does not mean, however, that a sample at 10 °C and the same sample at 40 °C necessarily have the same pH value.

Temperature compensation primarily corrects the temperature-dependent response curve of the electrode.

The chemical equilibria within the sample can still change.

Metrohm therefore recommends performing comparative pH measurements at comparable temperatures wherever possible and taking the calibration conditions into account accordingly.

Practical rule

If reproducible results are important:

• Bring the buffer and sample to as similar a temperature as possible,
• Document the temperature,
• Use the temperature sensor if available,
• Perform measurements of different batches under comparable conditions.

Calibrate your pH meter correctly: step by step

The exact operation depends on the measuring instrument and the electrode. The manufacturer's instructions therefore always take precedence.

A typical procedure, however, looks like this:

1. Select suitable buffers

First consider:

What pH value do we expect in the sample?

Then select reference points that cover the relevant range.

2. Check the condition of the buffers

Check:

• expiry or use-by date,
• opening date,
• storage conditions,
• visible contamination,
• unusual turbidity.

Do not use buffers whose quality can no longer be reliably assessed.

3. Prepare a small amount of buffer

For calibration, it is advisable to put the required amount into a clean container.

This prevents the electrode from being inserted directly into the stock bottle.

4. Rinse the electrode

Rinse the electrode according to the manufacturer's instructions, typically with deionized or suitable ultrapure water.

This removes residues of the previous sample or buffer solution.

5. Carefully remove excess water

The electrode should not be vigorously rubbed dry.

Depending on the sensor type, careful blotting may be useful.

Rubbing can:

• mechanically stress the sensitive glass membrane,
• generate electrostatic effects,
• influence the measurement.

6. Place the electrode in the first buffer

The glass membrane and relevant areas of the electrode must be sufficiently immersed in the solution.

Metrohm points out that with appropriate electrodes, both the membrane and the diaphragm must be in contact with the solution.

7. Let the measured value stabilize

Do not confirm immediately.

Wait until the measuring instrument detects a stable value or the stability criterion defined by the manufacturer is met.

8. Rinse the electrode again

Before it goes into the next buffer, residues of the first buffer are removed.

This is especially important because even small amounts carried over can alter the next reference point.

9. Measure the second and, if necessary, third buffer

Repeat the procedure.

10. Check the calibration result

After calibration, many instruments show values such as:

• Electrode slope,
• Offset or zero point,
• Assessment of the sensor condition.

Unusual values should not simply be ignored.

They can indicate:

• an aged electrode,
• contaminated diaphragm,
• unsuitable buffers,
• incorrect handling

indicate.

How to measure a sample correctly

After calibration, the actual measurement begins.

Here, too, handling determines reproducibility.

Homogenize the sample

In inhomogeneous solutions, the pH value can vary locally.

Depending on the sample, gentle stirring can therefore be useful.

However, the following should not be done unnecessarily:

• to introduce air,
• to drive off or absorb CO₂,
• to heat the sample strongly.

Immerse the electrode sufficiently

The relevant sensor areas must have full contact with the sample.

Avoid contact with the vessel wall

The sensitive glass membrane should not press against the beaker.

Metrohm also recommends positioning the electrode as reproducibly as possible in the vessel during repeated measurements.

Wait for stabilization

A pH value that appears after two seconds is not automatically the final reading.

Depending on:

• Electrode,
• Sample,
• Temperature,
• Ionic strength,
• Viscosity

Stabilization can take varying amounts of time.

Rinse the electrode between samples

This prevents components of one sample from being transferred into the next.

This step is especially important with large pH differences.

Rinsing the electrode: water yes, rubbing no

A typical lab scene:

The electrode is taken out of the sample, rinsed with water, and then vigorously rubbed dry with a paper towel.

The first part makes sense.

The second can be problematic.

A pH glass electrode has a sensitive surface. Strong mechanical rubbing should be avoided.

Better:

• Rinse according to the manufacturer's instructions,
• Gently shake off or blot off excess liquid,
• Do not polish the sensor dry.

Another common mistake is rinsing with the next buffer solution directly over the stock bottle.

In this way, contaminants can enter the entire buffer stock unnoticed.

Why buffer solutions should not be poured back

You need 30 ml of buffer, but use only 20 ml.

Pouring the remaining 10 ml back into the bottle seems economical.

In the lab, that can get expensive.

The removed solution may already have had contact with:

• Electrode,
• Glass vessel,
• Pipette,
• Ambient air,
• Residues from another sample.

If it is poured back, the entire stock bottle can become contaminated.

Metrohm expressly recommends not pouring used calibration standards back into the original bottle.

Better:

Only take out approximately the amount actually needed and dispose of leftovers properly.

Why alkaline buffers are especially sensitive

Alkaline buffers in particular deserve special attention.

The reason is all around us:

Carbon dioxide from the air.

CO₂ can be absorbed by alkaline solutions and change their composition or pH value.

This becomes especially relevant when:

• Bottles are opened frequently,
• Vessels left open for a long time,
• small amounts of buffer are used multiple times.

Metrohm therefore points out that alkaline calibration buffers above pH 9 can be particularly sensitive to CO₂ uptake.

In practice, this means:

• Close the bottle immediately after taking out solution,
• Do not leave buffer standing open unnecessarily,
• Do not reuse the container used for taking out solution,
• Document the opening date,
• Observe the manufacturer's specifications on shelf life after opening.

Especially at high pH values, an apparently small change in the reference solution can subsequently affect the entire calibration.

How should a pH electrode be stored?

A pH electrode should not simply be cleaned and placed in a drawer dry.

Many glass electrodes require a suitable storage solution so that the glass membrane remains hydrated and the reference system functions correctly.

Which solution must be used depends on the electrode type and manufacturer.

Therefore the following applies:

Do not improvise; instead, follow the electrode manufacturer's specifications.

A common mistake is to store an electrode permanently in deionized water.

However, rinse water and storage solution serve different purposes.

What is suitable for cleaning does not have to be suitable for long-term storage.

After prolonged dry storage

A dried-out glass electrode often responds:

• slow,
• unstable,
• with poor reproducibility.

Depending on the sensor type, reconditioning may be required.

Before immediately ordering a new electrode, it is therefore worth checking the manufacturer's reactivation instructions.

Why does the pH reading drift?

A slowly drifting reading is one of the most common problems in the laboratory.

Possible causes are:

The electrode has not yet stabilized

Simply wait a little longer.

The temperature is still equilibrating

If a cold sample is measured with a warmer electrode, the value can change during temperature equilibration.

The glass membrane is contaminated

Deposits can slow down the response.

The junction is blocked

The electrical contact with the sample can be impaired as a result.

The electrode has aged

With increasing usage time, the response speed can decrease.

The sample itself changes

In open vessels, for example:

• CO₂ uptake,
• CO₂ loss,
• Evaporation,
• chemical reactions

change the actual pH value.

Insufficient ionic strength

Samples with very low ionic strength can be more difficult to measure stably and reproducibly.

The electrode was stored incorrectly.

A dried-out or improperly stored electrode can respond unusually slowly.

A drifting reading therefore does not automatically mean:

“The pH meter is broken.”

First, it should be systematically checked whether the error comes from:

• Calibration,
• Buffer,
• Electrode,
• Temperature,
• Sample

comes.

The most common errors in pH measurement

Error 1: Using just any buffer

A reference point near pH 7 is not sufficient for every measurement.

The calibration range must match the sample.

Error 2: Measuring a sample outside the calibration range

A calibration in the pH 4–7 range is not an ideal basis for a measurement at pH 11.

Error 3: Using old or unknown buffer

An open bottle without a documented opening date is problematic for quality-critical measurements.

Error 4: Reusing buffer multiple times

With every contact with the electrode and the environment, the risk of contamination increases.

Error 5: Pouring buffer back into the stock bottle

A small apparent saving can make an entire bottle unreliable.

Error 6: Not rinsing the electrode between buffers

This contaminates the next reference point with the previous buffer.

Error 7: Vigorously wiping the electrode dry

The sensitive glass surface should not be treated like ordinary laboratory glass.

Error 8: Ignoring temperature

Calibration at a temperature significantly different from the sample measurement can impair comparability.

Error 9: Placing the electrode against the wall of the vessel

This can damage the sensitive glass membrane in particular.

Error 10: Immediately recording the first displayed value

Wait for a stable reading.

Error 11: Simply recalibrating a dirty electrode

Calibration does not compensate for every mechanical or chemical problem with the sensor.

Error 12: Storing the pH electrode dry

Depending on the electrode type, this can significantly impair responsiveness.

Error 13: Using deionized water as a long-term storage solution

Cleaning and storage are two different steps.

Error 14: Accepting calibration despite unusual slope

If the device reports unusual calibration data, the cause should be investigated.

Error 15: Measuring all samples with the same procedure

A clear aqueous solution, a suspension, a highly viscous product, and a water sample with very low ion content can place different demands on the electrode and the measuring method.

How often should a pH meter be calibrated?

There is no single universally applicable frequency.

It depends, among other things, on:

• required accuracy,
• frequency of use,
• electrode type,
• sample type,
• internal quality requirements,
• Manufacturer recommendations.

In a laboratory where pH values are regularly documented as quality-relevant, calibration before a series of measurements or at the start of a workday can be useful.

For less critical orientation checks, other intervals may be sufficient.

More important than a blanket rule like “once a week” is a documented process.

If the results:

• batch-relevant,
• release-relevant,
• regulatory relevant

are, the calibration frequency should be part of the quality system.

What do German standards say about pH measurement?

In Germany, there are specific standards for professional pH measurements.

Currently relevant are, among others:

• DIN 19266:2026-03 – Reference buffer solutions for calibrating pH measuring equipment,
• DIN 19268:2025-12 – pH measurement of aqueous solutions using pH sensors with glass electrodes and estimation of measurement uncertainty.

In addition, DIN 19267 exists for technical buffer solutions.

This illustrates an important point:

A reliable pH measurement consists not only of "immersing the electrode and reading the value".

Buffer quality, measuring system, calibration, and uncertainty belong together.

Practical checklist for reproducible pH measurements

Before you document the next value, check these points.

Before calibration

• Is the electrode suitable for the sample?
• Is it stored correctly?
• Is the glass membrane clean?
• Is the diaphragm free?
• Are the buffers still within their intended use?
• Is the opening date documented?
• Do the buffers cover the expected pH range?
• Do the buffer and the later sample have comparable temperatures?

During calibration

• Use a clean vessel.
• Only take the required amount of buffer solution.
• Rinse the electrode between the solutions.
• Do not rub the glass membrane dry.
• Immerse the sensor sufficiently.
• Do not press against the vessel wall.
• Wait for stabilization.
• Check calibration values and slope.

During sample measurement

• Homogenize the sample if necessary.
• Take the temperature into account.
• Position the electrode in a reproducible manner.
• Wait for a stable reading.
• Rinse the electrode between different samples.
• Document the measurement result together with relevant conditions.

After the measurement

• Clean the electrode.
• Use suitable storage solution.
• Do not return buffer residues to the stock bottle.
• Close buffer bottles immediately.
• Document any anomalies.

A reliable pH measurement therefore begins long before the electrode is immersed in the actual sample.

The quality of the result depends on the entire measurement chain: buffer solution, calibration range, electrode, temperature, sample, and working method.

Even small habits make a big difference. An unrinsed electrode or a contaminated buffer solution may look harmless at first. But you may then spend a long time searching for an alleged production or sample error, even though the cause already arose during calibration.

Anyone who regularly checks pH values should therefore not treat suitable buffer solutions as trivial consumables. They are the reference on which every subsequent measurement is based.