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A test strip is briefly dipped, changes color, and provides a result within seconds. That sounds simple. Nevertheless, this method is not suitable for every task. Someone who only wants to know whether a cleaning solution is acidic or alkaline requires a different measurement accuracy than a laboratory that must document the pH value of a sample or control a process.

Available options include pH test strips, indicator papers, liquid acid-base indicators, and electronic pH meters. Each method has its own measuring range, typical sources of error, and sensible areas of application.

This guide helps you to select the appropriate method and determine pH values more reliably.

What does the pH value indicate?

The pH value describes whether an aqueous solution is acidic, neutral, or alkaline.

Simplified:

• A pH value below 7 indicates an acidic solution.
• A pH value of about 7 is considered neutral at room temperature.
• A pH value above 7 indicates an alkaline or basic solution.

The well-known scale from 0 to 14 is a practical guide for many aqueous solutions. However, it should not be understood as a rigid boundary. Under certain conditions, values outside this range can also occur.

The pH value is logarithmic. A change of one pH unit therefore does not correspond to a small linear change. Between pH 3 and pH 4, for example, there is a tenfold difference in the corresponding hydrogen ion activity.

In practical measurement technology, the pH value is determined using calibrated procedures and reference buffer solutions. The Physikalisch-Technische Bundesanstalt uses primary reference buffers according to internationally coordinated measurement methods.

What methods are there for pH measurement?

The suitable measurement method depends on how accurate the result needs to be and under what conditions the measurement is taken.

The most common methods include:

pH test strips

They provide an approximate pH range through a color change. The color is compared to a printed comparison scale.

Universal indicator paper

It usually covers a broad pH range and is suitable for quick orientation.

Narrow-range indicator paper

This paper covers only a limited pH range, but within that range it can enable finer gradation.

Liquid indicators

Certain dyes change color within a characteristic transition range. They are used, among other things, in acid-base reactions and titrations.

Electronic pH meters

A pH meter measures the electrical potential of an electrode and converts it into a pH value. In practice, glass electrodes are often used, which must be calibrated with reference buffer solutions before measurement.

pH test strips and indicator paper

pH test strips are among the simplest methods for determining the approximate pH value of a solution.

The application is quick:

1. Briefly wet the test strip with the sample.
2. Shake off or allow excess liquid to drain.
3. Wait for the specified reaction time.
4. Compare the color with the comparison scale.

The result is usually a visual estimate and not a highly precise measurement value.

Advantages of pH test strips

• quick to use,
• no electronic measuring device required,
• easy to transport,
• little preparation,
• suitable for individual measurements and orientational checks,
• no calibration necessary.

Disadvantages of pH test strips

• limited reading accuracy,
• subjective color perception,
• difficult evaluation with colored or turbid samples,
• possible influence by strong oxidizing agents or other reactive components,
• often only coarse gradations,
• dependent on proper storage and shelf life.

Universal indicator paper

Universal indicator paper covers a wide range, for example pH 1 to 14. It is useful when the expected pH value is not yet known.

However, the wide measuring range has a disadvantage: the individual color steps are often relatively far apart. This is sufficient for an initial classification, but usually not for precise process control.

Narrow-range indicator paper

If the approximate pH value is already known, a narrow measuring range can be advantageous.

A paper for, for example, pH 4.0 to 7.0 usually allows a finer reading within this range than a universal paper from pH 1 to 14.

The procedure can therefore be two-step:

1. First, determine the approximate range using universal indicator paper.
2. Then measure more precisely with a narrower range indicator paper.

Liquid pH indicators

Liquid indicators are chemical dyes whose color changes depending on the pH value.

Well-known examples are:

• Phenolphthalein,
• Methyl orange,
• Methyl red,
• Bromothymol blue,
• Bromocresol green,
• Litmus,
• Thymol blue.

Each indicator has a specific transition range. It therefore does not indicate the entire pH range uniformly.

Example: Phenolphthalein

Phenolphthalein is colorless in acidic and weakly neutral environments. In the alkaline range, it develops a pink to strong magenta coloration.

It is therefore not suitable for accurate measurement of any pH value. Rather, it indicates whether a solution has reached the characteristic transition range.

Example: Methyl orange

Methyl orange changes its color in the acidic range. It is used, among other things, in certain acid-base titrations.

What are liquid indicators suitable for?

• demonstration of acid-base reactions,
• titrations,
• checking whether a defined endpoint has been reached,
• qualitative classification of a solution,
• Training and laboratory instruction.

Where are the limits?

A single indicator usually does not provide an exact pH value. It only shows whether the sample is within, below, or above its transition range.

The color of the sample itself can also hinder assessment. For heavily colored, turbid, or fluorescent liquids, the color change may be barely recognizable.

pH meter and glass electrode

An electronic pH meter is the preferred method when a specific numerical value is needed.

The device usually consists of:

• a measuring device,
• a pH electrode,
• often a temperature sensor,
• Calibration or buffer solutions.

The electrode generates an electrical potential depending on the hydrogen ion activity of the sample. The measuring device processes this signal and displays the calculated pH value.

Advantages of a pH meter

• higher resolution than test strips,
• numerical measurement value,
• better suited for repeated checks,
• results can be documented and compared,
• suitable for process monitoring and laboratory analysis,
• automatic temperature compensation possible depending on the device.

Disadvantages of a pH meter

• regular calibration necessary,
• electrode requires care and proper storage,
• higher purchase costs,
• sensitive to contamination and drying out,
• not every electrode is suitable for every sample,
• Measurement errors possible due to incorrect temperature or insufficient stabilization.

Why must a pH meter be calibrated?

The properties of an electrode change over time. Deposits, aging, storage, and temperature can affect the measurement result.

Therefore, the device is calibrated with buffer solutions of known pH values. The PTB points out that commercial pH meters must be calibrated with reference buffer solutions in practice.

For many applications, at least a two-point calibration is used. Two buffers are selected between which the expected measurement value lies.

Examples:

• acidic sample: buffer pH 4 and pH 7,
• alkaline sample: buffer pH 7 and pH 10,
• unknown sample: first measure approximately and then select suitable calibration points.

The specific calibration values depend on the device, method, and required accuracy.

Test strips or pH meter: the direct comparison

pH test strips

Result: approximate pH range
Accuracy: low to medium
Effort: very low
Calibration: not required
Cost: low
Suitable for: quick orientation and simple checks

Narrow-range indicator paper

Result: narrower pH range
Accuracy: better than wide-range universal paper, but still visual
Effort: low
Calibration: not required
Cost: low
Suitable for: known measurement ranges and quick comparative checks

Liquid indicator

Result: color change within a specific range
Accuracy: depends on indicator and sample
Effort: low to medium
Calibration: not required
Cost: low to medium
Suitable for: Titrations, reaction control, and demonstrations

pH meter

Result: numerical measurement value
Accuracy: significantly higher with correct calibration
Effort: medium
Calibration: required regularly
Cost: higher
Suitable for: Laboratory, quality control, documentation, and process monitoring

Which method is suitable for which application?

Rapid testing of a cleaning solution

For the quick classification 'acidic, neutral, or alkaline', a universal indicator paper is usually sufficient.

If it is to be checked whether the solution is within a narrower target range, a correspondingly graduated indicator paper is more useful.

Water control

For an orientational check, indicator paper can be used.

Anyone who wants to regularly document changes, detect small differences, or monitor a process should use a calibrated pH meter. For professional measurements in water samples, sampling, temperature, calibration, and measurement time must also be considered.

Soil and substrates

A test strip should not simply be pressed directly onto dry soil. Typically, a suspension or extract is first prepared according to a defined method.

The result depends, among other things, on:

• the ratio between sample and liquid,
• the liquid used,
• the waiting time,
• the homogenization,
• the temperature.

Anyone who wants to compare measurement values must use the same procedure for each measurement.

Titrations

For a simple acid-base titration, a suitable color indicator may suffice. It is crucial that its transition range matches the expected equivalence range of the reaction.

For colored samples, automated processes, or higher accuracy requirements, potentiometric determination with an electrode may be more suitable.

Quality control and production

As soon as pH values are documented, compared between batches, or used as a release criterion, a pH meter is usually the better choice.

Additionally, the following should be defined:

• Calibration frequency,
• Permissible buffer solutions,
• Measurement temperature,
• Waiting time until stable measurement value,
• Cleaning procedure,
• Documentation,
• Acceptance limits.

School, training, and demonstration

Liquid indicators are particularly suitable for visible acid-base reactions. Universal indicator paper also enables rapid classification of multiple samples.

Measuring pH correctly: step by step

The exact procedure depends on the chosen method.

Measurement with pH test strips

1. Select the appropriate measuring range

Use universal paper if the pH range is unknown.

If the approximate value is known, choose a paper with a narrower measuring range.

2. Use a clean sample container

Take a small amount of the sample into a clean container. This prevents the entire product from being contaminated by the test strip.

3. Correctly moisten the strip

According to the manufacturer's instructions, the strip is:

• briefly immersed,
• moistened with a drop,
• or contacted with a clean glass rod.

The application instructions of the respective product take precedence.

4. Observe the reaction time

Do not read the result immediately and also not several minutes later. The correct waiting time is stated in the product instructions.

5. Compare color under suitable light

Use as neutral, bright light as possible. Colored lighting can alter perception.

6. Document the result as a range

A test strip often does not provide an exactly confirmed single value. Therefore, document e.g., "pH about 6–7" instead of "pH 6.4" if the scale does not allow this accuracy.

Measurement with a pH meter

1. Inspect the electrode

Check:

• Is the electrode clean?
• Was it stored correctly?
• Is it dried out?
• Are there any visible damages?
• Is sufficient electrolyte present, provided the model is refillable?

2. Prepare suitable buffers

Choose calibration buffers that cover the expected measurement range.

Contaminated buffer solutions can falsify the entire calibration. Therefore, do not pour used buffers back into the stock bottle.

3. Calibrate the device

Rinse the electrode between buffers with suitable water and gently pat it dry. Vigorous rubbing can electrostatically affect or damage the electrode.

4. Consider the temperature

Buffer and sample should be measured at comparable temperatures if possible. Temperature affects both the electrode and the actual pH value of the solution.

A PTB publication indicates that calibration and subsequent pH measurement should be performed at the same temperature to reduce temperature-related deviations.

5. Immerse the electrode in the sample

The sensitive measuring surface must be sufficiently covered. The electrode must not hit the bottom of the container.

6. Gently move the sample

Gentle stirring can help obtain a uniform reading. Excessive stirring may introduce air or alter the composition of sensitive samples.

7. Wait for a stable reading

Do not read the first displayed value. Wait until the display stabilizes or the device signals a stable endpoint.

8. Clean and store the electrode correctly

After measurement, the electrode is rinsed according to manufacturer instructions and stored in the designated storage solution.

A classic glass electrode should normally not be stored permanently in distilled or deionized water, unless the manufacturer specifies otherwise.

Why temperature and calibration are important

Temperature is often underestimated in pH measurement.

It can cause two different effects:

1. The electrical behavior of the electrode changes.
2. The actual pH value of the sample can change with temperature.

Automatic temperature compensation primarily compensates for the temperature-dependent electrode response. It does not automatically mean that a value measured at 10 °C can be directly equated with a value measured at 30 °C.

Reference buffers therefore have temperature-dependent setpoints. For a reproducible measurement, buffers, sample, and instrument should be adequately tempered. Official reference measurements by the PTB are performed in defined temperature ranges.

Typical errors in pH measurement

Using too broad a test range

If someone expects a value around pH 6, using a test paper from pH 1 to 14 may only give a rough classification.

A narrower measurement range usually provides a more distinguishable color scale.

Visually measuring colored samples

With dark juices, dye solutions, or turbid suspensions, the natural color can mask the indicator.

A pH meter may be more suitable here, provided the electrode is intended for the sample.

Dipping the test strip directly into the storage container

This can introduce indicator substances or contaminants into the product.

It is better to take a small sample into a separate container.

Using a dirty electrode

Oils, proteins, salts, or solid deposits can extend the response time and shift the result.

The cleaning must match the type of contamination and the electrode used.

Not rinsing the electrode between samples

Residues from the previous sample alter the next measurement. This is especially critical with strongly acidic, alkaline, or buffered solutions.

Calibrating with unsuitable or old buffers

Opened, contaminated, or improperly stored buffers can change their setpoint.

Pay attention to expiration date, storage temperature, and manufacturer specifications.

Incorrectly select calibration points

If someone expects an alkaline sample but calibrates only in the acidic range, they are working outside the reasonably covered range.

The buffers should ideally bracket the expected measurement value.

Accept the first displayed value

Electrodes need time to adapt to the sample. Stabilization can take longer at low temperature, low conductivity, or with a dirty electrode.

Specify measurement results with too many decimal places

A reading of 7.00 does not automatically mean that the entire measurement procedure achieves an accuracy of ±0.01 pH.

The actual measurement quality also includes:

• Calibration,
• Condition of the electrode,
• Quality of the buffers,
• temperature,
• Sample preparation,
• Repeatability.

Directly compare different measurement methods

A test strip value and an instrumentally determined value may differ. This does not automatically mean that one of the results is wrong. Both methods have different resolution and sources of error.

Safety when handling acidic and alkaline solutions

A neutral or only slightly deviating pH value does not automatically mean that a sample is harmless. Conversely, the danger of a product cannot be deduced solely from the pH value.

Other relevant factors include:

• concentration,
• chemical composition,
• oxidation effect,
• toxicity,
• temperature,
• reactivity,
• amount used,
• possible vapor or aerosol formation.

Therefore, before measuring chemical products, the label and safety data sheet should be checked.

Depending on the sample, the following may be required:

• safety goggles,
• suitable protective gloves,
• lab coat or protective clothing,
• face protection,
• technical ventilation,
• suitable sample containers.

Unknown solutions should not be tasted, directly inhaled, or neutralized without testing.

Decision aid for the appropriate measurement method

Choose universal indicator paper when:

• the approximate pH range is unknown,
• a quick orientation is sufficient,
• no high accuracy is required,
• only measured occasionally.

Choose indicator paper with a narrow range when:

• the expected pH value is already approximately known,
• you want to better differentiate within a specific range,
• a quick visual check is sufficient.

Choose a liquid indicator when:

• a specific chemical transition point is to be observed,
• an acid-base titration is performed,
• the color change is part of a demonstration or reaction control.

Choose a pH meter when:

• a numerical measurement value is required,
• results are documented,
• small changes are relevant,
• is measured regularly,
• batches or processes are compared with each other,
• the sample's own color makes visual evaluation difficult.

For many tasks, a combination makes sense: First, the approximate range is determined with test paper. Then, more precise measurement is carried out with a suitably calibrated pH meter.

What matters is not which method is technically most elaborate. What matters is which measurement quality the specific task requires.

At ChemMarkt.de you will find indicator papers, acid-base indicators, buffer solutions and chemical reagents for laboratory, industrial, educational and technical applications.

Discover products for pH determination and choose the right solution for your measuring range.