pH Meter / pH Tester

The pH value on a pH meter’s display describes the character of an aqueous solution with respect to an acidic or basic alignment. The latter, either acidic or basic, is dependent on hydrogen ion activity. The pH is an important factor in our ecosystem. Here are some examples:

In the plant world, the pH of the soil has an impact on the intake of nutrients by the roots and therefore on the overall plant growth.

The human blood usually has a pH between 7.35 and 7.45. Should the pH of the blood drop, the hemoglobin will absorb less oxygen.

In the area of the underwater world some fish species need a rather acidic and the others – a rather alkaline environment.

In the field of chemistry, the pH value is an indicator of the strength of an acid or base.

Various methods or a measuring device / ph meter is used to determine the pH value.

PH-photometers also use the effect of the color change. One water sample should be taken for the measurement, and then measured by the pH meter in two steps. During the measurement step 1, a blank test is carried out (only the sample taken). Before the second measurement step, a reagent is added to the sample which causes a color change of the sample. Here the color change is measured with an optical measuring system and converted directly into the pH value. The maintenance expenses for a pH meter based on photometric procedure are relatively low, but the reagents are required for each measurement.

A fast and simple measurement is carried out with a pH meter, which has got an electrode. These measuring devices are available as a table or hand-held measuring device, as well as a stationary measuring unit. Measurement with one electrode is the most commonly used measuring method for the pH value determination.

For applications in laboratories, tabletop devices with glass electrodes are generally used. These very thin-walled glass electrodes require very careful handling. For this reason, in laboratory operation, glass electrodes are used with an electrode holder, which holds the electrodes and is balanced by the springs. A tabletop pH meter has, in most cases, an internal memory or an interface via which the recorded or currently measured data can be transferred to the PC. As for power supply, in addition to the pH meter devices with normal mains operation, there are some pH meters models available that also provide operation via a battery or a battery pack. This kind of pH meter is very flexible and can also be used for on-site measurements.

Stationary pH measuring systems are used where the pH value must be measured or adjusted constantly. This pH meter is mostly used as transmitters and send the measured values as analogue signals further. The analog signals, e.g. 4 ... 20 mA or 0 ... 10 V, are processed in electronic controls and possibly passed on to displays, dosing devices or pumps and valves.

The largest proportion of the pH measuring devices on the market are the mobile handheld instruments.

A mobile pH meter is offered in many different versions and with different equipment. The pH measuring devices, which form a solid unit with the electrode, are generally referred to as pH testers. These pH testers are the most cost-effective devices and are used in applications where a few measurements are required. Since all the pH electrodes are subject to drift and therefore also to wear, it should be considered when purchasing a pH meter that the electrode can be replaced. Mobile pH measuring instruments with a replaceable/ removeable electrode can be used in almost all areas, due to the possibility to rapidly change the electrode to a special one. A mobile pH meter often has measured value memories, which can record the current value or if the logger function is available, the whole measurement series. The measured value memories are designed as an internal memory or as a SD card recorder. From these pH-meters, the values can always be transferred to a PC and processed there. For the application in humid environments, the pH tester and handheld device should be equipped with an appropriate IP protection.

The selection of the pH electrode should always be based on the application. Thus, in the most common cases, for the mobile pH meter, the electrodes with a plastic housing and a gel filling is selected. These electrodes are mechanically relatively stable and are not directly destroyed when the container walls and edges are hit. For the area of foodstuffs, the electrodes with a conical tip or electrodes embedded in a stainless-steel blade are available. To avoid shortening the lifetime of the electrodes, they should always be cleaned after use. Depending on the application, special cleaning solutions are required. In case of applications in clean waters or comparable media, cleaning with tap water and rinsing with distilled water is sufficient. The storage of the pH electrodes should always be carried out in a storage solution for pH electrodes. In order to compensate for the above-mentioned drift of the pH electrodes, a calibration of the pH meter with the electrode is required from time to time. For this purpose, for example, calibration solutions with pH values 4, 7 or 10 can be used. If pH measurements are carried out in media at the temperatures above 25 ° C, pH meters with automatic temperature compensation (ATC) are recommended. For the pH measuring instruments with ATC, the measured value shift caused by the temperature is voided / removed.

DIN Guideline for pH value measurement in hard soil

The soil pH is a distinctive mark of its acid or base content. The pH of a soil, as in all the other aqueous systems, is affected by natural buffer systems. The determination takes place either in a suspension (extraction procedure) of soil in a neutral salt solution (0.01 N calcium chloride or potassium chloride) - lab measurement method. Here, since the absorbed H + ions on the ground exchanger are replaced by the CaCl₂-ions, the pH measured in such a way is about 0.3-1.0 units below the pH, which is measured in purely aqueous suspensions. However, not all the data in the literature are corrected accordingly.

Practical example: pH measurement in soil with a soil extraction solution - Laboratory measurement method according to DIN 19684 Part 1

Range: between pH 2 to pH 9
pHMeter: PCE-228
pH electrodes: pH electrode with spherical membrane, refillable or gel-filled, fabric or ceramic diaphragm
Required equipment: 1 electrode stand, 1 large-volume beaker (> 500 ml), 1 magnetic or glass rod for stirring

Carrying out a pH measurement - experimental method:

Make the soil mix sample (air-dry clay) taken from various representative sampling points. Remove foreign bodies (inter alia stones, roots). Slightly loosen the soil with hands.

Suspension with distilled water as the solvent

This method detects the normal acidity of the soil. The existing bound acid ions present in the soil matrix are not detected. Put 260 ml of distilled water to 100 g of a soil mix sample and stir / shake this suspension for 5 minutes. After this, the sample is left still for minimum 2 hours, but not longer than 24 hours.

Suspension with 0.01 M CaCl₂ solution

Take 75 ml of soil and mix it with 300 ml of 0.01 M CaCl₂ solution (0.01 M = 1.11 g of anhydrous calcium chloride or 1.47 g CaCl₂ * 2 H₂O for 1 liter distilled water), stir very intensely and let it stand without stirring for at least 60 minutes. Stir the sample shortly before you use the pH meter the measurement and immerse the pH electrode and temperature into the fluid to be measured.

The suspension with 0.01 M CaCl₂ solution the easily replaceable hydrogen ions are also incidentally detected, as these are exchanged from the bound sample matrix by the cations. This determination method is very close to the longer-term process in the soil. That is why, the pH of this suspension is 0.5 to 1.0 pH units lower than the pH of the aqueous soil suspensions.

Take a record of the measurement value with 0.01 pH accuracy when the measured value is stable.

Measured value indication in suspension:
- With distilled water: pH (distilled water) x.xx (y °C) pH (distilled water) 7.22 (23 °C)
- With CaCl₂: pH (CaCl₂) x.xx (y ° C) pH (CaCl₂) 7.22 (23 °C)

Please note that during the use of a pH meter any pH measurement the temperature constancy between the liquid under measurement and the electrode can take place. Finally, after the measurement clean the pH electrode of contaminants by rinsing with distilled water. pH sensor should not be affected mechanically. Then keep the electrode in (STORAGE-Solution).

pH Measurement in solvent besides than water

Application of a pH meter in Non-Aquaeous Solvents

We recommend to consider the following when using a pH tester or test kit:

- The results will be most accurate if you keep the sensor of the main electrode immersed in the liquid to be tested for 24 hours prior to the pH measurement.

- It is very important to use the following three electrodes: the main (glass) electrode of your pH meter, a reference electrode and an electrode for temperature compensation.

- Allow 5 minutes for stabilisation of the value before you read it out from the display of your pH tester.

- Change the unit of measurement of your pH meter if the pH value is below pH 0 or above pH 14 and repeat the measurement.

- When you have taken your measurement, immerse the sensor of the pH meter into hydro-chloric acid (0.1 mol/L HCl) for approx. 10 minutes before washing the sensor as well as the external cylinder of the reference electrode carefully with pure water.

One of the most important facts to know about pH meters is that pH values of non-aquaeous solutions, in contrast to those of aquaeous solutions, are more or less only an indication of the hydrogen ion activity in the sample.

The potassium chloride (KCl) that leaks from the liquid junction of the pH tester’s reference electrode containing the KCl solution does not normally dissolve in most non-aqueous solutions, which means that the liquid junction will be blocked. Therefore, lithium acetate (CH3COOLi) is used in some electrodes of a pH tester as this will dissolve in organic solutions.

pH Meter with ISFET Electrode

There is a wide range of the pH meter models in the market, starting from portable small inexpensive testers to complicated and pricy laboratory versions. The most well-known design of the pH meter is a device with the glass electrode. Not depending on the model, it is important to note that this kind of pH meter requires very careful handling and regular calibration. Glass is very fragile and once the electrode is broken or cracked there is no chance for getting accurate pH value measurement. The electrodes should be cleaned regularly, kept in a special solution under favorable ambient conditions (no high temperatures and humidity!), free from contaminants and fat (that is why it is recommended never to touch the device with the hands).

One of the options to the glass electrode, which as mentioned above, really needs an operator to be very concentrated, another type of electrode – based on ISFET technology – has been created. The main advantage of a pH meter equipped with an ISFET (on-sensitive field-effect transistor) electrode is that it is robust enough and has a short response time, at least not more that the glass electrode and can carry out measurements under low temperatures without being damaged. The ISFET pH meter finds its application especially in industrial field, where the operating conditions are quite rough sometimes and a certain resistance of the meters is absolutely advantageous. As against to the majority of pH meters, there is no glass element in the construction. Such a pH meter is widely applied in food / beverages and pharmaceutical industry, various branches where high hygienic requirement should be met. It ensures the safety of the products under testing, not only due to its resistance to contamination and compliance with all the hygienic norms, but also the risk of shards of glass getting into the material in case of the breakage of the glass electrode is brought to “0”. A pH meter with an ISFET electrode is resistant to shocks, provides very good, stable and accurate measurement, even under low temperatures, the pH meter is less sensitive to storage conditions and its electrode being out of solution does not influence the operation. The ISFET electrode can be sterilized, calibration intervals are longer, as well as the service life time.