pH Meter

What does the pH measure?

The pH value of a substance is directly related to the ratio of the hydrogen ion [H+] and the hydroxyl ion [OH-] concentrations.

The quantitative information provided by the pH tester expresses the degree of the activity of an acid or base in terms of hydrogen ion activity.

• If the H+ concentration is greater than OH-, the material is acidic; i.e., the pH measurement is less than 7.
• If the OH- concentration is greater than H+, the material is basic, with a pH value greater than 7.
• If equal amounts of H+ and OH- ions are present, the material is neutral, with a pH of 7.

Acids and bases have free hydrogen and hydroxyl ions, respectively. The relationship between hydrogen ions and hydroxyl ions in a given solution is constant for a given set of conditions, either one can be determined by knowing the other.

How to measure pH?

A rough indication of pH can be obtained using pH papers or indicators, which change color as the pH level varies. These indicators have limitations on their accuracy, and can be difficult to interpret correctly in colored or murky samples. More accurate pH measurements are obtained with a digital pH meter. A pH tester system consists of three parts: a pH probe, a reference pH electrode, and a high input impedance meter.

The pH electrode can be thought of as a battery, with a voltage that varies with the pH of the measured solution. The pH probe is a hydrogen ion sensitive glass bulb, with a millivolt output that varies with the changes in the relative hydrogen ion concentration inside and outside of the bulb.

The reference electrode output does not vary with the activity of the hydrogen ion.

The pH electrode has very high internal resistance, making the voltage change with pH difficult to measure. The input impedance of the pH meter and leakage resistances are therefore important factors.

The pH meter is basically a high impedance amplifier that accurately measures the minute electrode voltages and displays the results directly in pH units on either an analog or digital display.

In some cases, voltages can also be read for special applications or for use with ion-selective or Oxidation-Reduction Potential (ORP) electrodes.

pH Electrodes

pH electrodes are constructed from a special composition glass which senses the hydrogen ion concentration. This glass is typically composed of alkali metal ions. The alkali metal ions of the glass and the hydrogen ions in solution undergo an ion exchange reaction, generating a potential difference.

In a combination pH probe, the most widely used variety, there are actually two electrodes in one body. One portion is called the measuring pH electrode, the other the reference electrode. The potential generated at the junction site of the measuring portion is due to the free hydrogen ions present in solution.

The potential of the reference portion is produced by the internal element in contact with the reference fill solution. This potential is always constant. In summary, the measuring pH electrode delivers a varying voltage and the reference electrode delivers a constant voltage to the meter.

The voltage signal produced by the pH probe is a very small, high impedance signal. The input impedance requires that it be interfaced only with equipment with high impedance circuits.

pH electrodes are available in a variety of styles for both laboratory and industrial applications. All are composed of glass and are therefore subject to breakage.

Electrodes are designed to measure mostly aqueous media. They are not designed to be used in solvents, such as CCI4, which does not have any free hydrogen ions.

The pH electrode, due to the nature of its construction, needs to be kept moist at all times. In order to operate properly, glass needs to be hydrated.

Hydration is required for the ion exchange process to occur. If an pH sensor should become dry, it is best to place it in some tap water for a half hour to condition the glass.

pH electrodes have junctions which allow the internal fill solution of the measuring electrode to leak out into the solution being measured. This junction can become clogged by particulates in the solution and can also facilitate poisoning by metal ions present in the solution.

If a clogged junction is suspected it is best to soak the sensor in some warm tap water to dissolve the material and clear the junction. pH testers should always be stored in a moistened condition. When not in use it is best to store the electrode in either buffer 4.0 or buffer 7.0. Even if an electrode is not used it still ages. On the shelf, the pH probe should last approximately a year if kept in a moistened condition.

Electrode demise can usually be characterized by a sluggish response, erratic readings or a reading which will not change. When this occurs an electrode can no longer be calibrated. pH electrodes are fragile and have a limited lifespan. How long an electrode will last is determined by how well the probe is maintained and the pH application.

The harsher the system, the shorter the lifespan. For this reason it is always a good idea to have a back-up pH sensor on hand to avoid any system down time.

Calibration is also an important part of electrode maintenance. This assures not only that the electrode is behaving properly but that the system is operating correctly.

pH meter calibration

pH electrodes are like batteries; they run down with time and use. As an electrode ages, its glass changes resistance. This resistance change alters the electrode potential. For this reason, electrodes need to be calibrated on a regular basis.

Calibration in pH buffer solution corrects for this change. Calibration of any pH equipment should always begin with buffer 7.0 as this is the "zero point." The pH scale has an equivalent mV scale. The mV scale ranges from +420 to -420 mV. At a pH of 7.0 the mV value is 0. Each pH change corresponds to a change of ±60 mV. As pH values become more acidic the mV values become greater.

For example, a pH of 4.0 corresponds to a value of 180 mV. As pH values become more basic the mV values become more negative; pH=9 corresponds to -120 mV. Dual pH calibration using buffers 4.0 or 10.0 provides greater system accuracy.

Here is a general method for most digital pH meters. Some pH testers require slightly different techniques. Please read the instructions for their particular procedures.

1. The temperature setting on the meter must correspond to the temperature of the buffers used, or an automatic temperature compensator must be employed.
2. Turn pH meter to “pH” or “ATC” if automatic temperature compensation is used.
3. Place clean electrode into fresh, room temperature pH 7.00 buffer.
4. Adjust the pH reading to exactly 7.00 using the ZERO OFFSET,STANDARDIZED or SET knob.
5. Rinse the electrode with distilled or deionized water. (This would be the procedure for a one-point calibration. Continue through step 8 for a two-point calibration.)
6. Place electrode into the second buffer, either pH 4.00 or pH 10.00.
7. Adjust the pH reading to display the correct value using the SLOPE, CALIBRATE, or GAIN controls (coarse adjust).
8. Adjust the pH reading to read the correct value using the SLOPE knob (fine adjust).

Buffer Solutions

Buffers are solutions that have constant pH values and the ability to resist changes in that pH level. They are used to calibrate pH measurement systems (electrode and meter). There can be small differences between the output of one electrode and another, as well as changes in the output of electrodes over time. Therefore, the system must be periodically calibrated. Buffers are available with a wide range of pH values, and they come in both premixed liquid form or as convenient dry powder capsules. Most pH testers require calibration at several specific pH values. One calibration is usually performed near the isopotential point (the signal produced by an electrode at pH 7 is 0 mV at 25°C), and a second is typically performed at either pH 4 or pH 10. It is best to select a buffer as close as possible to the actual pH value of the sample to be measured.