pH-Meter

A pH meter or pH electrode is a measuring instrument for indicating the pH of a solution. The value is determined by an electrochemical method, and not acid -base indicators.

History

Between 1920 and 1940, the technical basis for the pH measurement were created. For example, in 1940 by Jena glass pH meter based on hydrogen electrode was patented. The first pH meter with glass electrodes were developed in 1935 by Arnold Orville Beckman for measuring the concentration of citric acid in citrus fruits and in the context of the company he founded, the National Technical Laboratories (currently: Beckman Coulter) produced. Other systems such as the quinone - electrode or electrode - antimony are also presented at this time.

The measuring principle used the most commonly used, the potential of a glass electrode, which is also referred to as the pH - electrode. A half-cell reaction at the glass membrane, where it forms an electric potential, which is directly dependent on the H -ion concentration. Of the potential difference to the reference electrode, a voltage substantially linearly maps the pH produced. As the reference electrode is, in most cases, a silver -silver chloride half-cell, which is assembled with the glass electrode, a so-called single-rod measuring chain. The reference electrode is connected via a diaphragm with the solution to be measured, which is usually made ​​of glass sponge, ceramic or platinum sponge. When not in use, the glass electrode is kept in a potassium chloride solution in order to keep the diaphragm potential neutral and conductive.

In the measurement can not be produced a resilient power. Therefore, the meter of an amplifier with very high input resistance and a downstream voltage indicator must be set up. Calibration both of the zero point and the amplification factor (slope) of the circuit must be adjustable. Because of the low capacity of the measuring potential, it is also very easy to disturbances of the measurement, for example, by the so-called streaming potentials. Contamination and leaching of the diaphragm also lead to measurement errors. Furthermore, a stable equilibrium is a measured value, the slower the lower the buffer capacity of the material to be measured.

A recent measurement principle is based on an ion sensitive field effect transistor ( ISFET). The measured values ​​of the ISFET are a little more stable and robust downstream electronics. Therefore, the service life ( durability, number of possible measurement processes ) of the sensor should be lower. The measurement values ​​are in between the glass electrode and the ISFET not particularly good match, since the deviations from linearity and accompanying measurement errors are different.

This illustrates the general problem of pH information. First, the pH value is theoretically defined as the negative logarithm of the hydronium ion concentration. On the other hand, the measurement of so many problems that often the operational definition is used: The pH was that of a glass electrode pH meter with a stationary measurement target of 20 ° C displayed value (plus other specifications of the calibration and measurement process ). On the awareness of the difficulties of the pH determination and the tacit agreement to this operational definition is based a common resistance measurement practitioners against the ISFET measurement.

The oldest technique for pH measurement is based on the pH dependence of the hydrogen potential ( see reversible hydrogen electrode ). Technically, such a pH meter were previously consuming - especially the hydrogen supply requires high technical complexity. This could be remedied in recent years, so that now the advantages

  • No alkali error ( see figure)
  • No acid error
  • Mechanically stable
  • Very low impedance ( input resistance)

With the disadvantages

  • Sensitivity to oxidizing or reducing species
  • Sensitivity to heavy metal salts (Cu, Ag, Hg) more noble than hydrogen must be weighed. However, the disadvantages cited here are theoretical in nature and need to be examined in detail.

Calibration

To obtain accurate results, it is common to calibrate the pH meter at least once every working day prior to use. This is done using standard solutions of known pH. Usually at least two, preferably three standard solutions are used at pH values ​​of 4, 7 and 10. These standard solutions (buffer solutions ) can be purchased ready for use generally in the Laborfachhandel and not recognized itself. The indicated by the pH meter reading is adjusted, if necessary, accordingly, ie adapted to the exact pH value of the standard solution.

Areas of application

PH meter can now find a wide range of applications, especially in industry, medicine, hunting and swimming pools. From today's everyday life, the pH meter are therefore not indispensable, above all, to ensure our safety in the application of cosmetics products like shampoo, detergent or regular pool visits. Too aggressive alkalis or acids could harm the people and therefore must constantly be scrutinized.

Electrochemistry Silver chloride electrode
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