Glass electrode
The glass electrode is the most common variant of a pH electrode and is used for pH measurement , i.e. H. to determine the hydrogen ion activity.
On the one hand, it is more suitable in daily laboratory practice than the hydrogen electrode ; on the other hand, all standardized processes are based on it. The potential established at a glass electrode depends on the hydrogen ion activity and follows the Nernst equation in an ideal system . In laboratory practice, however, there is no such system, so that the glass electrode has to be calibrated with buffer solutions whose pH values are known in order to establish a relationship between the measured voltage and the pH value of the sample solution. In order to minimize the measurement error, the expected pH value of the sample solution should be between the pH values of the buffer solutions used.
In principle, two electrodes are required for pH measurement, the reference electrode and the glass electrode itself. Mostly, however, a design is chosen in which both electrodes are combined in a so-called combination electrode . Since the temperature is an important parameter for measuring the pH value, a temperature sensor is often integrated into the combination electrode . Corresponding electrodes are often referred to as 3-in-1 electrodes.
The combined glass electrode
This combination electrode is made up of an inner tube and an outer jacket. The outer jacket serves as a reference electrode ( silver-silver chloride electrode ) and consists of a silver wire , silver chloride and an electrolyte solution (usually potassium chloride ). The inner tube also contains silver wire, silver chloride and potassium chloride solution, which also contains a buffer ( phosphate buffer ). The inner tube is connected to the glass membrane with the solution to be measured, the outer jacket in turn with a diaphragm. This gives the electrochemical series:
Ag | AgCl | KCl solution || Glass membrane || Measurement solution || Diaphragm || KCl solution | AgCl | Ag
In simplified terms, the potential of the glass electrode arises as follows:
The reference electrode is in electrical contact with the measurement solution via a diaphragm (usually platinum sponge or porous ceramic), but the diaphragm largely prevents material exchange with the solution in order not to change the potential of the reference electrode by foreign ions. The measuring electrode is located in the measuring rod in a buffered potassium chloride solution adjusted to pH 7. This is through a very thin glass membrane (≈ 50 µm) in conductive connection with the measurement solution, at which the potential used for pH measurement is created. The sodium and lithium ions in the glass membrane can move relatively freely, but the membrane is impermeable to hydrogen ions . Nevertheless, the hydrogen ions can occupy lattice positions on the oxygen anions of the supercooled silicate melt (see chemical structure of glass ), as this begins to swell on contact with the aqueous solution on the surface. A low pH value means that the hydrogen ions populate the lattice sites and the sodium and lithium ions “push back” into the membrane. Since these can move freely in the membrane, they tend to be shifted to the inside of the membrane, creating the measured potential difference. At a high pH value, the hydrogen ion concentration in the interior of the dipstick predominates, the process described takes place in a different direction, the potential arises with a different sign.
Components of the glass electrode
Glass electrodes consist of three main components: the glass membrane, the inner buffer and the measuring electrode. While the inner buffer and the measuring electrode can be used universally, the shape and properties of the glass membrane must be selected according to the respective sample type. Important criteria are the consistency, the volume and the temperature of the sample, which measuring range is expected and the concentration of the ions in the solution to be measured.
As standard, glass membranes have a cylindrical shape, but this is not suitable for all samples. For example, because of their resistance to contraction, spherical glass membranes are preferable for samples with low temperatures, while a hemispherical shape is more suitable for small volumes. In contrast, puncture electrodes have more robust needle membranes and electrodes for surface measurements using flat membranes.
In addition, the composition of the respective membrane glass should also be taken into account when selecting the electrode, especially if strongly alkaline solutions are to be measured.
Use and areas of application
The electrode must never be stored dry or in distilled water for a long time, but should be stored in a 3 molar aqueous KCl solution. Areas of application are:
- Measurement of the pH value in the laboratory
- Online measurement of the pH value in industrial processes, e.g. B. in chemistry, biotechnology, water and wastewater treatment, food and beverage industry etc.
- Carrying out acid-base titrations
- pH value determination in aquariums
See also
Web links
literature
- Ludwig Kratz : The glass electrode and its applications (= Scientific research reports. Natural science series. Vol. 59, ISSN 0084-0920 ). Steinkopff, Frankfurt am Main 1950.
- Daniel C. Harris: Textbook of Quantitative Analysis . 8th edition. Springer Spectrum, Berlin / Heidelberg 2014, ISBN 978-3-642-37787-7 , Chapter 14: Electrodes and Potentiometry , doi : 10.1007 / 978-3-642-37788-4_15 .
- Ralf Degener: pH measurement . Wiley-VCH, Weinheim 2009, ISBN 978-3-527-32359-3 , Chapter 2: Measuring devices .
- Mettler-Toledo (Ed.): Instructions for measuring pH: Theory and practice of pH applications in the laboratory . MCG MarCom, Greifensee 2007 ( full text [PDF; 876 kB ]).