pH combination electrode
The combination of a working electrode and reference electrode for measuring the pH value in one design is called a pH combination electrode .
Typical representatives are many types of pH glass electrode , which allow particularly uncomplicated pH value measurement in solutions. A silver-silver chloride electrode is usually used as the reference electrode . The electrical circuit is closed by a diaphragm that is soaked in 3 M potassium chloride solution, which also forms the internal electrolyte of the measuring chain. Potassium chloride is the only electrolyte that has the property that its cations (K + ) and anions (Cl - ) have practically the same ion mobility . Therefore, no additional potentials develop on the diaphragm with these electrolytes, which could falsify the measurement.
As a further functional compression, many pH combination electrodes also contain an integrated temperature sensor. Today, micro-versions of combination electrodes only have a diameter of approx. 3 mm.
Today, the redox measuring chains and many different ion-sensitive electrodes are usually also designed as single - rod measuring chains .
The pH combination electrode and how it works
The pH-sensitive element is the glass membrane made of silicate glass - usually a dome at the lower end of the pH combination electrode. The surface of the membrane is negatively charged. A low pH value results in high hydrogen ion activity (H + ). In this case, there is a greater potential at the glass membrane. The potential arising on the glass membrane is passed on to the plug contact via the internal buffer and the internal conductor.
The reference system begins with the diaphragm, which creates the electrical connection between the measuring solution and the reference electrolyte. The reference electrolyte is a saturated potassium chloride solution. The pH-independent comparison potential is fed to the plug contact via the reference recording system.
An ideal combination electrode has a zero point of pH 7 (at this pH value it has an output signal of 0 mV). The slope depends on the temperature; at 25 ° C it is −59 mV / pH. The electrodes must be connected to a transmitter , which carries out temperature compensation (either based on the measured medium temperature or on the basis of a fixed temperature value).
Versions of pH combination electrodes
Design and electrical connection
pH combination electrodes are standardized in terms of design; they usually have an installation length of 120 mm. Screwing into a fitting is possible via a screw-in thread PG13.5 . Electrodes without a temperature sensor have a two-pole electrode connection. Electrodes with temperature sensors (for temperature compensation of the pH measurement) are available with a six-pin Variopin connection , for example . Electrodes with a fixed cable are also used for laboratory applications.
Diaphragm
The diaphragm creates the electrical connection between the measuring solution and the reference electrolyte. An open and large-area diaphragm creates a reliable connection. On the other hand, in this case the reference electrolyte salt out relatively quickly as a saturated potassium chloride solution.
Nowadays, a point-shaped ceramic diaphragm made of zirconium dioxide is standard for high-quality industrial electrodes for generally aqueous media . The diaphragm is very suitable for clean to slightly contaminated measuring solutions. A long service life is achieved due to the relatively slow salting out . The diaphragm is not suitable for heavily soiled media because the soiling blocks the diaphragm.
The PTFE ring diaphragm is used for media with heavy contamination . Successful measurements are made possible by the large diaphragm made of dirt-repellent PTFE. The salting out of the reference electrolyte generally happens faster in electrodes with this diaphragm than in electrodes with a ceramic diaphragm.
Open diaphragms are used for very heavy soiling with dirt loads or even media such as paints and pastes. The open diaphragm has a hole in the shaft wall (similar to the ceramic diaphragm, but without a ceramic cartridge). Furthermore, electrodes with an annular gap diaphragm have an open ring between the outer shaft and the pH electrode. Electrodes with these open diaphragms enable measurements in media with extreme contamination.
In the order listed, the diaphragms are always more tolerant of contamination in the measuring solution (ceramic, PTFE, perforated and annular gap diaphragm). On the other hand, the electrolyte consumption increases and the period of use of the electrodes is shortened. PTFE or the open diaphragms should only be used if the application so requires.
Salt template
In order to keep the reference electrolyte saturated as long as possible, salt rings can be placed in the reference system. If the electrolyte is no longer saturated, part of the so-called salt reserve goes into solution. As long as there are salt rings in the reference system, the electrolyte is saturated.
Electrodes with a double chamber system
Metal ions in electroplating (such as copper and nickel ions ) get into the reference electrolyte through the diaphragm. When the reference cartridge is reached, the reference system is poisoned. The separation of the reference system into two chambers, which are separated by a diaphragm (double chamber), extends the diffusion time and thus the service life of the electrodes.
Electrodes with electrolyte refill
With these electrodes, the reference system is extended to a storage vessel with a hose. The storage vessel is placed approx. 50 cm above the electrode. The resulting hydrostatic pressure results in a continuous flow of electrolyte of a few milliliters per day through the diaphragm. These electrodes can offer a solution, for example, if the diaphragm is blocked by an oily measuring medium or, in other applications, a measuring substance enters the reference system and poisons it.
Wiring and fittings
The high resistance of the glass membrane results in a very high impedance in the measuring circuit. In addition to the required high input impedance of the transmitter, the design of the cable must take this into account. Coaxial cables with additional insulation are used. The inner core carries the potential of the glass membrane, the potential of the reference system is available via the copper braid.
Due to the high resistance of the measuring circuit, the connecting lines between the sensor and the transmitter should not be longer than approx. 20 m in practice. If larger distances have to be bridged, impedance converters or two-wire measuring transducers can be screwed between the electrodes and the measuring transducers. These reduce the measuring circuit resistance from the point of view of the measuring transducer (impedance converter) or provide an output signal that changes proportionally to the pH value (two-wire measuring transducer).
Due to the standardized design of the combination electrodes, a large number of fittings are available for them. These enable the sensors to be attached via the PG13.5 thread and protect them from mechanical influences. Several electrodes can also be placed in various fittings. For example, they enable redundant measurements or use together with redox potential combination electrodes and compensation thermometers - these have the same design. Flow fittings are placed directly in the material flow or in a bypass. As a rule, these are used with shut-off cocks, which enables the sensors to be removed during maintenance.
Process fittings made of stainless steel are placed in the process. The pH combination electrodes are screwed into this via the PG13.5 thread.
Manual retractable fittings are used when the process cannot be depressurized to remove the electrode. The sensor can be moved out of the medium thanks to the special mechanics of the retractable fitting. If the sensor is no longer in the medium, the process to the environment is pressure-tight and the sensor can be removed from the fitting.
Immersion fittings enable measurements in open containers or channels. They are used for tank wall mounting or as a hanging fitting.
Calibration and maintenance
As already mentioned, a pH combination electrode ideally has a zero point of pH 7 (at this pH value it emits an output signal of 0 mV). The slope of ideal electrodes is 100% (-59 mV / pH at 25 ° C). A changing zero point and a changing slope are compensated by a transducer to be used. Although it is actually an adjustment, the adjustment is called calibration. The standard procedure is two-point calibration, during which the electrode is placed in two buffer solutions of known pH. The pH values of the buffer solutions are indicated on the transmitter during calibration. When the two-point calibration is completed, the values of the zero point and the slope of the electrode used are displayed. The electrode used should have a zero point of around pH 6 ... 8, the slope should be at least 90% (90% of -59 mV / pH). Combination electrodes with transducers have to be calibrated during commissioning.
Combination electrodes must be kept clean during operation. Mechanical cleaning must be carried out carefully with regard to the glass membrane; the membrane glass must not be scratched under any circumstances. Glass cleaning agents, laboratory detergents (e.g. acetone, alcohol) or weakly acidic solutions (e.g. 1… 3% hydrochloric acid) can be used as auxiliary agents.
After cleaning, the electrodes must be checked with a buffer solution and, if there are deviations, the electrodes must be calibrated.
The storage time of combination electrodes is limited, 6 months can be given as a guide value, at the latest after this storage time they should be used. Upon delivery, the membrane of the pH combination electrodes is located in a wet holding cap filled with KCl. The electrodes must never be stored dry, the membrane glass should always be in a KCl solution.
Individual evidence
- ^ Daniel C. Harris: Textbook of Quantitative Analysis . Ed .: Gerhard Werner, Tobias Werner. 8th, completely revised exp. Edition. Springer, Berlin / Heidelberg 2014, ISBN 978-3-642-37788-4 , pp. 361 .
- ↑ a b Helmut Galster: pH measurement, basics, methods, applications, devices . Ed .: VCH Verlagsgesellschaft. ISBN 3-527-27836-2 , pp. 312 .
- ↑ a b c d e f g h i Öznur Brandt, Ulrich Braun, Matthias Kremer, Reinhard Manns, Jürgen Schleicher: Analytical measurement technology in liquid media: a manual for practitioners . JUMO, Fulda 2009, ISBN 978-3-935742-16-0 ( jumo.de - free full text).
- ↑ Ralf Degner, Stephanus Leibl: Measuring pH - This is how it's done . VCH Verlagsgesellschaft mbH, Weinheim, S. 272 .