Optimising electrode life without compromising measurement accuracy has become an important focus in most plants, considering the limited life span of most electrodes. The replacement of pH electrodes can account for a large chunk of a plant's maintenance cost and if the wrong electrode is used in the application, even higher maintenance costs can result. As a result, a lot of emphasis is now being placed on selecting the right type of electrode for an application, offering the best performance at the most affordable price.
Anatomy of a pH probe
A pH electrode consists of a measuring electrode, reference electrode and temperature sensor. The measuring electrode is made up of a glass bulb that is sensitive to hydrogen ion activity. Some electrodes even have a silicon dioxide chip in the place of the glass bulb to detect the ion activity. Essentially, the measuring electrode acts as the detector of the hydrogen ion's activity. The internals of the measuring electrode consist of platinum and silver wire, plasma-welded together, immersed in a pH 7 solution. This constitutes a half-cell.
In a conventional glass electrode, the glass bulb separates the electrode internals from the external process. There is no direct contact between the two sides. On the other half-cell - the reference electrode - the picture is quite different. The internal parts of the electrodes consist of a potassium chloride solution (usually 3 mol in solution strength) and a silver wire partially coated with silver chloride. A diaphragm of specific material separates the electrode internals and the external process. To ensure a good electrical connection, these diaphragms are permanently moist, but there is a danger that the external process can enter through the diaphragm and contaminate the reference electrode.
Diaphragm technologies
The most common diaphragm material is Teflon, which is an amorphous substance that can be manufactured with a very specific porous composition. Its main advantages are that a good electrical connection can be made and that there is no precipitation on the diaphragm surface. The main disadvantage, however, is that since the Teflon is hydrophobic (water repelling), solutions that contain oils and waxes will quickly stick to the diaphragm surface, effectively sealing the diaphragm. Further, with its large porous structure, 'memory effects' are not uncommon when moving from a low pH solution (pH<1) to a high pH solution (pH>13). It could take several hours for the diaphragm to release the retained ions and yield an accurate result. The bulk of industrial electrodes are Teflon diaphragm electrodes and they are used in virtually all industries.
A common alternative to Teflon is a ceramic diaphragm. The technology of ceramic diaphragms has advanced so far that several European research institutes focus purely on ceramic diaphragm composition. As with Teflon, there are several advantages and disadvantages. A sure advantage is the fast response given by ceramic diaphragms. They tend to be much smaller than their Teflon counterparts; hence the retained ions are much less. Ceramic diaphragms can be precision manufactured to ensure a specific flow rate through the diaphragm, which is very useful on liquid filled electrodes. A disadvantage of the ceramic diaphragm is that, due to its fixed crystalline structure, it provides a secure footing for crystalline precipitations, such as lime or calcium carbonate. To counter these precipitations, regular automatic cleaning is recommended. Ceramic diaphragms are ideal for applications with extreme pH swings, heavy poisoning environments - such as sulphides and cyanides - and similar heavy-duty measurements.
A last common alternative to Teflon and ceramic diaphragms is the option of no diaphragm, or open system. These give the fastest response of all the systems, purely because there is no barrier between the process and the reference system. The advantage of these fast responses is offset by the greatly increased weakness of quick reference degradation. Typical applications for the open junction system are emulsions and heavy precipitation applications.
Endress+Hauser's offering
In the Endress+Hauser product offering, all of the above diaphragm types are represented. The CPS11 and CPF81 electrodes are fitted with Teflon diaphragms, the CPS41 and CPS71 electrodes are fitted with ceramic diaphragms and the CPS91 is an open junction type electrode. The non-glass TopHit H electrodes are also fitted with ceramic diaphragms.
The rest of the reference electrode construction is made up from four different types of 2 mol potassium chloride reference systems. The most common type is the Polytex gel system found in the CPS11 and CPF81 electrodes.
The CPF81 electrode has the added feature of a primary containment area filled with potassium nitrate separated by a secondary Teflon diaphragm. This provides optimum protection for the gel reference system in heavy poisoning environments such as sulphides and cyanides.
The second type of reference is the liquid junction found on the CPS41. A liquid 3-mol potassium chloride solution fills the reference and it is continually replenished from an outside reservoir. Due to the precision construction of the diaphragm, it is rated at a flow rate of 5 ml/day at a 0,1 bar overpressure. Hence the 150 ml reservoir should only require filling once a month.
The third type of reference solution is a hybrid type gel. This is the reference solution for the CPS71. It is loosely based on the Polytex gel found in the CPS11. Through some extensive research and development, gels with different viscosities could be brought into contact with each other and still provide accurate potential transfer. The result was a double junction electrode without a secondary diaphragm. Contact with the medium through the diaphragm is limited to the less viscous primary gel. The contaminants that enter the diaphragm are free to move within this gel. The actual reference wire is encapsulated in a highly viscous gel that prevents contaminants from interfering with the measurement. A chemical law called 'Brownian Movement' also states that ion movement will be restricted to the area of lowest viscosity. Hence the reference wire stays completely protected. The last type of gel is almost solid. This is found on the CPS91 electrode. Due to its open junction construction, the gel needs to be almost solid to prevent it from falling out of the open junction. Also a Polytex derivative, it has excellent performance in soiling applications where direct contact to the medium is the only alternative for accurate measurement.
For more information contact Grant Joyce, Endress+Hauser, 011 262 8000, [email protected]
Endress+Hauser has multiple international patents for its diaphragms and reference gels. Several advanced chemical institutions assist Endress+Hauser to develop know-how in the chemical field of pH reference systems. The company's dedication to accurate and reliable measurement has helped Endress+Hauser secure the number 1 position in pH electrode manufacturing for five consecutive years.
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