The Measurement Electrode
The purpose of the measurement electrode is to determine the pH value of an aqueous solution.
Since 1897, the original platinum/hydrogen electrode was used to measure the hydrogen ion concentration in aqueous solutions. Today still serves as a reference standard for the electrometric pH determination. The hydrogen electrode consists of a platinized platinum plate or rod (coated with platinum black), subjected to a flow of gaseous hydrogen. A silver wire coated with silver chloride serves as a reference electrode.
The basic theory, when employing a hydrogen electrode, is as follows: If a metal rod (electrode) is immersed into an aqueous solution containing its own salt (silver electrode in silver nitrate), the atoms on the surface of that metal rod will ionize. The water molecules will attract the positively charged metal ions from the surface of the rod, which leaves the metal rod negatively charged. This charge exchange develops a potential difference at the phase boundary metal/solution. The potential depends on the ion concentration in the solution and is known as the galvanic potential.
Today the hydrogen electrode still serves as a reference standard especially as its measuring results are extremely accurate. However for practical reasons the hydrogen electrode has lost its importance because of its difficult and complicated handling.
Only the antimony pH electrode has survived out of various metal electrodes. Antimony is chemically resistant to hydrofluoric acid and can therefore be used for the pH measurement in solutions containing hydrofluoric acid. Hydrofluoric acid is well known to etch borosilicate glass thus antimony is often the only solution for certain applications such as those found in the semiconductor industry. Antimony pH electrodes have their own limitations. The measurement range is often limited compared to glass membrane sensors. Also, the accuracy and reproducibility of the measurement may have much larger tolerances. Finally, antimony requires special handling by humans as it is a potential carcinogenic material.
The Glass Membrane Electrode
It was not until the development of the glass membrane electrode that pH measurement became a simple and a reliable tool for all kinds of applications. In recent years the glass electrode has outgrown all other methods for pH measurements. The pH determination of an aqueous solution is today as common in industrial applications as temperature and pressure measurements, thanks to the reliability and accuracy of the glass electrode in combination with extreme stable electronic amplification. However, the successful application of the glass electrode requires some knowledge about its functionality and its maintenance, which this article will provide.
A glass membrane electrode consists of a shaft made from glass which should be highly resistant to hot alkaline solutions and its electrical resistance must be several times greater than that of the membrane glass. The pH sensitive part of the glass electrode is a hemispherically shaped electrode tip, the glass membrane. The membrane is made from special hydrogen ion sensitive glass and is fused to the electrode shaft. The glass electrode is partly filled with a buffer solution, normally having a pH value of 7.
A defined amount of potassium chloride (KCl) is added to this internal buffer. A silver wire, coated with silver chloride (Ag-AgCl) is inserted into the glass electrode right down into the internal buffer and serves as a conducting electrode. Via the core of the coaxial pH cable, the Ag-AgCl wire is connected to one terminal of a pH meter.
The pH Sensitive Glass Membrane
All types of glass possess the property of producing a potential difference relative to the hydrogen ion concentration in aqueous solutions. However only special types, such as the conventional Mc-Innes glass (Corning 015) produce galvanic potentials which satisfy the Nernst equation over a wide range of the pH scale.
Every manufacturer of pH electrodes is constantly researching for better pH sensitive glass. Through constant development Hamilton has achieved results which have not previously been available without unsatisfactory compromises. A general overview of Hamilton pH glass membrane types is listed in a separate article.
When the membrane glass of a measurement electrode comes into contact with an aqueous solution, it forms a thin gel layer of approximately 10-4 mm thickness between the glass surface and the solution. The thickness of the gel layer depends on the quality and composition of the membrane glass, as well as the temperature and the pH value of the measured solution. As the internal side of the glass membrane is in contact with the inner buffer (an aqueous solution of pH 7) a gel layer is also formed on the inside of the glass membrane.
A continuous exchange of H+ ions in the gel layers and H+ ions of the solutions takes place on both sides of the membrane. This ion exchange is controlled by the H+ ion concentration of both solutions.
If the hydrogen ion concentration of each solution is identical on both sides of the glass membrane, the ion exchange stops after an equilibrium has been reached between the H+ ions in the solutions and the H+ ions in the gel layers. Therefore, both sides of the membrane glass have the same potential and the potential difference is 0 mV.
If a difference of a hydrogen ion concentration exists between the inner buffer and the outer solution, a potential difference develops between the inner and outer sides of the membrane glass which is proportional to the difference in pH between the inner buffer and the outer solution. To be able to measure the membrane potential, the membrane itself has to be conductive. This is achieved by the mobility of the alkaline ions in the membrane glass (Li+ ions in most glasses today or Na+ ions in older membrane glasses).
The thickness and composition of the gel layer determine the response time and the characteristic slope of the glass electrode Therefore the gel layer is of critical importance to the electrode performance. Without the gel layer there can be no pH measurement. Unfortunately it takes approximately one to two days until a gel layer is fully developed. Therefore a measuring electrode needs to be hydrated (immersed into normal clean tap water) for at least 24 hours prior to use. Most manufacturers deliver their electrodes already hydrated (the membrane is kept wet with a KCl Solution in a plastic cap) which renders the electrode ready for immediate use.
Prior Article - pH Sensor Design
Next Article - The Reference Electrode
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