Instrumentation
be carried out. pH electrodes with the iodine/iodide reference system save this extra work. The differences between conventional and the iodine/iodide
reference systems can be observed by the pH measurement of a DIN buffer 4.01 pH at different temperatures. The buffer alters its pH value from 4.01 pH at 25°C to 4.09 pH at 60°C or 4.00 pH at 5°C. Prior to commencing the experiment, all electrodes are calibrated and at the outset indicate a value of 4.01 pH at 25°C. The electrodes are then immersed in the same buffer but
at different temperatures and the pH signal is recorded. The red line is the nominal value of the buffer at the three different temperatures, the black and blue curves are the pH signal of the electrodes with iodine/iodide reference systems while the green curve shows the measured values of an electrode with a silver/ silver chloride reference system.
sulphide with the traces of silver ions present. This leads to severe interferences that render any dependable measurement impossible. Furthermore, destruction of the reference system can occur. Nowadays, modern silver/silver chloride control systems
actually have encapsulated control elements from which practically no silver ions can escape. But even this ‘homeopathic’ silver content alone is enough to cause black discoloration of the diaphragms with silver sulphide. To illustrate the advantages of pH electrodes with the
iodine/iodide reference system over electrodes with a silver/ silver chloride reference system, the electrodes were exposed to a hot concentrated sulphide solution for several weeks in an endurance test. The silver/silver chloride reference systems were immediately affected and the zero point of the electrodes drifted greatly. However, the actual reference element was not contaminated immediately; rather the sulphide ions reacted directly on the diaphragm causing blockages due to the insoluble silver sulphide. This then led to higher potentials and to severe zero point shifts that equated to measurement errors up to the ultimate failure of the electrode. In contrast, the iodine/iodide reference system proved to
be much more robust with a longer lifetime in the sulphide solutions. Small traces of reactive iodine are actually found in the bridge electrolytes. Iodine is able to react with the sulphide ions to oxidize them. However, only soluble species occur that do not cause interference of the reference voltage. The reliability of the measured result of the pH electrodes with the iodine/iodide reference system is therefore significantly higher because the silver/silver chloride (Ag/AgCl) electrodes change so quickly and need to be recalibrated on an hourly basis and replaced daily.
Fig. 3. Zero point drift in hot sulphide solution.
The average pH deviation of the iodine/iodide electrodes over the entire temperature cycle is less than ± 0.02pH and at the end of the experiment the measured curve overlays the nominal value almost exactly. The measured deviations of the other electrode with a silver/silver chloride reference system are approximately a factor of 2 higher. The adjustment behavior of the conventional system lags more, especially at low temperatures, and shows an overshoot. Such overshoots are very critical in a measurement. The latest pH meters normally undergo an integrated
stability check. If the pH value varies only slightly in a specified time interval, as is the case if an overshoot passes through a maximum/minimum, the measuring instrument ‘thinks’ that the end value has been reached, declares that the measurement is finished and outputs the measured value. As Fig. 2 shows, this value can vary by as little as a hundredth of the pH.
Demanding measuring media Conventional silver/silver chloride reference systems can be contaminated by certain ions, which leads to significant interference potentials and, hence, measurement deviations. Sulphide solutions, for instance, constitute a special problem. The solubility product of silver sulphide is so low that virtually any ion sulphide that penetrates the diaphragm or reference system precipitates immediately as sparingly soluble black silver
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Conclusion pH electrodes with the iodine/iodide reference system offer dependable and rapid measurements even in the most challenging applications. They are the first choice when only metal ion-free reference systems can be used and provide users with the ability to conduct a successful and rapid pH measurement. The electrodes are able to provide rapid response behaviour in reaction to a change in the pH and/or temperature, independent of the sample composition. Precise measurement is guaranteed at different temperatures even if the measurement temperature is not the same as the temperature during the calibration. Matching electrode versions are provided for almost every application. In addition, the bridge electrolyte can be replaced and adapted to the measuring medium while compatibility with measuring media (Tris buffers) is ensured where silver ions in the reference system cause interference. ❒
Dr Michael Lange is with SI Analytics, part of ITT Analytics, MA, USA.
www.ittanalytics.com
References: 1. G Tauber, Referenssystem auf Iod-/Iodid-Basis, Fachvortrag ELACH7, Waldheim 2006; 2. G Tauber, Potentiometrische Messkette, Patentschrift DE 10 2006 012 799 B4; 3. J W Ross, Potentiometric electrode, UK Patent Application GB 2 088 565 A.
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