Instrumentation
the voltage. This shortcoming has been remedied by the development of an advanced iodine/iodide reference system, which is divided into three parts. The first part is a patented iodine storage reservoir (Figs 1 and 2). By means of a plastic plug permeable to iodine, this storage reservoir is used in conjunction with the middle chamber of the system. The iodine supply continuously releases iodine to the reference electrolytes and keeps the iodine concentration constant. This is important as, in contrast to iodide, iodine diffuses quicker from the middle chamber into the electrolytes of the third chamber that contains no iodine. The actual reference element is immersed in the middle
chamber, it is in contact with the reference electrolyte by means of a ceramic diaphragm. The same reference electrolyte as in the reference element is present in the middle chamber. However, the fluid volume is significantly greater. By stabilising the iodine content in the middle chamber, the concentration ratios in the actual reference element in which the reference voltage is generated remain constant, resulting in the voltage remaining stable for a prolonged period. The bridge electrolyte that comes in contact with the
measuring medium via the outer diaphragm is in the third and final chamber. As the bridge electrolyte does not affect the reference system voltage, it can be arbitrarily selected. The sole proviso is that the bridge electrolyte is ionically conductive and does not contain any substances that react with the iodine/ iodide-laden reference electrolytes. Apart from the usual 3 M potassium chloride solution, potassium chloride solutions of lower concentration, sodium chloride solutions or potassium nitrate solutions can be used. Solutions containing silver ions are, however, out of the question as they form insoluble silver iodide with the iodide ions of the reference electrolytes. Refilling of the bridge electrolytes using the refill hole
integrated in the electrode cap of the system is a simple procedure. The electrode can be filled with the electrolyte solution using any normal laboratory squeeze bottle. The reference space of the electrode can be tightly sealed using the integrated slide valve, which must be open for any measurement. This ensures that the bridge electrolyte can flow out and the sample measuring medium cannot penetrate.
High-speed pH measurements The novel iodine/iodide reference system offers a further important advantage over other ORP reference systems such as silver/silver chloride or mercury/calomel. The temperature dependency or temperature coefficient of this reference system is virtually zero. The temperature coefficient is a measure of how much the voltage changes with each degree Celsius relative to an identical reference system that is held at a constant temperature. If the ratio of iodine/iodide is adjusted accordingly, a reference voltage can be generated with a value of approximately +420 mV relative to the normal hydrogen electrode and which only demonstrates a difference of less than 1 mV (Fig. 3) at between 25-75°C. In contrast, a conventional silver/silver chloride reference system with 3M potassium chloride solution in this temperature range exhibits a voltage difference of approximately 6 mV. Conventional metal/metal chloride reference systems have
yet another disadvantage compared to the iodine/iodide system. If the temperature changes, the equilibriums between the
reference electrodes, the sparingly soluble salt and the solution must first be readjusted. Ions must cross over phase limits where dissolving or precipitating processes take place that require a specific period of time. In the iodine/iodide system, this adjusting process is spontaneous as both voltage-determining iodine and iodide ORP partners are in solution, namely there is a homogeneous system and the exchange current density on the platinum wire is very high. A sensor can be built on the basis of these two special characteristics of the iodine/iodide system which exhibits a response behavior to temperature changes that is no longer dominated by the inertia of the reference system, but is rather more dependent on the temperature response behaviour of the pH glass membrane.
Fig. 2. Adjustment behaviour when changing temperature.
The term ‘temperature compensation’ often arises in conjunction with the issue of pH measurement temperature dependency. Behind this issue is an automatic arithmetic function
that is integrated in all the latest pH meters. The instrument recalculates the temperature-dependent Nernst slope when calibrating the sensor to the current temperature. As a consequence, temperature compensation does not constitute a recalculation of the pH value from one temperature to another In addition, this compensation cannot take into account that the zero point of the pH electrode can also change with temperature. A zero point shift occurs in conventional electrodes if the reference voltages of the glass electrode and reference system change according to temperature, but do not compensate each other mutually. The process described here appears in the technical
literature under the expression ‘Isothermal point’. This characteristic of the electrode can contribute significantly to the uncertainty in the pH measurement. pH electrodes with the iodine/iodide reference system do not exhibit this behavior. The barely noticeable temperature step of the voltage of the iodine/ iodide system contributes significantly to the reliability of pH electrodes and achieves faster measurements. The effort involved in carrying out a pH measurement is also
clearly reduced. When precise pH measurements at different temperatures are required with a measurement uncertainty of ± 0.05pH or better, using standard reference systems necessitates that the measurement system (pH sensor – pH meter) is calibrated at the temperature at which the measurement is to
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