Dr Michael Lange provides an overview of pH electrode technology and discusses how the latest advances in iodine/iodide reference systems have changed the way pH measurements are performed.
The core principles of pH electrodes technology have not changed significantly since were first introduced in 1908, although technological innovations have enabled the development of a range of electrodes, including the most commonly used combination electrodes and electrodes with gelled electrolytes. Combination pH electrodes consist of two electrodes built into a single body, with one being a pH glass-measuring electrode and the other being a reference system. Using such a system, pH is measured by calculating the voltage difference between the two electrodes.
There are certain parameters that affect the performance of pH electrodes. The response time of an electrode is the length of time that is required to obtain a stable reading when the electrode is moved between solutions that exhibit different pH or temperature. The response time depends on the selection of a type of electrode, the measuring sample, the temperature of the solution being measured, the magnitude and direction of concentration change and the presence of interfering substances.
The thickness of the glass membrane of the electrode is particularly important as it determines the resistance of the electrode and affects its efficiency. High concentrations of contaminating metal ions greatly decrease the lifetime of glass membranes while also resulting in measurement errors. Metal ions interact with the glass membrane and can partially or completely replace the hydrogen (H+) ions present, generating false pH readings. This effect increases with increasing temperature. The sensitivity of pH electrodes is typically dependent on the temperature of the solution being measured, with the sensitivity increasing linearly with temperature.
The reference system also plays a crucial role in determining the accuracy and efficiency of combination pH electrodes. pH electrodes with an iodine/iodide reference system overcome the aforementioned challenges and deliver fast and dependable pH measurements.
Technological advances
pH electrodes with an iodine/iodide reference system, such as the IoLine electrode from SI Analytics, contain coloured solutions that are clearly visible in the pH glass measuring electrode and the reference system. The colours occur according to the specified ratio of iodine to iodide. The electrochemical voltage of the iodine/iodide ORP pair depends on the concentration ratio of both substances and not on their absolute concentration, as per the below equations. The ORP voltage can be used as the reference voltage in both the pH glass electrode and the reference system. A platinum wire is used in the control systems solely for determining the charge transport between the two iodine and iodide ORP partners and does not affect the voltage.
Nernst equation: E=E0 + (UN/2) log ([I3-]/[I-]3); E0=0.536 V;
Reaction equation: I3- + 2 e- - 3 I-
One key limitation of conventional iodine/iodide reference systems is that they continuously lose iodine from the reference electrolytes, thereby lowering 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 +420mV 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.
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 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 4.01pH at different temperatures. The buffer alters its pH value from 4.01pH at 25°C to 4.09pH at 60°C or 4.00pH at 5°C. Prior to commencing the experiment, all electrodes are calibrated and at the outset indicate a value of 4.01pH 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.
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 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.
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.