Reference system on iodine/iodide base (Presentation held at ELACH7 in Waldheim, September 2006)

Transcription

1 Reference system on iodine/iodide base (Presentation held at ELACH7 in Waldheim, September 2006) Introduction The reference electrode is a very important component in potentiometry. The standard hydrogen electrode is specified in electro chemistry with 0 V as a reference. Due to handling difficulties, standard hydrogen electrodes have no significant practical meaning. In practice, reference electrodes generally use mercurous chloride and thalamic electrodes followed by silver/silver chloride electrode Ag/AgCl at 3 mol/l KCl or KCl saturation because they can be used in a wide temperature range, are non-toxic and remain stable for a long time. There are disruptions to the diaphragm (liquid junction, "junction clogging" with ceramic diaphragms) from potential changes with temperature and reactions by silver ions with the component parts of some measuring solutions. Better suited reference electrodes have been sought after for such applications. As an alternative reference system, the long known (1) iodine/iodide redox system was used approx. 20 years ago for potentiometric (ph) measurement chains (2). Positive characteristics are the high exchange current seal, the low temperature progress of the potential and the lack of silver ions. The relatively short useful life of this redox reference is noted as a disadvantage as compared to the silver/silver chloride system. In the following presentation, the properties of the iodine/iodide reference system are described and options for improving its longterm stability are presented. Principles The redox reaction J e - <=> 3 J forms the basis of the iodine/iodide system. The reaction is upstream J 2 + J <=> J 3 The redox potential is described using the Nernst equation E H = E + RT/zF * ln ([J 3 ] / [J ]³) where E =0.536 V, R= J/(K*mol), T in K, z=2 and F= C/mol. The patent application (3) describes that the lowest temperature curve of the redox potential at 2.84 mol/l J - and 4.58x10-3 mol/l J 3 is observed, with a ratio of [J 3 - ]/[J - ] 3 = 2.0x10-4. Temperature curve The temperature curve of the iodine/iodide system was studied with different electrolyte compositions and with platinum as a redox electrode in comparison to the Ag/AgCl, 3n KCl electrode. The temperature curve of the redox potential is at approx. E H =420 mv in agreement with (3) at the lowest, as can be see in Figure 1. Useful life With customary reference electrodes (the second type of electrodes), the potential is stabilized in that an antisoluble component (e.g., AgCl) is used as supply and the reference electrolyte is continually refilled anew or held constant via a salt supply. The reference and bridge electrolytes are mostly identical here. With iodine/iodide redox systems, the potential of certain components J 2 or J 3 - and J are slightly soluble. They can be quickly diffused from the low amount of reference electrolyte in a ph measurement chain, wherein the potential and temperature curve change. This is the main cause for the low useful life of iodine/iodide references, which must additionally use a KCl bridge electrolyte. The goal of this project is to achieve potential stabilization and increase the useful life of the iodine/iodide system. This is achieved by using supplements to potential stabilizing components and suppressing the loss from diffusion.

2 Fig. 1 Temperature curve of the iodine/iodide redox potentials E H (red: Ag/AgCl, 3n KCl) With customary reference systems, the J concentration can be stabilized by saturating the reference electrolytes and adding solid KJ. In addition, adding low amounts (0.1 to 0.5 mol/l) KJ to the KCl bridge electrolytes slows the loss of KJ from the reference electrolytes without increasing the diffusion potential considerably. There are different options for stabilizing the J2 concentration. Iodine can be used in elemental or compound form via diffusion barriers with the reference electrolytes in equilibrium. There are different storage materials, such as starches (rice), epoxy and other plastics that bind in reverse to iodine. They release the iodine once in contact with an iodine solution until equilibrium has been reached. If the iodine-laden iodine solution is removed and fresh iodine solution is added to the iodine reservoir, the iodine is extracted again until equilibrium is reached. This process can be tracked by measuring the redox potential. Figures 2a and 2b show the equilibrium settings of the redox potentials on platinum as a function of time and reservoir type for the first and the tenth extraction. J2 release to 3n KCl + 1n KJ, 1st extraction Iodine supplies can be used as an elemental iodine reservoir, which comes into contact with the reference electrolytes via diffusion barriers made of iodine permeable plastics, such as silicon rubber. This allows for compensation of the slow iodine loss and stabilizes the redox potential. Another option is to connect the iodine in reverse to a material and save it in this way with the reference electrolytes maintaining equilibrium. This type of iodine storage will be described further below. Fig. 2a Potential setting for the 1st extraction

3 J2 release to 3n KCl + 1n KJ, 10th extraction After the 1st endurance test of 20 cycles showed almost no discernible changes, the endurance test was repeated twice. In the 36th test cycle in the 2nd endurance test, the commercially available electrode showed a significant increase in the temperature curve and a slight potential drift was already discernible. Fig. 2b Potential setting for the 10th extraction The iodine reservoir Epoxy 2 shows quick potential setting for relatively low potential shift. This means high potential stabilization in the time. For this reason, different reference electrodes were built with iodine reservoirs and subjected to endurance tests. In this automatic endurance test, the electrodes were built into a container that was repeatedly refilled with the buffer solutions ph=6.87, ph=4.01 and ph=9.18, then heated up to 90 C, then cooled to 25 C. For this test cycle, the electrode potentials were measured as compared to an external Ag/AgCl electrode, which was held constant at ambient temperature and which was connected to the test solution via a pressure-impinged electrolyte key. The test cycle was repeated twenty times during the endurance test. The results of the two test electrodes and a commercially available iodine/iodide electrode are shown in Figures 3a and 3b for the 2nd and the 36th test cycles. The temperature is shown in addition. It can clearly be seen that the temperature curves of all of the tested electrodes were low during the 2nd test cycle. Fig. 3a Potential and temperature curve Fig. 3b Potential and temperature curve in the 36th test cycle

4 Figures 4a and 4b show the complete measured data from the 2nd and 3rd endurance tests. Here it is clear that the sample electrode no. 505A with iodine reservoir has a significantly better potential stability and useful life than the commercially available electrode. Even the temperature curve is constant over the entire test duration. Fig. 4a 2nd endurance test Fig. 4b 3rd endurance test Potential curve of iodine/iodide reference systems that are commercially available and with iodine reservoir.

5 Summary By using reversible iodine reservoirs and KJ supplement to the bridge electrolytes, the useful life of the iodine/iodide redox system can be improved significantly. A new reference system for potentiometric measurement chains with the Literature 1. Brunner, E. : Z. physik. Chem. [Physical Chemistry] 47, 56 (1904) 2. Thermo Electron Corporation - Orion 81-02U ROSS Ultr(TM) Electrode, ( ) 3. J. J. W. Ross, Potentiometric Electrode, UK Patent Application GB A Author Dr. Günter Tauber SI Analytics GmbH Hattenbergstr Mainz Germany Tel +49 (0) Fax +49 (0)