Iodine-iodide electrode

The iodine-iodide electrode has been investigated and reported by many authors. Brunner [162] and Vetter [163] have given comprehensive and detailed descriptions of the chemical reactions, the equilibrium potential, polarization phenomena, and current density vs. potential curves of the iodine-iodide redox system, obtained with platinum as the noble metal electrode. 

This chapter will describe the iodine-iodide electrode with respect to properties that are relevant for practical applications, i.e., as a reference electrode for potentiometry as typically used in measuring chains with pH glass electrodes, which was introduced by Ross [164]. 

Equation (5.5.3) describes the reaction of practically used reference electrodes with high iodide concentrations. The potential of the iodine-iodide electrode is given by the Nemst equation... 

A reference electrode suitable for practical use must have a well-known potential in the specified temperature range, reproducible and stable over a long period of time. The liquid-junction potentials should be small and reproducible in the specified applications and finally its production should be easy and inexpensive. These expectations will be proved next for the iodine-iodide electrode. 

The temperature function of the iodine-iodide electrode was investigated by Ross [164], who described the optimum electrolyte composition with 2.84 mol KI and 0.00458 mol I+. This electrolyte results in an electrode potential with a temperature coefficient close to zero. 

Fig. 5.5.2 Temperature coefficient of the iodine-iodide electrode vs. the potential at 25 °C. The iodine-iodide electrode with the smallest temperature coefficient close to zero has a potential around = 415 mV at 25 °C and an electrolyte with a high KI concentrations of over 1.5 molL ...

If the reference electrolyte is diluted by 10 %, the potential changes by only 1.4 mV, half as much as the change of a silver-silver chloride electrode potential. This is explained by the two-electron reaction (5.5.3), which causes only half the slope. Therefore, the potential of the iodine-iodide electrode is less sensitive to the dilution of the electrolyte compared to the silver/sUver chloride, for instance. 

In most practical cases reference electrodes are in contact with the sample solutimi via a diaphragm, which is typically a porous ceramic plug. The diaphragm is filled with the usually used KCl soluticai as reference electrolyte to form a liquid junction at its outer surface. Details and characteristics of liquid junctions are described elsewhere. Especially for the iodine-iodide electrode it should be mentioned that... 

Hence, the iodine-iodide electrode is well suited for practical applications in a harsh environment with electromagnetic interferences. 

The main advantage is the zero temperature coefficient of the iodine-iodide reference system and the very small hysteresis. On the other hand, this reference electrode is free of silver ions to avoid problems due to interactions with sulfide ions or proteins at the liquid junction. In harsh environments the iodine-iodide electrode is almost insensitive to small currents and to electromagnetic interferences. 

The reversible iodine/molten silver iodide electrode was used for the first time by Sternberg, Adorian and Galasiu [108], To obtain this electrode it was necessary to construct an electrochemical cell which maintained iodine in the gaseous state from the moment it was generated until it was removed from the cell. The cell, which is shown in Figure 12, was constructed of heat resistant... 

Establishing the surface area is the next concern. Electrode surfaces are seldom flat. Instead, they tend to display a set of different crystal faces. Sliver Iodide electrodes, prepared by amalgamation of silver with mercury, followed by vapour deposition of iodine, look smooth and shiny to the naked eye but reveeil crystallites under the electron microscope. Surface Irregularities not only complicate the assessment of the real area, they may also Interfere in the analysis of impedance spectra In terms of equivalent circuits. After drying, the surface may be studied by the usual optical methods (sec. 1.2) with the famlllrir caveat that drying may change these properties. Anyway, for a number of oxides and silver iodide It Is now established that electrodes can be made which have... 

An advantage or disadvantage, depending on the situation, is that ion-selective electrodes are responsive to a particular chemical form of an element. For example, an iodide electrode responds only to I, not to the total iodine content which could include IOs or organically botmd iodine. 

Typically, a dye-sensitized solar cell (DSSC) is composed of a dye-sensitized mesoporous titania electrode, a platinum counter electrode, and an electrolyte containing iodine/ iodide. Indium tin oxide (ITO) glass was the commonly used conductive substrate for a DSSC, but its intrinsic rigidity largely limited the flexibility of the resulted device. Therefore, it is necessary to develop the other flexible and conductive substrates aiming at flexible DSSCs. 

M. Calvin This is a problem in surface electrochemistry. It is a question of why some molecules react reversibly with an electrode and others do not. I m afraid that I cannot answer the question. The only thing I can say is that the only hope for this disulfide system, at least electrochemically, lies in preventing the mercaptans and the disulfides from coating the electrodes and using a chemical system to which the electrode will respond. The 1 N HI looks so promising because I think in 1 A HI the mercaptan is prevented from forming a mercaptide layer on the electrode, and thus the electrode can respond in a reversible way to the iodine-iodide system. It also responds, apparently, to the permanganate-MnOz system on the alkaline side, but this is even more difficult. I m afraid I have no better answer than that. 

Based on AA oxidation to dehydroascorbic acid in acidic medium using iodine-iodide solution as oxidizing reagent. The iodine amount consumed in the redox reaction was detected Electrocatalysis of AA on a glassy carbon electrode chemically modified with polyaniline films AA was determined at a vitreous C electrode modified with 3,4-dihydroxybenzaldehyde AA was determined with a chemically modified with methylene green (electron mediator) carbon paste electrode... 

The temperature coefficient of the iodine-iodide redox electrode can be taken from the data of Fig. 5.5.1. Figure 5.5.2 shows a plot of vs. the electrode potential... 

Vetter [161] has investigated the polarization of platinum electrodes in the iodine-iodide system. A current density of 1 mA cm causes a potential shift of less than 1 mV, corresponding to a current of about 0.3 mA to a platinum wire with 0.3 mm diameter and a length of 3 cm. This is a very small polarization. A current of 0.3 mA would cause a potential drop of 300 mV at a 1 k 2 diaphragm. 

Most of the practical devices use iodine-iodide electrochemical systems with platinum electrodes. The electrolyte consists of a high-concentration aqueous solution of potassium iodide KI (lower temperature range boundary at —15 °C) or lithium iodide Lil (lower temperature range boundary at —55 °C) as neutral electrolyte and a small quantity of molecular iodine I2. If there is an excess supply of iodide, iodine enters a freely soluble complex compound (triiodide) according to the following scheme ... 

Reznikova LA, Morgunova EE, Bograchev DA, Grigin AP, Davydov AD (2001) Limiting current in iodine-iodide system on vertical electrode under conditions of natural convection. Russ J Electrochem 37 382-387... 

Silver-silver bromide and silver-silver iodide electrodes prepared in a similar manner behave as reversible bromine and iodine electrodes, respectively. 

Chlorine has a lower electrode potential and electronegativity than fluorine but will displace bromine and iodine from aqueous solutions of bromide and iodide ions respectively ... 

Iodine has the lowest standard electrode potential of any of the common halogens (E = +0.54 V) and is consequently the least powerful oxidising agent. Indeed, the iodide ion can be oxidised to iodine by many reagents including air which will oxidise an acidified solution of iodide ions. However, iodine will oxidise arsenate(lll) to arsenate(V) in alkaline solution (the presence of sodium carbonate makes the solution sufficiently alkaline) but the reaction is reversible, for example by removal of iodine. 

The pH must be kept at 7.0—7.2 for this method to be quantitative and to give a stable end poiut. This condition is easily met by addition of soHd sodium bicarbonate to neutralize the HI formed. With starch as iudicator and an appropriate standardized iodine solution, this method is appHcable to both concentrated and dilute (to ca 50 ppm) hydraziue solutious. The iodiue solutiou is best standardized usiug mouohydraziuium sulfate or sodium thiosulfate. Using an iodide-selective electrode, low levels down to the ppb range are detectable (see Electro analytical techniques) (141,142). Potassium iodate (143,144), bromate (145), and permanganate (146) have also been employed as oxidants. 

They form a monolayer that is rich in defects, but no second monolayer is observed. The interpretation of these results is not straightforward from a chemical point of view both the electrodeposition of low-valent Ge Iy species and the formation of Au-Ge or even Au Ge h compounds are possible. A similar result is obtained if the electrodeposition is performed from GeGl4. There, 250 20 pm high islands are also observed on the electrode surface. They can be oxidized reversibly and disappear completely from the surface. With Gel4 the oxidation is more complicated, because the electrode potential for the gold step oxidation is too close to that of the island electrodissolution, so that the two processes can hardly be distinguished. The gold step oxidation already occurs at -i-lO mV vs. the former open circuit potential, at h-485 mV the oxidation of iodide to iodine starts. 

The indicator electrode must be reversible to one or the other of the ions which is being precipitated. Thus in the titration of a potassium iodide solution with standard silver nitrate solution, the electrode must be either a silver electrode or a platinum electrode in the presence of a little iodine (best introduced by adding a little of a freshly prepared alcoholic solution of iodine), i.e. an iodine electrode (reversible to I-). The exercise recommended is the standardisation of silver nitrate solution with pure sodium chloride. 

Dilute solutions of sodium thiosulphate (e.g. 0.001 M) may be titrated with dilute iodine solutions (e.g. 0.005M) at zero applied voltage. For satisfactory results, the thiosulphate solution should be present in a supporting electrolyte which is 0.1 M in potassium chloride and 0.004 M in potassium iodide. Under these conditions no diffusion current is detected until after the equivalence point when excess of iodine is reduced at the electrode a reversed L-type of titration graph is obtained. 

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