Iodine-platinum electrodes

C. HIO is prepared by oxidation of iodine with perchloric acid, nitric acid, or hydrogen peroxide or oxidation of iodine in aqueous suspension to iodic acid by silver nitrate. Iodic acid is also formed by anodic oxidation at a platinum electrode of iodine dissolved in hydrochloric acid (113,114). 

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. 

The end point of the reaction is conveniently determined electrometrically using the dead-stop end point procedure. If a small e.m.f. is applied across two platinum electrodes immersed in the reaction mixture a current will flow as long as free iodine is present, to remove hydrogen and depolarise the cathode. When the last trace of iodine has reacted the current will decrease to zero or very close to zero. Conversely, the technique may be combined with a direct titration of the sample with the Karl Fischer reagent here the current in the electrode circuit suddenly increases at the first appearance of unused iodine in the solution. 

The platinum electrode was modified with an iodine adlayer of the specified symmetry. ) AC = acetate anion. 

The anode-cathode assembly generates the iodine. The dual-pin platinum electrode detects the end point. 

In other words, the small excess of HN02 present at the end-point can be detected visually by employing either starch-iodide paper or paste as an external indicator. Thus, the liberated iodine reacts with starch to form a blue green colour which is a very sensitive reaction. Besides, the end-point may also be accomplished electrometrically by adopting the dead-stop end-point technique, using a pair of platinum electrodes immersed in the titration liquid. 

End-point Detection The end-point of the Karl Fischer titration may be determined quite easily by adopting the electrometric technique employing the dead-stop end-point method. When a small quantum of e.m.f. is applied across two platinum electrodes immersed in the reaction mixture, a current shall tend to flow till free iodine exists, to remove hydrogen and ultimately depolarize the cathode. A situation will soon arise when practically all the traces of iodine have reacted completely thereby setting the current to almost zero or veiy close to zero or attain the end-point. 

Thus, the basis of the analysis rests upon the quantitative relationship existing between charge passed and iodine produced by the reagent according to the above reaction. Therefore, the generation of iodine is automatically stopped when an excess of it is detected by the indicator electrode. It essentially consists of two platinum electrodes across which an AC is applied and subsequently a marked drop in voltage between the electrodes takes place as soon as an excess of iodine is present. Normally such automated instruments make use of proprietory reagents exclusively. 

Worked Example 3.6. An old medicine bottle is discovered that once contained tincture of iodine. The bottle still contains a trace of solid iodine. Some of the solid is dissolved in an aqueous solution of potassium iodide of concentration 0.1 mol dm . The electrode potential of the I2,1 couple was determined at a platinum electrode immersed in the solution. If = —0.54 V and i,i for the resultant solution is —0.60 V, what is the concentration of the iodine Ignore the presence of the brown 13 ion. 

Because atmospheric humidity must be avoided, the reaction flask is isolated from the atmosphere with drying tubes. Moreover, since the solvent is rarely perfectly anhydrous and will contain traces of water due to its hygroscopic nature, its water content must be measured prior to the determination. The equivalence point of the titration reaction is detected by an electrical method instead of a visual method. The current intensity that passes between two platinum electrodes inserted in the reaction medium is measured (see Fig. 19.10). The reagent, which is a mixture of sulphur dioxide, iodine and a base, is characterised by the number of mg of water that can be neutralised by 1 cm3 of this reagent. This is referred to as the equivalent mass concentration of water, or the titre T of the reagent. 

From these equations it can be seen that each mole of water requires one mole of I2. In a visual endpoint Karl Fischer titration, a sample is titrated with the Karl Fischer reagent until a permanent iodine color (indicating that all water has been reacted) is observed. Because of other reaction products, the color change is usually from a yellow to a brownish color, which may be difficult to detect visually. Highly colored samples may affect the visual end point as well. A much sharper end point, known as the dead stop end point, can be obtained if the titration is done electrometrically. Here, two small platinum electrodes dip into the titration cell, a small constant voltage is impressed across these electrodes, and any current that flows is measured with a galvanometer. At the end point of the titration the current either goes to a minimum or else increases suddenly from nearly zero. Commercially available Karl Fischer instruments incorporate semiautomatic microprocessors based on this principle. 

Low PVs cannot be adequately measured by the official methods (AOCS, 1998 IUPAC, 1987a) because of uncertainly with the io-dometric titration endpoint the disappearance of the pale-violet color produced by the iodine and starch reaction is difficult to discern. The test has been modified by replacing the titration step with an electrochemical technique in which the liberated iodine is reduced at a platinum electrode maintained at a constant potential (Oishi et al 1992). PVs ranging from 0.06... 

Biamperometry — Whereas in amperometry the -> current is limited by the electrode process proceeding at one indicator electrode (and the -> counter electrode has no effect), in biamperometry the current flowing between two indicator electrodes is measured, i.e., both electrodes can limit the overall current. This approach is useful in following some -> titrations, and it may lead to zero current (dead-stop) at the equivalence point (dead-stop titration). Example iodine in an iodide solution is titrated with As(III). Two platinum electrodes with a potential difference of around 100 mV prompt iodine to be reduced on one electrode and iodide being oxidized at the other. The two processes maintain an almost constant current until the endpoint when iodine is exhausted. 

The above example was given to show the versatihty of the coulometric titration method. A more typical example would be one where the constant current generates electrolyticaUy, with 100 percent, a substance which reacts immediately with the analyte species in solution. A typical example is the generation of iodine between two platinum electrodes immersed in an oxygen-free solution of potassium iodide. The generating reaction at the anode is ... 

Figure 20.13 Measurement of water by the Karl Fischer volumetric technique. The titration ceU contains volumetric burettes (which may be automated) and two small platinum electrodes. As long as no iodide is present in solution the electrodes remain polarized and only a weak current circulates between them. Once the equivalence point is reached, an excess of iodine will cause the depolarization of the electrodes and a significant current is registered.

The interaction of iodine with palladium and platinum electrodes was studied in different electrolyte solutions and on single crystals with electrochemical techniques and UHV spectroscopic ex situ measurements [111]. The chemisorption of atomic iodine on palladium researched extensively because of its ability to protect the surface from air and water interactions. Moreover, it is able to induce a surface reconstruction from a stepped surface to a (1 x 1) unreconstructed one. 

Iodide in the solution in Problem 11.5 can also be determined by controlled-potential oxidation to iodine at a platinum electrode. What potential should be used for this oxidation (see Figure 11.10.1)7 How many coulombs will be passed ... 

The examples given in the table are based on chemical oxidation. By contrast, anodic oxidation of iodine in trimethyl orthoformate provides the iodonium ion which is effective for the iodination of appropriate substituted benzenoid compounds (ref.37) and has been considered superior to the iodonium ion developed in acetonitrile solution. Thus the additbn of the anolyte (4 moles) from trimethyl orthoformate (TMOF), elemental iodine and lithium perchlorate trihydrate, by anodic oxidation at ambient temperature in a divided electrolytic cell equipped with a ceramic diaphragm and platinum electrodes, to anisole (1 mole) in TMOF afforded iodoanisoles in 69% yield, in the proportions, 4-iodomethoxybenzene (87%), and the 2-iodo isomer (13%) with none of the 3-isomer. 

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