Iodide electrode

Elemental composition K 28.22%, Cl 25.59%, and 0 46.19%. An aqueous solution is analyzed for potassium by AA, ICP, and other methods (see Potassium). Perchlorate ion may be analyzed by ion chromatography or a liquid-membrane electrode. Iodide, bromide, chlorate, and cyanide ions interfere in the electrode measurement. Alternatively, perchlorate ion may be measured by redox titration. Its solution in 0.5M H2SO4 is treated with a measured excess standard ferrous ammonium sulfate. The excess iron(II) solution is immediately titrated with a standard solution of potassium dichromate. Diphenylamine sulfuric acid may be used as an indicator to detect the end point ... 

The oxidation processes of various anions, including halide ions, have been studied by voltammetry at solid electrodes [38 a]. For example, at a platinum electrode, iodide ion (I-) in various organic solvents is oxidized in two steps, i.e. 3I -> I3 + 2e and 2Ii-> 3I2+2e. From the difference in the standard potentials of the two steps, the dissociation constants of I3, p/f=-log ([yin/KD- have been determined as in Table 8.4. In aqueous solutions, p/f is 3 and I3 is not stable enough to use this method. The electrode oxidation of CN has been studied in several solvents using ESR spectroscopy [38b]. From the ultimate formation of the relatively stable tricyanomethylenimine radical anion, the following steps have been considered ... 

I Ion exchange -resin column On-line preconcentration electrode Iodide selective electrode InM [328]... 

The types of elements that can be determined by cathodic deposition at mercury electrodes are the amalgam-formers—Cu, Pb, Cd, Bi, Sn, Zn, Tl, and so forth. Using anodic deposition, halides and various sulfides can be determined at mercury electrodes, iodide at silver electrodes, and lead (as PbOa) and iron (as FegOg) at various solid electrodes. 

Urinary iodide concentrations were determined with an iodide-selective ion electrode(iodide-electrode, radiometer, Copenhagen, Denmark)The anti-thyroglobulin and -thyroid peroxidase antibodies were determined by highly sensitive ra-dioimmuno assay. 

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. 

An ion-selective electrode is available for chloride analysis chloride can be measured potentiometrically at 10 -1 M. Iodide and sulfide are the principal interferences. 

Detection of Bromine Vapor. Bromine vapor in air can be monitored by using an oxidant monitor instmment that sounds an alarm when a certain level is reached. An oxidant monitor operates on an amperometric principle. The bromine oxidizes potassium iodide in solution, producing an electrical output by depolarizing one sensor electrode. Detector tubes, usefiil for determining the level of respiratory protection required, contain (9-toluidine that produces a yellow-orange stain when reacted with bromine. These tubes and sample pumps are available through safety supply companies (54). The usefiil concentration range is 0.2—30 ppm. 

Another application is in tire oxidation of vapour mixtures in a chemical vapour transport reaction, the attempt being to coat materials with a tlrin layer of solid electrolyte. For example, a gas phase mixture consisting of the iodides of zirconium and yttrium is oxidized to form a thin layer of ytnia-stabilized zirconia on the surface of an electrode such as one of the lanthanum-snontium doped transition metal perovskites Lai j.Srj.M03 7, which can transmit oxygen as ions and electrons from an isolated volume of oxygen gas. 

Boujlel and Simonet used an electrochemical method to prepare a group of similar compounds, including compound ]5, shown in Eq. (3.41). In a typical case, benzil was reduced in DMF solution at the dropping mercury electrode in the presence of tetrabutylammonium iodide, used in this case as a supporting electrolyte rather than phase transfer catalyst. In the presence of diethylene glycol ditosylate, compound 15 (mp 77— 78°) was isolated in 10% yield. Using the same approach, acenaphthenedione was reduc-tively cyclized with triethylene glycol ditosylate to afford the product (mp 84—85°, 42% yield) shown in Eq. (3.42). 

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. 

Thus an almost complete separation is theoretically possible. The separation is feasible in practice if the point at which the iodide precipitation is complete can be detected. This may be done (a) by the use of an adsorption indicator (see Section 10.75(c)), or (b) by a potentiometric method with a silver electrode (see Chapter 15). 

Iodide. A 0.01 M solution of potassium iodide, prepared from the dry salt with boiled-out water, is suitable for practice in this determination. The experimental details are similar to those given for bromide, except that the indicator electrode consists of a silver rod immersed in the solution. The titration cell may be charged with 10.00 mL of the iodide solution, 30 mL of water, and 10 mL of the stock solution of perchloric acid + potassium nitrate. In the neighbourhood of the equivalence point it is necessary to allow at least 30-60 seconds to elapse before steady potentials are established. 

If the pellet contains a mixture of silver sulphide and silver chloride (or bromide or iodide), the electrode acquires a potential which is determined by the activity of the appropriate halide ion in the test solution. Likewise, if the pellet contains silver sulphide together with the insoluble sulphide of copper(II), cadmium) II), or lead) II), we produce electrodes which respond to the activity of the appropriate metal ion in a test solution. 

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. 

Place the prepared copper acetate solution in the beaker and add 10 mL of 20 per cent potassium iodide solution. Set the stirrer in motion and add distilled water, if necessary, until the platinum plate electrode is fully immersed. Use a saturated calomel reference electrode, and carry out the normal potentiometric titration procedure using a standard sodium thiosulphate solution as titrant. 

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. 

Dilute solutions of iodine, e.g. 0.0001 M, may be titrated similarly with standard thiosulphate. The supporting electrolyte consists of 1.0 M hydrochloric acid and 0.004M potassium iodide. No external e.m.f. is required when an S.C.E. is employed as reference electrode. 

The Karl Fischer procedure has now been simplified and the accuracy improved by modification to a coulometric method (Chapter 14). In this procedure the sample under test is added to a pyridine-methanol solution containing sulphur dioxide and a soluble iodide. Upon electrolysis, iodine is liberated at the anode and reactions (a) and (b) then follow the end point is detected by a pair of electrodes which function as a biamperometric detection system and indicate the presence of free iodine. Since one mole of iodine reacts with one mole of water it follows that 1 mg of water is equivalent to 10.71 coulombs. 

Mercury cyanide, 5, 1062 Mercury electrodes potential range aqueous solution, 1, 480 Mercury fluoride, 5. 1059 Mercury fulminate, 2, 7, 12 5, 1063 Mercury halides, 5, 1049 Mercury iodate, 5,1068 Mercury iodide, 5. 1059 Mercury ions Hgf... 

Other useful solid-state electrodes are based on silver compounds (particularly silver sulfide). Silver sulfide is an ionic conductor, in which silver ions are the mobile ions. Mixed pellets containing Ag2S-AgX (where X = Cl, Br, I, SCN) have been successfiilly used for the determination of one of these particular anions. The behavior of these electrodes is determined primarily by the solubility products involved. The relative solubility products of various ions with Ag+ thus dictate the selectivity (i.e., kt] = KSp(Agf)/KSP(Aw)). Consequently, the iodide electrode (membrane of Ag2S/AgI) displays high selectivity over Br- and Cl-. In contrast, die chloride electrode suffers from severe interference from Br- and I-. Similarly, mixtures of silver sulfide with CdS, CuS, or PbS provide membranes that are responsive to Cd2+, Cu2+, or Pb2+, respectively. A limitation of these mixed-salt electrodes is tiiat the solubility of die second salt must be much larger than that of silver sulfide. A silver sulfide membrane by itself responds to either S2- or Ag+ ions, down to die 10-8M level. 

FIGURE 5-18 Flow-through potentiometric cell-cap design. A, reference electrode B, iodide electrode C, flow-through cap D, inlet E, outlet. (Reproduced with permission from reference 49.)... 

Similarly to quantitative determination of high surfactant concentrations, many alternative methods have been proposed for the quantitative determination of low surfactant concentrations. Tsuji et al. [270] developed a potentio-metric method for the microdetermination of anionic surfactants that was applied to the analysis of 5-100 ppm of sodium dodecyl sulfate and 1-10 ppm of sodium dodecyl ether (2.9 EO) sulfate. This method is based on the inhibitory effect of anionic surfactants on the enzyme system cholinesterase-butyryl-thiocholine iodide. A constant current is applied across two platinum plate electrodes immersed in a solution containing butyrylthiocholine and surfactant. Since cholinesterase produces enzymatic hydrolysis of the substrate, the decrease in the initial velocity of the hydrolysis caused by the surfactant corresponds to its concentration. Amounts up to 60 pg of alcohol sulfate can be spectrometrically determined with acridine orange by extraction of the ion pair with a mixture 3 1 (v/v) of benzene/methyl isobutyl ketone [271]. 

Equation (40) relates the lifetime of potential-dependent PMC transients to stationary PMC signals and thus interfacial rate constants [compare (18)]. In order to verify such a correlation and see whether the interfacial recombination rates can be controlled in the accumulation region via the applied electrode potentials, experiments with silicon/polymer junctions were performed.38 The selected polymer, poly(epichlorhydrine-co-ethylenoxide-co-allyl-glycylether, or technically (Hydrine-T), to which lithium perchlorate or potassium iodide were added as salt, should not chemically interact with silicon, but can provide a solid electrolyte contact able to polarize the silicon/electrode interface. 

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