Iodides potentiometric titration

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. 

Dissolve 20 g of tetra-n-butylammonium iodide in 100 mL of dry methanol and pass this solution through the column at a rate of about 5 mL min - L the effluent must be collected in a vessel fitted with a Carbosorb guard tube to protect it from atmospheric carbon dioxide. Then pass 200 mL of dry methanol through the column. Standardise the methanolic solution by carrying out a potentiometric titration of an accurately weighed portion (about 0.3 g) of benzoic acid. Calculate the molarity of the solution and add sufficient dry methanol to make it approximately 0.1M. 

Gilbert and Hybart,189 from the results of potentiometric titrations, have suggested that iodine is bound as a complex of one or two iodine molecules together with none, one, or two iodide ions, depending on the concentration of iodide present. 

It has not yet been possible to obtain samples of amylopectin which do not show some slight evidence of uptake of iodine by linear material in the early stages of an accurate potentiometric titration. Although this effect is presumably due to contaminating amylose, the presence of some long branches in the amylopectin cannot be excluded. Anderson and Greenwood190 have shown that in 0.01 M iodide solution, for concentrations of total free iodine less than 1 X 10-6 M, the amount of iodine bound by... 

Potassium iodide prepared as described has been used in a very careful study of the absolute accuracy of the poten-tiometric iodide-silver titration,2 by comparing it directly against pure silver. The ratio KI Ag found in this way agreed to within 0.02 per cent with the theoretical ratio. This small deviation is to be attributed to a slight absorption of iodide ions by the silver iodide at the potentiometric end point and not to an impurity in the potassium iodide. 

Ten or so years later, in the 1930s, Edward Zintl conducted a series of more systematic studies of these systems. He carried out potentiometric titrations of liquid ammonia solutions of alkali metals with various p metal salts, typically halides. Thus, the titration of a sodium solution with lead(II) iodide revealed that the green anionic species in solution are Pbg. Zintl and coworkers also discovered that... 

The range of oxides suitable for investigations in this melt is appreciably narrowed as compared with the molten CsBr-KBr (2 1) mixture. The potentiometric titration method allows us to determine the solubilities of moderately soluble oxides in the iodide-melt, since the pO values in the section of the excess of the metal cations do not exceed 5-6. The solubility products of alkaline-earth oxides (excluding MgO), cadmium and lead oxides, were determined in Ref. [364], The values of solubility products obtained, and the derived oxide solubilities in molten Csl at 700 °C are presented in Table 3.7.15. 

One of the most probable reasons for such a behaviour of strongly acidic solutions in iodide-melts, under the potentiometric titration conditions is transfer of oxygen through the solid electrolyte membrane to the iodide-melt. The following processes could be assumed to occur in the said system. 

We have also investigated the solubilities of MgO and CaO in molten potassium halides at 800 °C [197] to elucidate the effect of anion composition of the halide melt on metal-oxide solubility. The MgO was found to be practically insoluble in chloride and bromide melts, and the iodide-melt could not be investigated owing to intense iodine evolution from strongly acidic solutions. In contrast, CaO solubility products were determined successfully in all the potassium halide melts at 800 °C, by the potentiometric titration method. The corresponding potentiometric titration curves are shown in Fig. 3.7.16. 

Yu.F. Rybkin and V.V. Banik, Acid-Base Potentiometric Titration in Molten Sodium Iodide, in Single Crystals and Technics (Inst, for Single Cryst., Kharkov, 1974) Issue 1 (10), pp. 111-114. 

The manganate ion is not reduced by bromide ion but is reduced slowly by iodide ion and quickly by vanadyl(IV) or hexacyano-ferrate(II) ions. When the latter two ions are used as reductants, especially with the potassium complex, green products are obtained rapidly and in high yield. The green species is unstable in solution and is apparently in equilibrium with the reactants. With potassium salts, the solubility of the product is low, and the reaction is driven to completion. Potentiometric titrations show that a one-electron reduction occurs to produce the green species, which has been characterized by analysis and optical and e.s.r. spectroscopy. It is a mixed-valence species similar to the heteropoly blues of molybdenum and tungsten. E.s.r. spectra suggest that the extra electron is fairly well trapped on a specific vanadium atom, and the complex is therefore a class II mixed-valence species.8... 

EXPERIMENT 20 POTENTIOMETRIC TITRATION OF A MIXTURE OF CHLORIDE AND IODIDE... 

In the procedure described, Dragendorff s reagent, potassium iodide bismuthate KBil i , forms a deeply orange-red-coloured adduct with many nonionic detergents (surfactants) from aqueous solutions. The adduct is filtered off and dissolved, and the bismuth contained in the adduct is determined by potentiometric titration or by AAS or ICP. 

That the pyrimidine nucleus is linked to the thiazole nucleus through the nitrogen atom of the latter soon became apparent. A comparison of the behaviour of thiamine and its thiazole moiety towards certain chemical reagents indicated the presence of a quaternary nitrogen in the former. Moreover, upon potentiometric titration of thiamine with alkali it behaved exactly as an addition product (VIII) of VII and methyl iodide. 

Generally, methods are based on solvent extraction of the additive followed by analysis for the extracted additive by a suitable physical technique such as visible spectrophotometry of the coupled antioxidant, redox spectrophotometric methods, ultraviolet spectroscopy, infrared spectroscopy, gas chromatography, thin-layer chromatography or column chromatography. In general, direct chemical methods of analysis have not foimd favour. These include potentiometric titration with standard sodium isopropoxide in pyridine medium or reaction of the antioxidant with excess standard potassium bromide-potassium bromate (ie. free bromine) and estimation of the unused bromine by addition of potassium iodide and determination of the iodine produced by titration with sodium thiosulphate to the starch end-point. ... 

The compendial method for assay of benzalkonium chloride is based upon reversal of the quaternization reaction. The compound is reacted with Nal, the benzyl iodide and tertiary amine are removed by extraction, and the iodide consumed is determined by titration with KIO3 (9). The alkyl chain length distribution is determined by HPLC. Other tests for characterization of benzalkonium salts are similar to those performed on alkyl quaternaries. These salts are also readily determined by two-phase or potentiometric titration with an anionic surfactant. 

Long chain amine oxides are sufficiently basic that they may be titrated directly with HCl in isopropanol. Any residual tertiary amine will also be titrated under these conditions, but modern potentiometric titration apparatus allows differentiation of the amine and amine oxide in a single determination (Chapter 2). Alternatively, reaction with methyl iodide to form the quaternary amine from the tertiary amine will eliminate interference (78). Amine oxides which cannot be determined in isopropanol can generally be titrated potentiometri-cally with perchloric acid in acetic acid/acetic anhydride solvent. Acetic acid alone is not a suitable solvent for this titration (79,80). 

The hydrogen ions thus set free can be titrated with a standard solution of sodium hydroxide using an acid-base indicator or a potentiometric end point alternatively, an iodate-iodide mixture is added as well as the EDTA solution and the liberated iodine is titrated with a standard thiosulphate solution. 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

Motonaka et al. [5] described a potentiometric method for the titration of ( -penicillamine with an iodide-selective electrode. An excess of methanolic 0.05 M iodine was added to a sample containing 8 pg to 6.2 mg of (/))-penicillamine. The solution was then diluted to 50 mL with H20, and titrated with 0.1 mM to 0.1 M AgN03. For 6 mg of (/))-penicillamine, the coefficient of variation was less than 0.3%. 

It is based on the addition of Mn2+ solution, followed by die addition of a strong alkali to die sample in a glass-stoppered bottle. Dissolved 02 rapidly oxidizes an equivalent amount of the dispersed divalent manganous hydroxide precipitate to hydroxides of higher valence states. In die presence of iodide ions in an acidic solution, the oxidized manganese reverts to die divalent state, with die liberation of a quantity of iodine equivalent to die original dissolved 02 content. The iodine is then titrated with a standard solution of thiosulfate. The titration end point can be detected visually with a starch indicator, or by potentiometric techniques. The liberated iodine can be determined colorimetrically. 

Another specialized form of potentiometric endpoint detection is the use of dual-polarized electrodes, which consists of two metal pieces of electrode material, usually platinum, through which is imposed a small constant current, usually 2-10 /xA. The scheme of the electric circuit for this kind of titration is presented in Figure 4.1b. The differential potential created by the imposition of the ament is a function of the redox couples present in the titration solution. Examples of the resultant titration curve for three different systems are illustrated in Figure 4.3. In the case of two reversible couples, such as the titration of iron(II) with cerium(IV), curve a results in which there is little potential difference after initiation of the titration up to the equivalence point. Hie titration of arsenic(III) with iodine is representative of an irreversible couple that is titrated with a reversible system. Hence, prior to the equivalence point a large potential difference exists because the passage of current requires decomposition of the solvent for the cathode reaction (Figure 4.3b). Past the equivalence point the potential difference drops to zero because of the presence of both iodine and iodide ion. In contrast, when a reversible couple is titrated with an irreversible couple, the initial potential difference is equal to zero and the large potential difference appears after the equivalence point is reached. 

The acid hydrolysis of the 2-aryl-1,3,4-oxadiazoles can be used for their analytical determination. The method used is either to break down the compound by heating with hydrochloric acid under reflux to give the acid hydrazide and then to titrate the hydrazide with iodide in bicarbonate solution,68, 69 or to titrate potentiometrically directly with sodium nitrite in a hydrochloric acid medium.128 In this way the acid hydrazide is formed in the first reaction step and is then converted into the insoluble azide by the sodium nitrite. 

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