Role of thiols (cysteine and teiourea) in the mechanism of the periodic decomposition reaction of hydrogen peroxide by potassium iodate in an acid medium

We have investigated the kinetics of the oxidation of cysteine and thiourea by an excess of potassium iodate under conditions corresponding to the production of auto-oscillations of concentration. First-order rate constants have been calculated for the disulfide oxidation of cystine by potassium iodate and of the disulfide of formamidine, the primary oxidation product of thiourea. It is shown that thorough oxidation of disulfides by iodate can occur and its stoichiometry is investigated. A new oscillating chemical system has been discovered, containing potassium iodate, hydrogen peroxide, thiourea, and a mixture of sulfuric and hydrochloric acids.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 26, No. 1, pp. 51–56, January–February, 1990.     

Colorimetric determination of inorganic iodine in fortified culinary products

The presented colorimetric procedure only requires simple laboratory equipment and is suitable as a routine procedure for checking concentrations of iodine in fortified culinary products. The Moxon and Dixon colorimetric procedure for iodine determination has been optimised for the determination of iodide and iodate in fortified culinary products, always containing high salt levels. The high sensitivity of the method permits a high dilution of the product solutions, thus reducing interferences from the inherent colour of the products. The calibration is linear in the range from 0 to 12 microg L(-1) of iodine with R2 > 0.99. A series of commercial culinary products were used to validate the method. Recoveries of iodine, added as iodide and/or iodate, were generally in the range 100+/-10%. High concentrations of chloride are essential to obtain a complete recovery of iodate. Limit of quantification was estimated to be 2 mg kg(-1) of product, based on 2-3 g of product. Concentrations of iodine determined with this method were similar to those obtained by an ICP-MS procedure.     

Rapid determination of thorium in ores : Separation as iodate in presence of oxalic acid

It has been shown that thorium could be precipitated quantitatively in 40% nitric acid solution as iodate, in presence of oxalic acid which complexes all those elements that interfere seriously in the usual iodate procedure. Thorium is thus freed from all the cations in a single operation from solutions containing sulphate and phosphate radicals. The method is very quick and thorium oxide in a sample of monazitc could be estimated with precision in 8–9 hours.     

Potentiometric microdetermination of nitro- and bromophenols by oxidation with lodate and periodate

Continuing the work started to study the redox reactions between iodate or periodate and monohydric phenols potentiometrically, standard solutions of oxidants are used for microdetermination of nitro- and bromophenols, by iodide reduction of unreacted iodate or periodate and back titrating excess iodide with Hg(II) using silver amalgam and SCE combination. The procedure is also evaluated in comparison with those mentioned in literature, indicating the reliability and high accuracy of the given procedure. The possible schemes for these redox reactions are suggested and compared with those mentioned in literature. These schemes also explain the formation of dimers as oxidation products at higher concentration of both reactants.     

Spectrophotometric determination of periodate or iodate ions by liquid-liquid extraction as an ion-pair using tetramethylammonium iodide

A simple and accurate extractive Spectrophotometric procedure was developed for the microdetermination of periodate and iodate in aqueous media. The determination of periodate was based upon the extraction of the ion-pair formed between the periodate and tetramethylammonium iodide at pH 4 in chloroform followed by direct Spectrophotometric measurements at 509, 358 and 288 nm. The optimum concentration range evaluated by Ringbom's plot, the molar absorptivity, the Sandell's sensitivity and the stoichiometry of the formed ion-pair were critically determined. Iodate could be determined quantitively by the proposed procedure after oxidation to periodate with potassium persulphate. The effect of the diverse ions on the determination of the periodate and/or iodate by the proposed procedures was also investigated. The application of the method for the analysis of iodate or periodate in the artificial fresh water was successfully carried out.     

Determination of iodate and sulphate in iodized common salt by ion chromatography with conductivity detection

A simple, rapid and accurate method for the determination of iodate in iodized common salts has been developed. The quantitative determination of iodate was accomplished by anion exchange chromatography with conductimetric detection. The method requires a sample pretreatment for the removal of large excess of chloride from the sample matrix. Onguard silver cartridges were found most suitable for this purpose. The sulphate content in the salt was simultaneously determined. The lower limits for the determination of iodate and sulphate in solution are 0.5 and 0.05 mug ml(-1), respectively. Quantitative recovery of the anions in synthetic samples has been obtained and the interferences from different cations and anions have been studied. The method has been successfully applied to the determination of iodate and sulphate in the commercially available salts. The concentrations of iodate measured by this method are in good agreement with those claimed by the manufacturer.     

The oxidation of iodide to iodate for the polarographic determination of total iodine in natural waters

A simple method has been designed for the oxidation of iodide to iodate in natural waters and subsequent determination of the iodate by differential pulse polarography. Iodide is oxidized to iodate with sodium hypochlorite and the excess of oxidizing agent is destroyed with sodium sulphite. The concentration of iodide is calculated as the difference between the concentrations of iodate in a sample before and after the oxidation.     

A simple colorimetric method for the assay of penicillin G salt and penicillamine by oxidation with iodate

A simple colorimetric procedure is described for determination of penicillin G salt and penicillamine. It is based on oxidation with potassium iodate at room temperature and measurement of the liberated iodine at 520 nm after extraction with carbon tetrachloride. Compared with other procedures, this method proved to be more rapid, highly reproducible and reasonably accurate. The relative standard deviation did not exceed 0.9% and 0.4% for penicillin and penicillamine, respectively. The procedure has been successfully applied to pharmaceutical preparations containing either of the two compounds.     

Reverse flow injection spectrophotometric determination of iodate and iodide in table salt

A very simple and sensitive reverse flow injection method is described for the determination of iodate and iodide. The iodate reacts with excess iodide in acidic medium to form tri-iodide, which can be spectrophotometrically monitored at 351 nm, and the absorbance is directly related to the concentration of iodate in the sample. The determination of iodide is based on oxidizing iodide to iodate. The calibration curve is linear in the range of 0.02-3.0 μg ml⁻¹ I with r²=0.9998, and the limit of detection is 0.008 μg ml⁻¹ I. The chemical and flow injection variables were studied and optimized to make the procedure suitable for quantitating iodate and iodide in table salts. It is shown that the reverse flow injection analysis could greatly improve the sensitivity and precision for determination of iodate with a relative standard deviation of 0.9%. A complete analysis, including sampling and washing, could be performed in 35 s. The procedure was applied successfully to the determination of iodate and iodide in table salts, and the results were statistically compared with results determined by standard iodometry method.     

Sequential flow injection determination of iodate and periodate with spectrophotometric detection

A flow injection (FI) system is described for the sequential determination of periodate and iodate based on their reaction with iodide at pH 3.5. Two sample plugs were injected into the same carrier stream sequentially. One injection is for the iodate determination and the other for the sum of iodate and periodate determination. For iodate determination, molybdate solution buffered at pH of 3.5 was used for selective masking of periodate. The influences of reagent concentrations were studied by a univariable method and the influence of FI manifolds was studied using univariable and simplex method. Periodate and iodate can be determined in the range of 0.050-5.0 and 0.050-10 microg/ml, respectively. The 3 sigma limit of detection was 0.030 and 0.050 microg/ml for periodate and iodate, respectively. The proposed method has been applied for the sequential determinations of periodate and iodate in water samples.     

Spectrophotometric determination of bromide (and iodide) in a flow system after oxidation by peroxodisulphate

The peroxodisulphate method for the determination of bromide has been modified. A flow-injection system for the spectrophotometric finish has been developed and the size of the ion-exchange column in the preconcentration step has been scaled down. The sum of bromate and iodate produced in the oxidation is determined by treating the oxidized sample with iodide in hydrochloric acid. The iodate is separately determined by applying the reaction in acetic acid. The working range of the spectrophotometric finish is 1-15muM and the limit of determination (10 sigma) is 0.7muM for iodate and for iodate plus bromate. The enrichment factor in the preconcentration step is 50, yielding a limit of determination of 15nM for bromide in natural waters. Eighteen samples of water from the Baltic, with salinity ranging from 3 to 33%. have been analysed. A Br/Cl ratio of (1.53 +/- 0.02) x 10(-3) was found. A comparative study of the original and the new preconcentration step has been made with three river waters, rich in humic substances. The results agreed within +/- 1. 5%.     

Oxidimetric determination of sulphides

Hypoiodite oxidises sulphides much more rapidly than does iodate under comparable experimental conditions. A procedure is reported for the determination of soluble and insoluble sulphides by oxidation to sulphate with hypoiodite. This is an improvement over Bethge's iodate method.     

A simplified method for the amplification of iodine

The sample solution is treated so that all iodine is present in the elemental state. This iodine is extracted into chloroform and thereby separated with very high selectivity from almost any matrix. Until now, in order to apply amplification via oxidation to iodate and reaction with iodide, a reextraction into a sodium hydroxide solution was necessary. In the new procedure the organic phase is shaken with bromine water. Thereby, the iodate formed moves completely into the water phase while the bromine accumulates in the chloroform. Remaining bromine in the water is destroyed with some formic acid. No buffer is needed, because the acid establishes the correct conditions for this reaction and also that between iodate and iodide. The iodine formed in sixfold amount can now be titrated visually or photometrically with thiosulfate or subjected to a second amplification cycle. The new procedure eliminates the reextraction, and the addition of some reagents especially sodium hydroxide which is the main contributor of extraneous iodine. Thus, the blank is reduced by a factor of 10 or more and is also more constant. Iodine at lower levels (< 1 μg/ml) can be determined and with higher reliability.     

Determination of the solubility product of Ba(IO(3))(2) by flow injection with amperometric detection

The solubility of Ba(IO(3))(2) has been determined in solutions containing a supporting electrolyte (KCl) to maintain the ionic strength in the 0-0.5 M range. The approach envisaged was based in the amperometric determination of iodine (or triiodide) generated in the reaction involving iodate in equilibrium in the saturated solutions and iodide contained in the solution carrier using a flow injection procedure, the electrode potential being maintained at +0.2 V in a wall-jet cell. The effect of the supporting electrolyte on the solubility of barium iodate was demonstrated and a good approximation of the value for the thermodynamic solubility constant of the precipitate was found by an appropriate correction of the solubility data using mean activity coefficients of barium iodate. The mean activity coefficients were estimated at each ionic strength by using the Debye-Hückel equation and the corrected solubility constant determined at 27 degrees C was found to be 2.7x10(-9) mol(3) l(-3).     

Simultaneous determination of iodide and iodate in seawater by transient isotachophoresis-capillary zone electrophoresis with artificial seawater as the background electrolyte

We developed capillary zone electrophoresis with transient isotachophoresis (ITP) as an on-line concentration procedure for simultaneous determination of iodide and iodate in seawater. The effective mobility of iodide was decreased by addition of 20 mM cetyltrimethylammonium chloride to an artificial seawater background electrolyte so that transient ITP functioned for both iodide and iodate. Limits of detection for iodide and iodate were 4.0 and 5.0 microg/l (as iodine) at a signal-to-noise ratio of 3. Values of the relative standard deviation of peak area, peak height, and migration times for iodide and iodate were 2.9, 1.3, 1.0 and 2.3, 2.1, 1.0%, respectively. The proposed method was applied to simultaneous determination of iodide and iodate in seawater collected at a pond at our university.     

Investigation of iodine liberation process in redox titration of potassium iodate with sodium thiosulfate

Potassium iodate is often used as a reference material to standardize a sodium thiosulfate solution which is a familiar titrant for redox titrations. In the standardization, iodine (triiodide) liberated by potassium iodate in an acidic potassium iodide solution is titrated with a sodium thiosulfate solution. The iodine liberation process is significantly affected by the amount of acid, that of potassium iodide added, the waiting time for the liberation, and light; therefore, the process plays a key role for the accuracy of the titration results. Constant-voltage biamperometry with a modified dual platinum-chip electrode was utilized to monitor the amount of liberated iodine under several liberation conditions. Coulometric titration was utilized to determine the concentration of a sodium thiosulfate solution on an absolute basis. Potassium iodate was assayed by gravimetric titration with the sodium thiosulfate solution under several iodine liberation conditions. The liberation process was discussed from the changes in the apparent assay of potassium iodate. The information of the appropriate titration procedure obtained in the present study is useful for any analysts utilizing potassium iodate to standardize a thiosulfate solution.     

Oszillopolarographischer nachweis und bestimmung von jodat und perjodat

The oscillopolarographic behaviour of periodate and iodate has been investigated in acid, neutral and alkaline electrolytes. The detection and determination of periodate in iodate may be carried out using iodate as electrolyte and the detection and determination of iodate in periodate may be carried out using 5 M sodium hydroxide as electrolyte.     

Decomposition of thiosulphate in acidified oxidants

The Wollack procedure has been used to study the competition between oxidant and acid for the thiosulphate ion. In concentrated hydrochloric acid solutions of iodine, iodate and chloramine-T, polythionates are the chief products, whereas in acid permanganate substantial amounts of other products occur. Both the concentration of the acid and the acid anion affect the course of the reaction in these solutions. Production of polythionate, an acid-decomposition product, suggests that acid decomposition successfully competes with oxidation.     

Determination of iodate and periodate at the microgram level by a kinetic method

A procedure is reported for the kinetic determination of iodate/periodate mixtures based on the reduction of these anions by the iron(II)/dipyridylglyoxal dithiosemicarbazone complex in an acidic medium. The reaction is monitored spectrophotometrically at 410 nm (absorption maximum of the iron(III) complex formed). Mixtures of these anions at μg ml^?1 levels for iodate/periodate ratios from 5:1 to 1:4 can be determined with a r.s.d, of ca. 3%. Molybdate is used to mask periodate to allow iodate to be determined alone. The sum of both anions is obtained in the absence of molybdate. Chromate, hypochlorite and hexacyanoferrate(III) interfere seriously.     

Determination of total inorganic iodine in seawater by cathodic stripping square wave voltammetry

A method has been designed for the reduction of iodate to iodide in seawater and subsequent determination of total dissolved iodine as iodide by cathodic stripping square wave voltammetry. The pH of the sample is lowered to about 1-2 and iodate is reduced to iodide with sodium sulfite under this acidic condition. The pH of the sample is then raised to 8-9 before the concentration of iodide is measured.