Determination of chlorates

Discussion. Chlorate ion is reduced by warming with excess of iron(II) in the presence of a relatively high concentration of sulphuric acid  

The excess Fe2+ ion is determined by titration with standard dichromate solution in the usual way. 

Procedure. To obtain experience in the method, the purity of analytical-grade potassium chlorate may be determined. Prepare a 0.02M potassium chlorate solution. Into a 250 mL conical flask, place 25.0 mL of the potassium chlorate solution, 25.0mL of 0.2M ammonium iron(II) sulphate solution in 2M sulphuric acid and add cautiously 12 mL concentrated sulphuric acid. Heat the mixture to boiling (in order to ensure completion of the reduction), and cool to room temperature by placing the flask in running tap water. Add 20 mL 1 1 water/phosphoric(V) acid, followed by 0.5 mL sodium diphenyl-amine-sulphonate indicator. Titrate the excess Fe2+ ion with standard 0.02M potassium dichromate to a first tinge of purple coloration which remains on stirring. 

Standardise the ammonium iron(II) sulphate solution by repeating the procedure but using 25 mL distilled water in place of the chlorate solution. The difference in titres is equivalent to the amount of potassium chlorate added. 

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Determination of chlorate as silver chloride Discussion. The chlorate is reduced to chloride, and the latter is determined as silver chloride, AgCl. The reduction may be performed with iron(II) sulphate solution, sulphur dioxide, or by zinc powder and acetic (ethanoic) acid. Alkali chlorates may be quantitatively converted into chlorides by three evaporations with concentrated hydrochloric acid, or by evaporation with three times the weight of ammonium chloride. 

Kim J, Marshall MR, Du WX, et al. 1999. Determination of chlorate and chlorite and mutagenicity of seafood treated with aqueous chlorine dioxide. J Agric Food Chem 47 3586-3591. 

A method for the determination of chlorate in water extracts of soil is based on its conversion to free chlorine upon reaction with hydrochloric acid, followed by spectrophotometric evaluation of chlorine at 448 nm by the spectrophoto-metric o-toluidine method [9]. A correction is made for interference by iron (III), nitrite, free chloride derived from hypochlorites and strong oxidising agents by subtracting the absorbance of a modified blank, containing a lower concentration of hydrochloric acid, from that obtained in the test. 

International Standard Organization. 1997. Water quality. Determination of dissolved anions by liquid chromatography of ions. Part 4 Determination of chlorate, chloride and chlorite in water with low contamination. ISO 10304-4. International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland. 

As discussed in Sections 2.2 and 2.3, any FIA readout is the result of two simultaneously occurring kinetic processes, that is, that of the physical dilution process and that of chemical reactions. Provided that the reaction rates of the chemical processes taking place vary significantly, these might be exploited for the purpose of kinetic discrimination. One such example is the determination of chlorate by reduction with tita-nium(III) in the presence of leukomethylene blue (LMB), according to the following reaction scheme [986] ... 

ISO 10304-4, Water Quality—Determinatwn of Dissolved Anions by Liquid Chrorruttography of Ions—Part 4 Determination of Chlorate, Chloride, and Chlorite in Water with Low Contamination. 1997. 

Methods for the determination of chlorates invariably rely on their strong oxidising power with simultaneous reduction to chloride. 

A formalin or nitrite reduction method may be used with advantage for the determination of chlorate in this lozenge. Standard mixtures of potassium chlorate with sugar and acacia have given theoretical yields of potassium chlorate by the following method ... 

Thermal decomposition of perchlorate salts to chloride, followed by the gravimetric determination of the resulting chloride, is a standard method of determining quantitatively the concentration of perchlorates. Any chlorates that are present in the original sample also break down to chloride. Thus results are adjusted to eliminate errors introduced by the presence of any chlorides and chlorates in the original sample. 

Fluoride ion, and weak acids and bases do not interfere, but nitrate, nitrite, perchlorate, thiocyanate, chromate, chlorate, iodide, and bromide do. Since analysis of almost all boron-containing compounds requires a preliminary treatment which ultimately results in an aqueous boric acid sample, this procedure may be regarded as a gravimetric determination of boron. 

Discussion. These anions are both determined as silver bromide, AgBr, by precipitation with silver nitrate solution in the presence of dilute nitric acid. With the bromate, initial reduction to the bromide is achieved by the procedures described for the chlorate (Section 11.56) and the iodate (Section 11.63). Silver bromide is less soluble in water than is the chloride. The solubility of the former is 0.11 mg L 1 at 21 °C as compared with 1.54 mg L 1 for the latter hence the procedure for the determination of bromide is practically the same as that for chloride. Protection from light is even more essential with the bromide than with the chloride because of its greater sensitivity (see Section 11.57). 

Determination of nitrate as nitron nitrate Discussion. The mono-acid base nitron, C20H16N4, forms a fairly insoluble crystalline nitrate, C20H 16N4,HN03 (solubility is 0.099 g L 1 at about 20 °C), which can be used for the quantitative determination of nitrates [see Section 11.11(E)]. The sulphate and acetate are soluble so that precipitation may be made in sulphuric or acetic (ethanoic) acid solution. Perchlorates (0.08 g), iodides (0.17 g), thiocyanates (0.4 g), chromates (0.6 g), chlorates (1.2g), nitrites (1.9 g), bromides (6.1 g), hexacyanoferrate(II), hexacyanoferrate(III), oxalates, and considerable quantities of chlorides interfere, and should be absent. The figures in parentheses are the approximate solubilities of the nitron salts in g L-1 at about 20 °C. 

The kinetics of the reduction of chlorate ion by Ir(III) have been determined by controlled-potential electrolysis to be ... 

Zhan and Mao [60] used a simple, fast, and selective alternating current oscilloscop-ic polarographic titration method for the determination of primaquine and other alkaloid phosphate in pharmaceutical preparation. The titration was carried out with a standard lead solution in hexamethylene tetramine buffer containing 1 M sodium chlorate (pH 5.5). The results obtained by this method are comparable to those obtained by pharmacopoeial method. 

For the determination of halogen in liquids weigh the substance in the manner described for C and H determination and introduce the weighing tube into the combustion tube in such a way that the former rests about 8-10 cm. beyond the front end of the long wire gauze roll. For liquids which burn with very great difficulty substitute ammonium nitrate for the potassium chlorate. 

Example Estimation of chlorate —an oxidant by iodometric determination. In this particular instance two things may happen, namely ... 

Barnett DA, Guevremont R, Purves RW (1999) Determination of parts-per-trillion levels of chlorate, bromate, and iodate by electrospray ionization/high-field asymmetric waveform ion mobility spectrometry/mass spectrometry. Appl Spectrosc 53(11) 1367-1374... 

Beitler MK, Chin HB. 1995. Improved determination of chlorite and chlorate in rinse water from carrots and green beans by liquid chromatography and amperometric and conductivity detection. J AO AC Int 78(3) 878-883. 

Dietrich AM, Ledder TD, Gallagher DL, et al. 1992. Determination of chlorite and chlorate in chlorinated and chloraminated drinking water by flow injection analysis and ion chromatography. Anal Chem 64 498-502. 

One last point. In the reaction of uranium(IV) where it is convenient to do a tracer experiment because there is only one metal ion product, we have actually determined the number of oxygens transferred to the uranyl ion product from the chlorite, and this number corresponds to 1.3 oxygen per chlorite transferred to the uranium. This is consistent with the results we reported some years ago (5) on the oxidation of uranium (IV) with Pb02 and Mn02, where indeed more than one oxygen is transferred. In conclusion, we feel that we have some direct evidence for two-electron transfer in these reactions and the formation of a chlorine(I) intermediate followed by the formation of chlorate. 

The first determinations of the at. wt. of chlorine were obtained by the analysis of the chlorates later determinations have been directed towards the quantitative synthesis of hydrogen chloride, or to the use of physical methods—mol. vol., limiting densities, and critical data. General calculations have been made by F. W. Clarke, B. Brauner, and others.1... 

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