Iodate primary standard

Method A With arsenic(III) oxide. This procedure, which utilises arsenic(III) oxide as a primary standard and potassium iodide or potassium iodate as a catalyst for the reaction, is convenient in practice and is a trustworthy method for the standardisation of permanganate solutions. Analytical grade arsenic(III) oxide has a purity of at least 99.8 per cent, and the results by this method agree to within 1 part in 3000 with the sodium oxalate procedure (Method B, below). 

The standardisation of thiosulphate solutions may be effected with potassium iodate, potassium dichromate, copper and iodine as primary standards, or with potassium permanganate or cerium)IV) sulphate as secondary standards. Owing to the volatility of iodine and the difficulty of preparation of perfectly pure iodine, this method is not a suitable one for beginners. If, however, a standard solution of iodine (see Sections 10.112 and 10.113) is available, this maybe used for the standardisation of thiosulphate solutions. 

Apparent indicator constant 264, 267 Apparent stability constant 59 Aqua regia 111 Arc alternating current, 764 direct current, 763, 771 sensitivities of elements, (T), 766 Aromatic hydrocarbons analysis of binary mixtures, 715 Arsenates, D. of (ti) 357 Arsenic, D. of as silver arsenate, (ti) 357 as trisulphide, (g) 448 by iodine, (am) 634, (ti) 397 by molybdenum blue method, (s) 681 by potassium bromate, (ti) 406 by potassium iodate, (ti) 401 in presence of antimony, (s) 724 Arsenic(III) oxide as primary standard, 261... 

Potassium iodate (KI03) for primary standard Potassium iodide (KI), iodate free, ACS grade 6.0 M HC1 (APPENDIX 2A)... 

Potassium iodate (KI03), potassium hydrogen iodate (KHI03), potassium dichromate (K2Cr207), and potassium ferricyanide (K3Fe(CN)6) are some of the primary standards commonly used to standardize sodium thiosulfate titrant. 

An accurately weighted amount of primary standard is dissolved in water containing an excess of potassium iodide. Upon acidification, stoichiometric amounts of iodine are liberated instantly, which are titrated with thiosulfate titrant of unknown strength, decolorizing the blue starch-iodine complex at the end point. With potassium iodate, the ionic reaction is as follows ... 

Potassium Iodate Solution (0.1000 N) Dry 4 or more grams of primary standard-grade potassium iodate (KI03) at 110° 5° for 2 h, and cool to room temperature in a desiccator. 

Potassium Iodate, 0.05 M (10.70 g KI03 per 1000 mL) Dissolve 10.700 g of potassium iodate of primary standard quality (KI03), previously dried at 110° to constant weight, in sufficient water to make 1000.0 mL. 

Although sodium oxalate is commonly used as a primary standard, its use is recommended only when oxalate is to be determined. For best absolute accuracy As(III) oxide is recommended. The direct titration of As(III) in acid solution does not proceed readily without a catalyst, probably because of the stabilization of Mn(III) by complex formation with As(V). With potassium iodate (1 drop of0.0025 M solution) as catalyst, the potentiometric end point was found to coincide with the visual end point, using ferroin, to within 0.01%, and the accuracy, tested against pure potassium iodide, was within 0.02%. 

Other suitable primary standards for thiosulfate include potassium iodate (Section 20-1), potassium bromate (Section 20-2), potassium ferricyanide, metallic copper, potassium acid iodate, and iodine. 

Potassium hydrogen iodate, KH(I03)2, is an excellent primary standard with a high molecular mass pier mole of protons. It is also a strong acid that can be titrated using virtually any indicator with a transition range between a pH of 4 and 10. 

Potassium iodate is an excellent primary standard for thiosulfate solutions. In this application, weighed amounts of primaiy-standard grade reagent are dissolved in water containing an excess of potassium iodide. When this mixture is acidified with a strong acid, the reaction... 

Other primary standards for sodium thiosulfate are potassium dichromate, potassium bromate, potassium hydrogen iodate, potassium hexacyanoferrate(III), and metallic copper. All these compounds liberate stoichiometric amounts of iodine when treated with excess potassium iodide. 

The oxidizing power (percent NaOCl or H2O2) of the solution is determined iodo-metrically by reacting it with an excess of iodide in acetic acid solution and titrating the iodine produced (13 in the presence of excess iodide) with standard sodium thiosulfate solution. The sodium thiosulfate is standardized against primary standard potassium iodate, and a starch indicator is used. 

S,Oj -(aq) -t- Ij(aq) 21 (aq) + S40t (aq) Sodium thiosulphate, NajSjOj SH]0, is not used as a primary standard as the water concent of the c stals is variable. A solution of sodium thiosulphate can be standardised against a solution of iodine, or a solution of potassium iodate(V) or potassium dichromate or potassium mangana (VlI). 

The lodlmetrlc method was, itself, calibrated by quantitative iodine liberation using excess iodide and standard lodate solution, prepared using dried iodate as a primary standard. Ozone in the gas phase was measured by UV absorbance, calibrated against the wet methods by absorbing the gas in reagent solution contained in the reactor. 

Arsenic(III) oxide (AS2O3) is available in pure form and is a useful (and poisonous) primary standard for many oxidizing agents, such as Mn04. AS2O3 is first dissolved in base and titrated with MnOJ in acidic solution. A small amount of iodide (F) or iodate (10J) is used to catalyze the reaction between H3ASO3 and MnO. The reactions are... 

Potassium iodate is used as a primary standard for thiosulfate solutions in quantitative analysis. Iodate reacts quantitatively with iodide to produce iodine, as shown in Equation (18.43) ... 

Halates are readily prepared by the base hydrolysis of the halogen. Bromates and iodates can also be synthesized by the oxidation of the halide. Only iodic acid can be isolated outside of aqueous solution. Chlorates are excellent oxidizing agents, lodate occurs naturally, is a source of elemental iodine, and is a primary standard for iodimetry. The perhalic acids of chlorine and iodine are much easier to prepare than that of bromine. Perchloric acid, like sulfuric and phosphoric acid in previous groups, has a tetrahedral structure around the central atom and involves dir-pir... 

As iodine and tri-iodide ions solutions are not primary standard solutions, the standardization of thiosulfate solutions may be achieved with the help of iodine solutions extemporaneously prepared from primary standards. Iodine is usually prepared by the iodate/iodide reaction or by the oxidation of iodides with potassium dichromate, but other possibilities exist. 

During this reaction, the oxidation state of iodate ions goes from +V to 0. It is a process sometimes called aftve-electron iodatometry (see the next chapter). In order to standardize thiosulfate solutions and since potassium iodate is a primary standard, its concentration must be the limiting factor of the reaction. This means that there must be an excess of iodide ions and of protons in comparison to iodate ions, when the reaction stoichiometry is taken into account. In other words, for exactly one mole of potassium iodate weighed, we must add more than five moles of iodide ions and more than six moles of protons. It is not necessary to know their exact numbers provided they obey the above conditions. In these conditions, exactly three moles of iodine are prepared from one mole of iodate. Figure 18.4 summarizes these considerations. 

The term iodatometry groups titration methods involving potassium iodate as the oxidant. Potassium iodate is available in at least a 99.9% state of purity. Before use in iodatometry, it must be dried at 120 °C. The anhydrous salt does not tend to absorb the ambient moisture. It is a good primary standard. However, because of the high... 

Since purest potassium iodate can be used as a primary standard, its solution can be used... 

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