Iodate standard

Schnepfe [83] has described yet another procedure for the determination of iodate and total iodine in seawater. To determine total iodine 1 ml of 1% aqueous sulfamic acid is added to 10 ml seawater which, if necessary, is filtered and then adjusted to a pH of less than 2.0. After 15 min, 1 ml sodium hydroxide (0.1 M) and 0.5 ml potassium permanganate (0.1M) are added and the mixture heated on a steam bath for one hour. The cooled solution is filtered and the residue washed. The filtrate and washings are diluted to 16 ml and 1ml of a phosphate solution (0.25 M) added (containing 0.3 xg iodine as iodate per ml) at 0 °C. Then 0.7 ml ferrous chloride (0.1 M) in 0.2% v/v sulfuric acid, 5 ml aqueous sulfuric acid (10%) - phosphoric acid (1 1) are added at 0 °C followed by 2 ml starch-cadmium iodide reagent. The solution is diluted to 25 ml and after 10-15 min the extinction of the starch-iodine complex is measured in a -5 cm cell. To determine iodate the same procedure is followed as is described previously except that the oxidation stage with sodium hydroxide - potassium permanganate is omitted and only 0.2 ml ferrous chloride solution is added. A potassium iodate standard was used in both methods. 

Place 1 mL of HC1 in an Erlenmeyer flask. Add 0.1 g sulfamic acid. Add 100 mL sample. Add 1 mL of starch indicator solution (or 0.1 g solid). Titrate with potassium iodide-iodate standard solution until a faint blue color develops. Run a blank using distilled water instead of sample. 

There is, therefore, a large and unaccountable difference between the absolute values of Paley et al. and other groups for the 0.01% solution. If these values were consistent with each other it would demonstrate the equivalence of the quartz and lithium iodate standards but the large discrepancy leaves the issue in doubt. In the second set of adjusted results in column B for Paley et al., the values have been recalibrated using 0.5625 x 10 esu as the reference for the 1% MNA solution. In the two cases the results of Paley et al. are, respectively higher and lower than the other sets of results which are in much better mutual agreement. 

Then 10.00 mL of the iodate standard solution are added with a caUbrated pipette or a precision piston burette. The bottle is filled to about the sample volume (to the neck) with pure water. 

Hien add l.OOmL of the iodate standard solution, fill up to just below the bottle neck (about 5 mL less) and titrate with the thiosulphate solution until the exact equivalence point. 

Add another 1.00 ml of the iodate standard and titrate again until the equivalence point. The reagent blank is the difference between the first and the second thiosulphate titration volume, i.e.,... 

VsTD. b- are volumes in mL. of the iodate standard, sample (bottle) and fixing reagents (manganese(/7) chloride plus alkaline iodide), respectively ... 

The concentration (C) of the thiosulphate is determined from the titration with iodate standard (Section 4.6.1) and calculated as... 

It is extremely difficult to estimate exactly the accuracy of the determination since the major contribution to the systematic error probably has its source in the sampling procedure itself (see Chapter 1). When attention is paid to all the sources of systematic errors (see Section 4.3), most of which result in an increased oxygen content, a field precision of 0.005 mL/L can be achieved using 100 mL samples and photometric endpoint detection and 0.03 mL with 50 mL samples and visual (starch) endpoint detection. The precision is about 25 % less for oxygen contents below 2 mL/L. If a good quality iodate standard is used for calibration the analytical accuracy is equal to the precision. 

About 50 mL of distilled water are filled into a 100 mL titration beaker 5mL of the sulphuric acid, lO.OOmL of the 1.667mmol/L iodate standard and l.OmL of the potassium iodide solution are added. The liberated iodine is titrated to a light yellow. One ml of starch indicator solution is added, and the titration is carried to the colourless endpoint. At the approach of the endpoint the solution becomes cloudy directly after the addition of an aliquot of the thiosulphate solution. The endpoint is reached when the cloudiness can no longer be observed. Diffuse illumination from below or moderate illumination facilitate detection of the endpoint. 

From the supernatant solution, 50 mL (V in Section 6.2.5) are pipetted carefully into the titration beaker. Care must be taken to avoid stirring up the precipitated zinc sulphide. Then 1 mL of the iodide solution and exactly 10 mL of the iodate standard solution are pipetted into the titration beaker followed by 5 mL of the diluted sulphuric add. The sample is shaken gently and the titration beaker is covered with a watch glass or plastic film and set aside for approximately 5 min. The surplus of iodine not consumed by the thiosulphate initially present in the sample is then titrated with the stardardized thiosulphate solution. The endpoint is determined as described in Section 6.2.4.L... 

A 10 mL volume of iodate standard solution corresponds to 5 mL of precisely 0.02 mol/L thiosulphate solution. If the volume of the thiosulphate solution consumed for its standardization is VmL, the factor/, with which this volume must be multiphed by in order to obtain the corresponding volume of precisely 0.02 mol/L thiosulphate solution, is /= 5/V. The volume of thiosulphate solution used for the sample titration is corrected as follows ... 

Generally the solubility of a given metal halate decreases from chlorate(V) to iodatef and many heavy metal iodates(V) are quantitatively insoluble. Like their parent acids, the halates(V) are strong oxidising agents, especially in acid solution their standard electrode potentials are given below (in volts) ... 

Iodine, Q.IN (0 to 1 —). Dissolve 12.690 g of resublimed iodine in 25 mL of a solution containing 15 g of KI which is free from iodate. After all the solid has dissolved, dilute to 1 L. If desired, check against a standard arsenite or standard thiosulfate solution. 

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. 

Ferrous Sulfdte Titration. For deterrnination of nitric acid in mixed acid or for nitrates that are free from interferences, ferrous sulfate titration, the nitrometer method, and Devarda s method give excellent results. The deterrnination of nitric acid and nitrates in mixed acid is based on the oxidation of ferrous sulfate [7720-78-7] by nitric acid and may be subject to interference by other materials that reduce nitric acid or oxidize ferrous sulfate. Small amounts of sodium chloride, potassium bromide, or potassium iodide may be tolerated without serious interference, as can nitrous acid up to 50% of the total amount of nitric acid present. Strong oxidizing agents, eg, chlorates, iodates, and bromates, interfere by oxidizing the standardized ferrous sulfate. 

Silver compounds, available from commercial suppHers, are expensive. Reagent grades of sHver(I) carbonate, cyanide, diethjldithiocarbamate, iodate, nitrate, oxide, phosphate, and sulfate are available. Standardized solutions of silver nitrate are also available for analytical uses. Purified grades of sHver(I) acetate, bromide, cyanide, and iodide can be purchased silver nitrate is also made as a USP XX grade for medicinal uses (6). 

Analytical Methods. A classical and stiU widely employed analytical method is iodimetric titration. This is suitable for determination of sodium sulfite, for example, in boiler water. Standard potassium iodate—potassium iodide solution is commonly used as the titrant with a starch or starch-substitute indicator. Sodium bisulfite occurring as an impurity in sodium sulfite can be determined by addition of hydrogen peroxide to oxidize the bisulfite to bisulfate, followed by titration with standard sodium hydroxide (279). 

Tin ores and concentrates can be brought into solution by fusing at red heat in a nickel cmcible with sodium carbonate and sodium peroxide, leaching in water, acidifying with hydrochloric acid, and digesting with nickel sheet. The solution is cooled in carbon dioxide, and titrated with a standard potassium iodate—iodide solution using starch as an indicator. 

Potassium iodate [7758-05-6] M 214.0, pK 0.80 (for HIO3). Crystd twice from distilled water (3mL/g) between 100° and 0°, dried for 2h at 140° and cooled in a desiccator. Analytical reagent grade material dried in this way is suitable for use as an analytical standard. 

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

In the second method a solution of the approximate strength required is prepared, and this is standardised against some standard alkaline substance, such as sodium tetraborate or anhydrous sodium carbonate standard potassium iodate or pure silver may also be used (see Section 10.84). If a solution of an exact strength is required, a solution of an approximate strength somewhat greater than that desired is first prepared this is suitably diluted with water after standardisation (for a typical calculation, see Appendix 17). 

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. 

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). 

It seems appropriate to refer at this point to the uses of a standard solution containing potassium iodide and potassium iodate. This solution is quite stable and yields iodine when treated with acid ... 

The standard solution is prepared by dissolving a weighed amount of pure potassium iodate in a solution containing a slight excess of pure potassium iodide, and diluting to a definite volume. This solution has two important uses. The first is as a source of a known quantity of iodine in titrations [compare Section 10.115(A)] it must be added to a solution containing strong acid it cannot be employed in a medium which is neutral or possesses a low acidity. 

For the preparation of standard iodine solutions, resublimed iodine and iodate-free potassium iodide should be employed. The solution may be standardised against pure arsenic(III) oxide or with a sodium thiosulphate solution which has been recently standardised against potassium iodate. 

B) With standard sodium thiosulphate solution. Sodium thiosulphate solution, which has been recently standardised, preferably against pure potassium iodate, is employed. Transfer 25 mL of the iodine solution to a 250 mL conical flask, dilute to 100 mL and add the standard thiosulphate solution from a burette until the solution has a pale yellow colour. Add 2 mL of starch solution, and continue the addition of the thiosulphate solution slowly until the solution is just colourless. 

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. 

Procedure, (a) Place 25 mL of the chlorate solution (approx. 0.02M) in a glass-stoppered conical flask and add 3 mL of concentrated hydrochloric acid followed by two portions of about 0.3 g each of pure sodium hydrogencarbonate to remove air. Add immediately about 1.0 g of iodate-free potassium iodide and 22 mL of concentrated hydrochloric acid. Stopper the flask, shake the contents, and allow it to stand for 5-10 minutes. Titrate the solution with standard 0.1M sodium thiosulphate in the usual manner. 

Procedure (iodometric method). Weigh out accurately about 5.0 g of the bleaching powder into a clean glass mortar. Add a little water, and rub the mixture to a smooth paste. Add a little more water, triturate with the pestle, allow the mixture to settle, and pour off the milky liquid into a 500 mL graduated flask. Grind the residue with a little more water, and repeat the operation until the whole of the sample has been transferred to the flask either in solution or in a state of very fine suspension, and the mortar washed quite clean. The flask is then filled to the mark with distilled water, well shaken, and 50.0 mL of the turbid liquid immediately withdrawn with a pipette. This is transferred to a 250 mL conical flask, 25 mL of water added, followed by 2 g of iodate-free potassium iodide (or 20 mL of a 10 per cent solution) and 10 mL of glacial acetic acid. Titrate the liberated iodine with standard 0.1M sodium thiosulphate. 

Discussion. The mercury is precipitated as mercury(I) chloride and the latter is reacted with standard potassium iodate solution ... 

Procedure. Weigh out accurately about 2.5 g of finely powdered mercury(II) chloride, and dissolve it in 100 mL of water in a graduated flask. Shake well. Transfer 25.0 mL of the solution to a conical flask, add 25 mL water, 2mL 1M hydrochloric acid, and excess of 50 per cent phosphorous(III) acid solution. Stir thoroughly and allow to stand for 12 hours or more. Filter the precipitated mercury(I) chloride through a quantitative filter paper and wash the precipitate moderately with cold water. Transfer the precipitate with the filter paper quantitatively to a 250 mL reagent bottle, add 30 mL concentrated hydrochloric acid, 20 mL water, and 5 mL carbon tetrachloride or chloroform. Titrate the mixture with standard 0.025M potassium iodate in the usual manner (Section 11.127). 

If the bulk of the iodate solution is added rapidly, atmospheric oxidation does not present a serious problem, but the method cannot be used in the presence of salts of antimony(III), copper(I), or iron(II). The solution, which should contain for example 0.15 g SnCl2,2H20 in 25 mL, is treated with 30mL of concentrated hydrochloric acid and 20 mL of water and is then titrated in the usual manner with standard potassium iodate solution. 

The liberated iodine and the excess of iodide is determined by titration with standard potassium iodate solution the hydrochloric acid concentration must not be allowed to fall below 7JVf in order to prevent re-oxidation of the vanadium compound by iodine chloride. 

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... 

---