Potassium iodate titration

USP XIX directs that hydralazine be determined in the raw material, tablets, and injections by potassium iodate titration in strongly acid solution, using chloroform to detect the presence of iodine (19). 

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. 

The determination of tin in metals containing over 75 wt % tin (eg, ingot tin) requites a special procedure (17). A 5-g sample is dissolved in hydrochloric acid, reduced with nickel, and cooled in CO2. A calculated weight of pure potassium iodate (dried at 100°C) and an excess of potassium iodide (1 3) are dissolved in water and added to the reduced solution to oxidize 96—98 wt % of the stannous chloride present. The reaction is completed by titration with 0.1 Af KIO —KI solution to a blue color using starch as the indicator. 

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. 

Weigh out accurately 0.14-0.15 g of pure dry potassium iodate, dissolve it in 25 mL of cold, boiled-out distilled water, add 2 g of iodate-free potassium iodide (Note 1) and 5 mL of 1M sulphuric acid (Note 2). Titrate the liberated iodine with the thiosulphate solution with constant shaking. When the colour of the liquid has become a pale yellow, dilute to ca 200 mL with distilled water, add 2 mL of starch solution, and continue the titration until the colour changes from blue to colourless. Repeat with two other similar portions of potassium iodate. 

Only a small amount of potassium iodate is needed so that the error in weighing 0.14-0.15 g may be appreciable. In this case it is better to weigh out accurately 4.28 g of the salt (if a slightly different weight is used, the exact molarity is calculated), dissolve it in water, and make up to 1 L in a graduated flask. Twenty-five millilitres of this solution are treated with excess of pure potassium iodide (I g of the solid or 10 mL of 10 per cent solution), followed by 3 mL of IM sulphuric acid, and the liberated iodine is titrated as detailed above. 

Alternatively, in this and all subsequent titrations with 0.025M potassium iodate, a 250 mL conical flask may be used and the carbon tetrachloride or chloroform indicator replaced by 0.5 mL amaranth or xylidine ponceau indicator, which is added after most of the iodine colour has disappeared from the reaction mixture (see Section 10.125). 

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). 

Copper(II) compounds. Many other metallic ions which are capable of undergoing oxidation by potassium iodate can also be determined. Thus, for example, copper(II) compounds can be analysed by precipitation of copper)I) thiocyanate which is titrated with potassium iodate ... 

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. 

Soluble sulphides. Hydrogen sulphide and soluble sulphides can also be determined by oxidation with potassium iodate in an alkaline medium. Mix 10.0 mL of the sulphide solution containing about 2.5 mg sulphide with 15.0 mL 0.025M potassium iodate (Section 10.126) and 10 mL of 10M sodium hydroxide. Boil gently for 10 minutes, cool, add 5 mL of 5 per cent potassium iodide solution and 20 mL of 4M sulphuric acid. Titrate the liberated iodine, which is equivalent... 

In the sodium borate solution containing bromide, when the pH 4 buffer is added before the potassium iodate solution, titrations give low total residual chlorine concentrations. This loss increases with the amount of stirring time between the addition of the reagents. Even for a stirring time of 10 seconds, there is a loss of about 17% of the total residual chlorine. If the solution were stirred for 30 min, 85% of the chlorine would have disappeared. The concentration of total residual chlorine determined by the reference methods does not change throughout the experiment. This implies that this loss of chlorine does not occur in the reaction vessel, but in the titration cell as a result of the analytical procedure. 

Titrations with Potassium Bromate, and (iii) Titrations with Potassium Iodate. 

Potassium iodate is a fairly strong oxidizing agent that may be used in the assay of a number of pharmaceutical substances, for instance benzalkonium chloride, cetrimide, hydralazine hydrochloride, potassium iodide, phenylhydrazine hydrochloride, semicarbazide hydrochloride and the like. Under appropriate experimental parameters the iodate reacts quantitatively with both iodides and iodine. It is, however, interesting to observe here that the iodate titrations may be carried out effectively in the presence of saturated organic acids, alcohol and a host of other organic substances. 

Procedure Weigh accurately benzalkonium chloride 4.0 g and dissolve it in sufficient DW to make 100 ml. Pipette 25.0 ml into a separating funnel, add 25 ml of chloroform, 10 ml of 0.1 N NaOH and 10 ml of potassium iodide solution. Shake the contents thoroughly, allow to separate and collect the chloroform layer in another separating funnel. Treat the aqueous layer with 3 further quantities each of 10 ml of chloroform and discard the chloroform layer. To the aqueous layer add 40 ml of hydrochloric acid, cool and titrate with 0.05 M potassium iodate till the solution becomes pale brown in colour. Add 2 ml of chloroform and continue the titration until the chlorofonn layer becomes colourless. Titrate a mixture of 29 ml of water, 10 ml of KI solution and 40 ml of hydrochloric acid with 0.05 M potassium iodate under identical conditions (Blank Titration). The differences between the titrations represent the amount of 0.05 M potassium iodate required. Each ml of 0.05 M potassium iodate is equivalent to 0.0354 g of C H CIN. 

Procedure Weigh accurately 0.5 g of potassium iodide and dissolve it in about 10 ml of DW. Add to it 35 ml of hydrochloric acid and 5 ml of chloroform. Titrate with 0.05 M potassium iodate till the purple colour of iodine disappears from the chloroform layer. Add the last portion of the iodate solution carefully and dropwise while shaking the contents of the flask vigorously and continuously. Allow to stand for 5 minutes. In case any colour still develops in the chloroform layer continue the titration. Each ml of 0.05 M potassium iodate is equivalent to 0.0166 g of potassium iodide. 

A host of other pharmaceutical substances, namely cetrimide, hydralazine hydrochloride, phenylhydrazine hydrochloride may be assayed by titration with potassium iodate as mentioned in Table 13.3. 

The bomb method for sulfur determination (ASTM D129) uses sample combustion in oxygen and conversion of the sulfur to barium sulfate, which is determined by mass. This method is suitable for samples containing 0.1 to 5.0% w/w sulfur and can be used for most low-volatility petroleum products. Elements that produce residues insoluble in hydrochloric acid interfere with this method this includes aluminum, calcium, iron, lead, and silicon, plus minerals such as asbestos, mica, and silica, and an alternative method (ASTM D1552) is preferred. This method describes three procedures the sample is first pyrolyzed in either an induction furnace or a resistance furnace the sulfur is then converted to sulfur dioxide, and the sulfur dioxide is either titrated with potassium iodate-starch reagent or is analyzed by infrared spectroscopy. This method is generally suitable for samples containing from 0.06 to 8.0% w/w sulfur that distill at temperatures above 177°C (351°F). 

Chlorine gas may be identified readdy by its distinctive color and odor. Its odor is perceptible at 3 ppm concentration in air. Chlorine may be measured in water at low ppm by various titrimetry or colorimetric techniques (APHA, AWWA and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington DC American Pubhc Health Association). In iodometric titrations aqueous samples are acidified with acetic acid followed by addition of potassium iodide. Dissolved chlorine liberates iodine which is titrated with a standard solution of sodium thiosulfate using starch indicator. At the endpoint of titration, the blue color of the starch solution disappears. Alternatively, a standardized solution of a reducing agent, such as thiosulfate or phenylarsine oxide, is added in excess to chlorinated water and the unreacted reductant is then back titrated against a standard solution of iodine or potassium iodate. In amperometric titration, which has a lower detection limit, the free chlorine is titrated against phenyl arsine oxide at a pH between 6.5 and 7.5. 

Potassium iodate is an oxiding agent in volumetric analysis. It releases iodine in KIO3-KI solutions for iodometric titrations. It also is a topical antiseptic and an additive to food to provide nutrient iodine. 

Elemental composition Concentration of sodium thiosulfate in aqueous solution can he measured hy titration with a standard solution of potassium iodate, potassium hiiodate, or potassium dichromate using starch indicator. The oxidant is added to an acidified solution of excess potassium iodide before titrating with the thiosulfate solution. 

This procedure is more widely used in pharmacopoeial assays than the dye extraction procedure. Excess potassium iodide is added to an aqueous solution of the analyte, which is a lipophilic cation. A lipophilic ion pair is formed between the cation and the iodide ion and is then removed by extraction into an organic phase such as chloroform. The excess iodide remaining in the aqueous phase is then titrated in concentrated HCl (> 4 M) with potassium iodate. The iodate oxidises iodide to E, which immediately reacts with Cl to give ICl resulting in the following equation ... 

For the use of the iodometric method in the analysis of mixtures containing sulphide, sulphite and thiosulphate, see Kurtenacker and others, Zeitsch. an.org. Chem., 1924, 141, 297 1927, 161, 201 and for mixtures of sulphide, polysulphide and thiosulphate, see Sohulek, Zeitsch. anal. Chem., 1925, 65, 352. For titration methods using potassium iodate, see Jamieson, Amer. J. Sci., 1915, 39, 639 also Ivanofi, J. Buss. Phys. Chem. Soc., 1914, 46, 419 Dimitrow, Zeitsch. anorg. Chem., 1924, 136, 189. 

Dissolve 0.10 to 0.15 gm. of dried potassium iodate in 20 cc. of water, add 3 gm. of potassium iodide and 5 cc. of hydrochloric acid. Dilute with 50 cc. of water, and titrate the liberated iodine with decinormal sodium thiosulphate. 

Quadrivalent cerium salts may be volumetrically determined by arsenious acid in aqueous sulphuric acid solution using a manganese salt as catalyst, with potassium iodate present as a promoter.2 Excess of arsenious acid is used and back-titrated with permanganate. If nitric acid is present in place of sulphuric acid, an alkali chloride and a trace of iodine are used to promote the action of the manganese salt. Direct potentiometric titration with arsenious acid may also be employed. 

There is a very simple and quick method that can be used to detect the reducing power of sulphur dioxide, developed in the last century and often called the Ripper titration (Ough, 1988). In this method, sulphur dioxide is titrated against iodine or potassium iodate/potassium iodide solution in the presence of starch. When all the sulphur dioxide has been oxidised, a blue colour is produced by the reaction of free iodine with the starch. This is a very quick method but will give only an estimate of the level of sulphur dioxide as other reducing substances, such as ascorbic acid, will interfere consequently, this method is not particularly appropriate for juices with high ascorbate levels. 

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