Potassium iodate assay

The contents of the ampule are diluted in water to a final volume of 50 mL. A 1-mL sample is then taken for the assay. To this sample 1.5 mL 0.667% (w/v) rhodanine in methanol is added. After exactly 5 min. 1 mL 0.5N KOH is added. After 2.5 min. water is added to a final volume of 25 mL. The absorbance is read at 520 nm after a 5-10 min. incubation. A standard curve is made by reaction of gallic acid in 0.2N sulfuric acid with the rhodanine solution. Hagerman and Butler (1989) argued that this assay is more suitable than the potassium iodate assay for the determination of hydrolysable tannins, although it has to be kept in mind that the rhodanine assay is sensitive to any gallic acid ester, including those in non-tannin compounds. 

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. 

Theory The iodine monochloride method described earlier employing standard potassium iodate is the basis for the official assay of potassium iodide. Vigorous shaking is a prime requirement, as the end-point is approached in this assay, because of the fact that both iodine and iodate in different phases attribute a heterogeneous medium. However, the reaction involving the oxidation of KI by iodate may be designated as shown below ... 

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. 

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 ... 

An alkaline borohydride solution may be assayed by addition of excess standard potassium iodate solution leading to its decomposition in accordance with the equation ... 

Assay Dissolve about 500 mg of sample, previously dried at 105° for 4 h and accurately weighed, in about 10 mL of water, and add 35 mL of hydrochloric acid and 5 mL of chloroform. Titrate with 0.05 M potassium iodate until the purple color of iodine disappears from the chloroform. Add the last portions of the iodate solution dropwise, agitating vigorously and continuously. After the chloroform is decolorized, allow the mixture to stand for 5 min. If the chloroform develops a purple color, titrate further with the iodate solution. Each milliliter of 0.05 M potassium iodate used is equivalent to 16.60 mg of KI. 

The name adrenochrome was proposed in 1937 by Green and Richter for the unstable red crystalline product obtained on enzymatic oxidation of adrenaline [22], In the same year Richter and Blashko obtained a crystalline iodo-derivative of adrenochrome by the oxidation of adrenaline with potassium iodate [23]. Thus the product responsible for the violet chromogen utilised in one of the original adrenaline assay procedures [10], was only isolated and characterised 30 years after the procklure was first described. 

The following official preparations contain iodide, which is assayed by the Lang iodate method described above equivalents are given for 1 ml of 0-05M potassium iodate ... 

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