Iodometric assays

The hberated iodine, as the complex triiodide ion, may be titrated with standard thiosulfate solution. A general iodometric assay method for organic peroxides has been pubUshed (253). Some peroxyesters may be determined by ferric ion-catalyzed iodometric analysis or by cupric ion catalysis. The latter has become an ASTM Standard procedure (254). Other reducing agents are ferrous, titanous, chromous, staimous, and arsenite ions triphenylphosphine diphenyl sulfide and triphenjiarsine (255,256). 

After stirring overnight at room temperature, the crystalline product was collected by filtration, washed with water and finally with acetone, and then dried at 45°C yield of dihydrate, 29.9 g or 79% of theory based on 6-APA iodometric assay, 922 meg per mg bioassay, 921 meg per mg, as described in U.S. Patent 3,478,018. 

It is, however, pertinent to mention here that iodometric assays are never performed in a strongly basic medium, because of the fact that the reaction between I2 and OH produces hypoiodide and iodate ions respectively as shown below ... 

B. Iodometric Assays i.e., release of iodine and subsequent titration with sodium thiosulphate. 

Steger, P. J. K., and Mtihlebach, S. F. (1997), In vitro oxidation of IV lipid emulsions in different All-in-One admixture bags assessed by an iodometric assay and gas-liquid chromatography, Nutrition, 13, 133-140. 

The iodometric assay for moxalactam involves the hydrolysis of moxalactam in aqueous base which results in the cleavage of the 3-lactam ring. This is followed by oxidation of the hydrolysis products with iodine, and the titrimetric determination of the iodine consumed. Since the unhydrolyzed drug does not react with iodine, an unreacted sample is used as a blank to compensate for iodine consuming impurities. 

Iodometric assay of amylose on cereal starches is most often done on lipid-extracted starch, and the amylose content from that assay is termed apparent amylose 145 or total amylose. 123 When the iodometric assay is performed on a lipid-extracted starch and the amylose content is corrected for interference from extra-long chains on amylopectin, the assay gives absolute amylose or real amylose. 145 Iodometric assay of starch that has not been lipid-extracted gives lipid-free (uncomplexed) amylose. Subtraction of total amylose from lipid-free amylose gives lipid-complexed amylose.123 The use of the term apparent amylose to designate lipid-free amylose is avoided in this chapter. All amylose percentages are calculated based on starch. 

Fig. 5.3. Apparatus used in the iodometric assay. G inert gas supply PS presaturator P pipettor M magnetic stirrer SD solvent delivery nozzle FR flow regulator D gas distributor CT connecting tube V vial S stopper with capillary opening SV side valve ST screw-top stopper holder N syringe needle. (Gebicki and Guide, 1989.)...

Ferric Ion Complexes Other chemical methods based on the oxidation of ferrous ion (Fe ) to ferric ion (Fe ) in an acidic medium and the formation of iron complexes have also been widely accepted. These methods spectrophotometri-cally measure the abihty of lipid hydroperoxides to oxidize ferrous ions to ferric ions, which are complexed by either thiocyanate or xylenol orange (23, 28, 29). Ferric thiocyanate is a red-violet complex that shows strong absorption at 500-510 nm (8). The method of determining PV by coloremetric detection of ferric thiocyanate is simple, reproducible, and more sensitive than the standard iodometric assay, and has been used to measure hpid oxidation in milk products, fats, oils, and liposomes (8, 23). 

The classical iodometric assay of penicillins [S9] is applicable to amoxicillin and was the required procedure in the US Pharmacopoeia [60] prior to introduction of HPLC in 1991. This method is based on the fact that the intact penicillin nucleus does not react with iodine, but reaction does occur after hydrolysis to the penicilloic acid. Any penicilloic acid or other iodine reactive compounds present in the sample as impurities are corrected for by a blank titration of unhydrolysed penicillin. A slight variation of the standard procedure is required for amoxicillin and ampicillin in which a small amount of HC1 is added to the blank to release bound iodine which would otherwise cause false results [61], The reaction between iodine and penicilloic acid does not have an exact stoicheiometry and results are calculated relative to the purity of a reference standard which is assayed simultaneously with the sample. 

Prior to the introduction of HPLC the US Pharmacopoeia included both the iodometric assay and titration of the acidic and basic Junctions, with a concordance requirement of not more than 6.0% difference between any pair of results [60]. Non aqueous potentiometric titrations in dimethylsulphoxide/methanol and glacial acetic acid, with lithium methoxide and perchloric acid titrants, were used for the acidic and basic... 

Bethel and Bond described two iodometric assay procedures for penicillin V in fermented broths. 

In lieu of microbiological assays described above, the iodometric assay is used as... 

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