Iodimetric Assays

To our knowledge no reaction of iodide ion as a nucleophile with a cation radical is known. Iodide ion reduces cation radicals very well and is frequently used for the iodimetric assay of cation-radical salts. Since the reduction is reversible and some compounds can be oxidized to the cation radical stage by iodine, an excess of iodide is used. Some cation radicals are also reduced by other halide ions for example, that of 9,10-diphcnylanthracene is reduced by bromide ion (Sioda, 1968), that of perylene by bromide and chloride ions (Ristagno and Shine, 1971b), and thianthrene radical cation to some extent by chloride ion (Murata and Shine, 1969). These reductions, particularly those by iodide ion, reflect again the competition between nucleophilicity and oxidizability of a nucleophile in reactions with cation radicals. 

The first iodimetric catalase test is attributed to Jolles (196) who, in 1903, described an end point technique the H 0 was titrated after 2 hours of reaction time and without using a buffer. Thirty years later Balls and Hale (45) described the first significant improvement in the iodimetric assay and their technique remains the most useful one to date. The historical development of the KI titration method can be followed in Table VI. The iodimetric estimation, however, is generally less satisfactory than the titration with permanganate because the iodine liberated in the test can both react with and be adsorbed on the proteins present. Also, the preparation of the KI and NasSjOs solutions is somewhat more tedious than that of KMn04. 

The determination of total penicillins is not easy. The U,S.P. uses the direct iodimetric assay as applied to benzylpenicillin after dissolving the sample in methanol, but this may lead to results which are 3 to 5 per cent low, due to interaction of procaine with iodine. Wild suggested the use of silicotungstate as a procaine precipitant before application of the iodimetric method but details given in his method required clarification. Based on the original, the following modification has been found by us to be satisfactory. 

Tablets of Phenoxymethylpenicillin, B,P., may contain phenoxymethylpenicillin or its calcium or potassium salt the content of active material may be determined by the iodimetric assay. Similar considerations apply to capsules.

Continuous research is being carried out to produce modified penicillins with more potent or more specific activity than the antibiotics at present available, or to overcome the penicillin-resistance which certain strains of organisms have built up. A number of these modified penicillins, such as methicillin and phenethicillin is already available and it seems likely that more will appear within the next few years. Little has been published on the analytical chemistry of these substances but it is probable that hydrolysis, with sodium hydroxide followed by titration of the excess alkali could be used for determination of the sodium or potassium salts of the substances mentioned above penicillamine hydrochloride may be determined by the iodimetric assay. 

How would you assay the following drags by iodimetric titrations ... 

Oxaziranes are active oxygen compounds and react with a wick variety of reducing agents- The meet important of these is the reaction with potassium iodide since this reaction is the basis of the iodimetric procedure for assay of oxazirane purity. The reaction has also been examined preparatively and forms a basis for reduction of oxaziranrs... 

Specifications for ascorbic acid, sodium ascorbate, calcium ascorbate, and ascorbyl palmitate are found in the UnitedSfates Pharmacopeia/NationalFormu/a (71) and the Food Chemicals Codex (72). The official assay for all four compounds is the iodimetric titration with 0.1 iodine solution and starch as the indicator. 

Instead of determining the purpurogallin colorimetrically it is possible to titrate it iodimetrically as shown by Bedford and Joslyn (55). Their quantitative assay is, however, quite tedious and involves the isolation of the purpurogallin. 

Repeat the iodimetric procedure using 10 ml of a standard solution of sodium benzylpenicillin in place of the 10 ml of filtrate, multiply the titration figure by 1-6520 to give the exact equivalent of each ml of 0-02N iodine in terms of procaine penicillin monohydrate and from this calculate the result of the assay. 

The iodimetric method of assay is favoured for many official purposes and it can be applied, with very little sample preparation, to injections, tablets and solution-tablets. For ointments and similar preparations it is not directly applicable since high results may be obtained due to iodine absorption by the base. For preparations of this type Boymond has recommended iodimetric determination after mixture with benzene and extraction with water. 

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