Iodine compounds, starch oxidized

Wet-Chemical Determinations. Both water-soluble and prepared insoluble samples must be treated to ensure that all the chromium is present as Cr(VI). For water-soluble Cr(III) compounds, the oxidation is easily accompHshed using dilute sodium hydroxide, dilute hydrogen peroxide, and heat. Any excess peroxide can be destroyed by adding a catalyst and boiling the alkaline solution for a short time (101). Appropriate ahquot portions of the samples are acidified and chromium is found by titration either using a standard ferrous solution or a standard thiosulfate solution after addition of potassium iodide to generate an iodine equivalent. The ferrous endpoint is found either potentiometricaHy or by visual indicators, such as ferroin, a complex of iron(II) and o-phenanthroline, and the thiosulfate endpoint is ascertained using starch as an indicator. 

Iodide can be determined spectrophotometrically (after oxidation to iodine), either as the blue iodine-starch adsorption compound, or as coloured organic extracts containing iodine. Iodide is oxidized to iodine with nitrite or iron(III). 

Iodides react with a number of oxidizing agents in acid solution to yield free iodine, which gives the familiar test with starch. The latter effect involves the production of a deep blue adsorption compound of iodine and starch. 1 When only small amounts of iodine are present, the insoluble blue starch-iodine compound remains in colloidal suspension. 

Elemental composition Pb 86.62%, O 13.38%. The compound may be identified by its physical properties and characterized by x-ray crystallography. Lead may be analyzed in the acid extract of the oxide by AA or ICP spectroscopy. It also may be analyzed by its oxidative properties. It hberates iodine from an acidic solution of potassium iodide, and the liberated iodine may be titrated against a standard solution of sodium thiosulfate using starch indicator (blue color decolorizes at the end point). 

Antimonious oxide, or any of its compounds, is dissolved in tartaric acid and water sodium carbonate is used to neutralise any excess of the acid used then a cold saturated solution of sodium bicarbonate is added in the proportion of lOc.c. to about O l gramme Sb.,03 to the clear solution starch is added, and iodine until the blue colour appears. This operation must be done quickly, as when the bicar- This is a useful method when c-ipper is absent.. 

The solutions required are iodic acid, 50 grms. in 250 c.c. deci-normal sodium thiosulphate 20 per cent, potassium iodide and starch paste. Into the flask of a Fresenius or Mohr s iodometric apparatus, 0-5-0-6 gramme of antimonious oxide is weighed, 20-25 c.c. of iodic acid are added, and 10 c.c. of potassium iodide solution are placed in the condensing apparatus. The contents of the flask are boiled until colourless, and the collected iodine determined with thiosulphate. From antimony compounds the sulphide is precipitated by sulphuretted hydrogen, then dissolved in HCl, and the oxide obtained by the action of sodium carbonate. The halogen acids, sulphurous acid, and sulphuretted hydrogen must be dissolved in... 

Determination of ozone in aqueous solution is perhaps the most problematic for a variety of reasons (1) ozone is unstable (2) ozone is volatile and easily lost from solution and (3) ozone reacts with many organic compounds to form products such as ozonides and hydrogen peroxide that are also good oxidants. Careful study of the use of iodometric methods for the determination of ozone in aqueous solution has revealed that the stoichiometric ratio of ozone reacted with iodine produced in the reaction varies from 0.65 to 1.5, depending on pFI, buffer composition and concentration, iodide ion concentration, and other reaction conditions. As a result, iodometric methods are not recommended. Ozone can be determined iodimetrically by addition of an excess of a standard solution of As(III), followed by titration of the excess As(III) with a standard solution of iodine to a starch endpoint. Methods using DPD, syringaldazine, and amperometric titrations have also been developed. 

In the final step of the analysis, the iodine is titrated with thiosulphate. The iodine is reduced to iodide, and the thiosulphate in turn is oxidized to the tetrathionate ion. The concentration of the thiosulphate solution used for the titration must be known precisely. The endpoint of the redox titration is commonly indicated by a starch indicator or by photometric or amperometric endpoint detection. The starch indicator forms an enclosure compound with iodine. The large electron cloud of the iodine interacts with the hydroxo dipoles in the starch helix resulting in an intensely blue colour of the iodine starch complex. Nevertheless, the iodine molecules can leave the starch hehx easily and thus can be reduced by thiosulphate. The endpoint of the titration is clearly marked by the change from blue to colourless. 

The free iodine can be detected by the starch test. When applying the redox reaction with iodide, it should be remembered that antimony-bearing organic compounds leave antimony pentoxide or calcium antimonate after ignition with lime and these compounds also set iodine free. The same is true when the ignition residue contains ferric oxide. 

Compounds whose structure possesses a weakly acidic NH group, such as primary amines, amides, imides, and lactam can be detected by performing a redox reaction through the formation of intermediary N-chloro derivatives. Potassium permanganate reacting with hydrochloric acid generates chlorine (probably at oxidation state +1), which transforms the invoked derivatives in N-chloroamines. The latter derivatives oxidize iodide ions into iodine detected with a starch solution ... 

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