Starch thiocyanates

A few substances indicate the presence of a specific oxidized or reduced species. Starch, for example, forms a dark blue complex with 13 and can be used to signal the presence of excess 13 (color change colorless to blue), or the completion of a reaction in which 13 is consumed (color change blue to colorless). Another example of a specific indicator is thiocyanate, which forms a soluble red-colored complex, Fe(SCN) +, with Fe +. 

Determination. The most accurate (68) method for the deterrnination of copper in its compounds is by electrogravimetry from a sulfuric and nitric acid solution (45). Pure copper compounds can be readily titrated using ethylene diamine tetracetic acid (EDTA) to a SNAZOXS or Murexide endpoint. lodometric titration using sodium thiosulfate to a starch—iodide endpoint is one of the most common methods used industrially. This latter titration is quicker than electrolysis, almost as accurate, and much more tolerant of impurities than is the titration with EDTA. Gravimetry as the thiocyanate has also been used (68). 

A simple test for ether peroxides is to add lOmL of the ether to a stoppered cylinder containing ImL of freshly prepared 10% solution of potassium iodide containing a drop of starch indicator. No colour should develop during one minute if free from peroxides. Alternatively, a 1% solution of ferrous ammonium sulfate, O.IM in sulfuric acid and O.OIM in potassium thiocyanate should not increase appreciably in red colour when shaken with two volumes of the ether. 

After the addition of the potassium iodide solution, run in standard 0.1M sodium thiosulphate until the brown colour of the iodine fades, then add 2 mL of starch solution, and continue the addition of the thiosulphate solution until the blue colour commences to fade. Then add about 1 g of potassium thiocyanate or ammonium thiocyanate, preferably as a 10 per cent aqueous solution the blue colour will instantly become more intense. Complete the titration as quickly as possible. The precipitate possesses a pale pink colour, and a distinct permanent end point is readily obtained. 

Iodine starch inclusion compounds 46 Iodine vapor reagent 46, 64, 78 2-Iodoestrone 66 Ipecacuanha alkaloids 46, 263 Ipecacuanhae radix extract 263 Iron cadons 144, 217, 311 Iron(IIl) chloride reagent 170,216,314 Iron(III) thiocyanate reagent 170 Isoascorbic acid 376 Isoleucine 246,247... 

Potassium permanganate and iodine, which are important redox reagents, are both self-indicating, i.e. the colour of the reagent in each case is intense and will impart a perceptible colour to a solution when present in very small excess. One drop of a solution of potassium permanganate (0.02 mol dm 3) can be detected in a titrand solution of 100 cm3, and a similar amount of iodine by shaking the titrand with 5 cm3 of chloroform or carbon tetrachloride to produce an intense purple colour. Specific indicators react in a specific manner with one participant in the reaction. The best examples are starch, which produces an intense blue colour with iodine and potassium thiocyanate, which forms an intense red compound with iron(III). 

Various microchemical tests are available for the detection of minute quantities of sulphur, both free and combined. The substance under examination may be treated with a little sodium hydroxide solution, the extract evaporated just to dryness, a few drops of aqueous sodium cyanide (0-1 per cent.) added and the evaporation repeated. The residue, moistened with dilute sulphuric acid and a drop of ferric chloride, gives the characteristic ferric thiocyanate colour if sulphur is present.6 In the ease of minerals, traces of sulphur dioxide produced on heating may be detected 6 by the colour change of an alkaline solution of Bromocrcsol Green or by the deeolorisation of starch-iodine solution. 

The sodium salt of these acids may be used to prepare aqueous solutions of indicators. Other examples of redox indicators include starch, potassium thiocyanate, methylene blue, and phenosafranine. Some selected general indicators in redox titrations are listed in Table 1.6.3. The properties of starch as an indicator in iodometric titration are discussed in the following section. 

Assay Dissolve about 1.5 g of sample, accurately weighed, in 100 mL of water in a 250-mL Erlenmeyer flask, add 2 mL of glacial acetic acid and 5 g of potassium iodide, mix well, and titrate with 0.1 A sodium thiosulfate to a light yellow color. Add 2 g of ammonium thiocyanate, mix, then add 3 mL of starch TS and continue titrating to a milk-white endpoint. Each milliliter of 0.1/V sodium thiosulfate is equivalent to 45.38 mg of Ci2H22CuOi4. 

Treat a piece of starch paper successively with a drop of 01m potassium thiocyanate solution and a drop of the acid test solution. A blue spot is obtained. Sensitivity 3 pg IO3. Concentration limit 1 in 12,000. 

Some redox indicators react specifically with one form of a redox couple to cause a visible color change. Two examples are starch as an indicator for iodine and thiocyanate for Fe(III). Such indicators, being limited in scope, are not amenable to general treatment. 

Specific indicators owe their behavior to a reaction with one of the participants in the titration. The best-known specific indicator is starch, which forms a dark blue complex with triiodide ion. Also, potassium thiocyanate is used as a specific indicator, for example, in titration of iron(III) with solutions of titanium(III) sulfate. 

Sodium and potassium thiocyanate are said to behave in a fashion similar to sodium acetate in starch acetylations/ ... 

Oxides and peroxides can occur in many essential oils by a photochemical reaction. 1,8-Cineol and linalool monoxide can be readily separated on silica gel thin layers with 1-nitropropane-hexane (1 1, v/v), as the mobile phase.In this case, they have exhibited Rf values of 73 and 8, respectively. " Another pair of compounds, ascaridole and 1,8-cineol, can be easily separated on a silica gel layer, obtaining a value for chloroform as the mobile phase of 63 and 54, respectively. The antimony chloride reagent gives a gray color. The potassium iodide-acetic acid-starch test is usually better than ferrous thiocyanate. 

Redox titrations require either specific indicators, which detect one of the components of the reaction (e.g. starch for iodine, potassium thiocyanate for Fe ) or true redox indicators in which the transition potential of the indicator between oxidized and reduced forms is important. The transition potential of a redox indicator is analogous to the transition pH in acid-base systems. 

---