Self-indicating

B. Self-indicating reagents. This is well illustrated by potassium permanganate, one drop of which will impart a visible pink coloration to several hundred millilitres of solution, even in the presence of slightly coloured ions, such as iron(III). The colours of cerium(IV) sulphate and of iodine solutions have also been employed in the detection of end points, but the colour change is not so marked as for potassium permanganate here, however, sensitive internal... 

The self-indicating granules of Carbosorb show when the absorbent is exhausted. 

The sodium hydroxide solution should be added slowly, since the reaction with the acidic ether extract is exothermic and may cause boiling of the ether. The ether extract should be washed with aqueous sodium hydroxide until the aqueous layer remains basic to litmus. This extraction is self-indicating the ether turns from a bright yellow to a light brown and color appears in the aqueous phase. 

The concept of reduction potential is introduced in Chapter 6. When the reduction potentials of two species differ by 0.1 V or more, the resulting redox reaction will proceed rapidly and stoichiometrically so that it may be used as the basis for a titrimetric procedure. The end point of a redox titration may be observed by following the potential of the titrand with an indicator electrode or with a visual indicator. In two special cases, the reagent (potassium permanganate and iodine) is self-indicating (vide infra). 

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). 

E Atherton, JL Holder, MMeldal, RC Sheppard, RM Valerio. 3,4-Dihydro-4-oxo-l,2,3-benzotriazin-3-yl esters of fluorenylmethoxycarbonyl amino acids as self-indicating reagents for solid phase peptide synthesis. J Chem Soc Perkin Trans 1 2887, 1988. 

Ammonium ceric sulphate serves as a powerful oxidizing agent in an acidic medium. The salt has a bright yellow colour and so its solution. On reduction, the resulting cerous salt obtained is colourless in appearance and, therefore, strong solutions may be considered as self-indicating. In general practice, 0.05 N solutions are employed invariably for estimations. As this concentration is very dilute for observation of the respective end-point, hence the inclusion of an appropriate indicator becomes necessary. The oxidation reaction involved may be expressed as follows ... 

Redox titration using potassium permanganate as the oxidising reagent and self-indicator. 

As noted earlier, these oxygen scavengers are self-indicating, so it is easy to determine when regeneration is necessary. However, there do not appear to be... 

It is usual to employ blue self-indicating silica gel crystals which turn a pale pink colour when regeneration by heating in an electric oven is required. 

Note. (1) If it is desired to follow the progress of the dehydrogenation, attach the side-tube through a U-tube packed with self-indicating soda-lime to a nitrometer filled with potassium hydroxide solution almost the theoretical quantity of hydrogen will be collected. 

Removal of peroxides.1 Self-indicating activated 4 A molecular sieve (J. T. Baker, Merck) effectively removes peroxides from various ethers. The effectiveness is not a result of adsorption but of interaction with the impregnated indicator. 

Many laboratory accidents have been ascribed to presence of peroxides in solvents, usually, but not exclusively ethers. Storage of the solvents for over-long periods, often under poor conditions, is a common feature of the incidents. When peroxides are removed from solvents by chromatographic adsorption on alumina columns, the concentrated band of strongly adsorbed peroxides at the top of the column may become hazardous if the solvent evaporates [1], The use of self-indicating molecular sieve under nitrogen is described as an effective method of de-peroxidising THF, diethyl and dipropyl ethers, suitable also for bulk-scale operations [2]. 

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