S.R. Sagi

A new reductimetric reagent: ironII in a strong phosphoric acid medium—II: Titration of molybdenumVI with ironII at room temperature

Abstract The formal redox potential of the FeIII/FeII couple has been determined in media of varying phosphoric acid concentration, and found to decrease steeply with increasing phosphoric acid concentration. Above a certain concentration of phosphoric acid, the difference between the formal potentials of the UVI/UIV and FeIII/FeII couples becomes sufficiently great to enable ironII to reduce uraniumVI even at room temperature. This is the reversal of the normal redox reaction—oxidation of uraniumIV by ironIII. A careful study of the various factors involved has enabled us to develop a new reductimetric titration of uraniumVI with ironII in strong phosphoric acid medium (11.0 to 13.5M) at room temperature using either a potentiometric end-point or a visual end-point with methylene blue or thionine as an internal redox indicator. Because the new procedure enables the titration to be carried out at room temperature, it is more convenient than procedures involving titaniumIII and chromiumII which require temperatures ranging from 60 to 90°. The new procedure has a further advantage in that it can be used for the determination of uraniumVI even in the presence of ironIII and tungstenIV, using an electrometric end-point. Also, the new procedure does not require the elaborate precautions which are necessary for the preparation and storage of titaniumIII or chromiumII solutions.

Sequential oxidimetric determination of thallium(I) and (III)

The reaction between thallium(III) and oxalic acid in sulphuric acid medium has been investigated. Spectrophotometric results show that thallium(III) can be quantitatively reduced to thallium(I) with oxalic acid in aqueous medium when heated to near boiling point. Conditions for the estimation of the excess of oxalic acid with cerium(IV) sulphate in the presence of thallium(I) and for the estimation of a mixture of thallium(I) and thallium(III) have been investigated. The method is simpler than many other redox methods reported for the determination of thallium(III) and is free from many interferences encountered in these titrations. The reagents are cheap and quite stable.

Thallimetric oxidations—V: Titrimetric and spectrophotometric determination of hydrogen peroxide

Titrimetric and spectrophotometric methods have been developed for the determination of hydrogen peroxide at mmole and mumole levels respectively. In these methods thallium(III) is used as the oxidant and the reduced thallium(I) is determined oxidimetrically with potassium bromate in the titrimetric method and by measuring the absorbance of thallium(III) at 260 nm in the presence of 0.1M hydrochloric acid and 1M perchloric acid in the spectrophotometric method. Photochemical redox methods for the estimation of hydrogen peroxide in the presence of a number of diverse ions are described.

A Photochemical redox method for the estimation of thallium(III)

A convenient photochemical redox method for the estimation of thallium(III) by reduction with oxalic acid followed by oxidation of thallium(I) with potassium bromate has been developed. The reduction is carried out in the presence of small concentrations of chloride and bromide as catalysts.

Tin(II) sulphate as a new reductimetric titrant—direct titration of thallium(III) with use of ion-pair and redox indicators

The use of tin(II) sulphate as a direct reductimetric titrant for thallium(III) has been investigated, with potentiometric and visual detection of the end-point. Some azure dyes are used as redox indicators and Methylene Blue is used as both a redox and an ion-pair indicator.

Photochemical reduction of thallium(III) with hydrogen peroxide

A convenient method for determination of thallium(III) is based on photochemical reduction with hydrogen peroxide in the presence of bromide as catalyst, followed by oxidation of thallium(I) with potassium bromate.

Titration of methylene blue with chloromolybdate(III).

The reaction of Methylene Blue with aquomolybdenum(III) chloride and chloromolyb-date(III) in hydrochloric acid media has been investigated. Chloromolybdate(III) can be satisfactorily used as a reductometric titrant for Methylene Blue in 6-8M hydrochloric acid medium. The end-point can be detected either potentiometrically or visually. Aquomolybdenum(III) chloride is not useful as a titrant for this estimation.

Dichromate titration of thallium(I).

The formal redox potentials of the thallium(III)-thallium(I) couple in different acids of varying strengths are reported. The minimum concentration of hydrochloric acid required for a direct titration of thallium(I) with potassium dichromate is 5M. Thallium(I) can be titrated directly with the primary standard oxidant, potassium dichromate, at room temperature, with ferroin as indicator, in 6M hydrochloric acid. Atmospheric oxygen must be excluded.

Thallimetric oxidations-VI Titrimetric and spectrophotometric methods for the determination of phosphite and analysis of binary mixtures of phosphite and oxalate.

Titrimetric and spectrophotometric methods have been developed for the estimation of phosphite at mmole and mumole levels, respectively. Thallium(III) is used as an oxidant and the thallium(I) produced is determined either oxidimetrically with potassium bromate or by measurement of the absorbance of thallium(III) at 260 nm in the presence of 0.1 M hydrochloric acid and 1 M perchloric acid. Based on the fact that phosphite and oxalate are oxidized under different conditions, methods are described for the analysis of binary mixtures of phosphite and oxalate. A method is also described for estimation of thallium(III) with phosphite as reluctant, and is applied for analysis of mixtures of thallium(I) and thallium(III).

Thallimetric Oxidations. Part III. Determination of glyoxylic acid and binary mixtures of glyoxylic, oxalic and formic acids by photochemical methods

Glyoxylic acid is oxidised photochemically to carbon dioxide and water by thallium(III) in the presence of Bras catalyst. Based on this, a convenient photochemical redox method is described for the determination of glyoxylic acid by titrating the thallium(I) formed bromimetrically in 1.5–2.0 M hydrochloric acid in the presence of Methyl Orange as indicator. Based on the fact that the photochemical oxidations of glyoxylic acid, oxalic acid and formic acid proceed differently in the presence of Br–, Cl– and Mn2+ and in combination, methods are described for the analysis of binary mixtures of these acids by titrating the thallium(I) formed under different conditions.