Dichromate ions reduction

The dichromate ion oxidises iron(II) to iron(III), sulphite to sulphate ion, iodide ion to iodine and arsenic(III) to arsenic(V) (arsenate). Reduction of dichromate by sulphite can be used to prepare chrome alum, since, if sulphur dioxide is passed into potassium dichromate acidified with sulphuric acid, potassium and chromium(III) ions formed are in the correct ratio to form the alum, which appears on crystallisation ... 

A voltaic cell consists of two half-cells. One of the half-cells contains a platinum electrode surrounded by chromium(III) and dichromate ions. The other half-cell contains a platinum electrode surrounded by bromate ions and liquid bromine. Assume that the cell reaction, which produces a positive voltage, involves both chromium(III) and bromate ions. The cell is at 25°C. Information for the bromate reduction half reaction is as follows ... 

As you might expect from the half-equation for its reduction, the oxidizing strength of the dichromate ion decreases as the concentration of H+ decreases (increasing pH). 

The small amount of mercury(I) chloride in suspension has no appreciable effect upon the oxidising agent used in the subsequent titration, but if a heavy precipitate forms, or a grey or black precipitate is obtained, too much tin(II) solution has been used the results are inaccurate and the reduction must be repeated. Finely divided mercury reduces permanganate or dichromate ions and also slowly reduces Fe3+ ions in the presence of chloride ion. 

However the second question, whether the Cr+3 species either underwent some chemical change so that they became inert in the solution or Cr+3 ions were not available to DPC for complexation from the existing dichromate ions remain to be explained. Since either oxidation (c) or reduction (b) would occur in the solution in the given set of experimental condition, another experiment was performed to ascertain the cause of decomposition of Cr-DPC complex resulting into the decolourisation. A current of N2 gas was purged into the decolourised solution for about 10 min to remove all dissolved 02 gas from the solution and create an oxidation free atmosphere in and above the solution in the flask. The solution was sealed and left for an hour. The colourless solution changed to feebly pinkish colour and intensified over night (about 10 h). This confirmed the restoration of chromium ions to +3... 

Example Potassium dichromate (VI) is an important oxidising agent that only works in an acidic medium. It is reduced to chromium (III) ions, whilst the H+(aq) ions from the acid end up as water. Each Cr atom undergoes a 3-electron reduction, but every dichromate ion contains 2 Cr atoms. Therefore 6 electrons appear on the left-hand side of the half-equation ... 

Oxidation-Reduction in Blood Analysis Demonstrating the Reaction in a Breathalyzer," J. Chem. Educ., Vol. 67,1990, 263. The oxidation of a primary alcohol by the orange dichromate ion is shown to first form an aldehyde, then a carboxylic acid, and green chromium(III) ion. The use of this reaction, principles of spectrometry, and gas laws in a commercial device for measuring blood-alcohol content are discussed. 

Figure 4.5 illustrates the titration curve that is obtained with an amperometric indicating system for the titration of lead ion with dichromate ion. If the applied potential is set on the plateau for the reduction of lead ion (approximately -0.5 V vs. SCE), curve a in Figure 4.5, will result. In contrast, if the applied potential is set at 0 V versus SCE, no current will flow until the point when excess chromate ion exists in the solution curve b is indicative of the titration curve that would be obtained. 

This reaction involves the reduction of the dichromate ion by ammonium ion. Chromic oxide is also made by heating sodium dichro-mate with sulfur, and leaching out the sodium sulfate with water ... 

Write equations for the reduction of dichromate ion by (a) sulfur dioxide ... 

Once a sample is in solution, the solution conditions must be adjusted for the next stage of the analysis (separation or measurement step). For example, the pH may have to be adjusted, or a reagent added to react with and mask interference from other constituents. The analyte may have to be reacted with a reagent to convert it to a form suitable for measurement or separation. For example, a colored product may be formed that wUl be measured by spectrometry. Or the analyte will be converted to a form that can be volatilized for measurement by gas chromatography. The gravimetric analysis of iron as FeaOa requires that all the iron be present as iron(in), its usual form. A volumetric determination by reaction with dichromate ion, on the other hand, requires that all the iron be converted to iron(II) before reaction, and the reduction step will have to be included in the sample preparatioii. 

In all three experiments, Cr(VI) underwent a substantive decrease in concentration, and the distribution of Cr(T) was modified, indicating the widespread mobilization of chromium and the extensive coeval reduction of Cr(VI) to Cr(III). Accumulation of chromium in the anode chamber shows that electromigration was the predominant driving force for the transport of ions (e.g. Mukhopadhyay, Sundquist, and Schmitz, 2007). Generally, under neutral or high pH conditions, Cr(VI) exists as the soluble and mobile CrOi ion (Reddy et al., 2003) and,to a lesser extent, the dichromate ion Cr207. Reaction of the dichromate ion and the chromate ion to chromic acid occurs only under strongly acidic conditions (e.g. Mukhopadhyay, Sundquist, and Schmitz, 2007) and were most likely not attained in this study. 

This dosimeter relies on the radiolytic reduction of the dichromate ion (Cr207) to chromic ion in aqueous perchloric acid solution. In this aqueous solution the radiolytically produced hydrogen atoms reduce Cr(VI), while the hydroxyl radicals oxidize Cr. Matthews (1981) suggested adding silver ions to the solution, since these scavenge the oxidizing hydroxyl radicals. 

Chromium metal can be electroplated onto a copper electrode from an aqueous solution of H2Cr2C>7, as shown in the Electroplating movie (eChapter 20.9). (a) Write the half-reaction for the reduction of dichromate ion to chromium metal, (b) What current would have to be applied for 15 minutes in order to plate out 0.75 gram of chromium metal from a solution of H2Cr2C>7 ... 

What reduction product is formed when dichromate ion is reduced in acidic solution When permanganate ion is reduced in acidic solution When permanganate ion is reduced in basic solution Write the electron reactions for these three cases. 

It was concluded that the leucoemeraldine form of PANI is oxidized by dichromate ions. By applying an electrode potential of Erhe = +0-2 Y, PANI film is reduced to its initial leucoemeraldine form. As a result, a net electroreduction of dichromate takes place at the PANI-modified electrode, wherein the PANI film acts as an electrocatalyst in the reaction sequence presented. Because PANI is present in its reduced form (as seen from spectra of Fig. 53, middle), it was concluded that a chemical redox reaction between dichromate and PANI is the rate-determining step, whereas the cathodic reduction of the PANI film proceeds relatively fast. 

The dichromate ion in acidic solution is a common oxidizing agent for organic compounds. Derive an expression for the potential of an electrode for which the half-reaction is the reduction of Cr20T ions to CH ions in acidic solution. 

The results and discussion of these reaction are found in M.A.Malati A.A.Abdul Azim Egypt.J.Chem.2 959)Al. It is suggested that dichromate ions are reduced by oxalic acid to Cr(III) which is then complexed with oxalate to form the complex-ion. The latter reaction is quantitative within a range of pH values. Mn ions were required to catalyse the reduction step. (See Am. Chem.Soc. 95( 1972)3181). 

First-Row Transition Elements Scandium to Manganese—Oxidation-reduction reactions are commonly encountered with transition metal compounds of Sc, Ti, V, Cr, and Mn. Two common types of oxidizing agents are the dichromates and permanganates. In aqueous solution, dichromate ion is in equilibrium with chromate ion. 

Why is it reasonable to expect the chemistry of dichromate ion to involve mainly oxidation-reduction reactions and that of chromate ion to involve mainly precipitation reactions ... 

Reduction of dichromate by strong reducing agents yields the chromiumfll) ion, Cr (see p. 383). 

The standard potential for the anodic reaction is 1.19 V, close to that of 1.228 V for water oxidation. In order to minimize the oxygen production from water oxidation, the cell is operated at a high potential that requires either platinum-coated or lead dioxide anodes. Various mechanisms have been proposed for the formation of perchlorates at the anode, including the discharge of chlorate ion to chlorate radical (87—89), the formation of active oxygen and subsequent formation of perchlorate (90), and the mass-transfer-controUed reaction of chlorate with adsorbed oxygen at the anode (91—93). Sodium dichromate is added to the electrolyte ia platinum anode cells to inhibit the reduction of perchlorates at the cathode. Sodium fluoride is used in the lead dioxide anode cells to improve current efficiency. 

The most favorable conditions for equation 9 are temperature from 60—75°C and pH 5.8—7.0. The optimum pH depends on temperature. This reaction is quite slow and takes place in the bulk electrolyte rather than at or near the anode surface (44—46). Usually 2—5 g/L of sodium dichromate is added to the electrolysis solution. The dichromate forms a protective Cr202 film or diaphragm on the cathode surface, creating an adverse potential gradient that prevents the reduction of OCU to CU ion (44). Dichromate also serves as a buffering agent, which tends to stabilize the pH of the solution (45,46). Chromate also suppresses corrosion of steel cathodes and inhibits O2 evolution at the anode (47—51). 

Chlorate Analysis. Chlorate ion concentration is determined by reaction with a reducing agent. Ferrous sulfate is preferred for quaHty control (111), but other reagents, such as arsenious acid, stannous chloride, and potassium iodide, have also been used (112). When ferrous sulfate is used, a measured excess of the reagent is added to a strong hydrochloric acid solution of the chlorate for reduction, after which the excess ferrous sulfate is titrated with an oxidant, usually potassium permanganate or potassium dichromate. 

The green colour due to the Cr3+ ions formed by the reduction of potassium dichromate makes it impossible to ascertain the end-point of a dichromate titration by simple visual inspection of the solution and so a redox indicator must be employed which gives a strong and unmistakable colour change this procedure has rendered obsolete the external indicator method which was formerly widely used. Suitable indicators for use with dichromate titrations include AT-phenylanthranilic acid (0.1 per cent solution in 0.005M NaOH) and sodium diphenylamine sulphonate (0.2 per cent aqueous solution) the latter must be used in presence of phosphoric) V) acid. 

Procedure. To obtain experience in the method, the purity of analytical-grade potassium chlorate may be determined. Prepare a 0.02M potassium chlorate solution. Into a 250 mL conical flask, place 25.0 mL of the potassium chlorate solution, 25.0mL of 0.2M ammonium iron(II) sulphate solution in 2M sulphuric acid and add cautiously 12 mL concentrated sulphuric acid. Heat the mixture to boiling (in order to ensure completion of the reduction), and cool to room temperature by placing the flask in running tap water. Add 20 mL 1 1 water/phosphoric(V) acid, followed by 0.5 mL sodium diphenyl-amine-sulphonate indicator. Titrate the excess Fe2+ ion with standard 0.02M potassium dichromate to a first tinge of purple coloration which remains on stirring. 

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