Potassium dichromate, 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 ... 

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. 

Chromium metal can be electroplated from an aqueous solution of potassium dichromate. The reduction half-reaction is... 

In acid solution, the reduction of potassium dichromate may be represented as ... 

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. 

In a review of the course and mechanism of the catalytic decomposition of ammonium perchlorate, the considerable effects of metal oxides in reducing the explosion temperature of the salt are described [1], Solymosi s previous work had shown reductions from 440° to about 270° by dichromium trioxide, to 260° by 10 mol% of cadmium oxide and to 200°C by 0.2% of zinc oxide. The effect of various concentrations of copper chromite , copper oxide, iron oxide and potassium permanganate on the catalysed combustion of the propellant salt was studied [2], Similar studies on the effects of compounds of 11 metals and potassium dichromate in particular, have been reported [3], Presence of calcium carbonate or calcium oxide has a stabilising effect on the salt, either alone or in admixture with polystyrene [4],... 

A sensitive spectrophotometric method was reported for the determination of niclosamide and some other antiamebic and anthelmintic drugs either in pure form or in formulations. The method was based on reduction with Zn and HC1 followed by reaction with metol and potassium dichromate at pH 3.0 to give a colored product having maximum absorbance at 530 nm for niclosamide [54]. 

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 ... 

Dimethylaminobenzaldehyde has been made by the condensation of chloral with dimethylaniline, and subsequent hydrolysis 1 by the hydrolysis of tetramethyldiaminobenzhydrol with acetic acid 2 by the condensation of dimethylaniline, formaldehyde and m-sulfo-/>-tolyI hydroxylamine followed by hydrolysis 3 by the electrolytic reduction of a mixture of sodium nitrobenzene sulfonate, dimethylaniline and formaldehyde, and subsequent hydrolysis 4 by the reduction of a mixture of dimethylaniline, formaldehyde and sodium nitrobenzene sulfonate with iron and hydrochloric acid, followed by hydrolysis 5 by the condensation of alloxan with dimethylaniline followed by hydrolysis 6 by the condensation of dimethylaniline, formaldehyde and sodium -toluidine sulfonate in the presence of hydrochloric acid and potassium dichromate followed by hydrolysis.7 The most satisfactory method, however, is the condensation of dimethylaniline, formaldehyde and nitroso dimethylaniline, followed by hydrolysis,8 a method which was first described by E. Noelting and later perfected in detail by L. Baumann. 

A( ueous solutions of chromium(ll) sulfate have Ijeen prepared from chromium(lll) sulfate by reduction with zinc powder and from potassium dichromate by reduction with amalgamated zinc and sulfuric acid. Solid chromitim(II) sulfate penlahyrlrate can be obtained from the reaction of highly purified chromium metal... 

The classical preparation16 of this salt involves the reaction of oxalic acid, dipotassium oxalate, and potassium dichromate, in which the reduction Cr(VI) — Cr(IH) is accompanied by complexation. The method described below has chromium in the +3 state in the highly reactive starting material. 

The hexammino-chromic salts are formed by reducing potassium dichromate in acid solution with alcohol. Zinc is added so that reduction... 

Twenty-five grains of potassium dichromate (or lTg of chromium (VI) oxide) are dissolved in 500ml of water. Sulfur dioxide is bubbled into the solution until reduction to the trivalent state is complete, as indicated by the pure green color of the liquid with no trace of yellow. The solution is then boiled to remove excess sulfur dioxide. 

If chromic chloride is not available as a starting material for this procedure, the compound may be prepared in solution by reducing BOg of potassium dichromate with a mixture of 250ml of concentrated hydrochloric add and either 100ml of 95% alcohol, or HOg of oxalic acid 2-hydrate. The reduced solutions are then used directly in the reduction procedure only the zinc need be added. 

OXIDATION-REDUCTION INDICATOR. A substance that has a color in the oxidized form different from that of the reduced form and can be reversibly oxidized and reduced. Thus, if diphenylamine is present in a ferrous sulfate solution to which potassium dichromate is being added, a violet color appears with the first drop of excess dichromate. 

Quinuclidine aldehydes are important synthetic intermediates. All three monoformylquinuclidines are known 2- and 3-formylquinucli-dines (75 and 76) were prepared 139,140 by reduction of JV-methyl-quinuclidinecarboxanilides with calculated amounts of LiAlH4 or by reduction of ethyl quinuclidinecarboxylates with NaAlH4. 4-Formyl-quinuclidine (77) was made by oxidation of quinuclidyl-4-carbinol (78) with potassium dichromate.87 3-Formylquinuclidine (76) was also synthesized from quinuclidin-3-one (2) by reaction with methoxy-methylene triphenylphosphorane and hydrolysis of the 3-methoxy-methylenequinuclidine (79) with hydrochloric acid.141... 

An alternative to the oxidation-number method for balancing redox reactions is the half-reaction method. The key to this method is to realize that the overall reaction can be broken into two parts, or half-reactions. One half-reaction describes the oxidation part of the process, and the other half-reaction describes the reduction part. Each half is balanced separately, and the two halves are then added to obtain the final equation. Let s look at the reaction of aqueous potassium dichromate (K2Cr2C>7) with aqueous NaCl to see how the method works. The reaction occurs in acidic solution according to the unbalanced net ionic equation... 

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