Dichromates

Of little use commercially except as a route to anthraquinone. For this purpose it is oxidized with acid potassium dichromate solution, or better, by a catalytic air oxidation at 180-280 C, using vanadates or other metal oxide catalysts. 

Diamond is very resistant to chemical reagents potassium dichromate and sulphuric acid attack it with the formation of CO2. It burns in air or oxygen at 700 C to CO2 leaving scarcely any ash some forms of bort may leave as much as 4-5% ash. 

Consider also the oxidation of iron(II) ions by dichromate(Vl)... 

Thus under standard conditions chloride ions are not oxidised to chlorine by dichromate(Vr) ions. However, it is necessary to emphasise that changes in the concentration of the dichromate(VI) and chloride ions alters their redox potentials as indicated by the Nernst equation. Hence, when concentrated hydrochloric acid is added to solid potassium dichromate and the mixture warmed, chlorine is liberated. 

Addition of dilute potassium dichromate(VI) solution, K2Cr207, to a solution of hydrogen peroxide produces chromium peroxide, CrOj, as an unstable blue coloration on adding a little ether and shaking this compound transfers to the organic layer in which it is rather more stable. 

In acid solution, dichromates(VI) (and also chromates(VI) which are converted to dichromates) are reduced to chromium(III) salts ... 

Hence the orange colour of a dichromate is converted to the green colour of the hydrated chromium(III) ion, Cr ", and sulphur is precipitated when hydrogen sulphide is passed through an acid solution.)... 

Sulphites react with oxidising agents, for example mangan-ate(VII) and dichromate(VI) ... 

This reaction is a useful test for a sulphite or for moist sulphur dioxide, which turns dichromate paper (filter paper soaked in potassium dichromate) from yellow to green. 

In what way does a solution of hydrogen peroxide react with (a) chlorine water, (b) potassium permanganate solution, (c) potassium dichromate solution, (d) hydrogen sulphide 50 cm of an aqueous solution of hydrogen peroxide were treated with an excess of potassium iodide and dilute sulphuric acid the liberated iodine was titrated with 0.1 M sodium thiosulphate solution and 20.0 cm were required. Calculate the concentration of the hydrogen peroxide solution in g 1" ... 

If the chloride is heated with sodium or potassium dichromate-(VI) and concentrated sulphuric acid, a red gas, chromium(VI) dichloride dioxide, CrOjClj, is evolved if this is passed into water, a yellow solution of a chromate(VI) is formed. 

Addition of an oxidising agent to a solution of an iodide (for example concentrated sulphuric acid, hydrogen peroxide, potassium dichromate) yields iodine the iodine can be recognised by extracting the solution with carbon tetrachloride which gives a purple solution of iodine. 

The most important compounds containing Cr(VI) are the oxide Cr03 and the oxoanions CrO, chromate(VI) and Cr20 . dichromate(VI). 

Chromium(VI) oxide is acidic, and the corresponding salts are the chromates and dichromates, containing the ions CrO and Cr207 . i.e. [Cr04 -I- CrOj] ". The oxidation state of chromium is -f6 in each ion (cf sulphur in and 8207 ). 

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

The dichromate ion has the following geometrical structure (single lines not necessary implying single bonds) ... 

If a metal ion of an insoluble chromate is added to a solution containing the dichromate ion. the chromate is precipitated for example with a soluble lead(II) salt ... 

Sodium dichromate is prepared on the large scale by heating powdered chromite with sodium carbonate, with free access of air the sodium chromate first formed is treated with acid ... 

Sodium sulphate crystallises out in hydrated form (common ion effect) and is filtered off on concentration, sodium dichromate is obtained. For analytical purposes, the potassium salt. K2Cr20-. is preferred potassium chloride is added and the less soluble potassium dichromate obtained. 

The dichromate ion is a useful oxidising agent in acid solution, and is used in volumetric analysis ... 

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 ... 

Chrome alum is also obtained if the acidified dichromate is boiled with ethanol, the ethanal formed distilling off. 

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

The addition of concentrated sulphuric acid to a solid dichromate mixed with a chloride produces a red vapour, chromium(VI)dioxide dichloride, Cr02Cl2 (cf. sulphur dioxide dichloride, SO2CI2). Chromium(VI) dioxide dichloride reacts with water immediately ... 

Thus, filler paper which has been dipped into a solution of potassium dichromate turns green in the presence of sulphur dioxide. This reaction provides the usual test for sulphur dioxide. 

Addition of hydrogen peroxide to a solution of a dichromate yields the blue colour of "peroxochromic acid. This is a test for soluble chromates and dichromates. 

Chromates and dichromates are used in industry as oxidising agents, for example in the coal tar industry, in the leather industry (chrome taiming), and in the dye industry as mordants. Some chromates are used as pigments, for example those of zinc and lead. Chromates and dichromates are poisonous. 

A reducing agent (for exampie sulphur dioxide) reduces the yellow chromate or orange dichromate to the green chromium(III) state. 

Hydrogen peroxide with a chromate or a dichromate gives a blue colour. 

---