Dichromate ions

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 ... 

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 ... 

Fig. 3. Chemistry of dichromated poly(vinyl alcohol) resist. Initially the dichromate ion absorbs light the light-activated species undergoes an...

Acidification of aqueous solutions of the yellow, tetrahedral chromate ion, Cr04 , initiates a series of labile equilibria involving the formation of the orange-red dichromate ion, Ct20i HCr04 Cr04 + H+... 

Consider the dichromate ion. It has no metal-metal nor oxygen-oxygen bonds. Write a Lewis structure for die dichromate ion. Consider chromium to have six valence electrons. 

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 ... 

Chromium in the +6 state forms two different oxoanions, the yellow chromate ion, Cr042-, and the red dichromate ion, Cr2072- (Figure 20.11). The chromate ion is stable in basic or neutral solution in acid, it is converted to the dichromate ion ... 

The dichromate ion in acidic solution is a powerful oxidizing agent,... 

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). 

Since the dichromate ion on the left side of the equation has been reduced to chromic ion, Cr+ on the right side, the conversion of methanol to formaldehyde must involve oxidation. To show more clearly that methanol has been oxidized, let us balance this reaction by the method of half-reactions. We have encountered the halfreaction involving dichromate and chromic ions before (Problem 20b in Chapter 12). It is... 

In a similar manner, ethanol can be oxidized by the dichromate ion to form a compound called acetaldehyde, CHaCHO. The molecular structure of acetaldehyde, which is similar to that of formaldehyde, is shown at the bottom in Figure 18-6. We see that the molecule is structurally similar to formaldehyde. The methyl group, —CH3, replaces one of the hydrogens of formaldehyde. The balanced equation for the formation of acetaldehyde from ethanol is... 

Write the balanced equation for the production of pentanone from pentanol, using dichromate ion as the oxidizing agent. 

The +6 state of chromium is represented by the chromates and dichromates. The chromate ion is a tetrahedral ion with Cr at the center dichromate ion may be visualized as two such tetrahedra having one oxygen corner in common. Figure 22-9 shows the arrangements. Chromates can easily be converted to dichromates by addition of acid,... 

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. 

Both lead ion and dichromate ion yield a diffusion current at an applied potential to a dropping mercury electrode of —1.0 volt against the saturated calomel electrode (S.C.E.). Amperometric titration gives a V-shaped curve [Fig. 16.14 (C)]. The exercise described refers to the determination of lead in lead nitrate the application to the determination of lead in dilute aqueous solutions (10-3 — 10-4lVf) is self-evident. 

Chromium in steel Discussion. The chromium in the steel is oxidised by perchloric acid to the dichromate ion, the colour of which is intensified by iron (III) perchlorate which is itself colourless. The coloured solution is compared with a blank in which the dichromate is reduced with ammonium iron(II) sulphate. The method is not subject to interference by iron or by moderate amounts of alloying elements usually present in steel. 

The above considerations will be illustrated by the simultaneous determination of manganese and chromium in steel and other ferro-alloys. The absorption spectra of 0.001 M permanganate and dichromate ions in 1M sulphuric acid, determined with a spectrophotometer and against 1M sulphuric acid in the reference cell, are shown in Fig. 17.20. For permanganate, the absorption maximum is at 545 nm, and a small correction must be applied for dichromate absorption. Similarly the peak dichromate absorption is at 440 nm, at which permanganate only absorbs weakly. Absorbances for these two ions, individually and in mixtures, obey Beer s Law provided the concentration of sulphuric acid is at least 0.5M. Iron(III), nickel, cobalt, and vanadium absorb at 425 nm and 545 nm, and should be absent or corrections must be made. 

One dichromate ion uses 6e, and two iodide ions liberate 2e hence the two partial equations apply in the ratio of 1 3 ... 

Because the oxidation number of Cr decreases from + 6 to +3, Cr is reduced and the dichromate ion is the oxidizing agent. 

FIGURE 16.8 The chromate ion, Cr04 . is yellow. When acid is added to a chromate solution the ions form orange dichromate ions. 

C19-0083. Dichromate ions, C r2 0-j, oxidize acetaldehyde, CH3 CHO, to acetic acid, CH3 CO2 H, and are reduced to Cr . The reaction takes place in acidic solution. Balance the redox reaction and determine how many moles of electrons are required to oxidize 1.00 g of acetaldehyde. What mass of sodium dichromate would be required to deliver this many electrons ... 

The authors believe that a similar mechanism to the above operates for the dichromate-ion dependent path, but that the equilibrium constant K is much smaller. This oxidation presents two novel features, namely the lack of an acidity dependence of the rate and the participation of a term involving dichromatic ion. 

In the case of high concentrations of chromate and low concentrations of arsenous acid, we must take into account that dichromate ions are also involved in reaction. Thus... 

The order of greater than unity with respect to chromate concentration suggests that here the active oxidizing agent is the dichromate ion. The concentration of this ion must vary as the square of the gross concentration of chromic acid, whenever that concentration is small. 

Over a range of ionic strengths, particles elute from the columns ahead of a dissolved marker species (dichromate ion) with particle residence time decreasing with increasing diameter. 

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