Oxidation chromate 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 ... 

Chromate ions, when used as inhibitors in aqueous solutions, passivate by maintaining a coherent oxide film on the metal surface. Passivation is maintained even in a boiling concentrated chromic acid solution, in which many of the oxides in bulk form are soluble. The passivity breaks down rapidly, however, once the chromate is removed. 

The Stability of the natural oxide film reinforced by the chromate ion determines the conditions of pH, ratio of activating anion to chromate, and temperature at which the oxide is broken down and a chromate film deposited. Thus magnesium alloys can be chromate-treated in nearly neutral solutions, whereas aluminium alloys can be treated only in solutions of appreciable acidity or alkalinity. 

Determination of chromium as lead chromate (precipitation from homogeneous solution) Discussion. Use is made of the homogeneous generation of chromate ion produced by the slow oxidation of chromium(III) by bromate at 90-95 °C in the presence of excess of lead nitrate solution and an acetate buffer. The crystals of lead chromate produced are relatively large and easily filtered the volume of the precipitate is about half that produced by the standard method of precipitation. 

The presence of the denominator term in the rate equation (17) suggests that the equilibrium (18) precedes the oxidation step. Two sequences of reactions are proposed (see below), depending on whether the sulphite radical ion dimerises (20) or attacks further acid chromate ion (21). It should be noted that of the species prevalent in dilute aqueous chromic acid, namely CrOj , Cr207, HCrO and H2Cr04, only the last is regarded as possessing oxidising powers. This fact, noted by Westheimer , is tacitly assumed in all recent discussion of... 

These conclusions receive support from the kinetic data . The rate of oxidation by the acid chromate ion is independent of the nature of the mineral acid but at higher acidity, when the oxidation by chromic acid becomes dominant, the rate depends not only upon the acidity but also upon the acid concerned, the oxidising ability of the species, HCr03A, increasing in the order... 

The observation of induced reactions involving chromate almost coincided with the discovery of the phenomenon of chemical induction itself. According to the the role of chromate ions in these reactions, two groups can be distinguished (i) Chromium(VI) plays the role of actor, whose reaction with various inductors listed in Table 1 results in the oxidation of several acceptor ions or molecules. 

Sometimes anodic protection is used, in which case the metal s potential is made more positive. The rate of spontaneous dissolution will strongly decrease, rather than increase, when the metal s passivation potential is attained under these conditions. To make the potential more positive, one must only accelerate a coupled cathodic reaction, which can be done by adding to the solution oxidizing agents readily undergoing cathodic reduction (e.g., chromate ions). The rate of cathodic hydrogen evolution can also be accelerated when minute amounts of platinum metals, which have a stroug catalytic effect, are iucorporated iuto the metaf s surface fayer (Tomashov, 1955). 

The most widely employed transition metal oxidants for alcohols are based on Cr(VI). The specific reagents are generally prepared from chromic trioxide, Cr03, or a dichromate salt, [Cr207]2-. The form of Cr(VI) in aqueous solution depends upon concentration and pH the pKx and pK2 of H2Cr04 are 0.74 and 6.49, respectively. In dilute solution, the monomeric acid chromate ion [HCr03] is the main species present as concentration increases, the dichromate ion dominates. 

For metallic iron and nickel electrodes, the transpassive dissolution causes no change in the valence of metal ions during anodic transfer of metal ions across the film/solution interface (non-oxidative dissolution). However, there are some metals in which transpassive dissolution proceeds by an oxidative mode of film dissolution (Sefer to Sec. 9.2.). For example, in the case of chromium electrodes, on whidi the passive film is trivalent chromium oxide (CrgOj), the transpassive dissolution proceeds via soluble hexavalent chromate ions. This process can be... 

Let us consider the chromate ion as the result of the stabilization of an oxide ion by one chromium tiioxide unit functioning as EPA the negative charges from the oxide ion are dispersed over the whole complex ion, forming one coordinate bond and making the other Cr-0 bonds more covalent. The chromium-oxygen distances within the CrOa units... 

The EPD functions at the oxygen atoms are also available for interaction with one more chromium trioxide molecule. Since the oxygen in the chromate ion has weaker EPD properties than the uncoordinated oxide ion, its interaction with the second chromium (VI)-oxide molecule is weaker than with the first one, and the chromium-oxygen bond distances in the newly attached CrOa unit show a less marked increase. At the same time the formation of a strong coordinate link from one oxygen of the chromate ion induces EA function in the chromium so that it... 

The bond distances between the chromium atoms and the terminal oxygen atoms become 1.629 A (27), which is longer than in chro-mium(VI) oxide but shorter than in the chromate ion (the bonds from the chromium to the central oxygen are 1.748 A). 

Chromium forms chromium(VI) oxide in which the metal is in -1-6 oxidation state. In acid medium it yields yeUow chromate ion, Cr04, and the red-orange dichromate ion,... 

In the above reaction the equilibrium, however, hes far to the left. Therefore the chromium(VI) oxide solution also contains trace amounts of chromate ion,... 

Chromium-Containing Ionic Species Chromate Ion, Hexavalent Chromium Cobalt Copper Cyclic Ethers Cyclic Ethers Dioxane-1,4 Ethylene Oxide... 

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

Similar mechanisms were postulated for the oxidation of glycols by periodate (32) and Ce(IV) (33, 34), and for the oxidation of glycerol by Ce(IV) (44). In these cases the existence of intermediate complexes was demonstrated. The oxidation of formaldehyde by Ce(IV) was also claimed to involve a pre-equilibrium of a Ce(IV)-formaldehyde complex (51). A similar complex was postulated in the formalde-hyde-Mn04 reaction (49, 87). The oxidation of isopropyl alcohol by chromate ions follows a similar mechanism, and a chromate ester was formed as intermediate (94). 

Starting with examples of Mechanism 10 in inorganic chemistry, one may cite the oxidation of cuprous thiosulfate by ferric, vanadate, molybdate, and chromate ions (38). 

In 1 M HC1, the formal potential is just 1.00 V and, in 2 M H2S04. it is 1.11 V so dichromate is a less powerful oxidizing agent than Mn04 or Ce4+. In basic solution, Cr20 is converted into yellow chromate ion (CrO -), whose oxidizing power is nil ... 

Tn aqueous media a different intermediate may well exist, especially nukc chromic oxide itself does not remain intact under these conditions X i e and oo-w-orkens have proposed that in aqueous acid solution the - we-determining step is oxidative addition of the chromate ion HCrO ... 

Aqueous sodium hypochlorite (NaOCl household bleach) is a strong oxidizing agent that reacts with chromite ion [Cr(OH)4 ] in basic solution to yield chromate ion (Cr042-) and chloride ion. The net ionic equation is... 

In the +6 oxidation state, the most important solution species are the yellow chromate ion (Cr042-) and the orange dichromate ion (Cr2072-). These ions are interconverted by the rapid equilibrium reaction... 

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