Hydrogen chromate ion

Fortunately the same audiors report a far more pleasant and less hazardous procedure utiliztng a polymer-siqtpoited hydrogen chromate ion (equation 40). The yieids are all in excess of 9S%, and the experiment simpUcity is commendable. 

There is also present in the solution some hydrogen chromate ion, HCrOr ... 

The ET step from AcrH2 to hydrogen chromate ions (P CrCU) is also reported to initiate the oxidation of AcrH2 to AcrH + in H20/MeCN (4 1, v/v) via a radical chain mechanism that is strongly inhibited by oxygen.70... 

The transpassive dissolution of chromium produces different hexavalent species chromic acid (H2Cr04), chromate ions (CrOl"), hydrogen chromate ions (HCrO ) and bichromate ions (Cr207 ). The relative concentrations of these species are determined by the following equilibria ... 

Chromium exists primarily as Cr(III) and Cr(VI). Speciation is very important because Cr(VI) is far more toxic than the trivalent species. The actual form of hex-avalent chromium is pH-dependent. Between pH 2 and 6 the yellow chromate, HCr04, and the more intensely colored orange-red dichromate, Cr207 , are in equilibrium. In very dilute aqueous solutions most of the chromium is present as the hydrogen chromate ion, but dichromate is the predominant species around 0.1 M. In basic solution, HCr04 is the major species. Under strongly acidic conditions, only dichromate ion exists. Cr(III) species in acidic solution exist as Cr(H20)5 ions and in concentrated alkali have been identified as Cr(OH)5 and Cr(0H)5(H20)2-. 

There is formation of hydrogen chromate ions (or acidic chromate ions) and also of dichromate ions (see Chap. 20). Due to the former equilibrium, if the pH value is too low, the concenttation [Cr04 ] is considerably lowered and the solubility product ATs(Ag2Cr04) = [Ag+] [Cr04 ] is never reached. The change at the final point is... 

In order that a chromate film may be deposited, the passivity which develops in a solution of chromate anions alone must be broken down in solution in a controlled way. This is achieved by adding other anions, e.g. sulphate, nitrate, chloride, fluoride, as activators which attack the metal, or by electrolysis. When attack occurs, some metal is dissolved, the resulting hydrogen reduces some of the chromate ion, and a slightly soluble golden-brown or black chromium chromate (CtjOs CrOs xHjO) is formed. 

Oxidation of isopropyl alcohol (H2R) by chromic acid has been studied in det ai by Westheimer and Novick , and it was found that acetone (R) is formed nearly quantitatively. The reaction proved to be first order with respect to hydrogen chromate and second order with respect to hydrogen ions. Measurements using 2-deutero-2-propanol under identical conditions as those for the oxidation of ordinary isopropyl alcohol showed the rate of reaction to be of that with the hydrogen compound. This fact is considered to prove that the secondary hydrogen atom is removed in the rate-controlling step and that the assumption of hydride-ion abstraction can be excluded. The data are consistent with the following mechanism... 

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). 

After decomposing the excess of hydrogen peroxide by boiling, chromate ions may be identified in the solution by one of its reactions (cf. Section IV.33). 

The Mohr titration must be carried out at a pH of 7 to 10 because chromate ion is the conjugate base of the weak chromic acid. Consequently, in more acidic solutions, the chromate ion concentration is too low to produce the precipitate near the equivalence point. Normally, a suitable pH is achieved by saturating the analyte solution with sodium hydrogen carbonate. 

The hydrolysis kinetics at low alkalinity were also measured, by using an anucleophilic buffer of lutidine/lutidinium ion at pH 8. The rate constant for this reaction (Cr O + H O 2 HCrO ) agreed well with that obtained recently by a relaxation method (8). Arrhenius parameters for this and for the ammonia-catalysed hydrolysed (7) are being obtained, and it is hoped to report on the acid-catalysed interconversion (9) of hydrogen chromate and bichromate ions. 

Iron becomes passive when it is dipped in very concentrated nitric acid. It then no longer displaces hydrogen from dilute acids. However, a sharp blow on the metal produces a change that spreads over the surface from the point struck, the metal once more becoming active. This production of passivity is due to the formation of a protective layer of oxide, and the passivity is lost when the layer is broken. Passivity is also produced by other oxidizing agents, such as chromate ion safety razor blades kept in a solution of potassium chromate remain sharp much longer than blades kept in air. 

The chromate ion has a tetrahedral structure. The formation of dichromate ion involves the removal of one oxygen ion 0 (as water), by combination with two hydrogen ions, and its replacement by an oxygen atom of another chromate ion. 

Barium nitrite, sodium hydrogen carbonate, aluminum nitrate, calcium chromate (chromate ion, Cr04 )... 

Oxidizing acids are aqueous solutions in which ions other than hydrogen react as oxidizing species, e.g. nitrate ion (nitric acid), chromate ion (chromic acid), or oxygen. 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

In metallic form, barium is very reactive, reacting readily with water to release hydrogen. In aqueous solution it is present as an ion with a +2 charge. Barium acetate, chloride, hydroxide, and nitrate are water-soluble, whereas barium arsenate, chromate, duoride, oxalate, and sulfate are not. Most water-insoluble barium salts dissolve in dilute acids barium sulfate, however, requkes strong sulfuric acid. 

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