Chromate , acidity

Synonyms Bis(tert-butyl)chromate acid, di-tert-butyl ester... 

HAZARD RISK Combustible reacts vigorously with oxidizing materials, fluorine, per-chromates, acids Ignites on contact with sodium peroxide and water decomposition emits toxic fumes ofNOx NFPA Code H 3 F 2 R 0. 

This reaction was initially reported by Etard in 1880. It is the oxidation of hydrocarbons by hexavalent chromium compounds (chromyl chloride, i.e., chromatic acid chloride) to form a mixture of alcohol, carbonyl compounds (aldehyde or ketones), chloro-ketones, or aldehydes and the starting material. Therefore, this reaction is generally known as the Etard... 

Clean the containers (Section 6) with a cleaning solution that will not impact the results, such as a chromatic acid substitute, rinse with tap water followed by distilled water, and dry in an oven set at a minimum of 105°C for at least 1 h. Likewise, clean all other glassware used in this test method. 

Lead(Il) chromate VI) is precipitated when a soluble chromate(VI) or dichromatelVl) is added to a solution of a lead salt in neutral or slightly acid solution ... 

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

Addition of silver nitrate to a solution of a chloride in dilute nitric acid gives a white precipitate of silver chloride, AgCl, soluble in ammonia solution. This test may be used for gravimetric or volumetric estimation of chloride the silver chloride can be filtered off, dried and weighed, or the chloride titrated with standard silver nitrate using potassium chromate(VI) or fluorescein as indicator. 

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. 

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 ... 

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 ... 

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. 

Addition of lead(II) nitrate in ethanoic acid solution gives a yellow precipitate of lead chromate. PbCr04. 

In the former, it gives precipitates with halides (except the fluoride), cyanides, thiocyanates, chromates(VI), phosphate(V), and most ions of organic acids. The silver salts of organic acids are obtained as white precipitates on adding silver nitrate to a neutral solution of the acid. These silver salts on ignition leave silver. When this reaction is carried out quantitatively, it provides a means of determining the basicity of the acid... 

In neutral solution, the indicator is potassium chromate(VI). In acid solution the CrOj" ion changes to CrjO (p. 378). and since silver dichromatefVI) is soluble, chromate(VI) is not a suitable indicator other methods can be used under these conditions. (In alkaline solution, silverfl) oxide precipitates, so silver(I) nitrate cannot be used under these conditions.)... 

Propionaldehyde. Use 34 g. (42-6 ml.) of n propyl alcohol, and a solution containing 56 g. of sodium chromate dihydrate, 300 ml. of water and 40 ml. of concentrated sulphuric acid. The experimental details are identical with those for n-butyraldehyde, except that the addition of the dichromate solution occupies 20 minutes, the temperature at the top of the column is not allowed to rise above 70-75°, and during the subsequent heating for 15 minutes the liquid passing over below 80° is collected the receiver must be cooled in ice. The yield of propionaldehyde, b.p. 47-50°, is 12 g. 

The allyiic oxidation with derivatives of chromic acid (K.B. Wiberg, 1965 H.G. Boscbe, 1975), c.g. di-/-butyl chromate (K. Fujita, 1961), CrOj/pyridine (W.O. Dauben, 1969), or KjCrO (C.Y. Cuiileron, 1970), gives similar products in occasionally very high yields. 

Chromic acid (H2Cr04) is a good oxidizing agent and is formed when solutions containing chromate (Cr04 ) or dichromate (Cr207 ) are acidified Sometimes it is possible to obtain aldehydes m satisfactory yield before they are further oxidized but m most cases carboxylic acids are the major products isolated on treatment of primary alco hols with chromic acid... 

Step 1 Reaction of the alcohol with chromic acid gives an alkyl chromate... 

Oxidation of primary alcohols to aide hydes (Section 15 10) Pyridinium di chromate (PDC) or pyridinium chloro chromate (PCC) in anhydrous media such as dichloromethane oxidizes primary al cohols to aldehydes while avoiding over oxidation to carboxylic acids... 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

Chromate(VI) Reduction with arsenate(III), ascorbic acid, hydrazine, hydroxylamine, sulfite, or thiosul-... 

Hydrazine Alkali metals, ammonia, chlorine, chromates and dichromates, copper salts, fluorine, hydrogen peroxide, metallic oxides, nickel, nitric acid, liquid oxygen, zinc diethyl... 

Cm ORINE OXYGEN ACIDS AND SALTS - DICm ORINE MONOXIDE, HYPOCm OROUS ACID, AND HYPOCm ORITES] (Vol 5) Calcium chromate [13765-19-0]... 

Dilead hexacyanokisferrate [14402-61 -0] Pb2[Fe(CN)g], is a white precipitate that forms when lead acetate is added to Ca2[Fe(CN)g]. It is insoluble in water or dilute acids but is soluble in hot ammonium chloride or ammonium succinate solutions. It has been used as a quaUtative analytical reagent in tests for cadmium and chromate. 

Lead—copper alloys are also used as tank linings, tubes for acid mist precipitators, steam heating pipes for sulfuric acid or chromate plating baths, and flashing and sheeting (see Tanks AND pressure vessels). 

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