Chromium compounds, general properties

Trivalent chromium compounds, except for acetate, nitrate, and chromium(III) chloride-hexahydrate salts, are generally insoluble in water. Some hexavalent compounds, such as chromium trioxide (or chromic acid) and the ammonium and alkali metal (e.g., sodium, potassium) salts of chromic acid are readily soluble in water. The alkaline metal (e.g., calcium, strontium) salts of chromic acid are less soluble in water. The zinc and lead salts of chromic acid are practically insoluble in cold water. Chromium(VI) compounds are reduced to chromium(III) in the presence of oxidizable organic matter. However, in natural waters where there is a low concentration of reducing materials, chromium(VI) compounds are more stable (EPA 1984a). For more information on the physical and chemical properties of chromium, see Chapter 3. 

General Properties of Compounds of Cobalt—Cobalt and the Halogens—Oxy-Halogon Derivatives—Cobalt and Oxygen—Cobalt and Sulphur—The Alums—Cobalt and Selenium, Tellurium, Chromium, and Molybdenum—... 

General Properties of Chromium Compound.s—Chromium and Hydrogen— Halogen and Oxyhalogen Compounds—Oxides and Hydroxide.s— Chromites—Chromic Acid—Chromates, Dichromates, and Polyohromates— Perohromio Acid and Perehromate.s—Compounds with Sulphur, Selenium, and Tellurium. 

General Properties of Molybdenum Compounds—Molybdenum and Hydi-ogcn— Halogen and Oxyhalogen Compounds—Oxides and Hydroxides—Molybdio Acid, Molybdates, and Polymolybdates—Permolybdio Acid and Permolyb-dates—Compounds with Sulphur, Selenium, and Chromium. 

BIOLOGICAL PROPERTIES not likely to migrate to groundwater uptake by plants is generally low chromium compounds are very persistent in water, half-life >200 days most in surface waters may be present in particulate form as sediment Cr (IV) is the major stable form in seawater can be detected in water by digestion followed by atomic absorption or by colorimetry analysis or by Inductively Coupled Plasma Optical Emission Spectrometry chromium (IV) can be detected by extraction and atomic adsorption or colorimetry dissolved forms can be detected by 0.45 p filtration followed by the previous methods... 

Reactions ofthe Group 2A Metals 27-6 Uses ofGroup2A Metals and Their Compounds The Post-Transition Metals 27-7 Group 3A Periodic Trends The d-Transition Metals 27-8 General Properties 27-9 Oxidation States 27-10 Chromium Oxides, Oxyanions, and Hydroxides... 

Much more important commercially are the 2 1 chromium(III) and 2 1 cobalt(III) complexes of tridentate azo compounds, which find a wider application, particularly as dyestuffs for wool, polyamide fibres and leather. These have been the subject of reviews23 24 which discuss their dyeing properties in detail. The patent literature on metal complex dyes of these types is vast but since this relates principally to the achievement of specific, desirable technical effects by appropriate substitution of the azo compounds it will not be considered in detail here. Rather will the emphasis be placed upon those aspects of dyestuffs of this type which are of general interest in the context of their coordination chemistry and, more particularly, on those areas where uncertainties exist or conflicting results have been reported. 

Common chemical properties The alkali metals are so chemically reactive that they are never found free in nature. Sodium and potassium react explosively with water to produce hydrogen gas. The alkaline earth metals are not quite as reactive as the alkali metals. The alkali metals react with water but not explosively. The transition metals are generally the least reactive of all the metals. However, when they combine with other elements, they form a large variety of colored compounds. Chromium oxide is green, titanium oxide and zinc oxide are white, manganese oxide is purple, and iron oxide is ochre. 

Chromic hydroxide is an amphoteric compound and exhibits acidic properties in combining with basic oxides to form chromites, to which the general formula M-jO.CrjOg is given, and which are isomorphous with the corresiiouding aluminium compounds known as spinels. They may be considered as derived from an acid, HCrOj the monohydrate, CrjOj.HjO, has this empirical formula. From a study of the action of sodium hydroxide on chromium hydroxide for prolonged periods and the rate of the formation of chromate by the oxidation of dissolved chromite, it would appear that the chronlic hydroxide acts as a poly basic acid. 

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