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|Section2= |Section3= }} The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the molecular species, H2CrO4 of which the trioxide is the anhydride. Chromic acid features chromium in an oxidation state of +6 (or VI). It is a strong and corrosive oxidising agent. == Molecular chromic acid == Molecular chromic acid, H2CrO4, has much in common with sulfuric acid, H2SO4. Both are classified as strong acids, though only the first proton is lost easily. :H2CrO4 ()− + H+ The pKa for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6.〔(IUPAC SC-Database ) A comprehensive database of published data on equilibrium constants of metal complexes and ligands〕 The value at zero ionic strength is difficult to determine because half dissociation only occurs in very acidic solution, at about pH zero, that is, with an acid concentration of about 1 mol dm−3. A further complication is that the ion ()− has a marked tendency to dimerize, with the loss of a water molecule, to form the dichromate ion, ()2−: :2 ()− ()2− + H2O, log KD = 2.05. Furthermore, the dichromate can be protonated: :()− ()2− + H+, pK = 1.8 The pK value for this reaction shows that it can be ignored at pH > 4. Loss of the second proton occurs in the pH range 4–8, making the ion ()− a weak acid. Molecular chromic acid could in principle be made by adding chromium trioxide to water (''cf.'' manufacture of sulfuric acid). :CrO3 + H2O H2CrO4 but in practice the reverse reaction occurs when molecular chromic acid is dehydrated. This is what happens when concentrated sulfuric acid is added to a dichromate solution. At first the colour changes from orange (dichromate) to red (chromic acid) and then deep red crystals of chromium trioxide precipitate from the mixture, without further colour change. The colours are due to LMCT transitions. Chromium trioxide is the anhydride of molecular chromic acid. It is a Lewis acid and can react with a Lewis base, such as pyridine in a non-aqueous medium such as dichloromethane (Collins reagent). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chromic acid」の詳細全文を読む スポンサード リンク
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