In basic solution abovepH 6, CrO_(3)forms the tetrahedral yellow chromate ion, CrO_(4)^(2-). BetweenpH 2 and 6, HCrO_(4)^(-) and the orange red dichromatedichromate ion, Cr_(2)O_(7)^(2-), arein equilibrium. At pH values below 1, themain species is H_(2)PCrO_(4). Theequilibria are HCrO_(4)^(-) hArrCrO_(4)^(2-) + H^(+), K = 10^(-5.9) H_(2) CrO_(4) hArr HCrO_(4)^(-) + H^(+), K = 4.1 Cr_(2)O_(7)^(2-)+H_(2)O iff 2HCrO_(4)^(-),K=10^(-2.2) In addition, these are base- hydrolysis equilibria Cr_(2)O_(7)^(2-)+OH^(+) iff HCrO_(4)^(-)+CrO_(4)^(2-) HCrO_(4)^(-)+OH^(-) iff CrO_(4)^(2-) iff CrO_(4)^(2-) +H_(2)O The pH dependentequilibria are quite labile and on addition of cations that form insoluble chromates (e.g., Ba^(2+),Pb^(2+) " and " Ag^(+)), the chromates and not the dichromates are precipitated. In the redox reaction involving Cr_(2)O_(7)^(2-) and Fe^(2+) ions, the number of electrons absorbed per chromium atom is