Layered double hydroxide stability. 2. Formation of Cr(III)-containing layered double hydroxides directly from solution

Solutions containing divalent metal [M(II) = Mg2+, Zn2+, Co2+, Ni2+, Mn2+] chlorides and CrCl3. 6H(2)O were titrated with NaOH to yield, for M(II) = Z n, Co, and Ni, hydrotalcite-like layered double hydroxides (LDHs), [[M(II)] (1-z)[Cr(III)](z)(OH)(2)[Cl](z). yH(2)O, in a single step, without intermed iate formation of chromium hydroxide. Analysis of the resultant titration c urves yields solubility constants for these compounds. These are in the ord er Zn < Ni approximate to Co, with a clear preference for formation of the phase with z = 1/3. With Mg2+ as chloride, titration gives a mixture of Cr( OH)(3) and Mg(OH)(2), but the metal sulfates give Mg2Cr(OH)(6). 1/2(SO4) by a two-step process. Titrimetric and spectroscopic evidence suggests short- range cation order in the one-step LDH systems.