Summary The acid, K 2 Cr 2 O 7 , has been titrated potentiometrically with the carbonates of Li, Na, K, Ca, Sr, Ba and Pb in fused KNO 3 at 350°. Both “forward” and “backward” neutralization curves (except for the case of Pb) consist of a single step corresponding usually to formation of CrO 4 2− . “Cyclic” neutralization curves with NaOH and KOH suggest that Na + is acid with respect to K + . The acidity of Ca 2+ , Sr 2+ and Ba 2+ decreases in the order given. CaCrO 4 and SrCrO 4 add an extra molecule of Na 2 CO 3 to give CaCrO 4 ·Na 2 CO 3 (SrCrO 4 ·Na 2 CO 3 ); BaCrO 4 does not. Pb 2+ is the most acid cation studied. PbCrO 4 adds another molecule of PbCO 3 and gives a second inflexion in the titration curves, corresponding to the formation of PbCrO 4 ·PbCO 3 . PbCO 3 decomposes thermally in fused KNO 3 to give white lead, 2PbCO 3 ·PbO. The reaction is zero-order with respect of PbCO 3 and has an activation energy of 38.2 kcal mole −1 .