Comparison of Stability Constants of Alkaline Earth Chlorides between Conductometric and Potentiometric Studies

Abstract

Abstract The stability constants of alkaline-earth metal chlorides, MCl+ (M2+=Ca, Sr, Ba), were determined by conductometric and potentiometric measurements in methanol. The conductometric study was made at 5, 15, 25, 35, and 45°C; the potentiometric study was at 25°C. The formation of MCl2 was neglected in the conductometric study, while in the potentiometric study it was taken into account, though it was too slight to determine a reliable value of the stability constant of MCl2. The stability constants of MCl+, K1, determined by conductometric measurements were compared with the previous conductometric results of the same metal(II) perchlorates, K1A. The magnitudes of K1 and K1A increase in the orders Ca \simeqSr<Ba and Ca<Sr<Ba, respectively. K1 is larger than K1A for calcium and strontium, though the K1 and K1A values for barium are similar to each other; that is, the degree of such an increase in K1A is larger than that in K1. Both enthalpy and entropy changes determined by conductometric measurements were positive for all the chlorides and perchlorates. However, the enthalpy changes of the chlorides were 1.2–1.4 times as large as those of the perchlorates. On the other hand, the K1 from potentiometry was smaller for every chloride and also increased more steeply (in the order Ca<Sr<Ba) than that from conductometry. Therefore, the difference of K1 between conductometry and potentiometry decreases in going from Ca to Ba.