Thermodynamic study of disorder in hexa-amminenickel(II) iodide and diamminenickel(II) iodide

Abstract

The heat capacity of crystalline nickel(II) iodide, diamminenickel(II) iodide and hexa-ammine-nickel(II) iodide has been determined from 10 to 300 K by conventional adiabatic low-temperature calorimetry, and from 300 to 550 K by differential scanning calorimetry. Measurements have also been made of (a) the heat of solution of the three salts in dilute acid at 298.15 K, (b) the ammonia dissociation pressure for the systems nickel iodide + diamminenickel iodide (from 490 to 523 K) and diamminenickel iodide + hexa-amminenickel iodide (from 469 to 503 K), (c) the magnetic susceptibility of nickel iodide and diamminenickel iodide from 4 to 230 K. The thermodynamic results have been used to obtain for the diammine and the hexammine two values of the molar entropy at 298.15 K, namely the third-law entropy S_cal, and the entropy S_eq derived from the appropriate equilibrium study. For each salt, S_cal and S_eq agree within experimental error, so that both salts appear to become completely ordered at 0 K. The implications of this result for hexa-amminenickel iodide are briefly discussed in the light of previous experimental and theoretical work on this salt.