The effect of temperature and pressure on the Gibbs standard free energy formation of ionic species, hence the equilibrium constant involving these ions has been reviewed and discussed. The role of the partial molal volume on the equilibrium constant was reviewed first. It was found that the effect of pressure is negligible unless the pressure applied is excessive, say over 200 atmospheres. The Gibbs standard free energy formation of various metal ions in water at various temperatures has also been calculated using a conventional method as well as the correspondence principles. The temperatures considered were 60, 100, 150 and 200℃. The results obtained for these temperatures based on these methods have been compared and discussed. It has been found that the results by these two methods do not differ much, mostly within 5%, which is contrary to what has been believed by many.