The most important influences on the width of the metastable zone are the kind and concentration of the impurities existing in the soultion. The presence of impurities in solution leads to different disturbances on the yield of a product. Only an experimental determination for the effects of impurities exists in the relevant literature without any interpretation of these results. Most explanations of crystal growth under additive effects start at the solid phase but do not take into account the change in the structure of the solution when additives are added to a solution.

Here it was investigated if the properties of intermolecular interactions cations and anions each for additives, solvent and salt in the solution are the key of the interpretation of influences of additives on the metastable zone width of inorganic compounds.

The polythermal method was used to measure the effect of the additives, e.g. Al³⁺, Fe²⁺, Ba²⁺, Li¹⁺ and K¹⁺ on the MSZ width of ZnSO₄. These additive ions were selected according to a suggested rule, which takes the amount of hydration enthalpy values into account. The results show that the additives affect the MSZ width not always as expected. Also it is explained that the effect of Al³⁺, Fe²⁺ and Li¹⁺ on the MSZ width of zinc sulfate can be given in terms of hydration enthalpy values and ion-ion interaction forces. This approach is, however, still not sufficient to interpret the effect of all additives e.g. not for BaCl₂ and K₂SO₄ on the MSZ width of ZnSO₄.