Nickel(II) sulfate, or just nickel sulfate, usually refers to the inorganic compound with the formula NiSO₄(H₂O)₆. This highly soluble turquoise coloured salt is a common source of the Ni²⁺ ion for electroplating. Approximately 40,000 tonnes were produced in 2005.At least seven sulfate salts of nickel(II) are known. These salts differ in terms of their hydration or crystal habit.The common tetragonal hexahydrate crystallizes from aqueous solution between 30.7 and 53.8 °C. Below these temperatures, a heptahydrate crystallises, and above these temperatures an orthorhombic hexahydrate forms. The yellow anhydrous form, NiSO₄, crystallizes in orthorhombic crystal system and in standard pressure decomposes to NiO in temperatures above 640 °C, before reaching the melting point. It melts only at high system pressure, using a constant volume method the melting point was found to be 1210 °C. The anhydrous sulfate is produced by heating the hydrates above 330 °C.X-ray crystallography measurements show that NiSO₄·6H₂O consists of the octahedral [Ni(H₂O)₆]²⁺ ions. These ions in turn are hydrogen bonded to sulfate ions. Dissolution of the salt in water gives solutions containing the aquo complex [Ni(H₂O)₆]²⁺.All nickel sulfates are paramagnetic.