Freezing temperatures of aqueous iron(III) sulfate solutions and crystallization of a new acidic water-rich sulfate

•Freezing temperatures of aqueous iron(III) sulfate solutions were determined.•Effect of excess sulfuric acid on freezing and phase formation at low temperatures.•A new water-rich acidic iron(III) sulfate was discovered. An important question concerning the possibility of life under martian conditions is the existence of liquid water at low temperatures. On the martian surface, the existence of iron(III) sulfate is expected. The influence of iron(III) sulfate salt on ice deposits in respect to the formation of liquid salt brines was not investigated in the past. In this contribution, the investigation of the phase diagram of the system iron(III) sulfate–water and the influence of sulfuric acid to this system are presented. A new crystalline acidic iron(III) sulfate hydrate has been found in the ternary system iron(III) sulfate–water–sulfuric acid, which represents the most water-rich iron salt phase ever detected.