Indium(III) sulfate (In2(SO4)3) is a sulfate salt of the metal indium. It is a sesquisulfate, meaning that the sulfate group occurs 1 times as much as the metal. It may be formed by the reaction of indium, its oxide, or its carbonate with sulfuric acid. An excess of strong acid is required, otherwise insoluble basic salts are formed. As a solid indium sulfate can be anhydrous, or take the form of a pentahydrate with five water molecules or a nonahydrate with nine molecules of water. Indium sulfate is used in the production of indium or indium containing substances. Indium sulfate also can be found in basic salts, acidic salts or double salts including indium alum. Properties In water solution, the indium ion forms a complex with water and sulfate, examples being In(H2O)5(SO4)+ and In(H2O)4(SO4)2−. Indium is unusual in forming a sulfate complex. The effect on the sulfate ion is revealed in the Raman spectrum. The proportion of sulfate complex increases with temperature showing the reaction that forms it is endothermic. The proportion also increases with concentration of the solution and can be over a half. The sulfate complex rapidly exchanges with water at a rate of over 10,000,000 per second, so that NMR cannot detect the difference that results from a complexed and noncomplexed indium ion. An indium sulfate water solution is quite acidic with a 0.14 mol/liter solution having a pH of 1.85. If the pH rises above 3.4 then a precipitate will form. The Raman spectrum of the solution shows lines at 650, 1000 and 1125 cm−1 due to a sulfur–oxygen bonds in sulfate bound to indium. A line at 255 cm−1 is due to the indium-oxygen bond to the sulfate. The water attached to the indium atom causes a band at about 400 cm−1. Solid anhydrous indium sulfate has two crystalline forms. When formed by chlorine gas chemical transport at 848 K, it has a monoclinic form with unit cell dimensions a = 8.570 Å, b = 8.908 Å and c = 12.0521 Å, β = 91.05°, and four formulae per cell. A high temperature form deposited at 973K has a hexagonal (or rhombohedral) form with cell dimensions of a = 8.440 Å, c = 23.093 Å and six formulae per cell. During extraction of indium, a sulfate solution of mixed metals, including indium sulfate, has trivalent metals partitioned into a kerosene solution of di-2-ethylhexyl hydrogen phosphate.