A method for the purification of spent sulfuric acid and particularly the purification of spent acid, from titanium dioxide rutile manufacture through a chloride route, is provided. In the chloride route of titanium dioxide manufacture, sulfuric acid is used to clean the un-reacted gaseous flow coming out of the oxidizer, so the spent acid mainly contains un-reacted Ti4+ ions, trace amounts of Fe3+ ions, and NO2 gas. Titanium phosphate can be precipitated using acidic and or alkaline phosphate-containing precipitants in stoichiometric amounts. The method can include the addition of cutting water to commence the precipitation in two or more steps, when an acidic precipitant is used, and in one step with half the volume of water when an alkaline precipitant is used.