A method and apparatus for production of a high-concentration H2SO5 solution is presented. This high-concentration H2SO5 solution is useful for producing a stable, non-hygroscopic triple salt. The invention includes a single-stage reactor that includes a reservoir for holding an oxyacid solution and an inlet to the reservoir for receiving a peroxide solution. The peroxide solution is added such that a gradient of peroxide concentration forms in the oxyacid solution as a function of distance from the inlet, and less than all of the oxyacid solution reacts with the peroxide solution at a given time. A stirring mechanism slowly dissipates the gradient, so that any H2SO5 formed as a result of reaction between the peroxide and the oxyacid is stirred away from the high-peroxide concentration zone, thereby preventing the decomposition of H2SO5 by an equilibrium reaction. The temperature of the reservoir is maintained at less than or equal to 20 ° C.