Abstract: A method of preparing mixtures of dilute acid of varying final specific gravities utilizes microprocessor controlled mixing in two in-line stages. Process recycled or production reject acid or water is initially mixed with concentrated acid and directed to an intermediate heat exchanger for removing the exothermic heat generated during mixing. The specific gravity of the mixture exiting from the heat exchanger is continuously measured and fed to a microprocessor which calculates the deviation between the measured specific gravity and a preselected set point specific gravity which is greater than the desired final specific gravity. The calculated deviation is used to generate a control signal which, in turn, is applied to adjust the flow of concentrated acid until the measured specific gravity corresponds to the set point specific gravity.