Abstract: There is described a method of preparing working strength solution in a processing apparatus which comprises adding a requisite volume of concentrated solution to a storage vessel until the liquid level on the storage vessel reaches the top of a standpipe in the storage vessel and continuing to add diluent to the storage vessel so that excess liquid in the storage vessel flows down the standpipe to a connected working vessel, stopping the flow of diluent when the volume of liquid in the working vessel reaches a predetermined volume, then commencing to pump liquid drawn in part at least from the bottom of the working vessel into the storage vessel, the excess liquid in the storage vessel flowing into the working vessel, and continuing to pump liquid from the working vessel to the storage vessel until the specific gravities of the liquid in the storage vessel and in the working vessel are the same.