I mix a 1% solution of the metals and a 5% citric acid solution, and mix immediatly prior to use. In my case this means 5ml of the metal, 5ml of the citric, and 40ml distilled H2O for an 8x10 image on 11x14 paper in a 11x14 flat bottem tray. This is for toning kallitypes.

I was a bit afraid to mix the acid and metals togather in larger quantities for the same reason, too expensive to screw up.