What I ended up doing is making a self dumping tub out of a small plastic parts tray and a pneumatic cylinder. Then I mounted the assembly on top of a 5 pound scale with an analog output, mounted so it dumps over the edge of the scale.
The PLC records the empty weight after each dump for tare and the weight while filling, stops the auger when the right amount and dumps the tray into the mixing tank below the scale.
If the solution gets low concentration the operator can adjust the powder weight on the HMI, within reasonable limits.
Also, the powder feels and acts exactly like powdered sugar used in baking and cake icing, except one cup of the stuff weighs almost 3 pounds/1.3 kg and it is a very dark gray color, as it's powdered copper.