Thanks for the information and for debunking my chemistry "theories". The solutions were definitely from the right bottles, so I guess that leaves contamination. I use a pipette with a rubber bulb on the end to measure out the concentrates, and although I try to wash it out thouroughly, a few drops of tap water are always left inside. The rubber bulb is also hard to control, and if I suck up too much concentrate, I squirt the excess back into the bottle, but maybe I shouldn't do that! I'll try to mix up some new sol'n B, and see what happens...