During the last week, your grab-sample DPD colorimetric tests for free chlorine residual at the effluent suddenly dropped from about 1.0 mg/L to near 0.0 mg/L, even though the chlorine feed rate and contact time have not changed. The disinfected effluent still shows good fecal coliform kill and no increase in process upsets. When opening a new packet of DPD powder, you notice the powder has a pinkish tint before it touches the sample. What is the most likely cause and best immediate action?