DS-3 is pure, as in not mixed with sand or any of the other things you find added to kettle descaler sachets to make the active ingredient in the things fill more horribly over-priced packets. However, it has a coloured dye and a smell added, so you don't mistake it for something else. The dye is also an indicator, changing colour when the acid is exhausted. I have noticed the dye stain plastic brush bristles -- I guess there's a possibility of it staining some more important plastics. It's not suitable for use on enamelled baths.