Test discs and reagent for the semi-quantitative determination of fluoride ions. The fluoride test is especially suited for rapid control of fluoride concentrations. Its purpose is to define concentration ranges and to detect values which fall short of or exceed given limits. Chlorates and bromates cause white discolourations of the test paper. Addition of sodium dithionite (Na₂S₂O₄) will circumvent this interference without affecting the fluoride test.