Refer to CLEAPSS Hazcard and CLEAPSS Recipe Book.
a Dissolve 5 g of hydrated cobalt(II) chloride in 100 cm3 of water (solution is TOXIC).
b Soak filter paper in this solution, drain and dry in an oven (set at no more that 100°C), so that it is a definite blue colour.
c The filter paper can be cut into small strips and stored in a desiccator with dry silica gel.
Cobalt chloride is TOXIC and DANGEROUS FOR THE ENVIRONMENT and May cause sensitisation. The indicator papers should be handled as little as possible and hands washed after use.