Make your own cobalt chloride indicator papers, which can be used to test for the presence of water

This resource provides instructions for preparing your own cobalt chloride indicator papers. You can use the papers to test for the presence of water, where the colour changes from blue to pink.

Cobalt(II) chloride is an inorganic compound of cobalt and chlorine, with the formula CoCl2. It is usually supplied as the hexahydrate CoCl2·6H2O. The hexahydrate is deep purple in color, whereas the anhydrous form is pale blue. Cobalt chloride paper is useful because the hydration/dehydration reaction occurs readily making the paper an indicator for water.



  • Filter paper
  • Drying oven set at 100 °C
  • Desiccator


  • Hexahydrate cobalt(II) chloride CoCl2·6H2O (TOXIC, DANGEROUS FOR THE ENVIRONMENT, SENSITISER ), 5 g
  • Water, 100 cm3

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Cobalt chloride (TOXIC, DANGEROUS FOR THE ENVIRONMENT, may cause SENSITISATION) – see CLEAPSS Hazcard HC025 and CLEAPSS Recipe Book RB030. The indicator papers should be handled as little as possible and hands washed after use.


  1. Dissolve 5 g of hydrated cobalt(II) chloride in 100 cm3 of water (solution is TOXIC).
  2. 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.
  3. The filter paper can be cut into small strips and stored in a desiccator with dry silica gel.