Humidity Indicator
Difficulty: Easy | Time: 20 minutes (plus drying time) | Visual Impact: Very High
TOXIC: Cobalt chloride is a Category 1B carcinogen. Wear gloves throughout. Do not let children handle the solution. Adult supervision required.
Historical Context
The color change of cobalt chloride with water was known to early chemists, but its practical use as a humidity indicator came in the 19th century. The reversible pink-blue transition made it ideal for desiccant indicator cards placed in sensitive equipment packaging - a fresh blue card confirms the contents are dry; pink means moisture has entered.
Industrial and laboratory desiccant products (silica gel, molecular sieves) commonly included cobalt chloride-impregnated indicator beads. The familiar blue-to-pink change of silica gel “indicator” products is cobalt chloride chemistry. However, due to its carcinogenic classification, many modern products have replaced it with less hazardous indicators like methyl violet or manganese-based compounds.
Making indicator paper in the lab captures the same elegant chemistry and makes the concept of coordination chemistry visible and tactile.
Materials
- Cobalt chloride - 1g dissolved in 20mL water
- Filter paper or thick paper cut into strips
- Small bowl or dish
- Tongs or gloves for handling soaked paper
- Hair dryer or oven (60-80°C) for drying
- Gloves (required)
Procedure
- Put on gloves.
- Dissolve cobalt chloride in water to make a pink solution.
- Soak paper strips in the solution for 30 seconds.
- Lift strips with tongs and lay on a clean surface.
- Dry thoroughly with a hair dryer or in a warm oven until the paper turns bright blue.
- The indicator paper is ready. Store dry.
Testing the indicator:
- Hold the blue paper over a cup of hot water - it turns pink where steam contacts it.
- Breathe on it - it turns pink from the humidity in your breath.
- Bring in from outdoors on a humid day vs. a dry day and compare how quickly it changes.
- Dry it with a hair dryer to restore the blue color and repeat.
Reaction
\[\ce{CoCl2 (blue) + 6 H2O <=> CoCl2·6H2O (pink)}\]
The Science
Anhydrous cobalt chloride is blue; the hexahydrate is pink. On paper, the cobalt chloride equilibrium responds to ambient humidity: in dry conditions the anhydrous (blue) form dominates, in humid conditions the hydrated (pink) form dominates.
The color shift reflects a change in coordination geometry. Dry: chloride ligands coordinate in a tetrahedral arrangement (blue). Wet: water molecules displace chloride and coordinate octahedrally (pink). The same cobalt ion, same solution - just a different geometric arrangement of surrounding molecules.
This is qualitative, not quantitative, but the change is fast and very visible. On a dry winter day the paper stays blue; on a humid summer day it stays stubbornly pink even after drying.
Safety
Cobalt chloride is classified as a probable human carcinogen (Category 1B). Wear gloves at all stages, especially when handling wet paper soaked in solution. Keep solution away from skin and eyes. Dispose of the cobalt chloride solution as chemical waste. Finished indicator paper should also be treated as chemical waste and not left where children or animals might handle it.