Humidity Indicator
Summary
Cobalt-chloride paper is blue when dry and pink when damp, so it reads the humidity of the air — a reversible switch between two complex-ion1 geometries. You’ll be able to explain why the coordination geometry changes the colour, why water displaces chloride when it’s humid, and why the change is fully reversible.
History
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.
Hazards & preparation
Toxic — adult supervision required. Cobalt chloride is a Category 1B carcinogen.
- Wear gloves throughout, especially when handling the wet, soaked paper. Keep the solution off skin and out of eyes, and don’t let children handle it.
Disposal: cobalt is a heavy metal — do not pour the solution down the drain. Hold the solution and the finished indicator paper for hazardous-waste collection, and keep the paper away from children and pets. See the Safety page.
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 the cobalt chloride in water to make a pink solution.
- Soak the paper strips for 30 seconds.
- Lift the strips with tongs and lay them 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 it dry.
Testing the indicator: hold the blue paper over a cup of hot water (it turns pink where steam touches it), breathe on it (pink from your breath’s humidity), or compare how fast it changes on a humid vs. dry day. Dry it again to restore the blue and repeat.
What you should see
Wet, the paper is pink; dried hard it flips to a clear bright blue. Held over steam or breathed on, the blue washes back to pink within seconds wherever moisture reaches it — and a blast from the hair dryer turns it blue again. It’ll cycle as often as you like.
| Symptom | Likely cause | Fix |
|---|---|---|
| Paper won’t turn blue when dried | Not dry enough, or humid room | Dry longer/hotter; store with a desiccant |
| Colour weak | Solution too dilute | Use a stronger cobalt-chloride solution |
| Blue won’t return to pink | Very dry air | Breathe on it or hold over steam; moisture brings the pink back |
The reactions
\[\ce{CoCl2 (blue) + 6 H2O <=> CoCl2·6H2O (pink)}\]
dry blue cobalt chloride absorbs water to become the pink hydrate, and gives it back when dried — a reversible, humidity-driven colour switch.
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.
Questions to Explore
Why does geometry change the colour? The same cobalt ion is pink octahedral or blue tetrahedral. Why do the two geometries absorb different light?
Hint / answer
The surrounding ligands split the cobalt’s d-orbitals, and the size of that split depends on how many ligands and in what geometry. Octahedral (six waters) and tetrahedral (four chlorides) split them by different amounts, so each absorbs a different colour of light — pink vs. blue.
Why water specifically? Water displaces chloride when it’s humid. Why does the equilibrium favour the hydrate when water is available?
Hint / answer
When water is plentiful, there’s simply a lot of it to bind the cobalt, and the octahedral water complex is the more stable arrangement under those conditions. By Le Chatelier’s principle, abundant water pushes the equilibrium toward the pink hydrated form.
Could you make a quantitative meter? The paper says only pink vs. blue. What would you calibrate to read a percentage?
Hint / answer
You’d measure the shade (say with a colour sensor or photo) and calibrate it against known humidities from a reference hygrometer, building a colour-to-% lookup. The paper’s colour varies continuously between the two extremes, so with calibration it could report a number.
Why is the change reversible? Many reactions don’t reverse easily. What makes this one cycle?
Hint / answer
Only weak bonds to water are being made and broken — no strong bonds to the cobalt are permanently altered — and the two forms are close in energy. So a small nudge (humidity or heat) tips it either way, back and forth, indefinitely.
Where else is this chemistry used? Silica-gel packets once used cobalt chloride. Why are products switching, and to what?
Hint / answer
Cobalt chloride is a carcinogen, so its use in consumer desiccants has been restricted. Makers now use safer moisture indicators like methyl violet or manganese-based dyes that show a similar colour change without the toxicity.
Going further
- Make a keepsake indicator. Seal a dried blue strip in a small clear bag with a little silica gel and watch it stay blue — a homemade “keep dry” card.
- Same chemistry, hidden message: use the cobalt colour switch as reversible Cobalt Invisible Ink.
Footnotes
Complex ion — A central metal ion surrounded by bound molecules or ions (ligands), often intensely coloured.↩︎