Cobalt(II) Chloride Hexahydrate

CoCl₂·₆H₂O hydrate

Molar Mass

237.929 g/mol

Properties

State	Solid at room temperature
Color	Deep rose-pink (hexahydrate); blue (anhydrous)
Solubility	Very soluble in water (52.9 g/100 mL at 20 °C)
Melting Point	86 °C (hexahydrate, loses water)
Boiling Point	1049 °C (anhydrous)

About Cobalt(II) Chloride Hexahydrate

CoCl2·6H2O is the deep-pink crystal that turns blue when you heat it — and that color change is the cleanest visible demonstration of crystal-field theory in any undergraduate inorganic curriculum. The pink is octahedral [Co(H2O)6]²⁺, where six water ligands give a moderate field splitting that puts the d-d ⁴T1g(F)→⁴T1g(P) transition around 510 nm (absorbing green, transmitting pink). Heat it, and waters strip off; in concentrated chloride solutions the cobalt switches coordination from octahedral six-coordinate to tetrahedral four-coordinate [CoCl4]²⁻, because for a high-spin d⁷ ion the loss of crystal-field stabilization energy on going from Oh to Td is small while the entropy gain from solvent reorganization is large. The Td complex absorbs at ~690 nm with a much higher molar absorptivity (the Td geometry has no center of symmetry, so the d-d transition is no longer Laporte-forbidden), giving it that intense royal-blue color. Two chemically identical cobalt centers, one absorption band each, but the Oh→Td switch shifts both the wavelength and the intensity dramatically. The same equilibrium drives temperature-sensitive 'mood ring' indicators, the cobalt-blue dehumidifier crystals you'll find in shoe boxes, and the classic invisible-ink demonstration. Industrially, the hexahydrate is the cobalt source of choice for Drierite humidity-indicating dessicant, ceramics under-glaze blue colorant, and a precursor to vitamin-B12 synthesis.

Where you'll encounter it

If you've ever opened a fresh tin of indicating Drierite (CaSO4 doped with CoCl2) and watched the blue beads turn pink as they pulled moisture out of the air over a few weeks, you've used the same chemistry as a quantitative drying check. In a teaching lab, the classic demo is to dissolve a few crystals in water (pink), then add concentrated HCl dropwise — the solution shifts from pink through purple to deep royal blue as you build up [CoCl4]²⁻, and adding water back reverses it. It's the most intuitive Le Chatelier visualization most students will see all term.

Common Uses

Humidity indicator on Drierite desiccant beads, switching pink-to-blue at relative humidity below ~35%
Le Chatelier demonstration via Oh [Co(H2O)6]²⁺ to Td [CoCl4]²⁻ equilibrium driven by HCl or temperature
Sympathetic-ink writing that vanishes on drying and reappears on warming with a heat gun or candle
Catalyst for benzoin-condensation analogs and oxidative cleavage of vicinal diols in research synthesis
Cobalt source for vitamin-B12 (cyanocobalamin) industrial bioconversion fermentation feedstock
Galvanic-deposition bath constituent for hydrogen-evolution-reaction electrocatalyst films
Underglaze blue pigment for ceramics fired above 1200 °C (forms cobalt aluminate spinel)
Stable colorimetric standard for visible-spectrophotometer calibration in undergraduate teaching

Safety Information

GHS H302 (harmful if swallowed), H317 (skin sensitization), H334 (respiratory sensitization causing occupational asthma), H341 (suspected mutagen), H350i (may cause cancer by inhalation, IARC Group 2B 'possibly carcinogenic to humans' for cobalt and cobalt compounds), H360F (may damage fertility), H410 (very toxic to aquatic life with long-lasting effects). OSHA PEL for cobalt and inorganic compounds is 0.1 mg Co/m³ as an 8-hour TWA. ACGIH TLV is 0.02 mg/m³. EU REACH SVHC list since 2010 — consumer-product use restricted. The clinically important effects are 'hard-metal lung disease' (interstitial fibrosis from chronic dust inhalation in tungsten-carbide grinding shops) and contact dermatitis. Handle in a fume hood; nitrile gloves are sufficient for the solid hydrate, but powder generation requires N95 respirator minimum.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

Co — Cobalt Cl — Chlorine H — Hydrogen O — Oxygen

Frequently Asked Questions

What is the molar mass of cobalt(II) chloride hexahydrate?

CoCl2·6H2O comes to 237.929 g/mol: cobalt at 58.933, two chlorines at 35.453 each (70.906), and six waters at 18.015 each (108.09). The anhydrous CoCl2 is 129.84 g/mol — useful when calculating how much hexahydrate you need to weigh out for a target Cl⁻ concentration in a Le Chatelier demonstration.

Why does cobalt chloride change from pink to blue?

Two different complexes, two different geometries. Pink is octahedral [Co(H2O)6]²⁺ — six waters in a moderate-field arrangement that places d-d transitions around 510 nm with low molar absorptivity (Laporte-forbidden in centrosymmetric Oh symmetry). Blue is tetrahedral [CoCl4]²⁻ — four chlorides, no center of symmetry, so the d-d transition gains intensity from p-d mixing and shifts to ~690 nm with much higher ε. The geometry change happens when water activity drops (heating, adding a chloride source like concentrated HCl, or adding ethanol).

How does cobalt chloride invisible ink actually work?

Write with a dilute (~1 wt%) CoCl2 solution and the wet ink looks pink, but as it dries the trace water-of-coordination evaporates and the writing fades to nearly invisible because the cobalt is so dilute. Hold the paper near a candle or hair-dryer and you push the equilibrium toward the dehydrated, intensely-colored blue species — usually a partially-hydrated hexagonal-blue or violet anhydrous form. Let it cool and pull moisture from room air and it goes back to pink-and-faint. It's the same chemistry as Drierite, just on cellulose.