Cobalt Chloride Anhydrous vs Hexahydrate: Formula, Color and Hydration

Anhydrous cobalt chloride and cobalt chloride hexahydrate are not interchangeable specification identities. The anhydrous material is CoCl₂, CAS 7646-79-9 and 129.839 g/mol. The hexahydrate is CoCl₂·6H₂O, CAS 7791-13-1 and 237.931 g/mol. Their water content changes molecular weight and theoretical cobalt fraction, while observed color can vary with hydration, coordination and environment. An RFQ should name the required form and current analytical basis.

Anhydrous vs hexahydrate comparison

References: NIST anhydrous cobalt dichloride, PubChem CID 24288, NIST cobaltous chloride hexahydrate and PubChem CID 24643.

Why are the CAS numbers different?

A hydrate contains water molecules in a defined stoichiometric relationship to the parent salt. Six water molecules are part of the hexahydrate formula unit, so CoCl₂·6H₂O has a different formula mass and identity record from anhydrous CoCl₂. Using CAS 7646-79-9 with a 24% cobalt target or a pink/fuchsia crystal description can signal that an anhydrous identity and a hydrate specification have been mixed.

How are the theoretical cobalt percentages calculated?

The cobalt atomic contribution is divided by the formula mass. This gives about 45.39% cobalt for ideal anhydrous CoCl₂ and about 24.77% for ideal CoCl₂·6H₂O. These figures explain why hydrate status matters, but they are not a current Longchang assay, purity limit or lot result. A commercial specification must define the test method, calculation basis, units, acceptance limit and applicable lot COA.

What color is cobalt chloride?

Reference sources often describe anhydrous or less-hydrated material as blue and the hexahydrate as pink or magenta. However, cobalt(II) color is sensitive to coordination, solvent, water activity, concentration, temperature and matrix. Dihydrate and other hydrated/complexed states can show additional colors. A visual observation is useful context, not a substitute for identity and water/hydrate testing.

Does CoCl₂ change color only because of “wet versus dry”?

Hydration is central to common indicator behavior, but the simple blue-equals-dry and pink-equals-wet rule is incomplete. The chemical environment around Co²⁺ affects its coordination and observed color. For a purchased material, define appearance under controlled conditions and pair it with analytical identity and water/hydrate criteria.

Qualification checklist for a Cobalt Chloride RFQ

Safety applies to both forms

ECHA states that Candidate List entries cover both anhydrous and hydrated forms and lists cobalt dichloride for carcinogenicity and reproductive toxicity. Hydration changes formula mass; it does not remove the need for current hazard communication, market review and site/process controls. This page is not an SDS. Request the current supplier SDS and use it with the buyer’s exposure scenario and local risk assessment.

See the ECHA Candidate List and cobalt dichloride substance information.

Use the correct Longchang page

• Read What is cobalt chloride? CoCl2 formula, uses and safety for the primary identity, naming, use-context and regulatory answer.
• Use this page for anhydrous-versus-hexahydrate, formula, molecular-weight, cobalt-fraction and color questions.
• Use the product page to request exact form, current documents, sample and quotation information.

Request current form and documents

Open the Cobalt Chloride CAS 7646-79-9 product page to request hydrate/form confirmation, the current specification, methods, applicable lot COA, SDS status, sample availability, packaging, MOQ, current availability and confirmed lead time.

Email info@longchangchemical.com with the required form, cobalt calculation basis, impurity limits, quantity, packaging, destination and document requirements.

Frequently asked questions

Is CoCl₂·6H₂O the same as CoCl₂?

No. The hexahydrate contains six water molecules per formula unit and has a separate CAS and molecular weight.

Why can a product table show about 24% cobalt?

Ideal cobalt chloride hexahydrate contains about 24.77% cobalt by formula mass. That numerical similarity is a reason to verify hydrate state and method—not proof that a historical table is a current hexahydrate specification.

Can color identify the hydrate?

Not by itself. Hydration affects color, but coordination, solvent, concentration, temperature and matrix also matter.