Cobalt

Elementary cobalt is a metal with a lustrous grayish tinge. It is attacked slowly by the mineral acids, forming divalent cobalt compounds. Cobalt metal is moderately reactive and can be used as a source for aqueous solutions of cobalt compounds. Cobalt metal of high purity is available from vendors on eBay, which sell items for element collections. Pure cobalt, however, is quite expensive. A better option is to purchase cheaply available compounds of cobalt. The picture shows a sample of pure cobalt.

In its compounds, cobalt usually is in its +2 or +3 oxidation state. Both of these oxidation states are accessible in aqueous solutions. The stability of the +2 and +3 oxidation states is strongly affected by ligands, to which cobalt can be coordinated. Free cobalt ion and hydrated cobalt almost exclusively exist in the +2 oxidation state. Cobalt in its +3 oxidation state is an extraordinarily strong oxidizer, which is capable of oxidizing water, giving off oxygen. However, in the presence of many ligands (e.g. ammonia, citrate, cyanide), the +3 oxidation state is strongly favored and the +2 oxidation state becomes reducing. E.g. when excess ammonia is added to a solution, containing aqueous cobalt (II), then a complex is formed, which absorbs oxygen from the air quickly, the cobalt being oxidized to its +3 oxidation state.

Cobalt compounds are readily available for the general public from pottery and ceramics suppliers. This is because cobalt can be used to give a deep blue color to glass and ceramics. The following compounds are available from pottery and ceramics suppliers:

• cobalt (II) carbonate, CoCO₃
• cobalt (II) sulfate, CoSO₄·7H₂O
• cobaltosic oxide, Co₃O₄
• From chemical supply houses, hydrated cobalt chloride CoCl₂·6H₂O can be obtained, but usually, this is quite expensive.
• From some art and paint stores, the pigment 'cobalt yellow', also called potassium hexanitritocobaltate (III), K₃Co(NO₂)₆·H₂O is available.

Cobalt carbonate is available as a purple fine powder, which easily dissolves in dilute acids on slight heating. These solutions are pink/rose. A solution in concentrated hydrochloric acid, however is deep blue, due to formation of the complex CoCl₄^2-. In dilute hydrochloric acid, the color can be anywhere between pink and blue. High temperatures favor the blue complex. Cobalt carbonate is a convenient source of cobalt (II) solutions in combination with any anion, depending on the acid in which it is dissolved. Cobalt carbonate also is cheap, hence, it is an interesting compound for the home lab.

Cobalt sulfate is a red/purple crystalline solid, which dissolves in water easily, forming pink/rose solutions. For experiments with divalent cobalt, this compound even is more convenient than cobalt carbonate. Preparing a neutral solution of a cobalt (II) compound, together with a non-coordinating anion, is really easy, when this compound is available. So, when one wants to study the properties of aqueous cobalt in more detail, this is the compound to start with. Cobalt sulfate also is a cheap compound.

Cobaltosic oxide, also called cobalt (II, III) oxide, is a mixed valency compound of cobalt, containing Co³⁺ and Co²⁺ in the ratio 2 : 1. It is available as a fine black powder, which is remarkably inert. This inertness makes the compound not interesting for the average home chemist. It does not dissolve appreciably in strong acids, nor in strong alkalies. Even after a day of treatment with concentrated hydrochloric acid, only a faint coloration of the liquid can be observed. Heating the acid also does not result in easy dissolving of the solid.

Cobalt chloride, hexahydrate, is a red/purple crystalline solid, which dissolves in water very easily. From the point of view of the home chemist, this compound is quite similar to cobalt sulfate. It, however, is much more expensive, and for some experiments, the presence of chloride may be interfering (chloride coordinates to cobalt (II) fairly well). If cobalt sulfate is available, then there is no real reason to purchase cobalt chloride.

Cobalt yellow, as the name suggests, is a yellow powder with a tinge towards orange/brown. This compound is only very sparingly soluble in water. In strong acids, it slowly decomposes, faster on heating. On decomposition, it gives nitrogen oxides and a solution, containing cobalt (II) ions. This compound is not really suitable to get clean solutions of cobalt (II). Quite strong acids are needed for its decomposition and there always is nitrogen oxide and nitrous acid present, together with the cobalt. This may interfere strongly in many experiments. Either cobalt carbonate or cobalt sulfate are more suitable for a home lab.

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