Cobalt blue hydroxide

For a cobalt(ll) salt, the precipitation of the blue->pitik cobalt(II) hydroxide by alkali, or precipitation of black cobalt(II) sulphide by hydrogen sulphide provide useful tests the hydroxide is soluble in excess alkali and is oxidised by air to the brown CoO(OH) . 

Cobalt(II) hydroxide is obtained as a precipitate when hydroxide ion is added to a solution containing cobalt(II) ions. The precipitate is often blue, but becomes pink on standing it dissolves in excess alkali to give the blue [Co(OH)4]2- ion, and in slightly alkaline solution is easily oxidised by air to a brown solid of composition ComO(OH). 

Transparent Cobalt Blue. Cobalt blue [1345-16-0] C.I. Pigment Blue 28 77346, is also produced as a transparent pigment by precipitating cobalt and then aluminum as hydroxides or carbonates from salt solutions using alkali. The hydroxides or carbonates are then filtered, washed, dried, and calcined at 1000°C [5.278]. It is important to carry out the precipitation with high dilution and to distribute the alkali uniformly throughout the entire reaction volume. 

The sparingly soluble Co(OH)2 may be pink or blue, with the pink form being the more stable freshly precipitated blue Co(OH)2 turns pink on standing. The change in colour is presumably associated with a change in coordination about the Co(II) centre. Cobalt(II) hydroxide is amphoteric and dissolves in hot, concentrated alkalis to give salts of [Co(OH)4]2- (21.37). 

A small excess of NaOH solution is added to a 0.1 M cobalt salt solution containing about 1% glucose. The precipitate is thoroughly washed in the absence of air with aqueous alcohol, aqueous acetone and finally with pure acetone. It is then dried. The resulting blue hydroxide changes color very readily on drying (oxidation). 

To determine the valence of Co, the sample is carefully heated with 2 N KOH until NH3 can no longer be detected. The resulting blue liquid, which contains suspended cobalt (III) hydroxide, is treated with KI and NaHC03 in a flask closed off with a glass stopper, and is then carefully acidified. After standingfor one day, the I3 which separates is back-titrated with NagSsOg solution. 

Barium selenate BaSeO 3.40 10 Cobalt(II) hydroxide (blue) Co(OH)3 5.92 10- ... 

Aluminium group Aluminium hydroxide, bayerite type Aluminium hydroxide, nordstrandite type Aluminium oxide, amorphous type Aluminium oxide, corundum type Bayerite Boehmite Calcium aluminium oxide Chromium aluminium cobalt oxide Cobalt aluminium oxide Corundum Diaspore Gibbsite Hercynite Lead aluminium oxide Nordstrandite Ochre Cobalt blue Emery Satin white Spinel pigments Turkish green... 

Cobalt oxides and hydroxides group Cobalt blue... 

Cbbalt green is said to be similar to cobalt blue q.v.) except that zinc oxide partially or wholly replaces the aluminium hydroxide. Cliurch (1901) describes making it by adding a solution of a cobalt salt to a paste of zinc oxide and water the mass is then dried and calcined. 

Aqueous cobalt(II) compounds also form complex ions. Whenever we write Co (aq), we are referring to the complex ion [Co(H20)g]. This ion gives an aqueous solution of cobalt(II) sulfate its pink colour. On adding sodium hydroxide solution, we see a blue precipitate of cobalt(II) hydroxide forming, which turns red when warmed if the alkali is in excess. Water ligands in [Co(H20)g] + can also be exchanged for ammonia ligands if we add concentrated aqueous ammonia drop by drop. 

Cobalt forms both divalent and trivalent cations. Like iron, it forms oxide/ hydroxide phases in both oxidation states as well as mixed oxidation state phases. Cobalt(II) oxide is used extensively in the ceramics industry to produce blue-coloured glazes. Cobalt(II) hydroxide has two forms, pink and blue, with the former being more stable. It is used as a drying agent or as a catalyst in the manufacture of battery electrodes. 

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