Cobalt blue hydrates

Cobalt II) halides can be obtained by direct combination of the elements, or by dehydration of their hydrates. Anhydrous cobalt(II) chloride is blue, and the solid contains octahedrally-coordinated cobalt the hydrated salt C0CI2. bHjO is pink, with each cobalt surrounded by four water molecules and two chloride ions in a distorted octahedron. 

Blue anhydrous cobalt salts hydrated copper(II) salts Prussian blue. 

Cobalt(III) forms few simple salts, but the green hydrated fluoride CoF3-3.5H20 and the blue hydrated sulfate Co2(S04)3-18H20 separate on electrolytic oxidation... 

Green Cr203, Hg2l2, Cr(OH)3, iron(II), nickel(II) and chromium(III) salts, CuCl2 -2H20, CUCO3, K2Mn04 Blue Hydrated copper(II) salts, anhydrous cobalt(II) salts, Fe4[Fe(CN)6] (Prussian blue)... 

Thenard (1803-1804) produced cobalt blue by calcining aluminium hydrate with acetified cobalt arsenate or acetified cobalt phosphate. Church (1901) gives a recipe for this, however, to our knowledge, no arsenate nor phosphate has been found in analyses of cobalt blue in paintings. 

Cobalt group Chromium aluminium cobalt oxide Cobalt aluminium oxide Cobalt arsenate Cobalt arsenic oxide hydrate Cobalt chromate Cobalt hexacyanoferrate(II) Cobalt iron oxide Cobalt magnesium oxide Cobalt tin oxide Cobalt tungstate Cobalt zinc oxide Cobalt(III) oxide Erythrite Cobalt black Cobalt blue Cobalt violet... 

Clays and metal oxides and salts such as talc, kaolinite, mica, zinc oxide, titanium oxide, iron oxide, hydrated chromium oxide, cobalt blue, ultramarine blue, calcium carbonate, barium sulfate, etc. are widely used as pigments for cosmetics. Since these pigments possess acidic and basic surface properties, and hence catalytic activity, cos-... 

MiscelEneous. Small quantities of cobalt compounds are used in the production of electronic devices such as thermistors, varistors, piezoelectrics (qv), and solar collectors. Cobalt salts are useful indicators for humidity. The blue anhydrous form becomes pink (hydrated) on exposure to high humidity. Cobalt pyridine thiocyanate is a useful temperature indicating salt. A conductive paste for painting on ceramics and glass is composed of cobalt oxide (62). 

Three color changes were observed blue, violet and red. From the information provided, the blue form appeared to be the anhydride, CoCl2. When exposed briefly to air containing water vapor, the anhydrous cobalt chloride turned violet, CoCl2 2H20. Upon further standing in air, the dihydrate was converted to the fully hydrated form, CoCl2 6 H20. The color of the anhydride-hydrate depended upon the relative humidity of the air to which it was exposed. 

Crystallization yields the commercial product, pink heptahydrate. Further oxidation of this salt in dilute H2SO4 with ozone or fluorine produces hydrated cobalt(III) sulfate, 002(804)3 I8H2O. This blue octadecahydrate, 002(804)3 I8H2O also is obtained by electrolytic oxidation of cobalt(II) chloride or any cobalt(II) salt solution in 8M sulfuric acid. 

Nickel sulfate forms double salts with ammonium or alkali metal sulfates. For example, blue-green hydrated ammonium nickel sulfate, (NH4)2S04 NiS04 6H20, crystallizes from a mixed solution of nickel sulfate and ammonium sulfate. Such double sulfates are isomorphous to corresponding alkali metal or ammonium double sulfates of iron, cobalt, magnesium, zinc, and other bivalent metals. 

The substance is prepared by treating the tetrammino-derivative with 50 per cent, hydrazine hydrate on a water-bath ammonia is copiously evolved and the hydrazino-derivative is precipitated. The precipitate is blue in colour, but the last of the precipitate is almost colourless. After washing with alcohol and ether it is obtained as a reddish-violet powder which is insoluble in water. It is more difficult to decompose with hot water than the corresponding cobaltous compound. 

Both the copper and nickel snlfamates (2-hydrates) are obtained by similar procedures and in comparable yield. The only losses are mechanical. The salts are respectively blue and green in color. Both the cobalt and the nickel salts dissolve in 96% alcohol but the copper compound does not. 

The color of the material is violet-brown. It is soluble in alcohol, ether, and acetone as well as esters to give a blue solution in each case—probably an auto-complex Co [Co (SON) 4]. In concentrated aqueous medium it is blue, but on dilution the color changes to the pink of hydrated cobalt (II) ions (dissociation of complex). 

A sample of about l.2g, when dissolved in water and titrated with iodine, gave 35.8% thiosulfate or 54.7% cobalt thiosulfate which corresponds to an 8-hydrate (54.3% COS3O0). The product prepared as described decomposes slowly in boiling water to give cobalt sulfide. The color in a cold concentrated aqueous medium is deep blue— possibly Co(Co(S 0 ) ). 

Thirty grams of mercuric chloride and 44.5g of potassium thiocyanate are dissolved in 500ml of water and mixed with a solution of 20g of cobalt nitrate 6-hydrate in 50ml of water. A shower of small deep blue crystals of the difficultly soluble complex salt begins to form almost immediately. The mixture is allowed to stand for twelve hours to complete the predpitation and then suction-filtered. The product is washed with three 30ml portions of cold water, drained well, and then rinsed with two 25ml volumes of 95% alcohol to aid in drying. The material is dried in air or in the oven at 100° C. 

A mixture of 0.05 mole of a cobalt salt (14.5g of the nitrate or 12g of the chloride—both 6-hydrate) and 21g of potassium thiocyanate are heated in 30ml of water until solution is complete. The dark blue liquid is cooled in ice until crystallization of potassium nitrate or chloride is complete and then suction-filtered. The residue is washed repeatedly with 10ml portions of ethyl acetate until it is... 

Coball(lI) hydroxide exists in two allolropic forms, a blue or-Co (OH) and a pink /l-Co(OH) . The hydroxide is prepared by precipitation from u cobaltous salt solution by an alkali hydroxide. When the alkali is in excess the pink ft form is produced—the blue a-furni is produced when the cobalt salt is in excess. The salt slowly oxidizes in air at mom temperature and changes to hydrated cobaltic oxide, Co-Oi - H 0. The hydroxide is practically insoluble in H 0 and in bases, but highly soluble in mineral and organic acids. The commercial salt is used as Ihe starting material in the preparation of drying agents. 

Cobalt (Co) Cobalt is a silvery white metal. Cobalt chloride absorbed onto a piece of felt can measure the relative humidity in air. When the air is humid, cobalt chloride picks up water from the air to form a reddish purple hydrate. When the air is dry, cobalt chloride releases the water of hydration and turns blue. 

General Properties of Compounds of Cobalt.—Cobalt, in its salts, behaves both as a trivalent and a divalent element, although the trivalent salts are unstable except in the case of double salts. The colour of cobalt salts varies greatly according to the degree of hydration, but it is usually pink or blue. Cobalt salts absorb nitric oxide,1 and with ammonia readily yield complex ammino derivatives. 

In the cold the saturated solution is rose-coloured, like the crystalline hexahydrated salt. On warming it becomes violet between 25° and 50° C., above which latter temperature it is blue. This is explained by some as due to a change in hydration of the cobalt salt in solution from the red hexahydrate, through the violet monohydrate, to the blue anhydrous salt. Certainly the changes in colour correspond to breaks in the solubility curve as shown in Fig. I.1 A similar change in colour from red to blue likewise occurs with increase of concentration of the solution. 

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