Cobalt blue colour

To the eye, cobalt blue and woad have a very similar colour. We say they have the same hue. But woad is considerably paler in intensity, even when the concentration c... 

Selenium oxychloride is a nearly colourless liquid having a boiling-point of 176-4° C. at 726 mm. Its melting-point is 10 8° C.7 At 20° C. it has a refractive index of 1-6516. The specific conductivity of the dry liquid at 25° C. is 8 2-0 ( 0-3) x 10 5 mho. The oxychloride is, however, extremely hygroscopic, and the presence of traces of water causes a considerable increase in the electrical conductivity. The presence of the merest trace of water can be shown 9 by sealing up a sample of the oxychloride with cobalt carbonate which has been dried at 200° C. for three hours a blue colour gradually develops if moisture is present. 

Cobalt oxide gives purple or blue coloured glass. 

A common test for zinc and aluminium is to heat together before the blow-pipe the salt suspected to contain the metal with cobalt nitrate it is probable that the green colour produced by zinc is due to the formation of a cobalt zincate, Co(Zn02), and the blue colour shown by alumina to a similar body, Co(A102)2. 

Uses.—Cobalt has not hitherto been utilised to any important extent in the industries, but Canada is making systematic efforts to develop its utility in view of the fact that rich stores of cobalt ores are found in that country. A few years ago practically the only use for cobalt was the production of the blue colour in the glass ana ceramic industries, and for the production of a blue paint. For these industries an annual production of 300 or 400 tons of cobalt sufficed.2... 

Cobalt chloride dissolves in alcohol to a blue solution, which becomes violet and then red on the addition of water. The alcoholic solution becomes red when very diluted or when cooled much below 0° C. The blue colour in these solutions has been attributed to the formation of double compound by Engel, to complex ion formation by Donnan and Bassett, ana to both these causes by Kotschubei. 

It is a bright yellow, crystalline powder, insoluble in water, and on account of its stability or inertness is used as an artists pigment. It is a very pure compound when prepared as indicated above, for neither iron nor nickel yields a corresponding derivative it is therefore valued for painting porcelain, because the blue colour produced on baking is correspondingly pure and beautiful. Furthermore, as has already been mentioned, the salt yields a valuable method of separating nickel from cobalt (see p. 24). 

Cobaltic Nitrate.—When a concentrated solution of cobaltous nitrate is electrolysed in a divided cell, it assumes a deep indigo-blue colour, but soon returns to its original state when the current is stopped.6 Presumably the colour change is due to the formation in solution of an unstable cobaltic nitrate. 

It is interesting to note that one part of cobaltous oxide will impart a distinctly blue colour to 250 parts of glass. 

Detection.—Dry Tests.—Salts of cobalt are usually of a rose-red colour when hydrated, and yield a beautiful blue colour in the borax bead test provided they are not present in excessive amount, otherwise the bead becomes opaque and appears black. When heated on charcoal with reduction mixture in the blowpipe test, metallic cobalt separates out in the form of small magnetic beads. When strongly heated with alumina a blue colour is produced, known as Thenard s blue. 

Potassium thiocyanate yields a blue colour with solutions of cobalt salts, a blue alcoholic layer being produced on addition of amyl alcohol or of alcohol and ether to the mixture (see p. 71) and shaking. 

Colorimetric Methods have frequently been suggested,1 but of these, that originated by Skey and studied by several others 2 appears to be the most useful. It hinges on the fact that potassium thiocyanate yields a blue colour with solutions of cobalt salts, due to the formation of cobalt thiocyanate. On adding alcohol and ether to the liquid, a blue layer is produced. This is destroyed by mercuric chloride, sodium acetate, or sodium thiosulphate, and is masked by the presence of iron salts in consequence of the intense red colour of ferric thiocyanate consequently these substances should not be present when the colorimetric test is applied. 

In carrying out the test a small quantity of standard cobalt sulphate solution is taken, and acidulated with dilute sulphuric acid. Ammonium thiocyanate is then added and the volume of the liquid made up to 10 c.c. A mixture of alcohol and ether is now added until the total bulk is 20 c.c. and the blue colour of the supernatant ethereal layer is compared with that from a solution of the liquid to be tested, and which has been treated in a precisely similar manner.3... 

Drop test Place a drop of the test solution on a spot plate, add a small crystal of ammonium thiocyanate followed by a little solid cobalt(II) acetate. A blue colour is produced in the presence of mercury(II) ions. 

Ammonium thiocyanate test (Vogel reaction) adding a few crystals of ammonium thiocyanate to a neutral or acid solution of cobalt(II) a blue colour appears owing to the formation of tetrathiocyanatocobaltate(II) ions ... 

Dry test (flame colouration) Potassium compounds, preferably the chloride, colour the non-luminous Bunsen flame violet (lilac). The yellow flame produced by small quantities of sodium obscures the violet colour, but by viewing the flame through two thicknesses of cobalt blue glass, the yellow sodium rays are absorbed and the reddish-violet potassium flame becomes visible. A solution of chrome alum (310 g 1), 3 cm thick, also makes a good filter. 

Dry test (flame colouration) The non-luminous Bunsen flame is coloured an intense yellow by vapours of sodium salts. The colour is not visible when viewed through two thicknesses of cobalt blue glass. Minute quantities of sodium salts give this test, and it is only when the colour is intense and persistent that appreciable quantities of sodium are present. 

Cobalt acetate solution When the reagent is added to a concentrated solution of potassium cyanate, a blue colouration, due to tetracyanatocobaltate(II) ions, [Co(OCN)4]2- is produced. The colour is stabilized and intensified somewhat by the addition of ethanol. 

Cobalt nitrate solution blue colouration, due to the formation of [Co(SCN)4]2- (Section III.26, reaction 6), but no precipitate [distinction from cyanide, hexacyanoferrate(II) and (III)] ... 

The sodium flame masks that of other elements, e.g, that of potassium. Mixtures can be readily detected with the direct vision spectroscope (see Fig. II.4). A less delicate method is to view the flame through two thicknesses of cobalt blue glass, whereby the yellow colour due to sodium is masked or absorbed, and the other colours are modified as listed in Table V.3. 

Antimony, zinc, and aluminium salts do not interfere aluminium should, however, be kept in solution by the addition of sufficient sodium hydroxide the influence of copper, nickel, and cobalt salts can be eliminated by the addition of potassium cyanide solution iron salts are masked by the addition of a tartrate but if aluminium salts are also present a red colour is produced. Magnesium salts give a similar blue colour, but beryllium can be detected in the presence of this element by utilizing the fact that in ammoniacal solution the magnesium colour alone is completely destroyed by bromine water. 

The components of this reagent consist of 0.1 g of cobalt acetate in 10 ml of 0.2% glacial acetic acid in methanol (1) and 5% isopropylamine in methanol (2). Testing is carried out by adding a small amount of component (1) to the test substrate, followed by a few drops of component (2). A blue colour will develop if barbiturates are present in the sample. 

This is a modification of the previous test, with the reagent consisting of a 2% (wl/vol) solution of cobalt isothiocyanate in water, diluted with an equal volume of glycerine (1), concentrated hydrochloric acid (2) and chloroform (3). Testing for cocaine is carried out as follows. A small amount of the material is placed in a test-tube and five drops of component (1) added, when a blue colour will develop if cocaine is present. One drop of concentrated hydrochloric acid (2) is then added and the blue colour, if this has resulted from the presence of cocaine, should disappear, leaving a pink solution. Further confirmation is provided by the addition of several drops of chloroform (3), whereupon an intense blue colour... 

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