IKON, COBALT, AND NICKEL 857 



If a solution of potassium hydroxide be added to a solution of a 

 cobalt salt, a blue precipitate of the basic salt will be formed. If a 



tension of solutions of chloride of cobalt with the temperature. We have repeatedly 

 seen that aqueous solutions (for instance, Chapter XXII., NotQ 28 for Fe 3 Cl 6 ) deposit 

 different crystallo-hydrates at different temperatures, and that the amount of water 

 in the hydrate decreases as the temperature t rises, so that it is not surprising that 

 CoCl 2 2H 2 O (or according to Potilitzin CoCl 2 H 2 O) should sepauate out above 55 and 

 CoCl 2 6H 2 O at 25 and below. Nor 'ia it exceptional that the colour of a salt varies 

 according as it contains different amounts of H 2 0. But in this instance it is character* 

 istio fflmt the change of colour takes place in solution in the presence of an excess of 

 water. This apparently shows that the actual solution may contain either CoC1^6H 2 O or 

 CoCl 2 2H 2 O And as we know that ar solution may contain both metaphosphoric PHOj 

 end orthophosphoric acid H 3 P0 4 = HP0 3 +H 2 O, as well as certain other anhydrides, 

 the question of the s.tate of substances in solutions becomes still more complicated. 



Nickel sulphate crystallises from neutral solutions at a temperature of from 15 to 20 

 in rhombic crystals containing 7H.jO. Its form approaches very closely to that of the 

 Salts of zinc and magnesium. The planes of a vertical prism for magnesium salts are 

 inclined at an angle of 90 80', for zinc salts at an angle of 91 7', and for nickel salts at 

 an, angle of 91 10'. Such is also the form of the zinc and magnesium selenates and 

 chromates. Cobalt sulphate containing 7 molecules of water is deposited in crystals 

 of the monoclinic system, like the corresponding salts of iron and manganese. The angle 

 of a vertical prism for the iron salt = 82 20', for cobalt = 82 22', and the inclination of 

 the horizontal pinacoid to the vertical prism for the iron salt = 99 2', and for the cobalt 

 salt 99 86' All the isomorphous mixtures of the salts of magnesium, iron, cobalt, 

 nickel and manganese have the same form if they contain 7 mol. H 2 O and iron or cobalt 

 predominate, whilst' if there is a preponderance of magnesium, zinc, or nickel, the 

 crystals have a rhombic form like magnesium sulphate. Hence these sulphates are 

 'dimorphous, but for some the one form is more stable and for others the other. Brooke, 

 "Moss, Mitscherlich, Rammelsberg, and Marignac have explained these relations. Brooke 

 and Mitscherlich also supposed that NiSO 4 ,7H 2 O is not only capable of assuming these 

 forms, but also that of the tetragonal system, because it is deposite'd in this form from 

 acid, and especially from slightly-heated solutions (80 to 40). But Marignac demon- 

 strated that the tetragonal crystals do not contain 7, but 6, molecules of water, NiSO 4 ,6H 2 O. 

 He also observed that a solution evaporated at 60 to 70 deposits monoclinic crystals, 

 but of a different form from ferrous sulphate, FeSO 4 ,7H 2 O namely, the angle of the 

 prism is 71 62', that of the pinacoid 95 6'. This salt appears to be the same with 6 

 molecules of water as the tetragonal. Marignac also obtained magnesium and zmo 

 salts with 6 molecules of water by evaporating their solutions at a higher tem- 

 perature, and these salts were found to be isomorphous with the monoclinic "nickel salt. 

 In addition to this it must be observed that the rhombic crystals of nickel sulphate with 

 7H 3 become turbid under the influence of heat and light, lose water, and change into 

 the tetragonal salt. The monoclinic crystals in time also become turbid, and change 

 their structure, so that the tetragonal form df this salt is the most stable. Let us also 

 add that nickel sulphate in all its shapes forms very beautiful emerald green crystals, 

 which, when heated to 280, assume a dirty greenish-yellow hue and then contain one 

 molecule of water. 



Klobb (1891) and Langlot and Lenoir obtained anhydrous CoS0 4 and NiS0 4 by 

 Igniting the hydrated salt with (NH4) 2 SO^ until the ammonium (Bait had completely 

 volatilised and decomposed. 



We may add that when equivalent aqueous solutions of NiX 2 (green) and CoX 2 (red) 

 ere mixed together they give an almost colourless (grey) solution, in which the green and 

 tred' colour of the component parts disappears owing to the combination of thecomple* 

 mentary colours. 



A double salt NiKF 3 fs obtained by heating NiCJ 2 withKFHPin a platinum'cruciblej 

 ECoFs is formed in a similar manner. The nickel gait occurs in fine green plates, easily 



