Element Nickel, Ni, Transition Metal
|Unlike cobalt, which is not employed in the metallic state, metallic nickel is a material which is greatly used. It was formerly used only for alloys; thus German silver is an alloy of nickel with zinc and copper. Some decades ago, however, the difficulties caused by the high temperature of fusion of nickel have been overcome (especially since it was found that it could be rendered more easily fusible by the addition of metallic magnesium or aluminium), and at the present day nickel is extensively employed in cases where it is required to use a tenacious and hard metal, and one which keeps well in the air and is difficultly fusible. It finds increasing use, therefore, for apparatus in the laboratory and for household utensils. |
Further, large quantities of nickel are deposited on other metals with the help of the electric current. It coats these with a resistant, almost silver-white layer, which keeps well in moist air, so that the nickel-plating of various objects made of iron and brass has become an extensive industry.
The electrical deposition of a metal depends on the fact that at the cathode of a circuit, the cations pass from the state of ions into the neutral state. In the case of nickelion, this passes into metallic nickel, which is deposited at all points where the current leaves the liquid. In this process various circumstances, such as strength of the current, nature of the solution, etc., have a considerable influence on whether the metal is deposited in a coherent, lustrous layer or as an incoherent powder. The practice of electro-plating, as this process is called, depends on the knowledge and application of the conditions which ensure the formation of a good deposit. This subject, which is very important in the arts, has been only very little investigated scientifically, so that no general rules can be given.
In order that the nickel-plating bath, which constantly gives up metal to the object to be plated, may not become exhausted, the anode is made of metallic nickel. By this means the anion is not discharged, but on the contrary, as much neutral or metallic nickel passes into the ionic state as is separated at the cathode, and the whole process consists in metal passing into ions at the anode, and being transported by the current to the cathode, where it again passes from the ionic state into the metallic. In this process the current would, theoretically, have practically no work to perform; as a matter of fact, however, a larger or smaller amount of work must be performed by the current on account of the differences in the concentration and other circumstances, a fact which finds expression in the so-called polarisation of the bath or the " bath potential."
Nickel forms a divalent elementary ion nickelion, Ni••, which is of a fine green colour; this colour is present in all solutions of nickel salts which contain this ion. Nickel, it is true, can also form a higher stage of oxidation, but this is extremely unstable, and does not behave as a salt-forming oxide. Nickel can form complex ions, but these are neither so varied nor so stable as in the case of cobalt; this forms the most essential difference between the otherwise very similar elements.
Nickel salts are obtained by the solution of metallic nickel in nitric acid; in the case of nickel, the decomposition of aqueous acid solutions with evolution of hydrogen takes place only very feebly and slowly. If aqua regia is employed, the chloride is obtained; by evaporating the nitrate with sulphuric acid, the former is converted into the sulphate.
From the green solutions of the nickel salts, soluble bases give a pale green precipitate of nickel hydroxide, Ni(OH)2, which loses water when heated, and is converted into grey nickel oxide, NiO. Nickel hydroxide is not soluble in alkalis, but dissolves in ammonia. As the liquid thereby becomes of an azure-blue colour, it must be concluded that a new ion is formed. The investigation of the solid salts has shown that we are possibly dealing with two different ions, one of which contains 4NH3, the other 6NH3, to one Ni; the ions, therefore, have the formula Ni(NH3)4•• and Ni(NH3)6••. They are both blue.
The complex ions of nickel containing ammonia differ from those of cobalt, not only in being derived from divalent nickel, but also in being much less stable. Whereas most of the cobalt-ammonia compounds can be brought together with bases, and even in some cases boiled with them, without ammonia being eliminated to any appreciable extent, the salts of the nickel-ammonia ions in the solid state slowly lose their ammonia even in the air, and quickly on heating. The dissociation pressure of these compounds therefore in respect of the ammonia has an appreciable value even at the ordinary temperature, while in the case of the cobalt compounds it is immeasurably small.
The nickel salts are similar to those of cobalt and generally isomorphous with them. Of these salts some importance is possessed by nickel sulphate, which is generally obtained in quadratic crystals with 6H2O, a form which is seldom found in the case of the other vitriols; it can, however, also crystallise in the forms of magnesium sulphate and ferrous sulphate. With potassium and ammonium sulphate, it forms double salts of the oft-mentioned type. Nickel sulphate and a double salt with ammonium sulphate are used in large quantities for the preparation of baths for nickel-plating.
With potassium cyanide, the nickel salts at first deposit a green precipitate of nickelous cyanide, which dissolves in excess of potassium cyanide and yields a yellow liquid. From this change of colour it can be seen that a new ion is produced; on evaporating the solution a yellow salt of the composition K2Ni(CN)4 • H2O crystallises out. The nickel-cyanidion which forms the basis of this salt does not have an analogous composition to the complex ions of iron, manganese, and cobalt, for it is only divalent. With regard to its stability, also, it differs greatly from these compounds. On acidifying the solution one does not obtain free hydronickelcyanic acid, but a greenish precipitate of nickelous cyanide is produced and hydrocyanic acid escapes. The acid, therefore, immediately decomposes according to the equation H2Ni(CN)4 = Ni(CN)2 + 2HCN. A separation of cobalt and nickel can be based on this reaction.
|Main article: The History of Nickel|
Nickel was discovered in 1751. However long before that Saxon miners were familiar with the rock which resembled the Copper ore and was of value for colouring glass green. All attempts to separate Copper from it failed. That was the reason why in the end of 18th century this ore was named German word "kupfernickel" meaning Devil's copper or St Nicholas's (Old Nick's) copper. In 1751, Swedish mineralogist Baron Axel Fredrik Cronstedt was attempting to extract copper from kupfernickel (now called niccolite), and obtained instead a white metal that he called nickel. When Bergman isolated purified nickel, he found out that it is a metal with properties close to Iron. Since that time nickel was the research object for all chemists starting from Proust.
Nickel is miners' swear-word. It originated from Nicolaus and bears various meanings, such as a double-faced man or goblin.
|Main article: The Occurrence of Nickel|
Nickel is deposited in the depth of the Earth. Its concentration in ultrabasic mantle layers is 0.2%. Most of the nickel on Earth is postulated to be concentrated in the Earth's core. Average Earth abundance is approximately 3%. The crustal abundance is 5.8x10-3%, mostly in deep basaltic layer. Nickel is associated with Iron and Magnesium, intercrystallised in their ores because of the same valence (II) and ionic radii. 53 nickel minerals are known, most of them originated at high temperatures and under high pressure from magma or hot water solutions. Nickel deposits are associated with magma and residual soil. Workable deposits (sulphide ores) consist of nickel and copper ores. Nickel is not abundant in the surface waters and biological substances. It is abundant in the soil of the areas rich by ultrabasic rocks.
Nickel plays a significant biological role as an important microelement. Its concentrations are: 5.0x10-5% in plants, 1.0x10-5% in organisms of terrestrial animals and 1.6x10-5% in sea creatures. Nickel is found in liver, skin and endocrine glands; it is deposited in keratinous tissues such as feathers. In fact arginase contains nickel. It is an important part of oxidizing process as well as some enzymatic reactions in plants.