To remove the copper from the nickel salt, the latter is first dissolved in water and acidulated by a few drops of sulphuric acid (commercial nickel sulphate is generally acid), then a current of hydrogen sulphide gas, which is prepared by pouring sulphuric acid over iron . sulphide in a flask, is passed through the solution.
The copper and other metals which are likely to be present are thrown down in the form of a black precipitate. When the odour of the gas is distinctlv recognised, its passage is stopped, and the solution is heated to expel the last traces of the hydrogen sulphide. It is then heated to boiling in a porcelain vessel with the addition of some metallic nickel. By this means the free acid is neutralised, and on evaporating to crystallisation there remains a salt sufficiently pure for nickel plating.
The articles which are to be plated are suspended in the solution which we have just described, and they are connected with the positive pole. A nickel plate, which also dips into the liquid, is connected with the negative pole; and from time to time the liberated acid is neutralised by the addition of a slight quantity of ammonium hydrate. It is better still, for practical results, to spread a layer of nickel oxide over the bottom of the vessel in which the nickelising is being carried on. This will dissolve in the free acid, and the solution will therefore remain neutral and of uniform strength.
The nickel oxide is prepared by completely saturating a solution of nickel sulphate with sodium hydrate (caustic soda), washing the precipitate, and then drying it. The nickel oxide thus formed is a heavy powder of an apple-green colour, and may be either spread over the bottom of the vessel, or it can be placed in a linen bag and suspended in the liquid. If a solution of nickel sulphate, acidified with sulphuric acid, is poured into a saturated solution of ammonium sulphate, crystals will separate out, consisting of the double salt of nickel ammonium sulphate. The crystals are washed with cold water, dissolved in hot water, and then the solution is completely neutralised with ammonium hydrate. It is then allowed to stand for several days at a temperature of 68°-77° F., until no more crystals separate out. It is also of importance that the liquid be maintained at this temperature during the nickelising, for otherwise the nickel will not adhere firmly to the metal.
During the operation of plating, a sheet of nickel, connected with the positive pole of the battery, is suspended in the solution. According as the nickel becomes separated from the solution the sheet dissolves, and thus the solution maintains its original strength. Plates of absolutely pure nickel are at present quite expensive, in consequence of the very high temperature which is required for their fusing and casting. By the addition of 1/5000th part of phosphorus its point of fusion may be considerably lowered. As the phosphorus is not objectionable in nickelising, the plates are generally made of metal containing phosphorus, and they are used to the best advantage in rather thin sheets, for, the larger the surface of the nickel plate, the less will be the strength of the current required; and when the pieces to be plated are not large, as will occur in the majority of cases, two or three Bunsen elements will be sufficient.
In addition to the above methods for nickel plating, others have been proposed which also give good results, but which require more expensive preparations than those previously mentioned; thus, for instance, the double salt of nickel potassium cyanide and solutions of nickel nitrate have been proposed. On account of the vapours which escape from the cyanide solutions, although only in small quantities, they are particularly objectionable, and therefore the employment of cyanide preparations, on account of their poisonous properties, should be avoided whenever it is possible to do so. The nickel nitrate gives a beautiful and durable coat of nickel. The solution is most effective when it is composed of 4 parts crystallised nickel nitrate dissolved in 150 parts water, to which 4 parts ammonium hydrate are added, and then 50 parts of acid sodium sulphite are dissolved in the above solution.
The acid sodium sulphite is prepared by heating copper with sulphuric acid in a retort; the gas produced is passed through a small quantity of water, which will retain the copper which has been mechanically carried over, and then the gas (sulphurous acid) is dissolved in water until the liquid smells distinctly of burning sulphur. The solution which has thus been obtained is divided into two portions: one part is saturated with sodium carbonate as long as effervescence takes place, the other half of the acid is then added, and in this manner sodium bisulphite is produced. This must be employed as it is, because it is impossible to crystallise the salt by evaporation, for in so doing one half of the acid would escape, and sodium mono-sulphite remain behind.
For nickel plating of the finest kind, such as is produced in American factories, a solution is prepared from the nickel nitrate and acid sodium sulphite. It sometimes happens that the nickel will strip or peel off from the metals on which it has been deposited. It is said that this objection can be overcome by placing the dried plated objects in a bath of oil and heating them up to 482°-518° F.
According to Weston, a plating of great beauty and durability is obtained by mixing a solution composed of 5 parts nickel chloride and 2 parts boracic acid with one made up of 2 parts nickel sulphate and 1 part boracic acid, and then adding, while continually stirring, sodium hydrate (caustic soda) until the precipitate is redissolved.
For the nickelising of iron or steel, it is best to first coat the objects to be plated with a thin film of copper, which is readily accomplished by dipping the material into a dilute solution of copper sulphate. - (Hartman.)
Palladium has of recent years been much used to plate watch movements with. According to Pilet, 4 mgrm. palladium are sufficient to coat the works of an ordinary sized watch. Pilet recommends the following bath: - Water, 2 litres; palladium chloride, 10 grm.; ammonia phosphate, 100 grm.; soda phosphate, 500 grm.; benzoic acid, 5 grm. This bath is suitable for all metals except zinc.