METALS, Solution of. See CHEMISTRY-Index.
On this subject Mr Keir has some curious observations in the Philosophical Transactions for 1790. He takes notice, that the word solution has two meanings; one expressive of the act of dissolving, as when we say that "solution is a chemical operation;" and the other, when it is put for the substance dissolved in the acid, as "a solution of silver in the nitrous acid." To avoid confusion, therefore, he uses the word solution to express the substance dissolved, together with its solvent; dissolution being the term made use of when the act of dissolving is meant. He continues the use of the terms phlogisticated and dephlogisticated to express certain states of the acids, but without reference to theory of any kind.
In dissolving metals, our author observes, that the properties of the several acids have been investigated with considerable success; and even one compound, viz. that of spirit of nitre and spirit of salt, commonly called aqua regia, is well known on account of its quality of dissolving gold. A vast field, however, yet remains for examination in the other acids, whether mixed together, or possessing various degrees of concentration, temperature, or phlogistication. Thus, tho' no two substances are more frequently in the hands of chemists than vitriolic acid and nitre, yet the properties of the mixture had not been investigated before Mr Keir made his experiments; and upon trial he found, that this mixture possessed certain properties which neither the vitriolic nor nitrous acids singly possess. The results of his experiments on this subject are as follow.
1. In a long-necked retort containing 1400 grain measures, 100 grains of oil of vitriol of the specific gravity of 1.844 were put along with 100 grains of pure nitre, and the salt dissolved in the acid by means of a water-bath. On applying a boiling heat, the silver began to dissolve, and the solution assumed a purple
Metal. or violet colour; but no air was extricated. An end was put to the operation (by the rushing in of the water used in the pneumatic apparatus applied to the retort) when 39 grains of silver had been dissolved.
2. To 100 grains of nitre previously dissolved in an equal quantity of vitriolic acid, 200 grains of standard silver were added; of which 92 grains were dissolved without the production of any air or gas: but upon pouring in 200 grains of water into the retort, a violent effervescence took place, and 3100 measures of nitrous gas were thrown off; and on adding 200 grains more, a farther emission of 600 grain measures ensued; but no more was emitted on the addition of more water, nor did any farther dissolution of the metal take place than two grains. The same phenomena took place in various proportions, according to the different quantities of acid and metal made use of.
On substituting tin for silver, none of the metal was dissolved or calcined by mixtures in the proportion of 200 grain measures of oil of vitriol to as much nitre, nor by 200 of the vitriolic acid to 150 of nitre: but with a proportion of 200 measures of the acid to 100 grains of nitre, the tin soon began to be acted upon, and diffused through the liquor; but no gas was extricated till the digestion had been continued in boiling water for two hours. The tin was still only calcined, not dissolved; 8500 grain measures of nitrous gas were extricated, and 73 grains of the metal reduced to a white powder. On pouring into the retort 200 grains of fresh water, a new effervescence took place betwixt the water and white powder, by which 4600 grain measures of nitrous gas were thrown off. The action of the menstruum was greatly promoted by augmenting the quantity of oil of vitriol, and adding water to the mixture.
By this mixture quicksilver was calcined to a grey powder, nickel was also partly calcined and partly dissolved; but no other metal was much affected, tho' the surfaces of some of them were tarnished. The mixtures themselves were very apt to congeal, especially where there was a large proportion of nitre; and their properties are much altered by the addition of water. Thus, in their concentrated state, they do not act upon iron; but by adding water, it acquires the property of acting upon that metal, and in different degrees according to the quantity of water added. Thus, by adding two measures of the compound acid to one of water, the liquor is rendered capable of calcining iron, and forming with it a white powder, but without any effervescence. An equal measure of water produces effervescence; and with a larger proportion of water the iron acquires a yellow or brown colour, such as phlogisticated nitrous acid acquires from iron, or communicates to a solution of martial vitriol in water. Dilution with water renders this compound acid capable of dissolving copper and zinc, neither of which it will touch in its concentrated state.
From this property of the compound acid not dissolving copper, but very readily silver, we have an easy method of separating the two metals from each other. This might be useful in many cases; but is particularly so in Birmingham, where great numbers of copper vessels covered with silver are manufactured. Thus there are always a great number of cuttings or
small bits called scraps, which are of no use but to separate the two metals from one another. The easiest method of doing this is an object of some consequence. Two methods have been generally practised for this purpose: one is by melting the whole mass of metal with lead, and separating them by means of eliquation and testing; the other is by dissolving both metals in vitriolic acid, and then separating the solution of copper from the vitriol of silver. The disadvantage of the former method is, the quantity of lead and copper wasted; and of the latter, that of vitriolic acid. The virtues of a mixture of oil of vitriol and nitre were some time ago communicated by Mr Keir to an artist at Birmingham; and it is now generally used there to effect the separation of the two metals. The method of using it is very easy; nothing more being requisite than to put the pieces of plated metal into a glazed earthen pan, and to pour upon them some of the acid liquor, which may contain about one pound of nitre to eight or ten of oil of vitriol. Stir them about, and assist the action by an heat from 100 to 200 of Fahrenheit's scale. When the liquor is nearly saturated, the silver may be precipitated in the form of luna cornea by common salt; or it may be obtained in its metallic form by adding to the liquor some pieces of copper, and as much water as will enable it to act upon them. He is of opinion, that the menstruum may be useful in all separations of silver from other metals. The name he seems to wish to impose upon it is aqua regina. The following are the conclusions drawn from the experiments on these mixtures by our author.
1. A mixture of the vitriolic and nitrous acids dissolves silver plentifully.
2. It acts upon tin, and mostly calcines it, as well as mercury and nickel; having little or no action upon other metals.
3. The quantity of gas produced while the metal is dissolving, is greater, relatively to the quantity of the metal dissolved, when the proportion of nitre to the vitriolic acid is small, than when it is large; and when the metals are dissolved by mixtures containing much nitre with a small production of gas, the solution itself, or the metallic salt formed in it, yields abundance of gas when mixed with water.
4. Dilution with water renders the concentrated mixture less capable of dissolving silver, but more so of acting upon other metals.
5. This mixture of highly concentrated vitriolic and nitrous acids, acquires a purple or violet colour when phlogisticated, either by the addition of inflammable substances as sulphur, or by its action on metals, or by very strong impregnation of oil of vitriol with nitrous gas.
6. By means of this phlogistication the mixture acquired a property of dissolving, though in small quantities, copper, iron, zinc, and regulus of cobalt.
7. Water expels a vast quantity of nitrous gas from a concentrated mixture of vitriolic acid and nitre impregnated with it; but this fluid unites with a mixture of oil of vitriol and nitre without any considerable effervescence.
By adding to the mixture of oil of vitriol and nitre a solution of common salt, a very powerful aqua regis is formed, capable of dissolving gold and platina; and
Metal. which, though perfectly free from all metallic matter, acquires at once a deep yellow colour. Dry common salt added to the concentrated mixture produces an effervescence, but no yellow colour.