Elements of the Theory and Practice of Chymistry, 5th ed.

by Pierre Joseph Macquer

To the success of this separation, by means of aqua fortis, several conditions are essentially necessary. The first is, that the Gold and Silver be in due proportion to each other; that is, there must be at least twice as much Silver as Gold in the metalline mass, otherwise the aqua fortis will not be able to dissolve it, for the reason formerly given. If therefore the mass contain too little Silver, it must either be melted down again, and a proper quantity of Silver added; or else, if the Gold be in a sufficient proportion to the Silver, they may be parted by means of aqua regis.

Secondly, it is necessary that the aqua fortis employed in this operation be absolutely pure, and free from any taint of the Vitriolic or Marine Acid: for, if it be adulterated with the Vitriolic Acid, the Silver will precipitate as fast as it dissolves, and so the precipitated Silver will again mix with the Gold. If the aqua fortis contain any of the Marine Acid, the Silver will be precipitated in that case also; and this inconvenience will be attended with another, namely, that the menstruum, being partly an aqua regis, will dissolve some of the Gold. You must therefore be very sure that your aqua fortis is pure, before you set about the operation. In order to discover its quality, you must try it by dissolving, in a small portion thereof, as much Silver as it will take up: if the aqua fortis grow opaque and milky as it dissolves the Silver, it is a sign it contains some foreign Acid, from which it must be purified.

In order to effect this, let the portion of aqua fortis used for the above trial stand to settle: the white milky part will gradually fall to the bottom of the vessel. When it is all fallen, gently decant the clear liquor, and pour a few drops of this decanted solution of Silver into the aqua fortis which you want to purify. It will instantly become milky. Let the white particles precipitate as before, and then add a few more drops of your solution of Silver. If the aqua fortis still become milky, let it precipitate again, and repeat this till you find that a drop of your solution of Silver, let fall into this aqua fortis, does not make it in the least turbid. Then filter it through brown paper, and you will have an aqua fortis perfectly fit for the Parting Assay.

The white particles that appear and settle to the bottom, on dissolving silver in an aqua fortis adulterated with a mixture of some foreign Acid, are no other than that very Silver, which is no sooner dissolved by the Nitrous Acid than it deserts that solvent to unite with the Vitriolic or Marine Acid, wherewith it has a greater affinity, and falls to the bottom with them. And this happens as long as there remains in the aqua fortis a single atom of either of those two Acids.

When therefore your aqua fortis hath dissolved as much Silver as it is capable of taking up, and when all the white particles formed during the dissolution are settled to the bottom, you may be assured that the portion which remains clear and limpid is a solution of Silver in an exceeding pure aqua fortis. But if the solution of Silver thus depurated be mixed with an aqua fortis adulterated with the Vitriolic or Marine Acid, a like precipitation will immediately ensue, for the reasons above given, till the very last particle of the heterogeneous Acid contained in the aqua fortis be precipitated.

Aqua fortis purified by this method contains no extraneous substance whatever, except a small portion of Silver; so that it is very fit for the parting process. But if it be intended for other chymical purposes, it must be rectified in a glass retort with a moderate heat, in order to separate it from the small portion of Silver it contains, which will remain at the bottom of the retort.

The third condition necessary to the success of this operation is, that your aqua fortis be neither too aqueous, nor too highly concentrated. If too weak, it will not act upon the Silver: and the consequence will be the same if it be too strong. Both these inconveniencies are easily remedied: for in the former case part of the superfluous phlegm may be drawn off by distillation; or a sufficient quantity of much stronger aqua fortis may be mixed with that which is too weak: and, in the latter case, very pure rain water, or a weaker aqua fortis, may be mixed with that which is too strong.

You may satisfy yourself whether or no your aqua fortis hath the requisite degree of strength, by dissolving therein a thin plate consisting of one part Gold and two or three parts Silver; which plate must be rolled up in form of a paper coffin. If, when all the Silver contained in the plate is dissolved, the Gold remains in the form of the coffin, it is a sign that your solvent has a due degree of strength. If, on the contrary, the Gold be reduced to a powder, it is a proof that your aqua fortis is too strong, and ought to be weakened.

The Gold remaining after the dissolution of the Silver must be melted in a crucible with Nitre and Borax, as hath already been said under the process for parting Gold and Silver by means of aqua regis. As to the Silver which remains dissolved in the aqua fortis, there are several ways to recover it.