The Preparation of Chemically Pure Gold
Commercial gold is never pure, although it answers for the majority of purposes such as coinage, jewelry or leaf. For fine gold plating, however, a chemically pure gold is frequently necessary and it may be of interest to know how it can be made.
Commercial gold bars usually run from 997 to 999 in fineness. They are more frequently found 997 than of a higher percentage. The reason for this apparently impure metal is that in the ordinary parting process, as practiced by the United States Assay Office, where practically all the gold used in our own country is refined, it is impossible, without a greater cost, to eliminate the last traces of impurities. As gold of the aforesaid fineness is suitable for the majority of purposes (particularly coinage) it is allowed to pass without any additional refining.
The impurities in commercial gold are silver, copper, tellurium, antimony, platinum, lead, bismuth, arsenic, iron, selenium, and occasionally traces of quite rare elements. Silver and copper, however, are the principal impurities, as they resist the nitreing process when a metal like lead would be thoroughly oxidized out. For this reason gold refining can be carried up to a certain point by the use of nitre, but it then must stop, as the silver and copper resist the action of this substance.
For completely purifying gold, the method employed by the British Government in the production of its pure gold Trial Plate, preserved by the Warden of Standards, is excellent. It can only be equalled (probably not surpassed) by the Wohlwill process of refining gold electrolytically. The Wohlwill process, however, is patented and a royalty must be paid in order to use it, and, too, special equipment is needed for working it. Those who wish to make small lots of chemically pure gold will find the following method capable of producing metal of the highest degree of purity. The process is as follows:
The purest gold obtainable is either granulated or rolled into sheet. It is then dissolved in aqua-regia (nitric and muriatic acids). For every ounce of gold use three ounces of strong, chemically pure muriatic acid, and one ounce of chemically pure nitric acid. If the gold is in large pieces, more acid may be needed at the end of the dissolving process; and if this is the case, then more can be added. The dissolving is preferably carried out in a glass flask, heated by a sand bath. The long neck of the flask prevents the loss of gold by spirting. When everything has gone into solution there will usually be a small quantity of flocculent residue left. This is chloride of silver that will not dissolve.
Now pour the solution into a porcelain evaporating dish and rinse out the flask with distilled water. Evaporate it until it becomes dark red and syrupy. A sand bath should be used for this purpose, and care must be taken not to evaporate too far; otherwise the chloride of gold will become "burnt" (i.e., it will change back to metallic gold). The dark red liquid thus obtained is chloride of gold and is allowed to cool.
The chloride of gold is then dissolved in pure grain alcohol (denatured alcohol will not answer). Add just enough alcohol to dissolve it (leaving the slight residue of chloride of silver which the alcohol will not dissolve) and then add I pennyweight of chloride of potassium to every ounce of gold. This is to precipitate the platinum. The platinum separates in the form of a yellow crystalline precipitate. It is soluble in water but not in alcohol, and it is for this reason that the alcohol is used.
The solution is allowed to stand over night and filtered to remove the platinum precipitate (potassium platinic chloride) and then water is added so that each gallon of solution contains half an ounce of gold. In this condition it is allowed to remain for three weeks, in order to allow all chloride of silver to separate out. After this period has elapsed the solution is filtered through fine filter paper or the clear liquid siphoned off.
To the clear solution thus obtained and heated to about 120 degrees Fahrenheit a strong solution of oxalic acid in water is added. The solution is maintained in a warm condition for some time and all of the gold will be precipitated as a spongy mass of brown color. After the solution has remained for a few hours all of the gold will be precipitated. It is allowed to settle and the clear liquid above it is siphoned off. Distilled water is now added and the whole stirred and the gold again allowed to settle. The clear liquid is siphoned off as before and the operation repeated five or six times. Dilute muriatic acid (1 part of acid and 10 parts of water) is now used instead of the water and the precipitate washed several times. The washing with water is then repeated once or twice to remove the acid left and ammonia water then is used for washing the gold. The washing is carried out several times and the gold is finally transferred to a fine, washed filter paper and washed repeatedly with water. In this manner all of the impurities are washed out.
The gold is now dried and removed from the filter paper to an unglazed porcelain crucible. In this it is melted under a small quantity of bi-sulphate of potash and borax (equal parts) for a flux. When melted it is poured out upon a slab of stone or in a stone mold in order to avoid contamination with iron.
The Trial Plate which the British Government made in this manner weighed 70 oz. and the average purity was 999.96. While the process is too expensive for many commercial uses, it illustrates the care that must be used in the production of a pure metal.
Source: The Brass World and Platers' Guide - January 1908
Trev.
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The Preparation of Chemically Pure Gold - 1908 Article
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