The Pamphlet Collection of Sir Robert Stout: Volume 69
The present high price of platinum, about 85 shillings per ounce, makes it worth looking for and saving. Platinum is generally found in small grains, of a greyish white colour approaching to that of tarnished steel. These grains are commonly flattened, and appear to have been polished by friction against other bodies. Their size usually varies from that of linseed to that of hemp seed, but fragments of much larger dimensions have occasionally been discovered.
It is frequently met with, especially in Otago, in alluvial deposits, associated with gold and being one of the heaviest of all known substances, heavier than gold, it is left behind with the gold and black sand; the latter can be partly removed with an ordinary magnet, and the residue tested for gold and platinum, thus:
The finely-powdered mineral is heated in a glass flask or porcelain dish to near boiling for some time with aqua regia, a mixture of four parts strong hydrochloric acid and one part page 16 strong nitric acid, till all but the sand is dissolved; as much as possible of the free acid is then evaporated off, and when cool the solution is diluted with water and filtered. If any silver is present, it will remain on the filter with the sand as insoluble chloride of silver, and after being well washed with water, can be tested by pouring over it a little ammonia water, collecting the liquid that runs through in a test tube and adding nitric acid in excess, that is till the smell of ammonia is removed. A white precipitate shows the presence of silver.
To the liquid from which the sand and silica have been removed, a solution of carbonate or bi-carbonate of soda is added until no more effervescence takes place; all other metals, except gold and platinum, will be precipitated, and the solution when filtered can be tested for these. If we wish to reduce the gold first, it can easily be done either by adding a solution of oxalic acid until it ceases to produce effervesence and has a sour taste, and then boiling; if any gold is present it will be precipitated as a dark powder, or by adding a solution of sulphate of iron (green vitriol) and letting stand for a few hours. Upon now filtering the liquid from which the gold has been precipitated, and testing a small portion with a solution of stannous chloride (commonly known as salts of tin), a dark brownish purple precipitate shows there is still some gold in solution, which must be got rid of by adding to the bulk of the liquid more oxalic acid or sulphate of iron, and, after boiling or standing, again filtering; if in a small portion no precipitate is produced, but a reddish brown colouring appears, platinum is present. Evaporate the remainder of the solution, add about three-fourths of its bulk of spirits of wine, and then a saturated solution of sal-ammoniac. The platinum will be thrown down as a yellow crystalline precipitate.
Or the platinum may be removed first by evaporating the aqua regia solution until it is much reduced in quantity, then adding about three-fourths of its bulk of spirits of wine, and after that a saturated solution of chloride of ammonium (sal ammoniac). By these reagents the platinum will be thrown down as a yellow crystalline precipitate, while the solution, filtered from this and treated with sulphate of iron or boiled with oxalic acid, deposits the gold.
- 20 grains = 1 scruple.
- 3 scruples = 1 dram or 60 grains.
- 8 drams = 1 ounce or 480 grains.
- 12 ounces = 1 pound or 5760 grains.
- 24 grains = 1 pennyweight.
- 20 penny weights = 1 ounce or 480 grains.
- 12 ounces= 1 pound or 5760 grains.
Avoirdupois Weight (the ordinary weights of the shops).
- 1 ounce = 437½ grains.
- 16 ounces = 1 pound or 7000 grains.
Note.—The grain is the same in all.
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