Table of Contents
Mercury occurs native and, occasionally, alloyed with gold or silver in natural amalgams; but its chief ore is the sulphide, cinnabar. It is comparatively rare, being mined for only in a few districts. It is chiefly used in the extraction of gold and silver from their ores (amalgamation); for silvering mirrors, &c.
Mercury forms two series of salts, mercurous and mercuric, but for the purposes of the assayer the most important property is the ease with which it can be reduced to the metallic state from either of these. Mercury itself is soluble in nitric acid, forming, when the acid is hot and stfftng, mercuric nitrate. Cinnabar is soluble only in aqua regia. Mercurous salts are generally insoluble, and may be converted into mercuric salts by prolonged boiling with oxidising agents (nitric acid or aqua regia). The salts of mercury are volatile, and, if heated with a reducing agent or some body capable of fixing the acid, metallic mercury is given off, which may be condensed and collected.
Mercury is separated from its solutions by zinc or copper, or it may be thrown down by stannous chloride, which, when in excess, gives a grey powder of metallic mercury, or, if dilute, a white crystalline precipitate of mercurous chloride. Nitric acid solutions of mercury yield the metal on electrolysis; and, if the pole on which the metal comes down be made of gold or copper, or is coated with these, the separated mercury will adhere thereto. It may then be washed and weighed.
The best tests for mercury next to obtaining globules of the metal are: (1) a black precipitate with sulphuretted hydrogen from acid solutions, which is insoluble in nitric acid; and (2) a white precipitate with stannous chloride.
DRY METHOD
Weigh up 5 grams, if the ore is rich, or 10 grams, if a poorer mineral. Take a piece of combustion tube from 18 inches to 2 feet long, closed at one end, and place in it some powdered magnesite, so as to fill it to a depth of 2 or 3 inches, and on that a layer of an equal quantity of powdered lime (net slaked). Mix the weighed sample of ore in a mortar with 10 grams of finely powdered lime and transfer to the tube; rinse out the mortar with a little more lime, and add the rinsings. Cover with a layer of six or seven inches more lime and a loosely fitting plug of asbestos. Draw out the tube before the blowpipe to the shape shown in fig. 47, avoiding the formation of a ridge or hollow at the bend which might collect the mercury. Tap gently, holding the tube nearly horizontal, so as to allow sufficient space above the mixture for the passage of the gases and vapours which are formed.
Place the tube in a “ tube furnace,” and, when in position, place a small beaker of water so that it shall just close the opening of the tube. The point of the tube should not more than touch the surface of the water. Bring the tube gradually to a red heat, commencing by heating the lime just behind the asbestos plug, and travelling slowly backwards. When the portion of the tube containing the ore has been heated to redness for some time the heat is carried back to the end of the tube. The magnesite readily gives up carbonic acid, which fills the tube and sweeps the mercury vapour before it. Some of the mercury will have dropped into the beaker, and some will remain as drops adhering to the upper part of the neck. Whilst the tube is still hot cut off the neck of the tube just in front of the asbestos plug (a drop of water from the wash bottle will do this), and wash the mercury from the neck into the beaker. The mercury easily collects into, a globule, which must be transferred, after decanting off the bulk of the water, to a weighed Berlin crucible. The water is removed from the crucible, first by the help of filter paper, and then by exposing in a desiccator over sulphuric acid, where it should be left until its weight remains constant. It should not be warmed.
Example:—5 grams of an ore treated in this way gave 4.265 grams of mercury, equivalent to 85.3 per cent. Pure cinnabar contains 86.2 per cent.
WET METHODS
Solution.—Since solutions of chloride of mercury cannot be boiled without risk of loss, nitric acid solutions should be used wherever possible. No mercury containing minerals are insoluble in acids; but cinnabar requires aqua regia for solution. In dissolving this mineral nitric acid should be used, with just as much hydrochloric acid as will suffice to take it up.
To separate the mercury, pass sulphuretted hydrogen in considerable excess through the somewhat dilute solution. The precipitate should be black, although it comes down at first very light coloured. It is filtered, washed, and transferred back to the beaker, and then digested with warm ammonic sulphide. The residue, filtered, washed, and boiled with dilute nitric acid, will, in the absence of much lead, be pure mercuric sulphide. If much lead is present, a portion may be precipitated as sulphate, but can be removed by washing with ammonic acetate. To get the mercury into solution, cover with nitric acid and a few drops of hydrochloric, and warm till solution is effected. Dilute with water to 50 or 100 c.c.
GRAVIMETRIC DETERMINATION
This may be made by electrolysis. The same apparatus as is used for the electrolytic copper assay may be employed, but instead of a cylinder of platinum one cut out of sheet copper should be taken, or the platinum one may be coated with an evenly deposited layer of copper. Fix the spiral and weighed copper cylinder in position, couple up the battery, and when this has been done put the nitric acid solution of the mercury in its place. The student had better refer to the description of the Electrolytic Copper Assay.
The mercury comes down readily, and the precipitation is complete in a few hours: it is better to leave it overnight to make sure of complete reduction. Disconnect the apparatus, and wash the cylinder, first with cold water, then with alcohol. Dry by placing in the water oven for two or three minutes. Cool and weigh: the increase in weight gives the amount of metallic mercury.
It must be remembered that copper will precipitate mercury without the aid of the battery; but in this case copper will go into solution with a consequent loss in the weight of the cylinder: this must be avoided by connecting the battery before immersing the electrodes in the assay solution. The electrolysed solution should be treated with an excess of ammonia, when a blue coloration will indicate copper, in which case the electrolysis is unsatisfactory. With a little care this need not happen. Gold cylinders may preferably be used instead of copper; but on platinum the deposit of mercury is grey and non-adherent, so that it cannot be washed and weighed.
VOLUMETRIC METHODS
Several methods have been devised : for the details of these the student is referred to Sutton’s “ Handbook of Volumetric Analysis.”
QUESTIONS.
- The specific gravity of mercury is 13.596. What volume would 8 grams occupy ?
- If 3.169 grams of cinnabar gave 2.718 grams of mercury, what would be the percentage of the metal in the ore ?
- Pour solution of mercuric chloride on mercury and explain what happens.
- On dissolving 0.3 gram of mercury in hot nitric acid, and passing sulphuretted hydrogen in excess through the diluted solution, what weight of precipitate will be got ?