Chemical Properties of Mercury

Mercury reacts readily with the halogens, whether moist or dry, especially with fluorine. In the presence of halogens or ozone it acquires the property of adhering firmly to glass. Pure, dry hydrochloric acid has no action, even at 200° C., nor hydrogen sulphide at 100° C. By the action of gaseous hydrochloric acid mixed with air, calomel is formed after some months, and hydrogen sulphide gas in the presence of air also attacks mercury. Gaseous hydrogen bromide only reacts slowly at ordinary temperatures, but rapidly at 100° C. Hydrogen iodide is readily decomposed in the cold, mercurous and mercuric iodides being formed. Mercury is unattacked by nitrogen, nitrous oxide, nitric oxide, and carbon dioxide, and is only superficially oxidised to a practically inappreciable extent in contact with air. In presence of ammonia, however, or if heated up to boiling-point, it is readily oxidised by oxygen. It is at once oxidised by ozone in the cold, but if heated to 250° C. it is not attacked, owing to the decomposition of the ozone. Mercury readily combines with sulphur. Hot concentrated sulphuric acid is decomposed by mercury with the production of sulphur dioxide and mercurous or mercuric sulphate, but with dilute acid there is no action. With nitric acid, even somewhat dilute, mercurous and mercuric nitrites and nitrates are formed with the evolution of oxides of nitrogen. Under certain conditions hydrogen is displaced from hydrochloric and sulphuric acids by the action of mercury. The mercury should impinge on the solution of acid, so that the concentration of mercury salt at the surface of the metal does not become large enough to inhibit the action. In the case of sulphuric acid, secondary effects, resulting in the production of hydrogen sulphide and free sulphur, have also been observed.

Mercury may act as a reducing agent towards such substances as potassium permanganate, dichromate, and persulphate, ferric salts, copper sulphate, nitrobenzene, and vanadic acid. Mercury also reacts with thionyl and sulphuryl chlorides, with phosphorus halides, and with halogen substitution products of the paraffins. The metal can apparently dissolve in and diffuse through water which contains air, nitrogen, carbon dioxide, sulphuric acid, or potassium hydroxide, and also through benzene, nitrobenzene, and alcohol. Mercury may act as a contact poison to catalysts - for example, to palladium or platinum.

Mercury is distinct from the other members of the same group in the periodic system, since it forms two well-defined series of salts, mercurous and mercuric. Solutions of mercurous salts contain the ion Hg₂^••, and of mercuric, the ion Hg^••. It is possible that at high dilution the mercurous ion splits up into two monovalent ions Hg^•, but probably only to an inappreciable extent.

The mercuric salts are peculiar in that the halides and the thio-cyanate are only very slightly dissociated. Those which are dissociated to a normal extent - that is, salts of strong inorganic oxygen acids and also salts of weak acids - are readily hydrolysed and form basic salts. Mercuric salts have a strong tendency to form complex salts with mercury in the anion. Mercurous salts, on the other hand, have very little tendency towards complex formation, are not readily hydrolysed, and are, many of them, only slightly soluble in water. Under the action of heat or light, mercurous salts tend to form a mixture of metallic mercury and the corresponding mercuric salt, especially if mercury be continually removed from the system, whilst mercuric salts, with excess of mercury, form mercurous salts.

Both the mercurous and mercuric ions are colourless.