Mercury

It is called corrosive because it is very poisonous and attacks (corrodes) living tissue, and sublimate because on heating the solid changes directly to vapour without melting, and the vapour condenses back to the solid on a cold surface. It is a covalent compound with weak forces between its molecules, so it sublimes easily: \(HgCl_{2(s)}\rightleftharpoons HgCl_{2(g)}\).

Mercuric chloride is soluble in water (sparingly in cold and readily in hot water), so it is easily absorbed into the blood and gives \(Hg^{2+}\) ions which bind to the proteins and enzymes of the body. Calomel, \(Hg_2Cl_2\), is insoluble in water, so very little of it is absorbed and it passes out of the body, which is why it is tasteless and almost non-toxic.

Mercury has a low boiling point (357 degrees Celsius) and is volatile, whereas the impurities (\(Pb, Cu, Cd, Zn\)) have much higher boiling points and are left behind in the flask. The distillation under reduced pressure (vacuum distillation) gives mercury which is about 99.9% pure.

The chief ore of mercury is cinnabar, \(HgS\) (mercuric sulphide).

Extraction (pyrometallurgy): the ore is crushed and powdered, concentrated by froth flotation, and then roasted with charcoal in excess of air in a shaft furnace. The sulphide is converted to the oxide, and the oxide, which is unstable to heat, decomposes into mercury vapour:

The mercury vapour is passed through water-cooled condensing tubes and collects as liquid mercury, which is then purified by filtration, washing with 5% nitric acid and vacuum distillation. No coke is needed as a reducing agent, because mercury is a very unreactive metal and its oxide decomposes on heating.

Nessler’s reagent is an alkaline solution of potassium tetraiodomercurate(II), \(K_2[HgI_4]\) (potassium mercuric iodide), made alkaline with \(KOH\).

Preparation: mercuric chloride solution is treated with potassium iodide. A scarlet precipitate of mercuric iodide forms, and it dissolves in excess of \(KI\) to give the complex. The solution is then made alkaline with \(KOH\).

Application: it is used to detect and estimate ammonia or ammonium ions in water. A trace of \(NH_3\) gives a brown colour or precipitate (iodide of Millon’s base), a larger amount a brown precipitate:

\(HgS\) (mercuric sulphide).

\(K_2[HgI_4]\) (potassium tetraiodomercurate(II), in alkaline solution).

\(HgCl_2\) (mercury(II) chloride).

\(Hg_2Cl_2\) (mercury(I) chloride).

Observation: a white precipitate forms at once.

Explanation: ammonia replaces one chlorine of mercuric chloride by the amino group, giving mercuric amino chloride, \(Hg(NH_2)Cl\) (also called infusible white precipitate), which is insoluble. The other chlorine goes to ammonium chloride.

(Equivalent to \(HgCl_2+2NH_3\rightarrow Hg(NH_2)Cl\downarrow+NH_4Cl\).) In contrast, calomel with ammonia turns grey-black, since it forms finely divided mercury together with \(Hg(NH_2)Cl\); this test distinguishes the two chlorides.

By heating mercury in an excess of chlorine gas (the chlorine must be in excess, otherwise calomel forms):

On heating, mercuric chloride sublimes (the solid changes into vapour without melting), and the vapour condenses on a cold surface: \(HgCl_{2(s)}\rightleftharpoons HgCl_{2(g)}\). The notes of the textbook give the reaction on strong heating as the decomposition into the elements, \(HgCl_2\xrightarrow{\Delta}Hg+Cl_2\uparrow\).

A scarlet precipitate of mercuric iodide is formed first, and it dissolves in excess of \(KI\) to give a clear solution of potassium tetraiodomercurate(II), which is the Nessler’s reagent (when made alkaline):

1. It is used as a pesticide, especially as a fungicide, and as a preservative of timber and leather.

2. Its 5% solution is used as a disinfectant. It is also used to prepare calomel and Nessler’s reagent.

Overall: \(HgS+O_2\xrightarrow{\Delta}Hg+SO_2\uparrow\).

Aqua regia is a mixture of 1 part concentrated \(HNO_3\) and 3 parts concentrated \(HCl\). It gives nascent chlorine, \(HNO_3+3HCl\rightarrow NOCl+2H_2O+2[Cl]\), and \(Hg+2[Cl]\rightarrow HgCl_2\). Overall:

The mercurous oxide formed makes mercury stick to glass and lose its meniscus, a change called the tailing of mercury.

Calomel is mercury(I) chloride, \(Hg_2Cl_2\). The formula is \(Hg_2Cl_2\) and not \(HgCl\) because mercury(I) exists as the \(Hg_2^{2+}\) ion, in which two mercury atoms are joined by a covalent bond.

Preparation: (i) \(Hg_2(NO_3)_2+2HCl\rightarrow Hg_2Cl_2+2HNO_3\) (laboratory method, a mercurous nitrate solution and a soluble chloride); (ii) \(Hg+HgCl_2\xrightarrow{\Delta}Hg_2Cl_2\); (iii) \(2HgCl_2+SnCl_2\rightarrow Hg_2Cl_2+SnCl_4\); (iv) \(Hg+HgSO_4+2NaCl\xrightarrow{\Delta}Hg_2Cl_2+Na_2SO_4\).

Physical properties: white amorphous solid, tasteless, insoluble in water and non-toxic, soluble in aqua regia.

Chemical properties:

Uses: as a laxative (purgative, the old bed pill) in medicine, as an antiseptic (now outdated), and to make the calomel electrode, a secondary reference electrode in electrochemistry.

1. Crushing and pulverisation: big lumps of cinnabar are broken by a jaw crusher and powdered in a ball mill.

2. Concentration: the powder is mixed with water and pine oil in a tank and agitated with compressed air (froth flotation). The sulphide ore particles stick to the froth and float, and the earthy impurities (gangue) settle.

3. Roasting and distillation: the concentrated ore, mixed with charcoal, is heated in excess of air in a shaft furnace (a reverberatory furnace is not suitable because the volatile mercury vapour would escape). The sulphide changes to the oxide, which decomposes at once into mercury vapour:

Impurities such as \(P\), \(S\), \(As\) and \(Sb\) go out as volatile oxides (\(4P+5O_2\rightarrow2P_2O_5\), \(S+O_2\rightarrow SO_2\), \(4As+3O_2\rightarrow2As_2O_3\)). The mercury vapour passes through Y-shaped iron tubes cooled by water, where it condenses to liquid mercury and collects in a receiver; the waste gases escape.

4. Purification: the liquid mercury is filtered through chamois leather, washed with 5% nitric acid, and finally distilled under reduced pressure (see Question 11).

Corrosive sublimate is mercury(II) chloride, \(HgCl_2\), a poisonous white crystalline solid.

Preparation:

(\(MnO_2\) prevents the formation of calomel.)

Physical properties: white crystalline solid, sparingly soluble in cold water and readily soluble in hot water, soluble in aqua regia, toxic, and it sublimes on heating.

Chemical properties:

Mercuric thiocyanate, when dried and ignited, swells into a snake-like tube called pharaoh’s serpent. With stannous chloride it is reduced stepwise, first to calomel and then to mercury:

Uses: pesticide and fungicide, 5% solution as disinfectant, preparation of calomel and Nessler’s reagent, and preservative of timber and leather. Traditionally egg white or milk is given as the antidote for poisoning.

The liquid mercury obtained from the condenser contains impurities such as copper, lead, zinc, bismuth and suspended particles (soot, \(HgO\)). It is purified in three steps:

(i) Filtration: the impure mercury is filtered through chamois leather or thick canvas, which removes the suspended particles and oxides.

(ii) Chemical washing: the filtrate is passed in fine drops through a long tube containing 5% dilute nitric acid. The mercury is oxidised a little to mercurous nitrate, and the more reactive metals (\(Zn, Pb, Fe\)) displace mercury from it and dissolve as nitrates:

(iii) Vacuum distillation: the mercury is distilled under reduced pressure. It boils at 357 degrees Celsius and distils over, leaving \(Pb\), \(Cu\), \(Cd\) etc., which have higher boiling points, behind. The mercury obtained is about 99.9% pure.

Iron impurity is removed in the same way, since iron is more reactive than mercury and dissolves in the dilute nitric acid.

\(M\) is mercury (\(Hg\)): atomic number 80, period 6, group 12, electronic configuration \([Xe]\,4f^{14}5d^{10}6s^2\), and the only metal that is liquid at room temperature (m.p. \(-38.8\) degrees Celsius, b.p. 357 degrees Celsius, density 13.6 g/cc).

The chief ore is cinnabar, \(HgS\).

The cinnabar ore is crushed and powdered, and concentrated by froth flotation. The concentrated ore is mixed with charcoal and roasted in excess of air in a shaft furnace, where the oxide formed decomposes to mercury vapour, which is condensed in water-cooled Y-shaped iron tubes:

The liquid mercury is purified by filtration through chamois leather, washing with 5% \(HNO_3\) and vacuum distillation (99.9% pure).

Mercury dissolves in aqua regia (3 parts conc. \(HCl\) and 1 part conc. \(HNO_3\)) to form mercuric chloride:

Mercury is not affected by air at ordinary temperature, but at about 350 degrees Celsius it forms red mercuric oxide: \(2Hg+O_2\xrightarrow{350^\circ C}2HgO\). Above 350 degrees Celsius the oxide decomposes back into mercury and oxygen, \(2HgO\rightarrow2Hg+O_2\).

1. As a thermometric and barometric liquid (thermometers, barometers) and in electrical switches and mercury vapour lamps.

2. In the extraction of gold and silver by amalgamation, in dental fillings (amalgam), and in the production of chlorine and caustic soda.

The element is mercury, \(Hg\), and its chief ore is cinnabar, \(HgS\).

Figure: Shaft furnace for roasting and distillation of cinnabar

1. Crushing and pulverisation of the ore (jaw crusher, ball mill).

2. Concentration by froth flotation: the powdered ore, water and pine oil are agitated with compressed air, and the sulphide particles rise in the froth while the gangue settles.

3. Roasting and distillation: the concentrated ore mixed with charcoal is heated in excess of air in a shaft furnace: \(2HgS+3O_2\rightarrow2HgO+2SO_2\), \(2HgO\rightarrow2Hg+O_2\). The mercury vapour is condensed in water-cooled Y-shaped iron tubes and collected as liquid.

4. Purification: filtration through chamois leather, washing with 5% nitric acid, and vacuum distillation.

Other compounds are \(Hg_2O\) and \(Hg_2(NO_3)_2\) (+1) and \(HgO\), \(HgS\) and \(HgI_2\) (+2).

1. In the extraction of gold and silver by the amalgamation process, and in the preparation of amalgams including dental fillings.

2. In thermometers and barometers, in fluorescent and mercury vapour lamps, and in electrical switches.