Mercury: a liquid metal

Mer­cury is a heavy, sil­very and very tox­ic met­al. One of the most well-known fea­tures of mer­cury is that in nor­mal con­di­tions this met­al is liq­uid and only freezes at –38.8°C! In na­ture there are only two el­e­ments that are liq­uid in nor­mal con­di­tions – mer­cury and bromine.

In the earth’s crust, mer­cury is a very rare el­e­ment, but rocks in which mer­cury is found usu­al­ly con­tain it in rather high con­cen­tra­tions. This is be­cause mer­cury mix­es poor­ly with el­e­ments that make up most of the earth’s crust, and so “ac­cu­mu­la­tions” of min­er­als con­tain­ing mer­cury are formed. The most abun­dant of them are cinnabar (mer­cury sul­fide HgS) and corderoit (Hg₃S₂­Cl₂).

The his­to­ry of met­al

Hu­man­i­ty has known about this met­al for a very long time. It was as­cribed both mys­ti­cal and heal­ing prop­er­ties. Cinnabar was used by hu­mans in the Pa­le­olith­ic era, 30,000 years ago. They paint­ed caves with it. The An­cient Greeks used mer­cury sul­fide in oint­ments, and An­cient Egyp­tians and Ro­mans used it to make cos­met­ics. Metal­lic mer­cury was found in the tombs of An­cient Egypt, dat­ed 1,600 BCE and the pyra­mids of Teoti­hua­can. The An­cient Egyp­tians and in­hab­i­tants of Mesoamer­i­ca may have used mer­cury to make a pro­to­type of the rivers of the af­ter­world.

Al­chemists be­lieved that mer­cury was the “pri­ma­ry ma­te­ri­al”, the “sub­strate” – the foun­da­tion of ev­ery­thing in ex­is­tence. They thought that mer­cury could turn or­di­nary met­als into gold.

The met­al takes its name from the Ro­man God, the pa­tron of traders and trav­el­ers, who is also re­spon­si­ble for com­mu­ni­ca­tion and luck. The chem­i­cal sym­bol Hg comes from the Greek word “ὑδράργυρος” (“hy­drar­gy­ros”), which lit­er­al­ly means “wa­ter-sil­ver”, as mer­cury is liq­uid like wa­ter, and shiny like sil­ver.

Chem­i­cal prop­er­ties of mer­cury

Mer­cury does not re­act with the ma­jor­i­ty of acids, but dis­solves in con­cen­trat­ed sul­fu­ric acid, in ni­tric acid and in aqua re­gia. If mer­cury is placed in ni­tric acid, mer­cury ni­trate Hg(NO₃)₂ forms:

Hg + 4H­NO₃ → Hg(NO₃)₂ + 2NO₂↑ + H₂O

Mer­cury is less re­ac­tive than cop­per, so cop­per forces mer­cury out of its salts. For ex­am­ple, if a cop­per coin is im­mersed in a so­lu­tion of mer­cury ni­trate, metal­lic mer­cury set­tles on the sur­face of the coin:

Cu + Hg(NO₃)₂ → Cu(NO₃)₂ + Hg↓

One fa­mous ex­per­i­ment with mer­cury is the mer­cury heart. Metal­lic mer­cury has a so­lu­tion of sul­fu­ric acid poured over it, and potas­si­um dichro­mate is added. In this ox­i­diz­ing medi­um, on the sur­face of the drop of mer­cury the film of its sul­fate forms, re­duc­ing the sur­face ten­sion that gives the drop its form. As a re­sult the drop of mer­cury spreads out a lit­tle.

2Hg + SO₄²⁻ → Hg₂­SO₄(film) + 2e⁻

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

If the sur­face of a drop of mer­cury is touched with an iron wire or nail, mer­cury sul­fate is re­duced to metal­lic mer­cury, the film is bro­ken, the sur­face ten­sion in­creas­es, and the drop “con­tracts”.

Hg₂­SO₄(film) + 2e⁻ → 2Hg↓ + SO₄²⁻

Fe → Fe²⁺ + 2e⁻

When the drop “con­tracts”, con­tact with iron is bro­ken, and the ox­i­da­tion process of the mer­cury starts again. The drop once more spreads out, touch­es iron, and then con­tracts. This process re­peats again and again, and the drop throbs like a beat­ing heart.

Where mer­cury is used

Ow­ing to its tox­i­c­i­ty, the use of mer­cury to­day has been se­vere­ly re­strict­ed. It can still be found in mer­cury ther­mome­ters, which are be­com­ing a thing of the past. Mer­cury ex­pands even­ly on heat­ing in a rather wide range of tem­per­a­tures, and as it ex­pands and ris­es up the glass cap­il­lary, it is easy to judge a change in tem­per­a­ture. The fact that it is opaque and has a char­ac­ter­is­tic shine makes it con­ve­nient to see where the “col­umn” of mer­cury in the ther­mome­ter is lo­cat­ed.

Ad­di­tion­al­ly, mer­cury fumes are used in some lu­mi­nes­cent lamps. Mer­cury fumes ra­di­ate ul­tra­vi­o­let light if an elec­tric cur­rent is passed through them. A spe­cial coat­ing on the sur­face of the lamp con­verts ul­tra­vi­o­let light into vis­i­ble light. One of these lamps con­tains around 10 mg of mer­cury, and in lamps with a low con­tent of this met­al, around 4 mg. Ad­di­tion­al­ly, mer­cury is en­coun­tered in some elec­tric switch­es and cur­rent rec­ti­fiers.

Elec­tric bat­ter­ies are made on the ba­sis of mer­cury, but nowa­days they have been re­placed by oth­er types of bat­ter­ies and ac­cu­mu­la­tors which do not con­tain mer­cury. There was even an at­tempt to use mer­cury as “fuel” for ion­ic en­gines of SERT (Space Elec­tric Rock­et Test) space­craft.

In in­dus­try, mer­cury and its com­pounds as used as cat­a­lysts, but one of their wide­spread ap­pli­ca­tions is the elec­trolyt­i­cal pro­duc­tion of ac­tive met­als, chlo­rine and al­ka­lis (the Cast­ner-Keller process). In this process, a very im­por­tant prop­er­ty of mer­cury is used – the abil­i­ty to form amal­gams, and al­loys with oth­er met­als. Only with iron, plat­inum, wol­fram and tan­ta­lum mer­cury does not form amal­gams.

Amal­gams of mer­cury played a ma­jor role in the de­vel­op­ment of chem­istry and the dis­cov­ery of new el­e­ments. The renowned Eng­lish chemist Sir Humphrey Davy used mer­cury ex­ten­sive­ly to ex­tract potas­si­um, mag­ne­sium, cal­ci­um, stron­tium and bar­i­um by elec­tro­chem­i­cal meth­ods – amal­gams of these met­als were formed in the elec­trol­y­sis of salts and ox­ides. Davy then re­moved the mer­cury and ex­tract­ed pure (or al­most pure) met­als. Ad­di­tion­al­ly, mer­cury was used to ex­tract gold from ore, as gold also formed an amal­gam with mer­cury.

Go­ing back in his­to­ry, we should dis­cuss oth­er spheres in which mer­cury was wide­ly used. Com­pounds of this met­al were used in medicine as di­uret­ics, lax­a­tives, and also medicine for syphilis. Medicine con­tain­ing mer­cury was tak­en by the Eng­lish king Hen­ry VIII, Ivan the Ter­ri­ble and Isaac New­ton. In Chi­na and Ti­bet, it was be­lieved that mer­cury ex­tend­ed life.

In the 20th cen­tu­ry, the com­pound Thiom­er­sal that con­tained mer­cury was used to ex­tend the shelf life of vac­cines. Mer­cury and its ni­trate were used by man­u­fac­tur­ers of felt hats in the 18th-19th cen­turies. Felt was soaked in a so­lu­tion of mer­cury ni­trate. Work­ers had to han­dle mer­cury and breath in its fumes, so poi­son­ing from this met­al was a pro­fes­sion­al ill­ness for them. As mer­cury poi­son­ing af­fect­ed men­tal health, the ex­pres­sion “mad as a hat­ter” arose. There is a leg­end that Lewis Car­rol based the Mad Hat­ter from “Al­ice in Won­der­land” on this type of work­er. How­ev­er, this is prob­a­bly not the case. The pro­to­type for the Mad Hat­ter was an ac­quain­tance of Car­rol, the fur­ni­ture deal­er Theophilus Carter.

Safe­ty mea­sures

Mer­cury is high­ly tox­ic. The max­i­mum ac­cept­able con­cen­tra­tion of mer­cury in a room dur­ing a work­ing day must not ex­ceed 0.05 mg per cu­bic me­ter of air. If a per­son in­hales mer­cury fumes over a long pe­ri­od of time, se­ri­ous poi­son­ing will oc­cur, dam­ag­ing the in­ter­nal or­gans and the ner­vous sys­tem. The per­son de­vel­ops symp­toms of “mad­ness”: ex­ces­sive shy­ness, in­de­ci­sive­ness, pho­bias, loss of self-iden­ti­fi­ca­tion etc. Ad­di­tion­al­ly, the gums and the res­pi­ra­to­ry tract be­come in­flamed, bron­chi­tis de­vel­ops and the di­ges­tive or­gans suf­fer.

What do you do if mer­cury is spilt at home or at work – if a mer­cury ther­mome­ter or lamp con­tain­ing mer­cury breaks? If a mer­cury lamp breaks, ev­ery­one not in­volved in clean­ing up the mess should leave the room, and take pets with them. Open the win­dows and turn on the air con­di­tion­ing. Put the lamp shards in a con­tain­er that can be closed or a plas­tic bag. Small shards can be gath­ered with sticky tape. Then air the room for sev­er­al hours. Don’t use a vac­u­um clean­er! This may cause the mer­cury fumes to spread even more!

If a mer­cury ther­mome­ter breaks, then you should also open the win­dows, and place the shards of the ther­mome­ter in a con­tain­er or plas­tic bag. Balls of mer­cury should be gath­ered very care­ful­ly with a pipette, as they have the prop­er­ty of mov­ing swift­ly in dif­fer­ent di­rec­tions. To make sure that no un­no­ticed drops of mer­cury have been left be­hind, turn off the light and shine a torch across the floor – you may no­tice shiny drops which were not vis­i­ble in nor­mal light.

The re­mains of the mer­cury which can­not be col­lect­ed with a pipette should be sprin­kled with sul­fur pow­der, which is sold in spe­cial sets for re­mov­ing mer­cury, or at an or­di­nary phar­ma­cy. You can also use iron(III) chlo­ride, which is sold at shops for ra­dio en­thu­si­asts. But if the mer­cury has fall­en onto por­ous or wooly sur­faces (for ex­am­ple a rug), then it is bet­ter to throw this item away, as it will be al­most im­pos­si­ble to re­move mer­cury from it.

Re­mem­ber that the gath­ered shards and re­mains of mer­cury should be dis­pensed with ac­cord­ing to the rules that ap­ply in your area. You may find col­lec­tion sta­tions for this type of waste.

Sources:

• Paul Par­sons, Gail Dixon — The Pe­ri­od­ic Ta­ble A vis­ual guide to the el­e­ments;
• Kristi Lew – Mer­cury. The Rosen Pub­lish­ing Group, 2008;
• Bar­ry R. Leopold – Use and Re­lease of Mer­cury in the Unit­ed States, Con­tract Nos. 68-C-0027 and 68-C7-0011, 2002;
• Wal­dron, H. A. (1983). Did the Mad Hat­ter have mer­cury poi­son­ing? British Med­i­cal Jour­nal (Clin­i­cal Re­search Ed.), 287(6409), 1961;
• John Burke Sul­li­van, Gary R. Krieger – Clin­i­cal En­vi­ron­men­tal Health and Tox­ic Ex­po­sures. Lip­pin­cott Williams & Wilkins, 2001;
• Sir Humphry Davy – The Col­lect­ed Works of Sir Humphry Davy …: Bak­e­ri­an lec­tures and mis­cel­la­neous pa­pers from 1806 to 1815. Smith, El­der and Com­pa­ny, 1840;
• Roy­al So­ci­ety of Chem­istry/Mer­cury;
• Wikipedia/Mer­cury;
• Wikipedia/Cinnabar;
• Wikipedia/Corderoite;
• Wikipedia/Cast­ner Kell­ner;
• Wikipedia/SERT-1;
• EPA/Mer­cury.