Ions of Mercury

Monovalent mercury is found only in dimeric salts such as Hg2Cl2 (calomel), vhich is sparingly soluble in vater and, again correspondingly, much less toxic than HgCl2 (sublimate). 

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The adherence of mercury to glass, i.e. tailing in presence of ozone, is probably due to the formation of an oxide. The oxidation of the iodide ion to iodine in solution is used to determine ozone quantitatively. 

C = C triple bonds are hydrated to yield carbonyl groups in the presence of mercury (II) ions (see pp. 52, 57) or by successive treatment with boranes and H2O2. The first procedure gives preferentially the most highly substituted ketone, the latter the complementary compound with high selectivity (T.W. Gibson, 1969). 

The isotope patterns for two simple organometallic compounds in the molecular ion region (a) dimethylmercury and (b) dimethylplatinum. The seven isotopes of mercury show clearly and appear quite different from the six isotopes of platinum. Since there are only two carbon atoms, the contribution from C is negligible. 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

The reactivity of mercury salts is a fimction of both the solvent and the counterion in the mercury salt. Mercuric chloride, for example, is unreactive, and mercuric acetate is usually used. When higher reactivity is required, salts of electronegatively substituted carboxylic acids such as mercuric trifiuoroacetate can be used. Mercuric nitrate and mercuric perchlorate are also highly reactive. Soft anions reduce the reactivity of the Hg " son by coordination, which reduces the electrophilicity of the cation. The harder oxygen anions leave the mercuric ion in a more reactive state. Organomercury compounds have a number of valuable synthetic applications, and these will be discussed in Chapter 8 of Part B. 

The halides are the most familiar compounds of mercury(I) and all contain the Hg2 + ion (see below). Hg2p2 is obtained by treating Hg2C03 (itself precipitated by NaHC03 from aqueous Hg2(N03)2 which in turn is obtained by the action of dil HNO3 on an excess of metallic mercury) with aqueous HF. It dissolves in water... 

The small amount of mercury(I) chloride in suspension has no appreciable effect upon the oxidising agent used in the subsequent titration, but if a heavy precipitate forms, or a grey or black precipitate is obtained, too much tin(II) solution has been used the results are inaccurate and the reduction must be repeated. Finely divided mercury reduces permanganate or dichromate ions and also slowly reduces Fe3+ ions in the presence of chloride ion. 

If nitric acid solutions of mercury(II) nitrate are used, the latter must be converted into mercury(II) chloride by the addition of at least an equivalent amount of chloride ion. 

Compact, ready-prepared calomel electrodes are available commercially and find wide application especially in conjunction with pH meters and ion-selective meters. A typical electrode is shown in Fig. 15.1(h). With time, the porous contact disc at the base of the electrode may become clogged, thus giving rise to a very high resistance. In some forms of the electrode the sintered disc may be removed and a new porous plate inserted, and in some modern electrodes an ion exchange membrane is incorporated in the lower part of the electrode which prevents any migration of mercury(I) ions to the sintered disc and thus... 

Chromium. 25.0 mL chromium(III) ion solution (0.02M, prepared by dilution of stock solution) + 50.0 mL 0.02 M EDTA + 50mL acetate buffer, boiled for 10 minutes, solution cooled, pH adjusted to 4.6 with hexamine, 1 drop of mercury-EDTA solution added, and then back-titrated with standard zinc ion solution. 

The amount of reddish-purple acid-chloranilate ion liberated is proportional to the chloride ion concentration. Methyl cellosolve (2-methoxyethanol) is added to lower the solubility of mercury(II) chloranilate and to suppress the dissociation of the mercury(II) chloride nitric acid is added (concentration 0.05M) to give the maximum absorption. Measurements are made at 530nm in the visible or 305 nm in the ultraviolet region. Bromide, iodide, iodate, thiocyanate, fluoride, and phosphate interfere, but sulphate, acetate, oxalate, and citrate have little effect at the 25 mg L 1 level. The limit of detection is 0.2 mg L 1 of chloride ion the upper limit is about 120 mg L . Most cations, but not ammonium ion, interfere and must be removed. 

Mercury(II) thiocyanate method Discussion. This second procedure for the determination of trace amounts of chloride ion depends upon the displacement of thiocyanate ion from mercury(II) thiocyanate by chloride ion in the presence of iron(III) ion a highly coloured iron(III) thiocyanate complex is formed, and the intensity of its colour is proportional to the original chloride ion concentration ... 

Rate law and mechanism. Suggest one reaction scheme for the oxidation of vana-dium(III) ions by mercury)II) ions on the basis of the rate equation given. 

Self-Test I.2B The mercury(I) ion, Hg22+, consists of two Hg+ ions joined together. Write the net ionic equation for the reaction in which colorless aqueous solutions of mercury(I) nitrate, Hg2(N03)2, and potassium phosphate, K3P04, react to give a white precipitate of mercurv(I) phosphate. 

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