Saturated Mercury

What volume (in liters) of a saturated mercury(Il) sulfide, HgS, solution contains an average of one mercury(II) ion, Hg2+ ... 

Fig. 5.6 (Left) Comparison of band energy levels for different II-VI compounds. Note the high-energy levels of ZnSe. Representation is made here for electrodes in contact with 1 M HQO4. The reference is a saturated mercury-mercurous sulfate electrode, denoted as esm (0 V/esm = +0.65 V vs. SHE). (Right) Anodic and cathodic decomposition reactions for ZnSe at their respective potentials (fidp, Fdn) and water redox levels in the electrolytic medium of pH 0. (Adapted from [121])...

The most frequently used calibration procedure is based on temperature dependence of pressure of saturated mercury vapour [19,39-41]. At 25°C this pressure is of 0.0018 mm Hg height it corresponds to the vapour density of 20 pg/1. To get in the measurement cell a mercury concentration of about 10 ng/1, the saturated vapour should be strongly diluted. Instead of dilution, a lower temperature can be used however, the density of saturated vapour of 10 ng/1 corresponds to the temperature of less than —40°C. Both dilution and temperature decrease can be realized easily in laboratory conditions but their incorporation into a miniaturized chemical sensor is rather complicated. An attempt to develop such a device is reported in Ref. [41]. An additional problem in application of these techniques in portable sensing devices with integrated calibration is the necessity to have a reservoir with mercury in the device it complicates recycling of these devices and does not correspond to modern trends in technology. 

Fifteen grams of pure zinc is shaken for a short time with 100 ml. of 1 N hydrochloric acid to which a trace of copper sulfate has been added. When the metal surface has become thoroughly clean, 50 ml. of a saturated mercury(II) chloride solution is added and the mixture is shaken until the zinc is well covered with a deposit of mercury. The metal is quickly washed by decantation, 50 ml. of clean mercury is added, and the solution is covered with more dilute acid. After a few hours the zinc dissolves and the 2% (by weight) amalgam is ready for use. It may be preserved under 0.1 AT hydrochloric acid in a loosely stoppered bottle. 

Place a drop of the test solution and 2 drops of saturated mercury(II) chloride solution in the same apparatus (Fig. 11.53) and, after a minute, acidify with 2m hydrochloric or m sulphuric acid, and proceed as above. 20 pg Na2S03 can be detected in the presence of 900 pg Na2S203 and 1,500 pg of Na2S. 

We shall see, however, that this value is some 2 per cent, too low. The deviation of saturated mercury vapour from the ideal gaseous state reduces its specific heat (cf. infra), and therefore increases A, on a rough estimate, by some 80 cals. Although the error still remaining appears inconsiderable in view of the difficulty of the measurement, it is sufficient to throw out the result we arc calculating by a large amount. 

Saturated mercury vapour contains approximately 15 mg Hg(g) per m at 20 °C, and approximately 2500 mg Hg(g) per m at 100 °C. The occupational exposme limit value for mercury vapour in air set by the American Conference of Governmental Industrial Hygienists is 0.025 mg/m . Thus, saturated mercury vapour at 20 °C contains approximately 600 times the permitted value, and at 100 °C, it contains approximately 100000 times the permitted value ... 

Weigh out 8 g purest iron(lll) chloride hexahydrate, into a 400 cm beaker. Add water containing a few cm cone. HCl and dissolve by stirring. Transfer to a volumetric flask and make up to 250 cm Pipette a 25.0 cm aliquot into a 250 cm conical flask, add 3 cm cone. HCl and then add, dropwise 0.25 M tin(II) chloride solution until the yellow colour just disappears and then gradually add a saturated mercury(Il) chloride solution to remove the excess tin(II) when a slight silky mercury(l) chloride is formed. Allow to stand for a minute, add 5 cm of orthophosphoric acid, a few drops of N-methyldiphenylamine indicator and titrate with standard M/60 dichromate solution as above. 

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