Mercuric atmospheric concentration

In the atmosphere, particulate bound mercury constitutes only 2% of total mercury in the air and has normally been found to be less than 0.1 ngm in regions unaffected by local sources. Some other mercury compounds, which may exist in the atmosphere, are mercuric chloride, mercuric bromide, mercuric hydroxide, mercuric sulfide, and mercuric cyanide. The rest is elemental mercury in the gaseous phase. In remote areas over the Atlantic and Pacific oceans, mercury bound to particulate matter concentrations are generally at or below the picogram per cubic meter level. 

The accepted primary reference electrode is the hydrogen half cell described in association with Fig.2.1 (Ref 5). It consists of platinum (which serves as an inert conductor) in contact with a solution at 25 °C, saturated with hydrogen gas at one atmosphere pressure, and containing hydrogen ions at pH = 0 (aH+ = 1). In practice, the major use of the standard hydrogen electrode (SHE) is for calibration of secondary reference electrodes, which are more convenient to use. Two common reference electrodes are the calomel or mercury/saturated-mercurous-chloride half cell with a potential of +241 mV relative to the SHE and the sil-ver/saturated-silver-chloride half cell with a relative potential of+196 mV. Both of these electrodes are saturated with potassium chloride to maintain a constant chloride and hence metal-ion concentration. 

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