Mercury, distillation impurities

Triple-distiUed mercury is of highest purity, commanding premium prices. It is produced from primary and secondary mercury by numerous methods, including mechanical filtering, chemical and air oxidation of impurities, drying (qv), electrolysis, and most commonly multiple distillation. 

We start by assuming that the electroanalyst has a ready supply of freshly distilled mercury. The usual contaminants in impure mercury are other metals, so the mercury represents a dilute amalgam. Distillation removes these metals. 

A liquid which has a vapor pressure of as much as 760 mm. of mercury at any temperature which permits the use of a suitable container may be purified from nonvolatile impurities by the process of distillation at atmospheric pressure. By working under reduced pressure, the vapor pressure of the liquid need not be nearly so great as 760 mm. As a rule, a liquid may also be separated from other liquids provided the vapor pressures of the two differ enough to cause them to boil at considerably different temperatures. To effect complete separation, the process of distillation must be repeated a number of times in a systematic way and is then called fractional distillation. 

The mercury of commerce, which usually is contaminated with variable portions of lead, tin, and bismuth, may be distilled with one-tenth of its weight of cinnabar and provided the distillation be carefully effected, the foreigu impurities will he found in the retort, in the form of sulphides, after tho pure mercury has passed over into the receiver. A purer product is, however, obtained when carefully prepared cinnabar or corrosive sublimate—chloride of mercury —that has been cautiously sublimed, is distilled with one part of quicklime or iron filings. The principal part of the impu-... 

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