Cold vapor

AA

INSTRUMENTS AT WORK

The Determination of Mercury by Cold Vapor Atomic Absorption
Douglas E. Shrader and William B. Hobbins, Varian AA Resource Center, Park Ridge, Illinois, U.S.A.

Number AA-32 September 1983

Introduction
Mercury contamination of the environment and its toxic effects have been known for centuries. The novel “Alice in Wonderland” refers to “mad hatters” who werehat makers gone “mad” due to exposure to mercury used in cleaning felt hats. In 1810, a British man-of-war salvaged 130 tons of mercury from a Spanish wreck off Cadiz. Parchment bags had been used to confine the mercury within wooden barrels, but owing to the sodden condition of the bags after salvage and the prevailing heat, mercury vapor very quickly diffused throughout the man-of-war. Almosteveryone on board soon showed signs of mercury poisoning, several members of the crew died, and the ship’s livestock was wiped out(1). Since then we have learned to handle and transport mercury itself with more caution, but disposal of industrial wastes containing mercury has often been less than responsible. Over the last two decades, a growing concern about increasing mercury concentrations inwateways and lakes has resulted in extensive surveys and large scale monitoring. This resulted in fishing bans in areas where mercury levels as high as 7 ppm had been recorded. During this same time, studies on the determination of mercury were carried out. Because of the relatively poor sensitivity for mercury provided by traditional flame atomic absorption, alternative atomization techniques for theAA determination of mercury have been developed. Furnace methods for mercury are not recommended due to the extreme volatility of mercury, which has a significant vapor pressure even at room temperature. The cold vapor atomic absorption technique for mercury has received the greatest attention. The cold vapor principle was first proposed by (2) Poluektov and co-workers in 1963 . However the mostpopular method is that usually credited to Hatch and Ott published in 1968(3). In this method, an acidified solution containing mercury is reacted with stannous chloride in a vessel external to the AA instrument. Ground state mercury atoms are produced

which subsequently are transported by an air or inert gas flow to an absorption cell installed in the AA instrument. This method providessensitivities approximately four orders of magnitude better than flame AA. The U.S.A. EPA has set a limit for mercury in drinking water of 2 µg/L or 2 ppb. The cold vapor technique is the only EPA approved method for determining mercury at this level(4).

The Cold Vapor Techinque
The processes occurring in the mercury cold vapor technique are simple but they can be affected by numerous factors. Ionicmercury in acidic solution will be reduced by stannous ions producing ground state atomic mercury. After equilibration the mercury vapor is swept from the reaction vessel with air, nitrogen or argon. In most instances the atomic mercury is swept through an absorption cell placed in the optical path of the AA instrument, atomic absorption takes place, and a transient signal is recorded.Alternatively, a closed system can be used, producing a somewhat lower “steady-state” signal. Results obtained can be affected by the type and concentration of acids employed, the actual form of mercury in the sample, and other reagents or matrix components of the sample. In order for the reaction to proceed efficiently, samples containing organically bound mercury must be subjected to an acid digestionproducing ionic mercury in solution. Various digestion schemes have been reported. Among them are (5) potassium permanganate and perchloric acid , nitric acid(1), and sulphuric acid and sodium hydroxide(6). The EPA recommends a rather extensive digestion scheme for waters and wastes which will be outlined later. A study was performed to determine optimum acid concentrations and types, sample...