STUDIES OF SOME ASPECTS OF ANALYTICAL CHEMISTRY OF MERCURY

Abstract

Mercury is one of the earliest elements known to mankind. The metal, amalgams, and its different compounds have found important uses In various spheres of human life. This element is unique in forming a very large number of organometallic compounds. The organo-mercurials have been used for variety of purposes. Typical of which is disodium salt of dibromohydroxymercurifluoroscein(mercurochrome) •— a common antiseptic. The mixed derivatives of mercury such as phenylr»ercurlc chloride are amongst the most effective commercial fungicides. The determination of mercury in different commercially important compounds, rocks and minerals has always posed a challenge to analytical chemists. In the last two decades, a good deal of Interest has been shown in various aspects of analytical chemistry of mercury because of following important reasons. Mercury is known to be a serious pollutant of the biosphere. People were aware of mercury poisoning even long back. But no serious attention was paid to the threat of mercury poisoning till around sixties when mercury-polluted fish claimed several lives in Japan. Sweden became aware of the mercury pollution problem as a result of a marked dieoff seed eating bird and their predators. These animals as well as many Swedish agricultural products were found to contain elevated levels of mercury which was attributed to 1 the large scale use of alkyl mercurial seed dressings in Swedish agriculture. In March, 1970 the Canadian government announced a ban on the sale of fish from the waters of Lake St. Clair because these fish were contaminated with mercury. Serious public concern about mercury pollution developed in U.S. in 1970 when the findings of Fedral Water Quality Agency and Food and Drug Administration indicated that mercury in a methylated form was leaving aquatic reservoirs in sufficient quantities to threaten both the quality of fresh water fish and their growth pattern. Different chemical industries are the main culprit for dumping tons of mercury in aquatic reservoirs where mercury is being converted to methyl mercury. These different reports have proved to be an eye opener. Now the interest in mercury pollution is international in scope. Excellent articles have appeared on this problem in several popular magazines1"3. Because of the foreseen hazards, mercury concentration limits of 0.005, 0.05 and 0.5 ppm have been prescribed for water, different food stuffs and fish, respectively. All this has prompted a lot of interest in the determination of mercury in different . 7,8 edibles4'5, water and atmospheric samples . People are equally interested in the determination of mercury in different biological samples9**11. Those who work with mercury and its compounds are recommended to undergo a regular clinical check-up for the mercury content in their urine, nails and hair samples. This practice has definitely avoided the occurrence of a number of fatal incidents. Recently cosnochemists have also become interested in the determination of mercury in different types of 12-14 meteorites . The abundance of mercury in ordinary chondrites is much lower than expected from current theories of nucleogenesis. The concentration pattern of mercury In different types of meteorites may provide a clue to their thermal history. It is evident from the above mentioned examples that for one reason or other the analytical chemistry of mercury has attracted the attention of workers in different disciplines. The two important aspects of the analytical chemistry of an element ar^ separation and determination. In the analysis of complex materials, many a times the separation becomes a necessity. Out of the different methods used for separation, liquid-liquid extraction and ion exchange chromatography are the most commonly used techniques. These methods are quick, convenient and effective. Moreover, the techniques can be extended from tracer concentrations to macro levels. Unlike precipitation methods of separation these do not present any problem at trace concentrations of an element. 4 Generally, the amounts of mercury present in different edibles, biological samples and meteorites are very low (of the order of ppm or lower). Neutron activation analysis is one of the best techniques for the determination of mercury at these concentration levels. In activation analysis, if mercury activity has to be separated, solvent extraction and ion exchange chromatography provide the most effective methods of separation. Looking at the versatility of these two techniques a systematic investigation of some of the aspects of liquid-liquid extraction and Ion exchange behaviour of mercury was planned. This, in tum, would be helpful to propose effective separation of mercury from other associated elements like zinc, cadmium, gold, and thallium. The separat ion of mercury from zinc and cadmium is of importance as they are the members of the same sub-group of the periodic table. Gold and thallium are the adjoining neighbours of mercury and, therefore, the separation of these elements from mercury will also be important from the point of view of nuclear reaction studies. In separations by liquid-liquid extraction, the extraction of halo and thiocyanato complexes is extensively used. The difference in the extraction behaviour of various * elements is explored for separation. Sore earlier workers 1 fi lfi 1*718 like Mylius and Huttner , West and Duff , Bock et al. ' , 19 20 21 Kitahara , Irving and Kossotti , and Moser and Voigt have studied the extraction behaviour of mercury(II) halo and thiocyanato complexes. But systematic and useful data was not available to propose effective separations and suggest some possible mechanism for these extractions. The data on these extractions were generally available in diethyl ether which is not a convenient solvent. Moreover, in some cases it existed only at a particular molarity of the acid. From the trends of the existing data it was apparent that mercury(II) halo complexes have a tendency to show higher extractions at low molarity (< 1) of the acid. This behaviour is of considerable significance for the separation of mercury(II), as a number of elements like Fe(III), Ga(lII);and As(III) which are 22 extracted as halo complexes , show a higher extraction only at higher molarity of the acid. Therefore, to propose separations a detailed study of the extraction behaviour of mercury(II) has been carried out. The extraction behaviour has been studied in various oxygenated and non-oxygenated solvents. To evaluate the role played by the hydrogen Ions of the acid the extraction from different acids has been compared with their corres ponding sodium or potassium salts. The effects of temperature, concentration of mercury(II) ions, and masking agents like cyanide, oxalate, tartrate, citrate, and EDTa on the distribution ratios have been investigated. These studies not only help to propose separations but also throw some light on the possible mechanism of extraction. The mercury(II) extraction data suggested ethyl acetate, n-butanol, and benzene as good solvents for the separation of Hg(ll) from Zn(IT), Cd(II), Au(III), Tl(III), andTl(l). But sufficient data on the extraction behaviour of these elements In these solvents did not exist in the literature?3',24 . Therefore, the extraction behaviour of these elements in these particular solvents has also been studied. The separation of Hg(II) from Zn(II), Cd(II), Au(III), Tl(l)and Tl(IIT) has been proposed with high separation factors. 4 The thesis embodies another interesting study on liquid-liquid extraction behaviour of mercury(II) concerning its extraction from various buffer media. The problem came in mind while studying the effect of pH on the extraction of Hg(II) 1,5-dlphenylcarbazide complex using different buffer solutions. In the case'of certain buffers, large blanks were observed due to the extraction of Hg(II)-buffer species. This suggested to take up a detailed study of extraction behaviour of mercury(Il) from various buffer media in different organic solvents. Generally, higher extractions were observed in lo«er pH range. It was thought worthwhile to use these higher extractions of mercury(II) for its separation from elements mentioned earlier. Therefore, the extraction behaviour of these elements has also been studied under identical conditions. The extraction data suggests that Hg(II) can be separated from Zn(II), Cd(II), AU(III), T1(I) and Tl(III). Though the constituents of some buffers are quite complex, some attempts have been made to identify those responsible for the extraction. 7 In the last decade there has been a breakthrough in ion exchange chromatography by proposing numerous AC separations in mixed solvents (organic + aqueous). Korkisch pioneered this idea and coined the name Combined Ion Exchange Solvent Extraction (abbreviated asCIESE) for the technique. A number of separations which are not possible or difficult in pure aqueous media have been carried out in mixed solvents25. The commonly used organic solvents for mixing with aqueous solutions are THF, acetone, methanol, and ethanol. It is assumed that in this technique two processes, ion exchange and liquid-liquid extraction, are operative simultaneously. The addition of organic solvent sometimes drastically changes the distribution coefficient of an element. This technique has been applied to both cation and anion exchange chromatography. The cation and anion ion exchange behaviour of mercury(II) in various aqueous media has been investigated by different workers 6~32. But no systematic studies were done on the behaviour of mercury(II) in mixed solvents which, in turn, may provide high separation factors for its separ ation from Zn(II), Cd(ll), Au(III), Tl(I)and Tl(III). Generally, these separations are difficult to be attained in aqueous media. In the present investigation the cation and anion exchange behaviour of Hg(II) has been studied in a mixture of HC1 or HN03 and methanol, acetone, or THF at a constant (0.6M) acid strength. To propose separation from Hg(ll), the ion exchange behaviour of Zn(II), Cd(II), Au(III), 8 T1(I), and Tl(III) has been studied under identical conditions. Some of the separations, where the separation factor is of the order of 10 or greater, have actually been performed using column method. The studies have resulted in proposing separation of mercury(II) from these elements with separation factors much higher than those 33—35 reported by earlier workers