The use of auric ion (Au3+)as a preservative for dilute mercury solution was reported a t a recent meeting ( I O ) . We found that 0.2 ppm of Au3+ (in the form of HAuC14.3 H2O) was sufficient to prevent mercury loss by volatilization but did not seem to affect mercury adsorption by the container walls. A solution of nitric acid containing 0.2 ppm of Au3+ a t pH 0.5 was as good as the acid dichromate solution described above for the prevention of mercury loss from water. Hydrogen peroxide has been mentioned in a recent article to be an effective preservative for dilute mercury solution (8). Our experimental results indicate that mercury loss from a 0.1% by volume of H202 solution was as high as 87% after 2 1 days of storage in polyethylene bottles. Less than half of the mercury lost from the solution was found on the walls of the container. Hydrogen peroxide appeared to increase mercury loss by volatilization from the solution. Its use to prevent mercury loss from water for long term storage seems of doubtful value. The disproportionation of mercury(1) to mercury(I1) and elemental mercury in aqueous solution has been suggested as a possible mechanism for the loss of mercury by volatilization (6). The vapor pressure of elemental mercury at 20 O C is 1.2 X lov3 mm and that at 26 OC is 2.0 X mm (11). If the air is saturated with mercury vapor at 20 "C, it would contain about 13 nanograms gram) of mercury per cm3. In a dilute mercury solution a t ppb level, the presence of trace quantities of reducing agents as impurities in the system (e.g., container surfaces) could cause reduction of mercury(I1) to mercury(1). The volatile mercury formed by the disporportionation reaction of mercury(1) could be lost to the vapor phase of the container. The reason that potassium dichromate can prevent mercury loss by volatilization is probably that its high oxidizing power keeps mercury ions at mercuij(I1) in the solution. Auric (Au3+) compounds are also powerful oxidizing agents. For example, the oxidation of mercury(1) to mercury(I1) by auric ions (Le., Hg22+ Au3+ = 2 Hg2+ Aul+) is a spontaneous reaction with Eo = 0.49 V.
+
+
When nitric acid-potassium dichromate solution at the specified concentration and pH was added to a natural water sample, the loss of 203Hgat 5 ppb mercury was found to be less than 3% after 21 days of storage in polyethylene bottles. The nitric acid-chloroauric acid solution described above was as effective as the acid dichromate solution when added to the same natural water sample. The low concentration of chloroauric acid required in the latter case may be an advantage for certain environmental samples. The fraction of mercury lost by adsorption t o container walls can be recovered by washing the container surfaces with concentrated nitric acid. However, correction of mercury loss from water by volatilization is by no means trivial because it would depend on the composition of the solution and the impurities present in the system. For instance, humic acid has been shown to cause volatilization of mercury(I1) from aqueous solution (12). The use of chemical preservatives appears to be necessary in order to obtain accurate data for environmental mercury in dilute aqueous solutions. LITERATURE CITED R. V. Coyne and J. A. Collins, Anal. Chem., 44, 1093 (1972). R. M. Rosain and C. M. Wai, Anal. Chim. Acta, 65,279 (1973). C. Feldman, Anal. Chem., 46, 99 (1974). P. Benes and i. Rajman, Collect. Czech. Chern. Commun., 34, 1375 (1969). (5) P. Benes, Collect. Czech. Chem. Commun., 35, 1349 (1970). (6)T. Y. Toribara, C. P. Shields, and L. Koval, Talanta, 17,1025 (1970). (7) S . Shimomura, Y. Nishihara, and Y. Tanase, Jpn. Anal., 18, 1072 (1969). (8) H. J. issaq and W. L. Zieiinske. Jr., Anal. Chern., 46, 1436 (1974). (9) W. G. King, J. M. Rodriguez, and C. M. Wai, Anal. Chem., 46, 771 (1974). (10) H. L. Rook and J. Moody, "Stabilization and Determination of Nanogram Quantities of Mercury In Water", 2nd International Conference on Nuclear Methods in Environmental Research, Columbia, Mo., July 1974. (11) "Handbook of Chemistry and Physics", 47th ed.. Chem. Rubber Co.. Cleveland, Ohio, 1967. (12) J. J. Alberts, J. E. Schindier. R. W. Miiier, and D.E. Nutter, Jr., Science, 184, 895 (1974). (1) (2) (3) (4)
RECEIVEDfor review March 24, 1975. Accepted May 27, 1975. This work was supported in part by a grant from the Idaho State Office of Higher Education.
2,2'-[2,6-Pyridinediylbis( methylidynenitrilo)]diphenol: A Highly Selective Reagent for the Detection of U(VI), Sb(lll), and Bi(lll) S. K. Thabet, S.
M. Adrouni, and H. A.
Tayim'
Department of Chemistry, American University of Beirut, Beirut, Lebanon
In an investigation to develop new specific and sensitive reagents for the detection of U(VI), Sb(III), and Bi(III), various chelating agents were tried. The chelating agent which gave the most satisfactory results was the Schiff base 2,2'-[2,6-pyridinediylbis(methylidynenitrilo)]diphenol (I) obtained from the condensation of 2,6-pyridinedicarboxaldehyde with o-aminophenol. This reagent gave intense red products with Bi(III), Sb(III),and U(V1) in acidic medium. It seemed therefore desirable to investigate further the application of I as a highly selective reagent for the detection of the three cations under different experimental conditions.
I EXPERIMENTAL
On Sabbatical leave at Kuwait Institute for Scientific Research, P.O. Box 12009, Kuwait. 1870
*
Reagents. The Schiff base (I) was prepared by dissolving 0.44 g of freshly sublimed o-aminophenol in 7 5 ml of water a t looo, and
ANALYTICAL CHEMISTRY, VOL. 47. NO. 11, SEPTEMBER 1975
adding to this 0.27 g of 2,6-pyridinedicarboxaldehydedissolved in 25 ml of water. The mixture was well shaken, kept for 30 min in a water bath (about SOo). and then stored overnight in the refrigerator. The yellow precipitate that formed was filtered, washed with water, and recrystallized from methanol. The yield was 78%. The Schiff base was stable for a t least one year in a dark brown bottle. A 0.4% solution of the reagent in chloroform was used for the tests. Saturated aqueous thiourea and 20% aqueous tartrate solutions were used for the U(V1)test. The buffer solution used for the Sb(II1) and Bi(II1) tests was prepared by mixing three parts of 9% aqueous ammonium acetate solution and one part of 6h‘ HCl (pH 0.235). Procedures. To Test for U(V1).One drop of thiourea solution and one drop of glacial acetic acid were added to one drop of the test solution. The mixture was shaken, and three drops of the reagent were added, followed by 1 ml of water. The red color produced was extracted into the chloroform layer by vigorous shaking. Limit of identification: 0.05 pg U(VI), limit of dilution: 1: 1,000,000. To Test for Sb(II1). Four drops of the buffer solution and four drops of the reagent were added to one drop of the test solution. The red color produced was extracted in the chloroform layer. When Bi(II1) was present, a drop of saturated thiourea was used to block it. Limit of identification: 0.05 pg Sb(II1); limit of dilution: 1: 1,000,000. To Test for Si(111).Four drops of the buffer solution and four drops of the reagent were added t o one drop of the test solution. The red color produced was extracted in chloroform. When Sb(II1) was present, its interference was avoided by first treating one drop of the test solution with two drops of 15% hydrogen peroxide. The mixture was kept in a water bath for one minute. The buffer and reagent were then added as above to produce a red color which was extracted in the chloroform layer. Limit of identification: 0.5 pg Bi(II1); limit of dilution: 1:100,000.
RESULTS AND DISCUSSION U(V1). The addition of a solution of (I) in chloroform to an aqueous solution of U(V1) salt results in the formation of an intense red precipitate. In the presence of glacial acetic acid, this precipitate could be extracted into chloroform. Aqueous solutions of the following ions were tested in the presence of glacial acetic acid: Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Zr(IV), Cr(lI1). M O W ) , W(VI), Mn(II), Fe(III), Co(II), Ni(II), Pd(IV), Pt(IV), Cu(II), Ag(I), Au(III), Zn, Rh(III), Cd(II), Hg(1, 11), Al, Tl(I), Sn(II,IV), Pb(II), As(III,V), Sb(III,V), Bi(III), La(III), Ce(III,IV), Th(IV), and V(I1). Of all the above-mentioned ions only U(V1) and Bi(II1) gave a red color, while with the others only the yel-
low color of the reagent was observed in the chloroform layer. To remove the interference of Bi(II1) with U(VI), saturated thiourea solution was used, which also improved the limits of the identification of U(V1) in the presence of other cations. Sb(II1) also blocked the color formation; its interference was removed completely by the addition of tartrate. The test for U(V1) was satisfactory in the presence of the following anions: F, C1, Br, I, Nos, SO4, S, s&3,CN, COa, Po4, citrate, tartrate, malonate EDTA, CNS, Cr04, Mn04, IO3, C103, Br03, and C104; but it failed in the presence of oxalate. This test for U(V1) made it possible to identify 0.1 g U(V1) in 500 g Be, Zr(IV), Mn(II), Ag(I), Cd, Hg(1, 11), Sn(I1, IV), and Bi(II1). Moreover, 0.5 g of U(V1) could be detected in 500 g of V(V) and Ce(1V). When Sb(II1) was present, two drops of 20% sodium, potassium tartrate was substituted for the thiourea of solution. I t was thus possible to detect 0.5 g U(V1) in 500 g Sb(II1). Sb(II1). The addition of a solution of Schiff base(1) in chloroform to an aqueous solution of Sb(II1) buffered by ammonium acetate produced an intense red color at pH 0.235. Aqueous solutions of the cations mentioned in the test for U(V1) were tested with the reagent in buffer; none produced a red color in the chloroform layer besides Sb(II1) and Bi(II1). However, the color due to Bi(II1) disappeared when a drop of saturated aqueous thiourea solution was added. The test was satisfactory in the presence of the following anions: F, Cl, Br, I, Nos, SO4, s&3,CN, C03, Po4, citrate, tartrate, malonate EDTA, CNS, oxalate, IO3, BrOa, C103, C104, Mn04, S 2 0 3 , and CrzO:. Sulfide reduced the limit of the sensitivity of the test to 50 g Sb(II1) in 500 g sulfide. It has been possible by this test to identify 0.2 g Sb(II1) in 500 g U(V1) and 0.5 g Sb(II1) in 500 g Ce(IV), V(V), Au(III), Hg(I), Pb(II), and Bi(II1). The remaining cations mentioned under U(V1) did not affect the limit. Bi(II1). The test for Bi(II1) works the same as for Sb(II1) except for the fact that thiourea blocks the Bi(II1) reaction. Thiourea produces a yellow color with Bi(III), which is the same color as that of the reagent itself. RECEIVEDfor review January 16, 1975. Accepted April 10, 1975.
Study of Liquid-Powder Interfaces by Means of Solvent Strength Parameter Measurements Claude H. Eon Laboratoire de Chimie-analytique-physique, Ecole Polytechnique, 17, rue Descartes Paris
Since the Forties, the need for optimizing adsorption chromatography has given rise to many attempts to classify solvents (and adsorbents) leading to scales of “polarity” often referred to as eluotropic series. Most of them, arbitrarily derived by simply ranking parameters like dielectric constants, dipole moments, heat of wetting, solubility in water . . . ( I ) can be considered as only very rough guidelines for they have very little theoretical foundations. A far more realistic approach, described by Snyder (2), consists in measuring the “strength” of the solvent from
50,France
chromatographic behavior; a proper correction for the solute effect leads to a solvent strength parameter t o which exhibits the remarkable property of being only a function of the pair solvent/adsorbent. Despite the fact that these series have quickly become popular, the rationale of this classification still escapes many analysts for t o is often considered to be an empirical parameter. In fact, t o is a rather fundamental parameter which can be related to the adsorption energy of the solvent ( 2 ) . It will be shown here that this view is consistent with the thermodynamics of the in-
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