Mercury Loss from Water during Storage: Mechanisms and Prevention J. M. Lo and C. M. Wail Department of Chemistry, University of Idaho, Moscow, Idaho 83843
Loss of mercury from water during storage in various containers has been discussed in several recent articles (13). This phenomenon creates a problem for analyzing mercury in natural water samples when storage is involved. I t also raises a question of how t o preserve standard mercury solutions for analytical purposes. For many environmental monitoring programs, the significant level of mercury is in the range of 1-100 ppb. Any minute loss of mercury during storage might represent a significant fraction of the aggregate and could easily affect the environmental quality data. T h e mechanisms by which mercury disappears from aqueous solution during storage are not well understood. Adsorption of mercury has been reported t o occur on the walls of polyethylene and glass containers (4, 5 ) . Volatilization has also been proposed as a mechanism responsible for the loss of mercury from dilute solution (6, 7). Acidification of water samples with nitric acid to p H < 1 has been suggested as a method to curtail mercury loss ( I , 2). Oxidizing agents such as dichromate and hydrogen peroxide have also been reported to be effective preservatives for mercury in dilute solution (3, 8). The mechanisms of mercury loss from aqueous solution and the effectiveness of the suggested preservatives have not been confirmed.
EXPERIMENTAL We have recently re-examined these problems using radioactive 203Hg (half-life 47 days) as a tracer. Demineralized water samples (450 ml each) spiked with 203Hga t 5 ppb mercury (prepared from Hg(NO&HzO) were stored in 16-02 Nalgene polyethylene narrow mouth bottles. All bottles were new to start with and were washed with concentrated nitric acid and rinsed with demineralized water prior to the experiment. The initial mercury activity in solution was measured by taking 1-ml aliquots from each bottle and counted on a Packard liquid scintillation counter. The details of counting procedure are described elsewhere (9). Various chemical preservatives were then added to the solution and the bottles were capped immediately for storage for 21 days. Our previous study indicated that mercury loss from demineralized water when stored in polyethylene bottles appeared to follow first-order kinetics with a rate constant of 1.8 X lo-* hr-' (2). A storage period of 21 days is sufficiently long for the system to approach a steady-state. A t the end of the storage period, mercury activity in the solution of each bottle was measured. The polyethylene bottles were cut and the activities on container walls were washed off with concentrated nitric acid and measured. Concentrated nitric acid was very effective at removing adsorbed z03Hgfrom polyethylene surfaces. After soaking the polyethylene bottles in concentrated nitric acid for one day, we found no detectable mercury activity remaining on the surface. The cap of each bottle was also washed with concentrated nitric acid and the activity in the acid solution was measured. RESULTS AND DISCUSSION The results of this study are summarized in Table I. Loss of mercury from demineralized water after 21 days of storage in polyethylene containers reached a value of about 95% of the initial concentration. Several samples taken from various locations of the solution indicated that mercury activities in the solution were uniform within the statistical fluctuation of counting. About 77% of the initial Author to whom correspondence should be addressed.
~
~~
Table I. Losses of Mercury from Aqueous Solution During Storage in Polyethylene Bottles for 21 days
1. 2. 3. 4.
None "0,. pH 0.5
KzCrz0,(0.05~ by wt)
K,Cr20, (0.OSc;) "0,. pH 0.5 5. Au3' (in HAuC1,.3 H 2 0 ) 0 . 2 ppm 6. Au3+0.2 ppm (in
HAuC14.3 H,O) "03,
-
pH 0.5 by vol.)
7. H2O2(0.1';
95+2 16 I 1 25 i 1 2 1 1
77*1 4*1 24 1 2 3 1
18*2 12*1 1*1 O i l
9012
86*1
4 1 2
2 x 1
2 x 1
O i l
87i2
37*2
50+3 ~
mercury activity was adsorbed on the walls of the container. We noticed that mercury activities on different sections of the polyethylene bottles were also fairly uniform with less than 3% fluctuation. The remaining 18%of the initial mercury activity, which could not be located either in the solution or on the container walls, was assumed lost by volatilization. An indication of the volatility of the mercury came from the fact that a high mercury activity was always detected on the surface of the cap, which was not even in contact with the solution during the experiment. Two types of caps were used; one was a Nalgene polypropylene screw closure with a ridge seal and the other, a black plastic cap with pulp and vinyl liner. Adsorption of volatile mercury was more pronounced in the latter case. T h e mercury activity found on the liner surface (about 2.5 cm2 in area) amounted to about 3% of the total initial activity in the system. Most of the volatile mercury in the system apparently could not be captured by the liner surface and was lost to the air during sampling. Acidification of water with nitric acid to p H 0.5 reduced the loss of mercury from solution to 16% of the initial concentration. Only 4% of the initial mercury was found on the walls of the container and the remaining 12% was lost by volatilization. This result indicates that nitric acid a t p H 0.5 is capable of stopping mercury adsorption to the container walls but does not effectively inhibit the volatilization of mercury. When potassium dichromate (0.05% by weight) was added t o the water, loss of mercury from solution was around 25%. In this case, virtually all of the lost activity was found on the walls of the container. No detectable amount of mercury was found on the liner surface of the' cap in this experiment. The results clearly indicate that potassium dichromate can effectively inhibit mercury loss by volatilization. A combination of nitric acid and potassium dichromate solution is expected to prevent mercury loss by both mechanisms, Le., volatilization and adsorption to container walls. Using this acid dichromate solution a t p H 0.5, we found that the loss of mercury from water was reduced to only 2 f 1%after 21 days of storage in polyethylene bottles.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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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
