Variables in the Determination of Mercury by Cold Vapor Atomic Absorption S. R. Koirtyohann' and Moheb Khalil' Department of Biochemistryand Environmental Trace Substances Research Center, University of Missouri, Columbia. Mo. 6520 1
Reduced mercury follows a well behaved partition function when equilibrated between a gas and aqueous phase. The numerical value of the constant Is about 0.4 and it is independent of most variables tested. Sulfuric acid concentration and temperature show a significant effect. Losses of mercury vapor caused by premature reduction were encountered In both manual and automated methods. The reduction is caused by components in common plastics and may be prevented by the presence of an oxidizing agent or by excess chloride. Lowered response was also observed when the solutions contained more than about 1 ppm of Pt, Au, or Ag. The reduced metal or precipitated chloride Interferes with proper partitioning of the mercury vapor.
Various modifications of flameless or cold vapor atomic absorption (AA) for the determination of mercury have become standard in many laboratories. The most popular method is that usually credited to Hatch and Ott ( I ) , although Poluektov et al. (2, 3 ) had employed a similar principle earlier. The mercury is reduced with Sn2+, partitioned by passing a gas through the solution, and measured as the reduced vapor is carried through a cell on an AA spectrophotometer. The procedure was simplified by Stainton ( 4 ) who carried out all reactions in a syringe. Automated procedures based on the same reactions have been developed by several groups (5-7). In spite of their apparent simplicity, these methods gave results which were sometimes unreliable in our laboratory, especially for standards. Some results from our investigation into the reasons for the poor reliability are presented here.
EXPERIMENTAL Apparatus. All atomic absorption measurements were made on a Perkin-Elmer Model 290 spectrophotometer. The 253.7-nm resonance line from a P-E hollow cathode lamp was used for all measurements. Scale expansion up to 1OX and damping to give a time constant up to about 10 sec were used when appropriate. All readings were presented on a strip-chart recorder. The absorption cells were Pyrex tubes about 1 2 cm long with quartz windows. Side arms of capillary tubing were attached near each end of the cell for entrance and exit of the vapors. Cells were 10-mm i.d. for the syringe procedure and 3.5-mm i.d. for the automated method. The cell was wrapped with a heating tape powered through a variable transformer to keep the cell temperature at -35 "C to prevent condensation on the windows. Disposable polypropylene syringes of 10- and 50-ml capacity (Sherwood Medical Industries, Deland Fla.) and a 50-ml all-glass syringe were used in the study of the Stainton ( 4 ) procedure. The tip of the 10-ml syringe fit inside the 50-ml plastic syringe tip to form an air tight seal, thereby eliminating any need for the special fittings described by Stainton ( 4 ) . The automated method used a Technicon Automated sampler I1 and proportioning pump I along with appropriate pump tubes, mixing coil, and a locally fabricated gas separator. The basic system which resulted from optimization experiments is shown schePresent address, Thompson-Hayward Chemical Company, Research and Development Department, P.O. Box 2383, Kansas City, Kan. 66110. 136
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
matically in Figure 1. Minor modifications of the basic system were used for some experiments. Reagents. Reagent grade chemicals were used throughout except for hydrochloric and nitric acids which were redistilled. The reducing solution initially used was similar to that described by Stainton ( 4 ) and consisted of 10 cm3 of concd H2S04,0.5 g NaC1, 2.0 g of (NHzOH)2"2S04, and 2.0 g of SnC12 per 100 ml of solution. The concentration of all components except the SnClz were varied for some experiments. The reducing solution was prepared fresh daily. A stock standard mercury solution containing 1.00 mg/ml was prepared by dissolving the metal in dilute "03. Working standards were prepared by appropriate dilution from this solution. Two types of standards were used. In one, the acid strength was maintained a t 0.16 M "03. The other was maintained a t 0.06 M HC1 in addition to 0.16 M "03. Standards below 10 kg Hg/ml were prepared fresh daily. For later work, standards were prepared according to the method of Feldman (8) which calls for storage in 5% "03 and 0.5% KzCr20i. This greatly reduced the standards stability problem. The noble metal solutions used for the interference experiments were prepared by dissolving the metal in "03 or aqua regia. 2,6 Di-tert- butyl-4-methyl phenol, commonly called butylhydroxytoluene or BHT, was obtained from Chemical Service Co., Media, Pa. Procedure. The basic syringe procedure consisted of placing the sample or standard contained in 10 ml of aqueous volume in a 50-ml plastic syringe. The tip of a 10-ml syringe containing 5 ml of reducing solution was then placed inside the 50-ml syringe tip to form a gas-tight seal and the reducing solution added. As the syringes were separated, air was immediately drawn into the larger syringe to prevent the escape of reduced mercury vapor and to give a total gas volume of 30 ml. The syringe was quickly capped and shaken for 30 sec. The air and partitioned mercury vapor were injected into the cell through a small magnesium perchlorate drying tube to prevent condensation of water vapor on the cell windows. Alternatively, the cell was wrapped with a heating tape to warm it slightly (-35 "C) for the same purpose. After the absorption signal was recorded, clean air was drawn back into the syringe through the cell and the recorder returned to the base line. The primary deviation from Stainton's procedure ( 4 ) was in the syringe sizes used. The above procedure was modified by adding HC1 or permanganate to the sample solution, altering the composition and method of addition of the reducing agent, substituting a glass syringe for the polypropylene normally used, by placing pieces of polypropylene inside a glass syringe, and by adding BHT to the aqueous solution. The partition constant for reduced mercury vapor between the aqueous and gas phases was calculated from measurements on two or more equilibrations of the same aqueous phase with air. The first injection was carried out in the normal way with care to inject all of the gas phase. The syringe was then removed from the cell, a second 30 ml of air drawn in, the syringe capped and shaken for the second injection. This procedure could be repeated for as many equilibrations from a single aqueous phase as desired. The value of the partition constant was studied with respect to shaking time, concentrations of HC1, "03, HzS04, and temperature. After initial optimization, the automated system was studied for possible effects of plastic pump tubing on standard readings. This was accomplished by adding extra segments of pump tubing in the flow system before and after addition of the reducing agent. The effect of adding noble metal ions to the sample solution was investigated with this system. It was felt that such ions might be reduced along with the mercury, form an amalgam, and interfere with the transfer of mercury vapor to the gas phase. Silver, gold, platinum, lead, and copper were used a t various concentrations.
-
SAMPLER II
AA SPECTROPHOTOMETER RECORDER
-m
AIR
r
3.4
,*::
I
3.4
RATE 20/HR 1 SAMPLE
L
W
3.4
I
5% SnCI, waste
-
P
0.6
O PROPORTIONING PUMP
Flgure 1. Diagrammatic representation of automatic instrument
Both the automated and syringe procedures were investigated for possible premature reduction of the mercury due to interaction with some part of the system. The effect of various oxidizing agents and chloride ion on the reduction was studied.
a7 0 I 6NI
HCI
a6
RESULTS AND DISCUSSION
Only that fraction of the reduced mercury vapor which partitions to the gas phase is measured by cold vapor atomic absorption. Estimates of the fraction of the mercury actually in the measurement cell have been made (41, but there appears to be no published study of the partition between the phases or the variables which affect it. The partition constant may be calculated without measuring absolute quantities of mercury if measurements are made on two separate portions of gas phase after equilibration with a single aqueous phase containing reduced mercury. If x = total absorbance for all of the mercury in the sample measured under the conditions of the experiment, y = the fraction of mercury found in the vapor phase after equilibration, a = measured absorbance from the first equilibration with air, and b = measured absorbance from the second equilibration with air; then xy = a and ( x - a ) y = b. Solving the equations simultaneously: a2
Y = -a - b
x=a-b a The Partition Constant is the ratio of mercury concentration in the two phases
K=
Concn of Hg in air
Concn of Hg in liquid The numerical value of K is independent of the units chosen to express concentration; therefore absorbance values which are proportional to the amount of mercury may be substituted.
where Va and VL are the volumes of air and liquid, respectively. Substituting from above:
a5 Y
5 5 0.4 r
a3
ai a2 0
t
1 I
,
1
I
2
I
3
I
4
1
I
I
I
5 6 7 8 9 mi A C I D ADDEDflO ml TOTAL VOLUME
1
IO
Figure 2. Effect of acid concentration on the partition constant
The numerical value of the partition constant is about 0.4 and it is independent of the amount of mercury used over the range of 10-40 ng. It is also independent of the volume ratio between the phases over the range of 1:3 to 4:l air-liquid. The effect of the concentration of several common acids is presented in Figure 2. Sulfuric acid is the only one showing a significant effect. The effect of temperature on the partition constant is shown in Figure 3. Near 25 OC, a temperature change of 5 "C produces a change in K of only about 1%.No temperature control should be required for most work. The partition takes place quickly. Five seconds of vigorous hand shaking gave the same yield of mercury vapor as longer times. If ideal behavior is assumed, it should be possible to calculate the Dartition constant from solubilitv data. According to Moser and Voigt (9), water saturated with reduced mercury contains 6.0 X glml. The concentration of ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
137
as
I
/
Table I. Mercury Absorption in the Absence of Added Reducing Agents, 50 ng Hg-10 ml Aqueous Phase-30 ml Air Aqueous phase composition
0.16 M HNO, 0.16 M HNO, + M KMnO, 0.16 M HNO, 0.16 M HNO, + 0.06 A' HCI 0 . 1 6 M HNO,
Syringe material
Polypropylene Polypropylene
0.184 0.000
Glass Polypropylene
0.000 0.000
Glass, containing cutup polypropylene parts 0.16 M HNO, Glass, containing parts of rubber plunger from disposable syringe 0.16 M HNO, Glass, containing 0.1 g BHT 'Average of 5 runs. Absorbance for 50 ng Hg run ducing agent added = 1.00. 10
0
3l
Q
50
M
70
80
Absorbance'
0.180 0.000
0.150
with re-
90
TEMPERATURE PC
Flgure 3.
Effect of temperature on the partition constant
mercury in saturated air a t 299 K is 2.16 X g/ml. The partition constant calculated from these data is 0.36 which is in quite good agreement with the measured values. Losses of mercury from dilute solution have been observed by several workers. Bate ( 1 0 ) clearly showed a loss from polyethylene containers during neutron irradiation. Grossoleil and Roy (11) studied the volatility of mercury using 203Hgtracer. They found that the loss was prevented by the presence of excess HC1 and attributed it to vaporization of HgC12. Formation of HgC1d2- in the presence of excess chloride was thought to stop the loss. Gutenmann and Lisk (12) reported a response from mercury in the Hatch and Ott procedure without adding the reducing agent. They suggest that the mercury was vaporized as HgCl2 which was then photolytically decomposed in the absorption cell to yield atomic mercury. They agreed with Grossolei1 and Roy on the apparent stabilization due to formation of HgC142-. In the present work using the syringe procedure, low and erratic values were sometimes obtained for standards but not for samples which contained excess oxidant from a digestion. In the automated method, standards were also sometimes erratic giving reduced peak heights and some peak tailing. The problem was much more severe with new pump tubing than with tubes that had been in use for several months. In both cases, the problem was eliminated if either excess HC1 (0.06 N ) or an oxidizing agent such as KMn04 was added to the standards. Also, using the syringe procedure, mercury absorption could be observed for standards prepared in dilute H N 0 3 even though no reducing agent was added. The signal was about 20% as large as that observed in the presence of the reducing agents. These observations cannot be explained by volatility of HgC12 as suggested by previous workers (11, 12) for several reasons. Data to support these reasons are presented in Table I. 1) Hgo is required for absorption a t 253.7 nm. Photolytic decomposition of significant amounts of HgClz by a weak hollow cathode source seems very unlikely. 2) Addition of an oxidizing agent prevented the loss of mercury without change in chloride ion concentration (See Table I). 3) Substitution of a 50-ml glass syringe for the disposable polypropylene one eliminated both the erratic behavior for standards and the mercury signal observed in the absence 138
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
of a reducing agent. Excess chloride had no effect with the glass syringe (Table I). 4) At a chloride ion concentration of 6 X M , the fraction of the mercury present in the various forms is readily calculated from the formation constants. Log Kf values for the steps of the reactions leading to HgCld2- are, Kfl = 5.3, Km = 7.5, Kf3 = 1.1,Kf4 = 1.0. The calculated values, expressed as the fraction of the total mercury existing in the various forms, are: Hg2+,2 X HgCl+, 2.4 X HgC12, 4.4 X 10-l; HgCls-, 3.5 X 10-l; and HgC142-, 2.1 x IO-'. These fractions apply at all mercury concentrations so long as the chloride ion is present in large excess. One concludes that HgC12 is the most abundant species at the chloride concentrations used, accounting for nearly half the total mercury. Since the losses were apparently reduced by much more than a factor of two when chloride was added, the formation of HgC142- cannot be the major factor in preventing the loss. A much more satisfactory explanation for the observation is that the mercury was reduced to Hgo by some component of the system prior to addition of Sn2+.The erratic standard readings and the absorption in the absence of added reducing agent were due to escape of the vapor thus produced. This reduction was prevented by the excess oxidant in digested samples or oxidant added to standards. Chloride ion probably acts by lowering the Hg2+concentration. Hg2+ is reduced by nearly 1O'O at 0.06 M C1- which is evidently enough to prevent its reduction by mild conditions but not by Sn2+. The reduction evidently occurred in the disposable syringe and had to be due to the polypropylene body or the rubber plunger face since these were the only two parts which the solution contacted. A new syringe was cut up, pieces were placed inside a 50-ml glass syringe and standards in dilute H N 0 3 run. A mercury signal was observed due to reduction by the polypropylene but not the rubber parts (Table I). According to the manufacturer (131, 2,6di-tert-butyl methyl phenol (BHT) is added to the polypropylene formulation as an antioxidant. This compound was added to a glass syringe and a standard in dilute "03 run. A significant signal was observed indicating reducing power for this additive preparation and leading to the conclusion that it may be responsible for the reducing power of the polypropylene syringe. A similar reduction mechanism was suspected for lowered peak heights and tailing in the automated procedure. However, it was impractical to remove the plastic pump tubing from the system. To test the effect of the tubing,
Table 11. Effect of Additional Lengths of Pump Tubing on Mercury Response, 5 ng/ml Hg Standards Relative absorbances Standard composition
0.15 N " 0 , 0 . 1 5 N MNO, + 0.06 N HC1 0.15 N " 0 , + 0.01 N KmnO, UAverage of 4 determinations.
One tube
Two tubes
Three tubes
10.5 22.1 22.8
8.9 22.5
7.4 22.7 22.9 1
2
3
4
5
6
7
8
9
IO
11
12
CONCENIRAiICN. ppm
Figure 4. Effect of platinum and gold on recovery of mercury
two additional pieces were placed in the sample line to see if peak heights were more severely affected. The data, which are presented in Table 11, indicate that the tubing played a role in the behavior and that either HCl or KMn04 in the standards offered adequate protection. The system was then modified so that the sample passed through a length (42.5 cm) of pump tubing after addition of Sn2+.The loss in peak height was 53% compared with normal operation. Peak tailing was also evident whenever standards were not protected from premature reduction. However, the total peak area was still reduced. In one set of experiments, the peak height was reduced 37% and the peak area 20% when HC1 was omitted from the standards. In the automated system, traces of mercury were apparently reduced by some component of the plastic pump tubing and a portion of the reduced mercury was sorbed by the tubing to be desorbed later and lead to peak tailing. Some of the mercury was either lost or was desorbed so slowly that it did not appear in a peak area measurement. Mercury vapor has been shown to pass through polyethylene (10) and a similar mechanism may be operating here. Cold vapor atomic absorption methods are generally quite free from interferences but the effects of noble metals which might be reduced, amalgamate with the mercury, and interfere with the partition have not been investigated. Figures 4 and 5 show the effect of added platinum, gold, and silver on a 10 pgh. mercury solution. If these metals are present at greater than about 1 pg/ml in solution, the mercury signal is markedly reduced. Either the reduced metal (Au, Pt) or the insoluble chloride (Ag) interferes in the determination. This observation is particularly significant since gold has been suggested as a stabilizing agent for a mercury standard being prepared by the National Bureau of Standards (14). Also, some industrial effluents may contain enough noble metal to interfere. Copper and lead at concentrations up to 20 ppm had no effect on the mercury signal. Both the syringe and automated mercury procedures described here have proved quite satisfactory for a variety of digested samples. The automated method gives performance which is superior to some that have been described. The detection limit is about 0.05 wg/l. and replicate analyses of the digest from two different fish samples gave relative standard deviations 0.68 and 1.0%. For standards, the re1 std dev was 0.75% at 30 pg/l. and 10% a t 1 FgA. Analysis of N.B.S. Orchard leaves S.R.M. No. 1575 gave 0.162 f 0.003 ppm and the N.B.S. bovine liver, S.R.M. No. 1577, gave 0.0167 f 0.006. The certified values are 0.155 f 0.015 ( 1 5 )and 0.016 f 0.002. For both methods, standards must contain an oxidizing agent or HCl or both. The method described by Feldman (8) for storing dilute mercury solutions was modified to in-
Hg concentration 10 pgll. ( 0 )Pt; (A)Au
5
10
15
20
25
30
CONCENTRATION OF Ag apm
Figure 5. Effect of silver on mercury recovery Hg concentration 10 pgll,
clude 0.1 N HCl as well as "03 and K2Cr207. Standards thus prepared have been stored for six months in polypropylene bottles with no sign of deterioration even a t the 1-ppb level. The oxidant does not interfere in either procedure. ACKNOWLEDGMENT
The authors thank E. E. Pickett for numerous helpful suggestions throughout the course of this work. LITERATURE CITED (1)W. R. Hatch and W. L. Ott, Anal. Chem., 40, 2085 (1968). (2)N. S . Poluektov, R. A. Vikun, and Y. V. Zelyukova, Zh. Anal. Khim., 18, 33 (1963). (3)N. S. Poluektov, R . A. Vikun. and Y. V. Zelyukova, Zh. Anal. Khim., 10, 873 (1964). (4) M. P. Stainton. Anal. Chem., 43, 625 (1971). (5) G. Lindstedt and I. Skare, Analyst(London), 96, 223 (1971). (6)P. D. Goulden and B. K. Afghan, Technicon Adv. Automated Anal., 2, 317 (1970). (7)B. W. Bailey and F. C. Lo, Anal. Chem., 43, 1521 (1971). (8)C. Feldman, Anal. Chem., 48, 99 (1974). (9)H. C. Moser and A. F. Voigt, J. Am. Chem. Soc., 70, 1837 (1957). (10)L. C. Bate, Radiochem. Radioanal. Lett.,6, 139 (1969). (11) J. Grossoieil and J. C. Roy, Can. J. Chem.. 48, 705 (1970). (12)W. H. Gutenmann, D. J. Lisk. and N. Grier, Bull. Environ. Contam. Toxicol., 8, 138 (1972). (13)Sherwood Medical Industries, Inc., Deland. Fla., private communication, 1973. (14)H. L. Rook and J. Moody, "Proceedings of the Second international Conference on Nuclear Methods in Environmental Research, University of Missouri, 1974," U.S.A.E.C. Information Center, Oak Ridge, Tenn., In press.
(15)R. Alvarez. Anal. Chlm. Acta, 73, 33 (1974).
RECEIVEDfor review May 21, 1975. Accepted September 22, 1975.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
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