2964
Anal. Chem. 1984, 56,2964-2966
Mercury Displacement in the Determination of Sulfur Dioxide with a Piezoelectric Crystal Detector A. A. Suleiman and G. G. Guilbault*
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
A plezoelectrlc crystal detector utlllrlng a mercury displacement reaction Is descrlbed for the detection and determlnatlon of sulfur dloxlde. The mercury vapor produced by bubbllng the sulfur dloxlde through a mercurous nltrate solutlon Is detected by a gold-coated plezoelectric crystal due to the formatlon of a mercury amalgam. This detector exhlblts good sensltlvlty and selectivity and can be used In the ppb and ppm concentratlon ranges of sulfur dioxide.
It is generally accepted that the mercury(1) ion undergoes a disproportionation reaction to form mercury(I1) ion and mercury(0) ( I ) .
Hg2+ + Hg2++ HgO
(1)
The disproportionation of the mercury(1) ion is accelerated in the presence of ligands which form stable mercury(I1) complexes ( 2 ) . This proposed method is based on the principle that equilibrium ( I ) will be shifted to the right by the complexation reaction of sulfur dioxide dissolved in an aqueous solution with the mercury(I1) ion.
2S02 + 2H20 + Hg;+
~1 Hg(S03)22-
+ Hgo + 4H+
(2)
The theoretical principles and an analytical method for the determination of sulfite ion or sulfur dioxide have been discussed by Marshal and Midgley (3). The ability of gold to adsorb and amalgamate mercury is well documented, and this principle has led to the development of detectors for mercury based on a gold-coated piezo-electric crystal in air (4) and in water (5). In this investigation a gold-coated piezoelectric crystal was used to monitor the elemental mercury released in reaction ( 2 ) . Thus, the concentration of sulfur dioxide in air can be determined.
EXPERIMENTAL SECTION Apparatus. A typical schematic diagram of the experimental setup is shown in Figure 1. The piezoelectriccrystal was driven by a low-frequency transistor oscillator Type OT-13 supplied by International Crystal Manufacturing Co., Inc., OK. The oscillator was powered by a Heathkit regulator power supply (Model IP-28) set at 9 V dc. The frequency of the vibrating crystal was monitored by a frequency counter (Model SM-2420)supplied by Heath Company, MI. The counter was modified by a digital-to-analogue converter (DAC) which provided a voltage output signal directly proportional to changes in the vibrating frequency of the crystal. The output of the DAC was connected to a recorder, and hence the change in frequency can be translated to a permanent record. The reaction cell (20-mLcapacity) was made from a Pyrex 24/40 ground joint closed with another 24/40 male joint connected to a drying tube. The base of the cell is a medium coarse fritted disk located just above the carrier gas inlet. The cell was immersed in a thermostatically controlled water bath. The flow rate was kept constant by a flow controller (Brooks Instrument Division, Hatfield, PA) and was monitored by a calibrated flow meter (Matheson, East Rutherfort, NJ). Reagents. A 0.01 M mercury(1) ion stock solution was prepared from analytical reagent mercurous nitrate dissolved in 0.01 M 0003-2700/84/0356-2964$0 1.50/0
Table I. Effect of Mercury(1)Ion Concentration on Resgonse" response, Hz 1x 1x 5x 1x
10-7
5 12
106 10+
34
10-5
44
response, Hz 2 4 6 1
x 10-5 x 10-5 X
x 10-4
48 55 50 43
"Temperature = 23 "C; purge time = 10 min; SOz concentration = 0.5 ppm; Ht concentration = 1 X M; volume injected = 2.5 mL; flow rate = 65 mL/min. nitric acid. The mercury(1) ion reagent was prepared daily from the stock solution and a 0.1 M nitric acid solution. Dilution to appropriate volumes was made with double-distilled deionized water. Nitrogen was used as a carrier gas and was passed through absorption tubes of silica gel and activated charcoal. Drying Tube. A drying tube was placed after the reaction cell to prevent water vapor from entering the detector cell. Anhydrous magnesium perchlorate was used initially, but it was replaced by molecular sieve Type 3A. The sieve proved to be more efficient and convenient since it can be regenerated by heat treatment, and tube blocking can be eliminated. Procedure. A 3-mL portion of mercury(1) ion reagent solution was added to the reaction cell kept at constant temperature. Carrier gas was bubbled through the solution at a controlled flow rate for 10 rnin unless otherwise indicated. A 2.5-mL sample gas of known SOz concentration prepared by the syringe dilution method described by Karasek (6) and Guilbault (7) was introduced into the carrier gas stream at the injection port. The response was the decrease in frequency after 2 min of introducing the sample. Reversibility which is the ability of the detector to reattain the base-line frequency after it has been exposed to a gas sample, was achieved by heating the crystal at 170 "C for 5 rnin with passage of the carrier gas. Mercury was thermally desorbed and the base-line frequency was reestablished.
RESULTS AND DISCUSSION Mercury(1) Ion Concentration. The effect of mercury(1) ion concentration on the response is shown in Table I. The results indicate that maximum response was obtained at 4 x M. At higher concentrations there will be a relatively high background of Hgo and the system equilibrates slowly. Since the sensitivity of this detector depends on the mass of mercury adsorbed on the gold surface, a higher background of Hgo will cause a decrease in the noncontaminated gold area and consequently decreases the collection efficiency of mercury in the measurement step. At lower concentration it seems that the reagent solution becomes exhausted faster, resulting in a low response. Effect of Temperature. Greater response was obtained at higher reaction cell temperatures. However, the purge time has to be varied constantly to suit the new temperature, and poor reproducibility was obtained due to difficulty in system equilibration. Also, at elevated temperatures the evaporation of water was enhanced and more frequent reconditioning of the drying tube became necessary especially in the case of Mg(C10&. At a fixed purge time of 10 rnin the best response was obtained a t 27 " C (Table 11). 0 1984 American Chemlcai Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
2965
Table IV. Effect of Flow Rate on the Mercury Displacement Detection Method" flow rate, mL/min
response, Hz
flow rate, mL/min
response, Hz
40 50 55
128 153 160
60 65 70
156 153 147
M; purge time = 10 min; SO2 "Hg;+ concentration = 4 X M; volume concentration = 0.5 ppm; Ht concentration = 2.5 X injected = 2.5 mL; temperature = 27 OC.
Flgure 1. (1) Absorption tube, (2) injection fort, (3) fritted disk, (4) reaction cell, (5) drying tube Mg(CIO,), or molecular sieve Type 3A, (6) detector cell, (7) oscillator, (8) power supply, (9) frequency counter, (10) digital-to-analogue converter, (1 1) recorder, (12) flowmeter, (13) KMNO, and H2S04trap solution.
Table V. Effect of Interferences on the Mercury Displacement Detection Method" interferentb
response, Hz
0 3 21 0 0 0 0 18
air 3"
HCl
Table 11. Effect of Temperature on the Mercury Displacement Detection Method" temp, OC
response, Hz
temp, OC
response, Hz
20 23 25 27 29
9 55 126 136 115
31 33 35 40
106 85 80 48
"HgZ2+concentration = 4 X 10" M; purge time = 10 min; SOz M; volume concentration = 0.5 ppm; Ht concentration = 1 X injected = 2.5 mL; flow rate = 65 mL/min.
Table 111. Effect of HNOa Concentration on the Mercury Displacement Detection Method" ["OS],
M
1.0x 10-4 1.0x 10-3 2.5 x 10-3
response, Hz 75 126 139
["OB],
M
5.0 x 10-3 7.5 x 10-3 1.0 x 10-2
response, Hz 129 118 86
"Hg;" concentration = 4 X M; purge time = 10 min; SO2 concentration = 0.5 ppm; temperature = 25 OC; volume injected = 2.5 mL; flow rate = 65 mL/min.
HN03Concentration Effect. It is necessary, as discussed elsewhere (2),that acid be added to suppress the hydrolysis of mercury(I1) ions. However, it was pointed out that an optimum H N 0 3 concentration is one which maximizes disproportionation and minimizes hydrolysis. The acidity of the reagent solution was adjusted with 0.1 M nitric acid. The results shown in Table I11 indicate that maximum response was obtained at a 2.5 X M nitric acid concentration. Effect of Flow Rate. Flow rate affects both the collection efficiency of mercury(0) on the gold surface of the crystal electrode and the efficiency of its removal from the aqueous phase. A t high flow rates the collection efficiency will decrease because of insufficient adsorption while the removal of Hgo from the solution is enhanced. The reverse is true at lower flow rates. As shown in Table IV the optimum flow rate was 55 ml/min, and this flow rate was used for all further studies. Interferences. The results in Table V indicate that the only major tested interferents at their threshold limit values set by OSHA are sulfide gases which promotes the disproportionation reaction by forming the very stable compound HgS. Sulfide gases can be trapped in acidic silver nitrate solution, in which the sulfide precipitates in the form of silver sulfide. The loss of sensitivity if a sample containing SO2and H2S or CS2 is passed through the silver nitrate solution is about 18%,but this appears to be tolerable if it is necessary to eliminate the interference effect of sulfide gases.
interferentb
response, Hz
H2S HZS, 20 PPm CSZ CSz 20 ppm
142 68 113 52 17 87 6
0 3
HN03 HNO3,2 ppm
"Purge time = 10 min; flow rate = 55 mL/min; Hg;+ concenM. *Tested at tration = 4 X 10" M; Ht concentration = 2.5 X 320 ppm.
Table VI. Effect of Purge Time on Response" purge time, min
temp, "C
response, Hz
5 10 30 40 5 10 25 30 5 10 30 5 10 20 30 90
23 23 23 23 29 29 29 29 25 25 25 27 27 27 27 27
76 75 66 64 133 141 109 102 135 139 127 155 160 138 127 99
"Hg;+ concentration = 4 X 10" M; flow rate = 55 mL/min; SO2 concentration = 0.5 ppm; Ht concentration = 2.5 X M; volume injected = 2.5 mL.
Table VII. Sensitivity Study for SO2 in the ppb Concentration Range" [Sod, ppb
response, Hz
re1 std dev, 70
20 50 100 150 200 300 400 500 600 700 800 900
14 24 46 51 75 89 144 160 181 184 196 202
9.8 8.1 5.2 3.6 3.2 2.9 2.8 2.6 2.1
1.9 1.8 1.6
"HgZ2+concentration = 4 X 10" M; volume injected = 2.5 mL; flow rate = 55 mL/min; Ht concentration = 2.5 X M; temperature = 27 "C.
2966
Anal. Chem. 1984,56,2966-2970
Table VIII. Sensitivity Study for SO, in the ppm Concentration Rangea [SO,], ppm
response, Hz
re1 std dev, 9i
38 59
7.8
89
5.6
0.5
5 10 25 50
6.1
241
4.3 3.8 2.9 2.7 2.6 2.4 2.3 2.1 1.8
446 488 641 710
64 80 100 110 150 180 200
805 913 940
972
'Hgz2+ concentration = 4 X M; volume injected = 1 mL; flow rate = 55 mL/min; HC concentration = 2.5 X M; temperature = 27 "C. Table IX. Lifetime of the Mercury(1) Nitrate Solutiona reagent vol, mL
no. of assay
3 3
160 51
2.5
2nd 1st 2nd 3rd 4th
177 172 161 84
2.5 2.5 2.5 2.5
1st 2nd
39 38
3rd 4th
38
1.0 1.0 1.0 1.0
6 6 6 6 3 3 3 3
1st
response, Hz sample vol, mL
29
2.5
'Hg;+ concentration = 4 X M; SO, concentration = 0.5 ppm; flow rate = 55 mL/min; H+ concentration = 2.5 X M; temperature = 27 OC.
Effect of Purge Time. The purge time is the time the carrier gas is bubble through the mercurous nitrate solution before the sample gas is injected. The results in Table VI indicate that the largest response was obtained after a purge time of 10 min. The response was slightly lower a t shorter
purge times due to the higher background of Hg(0). The decrease was more pronounced a t longer purge times since the mercurous nitrate solution is depleted with usage. Calibration Curve. The responses for different volumes of sample gas are listed in Tables VI1 and VIII. When 2.5 mL of the sample gas is injected, the response curve was linear over a concentration range of 20-450 ppb. Similarly, when 1mL of sample gas was injected, the response curve was linear over a concentration range of 2.5-85 ppm. The response reported is the average for five separate determinations. Since the detector responds to mass, larger sample volumes will produce larger amounts of mercury, causing an increase in sensitivity. It was concluded that it is possible to analyze for SO2 concentrations below and above the indicated limits using different sample gas volumes. The detection limit using k = 3 in the equation CL = k s B / m (8) was estimated to be approximately 0.08 ng of SO2 The advantage of the piezoelectric crystal detector over existing techniques is its sensitivity, low cost, and versatility. Portable detectors for field use can be easily developed. Lifetime of Mercury(1) Nitrate Solution. The effects of different volumes of injected sample and mercury(1) nitrate solution used were investigated. The results in Table IX indicate that the number of successive assays possible increased as the volume of the mercury(1) nitrate solution increased and when the volume of the sample injected decreased. It is suggested that especially in field studies it is more convenient to use larger volumes of the reagent solution with the ability to obtain satisfactory results.
LITERATURE CITED (1) Wolfgang, R. L.; Dodson, R. W. J . Phys. Chem. 1952, 56, 872. (2) Sanemasa, I. Inorg. Chem. 1978, 15, 1973. (3) Marshal, G.; Mldgley, D. Anal. Chem. 1981, 53, 1760. (4) Scheide, E. P.; Taylor, J. K Environ. Sci. Technol. 1974, 8 , 1087. (5) Guilbault, G. G.; Ho, M. H. Anal. Chim. Acta 1981, 730, 141. (6) Karasek, F. W.; Tiernay, J. P. J . Chromatogr. 1874, 8 9 , 31. (7) Guilbault, G. G.; Karmarkar, K. H. Anal. Chim. Acta 1974, 7 1 , 419. (8) Winefordner, J. D.; Long, G. L. Anal. Chem. 1983, 55, 713A.
RECEIVED for review August 11, 1983. Resubmitted August 13,1984. Accepted August 31,1984. This work was supported by the Army Research Office through Grant DAAG29-77-G0226-DR.
CORRESPONDENCE Electron-Impact-Induced Fluorescence-The Basis for a Universal/Selective Detector for Gas Chromatography Sir: As the complexity of problems involving separations increases, the need for selective detectors has become critical. Selective detectors, responding only to species containing specific heteroatoms, functional groups, or structural features, have had a tremendous impact in chromatography (1). Perhaps the ideal detector would be one which could be used in either a selective or universal mode. The detector used in gas chromatography which is closest to this ideality may be the mass spectrometer. Since electron impact (EI) on all compounds generates ions, total ion currents (TIC) can be monitored in a GC/MS analysis for universal response. Monitoring specific ions typical of certain molecular substructures (with E1 or chemical ionization (CI)) allows the use
of the mass spectrometer as a selective detector. This mode of operation is referred to as selected-ion monitoring (SIM) (2). Also, in GC/CIMS analyses, reactant-ion monitoring (RIM) provides the basis for selective detection ( 3 ) . We present here the basis for a new detector for gas chromatography. We will refer to the technique as optical mass spectroscopy, to emphasize the fact that it is an optical method which parallels mass spectrometry in many ways. The basic concept is as follows: electron impact on gaseous molecules produces a variety of products, both ionic and neutral. Some products are formed in electronically excited states. Many of these states fluoresce and can be optically detected. Thus, E1 on molecules produces molecular frag-
0003-2700/84/0356-2966$0 1.5010 0 1984 American Chemical Society
