1490
Anal. Chem. 1982, 5 4 , 1490-1494
Continuous Atomic Spectrometric: Measurement of Ambient Levels of Sulfur Dioxide in Air by Mercury Displacement Detection Geoffrey Marshall* and Derek Mldgley Central Electrlcitv Research Laboratories, Kelvin A venue, Leatherhead, Surrey KT22 7SE, United Kingdom
The analyticai atomic spectrometric technique of mercury displacement detection has been adapted so that sulfur dioxide can be determined at natural background levels in ambient air on a continuous basis with a 90% response time of 1-2 mln. Sample air is drawn Into the reaction vessel containing mercury(I ) ion reagent and any sulfur dioxide present reacts to form elemental mercury which is measured, after being swept out of the solution by the same flow of sample alr, by a mercury vapor detector. Reagent is continuously pumped through the analyzer and the instrument is calibrated with a permeation tube calibrator. The apparatus has a linear concentration range up to 100 ppb sulfur dioxide and a detection limit of approximately 0.2 ppb sulfur dioxide; this is much lower than can be obtained with existing commercial instruments. The apparatus is very precise and 6, 11, and 20 ppb sulfur dioxide can be measured with coefficients of variation of 1-2 % ,
In an earlier paper (1) we described a manual analytical method, mercury displacement detection, for the determination of picogram amounts of sulfite ion or sulfur dioxide. This paper describes the adaptation of the method to the measurement of sulfur dioxide in air on a continuous, unattended, basis. The development of an analytical apparatus, having the same advantages as the manual method, is described together with its performance characteristics. Comparisons are made with other instruments for low-level sulfur dioxide determination. The theory of the application of mercury displacement detection to sulfur dioxide determination has been fully described ( I ) . In brief, sulfur dioxide promotes the disproportionation of mercury(1) to give mercury vapor which is then determined with great sensitivity by atomic spectrometry using a simple and inexpensive mercury vapor detector. 2S02 2H20 Hgz+ Hg(S03)22- HgO 4 Hf (1)
+
+
-
+
+
EXPERIMENTAL SECTION The operating principle of the continuous analyeer is shown in Figure 1. The sample air enters the reaction vessel via the sample inlet and any sulfur dioxide present reacts with the mercury(1) ion reagent according to reaction 1. The liberated mercury is swept out of solution by the same flow of sample air, dried, and drawn into the mercury vapor detector for atomic spectroscopic measurement. The reagent is pumped first to a conditioning vessel, where sulfur dioxide free air is continuously bubbled through to reduce the blank, then to the reaction vessel, and finally to waste. The instrument is calibrated by passing known concentrations of sulfur dioxide in air from a permeation tube calibrator through the apparatus in place of the sample air. Apparatus, Mercury Monitor. Mercury vapor was measured with an L.D.C. Mercury Monitor, Model 1235, obtainable from Laboratory Data Control, European Sales Office, 1 Newcastle Street, Stone, Staffordshire. Reaction Vessel. A typical configuration of the glass reaction vessel is shown in Figure 2. The reaction vessel (1)is surrounded 0003-2700/82/0354-1490$01.25/0
by a water jacket (2) through which water at constant temperature is circulated via entry port (4) and exit port (5). The sample air (which also acts as the carrier gas) enters at port (6) and is drawn into the reagent solution through the frit (7) of porosity 2. The reagent solution enters at port (8) and leaves at port (9). The released mercury vapor exits at port (lo), and the optional bubble break device (ll),constructed of a jagged glass tube set into a glass disk, serves to burst any bubbles rising up the reaction vessel and so to prevent solution from entering the measuring system. Conditioning Vessels. A typical configuration of the glass conditioning vessel is shown in Figure 3. The conditioning vessel (1)is surrounded by a water jacket (2) through which water at constant temperature circulates, entering at port (3) and leaving at port (4). Sulfur dioxide free air at port (5), passes through the porosity 2 frit (6) to the reagent solution and leaves at exit port (7) to waste. Reagent enters from the reagent supply at port (8) and leaves for the reaction vessel (via the proportioning pump) at port (9). Excess reagent is drawn off to waste from exit port (lo), and any bubbles rising up the side of the vessel are broken at the bubble burst device (11). Drying Tubes. The moist mercury vapor generated in the reaction cell is dried before it reaches the mercury monitor by passage through two successive Perma Pure Dryers (Perma Pure Products Inc., Box 70, Oceanport, NJ). Water permeates selectively through a tubular membrane (2) and is continuously removed on the outside by a countercurrent of dry air. The flow rate of the dry air should be greater than that of the stream of air containing the mercury vapor. Sulfur Dioxide Calibrators. The apparatus was standardized by means of sulfur dioxide permeation tube calibrators. Two models were used, depending on the concentration of sulfur dioxide required: the Analytical Instrument Development, Model 330, was used for low levels (4-20 ppb) and the Meloy Laboratories, Model CS10-2, for levels in the range 20-100 ppb. In addition to these, the zero standard was obtained by passing air through activated charcoal. Reagents. Mercury(Z) Zon Stock Solution, 0.01 mol L-l. A 5.6-g portion of mercury(1) nitrate, Hg2(N03)2.2H20,B.D.H. Analar Reagent, was dissolved in 100 mL of 0.1 mol L-' nitric acid and made up to 1L with deionized distilled water. This solution was stable for at least 2 months. Mercury(Z) Zon Reagent Solution 10-6 mol L-l. A 10-Lsample of reagent was normally used in the reagent supply vessel. This was prepared by pipetting 1mL of the 0.01 mol L-' mercury(1) ion stock solution into the vessel, adding 100 mL of 0.1 Id-' nitric acid, and then making up to 10 L with deionized distilled water. Procedure. The following procedure was used in most of the tests, but in the preliminary work certain conditions were varied as described later. The apparatus was arranged as in Figure 1. The mercury vapor detector was operated on the 0.01 absorbance range, with its output connected to a 10-mVpotentiometric chart recorder. The thermocirculator (Grant FH 15) was set at 40 OC. Air freed of sulfur dioxide by passage through an activated charcoal trap was drawn through the conditioning vessel at 2.5 L min-' by a diaphragm pump (P). Sample air (or sulfur dioxide standards) was drawn through the reaction vessel at 100 mL min-' by a diaphragm pump (Q). Air was supplied to the drying tubes at ca. 800 mL m i d (pump R). Control of the above flow rates was achieved by means of adjustable flowmeters (GEC - Marconi Process Control), but to ensure a more constant supply of sample air, a Flostat (G A Platon, Basingstoke, Hants) was included 0 1982 American Chernlcal Society
ANALYTICAL CHEMISTRY, VOC. 54, NO, 9, AUGUST 1982
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4
lnstrumem for cOntlnuous determinatlOn 01 low levels of sulfur dioxide. Figure 1.
6-
I Figure 3.
5
8
v
6
Flgure 2. Reaction vessel.
downstream of the appropriate flowmeter. PTFE sample lines of internal diameter of ca. 4 mm were used to transport sample air, and sulfur dioxide standards, to the reaction vessel. All other gas lines were of PVC. The length of sample line was kept to a minimum between the reaction vessel and the mercury detector in order to reduce the dead volume (3). Reagent solution was pumped at rate R, from the conditioning vessel ria exit port A into the reaction vessel at port D. A mixture of waste reagent and entrained gas was drawn by the peristaltic pump from port B of the reaction vessel to waste at a rate R,, and R, > R, so that the reagent solution was maintained a t the level of the exit port B. Reagent was pumped from the supply reservoir to the inlet port C of the conditioning vessel a t a rate Ri. Surplus reagent (entrained with air) left the exit port E of the conditioning vessel at rate R.. The rates were chosen so that Ri > R. and R, R. > Ri. This arrangement ensured that the level of solution in the conditioning vessel was virtually constant at the height of the exit port E. For convenience, a multichannel peristaltic pump (Technicon Mk 1) was used. The pump rates were chosen as above as a precaution against flooding of the vessels with reagent solution, since peristaltic pumps are not sufficiently reliable for a system of balanced flows to be used (Rj = R, = R,), The choice of flow rates depends on two factors: the length of time during which it is desired to leave the apparatus unattended and the range of concentration over which analysis is required. The compromise flow rates adopted were Ri = 0.6 mL
+
6tm
2
Conditioning vessel.
min-', R, = 0.42 mL min-', R. = 0.6 mI. min I , and R. = 0.42 mL min-l and these gave a concentration range of 0-100 ppb SO, for an air sample rate of 100 mL m i d , wirh a 101. reservou of reagent lasting fur 11 days of operations. Calibration. The analyzer is calibrated hy either of the calibratnra previously described by drawing known concentrations of sulfur dioxide in air through the sample inlet in place of the sample air and constructing a calibration line by plotting the absorbance signals so obtained against sulfur dioxide concentration. The frequency of calibration depends on the precision and accuracy required hut the apparatus should always be recalibrated after reagent replacement. It was fuund that it was more important to rezero the instrument rather than recalibrate.
RESULTS AND DISCUSSION System Development. Studies wilhoul a Conditioning Vessel. Attempts were made to dispense with the ronditioning vessel and so simplify the analyzer. This resulted in a large increase in the hlank, and a noisier, drifting, base line, although the size of the signal above the base line for a given level (if sulfur dioxide was unaffected. Became a continuous analyzer with unattended operation WBS required, a Conditioning vessel was included in all further work. Concentration Range. The concentration range of the analyzer depends on the flow rate of reagent through the system, which itself affects the length of time the analyzer will operate unattended. The concentration range is also dependent on the flow rate of sample air through the reaction vessel, and this will affect the response time of the analyzer. An ideal analyzer should operate unattended fnr a ronsiderahle period of time over a large concentration range, and have a short response time. These ideaLq make contradirtory demands of this apparatus and, as a compromise. it was decided to aim for an analyzer capable of operating without attention (i.e., reagent addition) for at Iewt IO days and having an operating range from 0 to 100 ppb and a 90% response time of about I min. E//Qc~ of Flow Rate of Sample Air. Experiments were conducted to ascertain the effect of different sample air flow rates on sensitivity and concentration range. The flow rate of reagent was as specified in the Experimental Sertion. Results are given in Figure 4. where it is shown that the
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
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Table 11. Effect of Temperature on Sensitivity and Response Time e m p , "C
net signal (for 4 . 2 ppb SO,)
response time
25 40 50 60
110 115 120 128
10 min 1-2 min 1min 30 s
9 0%
0 SJlcL9 0l"il:
14 A I R p p b
Figure 4. Effect of flow rate of sample air on analyzer response (2 cm reaction vessel): (-1 100 mL min-I; (- --) 200 mL min-'; (- - -) 300 mL min-I.
Table I. Effect of Reagent Volume
10 -
vol of reagent upper linear 90% solution, concn limit, response ppb time, min mL
mL min-'
36 56 105 36 56 105 46 56 105
300 200 100 300 20 0 100 300 20 0 100
1
5
10
-1 -1 -1 2 2 4 3 4 6
60 -
flow rate,
calibration line of signal (in this case 100 0.01 absorbance units) vs. sulfur dioxide concentration tends to level off at a concentration which is approximately inversely proportional to air flow rate. The plateau is reached when the molar rates of input of sulfur dioxide and mercury(1) ion are approximately equal. According to reaction 1,the limit in a closed system would occur when [SOz]/[Hg22+]= 2, but in the flow system some of the reagent will be pumped to waste before it can react and the useful concentration range is smaller than expected from the stoichiometry of reaction 1. Flow rates below 100 mL min-l were not investigated because previous work (1) showed that below 100 mL m i d the released mercury could not be effectively purged from the solution in the reaction vessel. The flow rate did not affect sensitivity in the initial linear segment of each calibration. An air flow of 100 mL min-l was selected because it gave a larger range of linear calibration even though its 90% response time-see Table I-was longer than with higher air flow rates. Effect of Solution Depth. A specially designed reaction vessel was constructed to ascertain the effect of the depth of solution in the reaction vessel on concentration range and response time at different sample flow rates. It had three exit ports placed so that the volume of solution would be approximately 1 mL with the lower exit in use, 5 mL for the middle exit, and 10 mL for the upper exit. In all other respects conditions were kept the same as detailed in the Procedure section. The results in Table I show that increasing the volume of solution in the reaction vessel increased the response time without changing the useful concentration range and the final design of the analyzer employed a reaction vessel with an exit port giving an approximate solution volume of 1 mL. Design of Reaction Vessel. Previously (1) we reported that quicker response times could be achieved with a 1cm diameter reaction vessel. A limited amount of work was done with a reaction vessel of this diameter but it was found that the base line was more noisy than with the 2 cm reaction vessel. Ac-
40
-
30
-
20
-
10
HOURS TIME
1
1
1700 2 5 7 80
0100
1
.
17W 26 7 8C
1
,
I
17W
'0130 27
7 80
- 1 I
I
0100
22
28 7 80
Flgure 5. Effect of ambient temperature on base line drift: (-) base line vs. time; (- - -) ambient temperature vs. time.
zero
cordingly the 2 cm diameter reaction vessel was used in the final design. Effect of Temperature. Reagent Temperature. The temperature of the reagent is stabilized by circulating water at a constant temperature through the water jackets of the reaction and conditioning vessels by means of the thermocirculator. The manner in which changes of temperature affect the sensitivity and response time of the analyzer is shown in Table I1 where the net signal (100 0.01 absorbance units) and response time are given at different temperatures for a 4.2 ppb sulfur dioxide standard. The base line increases by about absorbance units per degree. The sensitivity increases only slightly with temperature, but the response time improves markedly. The improvement in response time was previously (I) explained in terms of the rate of transfer of mercury(0) between aqueous and gaseous phases. A temperature of 40°C was chosen as a good compromise for the final design because higher values would result in larger amounts of water vapor being released with consequent increased drying difficulties. Ambient Temperature. Changes in ambient temperature affect the light output of the mercury vapor discharge lamp. Figure 5 shows the changes in base line and ambient temperature with time during 3 days in summer. The very large peaks at 1800-1900 h on 25.7.80 and 27.7.80 were due to direct sunlight from the west facing window falling on the analyzer. In the figure, 10 divisions correspond to approximately 0.4 ppb. The base line drifted up or down as the temperature increased or decreased; when direct sunlight fell on the analyzer these changes were large and rapid. The detector should be shielded from direct sunlight and for the most accurate work should be kept a t constant temperature. Automatic means of compensation for the change of intensity of the mercury vapor discharge lamp with temperature and time could alleviate these requirements. This could be achieved either by using a more elaborate double beam mercury vapor detector or by frequent zero checks. Removal of Water Vapor. Water vapor and spray have to be removed from the air stream entering the absorptiometric cell, otherwise condensation occurs on the cell windows and
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
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Table 111. Precision of Determination of Sulfur Dioxide concn, PPb S,,a T
PPb
PPb
0
0.05
0.2
6 11 20
0.08 0.2 0.2
0.09 0 0.3
ST,^ PPb 0.2 0.1 0.2
0.4
Sw, Sg, and ST denote the within-batch, betweenbatch, and total standard deviations, respectively.
3 0
g
I 0
10
20
30
10
50
60
70
80
90
Typical recorder trace for sulfur dioxide determination (100 divislons 0.01 absorbance units). Flgure 6.
io
0 HOURS 1700 TIME
2100 11 1 2 6
:: I
I 0100
I
I
.-___-
05W
I
I
0900
12 12 80
Comparison of mercury displacement detection with a coulometric detector: (-) mercury displacement detector; (- - -) coulometric detector.
Flgure 7.
causes spurious absorption signals. In previous work (1) this was done effectively with a magnesium perchlorate dryer but its short lifetime (approximately 8 h) precluded its use with an automatic continuoudy operating system. At low temperatures (ca. 15 "C) no water entered the cell and it was therefore assumed that a t higher temperatures water entered not as spray but because of evaporation and subsequent condensation. This could be avoided by housing the entire apparatus in a temperature-controlled cabinet a t a temperature slightly in excess of that of the solution in the reaction vessels. It is possible to avoid condensation by the application of heat, and a heater of similar design to that of Christmann and Ingle ( 4 ) was constructed and placed between the reaction vessel outlet and mercury vapor detector inlet. This approach did not prove successful because condensation occurred further downstream in the system, i.e., in the flowmeter, pump, etc. In the final design of the apparatus drying was effected by the use of Perma tubes. In early work, problems were caused by water, which condensed upstream of the single dryer tube, being suddenly released and gushing through the dryer without being removed. This was cured by placing two Perma dryers in series in a vertical position and joined by a U-shaped length of PVC tubing (F in Figure 1). In this way any water gushing through the first dryer tended to collect in the PVC connecting tube but was effectively removed by the second dryer. One disadvantage of using Perma tube dryers was an approximately 40% loss in sensitivity compared with the use of magnesium perchlorate drying tubes, but in view of their convenience and indefinite operating life, the Perma tubes were preferred. Effect of Stirring. In an attempt to incrlease the efficiency of transfer of elemental mercury in the reaction vessel from the solution to the gas phase, the solution in this vessel was agitated with the aid of a magnetic stirrer bar. Since no improvement in sensitivity or response time resulted, stirring was not incorporated in the final design. Performance Characteristics. Calibration Line. A typical calibration line i8 shown in Figure 4 (100 mL min-l line). Over the concentration range tested1 (0-100 ppb SOz) peak signal varied linearly with sulfur dioxide. The sensitivity of the technique, defied as the concentration of sulfur dioxide giving a 1% absorption of incident light (0.0044 absorbance units) was approximately 22 ppb. Precision. Within-batch. between-batch, and total standard deviations for the determination of sulfur dioxide in air were calculated. Duplicates of Four standard sulfur dioxide in air
mixtures containing 0, 6, 11, and 20 ppb were measured in five batches, and the order of analysis was randomized within each batch. The blank (0 ppb) was obtained by drawing air through a charcoal absorber to free it of sulfur dioxide. The 6 ppb and 11 ppb standards were obtained from the A.I.D. calibrator and the 20 ppb standard from the Meloy calibrator. Approximately 5 min were given for each standard to reach and maintain a steady reading. A typical recorder trace for one batch of results is given in Figure 6. The recorder was equipped with a fly back control but peak heights are written on the figure for clarification. Absorbance readings were converted to parts per billion values by reference to a calibration line constructed from the mean values obtained from the four standard solustions. Results are given in Table 111. Limit of Detection. Limit of detection, for 95% confidence limits, is defined here as 4.625&, were S B is the within-batch standard deviation of the blank (5) and is calculated to be 0.24 PPb. Comparison of Mercury Displacement Detection Analyzer with a Coulometric Analyzer. A comparison was made of the mercury displacement detector with a coulometric analyzer (Philips PW 9750) utilizing the reaction of sulfur dioxide with halogen
SO,
+ 2H20 + Br, s H2SO4+ 2HBr
(2)
The reaction causes a depletion of bromine in solution and the current necessary to replace the bromine is measured and is proportional to the amount of sulfur dioxide absorbed and hence present in the air. The two analyzers were run in parallel for 10 days, sampling air from outside the building via PTFE sample lines. The mercury displacement detector was calibrated as previously described, but the coulometric analyzer was calibrated with its own internal calibrator. A selection of the results is given in Figure 7 , where the readings of both instruments are shown plotted every hour, these readings being taken from the continuous recorder traces. At low concentrations (Le.,
