In the Laboratory edited by
Topics in Chemical Instrumentation
David Treichel Nebraska Wesleyan University Lincoln, NE 68504
W
ISE Analysis of Hydrogen Sulfide in Cigarette Smoke Guofeng Li, Brian J. Polk, Liz A. Meazell, and David W. Hatchett*† School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400; *
[email protected] Many advanced undergraduate analytical laboratory courses focus on exposing students to modern instruments. However, students rarely have the opportunity to construct their own analytical tools for solving practical problems. We designed an experiment in which students are required to build their own analytical module, a potentiometric device composed of a Ag/AgCl reference electrode, a Ag/Ag2S ion selective electrode (ISE), and a pH meter used as voltmeter, to determine the amount of hydrogen sulfide in cigarette smoke. The techniques for constructing these electrodes are very simple. Cigarette smoke is collected into a 0.1 M NaOH solution using a gas washing bottle. The amount of sulfide in the cigarette smoke solution is analyzed by standard addition of sulfide solution while monitoring the response of the Ag/Ag2S ISE. The collected data are further evaluated using the Gran plot technique to determine the concentration of sulfide in the cigarette smoke solution. The experiment has been successfully incorporated into the lab course Instrumental Analysis at Georgia Institute of Technology. The class is made up of senior undergraduate students majoring in chemistry. Feedback from students has been very positive. They enjoy the idea of constructing an analytical tool themselves and applying their classroom knowledge to solve real-life problems. And while learning electrochemistry, students also get the chance to visualize one health hazard imposed by cigarette smoking. Principle Cigarette smoke has been intensively studied for relevant health effects. Several cigarette-smoke constituents have even been the subjects of student laboratory exercises (1–4). Adding to that list, hydrogen sulfide is investigated in this experiment. H2S is one of many toxic components of cigarette smoke (5). It is a weak acid with pKa = 7.1 and pKa = 17.1 (6 ). Thus the H2S solution is a mixture of H2S, HS, and S2, and the equilibrium fractions are very much dependent on the pH value of the solution in which H2S dissolves. A 0.1 M NaOH solution is used to collect the cigarette smoke, so that the H2S content in the cigarette smoke exists in the solution predominantly in the form of HS. Owing to the complex nature of the cigarette smoke, a highly specific detection method is necessary to prevent interference from species other than HS in the smoke solution. To achieve that, a silver wire coated with a very thin layer of Ag2S ion-selective membrane is used as an ISE for detection 1
2
† Current address: Department of Chemistry, University of Nevada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 891544003
of HS ions in the cigarette smoke solution (7). To find out the correlation between the response of the Ag/Ag2S ISE and the concentration of HS ion in the smoke solution, we apply the Nernst equation to the following equilibrium 2Ag + S2
Ag2S + 2e
EAg/Ag S = E°Ag/Ag S – RT ln S2 (1) 2 2 2F The concentration of S2 in the solution is governed by another equilibrium, HS H+ + S2 Ka = 2
H+ ⋅ S2 (2)
HS
Rearranging eq 2 to obtain the expression for [S2] and substituting that into eq 1 gives the expression for the half-cell potential of the Ag/Ag2S ISE:
EAg/Ag S = E°Ag/Ag S – RT ln 2 2 2F
K a ⋅ HS 2
+
H
(3)
° S and Ka are constants. Further, if the pH of In eq 3, E Ag/Ag the solution remains unchanged, it can be simplified into EAg/Ag S = E′ – RT ln HS (4) 2 2F in which Ka E′ = E°Ag/Ag S – RT ln +2 2 2F H 2
2
is a constant at constant temperature and pH. Equation 4 suggests that the Ag/Ag2S ISE is expected to give a Nernstian response specific to the HS ion in the cigarette smoke solution. This provides an effective means for detecting HS ions present. Two inherent advantages in the use of the Ag2S ISE membrane are worth mentioning. First, the interference from other species in the solution can be avoided owing to the highly selective nature of the Ag2S membrane. Second, the membrane is ultra thin, which gives the electrode a very fast response and it is ready for use without the need for preconditioning. The level of H2S in the cigarette smoke is rather low and the resulting concentration of S2 in the smoke solution is close to the detection limit of the Ag/Ag2S ISE. The electrode response under those conditions might be sluggish, which makes direct determination of the [HS ] in the solution rather difficult. To improve the accuracy of the detection, aliquots of HS standard solution are added to cigarette smoke solution
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while the response of the ISE is monitored. The collected data are then further processed by a Gran plot to graphically determine the initial concentration of HS in the cigarette smoke solution. Although the Gran plot technique is more often used to find the endpoint of a potentiometric titration (8, 9), here we apply it slightly differently. The rationale of this approach is detailed in the next few paragraphs. Consider the experimental setup shown in the Figure 1. The voltmeter reading E is given by E = Ecell = E+ – E REF E = E Ag/Ag S – E Ag/AgCl + Ej
(5)
2
RE F where both E Ag/ AgCl, the half-cell potential of the Ag/AgCl reference electrode, and Ej, the junction potential, are constants. Substituting eq 4 into eq 5 gives REF E = E′ – E Ag/AgCl + E j – RT ln HS (6) 2F During the measurement, aliquots of standard HS solution are gradually added into the cigarette smoke solution. The standard solution is prepared so that it has the same pH as the cigarette smoke solution. Hence, no matter how much standard solution is added, the pH of the solution in the cell remains constant. However, the concentration of HS in the smoke solution changes with the addition of the standard solution, which in turn changes the potential of the Ag/Ag2S ISE. This potential change is measured by the voltmeter connected in the circuit. Therefore, the response (E ) is solely dependent upon [HS ] if the temperature fluctuation during the measurement is negligible and the equation can be further simplified to E = E′′ – RT ln HS (7) 2F in which
E′′ = E′ –
REF E Ag/AgCl +
Figure 1. Experimental setup using ISE to determine the amount of HS in the cigarette smoke solution.
Figure 2. The construction of the Ag/AgCl reference electrode.
Ej =
Ka REF E°Ag/Ag S – E Ag/AgCl + E j – RT ln +2 2 2F H
vs Vadd will give a straight line. Next, locate on the Gran plot the intercept of the straight line with the x-axis, Vint. At Vadd = Vint the Gran function equals zero; therefore
is a constant. With Vadd mL of standard solution added to V0 mL of cigarette smoke solution, [HS ] is given by [HS] = ([HS]std Vadd + [HS]cig V0)/(Vadd + V0)
(8)
2F
0 = e RT E′′ ⋅ HS
E = E ′′ – RT ln 2F
HS
⋅ Vadd + HS
std
HS
⋅ V0
cig
2F
2F
std ⋅ Vadd +
2F
1050
HS cig
=
⋅ Vint
std
V 0
(12)
Experimental Procedure
HS
cig ⋅ V0
(10)
in which V0, E′′, T, and [HS ]cig are constants. Hence the Gran plot, a graph of the Gran function
Vadd + V0 ⋅ e RT E
(11)
(9)
Vadd + V0
Rearranging the above equation, we obtain
Vadd + V0 ⋅ e RT E = e RT E′′ HS
⋅ V0
cig
Rearranging eq 11, we obtain the final result:
By substituting eq 8 into eq 7, we obtain
⋅ Vint + HS
std
Preparation of Reference Electrode and ISE Ag/AgCl Reference Electrode. Cut a piece of silver wire (length ca. 6–7 cm, φ ca. 0.5 mm). Dip about 1⁄2 of the wire into a FeCl3 solution and hold it there for approximately 30 s. The wire in the solution turns dark gray as silver chloride is deposited on its surface. Rinse the wire with distilled water before use. The whole procedure for assembling a Ag/AgCl reference electrode is illustrated in Figure 2.
Journal of Chemical Education • Vol. 77 No. 8 August 2000 • JChemEd.chem.wisc.edu
In the Laboratory
of the aspirator and then light the cigarette. The resulting HS level should be sufficient for detection by smoking just one cigarette (Camel Cigarettes, unfiltered, R. J. Reynolds Tobacco Co.). Determination of [HS ] in the Cigarette Smoke Solution. Pipet 25.00 mL of cigarette smoke solution into a 100-mL beaker. Construct the experimental setup as shown in Figure 1. Continue adding 1.00-mL aliquots of standard HS solution to the cigarette smoke solution until the meter reading changes. Record the volume of standard HS added and the voltmeter reading. Repeat the above procedure to obtain at least 5 stable data points. Figure 3. Schematic of the gas washing bottle setup used for collecting cigarette smoke.
Figure 4. A typical Gran plot from students’ data.
Ag/Ag2S Ion-Selective Electrode. Obtain one 1.5-V, AA battery. Attach the “+” of the battery to the bare silver wire and the “” to a paper clip. Dip ca. 4⁄5 of the wire into the Na2S solution. The reaction is completed in seconds. Remove the silver wire quickly after it is coated with a layer of silver sulfide on its surface. Use Kimwipe tissue to remove all the loosely attached Ag2S particles from the electrode surface. Examine the electrode surface very carefully; if it is not completely coated with a thin, dark layer of Ag2S with no exposed silver surface, repeat the above process.
Standardization of HS Solution by Potentiometric Titration Standard HS solution is prepared by using 0.1 M NaOH to dissolve ca. 0.120 g of Na2S9H2O in a 500-mL volumetric flask. Use 5 mM AgNO3 standard solution to standardize the prepared HS solution by potentiometric titration. The detailed experimental setup can be found in the online supplementW or elsewhere (8). Determination of HS in Cigarette Smoke Solution Preparation of Cigarette Smoke Solution. Pipet 50.00 mL of 0.1 M NaOH solution and transfer it into a gas washing bottle. Set up the gas washing bottle as demonstrated in Figure 3. Attach a cigarette to the inlet of the gas washing bottle, and connect the outlet to an aspirator. Turn on the water supply
Results and Discussion A typical Gran plot from students’ data is shown in Figure 4, in which the intercept on the x-axis is Vint = 0.625 mL. From eq 12, the concentration of HS in the cigarette smoke solution is calculated to be 4.45 × 105 M. The students’ final results vary within the same order of magnitude, 105 M. This variance is caused by the inconsistency in collecting the cigarette smoke using the gas washing bottle. Better precision can be achieved by smoking several cigarettes and averaging the final result. However, we think this simple setup is adequate for educational purpose. Based on the above result, the amount of H2S is calculated to be 7.58 × 102 mg/cigarette. It has been reported that the odor threshold for H2S is 25 ppb (0.035 mg/m3); levels in the 3–5-ppm range cause an offensive odor; 5 minutes of exposure to 800 ppm has resulted in death (10). In comparison to those statistics, the H2S level resulting from smoking one cigarette is well below the lethal level, and one needs to smoke at least 5 cigarettes in a 10-m3 room to even reach the H2S odor threshold. It appears that the H2S in the cigarette smoke does not impose an immediate threat; rather it is the longterm exposure to H2S and especially to other more toxic components of the cigarette smoke that causes serious damage to human health. The technique we developed for making the Ag/Ag2S ISE is extremely simple. The electrode provides a very fast and stable response. It is obvious that the success of the experiment largely depends on the quality of the ISE electrode that students make. Therefore one has to examine the electrode surface very carefully, and make sure that it is coated with a thin layer of Ag2S with no silver exposed to the surface. Otherwise, there will be a loss of selectivity because the electrode is then responsive to redox changes in the solution. The ISE analysis of the hydrogen sulfide in cigarette smoke has been integrated into the undergraduate chemistry curriculum at Georgia Institute of Technology. Students work in teams of 2 or 3 and complete the experiment in a 6-hour lab session. The experiment is quite flexible; it can be broken into two parts if there are time or schedule constraints. Feedback from students has been very positive. They comment that the experiment is simple yet instructive and certainly adds a lot of fun to the laboratory course. W
Supplemental Material
Supplemental material for this article is available in this issue of JCE Online.
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Acknowledgments We thank Jiˇrí Janata for suggesting this experiment and for helpful discussions in the course of this work. We also thank the students from Chem 4211, fall 1998, for their contributions to the development of this laboratory exercise. Literature Cited 1. Wingen, L.; Low, J.; Finlayson-Pitts, B. J. Chem. Educ. 1998, 75, 1599. 2. Wong, J.; Ngim, K.; Eiserich, J.; Yeo, H.; Shibamoto, T.; Mabury, S. J. Chem. Educ. 1997, 74, 1100. 3. Zoller, U. J. Chem. Educ. 1979, 56, 518. 4. Chatfield, A. J. Chem. Educ. 1960, 37, A163.
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5. A complete list of the harmful chemicals identified in the cigarette smoke can be found on the Web site http://www. osha-slc.gov/FedReg_osha_data/FED19940405.html (accessed Apr 2000). 6. Giggin Back, W. Inorg. Chem. 1971, 10, 1333. 7. Koryta, J.; Stulik, K. Ion Selective Electrodes, 2nd ed.; Cambridge University Press: New York, 1983. 8. Harris, D. C. Quantitative Chemical Analysis, 5th ed.; Freeman: New York, 1998. 9. Gran, G. Analyst 1952, 77, 661. 10. Health assessment information on hydrogen sulfide can be found on the following Web sites: http://www.epa.gov/ngispgm3/ iris/subst/0061.htm and http://www.qrc.com/hhmi/science/labsafe/ lcsstxt/lcsstx53.htm (accessed Mar 2000).
Journal of Chemical Education • Vol. 77 No. 8 August 2000 • JChemEd.chem.wisc.edu
