E. E. van Tamelen and D. A. Seeley, J. Amer. Chem. Soc., 91, 5194 (1969). R. C. Reed, Ph.D. thesis, Wesleyan University, 1971. S.Bank and D. A. Juckett, manuscript in press.
Author to whom correspondence should be addressed.
RECEIVEDfor review June 5 , 1974. Accepted September 3,
1974. This work was presented in preliminary form a t the 139th Meeting of The Electrochemical Society in Washington, D.C., May 1971. Financial support was provided by the National Science Foundation. The Wesleyan University Computing Center provided computer time.
Comparison of High Precision Coulometric and West-Gaeke Methods with the Gravimetric Method for Preparation of Standard Sulfur Dioxide Gas Blends Using Permeation Tubes Anders Cedergren, Anders Wikby, and Kurt Bergner Department of Analytical Chemistry, University of UmeB, S-90 7 87 UmeB, Sweden
A high precision coulometric method has been developed to determine the permeation rate of sulfur dioxide from FEP permeation tubes. The relative standard deviation of the method was about 0.04% for titrations performed during a couple of hours. The relative standard deviation of titrations performed on different occasions over a period of one month was 0.1-0.2% which corresponds to a temperature control better than f0.02 OC during the whole period. The coulometric method has been compared with a modified West-Gaeke method according to Scaringelll ef a/. and with a gravimetric method. Some anomalities observed in the West-Gaeke method were explained from the kinetic behavior of the involved reactions. The relative standard deviation of the modified West-Gaeke method was found to be 3 % and this procedure was shown to constitute a significant improvement of the original West-Gaeke method. A comparison between the mean values obtained by the modified West-Gaeke and the coulometric methods and the mean value obtained gravimetrically gave ratios of 97.2 and 99.9 %, respectively. Because of its accuracy, the coulometric method made it possible to detect significant dlfferences in permeation rates for environmental gases such as nitrogen, oxygen, and air.
Several methods for the determination of sulfur dioxide in ambient air have been proposed. The analyses have been carried out by means of, for instance, spectrophotometry (1-4), gas chromatography (5-8), conductometry (9-12), coulometry (13-15), polarography (16), titrimetry (17, 18) and flame photometry (19).Among these methods, the colorimetric procedure developed by West and Gaeke (20) is considered to be the most selective, one of the most sensitive and suitable for field conditions (21 ). It includes a stable fixation of sulfur dioxide as dichlorosulfitomercurate ion by reaction with tetrachloromercurate ion. Nevertheless, there have been difficulties in obtaining reliable results with this method mainly because of variations in the quality of the pararosaniline dye ( I , 4,22). A general problem when comparing the various methods is the lack of reliable primary standards. The introduction of permeation tubes for various gases as proposed by O'Keeffe and Ortman (23) represents a significant step forward in this respect. Sulfur dioxide is condensed within a sealed tube and, provided that a steady state prevails, the gas permeates the tube in accordance with the permeation equation 100
where F is the rate of flow per unit length of the cylinder, D is the diffusion constant, S the solubility coefficient, and 1 and p 2 the partial pressures outside and inside, respectively. b and a are outer and inner radii, respectively. The permeation rate P is affected by the temperature, T , according to
p
P = Po exp (-E/RT)
(2 1
where E, the activation energy, is about 11 kcal as estimated from data given by Scaringelli ( 1 3 ) .A constant permeation rate in the steady state therefore requires a rigorous control of the temperature. The main advantage with this technique is that the weight loss of the tube per unit time corresponds to the rate of permeation of sulfur dioxide, i.e., the standardization procedure can be performed gravimetrically. To increase the versatility, however, several investigators have reported alternative methods for calibration of the tubes. Among these investigations, those of Saltzman et al. ( 2 4 ) and Scaringelli et al. ( I ) should be mentioned. Saltzman developed a simple and rapid microgasometric technique for calibration of sulfur dioxide permeation tubes. With careful work, the deviation from the average (at 95% confidence) was found to be fl% for measurement periods of 1-2 hours, and the average agreed within 1-2% with the gravimetric mean value. Scaringelli successfully modified the West-Gaeke procedure ( 1 ) and later (13), when this method was compared with the gravimetric method, he reported agreement in permeation rate for thick-walled tubes of 97.5 f 6.5% (at 95% confidence). The permeation rate was also determined with a coulometric procedure and he obtained 96.6 f 9.0%. Reports on calibration of permeation tubes for sulfur dioxide thus suggest an accuracy of about f1% in the most favorable cases. Within these limits, i t has been stated that minor variations in environmental conditions, i.e., relative humidity, gas composition, or pressure, do not cause significant changes in permeation rate. Earlier work ( 1 4 ) at our institute, with a coulometric determination of sulfur dioxide as a digestion product of sulfur in hydrocarbons showed that sulfur dioxide could be determined quantitatively and with high reproducibility. This suggests the possibility of developing a high precision method for standardization of permeation tubes. A high precision method would be of great value, partly because the permeation tubes are regarded as primary standards
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
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Figure 1. Schematic experimental arrangement 1 = gas cylinder. (nitrogen,air or oxygen), 2 = pressure regulator (Brooks Instrument),3 = thermostated copper wire, 4 = tubes filled with silica gel, 5 = thermostated U-tubes. 6 = scrubbers, 7 = by-pass, 8 = rotameters (Glass Precision Engineering Ltd)
and partly because such a method may enable us to study possible effects of different environmental conditions on the permeation rate. The coulometric method described in this paper has been compared with gravimetric analyses as well as with the West-Gaeke procedure modified by Scaringelli el al. ( 1 ) .
EXPERIMENTAL Gravimetric Procedure. Permeation tubes, F E P (fluorinated ethylene propylene), were prepared according to the procedure proposed by O'Keeffe and Ortman ( 2 3 ) .The tubes (0.25-cm i d . , 0.40-cm o.d.) were sealed with steel balls a t lengths of 3-4 cm. T h e tubes were stored in glass tubes containing silica gel (6-22 mesh B.S.S.) to remove moisture and sulfur dioxide. The glass tubes were immersed in a thermostat bath. The thermostat, provided with thyristor regulation and refrigerator was placed in a thermostated room, the temperature of which was 25.0 f 0.1 O C . In this way, the temperature constancy of the permeation tubes was 25.0 40.03 "C. The tubes were normally weighed every fourth day with a semimicro Mettler balance. The procedure was continued for about one month. West-Gaeke Procedure. A schematic figure of the experimental arrangement is shown in Figure 1. The scrubbers were filled with 30.0-ml tetrachloromercurate solutions. The procedure was started with the stop cocks in the by-pass position. The permeation tube was placed in one of the U-tubes. The parallel branch containing the empty U-tube was used for the determination of a blank. The temperature was kept a t 25.0 f 0.03 "C by surrounding the U-tubes with thermostated water. T h e system was preequilibrated by passing dry nitrogen gas. A gas flow of 60-80 ml/min was allowed to pass the scrubbers during 40.0 min. The solutions were analyzed for sulfur dioxide according to a modified West-Gaeke procedure, method B, described by Scaringelli ( I ) . A time scheme for reagent additions was constructed to obtain exactly the same time conditions for samples, blanks, and standard samples. The absorbances were measured a t 575 nm, 30 min after the addition of the pararosaniline dye in 1-cm cuvets with a Beckman DU Spectrophotometer provided with a digital read out. In some experiments the absorbance was measured as a function of time. Coulometric Method. General Procedure. The coulometric cell was connected after section 5 in Figure 1 with sections 6 and 7 removed. The nitrogen gas entered the cell via a glass capillary tube and the gas flow was 300-500 ml/min. In a few separate experiments, the nitrogen gas was replaced by clean air or oxygen. The temperature was kept constant a t 25.0 f 0.03 "C. The cell was connected to an LKB 16300 Coulometric Analyzer which has been developed according to principles given by Johansson (2.5). Continuous Coulometric Titration. The absorption medium was about 30 ml of an aqueous solution containing 0.40% potassium iodide and 0.68% acetic acid in a Metrohm titration vessel EA 880-20. Sulfur dioxide from the permeation tube enters the cell continuously and reacts with iodine, which is generated electrolytically a t a platinum electrode. A platinum spiral was used as auxiliary electrode and was separated from the sample solution by a dialysis membrane. The concentration of iodine was determined potentiometrically with a platinum wire (0.25 cm2) indicator electrode us. an Ag/AgCl reference electrode provided with liquid junction, Metrohm EA 217-A. The coulometric analyzer compares the redox potential (normally 375 mV) of the indicating electrode with a preset value. The difference obtained is amplified, and a current proportional to the difference is fed to the pair of generating electrodes in the cell. The gain and the preselected indicator potential value can be used to adjust the iodine 'level. A blank value is run by adjusting the instrument to a potential value which
ma'0
S
10
1s
time
20
25
30
days
Figure 2. The weight loss of the permeation tubes 0) No. 1 and 0)No. 2 as a function of time. The temperature was 25.0 f 0.03 OC
corresponds to the same iodine level as that used in the sample run. The amount of electricity required to titrate the absorbed sulfur dioxide was determined by an electronic integrator in the analyzer. T h e integrator could be read to 2 X lo-" equivalents which correspond to 1 X lo-" mole of SOz. Non-Continuous Coulometric Titration. For comparison, a few non-continuous coulometric titrations were performed. The aqueous medium consisted of 0.04% KI and 0.68% HAC,which is similar to that used by Scaringelli et al. ( 1 3 ) .At the beginning of the titration, there was an excess of iodine in the titration vessel. Sulfur dioxide was then allowed to pass for a certain time. The absorbed sulfur dioxide was then titrated back to the preselected potential value by generating iodine. A blank run was performed in an analogous way. Kinetic Studies. T o evaluate the kinetic data of the reaction between sulfur dioxide and iodine, we plotted the cell emf of the indicator system us. the iodine Concentration. A certain concentration of iodine was obtained by generating a discrete amount of iodine. The emf was measured with a Solartron 7040 DMM Digital Multimeter which can be read to 0.01 mV. Then 10-50 MI of a 0.4mM standardized sulfur dioxide solution was rapidly added to the cell solution containing different initial concentrations of iodine. The potential value was read from the digital multimeter after every five seconds. Using the above mentioned plot, the concentration of iodine and, consequently, from the 1:l ratio between reactants, the concentration of sulfur dioxide could be calculated as a function of reaction time. From the slopes obtained a t different times, the time derivative was determined. In this way, corresponding values of the iodine concentration, the sulfur dioxide and the time derivative d[Iz]/dt a t a certain time can be evaluated. The information was used to fit data into an Equation
where k is the rate constant, and nl,n2 equals 1 or 2
RESULTS Gravimetric Results. Weight losses for two permeation tubes are plotted us. time in Figure 2. The weight loss is a linear function of time. The mean weight loss for tube 1 over a period of one month was 1.009 pg/min and with a standard deviation of 0.013 pg/min. For tube 2, the mean value was 1.157 pg/min and the standard deviation 0.010 pg/min. Results of the West-Gaeke Procedure. Kinetic Results. The variation of the absorbance of the pararosaniline-formaldehyde-sulfur dioxide complex with time at 26.5 " C is shown in Figure 3. The Figure includes results for various initial concentrations of sulfur dioxide. For each concentration of sulfur dioxide, the influence of the age of the purified pararosaniline dye stock solution is also shown
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
101
r---
' 1
0.85
Table I. Time Derivatives of the Absorbance as a Function of the Age of Pararosaniline and the Concentration of Sulfur Dioxide. The Temperature Was 26.5 "C Concn of sulfur
OB0 0.7 5
060
zero, u g l m l
abs., min-'
0.42 0.84 1.25 0.42 0.84 1.25
0.5 1.4 2.6 0.8 2.0 3.1
1 month al " a
C
0
d A / d t x lo3'
dioxide a t time
Pararosaniline
1 day
c
-
Age of
~
The t i m e interval used for calculation was 30-60 min.
.a
51
n
0 7,
1D10-
E m
-e
1005 -
c
0.1
: 1.000 -
ol
41
-
0.05 -
-
-
-
*
!
2
1,.
0
i ,
,
, 50
,
,
,
,
,
,
,
100
,
,
, 150
,
,
,
,
, 200
,I
time min
I i
d
Figure 5. Typical coulometric determinations of the permeation rate for tube 1 performed on different occasions ( A 18/12/73, 0 8/11
1
74) The mean values obtained from the respective curves when corrected for blank values are symbolized by points C and D
440r.-
340 360
t L
4.5
5.5
65
- log E2+1J
7.5
Figure 4. Indicating electrode potential vs. AglAgCl as a function of -log[12
+
13-1
Potential value corresponding to point A required about 10 min for equilibrium: B, 20 sec; and the other,
