Anal. Chem. 1981, 53, 1799-1801
17’99
Continuous Flow Determination of Molecular Chlorine in Nonpolar Media Luuk Balt,” E k e J. Stamhuis, and Geert E. H. Joosten Department of Chemical Engineering, State University of Groningen, Nlenborgh 16, 9747 AG Groningen, The Netherlands
A continuous measuring system for chlorine In a nonpolar medium has been developed. Dllute (aqueous) methyl orange solution flows past a (Teflon) membrane mounted In a sensor. Chlorlne diffuses through the membrane and reacts with methyl orange, causlng decolorization. The extent of decolorlzatlon of the solutlon is measured colorimetrlcally. A hear dependence of the extinctlon on the chlorine concentratlon has been found. Chlorine concentrations of 5-200 mol/ma at 20-40 O C are measured with a standard deviation of the experimental points of 2%. The time constant (63% approach to equilibrium) is 22 8; the dead time Is 30 s In the system used. Calibration of the sensor should be carried out once a day.
Several methods are available for the batchwise determination of the molecular chlorine concentration in aqueous and nonpolar media (1-3). Also methods for the continuous measurements of dissolved chlorine in aqueous media are known (4-7). T o our knowledge no methods are available for the continuous measurement of chlorine in nonpolar media. In our research program on the chlorination of cyclic ketones with carbon tetrachloride as solvent, we needed a technique to determine continuously the local value of the steady-state concentration of chlorine in various continuous flow reactors. As the rate of reaction of chlorine with ketone was too fast for measurement, by manual sampling means, the present continuous and automated procedure was developed. The principle of the method is that molecular chlorine, which is present in the liquid reaction medium in the reactor, diffuses through a semipermeable membrane. At the other side of the membrane it reacts with a reagent, which flows a t a constant rate past the membrane. The reduction in concentration of the reagent is measured. It is proportional to the flux of chlorine through the membrane, which in turn is proportional to the chlorine concentration in the solvent. The membranes used were constructed from a number of fluorinated polymer films. Solvents and reaction products are not transferred in interfering quantities. The compound which reacts with chlorine and is decolorized is methyl orange (4). The methyl orange flows as a very dilute aqueous solution past the membrane. The reduction in the methyl orange concentration in the reagent flow is measured colorimetrically. Many more compounds, other than methyl orange, might have been used as reagent for the chlorine. Especially 4,4/-diamino-3,3’-dimethylbiphenyl(o-toluidine) and N,N-dimethyl-p-phenylenediamine (DPD) are suitable. However, these compounds are listed by NIOSH as severely toxic and/or carcinogenic and were therefore not used. EXPERIMENTAL SECTION Apparatus. Figure 1 shows the system. For pumps 1-4 a four-channel peristaltic pump was used which had the possibility to use tubes with different diameter and provision to engage 1-4 channels (Gilson Minipump). A continuous flow of methyl orange solution was metered by pump 3 to the chlorine sensor, which was placed in the chlo0003-2700/81/0353-1799$01.25/0
rine-containingsolution. Subsequently the methyl orange solution flows through a colorimeter to determine the extinction of t’he solution. To promote mixing at the membrane in the chloriine sensor and to avoid dispersion of reaction product in the lines to the colorimeter, the flow of methyl orange solution was segmented by air bubbles (slug flow) (8). The bubbles were removed in a gas-liquid separator before entering the meter. Pump 1 removed liquid from the separator at a rate, which was 80% of the feed rate to the separator. The ratio between feed and withdrawal was fixed by the choice of the diameters of the tubes in the peristaltic pump. The flow rate of the reagent could be adjusted between 6 and 60 X lo4 m3 s-l. The internal diameter m. To compensate the colorimeter of the tubes was 1.5 X for a slight drift and to set the recorder scale between 0 and max. % absorbance, a provision was made to feed reagent or pure water to the colorimeter, without interrupting the reagent flow past the membrane. The methyl orange solution could be fed directly from the storage vessel to the colorimeter with pump 4 and by manipulating valves A and B. At the indicated methyl orange concentration the minimal transmission was 68.5% (extinction E = 0.16). The maximum transmission was adjusted by feeding pure water with pump 2 to the colorimeter. The colorimeter used in the experiments is a Vitatron UC 200. Figure 2 shows the construction of the chlorine sensor. The semipermeable membrane (a) was kept in place over the top of the holder (c) by the ring (b). The screwcap (e) fastened the ring to the holder. The extension bar (f) was used to fix the sensor in the solution with the chlorine concentration to be measured. The air-segmented flow of reagent was fed through polyethylene tubes to the membrane. The effective surface area of the menim2. Various membranes were studied for their brane was 2 X suitability in the present procedure: the PTFE (polytetriifluoroethylene) membrane of the Beckmann 39553 O2sensor, thickness 2.5 X lo4 m; PTFE membrane, thickness 5 X lo4 m; FEP membrane (copolymer of tetrafluoroethylene and hexafluoroethylene),thickness 2.5 X 10” m. The first membrane was found to have the best properties and was used extensively. Reagent. The solution of methyl orange was prepared (4) by mixing 0.135 kg of a 0.0005% methyl orange solution with 4.365 kg of demineralized water and kg of 5 N hydrochloric acid solution. No exact data on the rate of reaction between chlorine and methyl orange are available. According to ref 4 the maximum rate is reached at pH 3. We found the reaction to be sufficiently fast for the extinction to reach its final value before entering the colorimeter. Development of the Procedure. The chlorine sensor and the procedure were tested in a thermostated vessel. The vessel was equipped with a bottom outlet tube to take samples, whiclh were analyzed by iodine thiosulfate (3). It was possible to feed pure solvent and solvent with chlorine dissolved in it, so that the chlorine concentration in the vessel could be varied at will. The liquid was stirred magnetically. After adjustment of the colorimeter with pure water and fresh methyl orange solution, the extinction of the methyl orange solution which flowed through the sensor was measured. This was done for various chlorine concentrations in the solvent. Furthermore the extinction as a function of time was measured when the chlorine concentration in the vessel was varied stepwise.
RESULTS AND DISCUSSION When chlorine is transferred from the nonpolar solution through the membrane to the methyl orange solution, thle concentration profile will in general look as given in Figure 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
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Figure 3. Schematic diagram of the concentration profile of chlorine near the membrane.
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Figure 1. Schematic diagram of apparatus used. Calibration of minimal transmission: valves A-8, pumps 1 and 4. Calibration of maximal transmission: valves A'-B', pumps 2 and 4. Measurement: A-B', pumps 3 and 4.
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Figure 4. Effect of temperature on the measured extinction of the reagent. POLKHENE TUBING
transferred per cubic meter of methyl orange solution which flows at a rate of 4" (m39-l) past the membrane of A(m2) is given by
DA 1 [Cl,] = L ZCb D is the diffusion coefficient (m2SI), L is half of the thickness
Flgure 2. Chlorine sensor. Ring (b) clamps Teflon membrane (a) on top (c) of the membrane holder (d). Screw cap (e) fastens the ring to the holder and extension bar (f). All measures are in m. From the measured rates of chlorine transfer it can be calculated that the concentration differences between the bulk of the solution and the outside wall of the membrane are virtually zero. The same holds for the inside of the membrane. The chlorine concentration in the methyl orange is virtually zero. Hence Cw,l = Cband C,,2 = 0. These concentrations were used when solving the differential equation describing the diffusion of chlorine through the membrane. By use of the solution it is easily calculated that the amount of chlorine 3.
of the membrane. The group DAIL is a constant for the sensor at a fixed temperature. The proportionality between the chlorine concentration in the solvent, Cb, and the amount of chlorine transferred per cubic meter of methyl orange solution is confirmed experimentally. The precision of the method, expressed as standard deviation is 2% of the measuring range. From the experimental extinction data and Tarras' (4) data on the extinction of the methyl orange solution to which chlorine is added, it is possible to calculate the value of the diffusion coefficient of chlorine in the membrane. The result is DclZ = 3.4 X m2 s-l at 20 "C in the Teflon membrane. No literature data on the diffusion coefficient of chlorine in PTFE are available, but for COz and NO2in PTFE at 25 "C reported values are 9.5 X and 3.7 X m2 s-', respectively (10). Our results seem in reasonable agreement with these data. Figure 4 shows the influence of the temperature on the rate of diffusion through the membrane. Brandrup and Immergut (10) express the variation of the diffusion coefficient with temperature by
DT = Doexp(
-2)
where ED is the apparent activation energy of diffusion (J mol-l) and R is the gas constant (J mol-' K-l). The valuee of ED calculated from Figure 4 in the temperature range 20-30
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
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"C are 39 kJ mol-', whereas they appear to be 111kJ mol-l in the range of 30-40 "C. Values of ED for C 0 2 and NO2 reported in the literature (IO) are 28.6 and 58.6 kJ mol-l, respectively. It was shown, as will follow, that the high value of DCI,at 40 "C is, in fact, due to some aging effect of the membrane (see Figure 6). In Figure 5 the results of a series of measurements taken in the course of 14 days using the same membrane are given. During the first 12 days the temperature was maintained at 20 "C. On the 13th day the temperature was raised to 30 "C and after 4 h to 40 "C. At the 14th day the temperature was reduced to to 20 "C. From the results it appears that the permeability of the membrane was virtually constant during the 12 days a t 20 "C. However, the increase to 40 "C apparently brought about a rapid increase in permeability which turned out to be in part irreversible. Chemical modification of the polytetrafluoroethylene membrane by C12or CCl, is not probable. Mechanical deformation of the membrane by the pressure of the methyl orange solution seems more likely. In fact, deformation was visually observed. It seems therefore advisable to attach a grid over the membrane to give it some mechanical support and. to operate the membrane at a lower temperature. This also decreases the chance of a membrane rupture. We have never observed rupture, but it should be avoided at all times, as the process fluid will be contaminated by the methyl orange solution in that case.
1801
In the experiments on the dynamic behavior of the sen~or, the time required to reach 63% of the final extinction after a stepwise change in the chlorine concentration was found to be 55 s. The time required for a 95% response was 169 s. Both times include a dead time of 31 s for the flow in the limes between sensor and colorimeter. Factors that determine the dynamic response time are (a) mass transfer resistance between the bulk of the nonpolar chlorine solution and the outside wall of the membrane, (b) diffusion through the membrane, (c) mass transfer resistance between the membrane and the bulk of the methyl orange solution, and (d) dead time and residence time distribution in the flow between sensor and colorimeter. It can easily be shown by calculation that factors a and c are negligible smd that the most important factor in the present setup is the diffusion through the membrane. The response time found experimentally is in good agreement with the value expected on basis of the diffusion coefficient. The mean residence time in the lines can be minimized by short tubes with small diameter. The slug flow reduces ithe residence time distribution to very small values (8). The range of chlorine concentrations over which the sensor was tested was 5-200 mol/m3. The range can easily be extended by choosing different flow rates of the methyl orange solution and surface areas of the membrane in the sensor. Also the concentration of methyl orange may be altered.
LITERATURE CITED (1) Sconge, J. "Chlorine-Its Manufacture Properties and Uses"; Rhelnhold: New York, '1962;pp 474-476. (2) Well, D. Z. Wasser Abwasser Forsch. 1975,8 , 85-87; Chem. Abstr. 1976, 84, 79420C. (3) Van den Berg, H. Thesis, State Unlversity of Groningen, The Nettierlands, 1973. (4) Tarras, M. Anal. Chem. 1947, l $ , 342-343. (5) Britlsh Patent 1428371,1978,Dow Chemlcal Corp. (6) U.S. Patent 3 959 087, 1976,Fisher & Porter Co. (7) Johnson, J. D.; Edwards, J. W. froc.-AWWA Annu. Conf., 95th 1975; Chem. Abstr. 1976, 85, 197924~. (8) Snyder, L. R.; Adler, H. J. Anal. Chem. 1976, 48, 1017-1027. (9) Tarras, M. J.-Am. Water Works Assoc. 1946, 38, 1146-1150.
(10) Brandrup, J.; Immergut, E. H. "Polymer Handbook"; Wiley: New York, 1975;p 111-236. (1 1) Maatman, H. Thesis, State University of Groningen, The Netherlands,
1960.
RECEIVED for review December 13,1980. Accepted June 8, 1981.
