Atomic Energy Comm. Rept. TID 7606 (1960). (4) Flaschka, H., Abdine, H., ChemistAnalyst 45,58 (1956). (5) Flaschka, H., Abdine, H., Mikiochiin. Acta 1956, 770. (6) Foreman, J. li., Smith, T. D., J. Chem. SOC.1957, 1758. (7) Gel’man, A. D., Artyukhin, P. I., Moskvin, A. I., Zh. Neorgan. Khim. 4, 1332 (1959); Nucl. Sci. Abstr. 13, 18993 (1959). (8) Kabanova, 0. L., Danuschenkova, hl. A., Paley, P. N., Anal. Chim. Acta 22,66 (1960).
(9) Kressin, I. K., Waterbury, G. R., ANAL.CHEM. 34,1958 (1962). (10) Larson, R. P., Seils, C. A., Meyer, R. J., U. S. Atomic Energy Comm. Rept. TID 7606 (1960). (11) Metz, C. F., ANAL.CHEM.29, 1748 (1957). (12) Metz, C. F., Waterbury, G. R., Ibid.. 31.1144 (1959). (13) Mil&, G. W. C.‘, Woodhead, J. L., Analyst 81,427 (1956). (14) Moore, F. L., Hudgens, J. E., Jr., ANAL.CHEM.29,1767 (1957). (15) Paley, P: N., Chang, W. C., Zh. Analzt. Khzm. 15, 598 (1960); Nucl.
Sci. Abstr. 15, 7305 (1961). (16) Ryan, J. L., Wheelwright, E. J., Ind. and Eng. Chem. 51,60 (1959). (17) Scott, F. A., Peekema, R. M., Proc. U.N . Intern. Conf. Peaceful Uses At. Energy, Geneva, AIConf. 28, 914 (1958). (18) Welcher, F. J., “The Analytical Uses of Ethylenediaminetetraacetic Acid,” Van Nostrand, New York, 1957. (19) Wilkins, D. H., Hibbs, L. E., Anal. Chzm. Acta 18, 374 (1958).
RECEIVEDfor review August 10, 1962. Accepted March 11, 1963.
A Continuous Photometric Fluorine Analyzer C. W. WEBER and 0.
H. HOWARD
Technical Division, Oak Ridge Gaseous Diffusion Plant, Union Carbide Nuclear Co., Oak Ridge, Tenn.
b A sensitive automatic analyzer has been developed for the continuous determination of elemental fluorine in gas mixtures. As the sample stream i s passed through hot sodium chloride, the fluorine quantitatively liberates chlorine, which i s measured at 360 mp in a pressure-controlled flow colorimeter cell. The range of the analyzer, calibrated with standard fluorine-nitrogen mixtures, i s 0.05 to 15 mole of fluorine, using a 10-cm. cell at 500 mm. of Hg. The relative standard deviation of the fluorine analyzer i s about h1.5% above O.8Y0 fluorine, and the standard deviation i s about h0.04 (absolute) at lower levels. Initial response lag i s 6 seconds, with full response in about 1 minute. The instrument i s applicable in the presence of hydrogen fluoride and other gases which do not absorb radiation near 360 rnp or liberate chlorine from sodium chloride.
for the low concentration range of interest. Staple and Grilly (3) determined fluorine continuously by measurement of the thermal conductivity of chlorine displaced from sodium chloride by the fluorine; this method also lacked
A
Figure 1 , Ultraviolet absorption spectra of fluorine and chlorine
70
determination of fluorine in a mixture of fluorine and nitrogen was needed for satisfactory control of a fluorination process. The operating range of concentrations was 0.3 to 10 mole % fluorine, to be controlled within 10.05 to *0.57,, respectively, throughout this range. Previous analytical control consisted of intermittent chemical analyses, which required constant manual effort to obtain an acceptable degree of process control. An automatic continuous analyzer was developed to reduce analytical costs and provide smoother process control. A few continuous fluorine analyzers have been described in the literature. Weber ( 5 ) developed an automatic, fluorine analyzer based upon pncumatic detection of the reduction of molar flow as fluorine reacts with sulfur dioxide; this analyzer \vas not sensitive enough CONTINUOUS
1002
ANALYTICAL CHEMISTRY
t
I CM. C E L L AT S.T.P. 330 m p
PRINCIPLE
Fluorine absorbs radiant energy only weakly in the ultraviolet and visible regions of the spectrum. Davis (1) reports a n absorbance of 0.3, for a depth of 1 em. at 0” C., 760 mm. of Hg, at the maximum absorption wavelength of 285 mp (Figure 1). Chlorine, on the other hand, gives a n absorbance of 3.0 at its maximum absorption wavelength of 330 mp. Therefore, to achieve the desired sensitivity in the photometric fluorine analyzer, the fluorine sample is passed through hot sodium chloride, where it displaces an equivalent amount of chlorine : Fz(g)
+ 2T\’aCl(s)
+
Clz(g)
220
260
300 340 WAVELENGTH, ~ ? p
380
the required sensitivity and precision. Staple, Schaffner, and Wiggin (4) utilized the liberation of bromine from sodium bromide by fluorine and the subsequent measurement of light absorption by the bromine; this method was not attractive because of reported bromine adsorption-memory effects. This report describes an automatic, continuous, fluorine analyzer which is basically similar to the bromine-displacement analyzer, except that the light-absorption measurement is made on chlorine displaced from sodium chloride by the fluorine. .Idequate scnsitivity :tnd prccision arc obtained and no adsorption difficulties are encountered under the operating conditions established.
+ 2XaF(s)
The resulting gas then flows through a chlorine-sensitive colorimeter, which generates a n electrical signal directly related to the concentration of fluorine in the sample. CONSTRUCTION AND OPERATION
Thc analyzer (Figure 2 ) consists basically of a heated sodium chloride reactor, flow colorimeter, pressure-control system, flow limiter, a vacuum pump for drawing a continuous sample through the analyzer, and a recorder. Sodium Chloride Reactor. The sodium chloride reactor is a nickel cylinder, 1-inch i.d. by 8 inches long. Approxiniately 100 grams of reagent grade, crystalline, sodium chloride is retained in the cylinder by nickel wool filters. It is designed so t h a t i t can be readily disconnected and one end removed for replacing the sodium chloride as needed. I n operation the reactor hangs vertically within a cylindrical heater which maintains i t a t 250’ (3. The gas stream passes upwart1 through the reactor. The sodium chloride is heated because fluorine, at low concentrations (< lye),
DIFFERENTIAL PRESSURE
Table I. Fluorine-to-Chlorine Conversion Efficiency in the Photometric Fluorine Analyzer CHEMICAL VACUUM
4
ROTAMETER
'El RECORDER
Figure 2.
I
I
UTUBE
L
I LENS' I
I
actor was determined by passing chlorine standards through the analyzer and comparing the responses with those for corresponding fluorine standards. The results (Table I) show that, within the accuracies of the analyzer and standards, the conversion efficiency is 100%. This conclusion is supported by the visible sharp demarcation between the reacted and unreacted zones of a sodium chloride column which has been removed from the analyzer after partial consumption. One 100-gram charge of the sodium chloride reactor is theoretically sufficient for approximately 300 hours of operation at an average fluorine concentration of 1% and a sample flow rate of 100 standard cc. per minute. However, the charge is usually renewed after 150 to 200%-hours of use; a larger reactor would require even less freauent attention. Colorimeter. The colorimeter used in tile analyBer is the Beckman Model 3700 flow colorimeter; a schematic diagram of the optical system is shown in Figure 3. The spectral filters are those provided b y the manufacturer for chlorine measurement. The peak transmittance of the filters is a t about 360 mp. *lthough
/SHUTTER-\
PHOTOTUBE/
38 67 84
38 65 83
with the colorimeter wits replaced with a nickel cell with '/,&ch-thick fluorothene windows, for resistance to fluoride corrosion (Figure 4). A thickness of fluorothene equal to that of the cell windows was also placed in the reference beam to compensate for the greater absorbance of the fluoroethene windows. For zeroing the analyzer, which is accomplished by adjustment of the shutter in the reference beam of the colorimeter. nrovision is made for
power line to the colorimeter minimizes the effect of line voltage fluctuations. Further signal stability was achieved by spring-loading the zeroing shutter so that it was less susceptible to vibration. The cooling water, required by the colorimeter for cooling the tungsten source lamp, is also used to cool the tube carrying the gas from the hot sodium chloride reactor to the colorirneter cell. A recording potentiometer, of greater sensitivity than the signal meter on the colorimeter, indicates and records the analytical signal; it has full scale ranges of 1 and 10 my., corresponding to approximately 1 and 15% fluorine, re spectively, at a pressure of 500 mm. of Hg at 25" C. in the 10-cm. colorimeter cell. The nonlinear response of
I I
I 1
Figure 3. Schematic diagram of optical system of Beckman Model 3700 flow colorimeter
Figure 4.
Optical cell
VOL. 35, NO, 8, JULY 1963
-
100 103 101
this does not coincide with the 330-mp ahsorotion neak for chlorine (Figure 1).
Schematic diagram of continuous photometric fluorine analyzer
does not react with pure sodium chloride at room temperature. Furthermore, some chlorine is adsorbed by sodium chloride below 200" C., introducing response lag and error. Low concentra tions of fluorine will react with sodium chloride at room temperature, pfovided the sodium chloride has been initially heated to 200" C. in the presence of >5% fluorine. This suggested that unused sodium chloride might be satisfactory a t room temperature if mixed with sodium fluoride; however, with 1% sodium fluoride in the sodium chloride, reaction was not complete at 0.4% fluorine. Kimhall and Tufts (3)report that the presence of a small amount of sodium fluoride in the sodium chloride initiates the fluorine-sodium chloride reaction, without the application of heat; however, they worked with high concentrations of fluorine, which generated relativelv high temneratures. Evidence i h a c sodium chloride adsorbs chlorine below approximately 200" C. was a temporary increase in chlorine emission occurring at about this temperature when a previously activatedsodium chloride reactor washeated from room temperature to 320O C. while 0.4% fluorine in air was passed through it, Consequently, the stipulated operating temperature of 250" C. was selected, -and has proved satisfactory. No other effects were seen on heating to 320" C., indicating that accurate temperature control of the sodium chloride reactor is not required. The efficiencyof conversion of fluorine to chlorine in the sodium chloride re-
5.0 10.0 15.0
1003
the 10-mv. range (see Figure 5) is peculiar to the electronic design of the colorimeter. Pressure Control System. Since the analytical signal depends, for a given fluorine concentration, upon the pressure of the sample in the colorinieter cell, provision is made for automatically controlling this pressure. -4 differential pressure transmitter (Taylor, Model 206-RD), range 0 to 10 mm. of Hg, continuously senses the pressure and actuates a pressure control valve (Hoke, Inc., Part 1148) in the sample feed line to maintain the pressure a t a preselected value. For a given desired operating pressure, the reference or “low” side of the transmitter is pressurized with dry air to 5 mm. of Hg below the desired operating pressure, and valved off. A manometer or Wallace-Tiernan gauge, temporarily attached to the system, serves to measure this reference (datum) pressure while it is being fixed. A small, inexpensive gauge permanently attached to the reference side of the transmitter will reveal any subsequent changes in the reference pressure. After the reference pressure has been so fixed, the desired operating pressure is obtained by appropriate adjustment of the pressure control valve until the receiver gauge in the transmitter output line reads midscale. Use of a differential pressure transmitter in the manner described permits optimization of the range of the fluorine analyzer by selection of operating pressures appropriate to the fluorine concentrations of interest. When the pressure control system is operating, the pressure oscillates with the opening and closing of the pressure control valve. To miriimize the amplitude of the oscillations, a needle valve, connecting a 2-liter surge bottle to the transmitter output line, is adjusted until the transmitter receiver gauge indicates minimum pressure oscillation. With this device the pressure oscillation was reduced from =tl% to k0.0270 of the pressure. Flow Limiter. The sample flow rate does not affect the accuracy of the analyzer, within reasonable limits. However, it does affect the response
Table II.
0.8 5.0 10.0 15.0
detns. 89 6
1004
0
L
0
0
L
04 4
-
-
I
L
08 8
-
I
12 I2
.
-
I
d
I6 16
(I-mv. scale) ( IO-mv. scale) ( IO-mv. scale)
(10-mv. scale)
71 39 69 86
Figure 5. Calibration curves
time and rate of consumption of the sodium chloride; therefore, the flow rate is limited to give an acceptable response time. The flow limiter is 20 feet of nickel tubing, 0.030-inch i.d., l/&ch o.d., in the form of a %inch diameter helix. It passes approximately 100 standard cc. of air per minute a t a forepressure of 500 mm. of Hg, and a backpressure of essentially zero. A glass rotameter, which can be connected in series with the flow limiter but is normally valved out, is occasionally valved in with clean air flowing through the analyzer, to detect possible plugging of the flow limiter by particulates from the sodium chloride reactor. A small amount of white powder, a mixture of sodium chloride and sodium fluoride, passes the nickel wool filter and accumulates in the bottom of the colorimeter cell over several weeks of operation of the analyzer; however, no significant plugging of the flow limiter has occurred during 10 months of use. Chemical Trap. The chlorine and other corrosive gases, such as hydrogen fluoride which may be present in the sample stream, are removed in a chemical trap before the vacuum pump. Two gas mask canisters (Army, M-11), connwted in series, are used for this purpose and are replaced when the sodium chloride reactor is recharged. Other types of traps, such as activated carbon or soda
70 38
os
86
71 38 67
85
7’2
*1.2
3s
h1.5
67 8-1
zk1.4
il.1
Comparison of Analyzer with Chemical Analysis hlean Fz concn., yo - Difference, yo (abs.) Chemical hnalyzer Mean Std. dev. +0.01 hO.04 0.30 0.34 5.48 5.56 $0.08 h0.22
ANALYTICAL CHEMISTRY
20 20
FLUORINE, percent
Precision of Photometric Fluorine Analyzer
Table 111.
No. of
I O C M CELL, 500 MM. H g A B S . , 25°C.
lime, would presumably serve equally well, and, of course, one of larger capacity would require less frequent attention. The particular selection was made for convenience and ex2ediency during the development of the analyzer, and has not been further evaluated. Calibration. Calibration curves for the analyzer are shown in Figure 5. The analyzer is calibrated with standard mixtures of fluorine in nitrogen, prepared in nickel cylinders on a n appropriate pressure-monitored manifold. A 2-liter cylinder a t a total pressure of 30 p.s.i.a. is sufficient for about 20 minutes’ operation of the analyzer a t a controlled pressure of 500 mm. of Hg. PERFORMANCE
Range. The range of the analyzer depends primarily upon the length of the colorimeter cell and the pressure of the gas in it. For example, under the conditions indicated in Figure 5Le., 10-cm. cell length and 500 mm. of Hg pressure-the lower limit is about 0.05% fluorine; the upper limit is about 15% because of the reduced sensitivity above this concentration. With an appropriately shorter cell, and/or lower pressure, up to 100% fluorine could be determined. Conversely a longer cell, and/or higher pressure, would provide a lower limit of detection. It is apparent from Figure 1 that a colorimeter filter with a transmittance peak a t 330 mp would provide more inherent sensitivity. Response Time. The response time of the analyzer, the time required to attain 75y0 of a signal change after a change in fluorine concentration, is about 1 minute a t a sample flow rate of 100 cc. per minute. Initial response, however, is noted in about 6 seconds. Interferences. The analyzer is free from interference by the common components of air, or from other gases which do not absorb radiant energy in the 360-mp region of the spectrum, and which do not react with sodium chloride t o liberate chlorine. Hydrogen fluoride has no effect. Chlorine, bromine, uranium hexafluoride, and certain oxides of nitrogen are examples of gases that cause optical interference. Ozone and trifluorides of chlorine and bromine would probably interfere by liberating chlorine. Servicing Requirements. Servicing the analyzer requires intermittent renewal of the sodium chloride, the chemical trap, and recorder chart; occasional replacement of the tungsten source lamp in the rolorimeter; and cleaning of the colorimeter cell as indicated by inspection. Precision and Accuracy. The relative standard deviation of the analyzer is about zkl.5y0 above 0.8%
fluorine (Table 11). Comparison of analyzer results with determinations by a thiosulfate-titra tion method is presented in Table 111, which indicates a standard deviation of a t least h0.04 (absolute) a t lower levels.
of the analyzer; R. D. High for providing standard gas mixtures; and the members of the Oak Ridge Gaseous Diffusion Plant Process Laboratory who made the comparative chemical determinations.
ACKNOWLEDGMENT
LITERATURE CITED
The authors thank P. R. Kuehn, S. A. MacIntyre, and J. G. Million for their assistance in the early field-testing
(1) Davis, W., U. S. At. Energy Comm. Rept. K-985 (Classified) (1953). ( 2 ) Kimball, R. H., Tufts, L. E., Ibid., MDDC-195 (1946).
(3) Staple, E., Grilly, E. R., Zbid., MDDC1565 (1947). (4) Staple, E., Schaffner, J. G., Wiggin, E., Zbid., MDDC-1610 (1946). ( 5 ) Weber, c. w., ANAL.CHEM.32, 387 (1960). RECEIVED for review February 5, 1963. Accepted April 19, 1963. Southeastern Regional Meeting, ACS, Gatlinburg, Tenn., November 1962. m'ork performed at the Oak Ridge Gaseous Diffusion Plant operated by Union Carbide Corp. for the U. S. Atomic Energy Commission.
The UIt ravio Iet S pect romet ric Dete r mina t io n of the Three Isomeric ToIuenesuIfonic Acids in Excess of Aqueous Sulfuric Acid HANS CERFONTAIN, HERMAN G. J. DUIN,' and LEO VOLLBRACHT2 Laboratory for Organic: Chemistry, University of Amsterdam, The Netherlands
b Mixtures of the three isomeric toluenesulfonic acids in excess concentrated aqueous sulfuric acid can be determined quantitatively b y multicomponent spectrophotometric analysis of the sulfuric acid solution on the basis of the ultraviolet absorption of the sulfonic acids. This determination is performed by subjecting the absorbances of the unknown mixture and of its constituents, gathered at a large number of wavelengths, to a least square treatment by an electronic computer. The para isomer is analyzed very accurately. The accuracy in determining the meta and ortho isomers is somemwhat less satisfactory because of their high spectral similarity. m-Toluene!;ulfonic acid can be determined more accurately b y multicomponent spectrophotometric analysis in oleum of about 10 weight SO,. In this solvent 0 - and p-toluenesulfonic acid are rapidly converted into toluene-2,4-disulfonic acid, so that only the sum of the amounts of 0 - and p-toluenesulfonic acid will b e found in the oleum analysis.
70
I
CONNECTION with a study on partial rate factors of the sulfonation of toluene (3, 16), a rapid and accurate method of analysis was needed for the quantitative determination of N
1 Present address: Research Laboratories of N.V. Philipci-Duphar, Weesp, The Netherlands. Present address: Central Research Institute of AKU and affiliated Companies, Arnhem, The Netherlands.
mixtures of the three isomeric toluenesulfonic acids in a large excess of concentrated aqueous sulfuric acid. The inverse isotope dilution method was successfully applied in preliminary experiments (16), but it did not seem attractive for routine analysis as the procedures are tedious. The cryoscopic method for the determination of the isomer ratio of a mixture of toluenesulfonic acids (4, 7, 9) was unfit to solve our problems in view of the relatively large quantities of sulfonic acid mixture required. The same difficulty applies to the isomer ratio determination proposed by Spryskov (11). The quantitative separation of small amounts of sulfonic acids from excess sulfuric acid seems very unattractive (6), thus making impossible gas chromatographic analysis of the sulfonic acids after their conversion to sulfonyl chlorides or to sulfonic esters (8). A spectrophotometric determination of the total amount of toluenesulfonic acids in an excess sulfuric acid was recently described (10). The spectrophotometric method, described for the determination of the ortho and para isomers in commercial mixtures of toluenesulfonamides (IC), was not appropriate to solve our problem as no provision was made for the determination of the meta isomer. The preliminary results of spectrophotometric analysis of multicomponent systems, obtained by Herschberg and Sixma in our laboratory, were very promising (6). Therefore the possibility of analyzing the toluenesulfonic acids in sulfuric acid solution by this method was investigated. The principles of the multicomponent spectrophotometric
analysis have adequately been described (6, 13). The analysis is based on a linear least square resolution of the absorption spectrum of the mixture to be analyzed in terms of the spectra of the pure components. To obtain maximal precision in the analysis, it is necessary to determine the absorbances of the mixture and the components a t a large number of wavelengths (large relative to the number of components) and to perform the measurements a t each wavelength as simultaneously as possible. The observed absorbances define an overdetermined system which, for the evaluation of the unknown concentrations, is subjected to a least square treatment by electronic computer (6). The applicability of the method depends on the differences in shape of the absorption spectra of the Components. The absorption spectra of the three isomeric toluenesulfonic acids in 82.4 weight yo sulfuric acid are shown in Figure 1. The similarity in shape of the ortho and meta spectra may cause these two isomers to be subject to some interchange in the analysis. To minimize any possible interchange in the content of the meta and ortho isomer, the number of wavelengths, a t which measurements have to be made, must be extremely large. In weak oleum solution 0- and ptoluenesulfonic acid are both rapidly converted into toluene-2,4disulfonic acid (16),whereas the meta isomer forms a mixture of toluene-2,5- and toluene3,5-disulfonic acid (12). The spectra in oleum of toluene-2,4disulfonic acid and of the mixture resulting after sulfonation of m-toluenesulfonic acid show VOL. 35, NO. 8, JULY 1963
1005
