Simultaneous Determination of Bromine and Chlorine with Methyl Orange H. A. Laitinen and Kenneth W. Boyer Department of Chemistry, Uniuersity of Illinois, Urbana, Ill. 61801 The reaction between methyl orange and bromine or chlorine involves competition between aromatic ring substitution and azo link cleavage, both of which produce a large decrease in the molar absorptivity of the 505 nm methyl orange absorption peak. However, predominant ring substitution for the bromine reaction produces an increase in absorbance at 317 nm, whereas predominant azo link cleavage for the chlorine reaction produces a decrease in absorbance at 317 nm. By utilizing the changes in the UV-visible spectrum of aqueous methyl orange solutions upon reaction with gaseous mixtures of bromine and chlorine in nitrogen, total halogen content up to 5 peq/liter and individual halogen mole fractions were determined with an 8% relative error and &ll% relative standard deviation. The formation of BrCl limits the total halogen concentration that can be analytically distinguished to less than 5 beq/liter in nitrogen. THEQUANTITATIVE DECOLORIZATION of sodium 4’-dimethylaminoazobenzene-4-sulfonate (methyl orange) is a known analytical method for the determination of either free bromine (1-3) or free chlorine ( I , 4-6). The basis for the method is the Beer’s law decrease in absorbance at 505 nm of a pH 2 solution of methyl orange with addition of aqueous solutions of either bromine or chlorine. Because the decrease in absorbance at 505 nm is the same for molar equivalent quantities of bromine or chlorine, Boltz (1) and Sollo et ai. ( 2 ) warn that if both free bromine and free chlorine are present the methyl orange method can be used only for determination of total halogen. The manner of addition of halogen to working solutions has been noted to be of critical importance ( I , 2, 6) with the methyl orange solution being much less bleached by a rapid addition of halogen without stirring than with slow addition with vigorous mixing. Two mechanisms have been postulated for the reaction of bromine or chlorine with methyl orange, requiring either one (Rl) or two (R2) moles of halogen per mole of methyl orange (X indicates Br or Cl):
H03S@
+ X 0N ( C H J 2 + N2 + NaX 2x2
+X
I---
-
, N O
(1) D. F. Boltz, “Colorimetric Determination of Nonmetals,” Interscience, New York, N. Y . ,1958, p p 161-96. (2) F. W. Sollo, T. E. Larson, and F. F. McGurk, Encirorr. Sci. Technol., 5,240(1971). (3) A. I. Cherkesov, J. Arrat. Chem. U.S.S.R., 15,743 (1960). (4) M. Taras, J . Amer. Water Works Ass., 38,1146 (1946). ( 5 ) M. Taras, ANAL.CHEM.:19,342 (1947). (6) F. W. Sollo and T. E. Larson, J. Amer. Water Works Ass., 55,1575 (1965). 920
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
Reaction (R2) is written for chlorine by Taras (4) based on data obtained by titrating acidic aqueous chlorine solutions with 0.005 methyl orange solution, clearly resulting in local excesses of halogen with respect to methyl orange. The same reaction is written for bromine by Cherkesov (3) who studied the analytical utility of using various azo dyes for the quantitative determination of several inorganic oxidizing agents, but he states that further investigation is needed to determine the composition and structure of the oxidation products. Both reactions (Rl) and (R2) are written by Boltz ( I ) depending on the manner of halogen addition. Sollo et al., who used the methyl orange method for determination of bromine ( 2 ) and chlorine (6), were careful to add aqueous bromine or chlorine solution slowly to an excess of well-stirred methyl orange solution, and to avoid any local excess of halogen. It is obvious from Equations (Rl) and (R2) that the above studies assume that the azo link is cleaved during the reaction with halogen. These studies reported only the effect of bromine and chlorine addition on the methyl orange peak at 505 nm. However, as shown by Figure 1, methyl orange in aqueous solution at pH = 2.1 has four absorption peaks: ( A ) 505 nm, log E = 4.66; ( B ) 317 nm, log E = 3.89; ( C ) 277 nm, log E = 3.94; and ( D )222 nm, log E = 3.86 (E = molar absorptivity). It is well established (7) that proton addition to aqueous solutions of 4-dimethylaminoazobenzene or its substituted derivatives results in an equilibrium mixutre of two tautomeric forms. H
H
I
I1
By comparison with spectrophotometric studies on substituted Camino- and 4dimethylaminobenzenes (7, s), the absorption blinds of methyl orange at 505 and 277 nm can be attributed to an azonium species of the form I and those at 317 and 222 irm to an azoanilinium form similar to 11. The intensity relationships of the peaks associated with the two tautomeric forms can be drastically altered without cleavage of the azo link, namely by ring substitution with resulting steric hindrance or hydrogen bonding (8) or by salt formation with the amino group (9, IO), whether it be substituted or unsubstituted with alkyl groups. Of course, cleavage of the azo link would result in an absorption spectrum characteristic of the reaction products. An analytical method was needed for simultaneous determination of bromine and chlorine released in the photodecomposition of PbBrC1. The possible utilization of the con(7) H. Zollinger, “Azo and Diazo Chemistry,” Interscience, New York, N . Y., 1961, pp 332-3. (8) G. Cilento, E,. C. Miller, and J. A. Miller, J. Amer. Chem. SOC., 78,1718 (1955). (9) E. Sawicki, J. Org. Chem., 22,365 (1957). (10) H. H. Jaffee and Si-Jung Yeh, ibid., p 1281.
siderable differences in spectra between 200 and 400 nm for reaction between methyl orange and bromine or chlorine (Figure 1) for simultaneous determination of these two halogens was the basis for this investigation.
I
I -50m1. D-----I
EXPERIMENTAL
Scope of the Experimentation. The experimental work was carried out in two phases. The objective of the first phase was the development of calibration curves for the effect of bromine and chlorine on the absorbance of methyl orange working solutions. In the second phase, emphasis was placed on characterizing the reaction products for specific reaction conditions and on determining the experimental factors affecting the precision and accuracy of using methyl orange for the analytical determination of bromine and chlorine. Reagents. Reagent grade chemicals and doubly distilled water were used throughout except where noted otherwise. For initial development of calibration curves, Fisher certified methyl orange (Lot 74021) was dried overnight at 105 "C, stored over PzOs, and used without further purification in a manner previously described ( I , 2) to make working methyl orange solutions 3.02 X lW5M in methyl orange, 0.02M in KC1, and O.01Min HCl. These solutions had pH 2.10 and an absorbance of 1.300 at 505 nm and 0.212 at 317 nm at 30 "C. For characterization of the reaction products, a different lot of Fisher methyl orange (Lot 753470) was used in the same manner as above to make working solutions, having the spectrum shown in Figure 1, that were 2.07 X lW5M in methyl orange at pH 2.10. Although the absorptivities of the two lots of methyl orange differed slightly, their calibration plots, Le., the change in absorbance per micromole of halogen added, agreed well within experimental error. Solutions of bromine in CC14 were made by bubbling vaporized Mallinckrodt bromine into Baker CCl,, and standardized by displacing iodine from a solution of potassium iodide and titrating the liberated iodine with standardized sodium thiosulfate solution using starch as an indicator. Solutions of chlorine in CCl, were made by bubbling Matheson chlorine into CC14, and standardized in the same manner. Because chlorine will freely volatilize out of CC14, changing the titer of an unstoppered chlorine solution significantly within a few minutes, chlorine solutions were standardized just prior to use, were kept tightly shut when not in use, and exposure to the open atmosphere was minimized to prevent erroneous results. This problem was not encountered with bromine solutions in CC1,. Solutions of 2.07 X lO-5M N,N-dimethylaniline (Mallinckrodt), 4.20 X 10-5M p-bromo-N,N-dimethylaniline (Eastman), and 4.06 X l W 5 M4-hydroxybenzenesulfonic acid sodium salt (Eastman), all at pH 2.10, were prepared by dissolving the respective reagent and 200 ml of acidic KC1 buffer solution in distilled water and diluting to 500 ml. All ether extractions were carried out using freshly distilled Mallinckrodt diethylether. Apparatus. Absorption spectra were obtained with a Beckman DB-G spectrophotometer and were recorded with a Beckman Model 1005recorder. pH measurements were made with a Leeds and Northrup pH meter (Cat. 7664) using a Sargent S-30072-15 combination glass electrode. A Sargent Coulometric Current Source Model IV (current regulation =t 0.1 %) was used for coulometric generation of bromine and chlorine. Bromine was generated at a rate of 0.05 peq per second and chlorine at a rate of 0.2 peq per second. Mass spectral data were obtained with a Varian CH-5 mass spectrometer. Samples were first run at a sample probe temperature of 20 "C, analyzer pressure of le7Torr and electron energy of 70 volts; some samples were also run at 18 volts electron energy. All samples were then run at a
2.07 X IO%
METHYL ORANGE p H 2.1 30 %
lu 0 2
+ 2.07pe9 C$
n
iI \ I
Figure 1. Effect of bromine and chlorine on methyl orange spectrum For the methyl orange spectrum (I), peaks A and C are due to the azonium tantomer, while peaks B and D are due to the azoanilinium tantomer
sample probe temperature of 70 "Cto analyze for less volatile components. Procedure. CALIBRATION OF METHYL ORANGEWITH STANDARD SOLUTIONS OF BROMINE AND CHLORINE IN CCl,. For the first method of calibration, 10-p1syringes were used to inject up to 7 p1 of standard bromine or chlorine in CC14 solution into a nitrogen stream being dispersed into 25 ml of methyl orange working solution. All halogen solutions used to 2.8 X 10-4M halogen in CCla. were in the range 2.1 X To determine the response of gaseous mixtures of bromine and chlorine, microliter aliquots of bromine solution and chlorine solution were simultaneously injected into the nitrogen stream with separate syringes. Injection of 20 p1 of blank CClr into the nitrogen stream produced no detectable change in absorbance of the methyl orange solution in the 620 to 310 nm range. To determine the effects of the interhalogen compound BrCl on methyl orange, standard solutions of bromine in CCh and chlorine in CC14 were mixed and irradiated by ultraviolet light to facilitate the formation of equilibrium concentration of BrCl (11). Microliter quantities of the BrCl solution were then injected into the nitrogen stream in the same manner as the bromine and chlorine solutions. The volatilization time of the microliter quantities of CC14 solution used was about 5 seconds with reaction between the halogen and methyl orange occurring immediately thereafter. At a gas flow rate of 1.5 ml/sec, the concentration of halogen in the gas stream being dispersed into the methyl orange solution would be in the range 30 peq/liter to 210 peq/liter for the first method of producing calibration curves. CALIBRATION OF METHYL ORANGE BY COULOMETRIC GENERATION OF BROMINE AND CHLORINE.A Sargent Coulometric Current Source Model IV (current regulation +O.l%) was used to generate bromine and chlorine coulometrically in individual 25-ml generator cells: bromine at a 1.76 cmz Pt anode in 15-ml solution of 0.1M NaBr and 0.1N HzS04; and (11) A. I. Popov and J. J. Mannion, J. Amer. Clzern. SOC.,74, 222 ( 1952). ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
921
0.0
-------
I
-0.05 0
xc,= i.0
0.3
Id
peq
/ :5 H A LOGEN
Figure 2. Determination of mixtures of electrogenerated ha1ogen See text for explanation of straight lines
chlorine at a 2.16 cm2Pt anode in 15-1111 solution of 5M NaCl and 0.02M NaHSOa (pH 1.9). The Pt cathodes were physically protected from, but electrically connected to, the rest of the solution via a porous glass frit. Nitrogen, presaturated with water vapor by being bubbled through a solution of electrolyte, was dispersed into the generator solutions to deaerate the bromine and chlorine into an 11.3 cma common mixing line at a combined gas flow rate of 1.49 cma/sec, resulting in a maximum halogen concentration of 3.5 peq/liter per microequivalent of halogen generated with a mixing time of 7.6 sec. before reaction with the working methyl orange solution. Equilibrium saturation of the generator solutions with halogen was assured by generating bromine and chlorine in excess of that required for saturation of the generator solution and then deaerating the excess to a waste liquid tube. The temperature of the generator cells was maintained at 27.0 f 0.5 "C with a constant temperature water bath. The gas flow rate was maintained constant at 1.49 cma/sec by a nitrogen pressure regulator and by maintaining a backpressure on the system of 1.6 f 0.05 mm Hg above atmospheric pressure. The time to generate 1.0 peq of halogen at equilibrium conditions and to deaerate back to equilibrium saturation was 30 minutes maximum for bromine and 20 minutes for chlorine. The assumption was made that atmospheric pressure would not change significantly over the 30-minute period from equilibrium to equilibrium. According to Lingane (12), 100% current efficiency for the electrogeneration of bromine by oxidation of bromide ion at a platinum anode is easily accomplished, because the potential of the bromine-bromide couple is more than 0.5 volt below the potential at which water is oxidized. However, (12) J. J. Lingane, "Electroanalytical Chemistry," 2nd ed., Interscience, New York, N. Y., 1958, p 536. 922
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
achievement of 100% current efficiency for electrogeneration of chlorine is more complicated because the potential of the chloride-chlorine couple is about the same as that for oxidation of water to oxygen. Since the change in absorbance per equivalent of electrogenerated bromine (from a solution of 0.1N NaBr and 0.1N H2S04 at a current density of 2.74 mA/cm2 at a Pt anode) agreed well with results obtained by injection of standard bromine in CC14solution, the assumption was made that 100% current efficiency was being achieved fpr electrogeneration of br omine. One hundred per cent current efficiency for electrogeneration of chlorine was not so easily achieved. Chloride ion concentration, acid concentration, and current density were varied independently with both carbon and platinum generator anodes to maximize current efficiency for each parameter. Since no combination of parameters gave a consistently higher absorbance change for chlorine than for bromine, a current efficiency of 100% was assumed for chlorine when the methyl orange absorbance change at 505 nm was the same per equivalent of chlorine generated as for bromine. Temperature and pH Effects. Sol10 noted that the absorbance of methyl orange solutions is temperature dependent, but did not exactly specify the dependence. We found 2.07 x l k 5 M methyl orange solutions at pH 2.10 to have a linear temperature coefficient between 10 and 60 "C of -0.0043 A A / T at 505 nm and +0.0004AA/"C at 317 nm, shifting from the azonium form to the azoanilinium form with increasing temperature. Likewise, the tautomeric equilibrium shifts from the azonium form to the azoanilium form with increasing pH (7). Thus temperature and pH must be carefully controlled for precise determinations of bromine and chlorine with methyl orange. For this study, all solutions were pH 2.1 =t 0.1. All spectra were obtained for solution temperatures in the spectrophotometer of 30 f 2 "C. Characterization of the Reaction Products. To determine the ether extractable reaction products, 500-ml solutions of 2.07 X l t 5 Mmethyl orange at pH 2.10 were each allowed to react with 17.5 peq of electrogenerated bromine and chlorine. At a reaction ratio of 2 eq halogen per mole of methyl orange, this would still leave a small excess of unreacted methyl orange. Also, 500-ml solutions each of 2.07 X lk5M N,N-dimethylaniline and 4.2 x lO-5M p-bromo-N,N-dimethylanilinewere allowed to react with 20.7 peq and 42.0 peq bromine, respectively. Immediately after each solution was halogenated, an ultraviolet-visible absorption spectrum was obtained in the region 620 nm to 200 nm. The solution was then saturated with diethyl ether and extracted with at least four 25-ml aliquots of ether. The ether was evaporated to a volume of about 3 ml, any water which separated out was removed, and the remaining solution was evaporated to near dryness just before mass spectrographic analysis. The last few tenths of a milliliter were evaporated in a mass spectrographic gold sample crucible and the resulting residue was analyzed with a Varian CH-5 mass spectrometer. A 50-ml sample of 2.07 X 10-5M methyl orange was sequentially halogenated with five 2.07-peq portions of bromine and a UV-visible spectrum obtained after each halogenation. Similarly, a 50-ml solution of 2.07 X lO+M methyl orange solution was sequentially halogenated with three 2.07-peq portions of chlorine and a UV-visible spectrum obtained after each addition. In each case, after the last halogen addition, portions of the solution were heated at various temperatures for various lengths of time, cooled back to 30 "C, and a UVvisible spectrum was obtained to follow the decomposition of the reaction products. RESULTS AND DISCUSSION Response of Methyl Orange to Bromine and Chlorine Individually. In agreement with the data given by Sollo et nl. ( 2 , 6), our data showed that the change in absorbance of 2.07
x lO-5M methyl orange solutions at 505 nm is nearly the same per equivalent of bromine or chlorine. The Beer’s law plot had a least squares straight line equation of AA505 = -0.034-0.773 (peq Br2) for bromine and AA505 = -0.0710.728 (peq Clz)for chlorine. However, as shown by Figure 1, the absorbance at 317 nm increases for reaction with bromine, 0.239 having a least squares equation AA317 = -0.002 (peq Br2), but decreases with reaction with chlorine, having a least squares equation AA317 = -0.001-0.038 (peq Ch). Beer’s law was obeyed both at 505 nm and 317 nm for simple dilution of the working methyl orange with a solution of acidic KC1 buffer at pH 2.1. Unless the water used was doubly distilled and all glassware carefully kept clean, enhanced absorbance was observed at the 317 absorbance maximum for the methyl orange-chlorine system. In the case of singly distilled water or of intentional use of dirty glassware, the absorbance enhancement in some cases was as large as 0.07 absorbance unit. As a result, this procedure probably could not be used for natural water samples, such as drinking water, for quantitative differentiation of bromine and chlorine. Calculations from Sollo’s data for reaction of aqueous solutions of bromine (2) and chlorine (6) with methyl orange give 0.94 mole bromine/mole methyl orange for bromine in the 0-2 ppm range, 1.09 moles bromine/mole methyl orange for 2-4 pprn bromine range, 1.01 moles chlorine/mole methyl orange for 0-1 ppm chlorine, and 1.05 moles chlorine/mole methyl orange for 1-2 ppm chlorine. Calculations from this study give a reaction ratio of 1.08 moles of bromine per mole of methyl orange and 1.12 moles of chlorine per mole of methyl orange. Thus, Sollo’s work and this study indicate that the ratio of bromine or chlorine reacting with methyl orange is near one mole of halogen per mole of methyl orange when no local excess of halogen is allowed to occur during the reaction. Response of Methyl Orange to Mixtures of Bromine and Chlorine. If it is assumed that bromine and chlorine each react independently with methyl orange, that the reaction products have very small molar absorptivities at 505 nm, but at 317 nm have distinctly different molar absorptivities, and that the concentration weighted absorbances of the reaction products and the residual methyl orange add linearly, then the reaction of a mixture of the two halogens with excess methyl orange should result in an absorbance at 505 nm characteristic of the total halogen present, but in an absorbance at 317 nm intermediate between the bromine reaction product and the chlorine reaction product plus residual methyl orange. Figure 2 is a grid of lines obtained by dividing the absorbance difference at 317 nm noted above into ten equal parts and assigning a mole fraction of bromine ranging from XBr = 1.O for reaction of bromine alone down to X B = ~ 0.0 for reaction of chlorine alone with methyl orange. Using this set of curves based on the above assumptions, the theoretical response of methyl orange to mixtures of bromine and chlorine was compared with the actual response. The total halogen was first determined from the absorbance change at 505 nm and then Figure 2 was entered to obtain the bromine mole fraction. The experimental results are summarized in Table I and are also plotted in Figure 2 for electrogeneration of chlorine and bromine mixtures. As shown by Table I, simultaneous injection of bromine and chlorine in CC14 solutions gave, in general, results for total halogen that were an average of 1 5 % too low. Using the “found” value of total halogen to evaluate the mole fraction of bromine, as would be done in practice where the total halo-
+
Table I. Results of Bromine-Chlorine Determinations Halogen Number peq total generation of halogen method trials “foundiactual 10 0.85 f 0.08 Volatilized ccl4 solutions of Brl and Clz Volatilized 3 0.86 0.05 CC14solutions of BrCl Electrogenerated 12 0.97 =t0.04 BrS and Clz Mean =tsample standard deviation.
Mixture
XBr
afound/actual 1.16 i 0.10 1.43
+ 0.03
1.08 f 0.11
gen would be unknown, gave a value for the mole fraction of bromine that was 16% high. However, if the known total halogen was used to evaluate X B ~then , the average “found” mole fraction of bromine was only 3 % high. Since independent sequential injection of bromine and chlorine solutions, with no possible opportunity for mixing in the gas phase, did not produce the low result in total halogen or high result of X B noted ~ in Table I, it was suspected that formation of the interhalogen compound might be interfering with the reaction. Injection of solution of BrCl in CC14gave an average “found” total halogen error of 14% low but enhanced the error in the “found” bromine mole fraction to 43 % high. If the actual total halogen value was used to enter Figure 2, the results obtained were still an average of 3 4 z high. The rate of bromination of aromatic compounds is known to be faster with BrCl than with Brz (13). BrCl dissociates according to the equation 2 BrCl $ Brz
+ Clz
z
being 43 % dissociated in CCl, solutions at 25 “C (11)and 21 in the gas phase (14). The formation of BrCl from Br2 and CIS in the dark is known to be a heterogeneous bimolecular reaction, but the rate of formation increases in the presence of light because of a photochemical chain reaction (15) with the initial rate of formation varying directly with the square root of the absorbed radiation intensity. Because no exact light intensity measurements were taken, the rate of BrCl formation was not known. However, the initial rate of BrCl formation would vary directly with bromine and chlorine concentrations with the rate decreasing as the BrCl concentration increased (15). Solvation of any undissociated BrCl by water should favor Br+ as the attacking electrophilic particle in a manner similar to solvation of IC1 (14) accounting for the high mole fraction of bromine found when BrCl is known to be present and for higher concentrations of bromine and chlorine. Oxidation of the bromide ion from the bromination reaction by free chlorine to produce free bromine and chloride ion with the free bromine reacting with methyl orange would not be expected to give a low total halogen content, but would give a high bromine mole fraction. Table I also summarizes the results of reaction of mixtures of electrogenerated chlorine and bromine with methyl orange. (13) E. Schulek and K. Burger, Magy. Tud. Akad., Kern. Tud. O u t . Kzzlem., 12,15 (1959); Chem. Abstr., 54,12903~(1960). (14) N. N. Greenwood, Rec. Pure Appl. C/zem., 1,84 (1951). (15) M. I. Christie, R. S . Roy, and B. A. Thrush, Trans. Faraday S O C . , 1139(1959). ~~, ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
923
.f
(
.I -
1
1
!
0
I
c
400
300
500
WAVE L ENG T H nm
600
Figure 3. Comparison of brominated methyl orange spectrum (I) with spectrum predicted by literature reaction (11) I. 50 ml of 2.07 X 10-5M methyl orange plus 2.0 peq of bromine 11. Solution at pH 2.10 with following composition: 1.83 X 10-5Mp-bromo-N,N-dimethylaniline 1.83 X 10-5M p-hydroxybenzene sulfonic acid
0.24 X 10-5M methyl orange
0.~1
A
I- 5 0 r n l . 2 . 0 7 ~ / 0 - ~ M M E T H Y L ORANGE
Figure 4. Effect of exhaustive bromination on methyl orange spectrum The maximum halogen concentrations are on the order of 100 times lower than for volatilization of CC1, solutions of halogen. In this case, the average error between “found” and “actual” total halogen has been reduced to an average 3% low and to 8 high for the bromine mole fraction. The significance of BrCl formation at lower total halogen concentrations could be experimentally tested by providing a larger mixing volume before reaction with methyl orange. However, this would require a significantly longer time to reestablish equilibrium after electrogeneration of the halogens, thus requiring more precise temperature and pressure control and was not attempted in this study. Reaction of Bromine with Methyl Orange. As shown by Figure 1, the addition of up to one mole of bromine per mole of methyl orange results in a large decrease in the absorbance at 505 nm, while the absorbances at 317 nm and 222 nm increase linearly as bromine is added. The absorbance at 277 924
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
nm increases only slightly and between 1 and 2 peq of bromine per mole of methyl orange is not resolved from the peak at 317 nm. Figure 3 compares the spectrum resulting from the reaction of 2.0 peq of bromine with 50 ml of 2.07 X 10-5M methyl orange (Le., 1.035 pmol of methyl orange) with the spectrum of the reaction products predicted by Reaction (Rl) in the introduction. Because none of the products predicted by (Rl) absorb at 505 nm, (Rl) would predict that the absorbance at 505 nm (Curve I, Figure 3) would be due to unreacted residual methyl orange. Curve I1 of Figure 3 was obtained by making up a solution at pH 2.1 containing the concentration of methyl orange corresponding to A505of Curve I, Figure 3. The known concentration of the unreacted methyl orange solution minus the residual methyl orange gave the concentrations of the reaction products predicted by (Rl), which were also added to the residual methyl orange solution. Curve I1 is the spectrum obtained from that solution. Comparison of these two spectra provides conclusive proof that (Rl) is not the reaction occurring under the experimental conditions of this investigation. Repeated extraction with 25-ml aliquots of ether of the 500 ml 2.07 X 10-5M methyl orange (10.35 pmoles) solution that had been allowed to react with 17.5 peq of bromine did not cause a reduction of the peaks at 505 nm or 317 nm that could not be accounted for simply by the dilution factor from the volume increase to 530 ml due to saturation of the solution with ether. The peak at 222 nm was reduced by about 5 in excess of that accountable for because of dilution. By contrast, extraction with three 25-ml aliquots of ether of solutions of N,N-dimethylaniline and p-bromo-N,N-dimethylaniline, which were saturated with ether after bromination with two moles of bromine per mole of substituted aniline, was sufficient to reduce to zero the major absorption peak in the vicinity of 220 nm of these brominated anilines. Taking into consideration the molar absorptivities of N,N-dimethylaniline or of its Cbromo, 2,Cbromo, and 2,4,6-bromo derivatives leads to the conclusion that neither N,N-dimethylaniline or its brominated derivatives are major reaction products for reaction of up to one mole of bromine per mole of methyl orange with methyl orange being kept in excess during the reaction. However, mass spectral analysis of the ether extract residue from the bromine-methyl orange reaction showed that all three brominated derivatives mentioned above do exist as minor reaction products. Figure 4 shows the effects on the UV-visible spectrum of adding in succession up to 5 moles of bromine per mole of methyl orange and of heating the solution after the last bromine addition. For addition of up to one mole of bromine per mole of methyl orange, the linear increase in absorbance at 317 nm strongly suggests ring substitution by bromine rather than cleavage of the azo link for the following reasons: 1. Mono- and disubstituted benzene derivatives do not characteristically have molar absorptivities in the 250 to 600 nm region which are larger than in the 200 to 250 nm region (16). 2. The existence of an absorption band in the vicinity of 320 nm is a characteristic of derivatives of Caminoazobenzene (8). 3. No change in spectrum I of Figure 4 results even after heating 45 minutes at 90 “C, in which case any parasulfonate diazonium ion formed by azo link cleavage (16) H. H. Jaffee and M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy,” John Wiley, New York, N. Y., 1962.
should have decomposed producing a corresponding absorbance decrease at 270 nm (17). 4. The absorbance at 505 nm was not completely reduced to zero for reaction of one mole of bromine per mole of methyl orange. However, this could result from inaccuracy in methyl orange solution concentration, less than 100% current efficiency for bromine generation, or some interfering side reaction. If ring bromination rather than azo link cleavage is occurring, then the most logical place for addition of the first bromine is ortho to the N,N-dimethylamino group, because the protonated azo link and the sulfonate group are both deactivating meta directors for aromatic electrophilic substitution, while the N,N-dimethylamino group is a strongly activating ortho, para director. Bromination in position ortho to the N,N-dimethylamino group should result in a large decrease in the molar absorptivity at 505 nm and an increase at 317 nm [compare with fluorinated derivatives in reference (S)]. Addition of a second mole of bromine per mole of methyl orange must result in bromination of another available position on one of the two rings and, to a lesser extent, in azo link cleavage. These suppositions are supported by the decrease in absorbance at 317 nm and emergence of a new peak at 270 nm. During the second and third bromine additions (I1 and 111, Figure 4), the solution is light green indicating that a chromaphoric system is still present. The green color disappears during the fourth and fifth bromine additions (IV and V, Figure 4). During the third, fourth, and Mth bromine additions, the peak at 270 nm increases in intensity, while those at 317 nm and 410 nm are reduced practically to zero. The smell of bromine being deaerated from the reaction solution near the end of the fifth bromine addition indicated that not all of the last bromine addition reacted with the solution. The reaction product causing the peak at 270 nm is definitely unstable to heating. After the last bromine addition, heating of the solution 15 minutes at 90 OC results in a change in log E at 270 nm of 4.15, which is in good agreement with the literature value of log E = 4.10 at 270 nm for psulfonate diazonium ion given by Kortum (17). The agreement of the molar absorptivities, the wavelength of absorption, and the instability to heat strongly support the conclusion that the reaction product responsible for the band at 270 nm is due to the psulfonate diazonium ion or, less likely, because of the strongly deactivating nature of the sulfonate and diazonium groups, to a brominated derivative of psulfonate diazonium ion. The increase in absorbance of the solution in the vicinity of 220 nm and the residual absorbance in the vicinity of 270 nm is due to the aromatic decomposition products of the substituted diazonium ion. Reaction of Chlorine with Methyl Orange. As shown by Figure 1, addition of up to one mole of chlorine per mole of methyl orange results in a decrease in absorbance at 505 nm similar to the bromine-methyl orange reaction. However, instead of an increase in absorbance at 317 nm and no change in absorbance at 270 nm, there is an absorbance decrease at 317 nm and an increase at 270 nm. Repeated extraction with 25-ml aliquots of ether of 500 ml of 2.07 X 10-5Mmethyl orange that had been reacted with 20 peq of chlorine did not cause a reduction of the peaks at 505 nm, 317 nm, or 270 nm that could not be accounted for by the volume increase to 530 ml due to saturation of the solution with ether. However, the peak at 222 nm was reduced by 35% in excess of that accounted for by dilution, indicating ~~
(17) G . Kortum,Z.Phys. Ckrrn. 8,50,361 (1941).
,,t
I- 50 &I. 2.07 x I\
M E T H Y L ORANGE-
+ 2.07peq c$
a,lyo
AF T€:d€AT/NG5iI A T 9o°C
0200
MIN. 6b0
WAVELENGTH nrn
Figure 5. Effect of exhaustive chlorination on methyl orange spectrum
that a significant portion of the reaction products were ether soluble. Mass spectral analysis showed the chlorine-methyl orange reaction ether extract residue to be a mixture of 2chloro-N,N-dimethylanilineand 2,4-dichloro-N,N-dimethylaniline, but did not include the 2,4,6-trisubstituted derivative. The reaction product responsible for the peak at 270 nm is unstable when heated, again indicating the p-sulfonate diazonium ion. Figure 5 shows the results of adding one, two, and three moles of chlorine per mole of methyl orange and of heating the solution after the last chlorine addition. The smell of chlorine being deaerated from the reaction solution near the end of the third chlorine addition indicated that not all of the last chlorine addition reacted with the solution. The change in log E at 270 nm after heating at 90 OC for 15 minutes is 4.12, which again is in good agreement with the literature value of 4.10 (17)for thep-sulfonate diazonium ion. CONCLUSIO~IS
The results of this study indicate that the reaction of up to one mole of bromine or chlorine per mole of methyl orange kept in excess involves competition between aromatic ring substitution and azo link cleavage as depicted by the following (X indicates Br or Cl):
H+-O,S
Q Q
1 \ N=N 1 \ N(CHJ~+ H+XX
The reaction of bromine with methyl orange occurs greater than 95% by Reaction (R3) as evidenced by the very small change in the UV-visible spectrum with ether extraction, the ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
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stability of the reaction products and the results of continued bromination. That Reaction (R4) does occur as a minor reaction is evidenced by the fragmentation patterns of mono-, di-, and tribromosubstituted N,N-dimethylanilines found by mass spectral analysis of the ether extract residue. Reaction (R4) for bromine can be “forced” upon the system to an appreciable extent only by addition of more than two moles of bromine per mole of methyl orange. The reaction of chlorine with methyl orange occurs about 70W by Reaction (R4) and about 30% by Reaction (R3) as estimated from the ratio of the p-sulfonate diazonium ion concentrations obtained for reaction of one and three moles of chlorine per mole of methyl orange, respectively. Both reactions (R3) and (R4) would be expected to result in a large decrease in absorbance at 505 nm; reaction (R3) because of steric hindrance preventing the dimethylamino group from assuming a coplaner configuration with the aromatic ring to which it is attached (2,9), and reaction (R4) because of destruction of the chromaphoric system due to azo link cleavage. Since the reaction products can themselves be halogenated, methyl orange should always be kept in excess when the decrease in absorbance at 505 nm is used for analytical determination of bromine and chlorine, otherwise, an erroneously low value of total halogen content will be obtained. This explains
why the manner of addition of halogen to working solutions is of such importance for accurate determination of true halogen content. By considering the total absorption spectrum in the region 200 nm to 600 nm, micromolar quantities of bromine and chlorine can be analytically differentiated from each other, provided that reaction conditions are closely controlled, namely, if very clean glassware is used throughout, if doubly distilled water is used throughout, if the concentrations of bromine and chlorine are low enough (or are in aqueous solution prior to reaction with methyl orange) to make the formation of BrCl unfavorable, and if the pH and temperature are held constant and are the same for which the calibration curves were produced. ACKNOWLEDGMENT
We wish to thank Jack Chang of Eastman Kodak for supplying thep-bromo-N,N-dimethylanilineand phydroxybenzenesulfonic acid sodium salt used in this investigation and for helpful suggestions for characterization of the reaction products. RECEIVED for review October 20, 1971. Accepted January 6, 1972. This investigation was supported by fellowship AP 48,933-01, Air Pollution Control Office, Environmental Protection Agency.
Determination of Alkylbenzenesulfonate and Alkylsulfate Homologs, after Electrophoretic Separation Using Aqueous Dioxane Agarose Gels John R . Bodenmiller and Howard W. Latz Department of Chemistry, Ohio Uniuersity, Athens, Ohio 45701 Alkylbenzenesulfonate and alkylsulfate homologs were quantitatively determined after separation by electrophoresis. The electrophoresis was carried out using aqueous dioxane/agarose gels in a cell which permitted direct contact between the gel and electrolyte. The gel strip containing the separated component was removed from the cell. The sample was extracted from the gel with water and was quantitatively determined by the methylene blue method of analysis. There were no significant differences in the amount of sample recovered from the gel after runs of 5.0 minutes, 1.0 hour, and 2.0 hours for 14 pg of pure p-3-124. The amount of recovery was 88.4% with a relative standard deviation of 0.6%. The relative standard deviation for direct methylene blue analysis of the same comWhen the gel strip was accurately pound was 1.4%. selected, values within 2.0% of the correct value were obtained.
THEPROGRESS MADE by various workers in their attempts to analyze sulfonic acids was described by Siggia and Whitlock ( I ) . A pyrolytic gas chromatographic method, for the determination of arylsulfonic acids and salts, based on the measurement of sulfur dioxide or parent hydrocarbon was recently developed by the stime authors ( I ) . This method gives ac( I ) S. Siggia and L. R. Whitlock, ANAL,CHEM., 42, 1719 (1970). 926
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curate results for the determination of total sulfonate. However, this method cannot be used to determine the concentration of homologs in alkylbenzensulfonate mixtures because the alkyl group undergoes pyrolysis. The pyrolytic gas chromatographic method introduced by Lew (2) for the determination of homologs and isomer distribution of various anionic surfactants gives quantitative results within +5%. While this method is quite versatile and analysis time relatively short, it is certainly not applicable to all situations. The same can be said of the gas chromatographic method recently developed by Puchalsky (3) for determintion of alkylsulfates. Bodenmiller and Latz ( 4 ) have shown that anionic surfactant homologs can be separated for qualitative identification by electrophoresis using aqueous dioxane agarose gels. The present paper describes a method for the quantitative determination of alkylsulfate and alkylbenzenesulfonate homologs based on their electrophoretic separation with an improved cell and subsequent analysis with methylene blue. No prior treatment of the sample is necessary because nonionic impuri~.
(2) H. L. Lew, J. Amer. Oil Cliem. SOC.,44, 359 (1967). (3) C. B. Puchalsky, ANAL.CHEW,42, 803 (1970). (4) J. R. Bodenmiller and H. W. Latz, ibid., 43, 1354 (1971).
