(3) Box, G. E. P., Hunter, J. S., Biometrika 41, 190 (1954). (4) Campbell, W. J., Brown, J. D., ANAL. CHEM.36, 312R (1964). ( 5 ) Davies, 0. L., ed., “Design and Analysis of Industrial Experiments,” 2nd ed., p. 247, Hafner Publishing Co., New York, 1956. (6) Ibid., p. 440. ( 7 ) Eisenhart, C., Ann. Math. Statist. 10, 162 (1939). (8) Kramer, C. Y., Ind. Quality Control 13, 8 (1957). (9) Lamborn, R. E., Sorenson, F. J., Advan. X-Ray Anal. 6 , 422 (1963).
(10) Lucas-Tooth, J., Pyne, C., Ibid., 7, 523 (1964). (11) Mitchell, B. J., “Encyclopedia of Spectroscopy,” p. 736, Reinhold, New 1960. York. ~.~~~ (12) Mitchell, B. J., “Encyclopedia of X-Rays and Gamma Rays,” G. L. Clark, ed., p. 531, Reinhold New York, 1963. (13) Myers, R. H., Alley, B. J., “Use of Regression Analysis for Correcting Matrix Effects in the X-Ray Fluorescence Analyses of Pyrotechnic Compositions,” Proceedings of Tenth Annual Conference on the Design of Experiments, 1964.
(14) Ostle, B., “Statistics in Research,” 2nd ed., p. 159, Iowa State University Press. Ames. 1963. (15) Sugimotd, AI., Bunseki-Kagaku 11, 1168 (1962); C.A. 58, 30769 (1963). (16) Sugimoto, M., Bunseki-Kagaku 12, 475 (1963); C.A. 59, 12152h (1963). (17) Williams, E. J.. “Regression Analvsis,” D. 162, Wilev “ , New York, 1959. (18jZbid.,-p. 166. (19) Youden. W. J.. “Statistical Methods y, New York, IYOl.
RECEIVEDfor review May 20, 1965. Accepted September 15, 1965.
Spectrophotometric Determination of Bromine and Hydrogen Bromide E. C. CREITZ Fire Research Station, National Bureau o f Standards, Washington, D.
b A method is described for determination of Brz and HBr in concentrations ranging to 5 pg. of bromine per rnl. The analysis depends on oxidation of o-tolidine by Brz and subsequent colorimetric determination of the yellow oxidation product. Total bromine was determined, after reduction of Brz, by measurement of the optical absorption of AgBr suspensions, and HBr was obtained by difference. An overall reproducibility of about 0.002 absorbance units (approximately 0.07 pg. per rnl.) was obtained. Some oxidizing or reducing substances may produce interferences, as well as other halogens and S-’.
C.
powders as supports for perfluoro-oils and waxes as recommended by Lysyj and h’ewton (6) produced columns lacking in resolution. The use of conventional bubblers for absorption of HBr and Br2 in water is feasible if the concentration of the resulting solution can be kept low enough so that hydrolysis will help prevent loss of the absorbed compounds. Conventional titration methods (4) would, however, be impractical a t these low concentrations. The sensitivity of colorimetric methods and the availability of a spectrophotometer suggested the approach to be described. EXPERIMENTAL
S
of combustion products of flames burning mixtures of fuel, air or oxygen, and small quantities of brominated compounds required a sensitive method for the determination of 51’2 and HBr. Gas chromatographic methods, which have proved satisfactory for Clz and HC1 in combustion products ( 3 ) , have not been usable for the corresponding bromine compounds. Adsorption with incomplete elution, reaction with the column packing, and poor resolution are characteristic of the columns tried for separating Br2 and HBr from the other combustion products. I n the case of HBr, formation of the mono- and dihydrates add to the difficulties. While dry HBr could be separated with some degree of satisfaction, it was expected that only the hydrates would be found in combustion products containing water vapor. Attempts to dehydrate them invariably resulted in some free Brg. Our lack of success with chromatographic procedures was confirmed by Fish (a). The use of perfluorocarbon molding TUDIES
1690 *
ANALYTICAL CHEMISTRY
Method. It is generally believed t h a t Br2 hydrolyzes, in dilute solution, according t o the equation: Br2
+ HzO ,-
+
HBr
H+
HBrO
it H + +it BrO-
+ Br-
(1) 0.100
0075
4
o
o
i
i
i
J
o
o
WAVELENGTH, rnp
Figure 1. Absorbance of a silver bromide suspension as a function of wavelength
followed by partial or complete ionization of the respective acids. The analyst is thereby provided with approaches based on the chemical properties of HBr and on the different chemical properties of HBrO. I n the absence of other ionizable bromine-containing compounds, either approach is adequate for the determination of Br2. However, in the presence of such compounds (which would add to the Br- concentration), only the properties of HBrO may be used for the determination of Br2. One may obtain a relationship between analyses based on the two approaches, so that, if additional Brion is present, its concentration may be determined by difference in the results obtained by the two approaches, one based on Br-, the other based on BrO-. The relationship between the two analytical approaches must be determined on solutions containing Br2 alone. However, very dilute Brz solutions of accurately-known concentrations cannot be prepared directly, but require standardization against a solution containing a known concentration of Brion. Potassium bromide was used for this purpose. Total bromine from such a dilute Br2 solution was determined by reduction of HBrO followed by addition of XgNO3-HKO8 solution and turbidimetric measurement of the resulting AgBr suspensions. The Brz was also determined colorimetrically by measurement of the transmittance of the yellow oxidation product of o-tolidine (4,4’diamino-3,3‘ - dimethylbiphenyl). I n acid solution, its absorbance maximum occurs a t a wavelength of 432 mp. A calibration curve for total bromine was produced by using a solution of KBr of k n o m concentration. These data were then used t o relate the two analytical methods and to standardize them. The absorbance of AgBr suspensions is related to both \vavelength and par-
0.15
I
I
50 100 TIME, MINUTES
1
I50
Figure 2. Effect of concentration and aging on absorbance of silver bromide suspensions Final volume 25 ml. in each case
MILLILITERS Bn SOLUTION
Figure 3. Absorbance as a function of bromine concentration for 1 hour aging Data obtained from Figure 2
tide size, the latter being related through growth rate to the Br- ion concentration from which it is precipitated. The absorbance of such a suspension as a function of wavelength is shown in Figure 1. A twofold gain in sensitivity can be obtained by making measurements at the shorter wavelengths. However, a certain amount of darkening occurs in silver bromide exposed to light, and it occurs more rapidly a t the shorter wavelengths. Analytical measurements were made a t 500 mg in spite of the reduced sensitivity, and no trouble with darkening in the spectrometer was experienced. Particle size of a silver bromide suspension depends on time and growth rate. Growth rate is a function of concentration of Br- ion in the solution from which the precipitate was formed. The data of Figure 2 were obtained by measuring the absorbance, as a function of time, of AgBr suspensions formed from aliquots of a dilute Br2 solution with reducing agent present. The suspensions were aged in the dark. A plot of the absorbance for 1 hour of aging is shown in Figure 3. Similar plots for other times of aging gave less satisfactory results. .4ccordingly, all XgBr suspensions were aged for 1 hour in the dark prior to measurement. Sourisseau ( 7 ) has shown that the addition of AgN03 to Br2 solutions leaves HBrO in solution. At the low concentrations encountered in these analyses, it was found that the decomposition of HBrO and its conversion to HBr was quite slow. To realize the full yield of AgBr in a reasonable length of time, i t was necessary to add a small amount of a reducing agent, Sodium sulfite was used for this purpose, Only dilute solutions of NazSOs could be used because silver sulfite is only moderately soluble and more concentrated solutions caused it to be precipitated along with AgBr. The addition of nitric acid helped to keep it in solution. With oxidizing substances, including aqueous solutions of bromine, o-tolidine reacts to give a yellow oxidation product ( I , 8 ) . The yellow oxidation product
is also a pH indicator, remaining yellow only below pH 3, turning to green a t pH 4, blue a t pH 5, and becoming colorless a t pH 6 and above. An absorption spectrum, obtained with a recording spectrophotometer from a solution containing Brz to which had been added a small amount of saturated o-tolidine solution in 3N HzSO4 indicated a single absorption in the region between 350 and 650 mp. The maximum absorption occurs a t 432 mp. All absorbance measurements were, therefore, made a t this instrumental setting. I n water solutions of Brz, the resulting oxidant decomposed to HBr a t varying rates. A rough kinetic determination showed that the predominant decomposition reaction was of zero order (not concentration dependent) and was traced to photolysis by radiation from the fluorescent room-lights. The use of subdued illumination from incandescent lamps reduced the rate of decomposition to a concentration dependent, but usable, level. (Part of the remaining decomposition was later attributed to the use of a softglass volumetric flask.) It was still necessary, however, to make several determinations a t selected times (usually 1 hour apart) and extrapolate back to the time of preparation of the Br2 solution t o obtain an accurate value for the initial concentration of oxidant. The presence of added HBr appeared to have little, if any, effect on the rate of decomposition of the oxidant. I n addition, the yellow color of the oxidized o-tolidine solution was found to be fugitive and also required a backward extrapolation to time of preparation. The order of this decomposition reaction was not determined but is known to be concentration dependent. Absorbances were conveniently measured 5 minutes after preparation and a t additional %minute intervals sufficient for the backward extrapolation to time of preparation. Procedure. Total bromine : ,4 calibration curve was prepared using
several aliquots of a stock solution of KBr (ACS, 99.0%) containing 18.6 mg. per liter. Aliquots were added to 25-ml. volumetric flasks to each of which was added 1 ml. of Na2S03 solution (1 gram/100 ml.). After mixing and diluting to approximately 20 ml., 1 ml. of solution (1 gram/100 ml. of 1X HNOa) was added to each. The solutions were made up to 25 ml. and aged 1 hour before measurement of the absorbance a t 500 mM. h plot of these data is shown in Figure 4. To establish their concentrations, similar measurements of total bromine were made on 20-ml. aliquots of a series of Brz solutions which contained up to 10 drops of saturated (room temp.) bromine-water per liter. Aliquots of these same Bra solutions were used for measurement of the starting concentration of oxidant by the double extrapolation as described below, thus giving the required data for standardizing and relating the two analytical approaches. Oxidant (HBrO) was determined on each of the above series of Br2 solutions as follows: 5 ml. of ethyl alcohol and 1 ml. of a saturated solution of o-tolidine in 3 N H2S04were placed into a 25-ml. volumetric flask. (The addition of alcohol improved the reproducibility. I t appeared that the oxidized o-tolidine was on the verge of being insoluble in solutions without alcohol.) A 5-ml. aliquot of the given Brz solution was added and the mixture immediately made up to 25 ml. Absorbances were measured on duplicate samples of the mixture at 5 , 10, 15, and 20 minutes after preparation. The averaged values of absorbance were plotted as a function of time and the curve extrapolated back to the time of preparation of the o-tolidine Brz mixture. By repeating this procedure at hourly intervals (usually 4) after preparing a Brz solution, a curve showing the change in oxidant concentration with time was obtained and extrapolated back to find the initial oxidant concentration (in terms of absorbance) at the time of preparation of the Brz solution. VOL. 37, NO. 13, DECEMBER 1965
1691
1
OO
0.I
0.2
0.3
0.5
0.4
0.6
A B S O R B A N C E OF o-TOLIDINE O X l O A T l O N P R O D U C T
Figure 5. Curve relating total bromine, as measured by AgBr absorption (20-ml. aliquot diluted to 25 ml.) to oxidant, as measured by absorbance of o-tolidine oxidation product (5-ml. aliquot diluted to 25 ml.) for solutions of Brz in water MICROGRAMS & - I O N
P E R MILLILITER
Figure 4. Absorbance of silver bromide suspensions as a function of concentration for KBr solutions of known concentration
RESULTS AND DISCUSSION
The calibration curve of Figure 4 shows a small amount of curvature, whereas Figure 3, which is plotted from essentially the same kind of data, is a straight line. No reason can be given for the difference in behavior, unless perhaps there was a residual, uncompensated dependence of absorbance on aging time. The curve of Figure 4 does not pass through zero a t the
MICROGRAMS OF Br PER MILLILITER
Figure 6. Relation between the absorbance of the oxidation product of o-tolidine and the (atomic) bromine concentration Data obtained from Figurer 4 and 5
1692
ANALYTICAL CHEMISTRY
abscissa, which indicates that probably some AgBr particles were too small to scatter effectively a t 500 mp. The same reasoning applies to Figure 3. The solubility of AgBr a t 20’ C. is given by Hillebrand and Lundell (6) as 0.107 rng./liter which can account for about a third of the unmeasured AgBr. The points on Figure 4 are of single (not replicate) samples and give an estimated reproducibility of about 0.001 absorbance unit (about 0.035 mg./ml.). The accuracy of weighing for the preparation of the KBr for the solution of known concentration was about 10.25%. The colorimetric sensitivity for the determination of the o-tolidine oxidation product was considerably higher than that for the determination of AgBr suspensions. This was partially compensated for by using 20-ml. aliquots of the dilute Brz solution diluted to 25 ml. for measurement of the absorption of AgBr suspensions and a 5-ml. aliquot diluted to 25 ml. for measurement of the absorbance of the o-tolidine solutions. Data for solutions of these respective dilutions are shown in Figure 5, which indicates that the overall sensitivity for the o-tolidine oxidation product was some 20 times that for the AgBr suspensions. The abscissas of Figures 4 and 5 were related through their respective ordinates to produce Figure 6, which relates the absorbance of the oxidation product of o-tolidine to the bromine concentration. If analytical results are to be reported in terms of ratios of Brz to total bromine, the required information may be obtained directly from the ordinate values of Figure 5, proper allowance being made for a zero correction and dilution factors. It is expected that difficulties in the complete recovery of bromine compounds from the products of combustion
of inhibited flames will make it preferable to report ratios, rather than absolute concentrations. It has been assumed that in saturated bromine-water the equilibrium shown in Equation 1 is far to the left. It has also been assumed that in the very dilute solutions the equilibrium is far to the right. Some evidence to substantiate the latter assumption is the observation that the rate of loss of bromine from a dilute solution stored in a volumetric flask, which was opened periodically for removal of samples, amounted to about 1% per 24 hours. No information has been obtained on interferences which depend primarily on the system sampled. However, since o-tolidine is an oxidation indicator, interferences by oxidizing and reducing compounds will have to be determined as the composition of the system indicates. Compounds producing insoluble silver salts will also interfere. LITERATURE CITED
(1) Clark, W. M.,, et al., “Studies on
Oxidation-Reduction” series, Hyg. Lab., Treas. Dept., U. S. Public Health Service, 1928-1931. (2) Fish, A., Imperial College, London, private communication, 1964. (3) Fish, A., Franklin, N. H., Pollard, R. T., J. Appl. Chem. (London) 13, 506 (1963). (4) Hashmi, A I . H., Ayaz, A. A., ANAL..
CHEM.35, 908 (1963).
(5) Hillebrand, W. F., Lundell, G. E. F.,
“Applied Inorganic Analysis,” 2nd ed., revised by Lundell, G. E. F., Br,ight, H. A., Hoffman, J . I., p. 734, Wiley, New York, 1953. (6) Lysyj, Ihor, Newton, Peter R., ANAL. CHEM.35, 90 (1963).
(7) Sourisseau, G., Ann. Chim. (Paris)8 ,
349 (1953).
(8) Tomicek, O . , “Chemical Indicators,” A. R. Weir, tr., Butterworths, London,
1951. RECEIVEDfor review June 4, Accepted September 13, 1966.
1965.
