chloric acid. The emission intensity of aluminum monoxide at 4867 A was then measured as a function of height in the flame, both in the solution containing aluminum and in the solution containing aluminum and titanium. The aluminum monoxide emission intensity was corrected for background from the hydrochloric acid and the considerable emission from titanium at the aluminum monoxide bandhead. In a similar manner the emission intensity of titanium monoxide was measured at 5004 A in the solution containing only titanium and in the solution containing both titanium and aluminum. The results of this study are shown in Figure 7. The titanium monoxide emission remains unaltered by the presence of aluminum. However, the aluminum monoxide emission intensity does show a definite decrease in the presence of titanium at heights between 0 and 12 rnm above the burner. It can be concluded that competition for oxygen may account for some of the enhancement effects observed between refractory elements. The competition probably occurs in the solid or liquid phase before salt evaporation is complete. In this state the effective concentration of both concomitant and analyte would be greatly increased, thus increasing the likelihood of reaction.
variables, which probably account for many of the discrepancies in interference studies reported in the literature. When constant flame conditions are used, the most probable explanation for the interferences reported in this study is the difference in volatility of the analyte when accompanied by other metal species, most concomitants increasing the volatility of the analyte. It is important to note that the volatility of the matrix in terms of boiling point and heat of vaporization is not necessarily of prime importance, but that the volatility of analyte is in some manner increased or decreased in the presence of concomitants. The evaporation rate of small salt particles in the flame is a complex function of many factors including drop size, diffusion coefficient of the evaporating species, surface tension, and heat transfer characteristics. Many interferences can be reduced or eliminated by the proper selection of flame conditions and salt concentration. A more descriptive study of atomization processes is the subject of a later paper. ACKNOWLEDGMENT
CONCLUSIONS
The authors thank J. G . Tschinkel for the many helpful discussions and suggestions. The authors also thank 0. H. Kriege for his critical reading of this manuscript.
These studies suggest the complexity of cation interference effects. The magnitude, and in some cases the direction of interferences, is dependent upon a variety of experimental
RECEIVED for review March 26, 1970. Accepted May 18, 1970.
Flame Emission Method for Determining Heats of Combustion of Selected Compounds J. J. Kroeten,’ H. W. Moody, and M. L. Parsons2 Department of Chemistry, Arizona State University, Tempe, Arizona 85281
A new method for determining heats of combustion of organic compounds was found using flame emission spectroscopy. Solutions of alcohols, carboxylic acids, and amines in methanol were introduced into an entrained air-hydrogen or oxy-hydrogen flame. The most intense CH emission bandhead at 431.5 nm was measured and a computer was used to correlate the data. A linear response for emission intensity vs. the heat of combustion was found for compounds in a homologous series. The slopes were the same for compounds containing the same functional group. The slopes for solutions containing different functional groups were quite different. When comparing the experimental values to the literature values, an average agreement of 3.6% was found. The standard deviations of the data were obtained from the calculated curves and found to be 3.3, 23.3, and 48.3 kcal for the amines, alcohols, and acids, respectively. The time required to complete the entire procedure i s about 1 hour.
tion (2) and the use of additivity rules (2). Some of the calorimetric approaches are the use of rotating bomb calorimeters (3), bomb calorimeters (4, and flame calorimeters (5). The precision of these techniques is usually around 0.5 to 1.5 %. These methods have several disadvantages, such as the need for expensive equipment, the length of time needed to obtain suitable results, e.g., some take 24 hours or longer (3), and the inability to distinguish between some geometrical isomers (2). The flame spectroscopy method described in this paper appears to be a fast and simple way of determining heats of combustion. The technique described here is empirical; however, there is a physical basis for the phenomenon. It is known that the emission intensity from atoms and molecules, e.g., the CH molecule, produced from the combustion of species in the hot flame gases is directly proportional to the number of these molecules existing per unit volume of hot
BASICALLY, there are only two general techniques for determining the heat of combustion of organic molecules-theoretical calculations and experimental calorimetric methods. The theoretical approaches include the concept of correlating the displacement of valence electrons to the heat of combus-
(1) S. Morris, Kharasch, and Ben Sher, J. Phys. Chem., 29, 625658 (1925). (2) S. W. Benson and J. H. Buss, J. Chem. Phys., 29, 546-572 (1958). (3) W. D. Good, D. W. Scott, and G. Waddington, J. Phys. Chem., 60, 1080-89 (1956). (4) F. D. Rossini and E. J. Prosen, J. Res. Nut. Bur. Stand., 33, 255 (1944). (5) G. Pilcher, H. A. Skinner, A. S . Pell, and A. E. Pope, Trans. Faraday SOC.,59, 316330 (1963).
Present address Syntex Corp., Pharmaceutical Analytical Department, Stanford Industrial Park, Palo Alto, Calif. To whom all correspondence should be addressed. 1040
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
Table I. Operating Conditions for Flame Emission Measurements Parameter Value 0 . 5 ml/min Sample solution flow rate Monochromator slit width 0.250 mm Monochromator slit height 2.0 cm Photomultiplier tube voltage 6ooV Wavelength setting (CH band-head) 431.5nm Grating blaze 6000'4 Entrained air-hydrogen flame 3 .O l./min Hydrogen flow rate Dc electrometer setting (full scale) 0 . 5 x 10-7 to 0.5 X
Oxy-hydrogen flame Hydrogen flow rate Oxygen flow rate Dc electrometer setting
Figure 1. Schematic diagram of experimental apparatus A. Hz B. Regulators C. Needle valve D. Rotameter E. Burner F. Flame G . Monochromator cf/3.6)
A
3 . 5 l./min 1 .O I./min 1 x 10-6 to 1 x 10-7~
flame gases (6). This has been supported experimentally in specific cases by showing that sample concentration was linear with emission intensity (7). This has also been confirmed by the authors using the experimental apparatus described below, The authors believe that the number of these molecules is a function of the bonding energy of the parent species as well as the collisional processes in the hot flame gases. Thus, the intensity measurement should be an indication of the heat of combustion or any other bonding parameter. EXPERIMENTAL Apparatus. A simple flame photometer setup (see Figure 1) was used. The optics consisted of a '/(-meter Jarrel-Ash monochromator (Model No. 26-780) with an RCA IP28 photomultiplier tube. The hydrogen-air flame used with the amines and alcohols was produced by introducing hydrogen gas in the oxygen port of a medium bore Beckman burner (Model No. 4020). An oxy-hydrogen flame was used in conjunction with a Hecto (Model V-10-S) total consumption atomizer burner for the carboxylic acid data. The burner was positioned such that the nozzle tip was level with the bottom of the monochromator slit. The optical arrangement produced a resolution of about 0.8 nm. The fuel flow rate was controlled by a five-stage regulation system. The sample solutions were force-fed into the burner by means of 10-ml glass syringes which were controlled by a variable speed Sage pump (Model No. 255-1). The samples were injected through a 19-gauge needle into a polyethylene tube which connected the burner capillary to the syringe needle. The output of the photomultiplier tube was measured by a dc electrometer (Jarrel-Ash, Model No. 26-780) and displayed on a Varian strip chart recorder (Model No. G-2000) with a Disc Integrator (Model No. 244). The operating conditions for the system are as given in Table I. Procedure. PREPARATION OF SAMPLES. Only the highest grade commercially available chemicals were used in conjunction with the air-hydrogen flame. No effort at further purification of the alcohols or acids was attempted. Almost all of the amines were purified just prior to their use because of the air oxidation that often occurs. They were distilled from KOH in a nitrogen atmosphere. Only the middle fraction of the distillate was stored under a nitrogen atmosphere in order to reduce oxidation. It was found by gas
H. I. J. K. L. M.
Phototube Power supply/electrometer Strip chart recorder Syringe of sample solution Variable speed pump Slit height, 2 cm
chromatographic analysis (Carle, Model No. 6500) that all the amines thus treated were at least 98% pure except for isopropylamine and isopentylamine which were about 97 % pure. All samples were prepared as 2, 5, or 10 mole % solutions in methanol. Mole per cent solutions were prepared so that equal numbers of molecules would be introduced into the flame per unit time; thus a molar response was obtained. SAMPLING.It was statistically determined (8) that the glass syringes used were matched within experimental error ; therefore, a separate syringe was used for each sample. The syringe and burner capillary tube were connected together by a polyethylene tube and a 19-gauge needle. A single needle was used in conjunction with all the syringes. The syringe was placed in the Sage pump which controlled the sample flow rate. This method produced the steady flow of sample necessary to ensure a molar response regardless of the sample's density and viscosity. Each solution was sampled for 2 minutes. After every 2 sample solutions, a run was made with the methanol blank to correct for drift during the time the measurements were taken and to provide intensity data from which the emission due to methanol was subtracted to obtain the emission due to the sample. Four to ten readings were taken for each solution. MEASUREMENTS. A spectrum (300 nm to 800 nm) was taken of the sample (see Figure 2). The most intense C H band-head (431.5 nm) was determined to be the best indicator of the heat of combustion. The monochromator was then manually adjusted to that particular wavelength. The sensitivity adjusted to give the maximum useful signal. The readings were obtained by means of a Disc Integrator whereby the noise signal could be averaged out. Reproducibility is affected by binding of the syringe plunger; consequently, the syringe plunger should be tested for signs of binding before the start of a run. If the plunger does not rotate freely, the clamps holding the syringe should be reset. As the experimental conditions are critical, care must be taken to assure that all parameters are reproduced each time measurements are made. It is suggested that the signal from a standard solution of high purity and known heat of combustion be used in setting the parameters from day to day; if this is done there is no danger of obtaining inconsistant results. Repeatability problems were not encountered when making measurements during the same run; however, it is best to let the flame burn for 15 to 20 minutes so that the characteristics of the burner are stabilized before making measurements.
(6) J. D. Winefordner and T. J. Vickers, ANAL.CHEM., 36, 1939-
1946 (1964). (7) P. F. McCrea and T. S. Light, ibid.,39, 1731-1736 (1967).
(8) M. L. Parsons, Anal. Lett., 2,229-237 (1969). ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
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I
OH
100
400
500
300
400
500
Figure 2. Spectra A . Typical spectrum of an alcohol or acid in nm B. Typical spectrum of an amine in nm
Table 11. Data for Compounds Measured Standard Intercept Slope error Compound (0) (b) of slope Correlatype kcal kcal + kcal tion. 0.7 0.999 Amines 295.2 49.4 0.4 0.996 10.9 Alcohols 226.1 11.3 0.994 298.5 Acids 164.8 a A perfect correlation is 1.000.
RESULTS AND DISCUSSION
Intensity data were taken for representative alcohols, amines, and carboxylic acids. Measurements of the various concentrations used were normalized to 5 mole % solutions. This was possible because of the linear nature of the sample concentration with emission intensity. These intensity data were correlated with known heats of combustion by means of a linear regression/statistical package computer program. A linear relationship was found t o exist differing only in intercept for different homologous series within each group of compounds. The statistical output data of the computer calculations for the normal series are given in Table 11. The compounds in each homologous series tested have the same slope within experimental error. The positions of these series are slightly displaced from one another. Therefore, all the amines and alcohols used were normalized t o the normal amines and alcohols, respectively. The normal series of compounds can be calculated by the following equation : AH,
= a
+ bIcH
AH,,
=
AH, - b(Ik - I,)
(2)
where A H , = known heat of combustion in kcal; AHu = unknown heat of combustion in kcal; = relative CH emission intensity of reference compound; I , = relative CH emission intensity of unknown compound; and b = slope in kcal. The results calculated with the use of these equations can be seen in Table 111. There are several compounds listed for which no literature value was obtained. There was no attempt a t parameter optimization in the study ; therefore, the reproducibility of observations made was probably not so good as could be expected. Reproducibility could be improved by optimizing the signal-to-noise ratio as suggested by Parsons and Winefordner (9). The precision of the intensity measurements in the entrained air- and oxy-hydrogen flames were calculated from the difference of the data points from the calculated curves, and standard deviations of 13.3,23.3, and 48.3 kcals were obtained for the amines, alcohols, and acids, respectively. The experimental values compared with the literature (IO) values within a n average range of 3.6%. This compares favorably t o the precision of present experimental methods (3). When solids were recrystalized and put into solution (2 mole %), they clogged the burner used with the entrained airhydrogen flame within minutes after starting the run. A smaller mole per cent solution could have been used, but the intensity produced would have been very weak and hard t o evaluate. However, this problem has not been encountered with oxy-hydrogen flame using the Hetco burner; its larger
(1)
where AHc = heat of combustion in kcal; a = constant in kcal; b = slope in kcal; and ICE = relative intensity of the CH molecule. 1042
Since all the other compounds are slightly displaced from the normal curve, they can be calculated by Equation 2.
(9) M. L. Parsons and J. D. Winefordner, Appl. Spectrosc., 21, 368-374 (1967). (10) “Handbook of Chemistry and Physics,” 48th Ed., The Chemical Rubber Publishing Co., Cleveland, Ohio, 1967.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
Table 111. Comparison of present Data with Literature Values AH (Kg-calimole) Intensity Literature (10) Predictedb Compound CH band-head Diff, 558.3 568.8 10.5 Propylamine 5.50 Butylamine' 8.37 710.6 710.6 ... ... 879.5 ... Pentylamine 11.79 Hexylamine 14.79 5.5 1022.2 1027.7 522.5 Isopropylamine 4.20 ... 728.2 713.6 Isobutylamine 8.37 14.6 Isopentylaminea 11.14 865.0 865.0 716.0 Tertbutylaminea 7.30 716.0 ... Tertpentylamine 10.62 880.0 ... ... Di-ethylamine" 7.97 716.9 716.9 ... ... Di-propylamine 14.11 1020.3 1209.3 Di-butylamine 17.90 480.5 486.1 Propyl alcohol 23.80 5.6 Butyl alcohol0 37.80 ... 638.6 638.6 Pentyl alcohol 50.00 22.2 793.7 771.5 Hexyl alcohol 63.30 916.3 ... Octyl alcohol 96.00 1272.4 1262.0 10.4 Isopropyl alcohol 20.50 443.3 474.8 31.5 Isobutyl alcohol4 38.40 638.2 638.2 ... Isopentyl alcohol 47.80 740.5 Butanoic acid 1.05 524.3 477.9 46.4 681.6 666.0 Pentanoic acid 1.68 15.6 831 . O 901.8 Hexanoic acid 2.47 70.8 1230.1 Octanoic acid 3.57 1343.6 Nonanoic acid 3.45 ... ... 1504.8 Decanoic acid 4.49 1458.1 46.7 Docecanoic acid 5.24 1771.7 1728.6 43.1 Tetradecanoic acid 6.39 2085.8 2071.9 13.9 Compounds used as standards. Calculated by Equation 1 or 2. .
nozzle and hotter flame produced overcome these difficulties and also give about a 100-fold more intense signal. The latter system is preferred. As previously mentioned, the time involved in some calorimetry experiments is 24 hours or more from start to finish (3). By the flame spectroscopy method it takes about an hour. The initial warm-up is 15 minutes, and the solutions can be made during this time. A single run takes about 10 minutes. Five runs would be considered adequate and could be made in 30 minutes. No cooling-down period is required. The conversion of data and calculations takes about 20 to 30 minutes. When making these measurements, it is necessary to run a standard solution containing the same functional group in
.
I
Diff., 1.88 ...
...
0.53 2.04 ...
... 1.17 ... 2.77
...
0.82 6.64
... 8.85 2.28 8.52 3.20 2.43 0.66
addition to the unknown. The standard should be from the same homologous series. Each spectrometric system will have to be calibrated with a series of standards containing the appropriate functional groups to determine the slope, b, for the individual system because of the critical nature of the experimental parameters. RECEIVED for review January 15, 1970. Accepted May 20, 1970. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research; also, we would like to acknowledge partial support by an Arizona State University Grant-in-Aid. Computer time was donated by the ASU Computer Center.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
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