T a b l e 111. Limit of Detection of Ferric Ions by Different R e a g e n t s Limit of RefReagent Detection erence 4,4’-Dicarboxydicyclohexylamine 1:0.87 x los Potassium thiocyanate 1:16 X lo6 ( 2 ) Dinitrosoresorcinol 1:2.85 X lo5 (6) 7-Iodo-8-hydroxyquinoline-51:100 x 105 ( 7 ) sulfonic acid Thiodiacetic acid 1:100 X lo5 ( 4 )
The results obtained with the present method are compared with those employing different reagents. Table I11 gives the limits of detection of ferric ions in parts of ferric ions per parts of solution, using different reagents. 4,4’ - Dicarboxydicyclohexylamine is a tridentate chelating agent and the coordination number of six of the ferric ion suggest that the coloring ion has the structure
Fe(CI4H&O4):
OH -
Fe( OH) 3
F&
H+
+
4,4’-Dicarboxydicyclohexylaniine as obtained here must be a mixture of isomeric compounds. It is suggestcd that the isomer having the conformation ae-ea (5) is the main chelating agent, the other isomers contributing little or not a t all, to the formation of the complex. LITERATURE CITED
considered. For instance, in the case of Ag+, Hg2+2, and Pb+2, the white precipitates are the insoluble chlorides, which can be separated from the solution. The phenomena observed with Bi+3 and Sb+3 are due to the formation of the oxychlorides. Antimony oxychloride will form when hydrochloric acid is added to a solution of the trichloride, while any bismuth salt in the presence of hydrochloric acid will precipitate as bismuth oxychloride. On the other hand cupric ions must complex to some extent with the reagent. T h a t is why high concentrations of this ion interfere with the test for ferric ions.
(1) Carswell. T. S.. Morill. H. L.. Znd. ‘ Ens. Chek. 29, 1247 (1939).
(2) Curtman, L. J., “Introduction to Semimicro Qualitative Chemical Analysis,” p. 162, MacMillan, Xew York, 1950. f3) Ferber. E.. Bruckner., H.., Ber. 76B. .
I
1019 11943).’ -, \
~
(4)Lyons, E., J . Am. Chem. SOC. 49, 1916 (1927). The formation of the ferric complex (5) Newman, M. S., “Steric Effects in with 4,4’-dicarboxydicyclohexylamine, Organic Chemistry,” p. 13, Wiley, depends greatly on the p H of the soluKew York, 1956. (6) Nichols, M. L., Cooper, S. R., J . tion. Thus the species formed a t Am. Chem. SOC.47,1268 (1928). different pH’s are products of reversible (7) Yoe, J. H., Hall, R. T., Ibid., 59, 872 reactions depending on the hydrogen (1937). concentration, EFTHIYIOS CHINOPOROS OHDepartment of Chemistry Fe+’ 2C14H23N04F-’ Suffolk University H+ Boston, Mass.
+
Gas Chromatographic Analysis of Fatty and Chlorinated Fatty Acids SIR: Although separation of volatile fatty acids by gas chromatography was first reported by James and Martin in 1952 (4), this procedure or its modifications ( I ) have received little application until very recently. As noted by Hunter et al. ( S ) , two factors contributing to this lack of progress were the need for strict maintenance of anhydrous conditions and the corrosiveness of acid vapors on metallic elements of the usual types of sensitive measuring equipment. Another important factor has been the pronounced tailing exhibited by the strongly adsorbed acids on most gas chromatographic substrates. Recently, Hunter and coworkers (6) have described a column packing t h a t performed very satisfactorily for the separation of organic acids encountered in food flavor analyses. An essential feature of this analytical procedure was the use of dichloroacetic acid to liberate the weaker organic acids from their sodium salts. An ionization detector cell was employed, and under the analysis conditions em438
ANALYTICAL CHEMISTRY
ployed, the dichloroacetic acid yielded only minor peaks in the early portion of the chromatogram. These were presumed to be due to decomposition products and caused only minor interference with the proposed analytical method. I n this work, the authors were concerned with the development of a gas chromatographic analysis method for the chlorinated acid acids. They were able to obtain appreciable response for these acids on a variety of column packings using B thermistor detector. Since ionization gage detectors are known to be insensitive to highly chlorinated compounds, i t was thought that the explanation of Hunter’s failure to observe peaks for these acids might lie in the fact t h a t he was using this type of detector. To test this hypothesis, a standard sample was prepared containing a known mixture of three fatty acids plus mono- and dichloroacetic acids. These acids were chosen to be easily separated on a 1-meter copper column packed with 10% LAC296 and 5% phosphoric acid (85%) on
acid-washed Chromosorb W. The standard sample was analyzed under identical conditions on a Perkin-Elmer 154-C Vapor Fractometer (thermistor detector) and a Barber-Colman Model 20 gas chromatograph (Srgo and Razz@ ionization detectors). The same column was used in both instruments. The Perkin-Elmer chromatograms were run first using the usual U-shaped column, and then the column was coiled for insertion in the Barber-Colman instrument. An appreciably higher pressure was required for the latter runs in order to obtain the same flow rate (60 cc. per minute), and both instruments were operated a t 150’ C. with FZ flash heater temperature of 240’ C. RESULTS AND DISCUSSION
Figure 1 shows a typical chromatogram of the standard acid mixture on each of the gas chromatographic instruments. Table I summarizes the individual acid responses in terms of peak areas. As expected, the response for the dichloroacetic acid was much
41)
A Perkin- Elmer Instrument
8
8. Barber- Colman instrument
Sr9’ or RaZZ6Ionizotion Detector
Thermistor Detector
I
P
T I M E IN M I N U T E S
Figure 1. Typical gas chromatograms of standard acid mixture 1. 2. 3.
4. 5.
Figure 2. Typical gas chromatograms of dichloroacetic acid (5) with repeat peaks for ( 1 ) acetic, (2) valeric, (3) chloroacetic, and (4) decanoic acids
Acetic Valeric Chloroacetic Decanoic Dichloroacetic
lower with the ionization detectors than with the thermistor detector. However, with both SrQO and R a 2 z detectors, there were distinct peaks for this acid, and the chloroacetic response was practically normal using these detectors. Since Hunter reported that only minor “impurity” peaks were obtained with these acids, it appears that the explanation of these apparently conflicting results lies in the long retention times for these acids under his operating conditions (120” C.) and in the relatively low concentrations used. The “minor impurity” peaks noted by Hunter were probably due to the so-called “repeater effect,” in which peaks are often observed for acids previously injected and adsorbed by carbon in the preheater section ( 5 ) . This effect is demonstrated in Figure 2,
which shows the results obtained on injecting several samples of pure dichloroacetic acid immediately after several samples of the test mixture indicated in Table I had been injected. The “repeat peaks” for the acetic, valeric, chloroacetic, and decanoic acids gradually decreased upon further injections and finally disappeared entirely. The effect of varying detector cell voltage was briefly studied (Table 11) to determine whether lower voltages might be used on the SrH and Ra226 detectors to eliminate or minimize the detector responses of the chlorinated acids. The relative response for chloroacetic acid was significantly reduced a t 500 volts, but showed little change a t the other voltages tested. The relative dichloroacetic acid response did not
appear to be affected greatly by the detector voltage using the Razz6 detector, but did appear to be significantly reduced at 500 volts using the Sr” detector. The vnlues in Table I1 are the averages of a t least two chromatograms, but no attpmpt was made to establish the significance of the other minor variations in response noted. Hunter has already pointed out the necessity for correcting for the minor “decomposition products” due to the dichloroacetic acid. I t is evident from this work that a major correction may also be required for the main dichloroacetic acid peak and that the amount of this correction is dependent on the type of detector used. While the need for this latter correction can be eliminated by exact neutralization of the alkaline extracts, i t is believed that elimination of the “repeat pcaks” will be more difficult. The cause and prevention of these interferences will be discussed in detail in a later communication. ACKNOWLEDGMENT
The authors are indebted to Irving Table 1.
Comparison of Thermistor, Strontium-90, and Radium-226 Detector Response to Various Fatty and Chlorinated Fatty Acids
Acid
Theoretical wt. %
Thermistor Response
Acetic Valeric Chloroacetic Decanoic Dichloroacetic
5.6 10.0 22.9 27.4 34.1
12.4 14.5 28.9 22.2 22.0
Table 11.
SrQ0, 1000 Volts 8.5 15.2 33.5 46.9 5.9
RaZ26,
1000 Volts 11.3 16.0 22.8 42.i 7.8
Effect of Detector Voltage on Response of SrgOand Ra*26Detector Cells
Akctual Acid Wt. 70 Bcetic 5.6 Valeric 10.0 Chloroacetic 22.9 Decanoic 27.4 Dichloroacetic 3 4 . 1
500 Volts SrgO Razze 7.1 15.0 13.9 60.7 3.3
8.5 15.4 15.6 54.5 6.1
1000 Volts Sr90 RaZ26 8.5 15.2 23.5 46.9 5.9
11.3 16.0 22.8 42.1 7.8
1500 Volts
2000 Volts
Sr9O
Razz6
Sr9O
Razz6
11.6 18.0 22.5 41.0 6.9
11.5 19.1 21.9 41.6 5.9
12.1 21.1 23.2 37.0 6.3
11.7 21.5 22.6 37.0 7.2
R.Hunter for suggesting the chromatographic packing which vas used in this study, as well as for his other helpful comments and criticism. LITERATURE CITED
(.1 .) Hardv. C. J.. Pollard, F. H., J. Chrorndlog. 2,22 11959). (2) Hunter, I. R., Ng, Hawkins, Pence, J. W., ANAL.CHEX.32,1757 (1960).
(3) Hunter, I. R., Ortegren, V. H., Pence, J. W., Ibid., p. 682. (4) James, A . T., Martin, A. J. P., Biochem. J. 50, 679 (1952). ( 5 ) Smith, E. D., Gosnell, A. B., South-
west-Southeast Regional ACS Meeting,
New Orleans, La., December 1961.
EDGARD. SMITH AUBREYB. GOSNELL
Graduate Institute of Technology University of Arkansas Little Rock, Ark. VOL. 34, NO. 3, MARCH 1962
439
