oxidation of silver (I) or chemical reaction with a nitrate oxidation product. The fact that the voltammetric curves for silver nitrate, lithium nitrate, and BuJV"N8 are identical, together with the fact that no anodic response is obtained for silver perchlorate, below the potential required for perchlorate discharge, indicates that the electrode reaction involves nitrate, rather than silver(1). Gas chromatographic examination of the head space vapor produced during oxidation of silver nitrate showed nitrogen to be a product, rather than oxygen as previously reported (6). A comment regarding this experiment is appropriate, since we are not in agreement with the previous workers. We sampled the head space through a septum using a gas-tight syringe. With this technique, some atmospheric contamination cannot be avoided; when oxygen and nitrogen are sought, care must be taken to avoid confusion. In these experiments, samples were drawn before and several times during the electrolysis. For comparison, chromatograms of air samples were made at the same time. A head space sample taken at the start of the electrolysis showed both oxygen and nitrogen, with an N/O peak height ratio of 3.0. The size of the peaks indicated that the sample was about 9 % air. Chromatograms of head space samples taken during the electrolysis showed little change in absolute magnitude for the oxygen and nitrogen peaks; however the N/O ratio gradually increased to 4.2, indicating that nitrogen is being produced in the reaction. We conclude that anodic oxidation of silver nitrate involves an initial electrode reaction by nitrate, followed by a chemical reaction with silver(1) which produces silver(I1) and nitrogen gas. Oxidation of Cadmium Nitrate in MeCN. The above results do not provide a complete description of the reaction;
however, they do offer some useful insight which has been lacking to the present. Water, carbon dioxide, and oxygen gas are formed. This means that the reaction scheme previously proposed ( 3 ) is at least a considerable oversimplification. The solvent is definitely involved in the reaction. The fact that mass spectrometric examination of the head space vapor shows N02, NO1, and N205 to be absent makes the reaction shown in step 3 seem somewhat unlikely. At the same time, we have not been able to identify the product which accounts for the nitrogen involved and oxygen is produced in about the amount demanded by reaction 3. It is possible that the unidentified product is responsible for the mass spectroscopic peaks observed at mje = 57 and 58. In each of the systems examined, nitrate has undergone a reaction at the electrode, presumably anodic discharge to produce nitrate radicals, which have reacted very rapidly with other components of the system. The very great reactivity or instability of nitrate radicals has made it impossible to make a direct observation of them in this work, although ESR spectra have been produced in low temperature experiments (13). It is an additional consequence of this reactivity that the nature of the reaction is apparently very strongly dependent upon the composition of the system. Not only will changes in solvents be expected to change the course of the reaction, but variation of solutes with the same solvent may have this effect. RECEIVED for review January 15, 1970. Accepted May 21, 1970. Work supported by the National Institute of Health through Grant GM-10064. (13) E. Hayon and E. Saito, J. Chem. Phys., 43,4314 (1965).
Identification and Determination of Freezingpoint-Depressant Anti-Icing Additives in Hydrocarbon Fuels by Infrared Spectrometry R. K. Ritchie and D. Kulawic Gulf Oil Canada Limited, Research and Development Department, Sheridan Park, Ontario
An infrared method has been developed to identify and determine freezing point depressant anti-icing additives-glycols, glycol-ethers, and alcohols-in hydrocarbon fuels. The method uses the OH stretching bands of the additives in dilute solution. A reference hydrocarbon fuel free of anti-icing additive is obtained by water-washing. Glycols and glycol-ethers are identified by bands resulting from the different types of intramolecular hydrogen bonds occurring in the molecules, and alcohols by the frequency of their free OH band. The method has been applied to hexylene glycol (2-methyl-2,4-pentanediol) and methoxy glycol (diethylene glycol monomethyl ether) in gasolines in the 0-0.25% range, to methanol and isopropanol in gasolines in the 0 4 % range, and to methyl Cellosolve (ethylene glycol monomethyl ether) in jet fuel in the 0-0.25% range. The effects of additives, base stock composition, and hydroperoxide formation are discussed. 1080
0
UNDER CERTAIN ATMOSPHERIC conditions (high humidity levels and temperatures slightly above freezing point) ice is likely to form on the throttle plate of carburetors in gasoline engines. A build-up of ice causes the carburetor to malfunction and can cause the throttle plate to close, resulting in a stalled engine. Ice formation in gasolines is prevented by the addition of either freezing point depressants that preferentially dissolve in water and thus lower its freezing point, or surface active agents that coat the inner surfaces of the carburetor and prevent water adhering to these surfaces. Freezingpoint-depressant anti-icing additives are also added to jet fuels to prevent the plugging of filters by ice crystals. Glycols and glycol-ethers (concentration zz 0.1 %) and alcohols (=1 %> are usually employed as freezing-point-depressant anti-icing additives; the chemical nature of the surface active additives
ANALYTICAL CHEMISTRY, VOL. 42, NO. , AUGUST 1970
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n
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3700 3500 3300 la HEXYLENE GLYCOL (0.12 %)
370.0
3500
3300 cm”
3700
16 METHOXY GLYCOL (0.12*rC1
IC
3500
3300
Cm
METHYL CELLOSOLVE (0.128%)
Figure 1. IR spectra of glycol and glycol-ether anti-icing additives Sample beam: Fuel diluted 5-fold with CCI, Reference beam: Water-washed fuel diluted 5-fold with CCI4 Cell pathlength: 1 cm Concentrations shown are (v/v) of the additive in the fuel
is more varied ( I ) . I n this paper we shall be concerned with only the freezing-point-depressant additives. In previous infrared methods the freezing-point-depressant anti-icing additives have been identified and determined using their characteristic CO stretching bands near 1100 cm-’. Powell ( 2 ) determined alcoholic anti-icing additives in gasolines using a water-washed gasoline as reference, and Hasegawa et ai. ( 3 ) have used an essentially similar method to determine an additive containing 98 % methyl Cellosolve (ethylene glycol monomethyl ether) and 2 % glycerol in jet fuel in the 0.05-0.30 concentration range. To overcome observed disadvantages of Powell’s method at additive concentrations of less than O S % , Jenkins and Scruton ( 4 ) determined the additives from the infrared spectra of water extracts of fuels; their method was applied to hexylene glycol (2-methyl-2, 4pentanediol) and isopropanol in gasoline, and to Cellosolve in kerosene. Nonspectroscopic methods include measurement of the freezing point depression of a water extract of the fuel (5), and chemical methods involving potassium dichromate (6) and hexanitrato ammonium cerate (7), also using water extracts of the fuels. These methods have been described only for the determination of the methyl Cellosolve additive in jet fuels. Gas chromatography ( I ) has also been used to analyze some commercially available anti-icing additives which appear to be predominantly simple alcohols; no application to the analyses of actual fuel samples was reported. We describe an infrared method in which the additives are identified and determined from their characteristic O H stretching bands between 3450 and 3650 cm-1. The method has been applied to the commonly used freezing point depressant anti-icing additives-hexylene glycol, methoxy glycol (predominantly diethylene glycol monomethyl ether), methanol, (1) M. E. Le Pera, J. Gas Chrornatogr., 6, 335 (1968). (2) H. Powell, J. Appl. Chem., 6,488 (1956). (3) K. Hasegawa, M. Kajikawa, M. Kawaguchi, and T. Nishijima, Bull. Jap. Petrol. Inst., 6 , 19 (1964). (4) G. I. Jenkins and M. R. Scruton, J . Inst. Petrd., 49, 176 (1963). (5) D. Loomer and A. L. Graham, Hydrocarbon Process. Petrol. ReJiner, 42, 158 (1963); Oil Gas J., 61(2), 82 (1963). (6) Federal Test Method, No. 5327.3, Defense Petroleum Supply Center, Washington 25, D. C., 1965; Canadian Government Specifications Board, 3-GP-0, Method 132.la, 1967. (7) L. Gardner, J. Inst. Petrol., 55,418 (1969).
and isopropanol in gasoline, and methyl Cellosolve in jet fuel. The advantages of the method over other infrared and nonspectroscopic methods are discussed. EXPERIMENTAL Reagents. Hexylene glycol was obtained from the Shawinigan Division of Gulf Oil Canada Ltd. Methoxy glycol and methyl Cellosolve (UCAR 600) were obtained from Union Carbide of Canada Ltd. Apparatus. Infrared spectra were obtained using a Beckman IR 12 infrared spectrometer and 1-cm NaCl cells. Spectra were scanned at 40 cm-l/rninute with a spectral slit width of 6 cm-1 (4 X standard) in the region of interest. The frequency scale of the spectrometer was calibrated using bands of ammonia and atmospheric water vapor (8). Reported frequencies are accurate to =tl cm-1 for the sharp bands of the anti-icing additives. Procedure. A 25-ml sample is shaken for 5 minutes with 5 ml of water to provide a reference fuel free of anti-icing additive. Five milliliters of the sample and reference are pipetted into 25-ml volumetric flasks and diluted to volume with carbon tetrachloride. A differential infrared spectrum of the diluted sample against the diluted reference is obtained between 3250 and 3800 cm-1 using 1-cm cells. The resulting spectra of gasolines containing typical amounts of hexylene glycol and methoxy glycol and a jet fuel containing methyl Cellosolve are shown in Figures la-c, respectively. The additives are readily identified from the frequencies and appearance of their OH stretching bands. If methanol or isopropanol are present in gasolines at typical concentrations, their spectra at this stage consist predominantly of very intense and broad bands near 3400 cm-1 due to intermolecular hydrogen bonding. When the presence of methanol or isopropanol is so indicated, a new reference is obtained by extracting the gasoline with an equal amount of water (to ensure complete extraction of the relatively high alcohol contents), and a fifty-fold dilution of both sample and reference with carbon tetrachloride results in the spectra, shown in Figures 2a and 6, respectively, of the essentially nonhydrogen-bonded alcohols. The frequencies of the bands for methanol and isopropanol are sufficiently different to positively identify either one. Hexyl(8) “Tables of Wavenumbers for the Calibration of Infrared
Spectrometers,” IUPAC Commission on Molecular Structure and Spectroscopy, Butterworths, London, 1961.
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6 0 .-
50 .-
50 3645
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3700 3500 2b ISOPROPANOL
METHANOL ( 0 . 8 % )
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1
Figure 2. IR spectra of alcoholic anti-icing additives Sample beam: Gasoline diluted 50-fold with C a r Reference beam: Water-washed gasoline diluted 50-fold with CCI, Cell pathlength: 1 cm Concentrations shown are % (v/v) of the additive in the gasoline
Table I. Calibration Data for Hexylene Glycol and Methoxy Glycol in Gasoline, and Methyl Cellosolve in Jet Fuel Absorbances Concentration Hexylene Methoxy Methyl in gasoline glycol, glycol, Cellosolve, or jet fuel, 3537 cm-1 3610 cm-l 3607 cm-1 % (viv) 0.020 0.040 0.048 0.080 0.120 0.128 0.160 0.200 0.240 0.320
0.034 0.065
0.028 0.042
0.130 0.192
0.070 0.100
0.254 0.318
0.134 0.165
0.070 0.186 0.350 0.468
Table 11. Calibration Data for Methanol and Isopropanol in Gasoline Absorbances Concentration Methanol, Isopropanol, in gasoline, 3645 cm-1 3629 cm-1 % (viv) 0.40 0.80 1.60 2.00 2.40 2.56 3.20 4.00
0.112 0.236 0.479 0.592 0.712 0.974
0.115 0.223 0.364 0.451 0.563
ene glycol, methoxy glycol, methyl Cellosolve, methanol, and isopropanol are determined quantitatively by measurement of the absorbance of the bands at 3537, 3610, 3607, 3645, and 3629 cm-l, respectively, from base lines drawn between 3250 and 3750 cm-l. The weak “negative” band near 3700 cm-1 in some of the spectra in Figures 1 and 2 is due to a small amount of water in the water-washed reference gasoline. Calibration curves were obtained by carrying out the above procedure on gasolines and jet fuels containing known amounts of anti-icing additive in the appropriate concentration range. The calibration data are given in Tables I and 1082
11. Linear calibration curves were obtained in all cases. The relative standard deviations of the slopes are 0.17, 0.68, 0.15, 0.89, and 0 . 0 7 x for hexylene glycol, methoxy glycol, methyl Cellosolve, methanol, and isopropanol, respectively. All curves pass within 0.003 absorbance unit of zero except for methoxy glycol and methanol where the deviation is 0.011. The curve for methanol could be improved by excluding the highest concentration value; further dilution to bring the band into a more accurate absorbance range is probably desirable. The above procedure for the water washing of gasolines and jet fuels to provide additive-free reference fuels was shown to be effective by obtaining differential spectra of waterwashed fuels, each of which had contained an anti-icing additive, against similar fuels known to be additive-free; no bands due to anti-icing additives were observed. RESULTS AND DISCUSSION The differences between the spectra of hexylene glycol and each of the glycol-ethers in dilute solution can be attributed to the different types of intramolecular hydrogen bonding which occur in the molecules (9). In the spectrum of hexylene glycol, the 3537 cm-’ band is due to either or both of the possible 1 , 3 intramolecular hydrogen bonds (because of two nonequivalent OH groups). The 3618 cm-l band and a reported high frequency shoulder (IO) are due to the OH bond of the proton acceptor 0 atoms (9); the shoulder on the 3618 cm-I band is not obvious in any of our spectra. In the spectrum of methoxy glycol the 3610 and 3485 cm-’ bands are due to 1,2 and 1,5 intramolecular hydrogen bonds, respectively, and a weak high frequency shoulder is probably due to the free OH bond of nonhydrogen-bonded molecules (9, 11). Similarly in methyl Cellosolve, the 3607 cm-1 band and a weak high frequency shoulder are due to the 1,2 intramolecular hydrogen bond and the free OH bond, respectively (12).
(9) M. Tichy, Aduan. Org. Chem., 5, 115 (1965). (10) H. Buc and J. Ntel, Compt. Rend., 252, 1786 (1961). (11) T. K. K. Srinivasan, C. I. Jose, and A. B. Biswas, Can. J. Chem., 47, 3877 (1969). (12) P. J. Krueger and H. D. Mettee, J. Mol. Spectrosc., 18, 131 (1965).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
The above spectral features show that hexylene glycol and the glycol-ethers exist in more than one molecular form under the experimental conditions of the analysis. However the linearity of the calibration curves and the apparent constancy of the relative band intensities for each molecule indicate that there are no significant changes in the relative amounts of the various molecular forms in the concentration range studied. There is evidence for intermolecular hydrogen bonding of the anti-icing additives in the spectra of gasolines and jet fuels before dilution with carbon tetrachloride, even for the relatively low concentration levels of hexylene glycol and the glycol-ethers. Five- or fifty-fold dilutions with carbon tetrachloride remove essentially all the intermolecular hydrogen bonds while retaining sufficiently intense bands for quantitative determination of the anti-icing additives at the relevant concentration levels. Dilution simplifies the quantitative analysis in that no account has to be taken of the effects of concentration and temperature (from the heat of the IR beam) on the equilibria between intermolecularly-hydrogen-bonded molecules and those which are not so bonded, and as a result all the calibration curves are linear. N o doubt intermolecular-hydrogen-bonding effects contributed to Hasegawa et al's ( 3 ) conclusion that the OH stretching region was not suitable for the determination of methyl Cellosolve in jet fuel. The only potential sources of interference with the method are molecules containing free or intramolecularly-hydrogenbonded O H and N H groups. Common fuel additives of this molecular type are the hindered phenol and phenylene diamine antioxidants. However, these antioxidants do not interfere since they are not readily extracted with water and thus make no significant contribution to the differential spectrum; also they are present in much lower concentrations (
