Robert E. Van Atta and R. Lewis Van Atta Ball State University Muncie, IN 47306
I Gas Chromatographic Determination of
II
Methyl Salicylate in Rubbing Alcohol An experiment employing standard addition
S t u d e n t experiments in gas chromatography employing three common methods (normalization, absolute calibration, a n d internal standard techniques) are described in most modern textbooks (1-3) or specialized references ( 4 ) on instrumental methods of analisis. T h e objectives of this experiment are t o provide a n exercise in a fourth important quantitative technique (standard addition) and t o use this technique in a rather unusual application of gas chromatograohv . . t o t h e analvsis for a minor comnonent in a commercid product. A number of ~ h a m a c e u t i c afirms l market a oroduct usuallv c n l l d ..winter&n ruhbiny alwhul," in u,h;h up 10 ahoui 2 rwrrent of meth\d s d ~ c v l a t eloil ~ d w i n t e r e r e e nis~ added. his ester imparts a pleasant odor to the preparation and is well-known for its penetrative properties. T h e product is generally colored green and is priced somewhat higher than ordinary rubbing alcohol, although t h e isopropyl alcohol content (70 percent) is t h e same. T h e experiment, a s subsequently descrihed, involves the of three sevarate test solutions. each containing" .oreoaration . a n identical volume of the unknown commercial sample, while two are also "spiked" with accurately measured volumes of reagent grade methyl salicylate; all are diluted to the same volume with solvent grade isopropyl alcohol. T h e solutions are chromatographed, in turn, with a carhowax column a t 175-200°C. T h e ester Deak, aopearina well after elution of the major components isopropyl &oh01 and water, is symmetrical and cleanly resolved (Fig. 1).T h e ester peak height, measured from t h e extrapolated background baseline, as shown in Figure 1, is proportional t o t h e concentration of t h e ester. T h e analysis is rapid and accurate, either by graphical interpretation (5), or direct calculation ( 6 ) from added concentrat i o n ( ~ )and measured peak heights according t o the equation Cr
h,c, = --h,+, - h,
where h, represents t h e measured ester peak height (in m m or chart divisions) for the solution containing no added ester, and h,+, that for a solution containing an added concentration of ester c, (volume-%) in identical total volumes. Experimental Apparatus A Gow-Mac Series 150 Gas Chromatograph associated with a Linear Instruments 252A strip-chart recorder was used for the measurements represented by Figures l and 2 and shown in the table. A 4 R X '/$in.15%Carbowax20M onchromsorb P 801100 mesh column and helium carrier gas were used for all measurements. The gas flow rate (60 mlimin) and column oven temperature (190°C)were adjusted hy eaeh student. Samples were measured and injected with a Hamilton 701-N 10.~1syringe. Preparation of Test Solutions Transfer 20.0 ml (pipet) of the commercial wintergreen rubbing alcohol to each of three 25-mI volumetric flasks.Add 0.20 and 0.50 ml (pipet), respectively, of U.S.P. reagent grade methyl salicylate to two ofthe flasks and dilute all three to the mark withsolvent grade isopropyl alcohol, mixing the resulting solutions properly. Calculate the concentration of ester in the standard addition solutions by the relationship 230 / Journal of Chemical Education
Figure 1. Typical chromatogram for methyl salicyiate in wintergreen rubbing alcohol.
Volume-%=
ml ester added X 100% 25.0 ml solution
Experimental Procedure Zero the recorder and the instrument, with appropriate attenuation (an Attenuator setting of 8 was used to secure the data shown) and a chart speed of 2 cmimin or equivalent. After thermal equilibrium has been established in the column, record three chromatograms for 2-4 samples of each of the three test solutions, measuring the samples as carefully as possible and including sufficient air to provide an air peak, if desired.,The recorder pen will go offscaleshortly after theair peak appears (see Fig. 1)and remain there for 30 sec or more, due to the presence of a large combined isopropyl alcohol-water peak. After all chromatograms have been recorded, construct baselines and measure the ester peak heights to the nearest 0.5 mm, as illustrated in Figure 1.Tabulate the measurements and compute average ester peak height for each test solution. Calculate the volume percentage of methyl salicylate by comparison with eaeh of the two standard addition solutions using the identical volume standard addition equation. Then construct a standard addition plot (Fig. 2), measured peak height versus concentration of added methyl salicylate. Extrapolate the resulting straight line to its zero intercept on the x-axis to determine the concentration of the ester in the unknown commercial product. Discussion Analvsis of known oreoared mixtures eontainine 0-3 volume-%
Methyl Salicylate Content of Wintergreen Rubbing Alcohols (Typical Student Results) Vo1.-% Methyl Salicylate Found
Peak Heights, mm Student
Product Brand
Calculatede average versus:
bs,
hx+,
57.0 54.0 53.0
88.5 83.0 80.5
59.0 57.0 57.0
88.0 84.5 84.0
18.0 17.5 37 n
47.0 46.0 AS n
hx
A
Haag's
B
Rexaii
C
Hook's
D
Gray's
Graphical
ST
$2
132.5 128.0 124.5
1.49
1.49
1.48
130.0 126.0 125.0
1.66
1.87
1.66
Hospital
Added concentration of methyl salicylafe war 0 8 and 2.0 uo1.-% for rl and s2, respectively 100
80
-
60 -
-40
40-
-20
Found.
v-9L
M
I I
l
l
Figure 2. Graphical evaluation for t h e analysis b y standard addition.
Addad. v - % of wintergreen rubbing
analvtieal results are obtained when a sin& standard and the direct calehatbm procedure are employed, the standard addition plot yields mare reliable results. Furthermore, student pipetting and syringe operation techniques are dramatically reflected by the plotting method, since the data yield perfectly linear plots if these techniques
for higher concentration samples are probably attributable t o slight variations in sample volumes, inasmuch as the students who performed this experiment had never before used a microliter syringe. T h e wen temperature for the instrument used drifted as much as 6 degrees over the time required t o record a set of 9 chromatograms (approximately 50 mi"). Although peak retention times shifted slightly, no significant effect was noted in the measured relative peak heights.
Acknowledgment
The a u t h o r s express t h e i r appreciation to t h e College of Sciences and Humanities of Ball State University for a CORE g r a n t which provided financial s u p p o r t and t o the s t u d e n t s of Chemistry 325, s o m e of whose e x p e r i m e n t a l results a r e s u m m a r i z e d in this article. Litelature Cited (11 ewina,G. w.,"lnrtrumentn1 Methodi .l Chemical Anal~sii."I r d Ed.. MrCraw-Hill. New York. 1969, pp, fiil2-603. (21 s t n h e l , H. A . "Chomicai i n ~ r r u m e n t s t i ~ A n : svrlematir ~ppn,arh." 2d Ed., Addinm-We&y. Reading, Massachusetts. 1973, p. 8fi9. I:!) Wiiisid,H. H..Meirit~. I,. I..,Jr..and Dean. J.A.."ln~trumentsl Methods oIAnnlyr1s." 5th Ed.. 11. Van Nmtmnd. New Ysrk. 1973. pp. 622 ff. Id) McNaii, H.M.. "Ra;ir Car Chrumatr,#mphg." ACS Audio Cmrse. American Chemical Society. Washinxtcln. 0.C.. 1971,pp. 9-12. I61 Pecsuk. K. I... Shields. I.. D..Cairns. T., and McWilliam. I.C.. "Mndern Mofhl~ds
