to H Iiich varying ainounts of silicon hnd tw:en atitled as stnntfnrtl solution to tltc plutoniutn solutions before dehydration with pcreliloric :icid. ?‘he rccoscriw, with an :iser:igc deviation from the $mount added uf & O S y of silicon, indicatr thnt plutonium docs not interfere, evcn though it forms n strong fluoride complcs. The nicthod has also bccn tested arid sucrcssf:iIiy npplicd to ur:iniuni nlloys, stcrl, niol~bdenum, and phosphoric acid. TnLles 111 and I\’ conipnre results on urnnium :~Iioysnncl on Sntionnl Bureau of Stnndnrtle stccls with v:ilucs . obtninctl by other ~ i w t ~ i o ~Inl ~l’iiblc V milyticnl c!ntn on rc:igcnt grade phosphoric acid (85%) nrc listcd. ?’lie dctcriuinntion of micro quantities of silicon in phosphoric acid and in fcrrous iiiet:iIs ifi of pnrticuiar interest because phosphorus n n d iron nrc common1:i; ciicountercti ns intcrfercnccs to the mol1.1,denuin blue mcthod for silicon ( 8 , 10). In plutoniuni snnlyscs, the dissolution of the sniiiple anti tlic rlistillntion of thc silicoti wcrc pcrforitictl i r i n glovctl funic hood tlcsigricrl for protection
SIR: Unconiugated trans unsaturatiori in Cm h t t y acid or cster mixtures prcpnred from natural lipidcs has been determined from infrarcd spectra by ii~casuringthe extinction of the trans peak a t 965 c n - 1 using the bnse line technique (1, 7) and the more sensitive differential rnetkod (3-6), Mixtures of Cu acids or esters from the lipides of feces, liver, vegetable mattcr, and other sources often contain complcx unsaturated material whose spectral interferepce makes i t difficultto compcnsatc correctly the bands of the acid or cster groups a t the region of the analytical peak when ’applying the differcntial iuethod. This communication describes horn compensation cnn be nchicved. The envelope of overlapping bands around 965 cm.-* thusobtnined can be separated graphically into its component bands, nnd there i g some evidence thnt 6onic of these bands also arise from trans unsaturation, although only that at 965 ern.'' can be definitely assigned, and i t is measured in terms of elaidate and the value verified. EQUIPMENT
Spectra were recorded on a Perkin.S
ANALYTICAL CHEMISTRY
against high-nlpha mdionetivity. After distillation the absorbing solution containing the silicon wis transfcrrcd from tlie plnstic rial in thc fume hood, through an ii~ncrconncctingport, into another vial in an adjacent, 1o\v-alphn arcn. Before proceeding with the pH adjustment a quick radioactivity countiug analysis for plutonium was madc on each solution. The totnl transfer pcr O.Sgrsm sample was about O y of plutonium. This nmount of plutoniuni apparcntly hnd no apprccinble effect on absorbance. Blnnks clistillcd from solutions of plutonium, previously tlepletcd of silicon, did not differ appreciably froin blanks involving no plutonium. Tfic color devclopnients 2nd the spectrophotometric measurements were mntle iri a well ventilnted open fume I1oori. ACKNOWLEDGMENT
The nuthor is inrlcbted to IC. J. Jcnscn for consultations contributing to this work and to Adolph Ycntrrs for :issist :i IICC in obtaining t 1le espcri rncn tal dntn prcsented.
Elmcr Model 21 tioubIc-i)cam spcctrophotonwt~cr. Thc slit \vas sct at 125 microns and the sample cclls werc 0.102 and 0,054 cm. long. A Perkin-Elmer variablc-length ccll wns used for conipcnsnting. PROCEDURE
Tile zero and Io ivere accurately fixed and the spcctrum \vas recorded betmccn 1020 nncl 890 cm.-’ of t!ic sample ccll containing 40 mg. of acid sample pcr ml. of carbon disulfide, compensated in the rcfcrencc beam with the correct thickness of solvent. The cxtinctiori of the interfering carboxyl band a t 933 cni,-l (Figure 1,A) was measured. This band mas cancelled by adjusting the thickness of a compensating solution of 40 mg. of stearic acid per ml. of carbon disulfide, according to the measured extinction. The diffcrential spcctrum wns then recorded (Figurc 1,D) using maximum instrument gain and sensitivity, and scannitig very slondy from the region of the analytical peak to beyond the new-blackout region at 933 cm.-l to avoid distorting the analytical band. I n the case of esters, B solution of 50 mg. of sample per ml.of carbon disulfide was used, compensated in the reference
UTEUTURE CITED (1) Bahko, A. K., Evtushenko, Zawd.hq/a Lab. 23, 423 (1957).
L. hf,,
(2) Cnrlson, A. B., Ranks, C . V., ANAL. CIim. 24,472(195?). (3) Cam, 0.P., IND. END.CHEU.,ANAL. ED.16 309 (1844).
(4) Codeil, M.,Clemency, C., Norwitz, G., ANAL. CHGhI. 25,1432 (1953). (5) H : J 1 , hl. T.,Chnnisf AnaIysl 46, 04 (1850.
(6) Jean, >I., Chim. anal. 38,37 (1956). (7) ICenyon, 0. A., Bewick, H. A., ANAL.CIIEJI.25,145 (1953). (8) King, E. J., Stocry, 13. D., Holt, 1’. F.,Y ~ i t e D. ~ , E., l’icklcs, D., Analyst 80,441 (1955).
(9) Luke, C. I,., ANAL. Cirxx, 25, 148 (1!Ei3). (10) hIenis, O., hliinning, D. L., Anal.
Chini. .4cta 16, F i (1957). (11) hlullin, J. B., Riley, J. P., Ibid., 12, 16? (1955). (12) Strickimd, J . D. SI., J , Am. Chen. SOC.74,872 (1952). (13) U. S. Atoniic Energy Comm.
Rept., ANL5124 (1953).
(14) Ibid., ANL-5974 (195!)).
(15) Volk, R. J., Weintntub, R. L., ANAL.CIIELI. 30, 1011 (1958). RECEIVEDfor rcvicw 3I:iy 11, 1959. Acccy~bd Octobcr !I, 1959. Work performed utidcr tbe nuspict:s of the U, 8.
Atomic Eticrgy Coniini~sion.
ccll \\,ith an cqual conccntistion and thickiiws of nicthyl stcnrate to eliminate intcrfrreiice from the 1015-cm.-l ester Ilantl. The Lackground from wliicli tlie extinction is mcasurctl was obtaincd by recording the spectrum of the sample cell containing carbon disulfide, with the reference cell containing thc samc thickness of solvent (Figure 1,J‘). Graptiicai methods (I)) .were then used to calculate the trans cloublc bond content of thc snmplc ns claidate. This mluc was vcrificd 1 ~ 5 - recording the differcntial spcctrum of the sample against a solution containing the estimated quantitics of claidic and stearic acids or mcthyl claidstc anti mcthyl stcarntc. RESULTS A N D DISCUSSION
Beer’s law plote of both eiaidic ncid and mcthyl clnidntc show that thc specific extinction cocficicnt, k, is 0.495.
Because of interfering bands a t 1015 cm.-i it was often not possible to calculate tho amount of methyl stearate needed for correct cornpentxition, but because of the ester bnnd’s compnratively lorn intensity, the un-
compensation a t 965 crn.-' resulting from the use of a n equal concentration and thickness of methyl stearate in the reference cell was slight, The intensity of the 933-cm.-l band of the acid group varied with the sample, and it was important to compensate it correctly because of its high intensity and proximity to the analytical band. The effect of overcompensating a n acid sample is shown in Figure 1, E, where, as when elaidic-stearic mixtures are estimated, the sample thickness and concentration of stearic acid were used to compensate. A check that the interfering band of the acid or ester group was compensated correctly can be made by subtracting the absorbance difference of the single-beam curves (Figure 1, C and A ) wave length by wave length from the differential curve. If correctly compensated, the background obtained graphically coincides with that obtained experimentally (Figure 1, F ) . The background was not a straight line, as the sample cell became dirty with the crude samples used. The differential curve recorded (Figures I,2, and 3) is the envelope obtained by the summation of overlapping bands. Interfering bands mere most often found a t 1022, 993, 984, 970, 944, or 912 cm.-l Their half-band widths are close to the value of 12 to 13 cni.-l found for the 9 6 5 - ~ n i . - ~band under the monochromator conditions used. The band a t 912 em.-' alone is TTery much wider, but it is remote from the analytical peak and generally of too low a n intensity to interfere. The component bands, nhich, when summed, give the experimental curve, were found after noting their half-band n idth and their frequency separation, so that they could be drarln, using the diagrams in (9) as a guide. The shoulders of a number of bands may be found to interfere at 965 cm.-l The band obtained graphically a t 965 cm.-' should always be the same as that obtained by subtracting from the experimental curve (Figure 2, A ) the curve that is obtained in verifying the estimated trans value (Figure 2, B ) . Table I lists unconjugated trans double bond contents expressed as elaidate, calculated from the curves in Figures 1, 2, and 3. These are typical experimental samples n ith varying degrees of interference near 965 cm.-l The effects of not conipensating correctly are less viith ester samples. The over- or undercompensation a t 965 em.-', observed TT hen checking with the calculated concentration of elaidic acid or methyl elaidate in the reference beam, can be measured to n-ithin 3=0.501, of trans unsaturation. The repeated appearance of the same interfering bands near 965 em.-' suggests they are not due to random impurities. Much of t,his interference
Table 1. Trans Unsaturation Expressed as Per Cent Elaidic Acid or Methyl Elaidate 015
-
yo trans
Curve 1, D 2, A 3, A 3, B 3, c 3, D
=t0.5
14.0 6.0 36.5 23.1 12.0 3.0
0 8. I O .
IO20 1000 %O
9 6 0 940 920
WAVE
Figure 1.
'xx)
NUMBER
Infrared spectra u"0
A.
Uncompensated sample of crude acids from fecal lipides ( 3 ) of sheep, 40 mg./cc. of CS2 in 1 -mm. cell B . Stearic acid, uncompensated, 40 mg./cc. of CS2 C. Stearic acid, sufficient to compensate sample at 933 c m - 1 D. Differential curve sample, correctly compensated E. Differential curve of equal concentrations of sample and stearic acid F. Background
O'O
r-l
0
- 0 6
090 980
970 960 9 5 0 9 4 0
WAVE
Figure 2.
NUMBER
Infrared spectra
A.
Correct differential curve of distilled methyl esters of horse fat acids, 50 mg./cc. of CS2 in 1 -mm. cell B. Slightly overcompensated (0.2% trans) with 6.3% methyl elaidate in reference beam C. Trans peak remaining after subtracting B from A
could be due to group vibrations in minor components of the sample mixtures, as they are weaker in purified samples. Linolenic acids from different sources were found to have the same interfering bands in different proportions. Some of these bands originate with C I S acids having trans unsaturation that is conjugated with other double bonds in the same molecule ( I ) , so that only unconjugated trans unsatura-
III 4 LL
in 4
0
0 8 1.0 1010
I
I
I
990
1
970 WAVE
950
030
NUMBER
Figure 3. Correct differential curves of esters and acids, with background Acids from fractionated esters (3) of horse fecal lipides 8. Methyl eloidate-methyl stearate, 23.2% trans unsaturation C. Distilled methyl esters, from fecal lipides of sheep D. Acids, from bacon fat €, Background
A.
tion is estimated a t 965 cni.-' The oxidation of unconjugated double bonds can give rise to conjugation and frequency shifts ( 6 ) . The most serious interference comes from the band sometimes found a t 970 cm.-' (Figure 2, A ) . Evidence so far points to this being due to an unconjugated trans double bond situated nearer the end of the molecule, or nearer to a branch chain. The trans peaks of long-chain trans monounsaturated esters such as trans-9-octadecenoic, trans-6-octadecenoic, and trans-11-octadecenoic are a t 965 cm.-l (7), but that of trans-2-octadecenoic acid and its methyl ester absorbs a t 980 cm.-* ( 8 ) . The trans peak of unsaturated long-chain hydrocarbons (2) usually s h o w a frequency rise under similar circumstances. Because of the uncertainty in assigning the band a t 970 em.-', unconjugated trans unsaturation a t 965 cm.-l, only, is measured, and in the case of CIS acids, expressed as elaidic acid. It is planned to study the effect of environment on the frequency and intensity of the trans peak, and of the carboxyl band near 933 cm.-l VOL. 32, NO. 1, JANUARY 1960
129
LITERATURE CITED
( 5 ) McDonald, I. R. C., Nature 174, 703
(1) Ahlers, N. H. E., Brett, R. A., McTaggart, N. G., J . A p p l . Chem. 3, 433 (1953). (2) ~~~~i~~~ petroleum Research project 44, Carnegie Institute of Technology. (3) Hartman, L., Shorland, F. B., Cleverley, B., Biochem. J. 69, 1 (1958). ( 4 ) Hartman, L., Shorland, F. B., McDonald, I. R. C., Ibid., 61, 603 (1955).
(1954). (6) SePhtOni H. H.1 Sutton, D. A.1 Chem.
& Znd. (London) 1953, 667. ( 7 ) Shreve, 0. D., Heether, A I . R., Knight, H. B., Swern, D., i l s a ~CHEX . 22, 1261 (1950).
( 8 ) Sinclair, R. D., McKay, A. F., >Iyers, G. S., Jones, R. IC.,J. Am. Chem. SOC. 74, 2578 (1952).
(9) Vandenbelt, J. &I.,Henrich, C., A p p l . Spectroscopy 7, 171 (1953).
BARRYCLEVERLEY Department of Scientific & Industrial Research '"*lingtonJ x e w Zealand. RECEIVED for review June 2 3 , 1959. Acc e p t d Octolier ?eti, 195!1.
Determination of Carbon in Organic Substances by an Oxygen-Flask Method SIR: Gotte, Kretz, and Baddenhausen (2) have used the Hempel-Schoniger oxygen-flask method (3, 6) for the determination of carbon-14 in organic materials. This method may also be used to determine the total carbon content in solid organic substances. The method is comparable in accuracy to the Pregl dry combustion method (4) and is much faster than the classical method. KO expensive apparatus is required. The sample in milligram amounts is placed on a glass-wool pad and is ignited electrically in a n atmosphere of oxygen. The carbon dioxide evolved is absorbed in aqueous sodium hydroxide and determined acidimetrically. A complete determination, excluding the time required to weigh the sample, may be made in less than 20 minutes. EXPERIMENTAL
The apparatus used is shown in Figure 1. Two platinum wires (B. & s. gage No. 22) sealed in two 6-inch lengths of 4-mm. glass tubing are used to make electrical contact with the ignition coil, and two other short platinum wires
Table I.
Substance Benzoic acido
Analyses of Known Compounds
Formula C~HGOI
Vanillin Dextrose Cholesterol 6-Nitrocholesteryl acetate Acetanilidea Azobenzene 2-Saphthyl phenyl sulfide N-benzenesulfonylN-carbethoxymethyltert-Butyl mesidine Cystinea Benzeneboronic anhydridepyridine complex Potassium acid phthalatea a Sational Bureau of Standards samples. 130
ANALYTICAL CHEMISTRY
fused at the ends of the tubing are used to support the glass-wool sample holder. A 15-cm. length of nickel-chromium resistance wire (B. 8: s. gage KO.32) with a resistance of ea. 5 ohms is made into a coil by winding around a wire 1 nim. in diameter. A few windings of the ends of the coil around the platinum conducting wires make satisfactory electrical contact, and no soldering is required. A 1 x 1.5 inch pad of Corning KO. 7220 borosilicate glass wool. thick enough to look ofaque to light, is placed betn-een the ignition coil and the supporting mires. A weighed amount of sample, preferably from 3 t o 25 mg., is transferred to the glass-wool pad close to the ignition coil by a Tveighing tube. The glass-wool sample holder is folded in the middle and the tvr-o halves are held together by the supporting wires. A 500-ml. thick-walled Erlenmeyer flask containing about 25 ml. of carbonate-free 0.5N sodium hydroxide solution is flushed with a rapid flow of oxygen for 2 or 3 minutes. Complete replacement of air is required for good results. The rubber stopper with the sample assembly is inserted, and the leads are connected to a Tariac. Although the optimuni
Sample Wt ., hIg. 3.478 7.763 11.385 12 042 6.400 17.310 24.975 11,120 9.390 13,940 12.460 4.458
% Carbon Calcd. Found 68.84
63.15
40.00 83. 87 73.53 71.09 79.09 81.31
Absolute Error,
70
68.43 69.28 68.42 68.84 63.47 63 26 40.31 84.07 73.84 71.31 79.91 80,50
-0.41 +O 44 -0.42 0.00 + O , 32 so.11 +0.31 +o. 20 +0.31 +o . 2 2 + O . 82 -0.81
14 400 i. 710 20.132
66.15 29.99
66.19 .. ~. 30.56 30.13
$0.04 +O. 57 $0.14
8.123 17.185
TO. 68
i1.00 46.61
+O. 32 -0.44
47.05
Figure 1.
Combustion apparatus
applied voltage required for a good combustion varies somewhat n i t h the conibustibility of the sample, 16 volts, which provides a current of about 3 amperes, is usually suitable. As soon as the sample begins to burn, the flask must immediately be inverted. I'io soot or snioldering occurs in a complete combustion. After combustion is complete, the flask is shaken vigorously for about 5 minutes. Most of the excess alkali may be neutralized by addition of about 10 nil. of 1 s hydrochloric acid solution. The solution is then brought to the phenolphthalein end point by careful addition of dilute hydrochloric acid. Thymol blue or cresol red-thymol blue mixed indicator maybe used, if preferred. The titration with 0.1A- hydrochloric acid is continued with a 10-ml. microburet to the methyl orange end point. A methyl orange-indigo carmine indicator has also been recommended for this titration (1). The hydrochloric acid is standardized against a neighed amount of sodium carbonate. Comparison color standards are recommended for both end points, and correction for the blank must be made. Carbonate-free water is used for all preparations. The carbon content is calculated from the equivalents of acid consumed between the tn-o end points by the equation: 1.201
%
=
x
103iv.v.
mg. of sample
where Sa is the normality of the acid, and T', is the milliliters of acid. corrected
