133
V O L U M E 2 3 , N O . 1, J A N U A R Y 1 9 5 1 Table 11.
Azeotroping Effect
Compound Saplithalene 1-Methylnaphthalene 2-hfethylnaphthalene Diphenyl 2-Ethylnaphthalene '.6-Diniethylna),hthalene 1,6-Diinethylnaphthalene 1,7-DirnethylnaphtIialene -~
Boiling Point Displacement, F. 22 28 27 25
41 19
17 17
lower than the boiling point of the pure hydrocarbon. The extent of this azeotroping effect is shown in Table 11. ACKNOWLEDGMENT
The authors are indebted t o R. L. Hopkins for making thr separat'ions of aromatic types by silica gel and picrate formation, and t o H. 11.Smith and H. T. Rall for their encouragement and helpful suggwtions in the course of the research. LITERATURE CITED
~~ ~-.. ..
d t i i i g subfrartions. That a separation of aronxttic typcs was effected and benzenoid aromat,ics are present is s1ion.n bl- the three distinct types of spectra. Subfractions 1 and 2 represent t,he p:iraffin-naphthene portion from the silica gel separation; 3 through 8 show spectra characteristic of benzenoid aromatics (including Tetralins); 9 through 11 show naphthalenic-type spectra; and 12 and 13 show spectra representing a mixture of naphthalenic and diphenyl derivatives. Although subfractions 3 through 8, containing benzenoid aromatics, const,itute roughly one third of the aromatic portion, their contribution t o the spectrum of the whole fract,ion is negligible. To study these alkylated benzenes and Tetralins, separations of the naphthalenes niust first be effected. I t was found that the maximum concentration of each arom:itic hydrocarbon identified occurred in a fraction boiling considcrably
(1) American Petroleum Institute, Research Project 44, Sational Bureau of Standards, "Catalog of Infrared Spectral Data," Serial Nos. 764 through 777, contributed by Research Lahora-
tories, Trinidad Leaseholds, Ltd., England. (2) American Petroleum Institute, Research Project 44, Kational Bureau of Standards, "Catalog of Ultraviolet Spectral Data." (3) Cary, H. H., and Brcliman, A . O., J . Optictrl Suc. Am., 31, 682
(1941). (4) Hopkins, R. L., and Adanis, ?;. G., Proc. Okluiioma A c a d . of S c i . 27 (1947).
( 5 ) Hougen, 0. h.,and TYatson, K. M., "Industrial Chemical Calculations," pp. 95-101, Xew l o r k , John Wiley & Sons, 1931. . and Streiff. 9.J.. J . Research S u t l . Bur. Stnndrirds. 27,'343 (19411. (7) W a d , C . C., Gooding. 11. AI., and Eccleston, B. H., Inrl. h'ug. them., 39,in5 (1947). (8) TYard, C. C., and 8chwal.ta. F. G., Petroleum P r o c c s s h g , 5, 164 (1950). May 27, 1950. Presented before t h e Division of Petroleum Chemistry at t h e 117th l l e e t i n g of t h e A v E R I c A N CHEMICAL SOCIETY, Houston, Tex. RECEIVED
Infrared-Transmitting Solvents Triethylamine Addition as a Means of Increasing Applicability J. S. 4 R D AYD THOMAS D. FONTAINE Rureau of Agricultural and Zndustrial C h e m i s t r y , Agricultural Research Center, Reltsville, M d . The solvents considered best for infrared investigations-carbon disulfide and carbon tetrachlorideare not suitable solvents for most organic acids. .4 nonaqueous solvent suitable for solubilizing acids was needed in order to characterize plant-growth regulating acids. When carbon disulfide and carbon tetrachloride are modified by the addition of 0.5 to 2.5% triethylamine, the resulting mixtures transmit sufficiently for easy use in all the regions from 2 to 15 microns, except those already obliterated by the unmodified solvents. Some plant-growth regulat-
HEN infrared spectra are desired of solids in the solution state, it is frequently impossible t o find a suitable solvent. Carbon disulfide and carbon tetrachloride are outstanding examples of the type of solvents t h a t have sufficient transparency for u5e over extensive portions of the salt region, but a large propoi tion of the organic substances do not dissolve t o the necessary extent in these liquids. The inability t o find a suitable solvent need not, however, prevent one from obtaining the spectra of a solid. Mull and film techniques give spectra that are satisfactory for many purposes, and those who have developed skill with these techniques state that they allow great flexibility in contrast to the limited applicability of solvent techniques. The mull and film techniques are not satisfactory for all purposes, however, and there remains a critical need for increasing the applicability of solvent techniques.
ing acids, for which no other suitably transmitting solvent could be found, readily dissolved in these mixtures. Analogous solvation methods offer a possible way for extending the solubility of other types of solutes in infrared-transmitting solvents. The method facilitates studies of the neutralized state of acids by offering a practical nonscattering medium for obtaining their spectra, and the spectra indicate the effects of neutralization on the vibration of bonds at specific locations along the molecular chains.
Barnes, Liddel, and Willianis ( 1 ) and Torkington and Thonipson ( 8 )have discussed some of the requirements of infrared-transmitting solvents. Because the desired infrared transparency of solvents is believed t o be associated with a simple moleculiir structure in which the bonds are as few as possible in both type and number, such as CXa,(CX,),, or partially appropriate structures, like X.( CX$),.X, all the solvents that seem appropriate fall into narrow portions of formula indexes and are readily searched out. From such a search and follow-up tests, it appears that important improvements can be expected only for applirations over limited spect,ral regions, or t o eliminate ccrtain skips, and that there is littlc chance t o discover new solwnts that would a l l o ~one t o get the fairly complete spectra of additional typcs of so1UtCP.
An alternative possibility
i3
t o w e a mixed solvent, in which a
134
ANALYTICAL CHEMISTRY
minor component is used to solubilize the sample. The choice of this component need not be highly restricted by transmission requirements, for the total transmission may be kept within practical hounds by selecting one of the few highly transmitting solvents for the bulk of the mixture. In the application of these principles, it was found that either carbon disulfide or carbon tetrachloride containing a small proportion of triethylamint~ would dissolve many acidic compounds which were insoluble in the unmodified solvents. With concentrations of triethylamine of from 0.5 t o 2.5%, the solubility of the acidic compounds usually reached an acid-base equivalent of the triethylamine added, and the resulting solutions were suitable for infrared purposes. Esprriments conducted along these lines with the intention of solubilizing bases have shown that a n analogous mixture with trichloroacetic acid as the additive has stabilit,y and some desirable rt,gions of transparency. Gore, Barnes, and Peterson ( 5 ) have used neutralization by hydrochloric acid, hydrochloric acid-d, sodium hydroxide, arid sodium hydroxide-d in infrared studitis of amino acids in aqueous solutions. These examples illustrate the broad general possibilities of using reactions, hydrogtw bonding ( 3 , 4,7 ) ,and a variety of types of intermolecular associations as a means of extending infrared solution techniquw. The work described here is chicfly concerned with the solubilization of acids in nonaqueous systems. Triethylamine JWS chosen as the solubilizing base for these investigations because it is a symmetrical molecule, or as nearly so as any other practical choica, whose bonds are saturated and restricted to as few as possible in both type and number. It, is the simplest, membrr of the 11,s series that can be conveniently handled as a liquid, and it is
obtainable pure and anhydrous. Also, the absence of an aminohydrogen group seemed important t o exclude solubilization attachments other than by neutralization, and t o prevent as much as possible any reactivity with carbon disulfide if this solvent should behave as an acid anhydride. EXPERIMENTAL
Recommended Procedure. The met'hod, as adapted for a cell depth of 0.5 mm. and for a sample concentration of 0.05 .V in respect to the acid group, may be illustrated as follows. After the sample is weighed, and immediately before use, the solvent is freshly prepared by pipetting 0.120 ml. of trieth 1 amine under the surface for 9.88 ml. of either carbon disulXde or carbon tetrachloride and mixing t o get a total volume of 10 ml. and a base normality of 0.086. This normality is considered high enough t o allow for unexpected variations of a 0.05 N acid sample. Portions of the same solvent' mixture are used both for filling the balancing cell and for making solutions for the sample cell. Choice of Solvent Mixture. Of the two solvent mixtures, triethylamine-carbon disulfide is preferred for most purposes. This solvent mixture remained clear for several weeks in contact with salt, polyethylene, or tin or lead and their amalgams, and no change in the infrared spectrum was observed after severd days' standing. Chiefly because the component? of the solvent misture have different evaporation rates, only fresh solutions are recommended. The triethylamine-carbon tetrachloride misture is less stable and must be used cautiously t o prevent fogging of the cell n-indon.s
100% T
0
0 IOOXT
I
I
I
I
I
I
,
I
I
1
-
-
- D 0 2
WAVE
LENGTH
IN
MICRONS
Figure 1.
Effects of Triethylamine Addition on Infrared Spectra
Examples of solvents and acids compared under parallel conditions of slit width (automatica!ly varied), cell depth (0.55 mm.), and temperature (25'), A . Carhon tetrachloride alone (dotted), and as modified b y 1 % b y volume of triethy!amine,(solid line) B. Carbon disulfide alone (dotted), and as modified by 1% by volume of triethylamine (solid line) C. Palmitic acid, 10 mg./inl., in carbon disulfide (dotted), a n d in carbon disulfide as modified with 1 % triethylamine (solid line) D . Acetic acid in unmodified solvents (dotted), 0.26% b y volume (0.016 N ) in carbon tetrachlofide from 2 to 7 . 2 micron?, and 0.34,% b y volume (0.060 .V) in carbon disulfide from 7.2 t o 15 microns; and correspondingly in modified solvents containing slight excess of triethylamine (solld line)
V O L U M E 23, N O . 1, J A N U A R Y 1 9 5 1
135
IO0
string slit-bvidth drive, was used in these investigations. The corresponding spectrograms of car80 bon disulfide, before and after modification with 1 triethylamine, a r e 60 shown in Figure 1, R. From these illustrations, it may be seen that the extra bands due to the 40 triethylamine addition are neither n u m e r o u s n o r deep, and that the resultA B D \ 20 S ing mixtures t r a n s m i t sufficiently for easy use \ from 2 t o 15 microns in MICRONS all regions except t,hose I I I I I I I I I I I I I I I I I I I I I I , I 0 which are already oblit4.2 4.5 4.8 4.2 4.5 4.0 2.7 3 0 3.3 2.7 3 0 3 3 erated by the unmodified Figure 2. Changes of Specific Bands of Acids with Neutralization s o l v e n t s . Because tht. Dotted lines show spectra of free-acid state, a n d solid lines triethylamine-neutralized state. uncorrected background A . Band of acetylene group adjacent t o carboxyl, 3-(p-chlorophenyl)-2-propynoicacid, 0.0% .VI, 0.55-1nru. cell in tetrahydropyran solution and In tetrahydropyran modified with triethylamine (0.086 A I ) (not shown) is already B . Another band of acetylene group adjacent t o carboxyl, 3-(l-naphthyl)-2-propynoicacid, 0.038 M, 0.55-mni. cell. in carbon tetrachloride solution a n d in carbon tetrachloride modified with triethylamine (0.054 .\I) complex with atmospheric C., Band pf hydroxyl group adjacent to carboxyl, dl-mandelic acid, 0.66 M , 0.10-mm. cell, in tetrahydropyran and solvent bands, thc fc.nsolution and in tetrahydropyran modified with triethylamine (0.80 iM) D.,Band of hydroxyl group in 12-position t o carboxyl, ricinoleic acid, 0.050 M , 3.2-mm. cell, in carbon tstrachanges of the curvaturt> chloride solution a n d in carbon tetrachloride modified with triethylamine (0.086 M ) caused by the triethylamine do not add sigriifiwith a precipitate. In salt cells having polyethylene spacers, cantly to the regraphing troubles except in the 3.4-micron region. this solvent mixture is suitable for use only for about 2 t o 2.5 When these mixed solvents are used to obtain the spectra of hours after its preparation, and it may have t o be used quicker acids, the corrected curve obtained is neither that of the free acitl than this in cells having amalgamated metal spacers, because these nor that of its triethylamine salt. Instead, it approachw :I representation of the neutralized acid without the pattern of the materials seem to catalyze the decomposition. After use, the neutralizing base. The spectrograms obtained are characteristic cells should be promptly cleaned; and if any precipitate has collected, it may be removed by filling the cell with benzyl alcohol of the compound, and are generally useful in the byays of normal infrared spectra. However, close comparisons are best made with and then rinsing with the uqual solvents. S o spurious bands from the precipitate were detected when searched for by comdat,a obtained in the same manner. In general, band locatioiis paring a slonly taken spectra ( 3 hours) with one which was comremain approximately the same, wit'h few added or subtracted pleted quickly and during which no visible precipitation occurred. peaks. There are, however, characteristic differences, some of When polyethylene spacers are used, a precaution is advisable which can give information which is an important supplement t ( J when changing from carbon tetrachloride t o carbon disulfide soluthat obtained from the spectra of the free acids. I t is notetions, irrespective of whether triethylamine is present. iilthough worthy that the location of the carboxyl carbonyl band is only the polyethylene spacers did not seem t o dissolve in any of the very slightly shifted by this type of neutralization, in contrast t o infrared solvents encountered, they have given trouble 1% hich was the large shifts which occur with neutralization by inorganic. suspected to be from the plastic imbibing and releasing carbon bases. tetrachloride. h carbon disulfide solution used afterwards in the The unusual features of this type of spectrum can be mad(, same cell may give confusing spectra showing the strong bands of clear with spectrograms of some of the acids that will dissolve in carbon tetrachloride, particularly the very intense doublet whose either the modified or the unmodified solvents. The accumulated peaks are at 12.i2 and 13.07 microns in that Rolvent. Aprolonged data, consisting of the more or less complete salt-region spectra of series of washings and aerations may be insufficient t o prevent ten acids, have been consistent with the following interpretations. the reappearance of these bands, and it is recommended that the Four important differences have been observed: cells with these spacers be stored in a vacuum desiccator over1. The general absorption between peaks has usually been night when changing from carbon tetrachloride to other solvents. greater for the neutralized state. An overlapping and nearly complete coverage of the spectrum 2. There is a decrease in the peak absorptions that arise from in the salt region can be obtained in two sections by using the bonds close t o the neutralization effect. 3. Bands due t o groups common to both the triethylamine triethylamine-carbon tetrachloride solution from 2 to i . 9 microns and the sample may be distorted. This distortion has been oband the triethylamine-carbon disulfide mixture from i.7 to 15 or served chiefly in the CH band, where neutralization affects the more microns, as seems advisable when using the unmodified absorption of the near-by CH bonds of both the sample and the solvents. Thcse two mixtures have dissolved nearly all the triethylamine, and the decrease in the peak absorption of the triethylamine band in one cell and not in the other leads t o imorganic acids for which they were tried, but apparently they do perfect compensation. I n some estreine cases, this causes the not usually dissolve those acids 1%hose active hydrogens are very CH band t o register as extending partly above the 100% transstrongly bonded t o another group in the same molecule-for mission line. example, they failed t o dissolve oxalic acid and indole-3-acetic 4. The lateral displacement of a triethylamine band may lead t o a distorted band or even to a n additional band not present acid. in the normal spectrum of the sample. If such a band is of CHARACTER OF THE SPECTROGRAMS moderate intensity, the displaced compttnsation will give it, an unsymmetrical contour with a high base on one side, occasionall\The spectrograms of carbon tetrachloride alone (dotted), and as higher than the 100% line, and thus its abnormal nature may bit modified by 1% triethylamine (solid line) are shown in Figure 1, revealed. I n a general way, conjugation, as in the aromatic. A . .4 Perkin-Elmer infrared spectrometer, 3Iodel 12-C with a series, seems t o intensify all these effects.
c
l'\. :.:I-
ANALYTICAL CHEMISTRY
136 IO0 % T
0
-
"
-
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O
- m--
(1
e
-
N
,
I I 1
-
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I
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0
(C)
Figure 3.
2,4-DICHLORO-5-10DOPHENOXYACETIC
ACID,
Io,.,
0.55 M M . ,
IN
I% EtJN-CSe
Spectrograms of Some Plant-Growth Regulating Acids, as Obtained by Using Modified Solvents
The nornlal spectrum of pulniitic acid dissolved in carbon disulfide (dotted) and t h a t obtained in the triethylamine-carbon disulfide mixture (solid line) are shown in Figure 1, C. I t can be seen how the general absorption between peaks is increased by neutralization. This is a n example of a very long molecule, with bonds both near by and far remoyed from the neutralizable hydrogen. The near-by bonds are rcpresented by the carbonyl band (5.84 microns), and by bands a t approximately 8 and 10.7 microns, which are in common with acetic acid and are therefore known to represent the very close bonds. These bands show a decrease in peak absorption. I n contrast, the more distant bonds are represented by a band a t 13.88 niicrons (t,he right shoulder only of the solid line doublet i, which represents the distant hydrocarbon end of palmitic acid, having originated from the group --(CH2)".CH3in which n is 3 or more (6, 9). A second example is the CII band a t 3.4 microns; most of these bonds are far away, and therefore, this band has nearly its full length. The spectrum of acetic acid in the two solvents ( D ) is a n extreme example of a case where all the bonds are very near the ncutralizable hydrogen. I n the curve of the free acid, the conditione were those in which acetic and othrr similar monocarboxylic acids of low molecular weight exist largely in a dimeric form which r t w l t s from association through their carboxyl groups. I n the 3.4-micron region of t.he curve for t,he free acid, there are absorptions due t o the associated carboxyl groups which are strong enough t o overshadow absorptions due to the CH bonds ( 2 ) . When the acid is neut'ralized, the change of the absorption of the CH groups is therefore not clear because of other greater changes due t o the breaking of the association, but the total effect on the :il)sorption is a significant decrease of the peaks in the vicinity of 3.4 microns. Other examples of the decrease in peak absorption are the carbonyl band (5.84 to 5.86 microns), the methyl band (7.05 t o 7.09 microns), the acetyl band (7.76 to 7.92 microns), ant1 :I band at 10.69 microns which is common t o many carboxylic
acids i l l the free state. .it either side of the 10.69-micron band there are weaker Iiands that rcmain promirwnt aftrr neutralization. In such cascs, it has been assumed that these are not independent of the strong band and that the energy of t.he strong band becomes spread somewhat symnietrically in this fashion. .inother possibility is that it represr:iits one form of the sample becoming more abundunt as another bcconies less so, such as a change of dimerization or of charge distribution. Well isolated on t h r solid line is an additional band a t 13.83 microiis which is not in the original acid. Such a band usually appears a t this location in the spectra of acids in the triethylamine-carbon disulfide mixture and arises from the lateral displacement of a triethylamine band as an effect of its neutralization b,v the sample. The changes of the bands of specific groups as they are affected by the neutralization of ti near-by or a far-removed carboxyl group are further illustrated in Figure 2. There was some difficulty in finding substituted acids for which suitable solvents were available to enable the authors to obtain their spectra in the absence of triethylamine. In two cases, tetrahydropyran used as a solvent, with and without triethylamine, was found t o allow spectra of the two st,ates of thc acids to be compared under a p . propriately similar conditions. Sections A and B show how neutralization greatll- decreases the peak absorptions of acetylene groups in the 2-position t o a carboxyl. After the spectrum of the neutralized acid (solid line) for A , was obtained, the solution was acidified with dilute hydrochloric acid, washed, and dried, and the spect,runi was again produced; the peak returned to its previous level, indicating that the decrease was associated with neutralization, possibly as a result of a change of symmetry around the acetylene group (11) instead of with irreversible effects such as polymerization. C shows that the band from an alcoholic hydroxyl group in the 2-position is decreased in peak absorption by neutralization, while D shorn t h a t the same band from a further away 12-position is not decreased. The contours
V O L U M E 2 3 , NO. 1, J A N U A R Y 1 9 5 1 of thcsc L m d s arc' affectod Iiy the 11)-drosyl Ix~ndabeing partially superimposed on absorptions from the carbolcyl groups, b u t suficiently valid comparisons should bc possible because the interfercncc's are from the same ratio of carboxyl t o hydroxyl in both caws. SPECTRA OF PLANT-GROWTH REGULATORS
The. solvent mixtures dcscrihed here have been of considerable liclp in obtaining characteristic infrared patterns of 2,4-dichlorophcfinosyacetic acid and some related plant-growth regulating acids, as shown in Figure 3, for which no other suitable infraredtrarisniitting solvent could be found. The lower curve represents two identical curves of the stable and of the radioactive iodine 131-labeled 2,4-dichloro-5-iodophenoxyaceticacids, which wore synthesized in this laboratory ( I O ) . As only a very small proportion of the iodine was in the radioactive form, and this had largely decayed a t the time, no isotopic differences could be expected. The method was able t o prove with niicrosamples that the trvo preparations of the arid wcsre i d ~ n t i c a lexcept for the radioactivity. T h c CII bonds of these acids are massed relatively near the itcid grc~up,and some of the connecting linkages are conjugated. The CII bonds are also comnion to both the triethylaniine and the mmple. Therefore the CII hands are both distorted and t1ecw~:isedin height, and in two cases t.liey extend above the 100% transmission line. The carbon?-l bands appear to have been decrc:1secl i n peak absorption and broadened. The additional Ixintl a t 13.8 microns is prominent. The most characteristic difft~rrncc~s in the spectra of thesr substances appear between 11 :tnd 13 microns. The: esters or isolated triethylamine salts of such acids could
137 have been studied alternatively, but the method described has some special features to recommend it. I t is simple, rapid, and uses the product directly a t hand, without the need to prepare a derivative. The spectrograms represent more esactly the actual state of purity of the sample. Though a frlir distortional by effects appear, the systcni of bands is considerably ~iinplific~il the compensative removal of most of the coniplic!ntionn atltlt.tl l)y the drrivativr nttachment. LITERATURE CITED
(1) Barnes, R . B., Liddel, U., and Williams, V. Z., IND.E K C . CHEM., .ANAL. ED., 15, 659-709 (1943). (2) Bushwell, A. 31.,Rodebush, W. H., and Roy, RI. F., J . A m . Chem. Soc., 60, 223944 (1938). (3) Coggeshall, N. D., ANAL.CHEM.,22, 381-95 (1950). (4) Coggeshall, N. D., J . Am. Chem. Soc., 72, 2836-44 (1950). (5) Gore, R. C., Barnes, R. B., and Peterson, Elizabeth, .Ix.- have been reserved primarily for research, inasmuch ae the analytical results could be obtained only days later, when they had become of academic rather thnri practical value to the clitiician.
By t,he t,echnique of flame phot,oinetry, data on sodium and pot,:tsium concentrations in plasma, urine, or other biological fluids can l)e obtained in a few minute?. I3cJcause important differences h e t w r n norinal antl pathologicxl valiies may tie of small m:ignit,ude, high :tnal!~tic:il precision is required. The method ( 1 ) consiqtq of at,omizing R dilut,e solution of the sample and mising t,he arrosol obtained with the gas utilizcd by a Bunsen burner. A4sthe niist,ure of gas, air, and aerosol is ignited by the burner, light, of characteristic wave length is emitted by the test niaterial. T h e intensity of the emitted light is measured antl compared with t h a t found for known standard solutions. .S more accurate technique is to add lithium as a n internal standard to all solutions taken for analysis and then to read the ratio of podium or potassium to lit'hium (,?I. T h e original publications of Barnes and Berry and their associates (1, 5)set forth these essentials for accurate flame photom-
