Determination of Phenolic Hydroxyl by Near-lnf ra red S pect$0photometry ROBERT F. GODDU Research Center, Hercules Powder Co., Wilmingfon
,Use of the 2.7- to 3.0-micron range for the qualitative and quantitative analysis of phenols and mixtures of phenols and the effects of substitution and intramolecular hydrogen bonding on the free hydroxyl stretching band are described. The molar absorptivity of most phenols i s of the order of 200 liter per mole-cm., which allows the analysis of samples containing as little as 25 p.p.m. of phenolic hydroxyl. Because intramolecular hydrogen bonding causes shifts in the hydroxyl band, it i s often possible to analyze samples which contain several phenolic species. The 2.7- to 3.0-micron region i s particularly applicable to the study and analysis of phenolic antioxidants. As with other near-infrared procedures, the rcpidity, selectivity, and sensitivity give the method advantages over more conventional methods of analysis.
S
advantages of the near-infrared region betneen 2.7 and 3.0 microns for phenolic hydroxyl group analysis are presented here. Previous papers have covered the near-infrared determination of unsaturation (6) and the deterniination of terlninal epoxides (8). Other quantitative functional group n ork in the near-infrared region has recently been published b y TV’hetsel, Roberson, and Krell (15-17). The 2.75-micron, free hydroxyl band in phenols has been nidely used with coni-entional infrared instruments for the quantitative determination of various types of phenols. This work was usually done n i t h rock salt optics and n ith no attempt a t gaining selectivity by the use of high resolution. Several spectroscopists have studied the funtlnniental hydroyyl-band region to gain information on hydrogen bonding, in particular intermolecular hydrogen bonding (1-3, 5 , 10, 13). These \\ orkers. in general, uqed thin cells and relstively concentrated solutions (greater than 10 mX); quantitative work n i t h free or intramolecularly bonded hydroxyl bands under their conditions n ould hare been difficult, if not impossible. Data on phenols in the 3-micron region appeared in t n o recent papers. Stone and Thonipson (14) studied the correONE
99, Del.
lation of band intensity, half-height band-width, and frequency of the free hydroxyl peak with the Hammett factor U, on numerous meta- and parasubstituted phenols. Flett ( 4 ) reviened his and others’ work on the 3micron band of a wide variety of hydroxyl compounds. His work is more extensive n ith respect to monohydroxy1 phenols than that reported here. It \vas pointed toward a n understanding of various types of internal hydrogen bonds and hoJv they may be formed. His data are reported in ternis of absolute band intensities but molar abs0rpti.r ities can be calculated from his data. The work was done on a Grubb-Parsons S-4 double-beam spectrometer n i t h a lithium fluoride prism. Many of his data can be used to extend the analytical applications discussed in thi3 paper, and conversely some of these data can be used to extend Flett’s work. K o r k has been done on the effect of hydrogen bonding on the first overtone band of the hydroxyl stretching vibration a t 1.4 microns. These studies, mainly by Wulf (18-21) and coworkers and Luttke and JIecke ( I I ) , were pointed toward determining the cis or trans orientation of the hydroxyl group relative to the orientation of other groups on the benzene ring (12). Their data are similar to those obtained b y this laboratory in the fundamental band region. By using the fundamental hydroxyl band, i t is possible to work a t lower concentrations than they, and a t these lorn concentrations it is far easier to distinguish intermolecular effects from intramolecular interaction. The data presented here indicate the value of the fundamental hydroxyl region for qualitative and quantitative work iyith phenols. EXPERIMENTAL
Instrumental. For work in t h e 2.7t o 3.0-micron region where t h e fundamental hydroxyl bands are found, t h e most convenient commercially available instrument is t h e Beckman Model D K spectrophotometer. T h e Perkin-Elmer Spectracord Model 4000 covers a portion of t h e range-up t o 2.85 microns. All work reported here was carried out using a Beckman Model DK-2 spectrophotometer. The following instrument settings
were used: scanning time 20; scale expanded 2X: time constant 0.2; sensitivity, such that a nominal slit of 0.12 mm. v a s obtained a t 2.763 microns with the reference cell filled with solvent in the reference beam. giving a half-intensity band width of 0.006 micron a t this wave length; range, 0-1 ab3orbance scale. The wave length measurerilents described were calibrated in terms of the data of Fox and Martin ( 5 ) , Kuhn (?), and Barnard, Hargrare, and Higgins ( I ) , who reported that the free hydroxyl absorption niaximum of phenol was 2.771 + 0.002 microns. The wave length scroll on the DK-2 used here read 0.012 to 0.014 micron low. The wave length reproducibility of this instrument was 10.002 micron or better. Absorbance measurements were made using the base line technique. For example, in Figure 1 the base line n a s drawn between 2.71 and 2.77 and 2.77 and 2.97 microns. All absorbance% were corrected for a small water peak at 2.770 microns by correlating t h r height of that peak n i t h the height of the water peak at 2.701 microns. K a t e r absorption is important mainly in n orh with 5- or 10-em. cells. Cells. T h e silica or quartz c d l q ordinarily obtained have strong I\ atel bands in t h e 2.75-micron region. For this work, 1- a n d 10-em. cells of special near-infrared transparent silica were obtained from t h e Pyrocell Manufacturing Co., Yew York City. These cells transmit 85 + 1% of t h e incident light, using air as a reference, over the range from 1.0 to 3.1 microns. Solvent. Reagent grade carbon tetrachloride dried over magnesium perchlorate was used in all work reported here. Carbon tetrachloride from t h e same batch was used in t h e sample and reference beams. Carbon disulfide could probably be substit u t e d in most cases. Methylene chloride m a y be used between 2.72 and 2.95 microns. The sources of the chemicals are indicated in the tables in which they are mentioned. The commercially available materials n-ere of the grade indicated and not purified further. The laboratory samples were the purest obtainable and believed to be better than 95% pure, unless otherwise indicated. PHENOLS WITH ONE HYDROXYL GROUP
Spectral data on a wide variety of VOL. 30, NO. 12, DECEMBER 1958
2009
Table 1.
Near-Infrared Absorption Data h
Phenol p-Cresol p-Isopropylphenol p-Nonylphenol p-Phen ylphenol
215 256 415 (2 OH’S)
2 7i3
202
206
2 773
186
185
Mono-ortho-Substituted Phenols Matheson, Coleman 2. i i 1 & Bell Lah. sample 2 774
172
171
173 14 -190 190
175 14
Matheson, Coleman & Bell Lab. sample
m-Isopropylphenol
2-tert-Butyl-4-methoxyphenol 2,5-Di-tert-butyl-p-cresol
Tennessee Eastman Lab. sample
2-Isobornyl-p-cresol o-Phenylphenol
Lab. sample Lab. sample
2( or,or-Dimethylbenzyl)-p-
Lab. sample
cresol
fi
194 192 195 200 245 425
m-Cresol
2-tert-Butyl-p-cresol
Max,
2.771 2 771 2 771 2 771 2 774 2 772
4,4’-Isoprop~-lidenediphenol (bisphenol --1)
o-Cresol
Source Phenols Merck, U.S.P. Lab. sample Lab. sample Jefferson Chemical Recrystallized Lab. sample
Molar Absorptivity E , Liter/Rfole-Cm. 2-5 mM 0.1-0.3 mM
2 745 2 771 2 775 2 746 2 772 2 808 2 776 2 833
Diortho-Substituted Phenols Lab. sample 2 769 2,4-Di-tert-butyl-o-cresol Lab. sample” 2 768 2,6-DiisopropyIphenol Ethyl5 2 766 2,6-Di-tert-butyl-phenol Ethyl 2 746 2,6-Di-tert-butyl-p-cresol Lab. sample 2 744 2,4,6-Tri-tert-butylphenol Lab. sample 2 746 2,6-Diisobornyl-p-cresol Lab. sample 2 771 2,6-(a, or-Dimethylbenzyl)-pLab. sample 2 843 cresol a Sample of questionable purity. 2-tert-Butyl-o-cresol
...
160 196 27 222 168 150 147 210 180 186 143 290
192 196
...
...
69 16 157 201 21 220 169 153 147 214 183 189 144
280
has essentially no effect on the wave length of the maximum. Thus, presuniably the isobornyl group does not hinder the hydroxyl group as much as the tert-butyl group, because the former is less free to move about. -4s with mono-ortho substitution, 2,6-dibenzyl substitution in the 2,6positions causes a shift to longer wave lengths for the reasons given above. The molar absorptivities of 2,6disubstituted phenols appear to be lower, in general, than for the other phenols in Table I. As expected, dilution causes little change in the absorptivity, especially n-ith the very hindered compounds. POLYHYDROXY PHENOLS
The high resolution spectrum of a typical polyhydroxyphenol in the 2.7to 3.0-micron region is shom-n in Figure 1. The absorption band a t about 2.75 microns is due to the free hydroxyl stretching vibration. The 2.85-micron band is attributed to intramolecularly bonded hydroxyl. It is apparent from Figure 1that there are several variables n-ith nhich to work in connection with the polyhydrosyphenols, such as the relative intensity of the free and intramolecularly bonded hydroxyl peaks and the frequency of wave length separation between these two peaks. Data in Table I1 show how relatively small changes in the structure of a compound can have a large effect on its free and intramolecularly bonded hydroxyl stretching vibrations. Data on catechol and resorcinol indicate that only lJ2-dihydroxybenzerles have intramolecular interaction. Coinpounds I11 through VI1 are formaldchyde condensates of various phenols. Those which are substituted similarly have similar hydroxyl spectra-for ex-
monohydroxy benzenes are included in I). Aliphatic substitution tends to Table I. Phenol and meta- and paracause shifts to lower wave lengths, substituted phenols all absorb a t alIyith the extreme being noted in 2,6most the same wave length, 2.772 5 di-tert-butyl substitution. I n this case, 0.001 microns, and have very close to the shift is so far that unsubstituted the same molar absorptivity, with the or mono-ortho-substituted compounds exception of p-phenylphenol, in which may readily be determined in the presthe phenolic hydroxyl is activated. ence of the disubstituted compound Dilution by a factor of 20 increases (Figure 2 ) . Substitution of t n o large the molar absorptivity very little. isobornyl rings in the ortho positions Thus in quantitative work, deviations from Beer’s law are not usually serious. I n the more concentrated solutions (2 to 0.605m Jf) there is no observable intermolecularly bonded hydroxyl stretching band in the region of 2.9 to 3.0 microns. 0.50Mono-ortho substitution does not change the position of the free hydroxyl 0.kOband. Exceptions to this are o-phenylY phenol and 2-(cu,~~-dimethylbenzyl)-pcresol. I n both compounds, interaction of the hydroxyl hydrogen with the electrons in the phenyl rings could be postulated thus forming a type of intramolecular hydrogen bonding. Similar interaction of hydroxyl hydrogen and a benzene ring is observed in aromatic hydroperoxides and aromatic I I I 2 . no 85 2.90 95 2.71 alcohols (1). The molar absorptivities MICRONS of the mono-ortho-substituted phenols Figure 1. Near-infrared absorption spectrum of 2.73 mM solution of 2,2’are comparable to those in Table I. methylenebis(4-methyI-6-tert-butylphenol) in carbon tetrachloride in 2.7Diortho substitution causes changes to 3.0-micron region (1-cm. cells) in the wave lengths of the absorption maxima of most compounds (Table Wave length scole not corrected for 0.01 3-micron instrument error 2.70
2010
ANALYTICAL CHEMISTRY
2
2
Table II.
NO.
Structure
Phenol
Ib Catechol
Data on Polyhydroxyphenols Tntramolecularly Bonded Hydroxyl Free Hydroxyl Band Band Molar Molar absorpabsorptivity, tivity, E, liter/ E, liter/ hx., hax., mole-em.n Reference mole-em: p P 216* 2.804 200 Dist. Products, 2.771 Eastman Grade
VFree VBonded,
€Free
+
Crn.-’
€Free SBmded
43
0.52
o O H I I c Resorcinol
...
...
...
1.00
170
2.886
100
138
0.63
2.777
152
2.888
79
140
0.66
American Cyanamid
2.757
84
2.851
202
120
0.29
Lab. sample
2.759
81
2.851
186
118
0.30
Lab. sample
2.759
Si
2.859
181
122
0.32
Lab. sample
2.772
425
...
...
...
1.00
Lab. sample
...
...
2.880
1035
...
0
Catalin Corp.
...
...
2.853 2.928
66 134
...
0
2.776
380
...
...
...
1.00
Merek, U.S.P.
2.772
>36&
Lab. sample
2.776
Lab. sample
O O H I11 2,2’-1\Iethylenebis(4-chlorophenol)
C1 @ OH CHr-O
IT 2,2‘-Methylenebis(4isopropylphenol)
-J~ H cB ’~ -J(~‘V 2,2 ‘-Methylenebis(4- B methyl-6-tertbut ylphenol)
c1
i-Pr OH
i-Pr OH
CH3
CHI
VI 2,2 ’-Methglenebis(4isopropyl-6-tertbutylphenol) i-Pr
i-Pr
VI1 2,2’-?\Ieth>-lenebis(4,6-di-tert-butyIphenol) t-BU t-Bu VI11 4,4’-IsopropylideneCHI diphenol (Bisphenol A) H O o - i - o O H
IX
2,2’-Isopropylidenebis(4methy1-6tert-butj-lphenol)
X 2,2’-Thiobis(4methyl-6-tert-
t-Bu
t-Buo-S$-Bu
but ylphenol) CHa
CHI
S I 4,4’-Thiobis(3methyl-6-fedhutylphenol)
Monsanto
2.748
41
HO t-Bu
c
t-Bu
Molar absorptivities determined in 2 to 3 mM solution, except I and 11. Some increase in E noted on dilution to 0 . 1 to 0 . 2 mM. Compound I 0.28 mM. Compound I1 not completely soluble a t 0 . 2 mM.
VOL. 30,
NO. 12, DECEMBER 1958
201 1
ample, compounds I11 and I T , and compounds I-, TI, and T’II. K h e n the hydroxyl groups are not ortho but para to the methylene, or other connecting group, as in compounds T’III and XI, no intramolecularly bonded peak is observed. This Jvould presumably be true with nieta condensates also, although none of proved purity were available. Perhaps the most striking example of intramolecular bonding observed is in the acetone condensate of 2-tert-butylp-cresol (compound IX). I n this compound, n hich is rather similar to compound T’, there is no free hydroxyl peak and the intramolecularly bonded hydroxyl band has the highest molar absorptivity observed in this region. -4molecular model indicates that the two hydroxyl groups must, of necessity, be in close proximity to each other, thus confirming the spectral observations. Another example is compound X, in n hich there is a sulfur bridge with hydroxyls ortho to it. Again there is no free hydroxyl peak, but there are tiyo intramolecularly bonded hydroxyl bands. By analogy to other compounds in the table, the peak at 2.840 microns is probably due to the bonding with the neighboring hydroxyl group, and the peak a t 2.915 microns is then most probably caused by bonding of the hydroxyl to the sulfur atom. Data were also obtained on several other types of phenolic compounds nhich are not included in Table 11. I n particular, i t appears that eo y o u n d s which have a phenolic hydroxyl intramolecularly bonded to a n ether oxygen, similar to compound X, also absorb in the region of 2.92 microns. I n compounds in which the hydroxyl hydrogen is intramolecularly bonded to a carbonyl group-for example, in salicylaldehyde or o-hydroxybenzophenone-no hydroxyl stretching band is observed in this region. Flett (4) gives a more thorough discussion of the interaction of a wide variety of functional groups with phenolic hydroxyl hydrogens. QUANTITATIVE ANALYSIS
Figure 2 s h o w that i t is possible to analyze mixtures of phenols containing hindered and free phenolic hydroxyl. Samples containing as many as tn-o or three different types of intramolecularly bonded phenolic hydroxyl compounds in addition to free and/or hindered phenolic hydroxyls can also be analyzed. Separate peaks in the 2.75-micron region may be resolved by the DK-2 if the peaks are sharp and are 0.015 micron or more apart. Rough quantitative n-ork is possible even iyith unresolved peaks if they are not too broad. For example, mixtures of compounds n hose intramolecularly bonded hy201 2
ANALYTICAL CHEMISTRY
M I C R 0N S
Figure 2.
Near-infrared absorption spectra of p-cresol and
2,6-di-tert-butyl-p-cresol in 2.7- to 2.8-micron region (1 0-cm. cells)
---0.262
0.261 mM p-cresol in carbon tetrachloride m M 2,6-di-tert-butyl-p-crerol in carbon tetrochloride Wave length scale not corrected for 0.01 3-micron instrument error
droxyl peaks are at 2.853 and 2.864 microns were analyzed with some success. Such a mixture has but one peak, SO that both constituents were determined by simultaneous equations. Phenolic niixtures containing u p to four components have been successfully analyzed using a single scan of the 2.7- to 3.0-micron region. I n favorable cases. six or more phenolic components could be determined sirnultaneously. Assuming that a n absorbance of 0.01 is easily measured, it can be calculated from the data in Table I that i t should be possible to detect down to about 0.05 p.p.m. of phenolic hydroxyl in the carbon tetrachloride solution under investigation, using 10-em. absorption cells. I n practice, this is difficult because even traces of water (a few parts per million) have some absorption at about 2.75 microns. A practical limit of detection might be better placed a t 0.25 p.p.m. of phenolic hydroxyl. Presuming a 1% solution of a sample in carbon tetrachloride, this would mean a limit of detection of the order of 25 p.p.m. of phenolic hydroxyl in most cases. By the use of larger samples, differential techniques, and extensive precautions, this limit of detection could undoubtedly be pushed loiver. INTERFERENCES
All other free-hydroxyl-containing compounds absorb in the 2.7- to 3.0micron region and may interfere with the phenolic hydroxyl bands if they are close together. A future paper will give data which have been presented on these compounds ( 7 ) . The absorption maxima of these other types of hydroxyl compounds and the average molar absorptivities obserred are sum-
marized in Table 111, $ 0 that they may be considered n-hcn a w s s i n g nearinfrared for the determination of phenolic hydroxyl. Because the first OT ertone of the carbonyl band and the fundamental K-H stretching band of most amides and aniines are in the 2.8- to 3.0-micron legion, they also must be considered as possible interferences in the determination of intramolecularly bondcd hydrouyl. The molar absorptil itks fcr most carbonyl compounds in the 5.0-micron region are from 2 to 4 liters per mole-cx. so that they must be present in large amounts to interfere. The molar absorptivity for a typical arolnatic amide, A--ethj 1toluamide. is 41 liters per mole-em. a t 2.89 microns, so that even small amounts of compounds with free N-H groups may sometimes cause difficulty. Formaldehyde absorbs a t 2.772 microns and has an absorptivity of 19 liters per molecm. and also may interfere. Large amounts of polar compounds may affect the molar absorptivity of the phenolic hydroxyl band, and thus calibration curves should be run using known samples that represent tlic expected sample composition as closely as possible. CONCLUSION
The near-infrared region betn-em 2.7 and 3.0 microns offers a convenient tool for the rapid, selective, and sensitive determination of phenolic hydroxyl compounds. Advantages over chemical methods include greater selectivity, rapidity, use of small samples, and good sensitivity. The accuracy and precision are equivalent to those which can be expected from other photometric procedures. I n addition to the qualitative and quantitative aepccts
of the technique. the information obtained on the orientation of the hydroxyl groups within the molecule is of value in studies of the reactivity of phenols slid phenolic antioxidants. LITERATURE CITED
(1, Ihrii:ird. D.. Hargrave, K . R., Higgins. G . 11.C.. J . Chetn. SOC.1956, 2845. (2) Comeshall. S U , , J . -4m.Chem. SOC. 69, l%20 (19471
13) ~, Unviea. 11 31.. Truns. Faradav SOC.
36, 11 14’(1O40‘1, (4) Flett. 31. Pt. C., Spectrochim. Acta 10, 21 ( 1 9 5 7 , ( 5 ) Fox, J. J.. Martin! A . E., Proc. Roy. SOC. (Loridon) A162, 419 (1937). (6) Gp_ddri. R . F.. ASAL. CHEX.29. 1770 (19or).
( 7 ) Goddu, R. F., Pittsburgh Conference
on A4nalytical Chemistry and Applied Spectroscopy. 3Iarch 1958. (8) Goddu. R . F., Delker, D. A., ANAL. CHEY.30, 2013 (1958,. (9, Kuhn. 1,. P.. J . -4m.Chem. SOC. 74, 2492 ( 19L52\.
Table 111. Near-Infrared Absorption Data on Other Hydroxyl-Containing Compounds Absorbing in 2.7- to 3.0-Micron Region
Av. Molar Region of AbsorpAbsorption tivity, Functional Maxima, e, Liter/ Group c1 Mole-Cm. Acids (monomer) 2.82-2 84 10-100~ 50 .4lcohols 2.74-2 78 .. Hydroperoxides Aliphatic 2.81 85 Aromatic 2.82,2.84 40,30 Oximes 2.77-2.78 200 a Molar absorptivity of acids a t 2 83 microns is a strong function of concentration. (IO) Lord, R . C., Merrifield, R. E.. J . Chem. Phys. 21, 166 (1953). (11) Luttke, W.,Mecke, R., 2. physik. Chem. Hoppe-Seyler’s 196, 56 (1950).
Determination of Terminal Epoxides N e a r-Inf r a red Spect ro photo metry
(12) Pauling, Linus, “Sature of the Chemical Bond,’’ 2nd ed., p. 316 ff., Cornel1 Univ. Press, Ithaca, S. I-., 1948. (13) Sears, W. C., Kitchen, J. J., J . A m . Chem. SOC.71, 4110 (1949). (14) Stone, P. J., Thompson, H . W., Spectrochlm. Acta 10, 17 (1957). (15) Whetsel, Kermit, Roberson, K.E., Krell, M. W.,ASAL. CHEM.29, 1006 (1957). (16) Ibid., 30, 1594, 1598 (1958). (17) Whetsel, Kermit, Roberson, W. E., Krell, M. W., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1958, (18) Wulf, 0. R., Jones, E. J., J . Chem. P h y s . 8 , 745 (1940). (19) Wulf, 0. R.: Jones, E. J.! Deming, L. S..Ibid.. 8. 753 11940) (20) \T;ulf, 0. R.,Liddel, Urner, J . A m . Chem. Soc. 57, 1464 (1935). (21) Wulf, 0. R., Liddel, Urner, Hendricks, s. B., Ibid., 58, 2287 (1936). RECEIVED for review l f a y 8, 1958. Accepted July 28, 1958. Presented in part a t the Pittsburgh Conference on -4nalytical Chemistry and iipplied Spectroscopy, March 1958.
by
ROBERT F. GODDU and DOROTHY A. DELKER Research Center, Hercules Powder Co., Wilmington 99,Del.
,Terminal epoxides have sharp absorption bands in the near-infrared region at 1.65 and 2.20 microns. The latter are several times more intense than the former. These bands are useful for the rapid determination of epoxides in a variety of mixtures. In particular, both terminal epoxides and terminal olefins may be simultaneously determined in the same sample. Other oxygen rings d o not interfere. The range covered b y the method is from 10 y per ml. of
’ ‘C-CH:
to pure epoxides.
The
\o.
accuracy and precision over most of the range are to =!=l to 2% of the amount present. If suitable standards are available, near-infrared can often replace chemical procedures which usually require more sample and more time, and in some instances, d o not give appreciably better accuracy or precision. Also, preliminary work indicates that it may be possible to improve the accuracy and precision of this method to *0.570 b y using high absorbance reference techniques and a 0.0- to 0.1-absorbance slidewire.
A
publication ( 2 ) described some advantages of near-infrared spectrophotometry n-hen applied to the determination of unsaturation-in particular, terminal unsaturation-in organic compounds. I n extending the uses of the near-infrared region (1.0 to 3.1 microns), another functional group method, similar to that described, has been developed. This concerns the determination of epoxides. It iq also applicable to the analysis of olefin-epoxide mixtures. Bearing in mind certain similarities between epoxides and olefins, the marinfrared region was investigated as a tool for the determination of epoxides. The spectrum of a typical terminal epoxide, epichlorohydrin, is shown in Figure 1. The sharp bands a t 1.65 and 2.20 microns are very similar to those usually found in terminal olefins ( 2 ) . The work reported here is based on the use of these bands to determine epoxides and differentiate them from terminal olefins. To the best of the authors’ knowledge, these bands have not been studied previously. Other ouygen rings such as those in oxetanes, furans, and dioxane do not absorb in this region. RECEXT
EXPERIMENTAL
Beckman Model DK-2 Spec’rophotometer. For qualitative s c m ning, t h e instrument was set u p as described ( 2 ) . For quantitative n-ork, it was desirable to operate a t a conqtant nominal slit rather than a t a constant sensitivity, because of the large varintions in nominal slit which occur as a function of both the fluctuations of room temperature during the qummer and the aging or replacement of the lead sulfide cell. Otherwise the instrument was set up as described prrvioiisly ( 2 ) . The effect of variations in nominal slits is more pronounced n-ith the DK-2 than with the Cary Model 14 because of the latter’s narrov-er spectral slits a t all operating conditions. A variable nominal or spectral slit may lead to a variable molar ahsorptirity. The slit width data are in Table I. Cary Model 14M Spectrophotometer. T h e Carv n a s used for quantitative work. The settings were: slit height o u t : speed 10 A . per second; and sensitiritv such t h a t t h e slit widths in Table I n e r e obtained. -4 -0.0 t o 0.1, 0.1 t o 0.2-absorbance slide-wire was used n.hen necessary. 3latched 1-em. cells !{-ere used for most of t h e work reported. Longer cells m a p be used t o obtain greater sensitivity and either fused silica or VOL. 30, NO. 12, DECEMBER 1958
2013
