applied to the absori3er. The oxygen flow is adjusted so that there is an excess exiting at the Pole in the stopper in the suction flask. A flow rate of 45 liters per minute is typical. The stopper is removed from the combustion flask. The fuse is ignited with a microburner, and the sample holder is quickly inserted into the bottle. The burning proceeds with a bright flame. A dl11 or yellowish flame shows too low a n oxygen flow rate, and usually a large amount of carbon is produced. K h e n the burning is complete, the apparatus is allowed to cool. The absorber solution is drawn into a beaker, and the rinsings from the absorber, the combustion bottle, the connecting tubing, and the sample holder are added to the beaker. When magnesium oxide is used, it is also added to the beaker containing t h e washings. When the desired element exists at low concentration, the solution in the beaker may be concentrated. T h e desired elements are determined by standard procedures. Phosphorus is determined colorimetrically by the molybdivanado procedure as described by Barney ( I ) . The sulfur which is converted t o sulfate in the absorbing solution is titrated with barium perchlorate as described by Wagner ( 4 ) . The chloride formed is titrated potentiometrically by argentometric means.
During combustion, the bottle is surrounded by a metal desiccator guard as a safety precaution. The entire apparatus is contained in a plywood box with one open side. The open side faces a wall to minimize the danger from fragments if a n explosion occurs. Although no explosions have occurred in any of our combustions, these precautions are felt to be necessary. DISCUSSION
T h e described burning technique has been applied t o solids and liquids with molecular weights above the lubricating oil range. Solids which melt or sublime above 175' C. are easily combusted after pelleting. Lower-melting solids and high-boiling liquids are mixed with magnesium oxide. Phosphorus, sulfur, and chlorine have been determined. Suitable absorbing solutions are used for each element, and the elements sought are determined as outlined above. Typical recoveries are shown in Table I. One of the advantages of the new technique is the compactness of the apparatus. The combustion flask takes up much less laboratory bench space than the larger sized Schoniger flasks, and their safety shields. I n the Schoniger procedure, a dis-
advantage of the larger containers is the formation of an aerosol which does not readily settle or dissolve in the bolution m the flask or bottle. K i t h the larger containers plus the larger samples, the water of combustion forms a mist which usually contains some of the desired element. Analysis cannot be completed until this aerosol settles or is dissolved, and this waiting period increases the analysis time markedly. With the new apparatus, the oxygen f l o w through the bottle and sweeps the aerosol into the absorber. Thui, there need be no waiting for the -elution of the aerosol. LITERATURE CITED
(1) Barney, J. E., Bergmann, J. G., Tuskan, K.G., ASAL. CHEM.31, 1394 (10593. ( 2 ) Barney, J. E., Tuskan, W. G., Hens-
ley, A. L. (to Standard Oil Co., Chicago, Ill.? a corporation of Indiana), U. S. Patent 3,058,813 (Oct. 16, 1962). (3) Schoniger, IT., Mikrochini. Acta 1955, 123. ( 4 ) Wagner, H., l b i d . , 1957, 19. ( 5 ) lT7ickbold, I?., dngew. Chem. 69, 530 (195i).
L. L. FARLEY
R.A . WIKKLER California Research Cow. Richmond, Calif.
Hydrogen Nuclear Magnetic Resonance Chemical Shift Correlations in Halogen Derivatives of Benzene and Alkyl Benzenes Ferdinand C. Stehling, Research and Development, Humble Oil and Refining Co., Baytown, Texas H E UETERMINATIO v OF molecular T s t r u c t u r e by nuc1e;ir magnetic resonance (XMR) general y requires chemical shift correlations obtained from compounds of known structure (2). Halogenated mono-nuclear aromatic hydrocarbons constitute an important class of compounds for which only a limited number of such corre ations are available ( I , 2, 4, 6, 7 ) . To facilitate the analysis of compounds of this type, the spectra of 55 compounlls were obtained, interpreted, and summinized in chemical shift charts. The application of such chemical shift charts to the determination of molecular structure is discussed in detail in ( 2 ) .
EXPERIMEbTAL
Compounds used in this study n-ere obtained from various commercial sources or synthesized in this laboratory. These include three fluorine, 29 chlorine,
50 bromine, and eight iodine compounds with from one to six ring substituents. Hydrogen N M R spectra were obtained with a modified 60-mc. Varian HR-60 NMR spectrometer equipped with proton control of the magnetic field. Band separations measured with this system were reproducible within 0.05 p.p.m. -411 samples were run at 50 =t15% (volume) concentration in carbon tetrachloride, with tetramethylsilane (TMS) added as an internal standard. Although data obtained on more dilute solutions would be of greater theoretical interest, a high solute-concentration level was chosen because many analytical samples must he run at high concentrations to detect and identify impurities. Chemical shifts in first-order spectra were selected by standard methods, whereas those in ilB, A2B, and A2B2 spectra were chosen using the published tables of Corio (3). No attempt was made to calculate chemical shifts exactly for more complex spectra. I n these cases the center of area of a
band was taken as an approximation of the chemical shift. I n spectra where there was extensive overlap between bands, the chemical shift was taken as a range which included the width of the superimposed bands. Chemical shifts are given in 7 units (8). RESULTS A N D DISCUSSION
Chemical shift correlations are summarized in Figure 1. The nomenclature and abbreviations in these charts are those used by Chamberlain (2). The analytical utility of this chart is apparent from the regularity of the chemical shifts and the relatively narrow ranges over which various hydrogen types resonate. Most of the data may be summarized by a few generalizations: Ring H. T h e chemical shift of a hydrogen a t o m on a benzene ring is determined primarily b y t h e number a n d t h e identity of t h e substituents VOL. 35, NO. 6, M A Y 1963
773
resonance (relative t o hydrogen substituent)
Group
F CHa CHXI. - , CH-Br -
+
+ 0 . 2 p.p.111. + O . 15 -0.10 -0.15 -0.20 -0.30 -0 45
c1
CHC12 Br
I
+
ortho to that hydrogen. I n Table I the effect of various substituents on the chemical shift of an ortho hydrogen is summarized. The chemical shift of an
T SCALE
RING SUBSTITUENTS
-+
I
CHzCI, CI
02
Ring H
CH CI CH3
I
Figure 1 .
WWn -yo
-CH3
02
ad m, P
o,m,p
Ring H
-
CHCICH3 Ref. Ring H
I
0,
m, P
I
-CH3
SHIFT,
I
PPM.
Hydrogen nuclear magnetic resonance chemical shifts in halogenated benzenes and alkyl benzenes
In 1 -chloro-3-methylbenzener
2-hydrogen o with reference to CI, 1,3-dichlorobenzene:
774
9
oz o m,p
CHEMICAL Examples:
8
7
0
m*p'o,m,p
CHzCI Ref.+ CI Ref.+
6
5 I
"1""1""1"'~1""1~''~1'~"1""~'"'1""~""1""1"'~1''''1"''~'1"r
Ring
CH3 Ref.+
I
4 I
CI Ref.+ GI, CH3
+
3
2
1
1 ~ " ' 1 " ' ~
F Ref. CH3 Ref.+
tity of X. Ortho CH2X, X, and CHB substituents cause the methylene absorption to shift downfield, relative to a CH2X group with no ortho substituents. There is, however, considerable overlap, so i t is not always possible to determine the number of ortho substituents from the chemical shift of the methylene band. Ar-CH3. T h e resonance of a methyl group attached t o a benzene ring is shifted downfield by ortho CHzX and X substituents, relative t o a methyl group with no ortho substituents. Again, there is considerable overlap. I n contrast, Chamberlain ( 2 ) has shon-n that a methyl group shifts a n ortho methyl substituent upfield, relative to a methyl with no ortho substituents. Several precautions in the use of
aromatic hydrogen may be estimated by taking the algebraic sum of the substituent effects and adding 3.00. For example, the hydrogens ortho to the iodine atom in iodobenzene resonate a t (3.00) = 2.55 T, about (-0.45) and the 2-hydrogen in l-chloro-3methylbenzene resonates a t about (-0.15) (0.15) (3.00) = 3.00 T. Application of the additivity rule and of the data given in the table reproduces all the aromatic-hydrogen chemical shifts presented in the charts within 0.20 p.p.m. and most of the shifts within 0.10 p.p.m. Ar-CH2X. For t h e halogen compounds included in this study, the chemical shift of the methylene hydrogens in Ar-CH2X is, t o a close approsirnation, independent of the iden-
Table I. Effect of Substituents on the Chemical Shift on an Ortho Hydrogen Effect on 2-H
ANALYTICAL CHEMISTRY
D
with reference to CHI;
4-hydrogen p to CI, o to CH3, -CH3
2-hydrogen designated o 2 to CI
m to CI.
these shift charts should he observed. Conil)ariuon of our data with those of Tier.; ( 7 ) for seven compounds s h o w that the shifts in ea. 50% carbon tetrachloride solution an: a n average of 0.16 p.l).m. greater than those measured in 2% solution. The direction and average magnitude of these solvent shifts are in accord with those observed for other aromatic solutes in nonaromatic solvents ( 5 ) . Experiment. n.ith 10 compounds h:tve indicated that estral)olation of our data t'o very low eonccntrations by subtracting 0.16 p.1i.m. gives a maxinium error of 0.10 p,ii,ni. Examination of a limited num1wr of halonaphthalenes and biphenyls
has shown that the effects of halogen substitution on the chemical shifts of ring hydrogen resonances for these compound types differ markedly from those observed in benzene derivatives. Consequently, the data given should not be extrapolated to polynuclear aromatic halides ithout further investigation.
for synthesizing some of the compounds used in this investigation.
ACKNOWLEDGMENT
stein, H. J., .'High Resolution Suclear Magnetic Resonance," p. 424,lIcGrawHill, New York, 1959. ( 6 ) Spiesecke, H.. Schneider, W . G., d . Chem. Phys. 35, 731 (1961). ( 7 ) Tiers, G. V. D., private communicntion. St. Paul. Minn.. 1958. (8) Tiers, G. 1'. D.. J . Phys. Chem. 62, 1151 (1958).
The author expresses his appreciation to Y. F. Chamberlain for helpful discussions concerning the interpretation of the spectra, to T. J. Denson and R. K. Saunders for securing the spectra, and to B. F. .Irniendt and J. L. Tveton
LITERATURE CITED
(1) Bothner-By, A. A., Glick, R. E., J . Chem. Phys. 2 6 , 1651 (1957). ( 2 ) Chamberlain, S . F., ANAL. C H E ~ I . 31, 56 (1959). (3) Corio, P. L., Chem. Rei?.60,363 (1960). (4) Corio, P. L., Dailey, B. P., J . Am. Chem. SOC.78, 3043 (1956). (5) Pople, J. A , , Schneider, W. G., Bern-
The Melt-in-Rod Technique for Spectrochemical Analysis of Aluminum Samples limited in Quantity Nicholas Christ, Reynolds Metals Co., Metallurgical Research Division, Richmond, Va.
laboratories are called upon to analyze materials which do not meet the sample requirements of the €'xisting analytical procedure. In the case of aluniinuni and its alloys, the uscal sample form is a chill cast disk prepared as described in AIST,II method E-2 811 7-10. Xl)prosimately 50 to 60 grams is the minimum quantity of metal that may be used tlJ prepare a satisfactory chill cast disk. When the sample n-eighs on the order of 5 to 10 grams, however. serious clifficult,ies are encountered if it is to be analyzed :,l~ectrochemically. A sain1)ling method 11x5 been developed for the qmtrochemical analysis of aluiiiinuni samples such as solid chunks, chips, sheet,. or foil xeighing a t least 5 to 7 grams. Dur.ng excitation of these sarn])les in the "as is" condition, overhcating occurs. In thin samples, such :is foil?, localized melting occurs a t the ,sirface undergoing escitation. IJccau~cof overheating, a change may occur in the inten.ity ratio of t,he analJ-tical iine to t h e internal standard litie, :is compared to a more massive sample such as a chill cast disk of the same composition. If a change does: occur in the intensit)- :.atio, the spectrochemical value obtaiiicd will he in error. The extent of the crrc'r delmids on the sample form anti the degree of overheating associated with it. Since the saniplcs oi interest are in the form of chunks, chips, sheet, 0.' foil, the analytical err'ors that result are not readily predictable. S e v ( ~ n 1e x a m l h may be given to illustrate the effects 0.'sample form on the accuracy of spectrochemical results. , of 3Cl03 sheet samples. are r.oted part,icularly
IMI-
ST SPECTROCHEMICAL
for Fe and hln. The mean error for 11 samples was -0.07% for Fe and -0.15% for bfn. For both Fe and M n the relatire error equals 14y0 of the amount present. Errors in analysis of 1100 alloy foil samples compacted into pellets, 1-inch diameter by inch thick, are noted particularly for Fe. The mean error for seven samples was
-
3d-
ALUMINUM
ROD WITH PREFORMED GRATERS
-0.227, and the relative error bas 45% of the amount present. While metallurgical differences may also contribute to the above discrepancies i t is felt that any metallurgical effect is minor in comparison to the overheating effect. The melt-in-rod technique attempts to niininiize overheating by placing the limited quantity of sample in close contact ~ i t ha large ma.? of metal. Some of the heat generated during the escitation of the sample is then conducted airay from the sample to the rod. The melt-in-rod method is applicable to the analysis of 3003-type alloys and pure aluminum alloys such as 1100- and 1200-type alloys. It may be applicable to other alloy types; however, only the alloys mentioned abore have been invedigated. The accuracy approaches routine spectrochemical accuracies when i)roper correction factors are assigned. DESCRIPTION OF METHOD
,
If
.4"-
_____(
Figure 1. Apparatus used to prepare melt-in-rod samples
The melt-in-rod method involves melting the sample and then forcing it into an aluminum rod with a preformed crater. Dimensions of the rod used in this technique are shown in Figure 1. The two '/*-inch holes a t the bottom of the crater allow air to escape as the rod is pushed into the molten metal and also provide support for the solidified sample during machining. Samples which may be analyzed by this method must weigh a minimum of 5 grams in the case of small solid samples, while thin samples such as foils require a t least 7 grams. The greater sample weight is required in the case of foils, since as the sample thickness decreases the ratio of oxides present in the melt increases and less metal is available for preparing the melt-in-rod samples. For VOL. 35, NO. 6, MAY 1963
775
