Flowing Oxygen Schoniger Combustion for Large Samples SIR: The problem of burning multigram samples of heavy organic liquids and solids for low-level elemental contents, for which the Schoniger (3) and Wickbold (6) techniques by themselves do not apply, was solved by combining appropriate features of each apparatus. This was done by providing flowing oxygen to a combustion-in-abottle technique followed by the Wickbold absorber. Samples as large as 8 grams have been burned, and some ash content of the sample is permissible. Liquids and low-melting solids are mixed with a flame retardant, and highmelting solids are formed into a pellet before combustion. The method is rapid, and the apparatus is easy to construct.
LEAD SHOT
Sulfur
772
-
2-LITER
BOROSILICATE
500 ML
Apparatus. T h e apparatus which was developed is shown in Figure 1. T h e platinum wire holder and the sample cup are similar t o those described by Barney et al. (1, 2) The combustion chamber consists of a 2-liter borosilicate glass bottle with a groundglass neck. Inlet and outlet tubes for the oxygen are fused into the shoulders of the bottle. The absorber is attached to the outlet with a short connecting tube. The stopcock near the bottom of the bottle is useful as a drain when washing the bottle after a combustion. The sample holder is weighted with lead shot which makes a clamp unnecessary. The connecting wire and the basket are made of platinum. The length of the connecting wire is such
SUCTION FLASK
/
//
I
\\ Figure 1 .
Assembly of apparatus
that the basket occupies the central area of the combustion bottle. The sample is placed in a sample cup which is in turn placed in the platinum basket. These sample cups are made from 19- by 90-mm. extraction thimbles and are cut to allow a thin piece of the thimble to extend above the cup. This thin section is a tab which is lighted for the ignition step. Various heights of thimbles are used for different size samples.
Analyses of Samples of Known Chlorine, Phosphorus, a nd Sulfur Content
Type of material burned White oil
Dibasic acid
ANALYTICAL CHEMISTRY
ABSORBER
FRITTED GLASS PLATE
EXPERIMENTAL
Element desired Chlorine
WICK BOLD
g COARSE
02
Table 1.
I!
Amount Amount added, recovered, Sample p.p.m. size, grams p.p,m.
Per cent recovered
3.111 3.017 3.055 2.392 2.452 2,396 3.201 2.515
540 540 540 201 20 1 20 1 200 200
540 530 550 197 195 198 196 20 1
98 102 98 97 99 98
4.124 3.589 2.657 2.636 2.656 2.591 4.662 5,025 2,695 4.654
1055 989 83 97 39 46 12 24 65 39
1040 968 77 98 36 47 12 24 69 39
98 98 93 101 92 103 100 100 106
100
101
100
The 500-ml. suction flask serves as a rough flow meter. Its stopper has a second hole in it, and the oxygen flow is adjusted so that there is an excess exiting from this open hole when the apparatus is fully assembled and the vacuum source is connected. The hole also serves as a safety valve if there is extra pressure built up in the combustion bottle during the burning. -4calibrated flow meter ahead of the suction flask is used to measure the oxygen flow and makes it easier to reproduce the rate. The absorber is identical with that used on the Wickbold apparatus (6). Various absorbing solutions are used for the different elements to be determined. Procedure. For the combustion of liquids and low melting solids, magnesium oxide is used as a flame retardant. To prepare a liquid sample for combustion, a thin layer of magnesium oxide is placed in the bottom of the sample cup. Up to 8 grams of sample is weighed into it; half that weight of magnesium oxide is added, and the two are mixed. The sample is then placed in the sample holder and is ready for burning. Solid samples are pelleted in a n Amlied Research Laboratories’ briq&ting press using a 0.5-inch diameter die. Ignition is obtained by laying the pellet on a short filter paper fuse in the sample cup. The sample is wet with two drops of isooctane, and the fuse is then ready for ignition. The apparatus is assembled as shown in Figure 1. Seventy milliliters of a suitable absorbent is placed in the absorber. The combustion bottle is closed with a stopper, and vacuum is
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 AND 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
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