Table 111. Carboxyl Group Determinations for Attrited Kerogen Concentrate and Trona Acids
Sample Kerogen concentrate" Trona acids
Determination Carboxyl, no. meq./gram 1
2 3 4 1
2 3 4 5
0.45 0.41 0.42 0.42 1.06 1.06 1.14 1.15 1.01
Corrected for pyrite and silicate interference.
boxyl determination brought about by the use of steam distillation to remove acetic acid in comparison with the previously used equilibrium conditions. Additional evidence for the applicability of the steam distillation procedure was obtained from the two ion exchange celluloses (Table 11). Results obtained were within the compositional limits specified in the manufacturers' literature. Table I11 shows the results of several determinations made by the steam distillation method on the kerogen concentrate and trona acid samples. The data for the kerogen indicate good
reproducibility of results and are believed t o represent accurately the carboxyl present in kerogen. The oxygen content of the kerogen concentrate used in this study was 8.5% or approximately 5.3 meq. per gram. As each milliequivalent of carboxyl represents 2 meq. of oxygen, the determined carboxyl content indicates that about 16% of the kerogen oxygen was present in carboxyl groups. Reproducibility obtained for the trona acid sample was less than ,for kerogen; however, the relative deviation of the five determinations was under 5y0. The oxygen content of the trona acid sample was 10.5%; so the determined carboxyl content indicates that about 35% of the oxygen was present in carboxyl groups. A major advantage of the steam distillation method is that it is rapid. The reaction was complete for the cellulose and pulp samples within 8 hours and complete for the organic acids of known composition in 4 hours. The advantage is particularly apparent for kerogen concentrate and trona acid samples where the calcium acetate exchange reaction was essentially complete after 6 to 8 hours as compared t o a 1- to 2-week reaction period by other methods. This relatively short reaction period also diminishes the mineral interference and improves the reproducibility.
LITERATURE CITED
( 1 ) Blom, L., Edelhausen, L., van Krevelen, D. W., Fuel 36, 135 (1957).
(2) Doering, H., Das Papier 10, 140 (1956); C.A . 50, 12467f (1956). (3) Fuchs, W., Brennstoff-Chem. 8, 337 (1027); C.A. 22, 3973 (1928). 14) Fuchs. W.. Fuel 22. 122 (194.1) ~ ~ . _ _ , ( 5 ) Fuchs; W., Freiderger Forschungsh 23C, 84 (1956); C.A.51, 13354e (1957). (6) Ihnatowicz, A., Prace Glowneao Inst. Gornichtwa, Communication Ro, 125 (1952). (7) Li, S., Parr, S., Ind. Eng. Chem. 18, 1299 (1926). ( 8 ) Ludtke, M., Angew. Chem. 48, 650 (1935); C.A.30, 855a (1936). (9) Ludtke, M.,Papier-Fabr. 32, 509, 528 (1934): C . A . 29, 5264' (1935). (10) Smith, J. W., Hiiby, L. W., ANAL. CHEM.32. 1718 11960). ( 1 1 ) Sobue,"., Okubo,' M., Tappi 39, 415 (1956). (12) Wilson, W. K., Mandel, J., Zbid., 44, 131 (1961). J. I. FESTER W. E. ROBINSON
Laramie Petroleum Research Center U. S. Bureau of Mines Department of the Interior Laramie, Wyoming Division of Analytical Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963. This work was conducted under a cooperative agreement between the Bureau of Mines, T i . S. Department of the Interior and the University of Wyoming. Trade names and references to specific commercial materials are for purposes of identification and do not imply endorsement by the Bureau of Mines.
Quantitative Determination of Vinyl Acetate Content of Ethy lene-Vi ny I Acetate Co poIy me rs by High-Reso Iutio n Nuclear Magnetic Resonance SIR: The recent appearance of an article on the analysis of ethylene copolymers by high-resolution nuclear magnetic resonance ( I ) prompted presentation of results on a quantitative method developed in this laboratory for determining the vinyl acetate content of ethylene-vinyl acetate ccpolymers by high-resolution nuclear magnetic resonance. The method developed here is based on the fact that the ratio of proton counts per unit weight of the monomers ethylene and vinyl acetate is approximately two to one. By measuring the proton content per unit weight of a copolymer, the vinyl acetate content of the copolymer can be determined. Since the method depends on a measure of the total proton count per unit weight of copolymer, overlapping of peaks is of no consequence. Therefore, the method can be used fpr copolymers with a wide range of vinyl acetate contents above S%, which is the practical accuracy limit of the method. 1394
0
ANALYTICAL CHEMISTRY
I n ethylene-vinyl acetate copclymers, the following monomer units are present: -CHz-CHz-
-CH-CHr-
A
L o AH* (1)
(11)
The spectral lines corresponding to the various protons in the monomer units (I) and (11) are shown in Figure 1. I n the same spectrum, the line at approximately 4.1 p.p.m. is caused by ferrocene, which is added to the solution as a quantitative internal standard for proton content determination of the copolymer. The larger peak on the left is caused by the solvent monochlorobenzene. The relationship between the weight fraction of vinyl acetate in the copolymer and the measured ratio of the peak areas is expressed :
where I ,
peak area of internal standard ferrocene. I , = peak area due to the protons in the copolymer. The area due to the tertiary methylene proton in rnonomer unit (11) which is located a t approximately 4.75 p.p.m. is not included. W , = weight of internal standard used. W , = weight of the copolymer used X = proton count per unit weight of the copolymer to be determined. Equation 1 may be rewritten as: =
But the proton count per unit weight of a copolymer composed of the monomer
units (I) and (11) with. proton counts 4 5 per unit weight of - and __ 28.03 86.05’ respectively, can be expressed as:
x
4
= --
28,03
(1
-
CY)
.t86.05 __
’ ’
(3)
where = weight fritction of vinyl acetate in the copolymlx. When X is determined from Equation 2, the weight fraction of vinyl acetate in the copolymer may be obtained from Equation 3. Since the relationship between CY and X is linear, the determination of a from X can be simplified with a graph of CY us. X where 0 and 1 are the limits for a. EXPERIMENTAL
Apparatus. The areas of t h e spectral lines were obtained a t a frequency of 60 me. per second with a Varian V-4302 DP-60 K M R spectrometer equipped with a l:!-inch electromagnet, a magnetic flux stabilizer, and a V-3521 N M R Integrator. T h e probe was maintained a t the desired temperature with a Varian V-4340 variable temperature probe accessory. A Moseley Model 2D2 X-Y recorder was used for recording the integrals. Reagents.) Ferrocene used as the internal standard was a C . P . grade from Matheson Coleman & Bell with a m.p. range of 173’ t o 174’ C. T h e monochlorobenzene u w d as a solvent was a certified reagent grade material from t h e Fisher Scientific Co. 130th chemicals were blank checked, and no detectable irnpurif y peaks were found in their NMR spectra. The noctacosane (&Ha) hydrocarbon was a reagent grade materid from Distillation Products Industries. Polyvinyl acetate was obtained from the Bakelite Division of Union Carbide. Procedure. Dissolve about 0.1 g r a m of t h e copolyme- and 0.2 gram of ferrocene, both weighed t o the nearest 0.1 mg., in a b o u t 3 ml. of monochlorobenzene. ‘I’o avoid the loss of ferrocene on heating, the copolymer is first dissolved in the solvent by heating i t gent1 on a hot plate and the ferrocene a d d e l to the cooled solution. For those copolymers containing insoluble solids, the sample solutions are filtered before use. Set rf. power level c’f the V-4311A unit to 70 db. and thc magnetic field sweep rate to about 15 c.p.s. per second. Balance the rf. reference phase carefully until i t is exactly in the absorption mode. Stop spinning 1 he sample tube before peak integration. Measure the heights of the integral curve to the nearest 0.1 mm. with a Vernier caliper. RESULTS AND DISCUSSION
Before the present met’hod was developed, another procedure which is very similar to the one reported recently was investigated ( 1 ) . The method was based on the principle tk!at the peak due to the methyl group in monomer unit
I
13)
I I
I1
i
I.?)
Figure 1 . Spectrum and integral of ethylene-vinyl acetate copolymer sample s o b tion Peak 1: -CHz-
0
I
II Peak 2: CH3-C-0-CH
I cH2 Peak 3: ferrocene
0
ll
Peak 4: CHa-C-0-CH
I I CHz I
values listed in Table I1 is an average of about 10 measurements. The method is based on the condition that both the internal standard and the copolymer to be analyzed should be completely dissolved in the solvent to give a homogeneous solution. Serious errors may result from improper preparation of the solution, which may cause loss of part of the weighed cocomplete solution of the copolymer sample. vinyl acetate contents at the concentrations used. Even a t room temperature, satisfactory results were obtained for copolymers with vinyl acetate contents of 30% or above. For copolymers with low vinyl acetate content, probe temperatures as high as 120’ C. are needed. Since the NMR absorption intensity is inversely proportional to the temperature, running a sample a t an unnecessarily high temperature should be avoided. For Some unknown reason it was found during this work that a transient ringing, which gave high integrated intenaities for ferrocene followed the
Peak 5: chlorobenzene
(11) was resolved from the peaks caused by the methylene protons. From the measured ratio of these two groups of peaks, the vinyl acetate content of a copolymer could readily be determined. However, several experimental problems were encountered which made the method unattractive. With increase in vinyl acetate concentration, the changing environment of two of the three methylene protons in the monomer unit (11) produced tailing and partial overlapping of the methyl peak. Difficulty was also encountered in preparing homogeneous solutions of high enough concentration. The method described in this paper was developed to eliminate these difficulties. The new method using ferrocene as an internal standard was first tested by using nine blends of polyvinyl acetate and pure n-octacosane. A correction was made to account for the terminal methyls in the calculation. The results of the analyses are shown in Table I. Each of the values listed in Table I is an average of about 10 successive integrations. As a typical example, the standard deviation for blend No. 5 was *1.3’%. To make sure that the riiethod is also applicable to commercial cq)olymer\, a series of six commercially available copolymers were analyzed. The results
Table I. Vinyl Acetate Content of Polyvinyl Acetate and n-Octacosane Blends
Present, weight Blend
yc
1 2 5
10 33 19 88 29 90 39 (35 50 05
6 7 8 9
90 00
3
4
Found, weight % 10 2
Difference,
u/o
13 22 20 --I 15 + O 05 - 1 20 + I 04
+O
20 1 30 1
+o +o
50 1 59 1 70 9 80 2 90 3
-0 38 + O 30
38 8
60 30 69 86 80 58
Table II. Vinyl Acetate Content of Commercial Ethylene-Vinyl Acetate Copolymers
Vinyl acetate _ _ _ Present, Found, Std. weight weight dev., c/c % %
~
Copolyniers Du Pont Elvax
31-33
33.4
1. I
220 I j u Pont Elvax 250
28-30
29.3
1 3
28-30
30 9
I 0
KA5021 Hayer Ievapren 450 Buyer Levapren
30
30 0
0 6
45
43 5
0 7
70
69 0
18
150
Du Pont Elvrtx Rayer T,evapren
KA3023
VOL. 36, NO. 7, JUNE 1964
1395
sharp peak of ferrocene. By stopping the spinning of the sample tube and slightly detuning the field homogeneity controls most of the ringing was eliminated. The audio modulator phase balance of the V-3521 Integrator was critical in measuring the area integrals, because the solvent peak and sample and reference peaks could not be adjusted to exact absorption modes siniultaneously with an improperly balanced 2KC audio modulation phase.
The residual intensity from the large solvent peak introduced error. In quantitative S M R , rf. saturation is always a problem. This is especially true of the present work in which the peak intensities of diluted polymer solutions are being measured. The possibility of error due to rf. saturation a t the rf. power level of 70-db. units a t the magnetic field sweep rate corresponding to about 15 c.p.s. per second was checked and found to be negligible.
LITERATURE CITED
(1) Porter, R. S., Nicksic, S. W.> Johnson, J. F., AKAL.CHEM.35, 1948 (1963). HVSGYrr CHEN' MARSHALL E. LEWIS
Research Division U. S.Industrial Chemicals Co. Division of Sational Distillers and Chemical Corp. Cincinnati 37, Ohio Present address, Research Division, Goodyear Tire& Rubber Co., Akron, Ohio
Com bustion-Conductometric Determination of Less Than 10 p.p.m. Carbon in Tungsten SIR: Tungsten metal which has been processed by floating-zone refining commonly contains less than 10 p.p.m. of carbon by weight. The quantitative determination of carbon in this material is of interest because it affords a method for measuring the effectiveness of the purification process and also because carbon apparently has an appreciable effect on the mechanical properties of the metal (6). However, quantitative determination of the carbon in this low range has proved to be a difficult task with the conventional combustion conductometric method. h limitation of the technique a t lorn concentrations is the magnitude and variation of the blank, a major part of which arises from materials such as iron and tin added to the crucible ostensibly to promote heating and to aid in the formation of a less viscous mixture. Haymes and Ollar (3) reduce contamination from this source by prefusing an electrolytic iron bath in a helium atmosphere. Although the presence of the additive materials in the crucible is apparently essential for the quantitative recovery of carbon from some metals and alloys (2, 4),it is questionable that it is necessary or desirable in the evolution of carbon from tungsten or other refractory metals. For example, it has been observed that when iron chips were fused in the presence of tungsten granules the unreacted tungsten metal was formed into an agglomerate apparently by the stirring effect of the high-frequency field. This has been observed by other investigators such as Huber and Chase (4)>mho recommend the crushing and optical examination of the fused sample for unreacted metal. In the preient work, it was found that rarely would a crushing procedure alone disclose the presence of unreacted tungsten. Careful sectioning of the crucibles, however, showed that in about SOYo of the fusions the tungsten metal could be found intact. Thus it appeared that the elimination of all additives to the crucible by heating the 1396
ANALYTICAL CHEMISTRY
sample directly would not only simplify the operation, but would be a major step toward achieving a reproducible blank and, therefore, a lower limit of determination for carbon. The purposes of this investigation were to achieve improved precision a t the lower carbon concentrations and, in addition, to prove the accuracy of the method when applied to refractory metals, particularly tungsten. The oxidation step was investigated, and a new combustion technique is recommended. Precision and accuracy are reported, and the resultant analytical procedure is outlined in detail. EXPERIMENTAL
Apparatus and Reagents. Induction furnace, Model 521; conductivity cell, illodel 515; aluminum oxide crucible, No. 528-35; crucible covers, S o . 528-42; zirconium oxide crucibles. N o . 501-45; iron chips, S o . 501-77; tin granules, S o . 501-76; tin capsules, Yo. 501-59; and quartzenclosed carbon crucible, No. 550-182 (Laboratory Equipment Corp.). Ba(OH)&olution, 1 gram of Ba(OH)2.8 H z 0 per liter. WC and W2C, 99.9% purity, (hdamas Carbide Corp.). Potassium acid phthalate, primary standard, aqueous solution containing 100 gg. of carbon per liter. Platinum disk susceptors, 1-inch diameter, fabricated from platinum foil and formed to fit the bottom of the sample crucible. Preparation of Samples. Samples obtained as solid metal are crushed in a hardened steel mortar until the bulk of the material passes a 16-mesh screen and is retained on a n 80-mesh screen. T h e small amount of material passing the fine screen is recombined with the sample granules because carbon segregation may occur. A considerable amount of iron may be introduced into the sample during this operation and may result in high values for carbon if not completely removed. This is accomplished by digestion in 1 : 1 HCI followed by thorough rinsing with water and drying with ether. Samples in powdered form do not require
special treatment, although the combustion conditions described later for pulverized metal are modified for powders. Procedure. Because instrumental reliability may vary from day to day, it is desirable to make some basic checks on the instrument prior to running samples. The variation in conductivity of the Ba(OH)2 solution subjected to a continuous flow of oxygen for a period of hour should not exceed 0.1 ohm. After completion of this test for instrument stability the furnace is conditioned by heating until readings for an 8-minute period do not exceed 0.3 ohm for multiple determinations, as determined with quartz-enclosed carbon or platinum susceptors of the type to be described. These readings were obtained with solutions that contained approximately 0.7 gram of Ba(OH)2.8Hs0 per liter. A sample weighing between 2 and 8 grams, depending on carbon content, is placed in an aluminum oxide crucible which has been previously heated in oxygen a t 900" C. for 15 minutes. hfter introducing the crucible into the furnace, pure oxygen is flushed through the furnace and conductivity cell for 2 minutes at a rate of 250 to 300 ml. per minute to divest the system of COn which may be picked up during the loading operation. The furnace is then energized for an 8-minute combustion cycle, although the formation of tungstic oxide is generally complete in 2 minutes as indicated by the rapid fall of the plate current. Dial readings are taken a t a selected period in the temperature cycle of the thermostatically controlled bath near the end of the collection period, The instrument used in this work gave the most reproducible readings 60 seconds after completion of a heat-on cycle. The instrument readings are converted to micrograms of carbon by u4ng a ])reviously prepared calibration curve. For powdered samples, place a plntinum disk, 1 inch in diameter and l'az inch thick under and in good thermal contact with the crucible containing the sample. An alternate procedure for igniting powders entails placing a
