the total contribution by the liquid phase. In Table V are shown the relative contributions of the gas phase and liquid phase to the observed plate height at an arbitrary carrier gas velocity of 6.68 cmjsec. Effects attributed to the gas phase
H
=
Bg+ HL
or or
(1 3)
contribute about 80% of total plate height with the protium forms and about 857% with deuterium forms of sugars. Column efficiency is dependent primarily on effects in the gas phase, therefore, and improved column performance in the fractionation of isotopic-substituted compounds of the types studied here will be obtained by careful control of the band broadening factors in the gas phase. Giddings (17) has made a thorough theoretical study of the nature of the gas phase term, H,, and Saha and Giddings (18) have shown experimentally how various column parameters and support characteristics affect the Ho term. These authors show that major contributions to ,U,are related to the physical nature of the support, especially pore size distribution, pore geometry, and particle size distribution, and by the uniformity of packing in the column. A further reduction in mesh size range and, perhaps, a decrease in the column diameter would effect ~~~
Table V. Relative Contributions to Plate Height by Gas Phase and Liquid Phase V = 6.68 cmisec, helium carrier Sugar a-D7-mannoseTMS a-H7-mannoseTMS p-D,-mannose TMS P-H7-mannoseTMS
Zcm 0.0408 0.0391 0.0416 0.0397
-
-
~1,cm ~,,cm
HJH
0.0056 0.0051 0.0068 0.0061
0.81 0.87 0.78 0.85
0.0329 0.0340 0.0326 0.0336
greater isotope fractionation efficiency by reducing band broadening in the gas phase. In our previous studies on separation of protium and deuterium forms of the TMS sugars on 50-foot packed columns of small diameter (9, we found that band width in the deuterium form was greater than that in the protium form by 11%. The reverse was found in this study, with short packed columns; that is, band widths were somewhat greater in the protium forms. The two forms were injected together on the long columns, and singly on the short columns, but the reversal of band width differences cannot be attributed to this factor. It is very difficult to account for this phenomenon at the present time because it might be related to almost any of the terms in the plate height equation.
~
(17) J. C. Giddings, ANAL.CHEM., 34,1186(1962). 37,830 (1965). (18) N. C. Saha and J. C. Giddings, ANAL.CHEM.,
RECEIVED for review May 2, 1966. Resubmitted May 29, 1968. Accepted June 10,1968.
Determination of Normal Paraffins in Petroleum Heavy Distillates by Urea Adduction and Gas Chromatography J. R. Marquart, G . B. Dellow, and E. R. Freitas Shell Development Co., Erneryuille, Gal$ A simple and efficient method has been developed for the determination of normal paraffins in heavy gas oils. The method consists of a urea adduction step for isolating the normal paraffins, which in turn, are analyzed with good resolution by gas-liquid chromatography. The final temperature of adduction determines the lower carbon number limit of applicability of the method, C1, at 25 OC and Clzat 0 O C . The method of analysis has been shown to be in excellent agreement with the composition of known calibration blends. DIRECT ANALYSIS of normal paraffins in petroleum distillates in the heavy gas oil range by gas-liquid chromatography (GLC) is generally unsatisfactory because of resolution problems encountered in such complex mixtures of hydrocarbon types. A number of methods using SA-Type Molecular Sieves or urea adduction to simplify the chromatogram have been proposed. Each of these, however, has some shortcoming, either in complexity of operation or inability to handle oils containing high molecular weight paraffins. The following are typical illustrations: 1. Subtractive GLC methods (1, 2 ) involve duplicate (1) N. Brenner and V. J. Coates, Nufure, 181, 1401 (1958). (2) B. T. Whittam, ibid.,182, 391 (1958).
chromatograms of the oil with and without a precolumn of 5A Type Molecular Sieves. Comparison of the chromatog r a m reveals the contribution of n-paraffins to the overall spectrum. The method is fairly satisfactory for oils in the kerosine range. For higher molecular weight oils, however, temperature programmed GLC is necessary and duplication of chromatograms is generally poor. 2. Subtraction and re-elution GLC methods (3, 4 ) involve chromatography of an oil while removing the n-paraffins on a molecular sieve precolumn and subsequent elution of the sieve bed at elevated temperatures to obtain a chromatogram of the adsorbed n-paraffins. This method is also limited to the kerosine range, because higher molecular weight paraffins cannot be removed readily from the sieves by heating. 3. Molecular sieve recovery methods (5,6) involve removal of the n-paraffins on molecular sieves and subsequent recovery of the n-paraffinsby destruction of the sieves. These methods have been used for heavy distillates, but are often laborious (3) F. T. Eggertsen and S. Groenning, ANAL. CHEM.,33, 1147 (1961). (4) G. C. Blytas and D. L. Peterson, ibid., 39, 1434 (1967). (5) J. V. Brunnock, ibid., 38, 1648 (1966). (6) J. V. Mortimer and L. A. Luke, Anal. Chim. Acta, 38, 119 (1967). VOL. 40, NO. 1 1 , SEPTEMBER 1968
e
1633
Squa lane
Internal Standard c16
in
?!'
0
b
C19 ir O
I
c20
h Program Temperature, OC
Figure 1. GLC spectrum from HGO-A and time consuming. Mortimer and Luke (6) have a modification that can be performed in 2 to 4 hours, but the repeatability of their results for a calibration blend was poor for light components and for components present in less than about 1%w. We have calculated the relative standard deviation as a function of n-paraffinic carbon number and find it to be poorer the lower the carbon number, suggesting losses of the lower homologs. The method seems adequate in the heavy distillate range, but is still rather laborious for use as a routine method. 4. Urea adduction recovery methods involve removal of n-paraffins as a urea clathrate and subsequent recovery of the n-paraffin by destruction of the clathrate. Because urea adducts can be destroyed simply by addition of water, the method is simpler than the molecular sieve recovery method that requires digestion in hydrofluoric acid. Also, because the urea clathrate forms about the paraffin molecule, rather than the paraffin having to "worm" its way into an existing cavity, the rate of n-paraffin uptake by urea is considerably faster, especially for heavy paraffins. Frehden and Lazarescu (7) and Orszag and Bathory (8) have successfully analyzed gasoline and kerosine range distillates by urea adduction, but their methods fail for heavy distillates. Karr and Comberiati (9) offer a complex method involving adsorption of n-paraffins on a solid bed of urea-coated silica gel for use with heavy distillates. Our technique is a modification of the first two urea adduction methods cited above and has proved to be a rapid, reliable, and accurate method for the determination of n-paraffins in heavy distillates. The method consists of a simple efficient urea adduction step for isolating the normal paraffins, which in turn, are analyzed with good resolution by gas-liquid chromatography.
4[
---
GLC
MS
Total Content:
i L
L
8
12
16
20 24 Carbon Numbers
28
17%~
32
Figure 2. n-Paraffins in HGO-A
Considerable discussion has been made of packed columns (10) and capillary columns (11) that give adequate resolution of n-paraffins. Instability of the stationary phase at the temperatures used to elute the high molecular weight paraffins (generally 300-320 "C) is generally the main problem. We have chosen to use a packed column for its simplicity and ease of preparing and handling. The proposed column offers good n-paraffin resolution and is stable above the temperature needed to elute Cas(250 "C). The column consisted of 9 feet of 'is-inch, thin-wall, stainless-steel tubing packed with 2 % ~ Union Carbide Carbowax 1000 on Johns-Manville Chromosorb W, 60/100 mesh. Single column operation was used because it was difficult to keep dual columns balanced as they gradually deteriorated under high temperature use. Helium was used as the carrier gas at about 50-60 cc/minute. The flame detector was operated with a hydrogen flow rate of about 50-60 cc/minute and a compressed air flow rate adjusted to optimize detector sensitivity., Typical column temperature programming covered the range from 75 "C to 250 "C at 6 "C/minute. The injector was held at about EXPERIMENTAL 250 "C and the detector at 300 "C. Apparatus and Materials. GAS-LIQUIDCHROMATOGRAPH. Data evaluation was done using a computer technique involving the recording of detector outputs on magnetic An aerograph Model 1520 gas chromatograph equipped with tape, but mechanical integration, while much more time a hydrogen flame ionization detector was used in this work. consuming, is applicable to the chromatograms. (7) . , 0. Frehden and G. Lazarescu, Rev. Chim. (Bucharest), 13, 491-3 (1962).
(8) I. Orszag and J. Barthory, Acta Chim. Acad. Sei. Hung., 40, 367 (1964). (9) C. Karr, Jr., and J. R. Comberiati, J. Chromatog., 18,394 (1965).
1634
ANALYTICAL CHEMISTRY
(10) G. Hildebrand, C. Peper, and B. Dahlke, Chem. Techn. (Berlin),15, 147 (1963). (11) D. H. Desty, A. Goldup, and B. H. F. Whyman, J. Inst. Petrol., 45, 147 (1963).
Squa la ne Internal Standard
o:"
e
C2r
Program Temperature, "C
Figure 3. GLC spectrum from HGO-B Reagents. Reagent grade chemicals were used whenever available. The purities and sources of the materials are listed below. Chemical Urea Methanol Methyl ethyl ketone n-Decane Squalane n-Paraffins up to c 3 2
Purity
1.6
t
-G LC ---MS
/1
Source
Reagent Grade, ACS Reagent Grade 99, 5 % Pure
Matheson, Coleman and Bell Allied Chemical Shell Chemical
Pure Grade 90-98% Pure High Purity
Phillips Petroleum Eastman Kodak American Petroleum Institute
Procedure. RECOVERY OF NORMALPARAFFINS BY UREA ADDUCTION.Weigh, accurately, enough oil into a 2-ounce bottle to give about 0.3 gram of n-paraffins. Add 8 grams of urea and 25 ml of methanol to the oil. Shake the mixture for approximately 30 minutes at 55-60 "C. Remove from bath and continue shaking for 1 hour while allowing to cool to about 25 "C. Separate the adduct crystals from the oil by filtration through number 5, 50, or 51 Whatman filter paper in a Buchner funnel at room temperature. Be sure the paper is properly sealed by prewetting with methyl ethyl ketone (MEK) that is saturated with urea at room temperature. Wash the adduct four times with 20-ml portions of the urea-saturated MEK per wash, mixing thoroughly to a slurry each time. Scrape the washed adduct into a 1-ounce bottle. Wash any residual adduct from the paper with water. Fill the bottle to the shoulder with water and make certain that all solid adduct has dissolved. Add about 2 ml of n-decane to dissolve the solid wax. A lower molecular weight solvent can be used if desired. CHROMATOGRAPHIC ANALYSIS.Weigh an internal standard into the layer of n-decane solution of decomposed adduct. Squalane is recommended as a convenient internal standard, although it often does not resolve well from the n-Cz7 peak. To minimize interference from n-C2,, sufficient amounts of squalane were used so that it was the predominate peak in the spectrum. Generally 0.1-0.3 gram was sufficient. The contribution from n-C2,was established by linear interpolation of the n-C26and n-Czs peak areas and subtracted from the squalane if necessary. Contributions from adductable non-normals are small and can be neglected.
I a
12
16
I 20
I
Total Content:
10%~
I
I
24
I
I 28
L 32
36
Carbon Numberr
Figure 4. nParaffins in HGO-B A sample of the marked hydrocarbon solution is injected into the GLC in amounts varying from 0.5 to 1.5 ml. The smaller portions give slightly better resolution, but for oils that are fairly low in n-paraffin content ( 1 0 % ~ larger ) samples are often necessary to get sufficiently large peak areas. Temperature programming from 75 "C to 250 "C at 6 "C/ minute gives good resolution of n-Clz through n-CBswith a reasonable running time of about 30 minutes. Base line drift for the chromatogram is, in general, negligible below about 200 "C. Above 200 "C, the base line rises gradually because of column bleed, but does not interfere with the resolution of peaks. Small peaks evolving between the regularly spaced normal paraffin peaks are presumed to be monomethyl branched paraffins that adduct along with the normals. These resolve from the major peaks and are not included in the count of n-paraffins. RESULTS AND DISCUSSION
Adduction Conditions and Completeness of Recovery. The adduction procedure as outlined above uses about half again the urea theoretically required for complete adduction (assuming 3.3 grams of urea per gram of n-paraffin adducted (12) and a solubility of urea in methanol of 2 0 z w at 25 "C). (12) 0. Redlich, C. M. Gable, A. K. Dunlop, and R. W. Millar, J. Amer. Chem. SOC.,72, 4153 (1950). VOL. 40, NO. 1 1 , SEPTEMBER 1968
1635
Carbon Number Blend By adduction Final temp 25 "C 0 "C
0.50
9 0.48
Table I. Analysis of Reblended Heavy Gas Oil Effect of Temperature on Recovery n-Paraffin content (%w) 10 12 14 16 17 20 21 0.51 0.53 0.55 0.51 0.54 0.38 0.56
0.04 0.08
0.06 0.12
0.07 0.28
8
Carbon Number Analysis 1 2 3 4 5 6 9 5 x Confidence limits Per cent relative error
0.38 0.50
0.43 0.54
Table 111. Properties of HGO-A and HGO-B HGO-A 36.1 469 500
541 613
HGO-B 28.2 387 570 660 700+
The excess urea precipitates, but is not harmful, because the amount of adduct is not determined gravimetrically. The initial heating at 55-60 "C for a period of 30 minutes serves to increase the rate of adduction of the heavier n-paraffins through increased solubility and diffusion to the methanol/ urea phase. Recovery of n-paraffins was examined as a function of temperature by adding known blends of high purity n-paraffins ranging from Cs through C ~ to Z a urea denormalized heavy gas oil. The blended oils were analyzed by the outlined procedure at the recommended final temperature of 25 "C and also by an additional 1-hour adduction period at 0 "C. Filtration and washing was also carried out at 0 "C in the latter case. Table I shows the extent of recovery of n-paraffins ranging from CSto Cas at the two adduction temperatures. It is apparent that the adduction at 25 "C gives virtually complete recovery of n-paraffin of C16 or above, but below Cl6 recovery becomes progressively worse. Lowering the final adduction temperature to 0 "C improves the recovery such that CI2and above are virtually completely recovered. However, because isoparaffins adduct increasingly under these conditions and may effect the base line, low temperature adduction has been used only if analysis is to include n-paraffins below c16. 1636
ANALYTICAL CHEMISTRY
0.52 0.54
0.40 0.39
0.58 0.58
Table II. Analysis of Reblended Heavy Gas Oil Repeatability of Analysis n-Paraffin content (%w) 17 19 21 22 25 1.46 0.56 1.94 2.06 0.61 1.93 2.06 0.57 1.44 0.60 1.53 2.02 2.11 0.58 0.63 0.61 0.56 1.50 1.98 2.05 1.43 1.89 2.00 0.58 0.61 1.97 2.05 0.55 0.61 1.47 &0.026 10.12 &0.092 &0.031 f O .10 6.8 4.2 6.1 4.5 5.5
16 0.19 0.18 0.19 0.18 0.18 0.19 10.014 7.6
API gravity ASTM distillation (D-1160) Initial boiling point, "F 10% V distilled, OF 50% V distilled, "F 90% V distilled, OF
0.49 0.51
22 0.42
30 0.25
31 0.18
32 0.34
0.45 0.43
0.25 0.23
0.18 0.17
0.32 0.32
27 0.32 0.33 0.33 0.33 0.32 0.32 &0.014 4.3
30 0. i5
0.14 0.14 0.14 0.15 0.14 10.014 9.8
Total 7.29 7.25 7.53 7.35 7.16 7.30 &0.32 4.4
The repeatability of the analyses has been found to be about 95% at 95% confidence limits. Table I1 shows data collected for analysis of a blend simulating a typical heavy gas oil. Mortimer and Luke (6) reported that repeatability became poorer as the GLC conditions changed over a period of a couple of weeks. We did not find this to be the case, providing the column was repacked when resolution between peaks was found to deteriorate, generally after 20 to 30 analyses. Calibration Factors. Calibration blends of pure normal 1 6 through were marked with paraffins ranging from c squalane as an internal standard and analyzed by GLC. The peak area was nearly proportionate to the abundance of the component ( Z w ) throughout the entire range. Thus, in general, there is no need for calibration factors. In cases where more accurate determinations are desired and especially where the distillate contains a broad range of nparaffins, a linear increase of 0.01 was applied to each successive carbon number. For squalane as the internal standard, the linear relationship between calibration factor and carbon number crosses unity at about CZ5. n-Paraffin Content of Heavy Gas Oils (HGO). The adduction and GLC procedures were tested on two heavy gas oils (HGO-A and HGO-B). The physical properties of these two oils are summarized in Table 111. HGO-A contained a total of about 1 7 z w n-paraffins ranging in carbon number from about CIOthrough CZ,. Figure 1 is the GLC spectrum of the n-parafins separated by adduction. The resulting carbon number distribution is shown in Figure 2 along with the results obtained on the same material by a highly developed mass spectrometric (MS) technique (13). The agreement is quite satisfactory. HGO-B contained about 1 0 % n-paraffins ~ ranging from
(13) D. P. Stevenson and A. G. Polgar, N A S A Contractor Rept., CR-519 (July 1966).
about C12 to about C32. Figure 3 is the chromatogram for HGO-B adducted paraffins and Figure 4 is the corresponding carbon number distribution. Again GLC analysis and mass spectral data are in good agreement. Comments. Although the analyses of only two gas oils are cited here, the technique has been used routinely by us on many other samples and found to be highly repeatable and accurate to better than 95%. We have found the method to be applicable to petroleum fractions either heavier or lighter than the heavy gas oils with only minor modifications of the procedures. Waxes that have been separated by other means can also be analyzed by applying the GLC techniques
directly. The isoparaffin content of such waxes can generally be determined by evaluation of the resolved non-normal peaks. ACKNOWLEDGMENT
The authors are grateful to H. S. Knight and P. A. Wadsworth, Jr., of this Company; Mr. Knight for valuable advice in designing the GLC techniques and Mr. Wadsworth for the mass spectral data. RECEIVEfor review April 8, 1968. Accepted June 14, 1968.
A Ring-Disk Electrode Study of the Deposition and Stripping of Thin Copper Films at Platinum in Sulfuric Acid G . W. Tindall’ and Stanley Bruckenstein’ Chemistry Department, Unioersity of Minnesota, Minneapolis, Minn. 55455
The deposition and stripping of very small amounts of copper from 0.2M H2S04 has been investigated. Procedures were devised for depositing a known, small amount of copper on the disk of a rotating platinum ring-disk electrode, either by constant current or constant potential electrolysis. During deposition and stripping of copper it was found that a significant amount of Cu(l) can form in 0.2M H2S04media. Quantitative agreement was found between the amount of Cu(l) and Cu(ll) produced during electrochemical oxidation of a copper-plated platinum disk and the disk oxidation current. Conditions are given for the quantitative deposition and stripping of copper without interference from Cu(l) formation.
THEELECTROCHEMICAL deposition and stripping of very small amounts of metals at solid electrodes have been investigated by a number of workers (1-6). In many such studies the amount of metal deposited appeared to exceed the amount of meial stripped. This apparent violation of Faraday’s law was attributed to mechanical losses, chemical stripping by impurities, alloy formation between deposit and electrode, and less than 100% current efficiency during deposition and stripping. In the case of copper, the difference between the amount of copper apparently deposited and the amount stripped was very large (2, 6). Bixler and Stafford (6) concluded that this large difference which occurred when copper was deposited and stripped from platinum in 0.1M NaC104 might be due to the formation of Cu(1) during deposition. However, they were unable to verify this hypothesis. Also, Address after September 1, 1968, Chemistry Department,State University of New York, Buffalo, N.Y. 14214 (1) S . S. Lord, R. C. O’Neill, and L. B. Rogers, ANAL.CHEM., 24, 209 (1952). (2) R. C. DeGeiso and L. B. Rogers, J . Electrochem. SOC.,106, 422 (1959). (3) A. R. Nisbet and A. J. Bard, J. Electroanal. Chem., 6, 332 (1963). (4) J. W. Bixler and S. Bruckenstein, ANAL.CHEM., 37, 791 (1965). (5) W. M. Krebs and D. K. Roe, J . Electrochem. SOC.,114, 892 (1967). (6) J. W. Bixler and W. F. Stafford, ANAL.CHEM., 40,425 (1968).
they found no evidence for Cu(1) formation during the stripping process. It has been known for a considerable time that equilibration of copper metal and Cu(I1) in noncomplexing media-e.g., H2S04and HC104-is fairly rapid and that the quantity of Cu(1) formed is governed by the equilibrium constant for the reaction Cu Cu(I1) 2 2Cu(I). Direct voltametric evidence for the formation of Cu(1) during the plating of copper in such media was first given by Nekrasov and Berezina (7), and later by us (8). We have studied the electrochemical deposition and stripping of copper from the disk of a rotating platinum ring-disk electrode (RPRDE) in 0.2M H2S04. Deposition from HC104 was attempted but it was found that the reduction of Cu(11) in HClO4 is highly irreversible. Deposition from neutral salts, such as N a N 0 3 and NaC104, was also considered, but solutions of these salts yielded prohibitively large residual currents which prevented the accurate study of copper deposition from very dilute Cu(I1) solutions. In 0.2M HzS04 we find that monolayer amounts of copper can be deposited and stripped from a disk electrode and that Faraday’s law is obeyed, providing corrections are made for Cu(1) formed during the experiment. The amount of Cu(1) produced during an experiment is determined by means of the ring of the RPRDE as is described below.
+
EXPERIMENTAL.
Figure 1 is a schematic of the instrument used to deposit and strip copper by constant current electrolysis at the disk of the RPRDE. The constant current at the disk electrode is given by i = -E 2lR The potential of the disk electrode equals the voltage measured between points A and B in Figure 1. The potential of the (7) L. M. Nekrasov and M. Berezina, Dokl. Akad. Nauk SSSR, 142, 855 (1962). (8) G. W. Tindall and S. Bruckenstein, ANAL. CHEM.,40, 1051 (1968). VOL. 40, NO. 11, SEPTEMBER 1968
1637
