-~
tained 58 mg. per liter of chloride, a total hardness of 154 per liter, 156 mg. per liter of sulfate, less than 0.1 mg. per liter of aluminum and total phosphate, and 0.020 mg. per liter of naturally occurring fluoride The average percentage recovery a t the fluoride level of 0 19 mg. per liter was 105% with a standard deviation of =kO0.O35mg. per liter. The precision obtained for the 0.87 mg. per liter concentration was +0.034 mg. per liter with an average recoveiy of 101%. Effects of Different Lots of Resin (Table 11). T o investigate the possibility of resin effect upon the method, five different lots mere obtained and eight replicate determinations were made on each lot. The standard deviations mere all less than those shown with the samples studied, except for lot C. Here the value of 0.090 exceeds the highest standard deviation value (0.072) found for the samples. This was not investigated further. Lack of data concerning regeneration and re-use of the resin became apparent during development of the procedure. For the samples analyzed, freshly prepared resin could be re-used three times, a t a dosage of 1.0 m g . per liter of fluoride, before it had to be regenerated. The resin in each c:olumn was regenerated by adding 2!5 ml. of 3M HCl, followed by 25 ml. of dutilled water and 25 ml. of 1.0121 NaC2H302solution, and finally rinsing with 50 ml. of distilled water. Each solution was allowed to drain to waste, prior to addition of the next solution. Our drka indicate that the regenerated resin columns may again be used three times before discarding.
____
Table II.
Concn. of fluoride added, mg./l.a
Comparison of Data Using Different Lots of Resin
Resin lot no.
0.50 0.50
A
1.00 1.00
B
No. of replicates analyzed 8 8 8 8 8
B
1 .oo
A
C
Mean,
mg./l. F 0.51 0.52 1.00 1.04 1.03
Av. % recovery
Std. dev.
102 103 100 104 103
0.040 0.027 0.047 0.055 0,090
Sample with all previously discussed interferences added at concentrations indicated in Table I. a
Table 111.
Series of Standard Curves
O.OOmg.F Mean Std. dev. Coeff. of variation,
0.351 0,009 2.6
7’
Additional research regarding extension of the life of the needed. Resin columns may served over extended periods of keeping the resin covered with water. Standard Curves (Table
further resin is be pretime by distilled
111). For
the 15 replicate sets of fluoride standards prepared during this study, good reproducibility was obtained, as indicated in Table 111. A11 standards were passed through ion-exchange columns and treated exactly as the samples. ACKNOWLEDGMENT
We acknowledge the interest and suggestions of S . A. Talvitie, Public Health Service, Salt Lake City, Utah, and of Ervin Bellack, Division of
Absorbance 0.02nig.F O.O1mg.F 0.313 0,010 3.2
0.270 0.006 2.2
0.06mg.F 0.232 0.007 3.0
Dental Public Health, Public Health Service, Bethesda, Md. LITERATURE CITED
(1) American Public Health Association,
American Water Works Association, and Water Pollution Control Federation. “Standard Methods for the Examination of Water and Wastewater,” 11th ed., 1960. (2) Bellack, E., Schouboe, P. J., ANAL. CHEM.30, 2032 (1958). (3) Feigl, Fritz, Schaeffer, A., Ibid., 23, (19.51 ).. _ _ ~ _
(4) J.A W L . Water Works Assoc. 559 (1962). (5) Talvitie, N. A., Brewer, L. A,, Am. Ind. Hyg. Assoc. J. 21, 287-95 (August 1960). (6) U. S . Public Health Service Cin-
cinnati, Ohio, “Water Fluoride,”’ Analytical Reference Service Report, Robert A. Taft Sanitary Engineering Center, 1961.
RECEIVED for review September 26, 1963. Accepted December 30, 1963.
..
Rapid and Precise Method for DetermI nI ng Surface A,reas JASPARD H. ATKINS Research Laboratories, Cabot Corp., Cambridge, Mass.
b A rapid and precise method for the determination of surface areas, developed by refinement of the apparatus and analytical procedures described by Nelsen and Eggertsen, involves adsorption of nitrogen from a stream of helium (2nd nitrogen a t liquid nitrogen temperatures and measurement of the nitrogen desorbed from the sample upon removal of the liquid nitrogen. Relative standard deviations for a series of singlepoint analyses on pt?lletized carbon blacks range from +o.25yOfor a low volatile black to k1.35% for one of high volatility. Use of a correction factor, which is a function
of relative pec.. areas, narrows the range of relative standard deviations To get good to *0.25 to *1.01%. agreement with multipoint volumetric BET determinations, it is necessary to analyze a sample in the apparatus a t two different relative pressures, so as to determine the intercept on the BET plot. A trained operator can do 25 single-point or 15 two-point determinations in an 8-hour work day.
T
most widely accepted method for determining surface area involves measuring the amount of gas adsorbed on a solid surface a t a temperaHE
ture close to the boiling point c- the gas. Kitrogen is most commonly used as the adsorbate. If the adsorption is measured a t several gas pressures, the Brunauer-Emmett-Teller (BET) (3) equation can be used to calculate the amount of adsorbate required to form a monolayer. This value multiplied by the proper factors for area covered per unit amount of nitrogen gives the surface area. Usually the amount of gas adsorbed is determined by measuring pressure differences in a calibrated glass vacuum apparatus. The new method, developed originally by Nelsen and Eggertsen ( d ) , is also based on gas adsorption and use of the VOL. 36, NO. 3, MARCH 1964
579
Figure 1 . stream
Schematic flow diagram of new apparatus with auxiliary outgas
B E T equation, but uses different adsorption measurement techniques. The amount of adsorbed gas is determined by thermal conductivity measurements of adsorbate concentration in a continuous helium-nitrogen stream rather than by pressure-volume measurements as in a static system. The procedure is rapid and is applicable over a wide range of surface areas. -4commercially available instrument of the Selsen-Eggertsen design was evaluated in this laboratory for use in quality control in the carbon black industry and was not considered adequate for the precision needed. A new instrument was designed, constructed, and evaluated. The analytical precision obtained with this instrument was superior to that of the commercial unit and was equivalent to that obtained with a volumetric apparatus by a highly trained operator. For precise measurements, it was necessary to consider the effects of changes in ambient temperature, barometric pressure, liquid-nitrogen temperature, and nitrogen concentration in the mixture. It was necessary to correct for the nonlinearity of the catharometer, by either adjusting sample size so that the desorption and calibration peaks were the same height or using a correction factor which was determined as a function of relative peak areas, desorption peak area divided by calibration peak area. I n measuring the variables to the necessary degree of accuracy, surface areas of eight carbon black samples were measured a t least 10 times each, and a new sample was Fyeighed for each determination. The relative standard deviation for singlepoint determinations varied from *0.250/, for a graphitized sample up to kl.35yGfor a sample with a large per cent of volatile material. These samples varied from 7.07 to 356.3 sq. meters per gram.
580
0
ANALYTICAL CHEMISTRY
Single-point surface areas can vary up to 10% from the values obtained by multipoint determinations, since it is assumed that the B E T plot goes through the intercept. Where it is desirable to have a multipoint determination, the instrument has a second cylinder of premixed gas connected to it which permits analyses at two different relative pressures, and using these results in a B E T plot, a correction for the intercept can be made. Two-point analyses were done on a series of samples including silica gel, FesOa, and TiOl and the results compared to multipoint volumetric B E T surface areas. The agreement in all cases was similar to the agreement between two volumetric multipoint B E T analyses. With this apparatus, it is possible to carry out 15 single-point or 10 two-point determinations in a day. A modification of this design using a n auxiliary gas stream and a second sample cell for outgassing and adsorption, x i t h a valve to interchange cells between the two gas streams, permits an operator to carry out 25 single-point or 15 two-point analyses in a day. EXPERIMENTAL
Apparatus. Figure 1 is a schematic drawing of t h e new instrument. The solid lines show the basic apparatus and the broken line shows a n auxiliary gas stream to permit the simultaneous outgassing and adsorption of one sample, while a second one is being analyzed. The basic apparatus has two flow controllers for the introduction of the carrier gas and the adsorbate, or where two premixed gases of nitrogen and helium are used, each is connected to a controller and only one is used at a time. The gas flows in sequence through: a tee joint connecting the two con-
trollers; a glass wool-packed tube submerged in liquid nitrogen, where it is dried; the first of four identical long coils, made from 12 feet of 1/8-inch copper tubing, submerged in a large stirred water bath, where it assumes the temperature of the bath; the first half of a hot-wire catharometer, where its thermal conductivity is measured; and the second coil. The gas stream is then split, with part of the stream going through one of the calibrated loops in the calibration valve and the remainder going through a bypass around the calibration valve, where the bypass has a needle valve for regulating the relative amounts of gas going through the calibrated loop. The gas then passes through the sample tube, the third coil, the second half of the catharometer, the fourth coil, a soap bubble flowmeter. The auxiliary gas stream has a third constant flow regulator connected to the same gas supply as the regulator helium or gas mixture, a glass woolpacked drying trap, place for a second sample cell, and connecting the two systems a double four-way selector valve for interchanging the sample cells between the two systems. Procedure. Experimentally i t has been determined t h a t for t h e highest precision i t is necessary to set u p a strict empirical procedure and t o follow i t for all samples. This mainly concerns time periods between different operations.
During an analysis, a dried sample is weighed into a sample cell with a vibratory spatula. The cell is attached to the instrument and the gas mixture of helium and nitrogen flows through the cell a t a flow rate of 25 to 30 cc. per minute while the sample is being heated by a small oven around the cell. The air in the sample tube and the volatile material appear as a peak with tailing on the strip chart recorder. Three minutes after the pen returns to its base line, the oven is removed and a Dewar of liquid nitrogen is placed around the cell. The cell should always be submerged to the same depth in the liquid nitrogen. Sitrogen is adsorbed onto the sample’s surface from the gas mixture and 5 minutes after the adsorption is complete, as shown by the recorder pen returning to its base line, the Dewar is removed and the oven replaced. -4desorption peak and integral, from a Disc Integrator, now appear on the recorder, xhich are a direct function of the amount of nitrogen desorbed. The calibration loop with the volume nearest the desorption volume is filled with nitrogen and injected into the gas stream, passing through the heated sample cell, which results in similarly shaped calibration and desorption peaks. By comparing the two peak integrals, and knowing the calibration volume, the amount of nitrogen desorbed is readily determined. Figure 2 is a recorder tracing of an analysis carried out as described above.
DISCUSSION
A commercially available instrument of the Nelsen-Eggertsen design, the Perkin-Elmer-Shell Model 212 Sorptometer, was evaluated in this laboratory for use as a quality control instrument in the carbon black industry and was not considered adequate for the precision needed. Table I is a represeritative sample of a large number of results, all obtained by the same operator, using the comIt s h o w the mercial apparatus. standard deviations obtained for two samples of carbon b1ac.k in consecutive determinations by an experienced operator. I n calculating the standard deviations, three values have been omitted for Vulcan 3, m d two for Vulcan 9, because these values differed from the average of thc other values by more than 4a (probzbbly because of leaks a t the grommet seals). The results agree favorably with those obtained by Daeschner and Stross (1) for desorption into flowing carrier. The poor reproducibility obtained was, to a large extent, due to apparatus design. Modification of the commercial unit did not appear to be feasible and a new unit was built following the schematic drawing shown in Figure 1. For precise analyses, the calibration peak should have a she,pe similar to the desorption peak and the peak heights should agree within t few per cent. This is done in the nc:m apparatus by locating the calibration loops before the sample cell and by Fplitting the gas stream so that only part of it sweeps the calibrated loop. To eliminate desorption temperature and pressure effects u p m the catharometer, l / d n c h copper coils, 12 feet long, were connected to each of the four gas ports on the cathai*ometer. These
Figure 2.
Table
I.
Single-Point Determinations of Surface Area with Commercial Nelsen and Eggertsen Apparatus
Sample Vulcan 3, Sample A Vulcan 9, Sample A
No. of
runs
Specific surface area, sq. m./g.
15 15
68.7 110.0
Std. dev., f1.21 f3.29
u
Rel. std. dev., yo fl.76 f2.99
II. Single-Point Determinations of Surface Area with N e w Apparatus Specific 70 So. of surface area, Rel. std. volatile Sample runs sq. m./g. Std. dev., u dev., % content Graphon" 10 81.1 &0.20 f0.25 0.06 Vulcan 9 10 118.4 0.6 0.5 2.05 Vulcan 3b 10 67.9 0.5 0.7 1.96 Regal300 11 78.3 0.5 0.65 2 19 1.22 10 106.2 i.3 SpKeron 6 4.3 1.35 4.85 Monarch 74 13 356.3 Sterling MTc 10 7.09 0.043 0.60 3.34 0.62 0.70 Sterling Sb 10 24.61 0.15 Table
b c
Spheron 6 heated to 2700" C. Samples not run a t same time as other six. One value (6.88) discarded as being > 4u.
supply sufficient heat exchange surface and, because of their large relative resistance to flow, minimize pressure effects equally to both reference and sample detectors. Experimentally a 31/2-gallon, stirred water bath has been found to be beneficial. O-ring vacuum connectors are used to attach the sample cell to the apparatus, and are relatively leak-free, especially if the O-rings are changed after every 25 samples. Gas leaks in the connectors, when they occur, are shown by discrepancies in the calibration peak. The flow control is regulated by use of a differential pressure controller set to give a constant pressure drop across a variable needle valve. There is no gas bleed in this controller, as there is in a good variable pressure controller so that
Recorder tracing of sample run
a cylinder of gas in constant use now lasts two to three months. Table I1 shows the reproducibility obtainable with the new unit. The results in this table were obtained by analyzing samples of the six pelletized carbon blacks in random order, weighing out a new sample each time, plus two others. The weights were adjusted so that there was close agreement (=t2%) between the sample peak heights and the calibration peak height. Examination of the results shows that the best reproducibility is obtained with low volatile carbon blacks and that the reproducibility becomes worse with increasing volatile content. Dannenberg and Opie (2) have shown that the volatile content of a carbon black, the groups that come off the sample dried a t 200" C., which is then heated to 1000" C., affects the amount and rate of moisture adsorption a t low relative humidities. The volatile content is mainly a measure of polar groups on the surface of the black and increasing volatile content increases the attraction of the carbon black for moisture. The sample of Sterling S was run to compare this instrument with the apparatus of Daeschner and Stross ( I ) , who used a similar grade of carbon black. Their relative standard deviation for desorption into a flowing carrier was 3.8%, compared to 0.62% for this apparatus using the same technique. Using a correction factor discussed below, the relative standard deviation becomes 0.56'%, which is half of the value reported by Daeschner and Stross for desorption into a static carrier plus the use of a family of calibration curves. Precision. To achieve precise measurements of surface area, it is necessary to measure a number of the VOL. 36, NO. 3, MARCH 1964
581
similar shape, it was m u m e d that the value of the specific surface area is correct where the relative peak height (sample peak height divided by calibration peak height) is unl ty. To correct for this nonlinearity, Figure 4 was replotted as a straight-line calibration curve, a factor (calculated specific surface area divided by correct specific surface area) us. relative peak area. This calibration curve is valid for a given nitrogen-helium ratio, a specific current setting, and an individual instrument. By repeating the above for different nitrogen-helium ratios, a family of such calibration curves can be obtained. Using the correction factors from such a plot, the specific suimface areas of the saniple in Table I1 Fiere recalculated. The relative standard deviation of Monarch 74 dropped from 1.35% to l . O l ~ cand for Spherori 6 from 1.22% to 0.99%. There was no change in the Graphon.
Accuracy. If t h e volumetric BET nitrogen surface area is accepted as correct, t h e accuracy of t h e singlepoint surface area is n o t so good as its precision, for in a single-point determination, it is necessary t o assume a n intercept of zero in t h e B E T plot, which is seldom correct. A number of samples were analyzed a t two different relative pressures, so t h a t their intercepts o n t h e B E T plot could be determined, and were then analyzed on a volumetric adsorption apparatus. A comparison of the results, shown in Table IV, indicates the importance of the intercept in computing an accurate specific surface area. A study of hundreds of volumetric B E T analyses run in this laboratory over a number of years shows only slight differences in the intercepts of samples of the same grade. For this reason, it is felt that for quality control, where relative changes in surface area are more important than accurate speci-
fic surface areas, single-point determinations are adequate. ACKNOWLEDGMENT
I am indebted to A. F. Cosman, R. F. Roche, C. T. Savage, and F. J. Patz for their diligence in the analyses of many hundreds of samples made during this study. LITERATURE CITED
(1) Daeschner, H. W., Stross, F. H., ANAL.CHEM.34, 1180 (1962). ( 2 ) Dannenberg, E. M., Opie, W. H., Jr., Rubber World 137. 849 (March 1958): ,, 138,85 (April 1958).
(3)7Emmett, P. H., Brunauer, S., Teller, hdward, J. Am. Chem. SOC.60, 309 (1938). (4) Selsen, F. ill., Eggertsen, F. T., AKAL.CHEM.30, 1387 (1958).
RECEIVEDfor review August 14, 1963. Accepted November 29, 1963. Division of Analytical Chemistry, 145th Meeting, ACS, Xew York, N. Y., September 1963.
Dimethoxy pro pa ne Ind uced T ra nseste rificati o n of Fats and Oils in Preparation of Methyl Esters for Gas Chromatographic Analysis MICHAEL E. MASON and GEORGE
R. WALLER
Agricultural Experimerif Sfation, Oklahoma State University, Sfillwater, Okla.
b A simple, convenient, and quantitative method for the preparation of methyl esters of fatty acids from fats and oils which does not require treatment a t elevated temperatures is described. Gas chromatographic analyses are made by injecting aliquots taken directly from the reaction mixture thus eliminating evaporation and extraction steps used in 'other procedures. The use of 2,2-dimethoxypropane (DMP) to drive trarisesterification to completion eliminates the need for elevated temperatures. DMP reacts with glycerol to form isopropylidene glycerol (IPG) which chromatographs readily and serves as a convenient marker in determining retention times. This method requires only a few simple operations and is especially adaptable to routine analyses of large numbers of sarrples.
S
have been devised for preparing fatty acid methyl esters from glycerides. Craig and Rlurty (1) described 51, method employing sodium methoxide as the catalyst and reflux times of the order of 30 minutes. Luddy, Bxford, and Riemenschneider ( 4 ) showed that the reflux EVERAL METHODS
time could be shortened to 5 minutes by using potassium methoxide as the catalyst. Under these conditions, even the steryl esters could be completely transesterified. Recently, Peterson, D e Schmertzing, and Abel ( 7 ) described a method which employed boron trichloride as the catalyst and a prescribed heating period. Gehrke and Goerlitz ( 2 ) prepared the esters by treatment of the saponified lipids with silver oxide and methyl iodide. The use of 2,2-dimethosypropane (DMP) in preparing methyl esters from lipid materials was reported by Tove (8) in studies on epidermal fats of rats and by Waller (9) in studies on peanut oils, but data to support the quantitative conversion of glyceride fatty acids to methyl esters were not presented by either author. Work in this laboratory indicated loss of the esters could occur when reaction mixtures were refluxed, when the esters were extracted over aqueous phases, and when solvents were removed by evaporation. This loss was especially pronounced with the lower molecular weight esters. Consequently, attempt3 were made to devise a procedure free of time consuming and error ridden manipulations involving refluxing, evaporations, or
extractions over aqueous phases. The described procedure is especially adapted to the routine analyses of large numbers of samples by gas liquid chromatography (GLC). EXPERIMENTAL
GLC analyses were performed with an F a n d 11 model 500 linear programmed gas chromatograph equipprd with a Disc Instruments, Inc., integrator, Bristol 1-mv. recorder, and a hot wire detector. Nost of the chromatograms were obtained with lIr-inch by 8-foot aluminum columns of 15% Lac-3R-728 (Cambridge Instruments Co., Inc.) on Chromosorb W, 60- to 80-mesh. However, some chromatograms were obtained with l/r-inch by 6-foot aluminum columns of 14.57, EGS (ethylene glycol succinate, AlppliedScience Laboratories) on Anakrom, 100- to 110-mesh, type A (hnalabs, Inc.). Helium a t a flow rate of 60 ml. per minute was used as the carrier gas. Reagents. Benzene, reagent grade, dried over sodium, 2,2-dimethoxypropane (Dow Chemical Co.) redistilled from 76" t o 79" C., methanol, super-dry with less t h a n 0.027" water (Britiah Drug Houses, Ltd.), triglycerides, 99,5yc purity ( l l a n n Laboratories) used without further purification. Apparatus.
VOL. 36, NO. 3, MARCH 1964
e
583
