Surface Area Determination by the Continuous Flow Method SIR: I n a recent communication, Daeschner and Stross ( 1 ) described their evaluation and further developments of the Nelsen and Eggertsen (2) flow method for the surface area determination of solids. Their publication prompts us to report our experience with this method and t o call attention to a possible source of error due to the accidental presence of water in the system. ITe have investigated the method using prepared mixtures of helium and nitrogen as suggested by Roth and Ellwood (3). This technique does not require the continuous gas flow measurements which. in the original method, are a great source of difficulty and error. EXPERIMENTAL
Apparatus. T h e apparatus, shown schematically in Figure 1, permits a simultaneous analysis of four samples. It consists of four main parts: t h e high pressure line, the main flow line,
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,_ -. - - - - - - calibration flow line
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t h e outgassing line, and t h e calibration line. The high pressure line includes the gas reservoirs, tumbler and vent valves, and copper tubing up t o valves L and G. M1 - M4 are the different gas mixtures containing from 7 . 5 to 2570 nitrogen. X~lz is the nitrogen used for peak area calibration; He, pure helium, is necessary for gas mixture analysis. All the gases are of 99.99% purity and their water content is less than 5 p.p.m. The main flow is monitored by a Kegretti and Zambra (London) pressure regulating valve (G) while the calibration flow is adjusted by a simple twoway valve ( L ) . The two-nay tumbler valves (1 to 12) permit the sending of any gas either through the main flow or through the calibration f l o ~line. Vent valves 13 and 14 are used t o purge these lines. The main flow line goes from valve, G, through the thermal conductivity reference cell, RC, the cold trap, E, through one of the sample tubes, A , B, C, or D , through part of the Beckman gas sampling valve, F , through the thermal
conductivity measurement cell, MC, its by-pass capillary, P, and out through the outgassing line. A well isolated Perkin-Elmer thermistor block is used a t room ternperat'ure for thermal conduct,ivity detection. The 13ecknian ga? sampling valve is equipped with two different sample t'ubes of known volume. The outgassing line starts at the exit of cell -1fCj goes t,lirough one of the sample tubes, A , B , C , or D, and out through the flowmeter H ("Rota," l a c h e n , Germanyj ; the outgassing line permits the measurement of one sample while the next is being outgassed. The calibration f l o ~line goes from valve L , through stopcock J , through part of the gas sampling valve: F , and out through the bubbler, K . Stopcock J is closed before each injection t o bring the gas in the calibration tube t o atmospheric pressure.
Errors Due to Water. T h e greatest source of difficulty was initially cn-
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outgassing line
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vent Valve
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joint Figure 1.
Flow diagram VOL. 35, NO. 2, FEBRUARY 1963
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1.00 2.00 3.00 4.00 VOLUME OF N I T R O G E N INJECTED, ML.
Figure 2. Change of peak area with calibration volume of nitrogen injected into gas stream
within the apparatus, thc procedure can be followed on the recording potentiometer. \Ye regularly observed an adsorption peak as soon as the furnace was removed from the sample tube. We attributed these facts to the presence of water in the gas stream and put several cold traps, connected by polyvinyl tubing, in series, in front of the sample tubes. I n spite of these precautions, the results remained unchanged, and mater was found condensed in each of the cold traps. The water content of the gases n as measured by the Beckman electrolytic hygronieter. When this apparatus \vab connected to the gas reservoir by polyvinyl or rubber tubing, the indicated water content was about 70 p.p.m. after seieral hours, whereas for copper
countered by the presence of water in t h e gas stream. Although our euperimental points almost perfectly fitted t h e BET plot, t h e surface areas were systematically too low as compared to the conventional volumetric measures. Upon closer examination, we noticed that the first point determined always fell out of the plot-Le.. the volume of nitrogen adsorbed at this pressure was relatively greater than that for the other pressures-in fact, adopting the “one-point” method. we connected this point by a straight line to the origin of the BET plot and obtained comparatively correct resultp. We then concluded that the surface of the solid was blocked partially after the outgassing procedure, and partially during the first adsorption measure. As the sample is outgassed directly
Table I.
Comparison of Surface Areas b y the Dynamic and Static Methods
Sample Desulfurization catalyst A Desulfurization catalyst B Desulfurization catalyst C Desulfurization catalyst D Desulfurization catalyst E Reforming catalyst Alumina A Alumina B Kaolinite clay derosil a Static method. Dynamic and static methods.
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ANALYTICAL CHEMISTRY
Specific surface area (sq. meters per gram) Dynamic Static Various methods other method this method this laboratory laboratories laboratory Pi5 276 278 225 254 165 285 127 42 185
2T7 268 264 216 246 164 118 40 157
281* 216a 25.3
tubing it fell to less than 5 p.p.m. It was concluded that the water diffused into the gas stream through the polyvinyl and rubber lines and connections; it was then sorbed after the outgassing step and during the first nitrogen adsorption at low temperature. Tygon lines as used by Nelsen and Eggertsen (2) had not been tried. This major difficulty was overcome when all tubing was replaced by copper and glass; metal-to-glass connections were made by Teflon sleeves. Enough emphasis cannot be placed upon the importance of eliminating all traces of water from the gas stream. Increasing t h e Limits of Linear Response. As did Daeschner and Stross ( I ) , we also established the curve: Chart peak area against volume of nitrogen injected (Figure 2) ; this plot is linear up to 1 ml. only. By the introduction of the capillary, P (Figure l), the thermal conductivity measurement cell, MC, is partially by-passed, so that only a constant fraction of the gas stream is analyzed. I n this may, the cell is not so rapidly saturated by the gas to be analyzed, and the linearity region is widened, in this case up to 2 ml. As this region is independent of the gas composition, we decided to work therein b y adequately choosing our sample size. For greater precision and to minimize the effects of flow rate changes, we maintained the relatively time consuming calibration steps following every desorption. Because under our conditions only a mauimum of 2 ml. of nitrogen are released from the sample into the gas stream, we never observed a substantial change in the gas flow rate as described by the above authors; in this case their proposed modification is not necessary. RESULTS
To check the accuracy of the method, comparative measurements were carried out as shown in Table I. All samples were powders, sieved to a particle size of 40- to 100-mesh, calcined for 1 hour a t 500’ C. in a muffle oven. For the volumetric method, the samples were outgassed a t 350’ C. for one night under vacuum better than mm. of Hg; for the flow method, at 350’ C. for 30 minutes under nitrogen-helium flow. LITERATURE CITED
(1) Daeschner, H. W., Stross, F. H., ANAL.CHEM.34,1150 (1962). (2) Nelsen, F. M . j Eggertsen, F. T., Ibid., 30, 1387 (1958). ( 3 ) Roth, J. F., Ellwood, R. J., Ibid., 31. 1738 11959). RAYYOXD AI. CAHEX JOSEPH XLRECHAL Labofina, S. A.
Centre de Recherches du Groupe Petrofina Brussels, Belgium
