the dyestuff user who must reproduce an exact shade. However, for use as chromatographic spray reagent, the instability is not so important. I n our experience, the colors produced by the first five stabilized diazo salts of Table I after standing a t room temperature for 3 years were essentially identical with those produced by fresh salts.
ACKNOWLEDGMENT
The authors thank the Antara Chemicals Division of General Aniline and Film Corp., the Organic Chemicals Department of E. I. du Pont de Nemours & Co., Inc., and the Koppers Co., Inc., for supplying the stabilized diazo salts used in this study. LITERATURE CITED
(1) Neu, R., 2. anal. ~Chem. 151, 321 (1956).
(2) Pearl, I. A,, Beyer, D. L., Laskoweki, D., Ta pi42,779 (1959). (3) \ , Pearr I. A.. McCov. .., P. F.. ANAL. CHEM.32,132 11960). (4) Ridgway, R., ''Co!x
Standards and Color Nomenclature, 1st ed., published by author, Washington, D. C., 1912. (5) Shand, E. W., Koppers Co:, Pittsburgh, Pa., private communication.
RECEIVED for review April 21, 1960 Accepted July 20, 1960.
Surface Areas from Adsorption of Methyl Red by Column Chromatography HANS A. BENESI Shell Development
Co., Emeryville,
Calif.
b The uses and limitations of a dye adsorption method for the determination of the surface area of cracking catalysts and allied solids have been explored. The method is based on the measurement of the length of the colored zone produced by the chromatographic adsorption from benzene solution of methyl red on a column of powdered sample. The results obtained indicate that this method can be used to determine the accessible surface area of alumina-silica and silica gel with a probable error of 6%.
D
methods for the determination of surface areas of solids have been used for many years (6-8). Their chief advantage is the simplicity of the experimental technique. Their disadvantage lies in the fact that adsorption of dyes is highly specific; consequently, no single dye can be used to measure surface areas of all solids. However, this type of method is well suited for measurement of the surface area of a given class of solids. Shapiro and Kolthoff (IO) have already shown that the area of silica gel surfaces can be determined from measurements of the adsorption of methyl red [o-(pdimethylaminophenylazo) benzoic acid i. Apparently, methyl red is chemisorbed on silica gel through an acid-base reaction between the basic azo group in the dye and the weakly acidic SiOH groups on the silica gel surface. If this is so, methyl red adsorption measurements should also prove useful in the determination of surface areas of cracking catalysts, because such solids are strongly acidic ( I , 11). The present study explored this possibility. Shapiro and Kolthoff measured methyl red adsorption from benzene
technique. I n the present study, this difficulty was avoided by using a chromatographic technique to measure methyl red adsorption. Employing such a technique, advantage was taken of the fact that the equilibrium methyl red concentration behind a zone front is essentially identical with the methyl red concentration in the standard solution itself. Another advantage of the
I
YE ARSORPTION
1410
ANALYTICAL CHEMISTRY
treme simplicity.
2 l
Apparatus and Materials. T h e chromatographic tube used is shown in Figure 1.
Catalyst
Composition, yo
MS-A-1 Aerocat
13 AlzOs, 87 s i o z 22 A1~01,78 SiOn 37 Alzo3, 63 si02 22 AlzOa, 78 SIOZ 31 MgO, 69 Si02
h1S-A-3 Aerocat Filtrol SR Alumina-silica Magnesia-silica
The wide range in surface area of the samples shown in Figure 2 was obtained by sintering fresh catalysts. Sintering was brought about by calcination at 900" to 1000" C., by heating to 565" C. in the presence of steam, or by long term use in the refinery. Most of the silica gel samples used
Manufacturer American Cyanamid Co. iZmerican Cyanamid Co. Filtrol Corp. Sational lluminate Co. Davison Chemical Co.
were obtained from Davison Chemical Co. The alumina and magnesia preparations have been described elsewhere (1). Surface areas of all samples used were determined from nitrogen adsorption measurements a t the boiling point of nitrogen using the Brunauer-Emmett-
methyl red solution. On the other hand, if 5 ml. of solution colors the entire column, repeat the above steps using a smaller volume of methyl red solution. If the volume used is less than 3 ml., correction must be made for t h e amount of methyl red remaining unadsorbed in the void volume of the adsorbent column.) From the weight of the sample in the colored zone and from the volume and concentration of the methyl red solution, calculate the number of millimoles of methyl red adsorbed per gram of sample.
l o 0
2CI ' 0 E T
1 3 a e *rea
--,--do
0
rn'll"rn
Figure 2. Methyl red adsorption as function of B.E.T. surface area
Teller (B.E.T.) method (4). Pore volumes were determined from measurements of carbon tetrachloride adsorption using a method described elsewhere (3). Average pore diameters, 2, were calculated using the relation 4 v p x 104
d(A.) =
where V p = pore volume, cc./gram S = B.E.T. surface area, sq. meter/gram All samples were calcined in air a t 550" C. immediately before making dye adsorption measurements in order to standardize the condition of the surface. This treatment also removed the carbon deposit from catalysts that had been used t o crack hydrocarbons. PROCEDURE
Methyl red adsomtion is measured as follon-s : Calcine 0.5 gram of sample in air at 550" C. for 2 hours. Transfer the calcined product to screw-cap vial (1-dram capacity) and obtain the total weight. (The sample should consist of particles that will pass through a 60but not through a 325-mesh screen.) Insert a cotton plug in the 2-mm. end of a chromatographic tube. Quickly transfer the calcined sample from the screw-cap vial to the top of the chromatographic tube, nearly filling the 2mm. capillary. T a p or vibrate the tube to pack the column. Reweigh the capped vial to obtain the sample weight in the chromatographic tube. The standard methyl red solution used for surface area measurements contains 0.6 gram of methyl red per liter of benzene solution. Pipet 5 ml. of the methyl red solution into the top of the chromatographic tube. If the adsorption process takes longer than 1 hour, apply 1 to 5 p.s.i. pressure through the ball joint a t the top of the tube. After the last of the methyl red solution has passed into the column, measure (to i l mm.) the length of the colored zone and the total length of the adsorbent column. From these two lengths and the total weight of sample, calculate the weight of the sample in the colored zone. (If the length of the colored zone does not exceed 30 mm., use more
? ,
\,e
DATA A N D DISCUSSION
If a suitable dye-solvent system is chosen, the equilibrium adsorption of a dye on a solid surface will approach saturation as its concentration is increased. It is reasonable to assume that the amount of dye adsorbed a t saturation corresponds to a monomolecular layer. Thus, if the surface area of the solid has been determined by an independent method, the area occupied by the adsorbed dye molecule can be calculated. The measurement of dye adsorption a t a suitable solution concentration can then be used to determine surface areas of other samples of similar solids. This is the principle underlying the present method. The data are given below. Solution Adsorption Isotherms. Solution adsorption isotherms of methyl red (in benzene solution) on alumina, silica gel, and alumina-
Table 1. Slat erial Magnesia-silica Magnesia-silica Magnesia-silica Silica gel Silica gel Alumina
_
i
i
3
