If the total amount of monobasic acids
solution is put in a cell, its absorbance must be determined quickly or serious errors \Till result. After a solution has been poured from a cell, rapid evaporation of the solvent leaves a thin film of acid on the cell, which is not easily removed by rinsing with cyclohexane. Cells are easily cleaned by rinsing a fe\T times with benzene immediately after solutions are poured from the cells. The absorption of p-tert-butylbenzoic acid a t 282.5 mp \vas found to follow Beer's law IT ithin concentration limits that will be used in the method. Vnriation in absorbance with concentration is shown in Table I. The absorptivity a t 300 mp of the acid within the concentration limits given in Table I was found to be 0.05. I n Table TI are listed absorptivities of some of the fatty acids vc-hich may be used nith p-tert-butylbenzoic acid in alkyd resins. Results of analyses of mixtures of the fatty acids listed in Table I1 xith p-tertbutylbenzoic are listed in Table 111. As would be expected, based on the absorptivities of the fatty acids, less accurate results were obtained with mixtures containing coconut fatty acids. I n Table IV are listed the results of alkyd analyses. p-tert-Butylbenzoic acid was added to all samples in known amounts. The results obtained with the lauric alkyd are higher than one might expect. This alkyd was much darker in color, however, than most lauric alkyds. These results indicate that color can be introduced into the fatty acids during alkyd manufacture.
is desired, cool the flask in a desiccator and weigh. Determination of p-tert-ButylbenDissolve all monobasic zoic Acid. acids in about 40 ml. of cyclohexane. Transfer the solution t o a 100-ml. volumetric flask, rinse the tared flask with three 10-ml. portions of cyclohexane, and add rinsings to volumetric flask. Dilute the solution in the volumetric flask t o the mark with cyclohexane and mix well. Pipet 10 ml. of the resulting solution into another 100-ml. volumetric flask, dilute to 100 ml. with cyclohexane, mix well, and determine the absorbance of the solution at 282.5 and 300.0 mp Calculate the amount of p-tert-butylbenzoic acid in the nonvolatile resin from the following equation.
yo p-tertbutylbenzoic acid
=
(Azaz.6 - 24800)100 ( a ~ .j asoo)(weightin grams of
nonvolatile resin) x b
RESULTS AND DISCUSSION
All absorbance values in this investigation mere obtained with a slit width of 0.15 mm. The slit width at 300 mp is unimportant. Absorbance obtained at 282.5 varies somewhat with slit width. p-tert-Butylbenzoic acid prepared by threefold crystallization of the technical acid from isopropyl alcohol, with a calorimetric purity of 99.8% (supplied by the Shell Chemical Corp.), was used in all work. Cyclohexane is very volatile. After a
The results obtained with the coconut alkyd are as accurate as one could expect and indicate that practically no color was introduced into the coconut acids during manufacture and analysis of the alkyd. The standard deviation ( 2 ) of the method based on seven duplicate determinations is 0.2%. It is concluded that the accuracy of the method for determining p-tertbutylbenzoic acid in alkyd resins will vary with the degree of unsaturation and oxidation of the separated fatty acids. Reasonably accurate analyses of alkyds manufactured n-ith coconuttype and similar slightly unsaturated acids can be obtained. For alkyds containing appreciably unsaturated fatty acids, an infrared method of analvzis n-ould be more feasible. ACKNOWLEDGMENT
The authors gratefully acknowledge the suggestions of C. A. Lucchesi in the development of this method and his help in the preparation of the manuscript. They also thank the Shell Cheniical Corp. for the preparation of the pure sample of p-tert-butylbenzoic acid. LITERATURE CITED
(1) Iiappelmeier, C. P. A,, VanGoor, it. R., Verfkroniek 16, 8-10, 17-20 ( 1943). (2) Youden, iT. J., "Statistical Methods for Chemists," Wiley, Sew York, 1951.
RECEIVED for review December 8> 1958. ;\ccepted dpril27;.1959.
Modification of the lnnes Equation for Determining Surface Areas of Low-Area Solids LOUIS BIN1 and R. 1. DISCH, Jr.' American Cyanamid Co., Bound Brook,
N. 1.
,A more general equation is formulated and used in the present determinations for calculating the dead space correction by the rapid, automatic gas adsorption method of Innes. Surface area data for a variety of materials, using the new procedure for calculating dead space, are compared with the data obtained by the conventional but slower, manual, volumetric Brunauer-Emmett-Teller method.
F
the surface area of solids, gas adsorption techniques, particularly that of Brunauer, Emmett OR MEASURING
1404
ANALYTICAL CHEMISTRY
and Teller ( I ) , have been most widely used. However, this method is time consuming and not particularly suited to routine york. Although Innes' method (3) gives results in reasonable agreement with those obtained by the B E T method for materials having a surface area of about 5 square meters per gram or higher, his equation requires modification when applied to solids of lower specific surface. This paper presents such a modification \\-hich permits the extension of the range of applicability. The Innes equation for evaluating the dead space correction (DSC) is
where Psbs is the absolute pressure at the end point of the measurement and 6.0 is the arbitrarily chosen end point pressure. C,, is the dead space in standard cubic centimeters (scc.) (volume a t standard conditions: 273" C. and 29.92 inches of mercury) of thta 1 Present address, Department of Chemistry, Harvard University, Cambridge 38, Mass.
sample tube a t 78" K. W/d, where W is the weight in grams and d is the density in grams per cc., accounts for dead space lost when the sample is introduced. Hence, the actual value of C, is highly dependent on the ambient temperature, and, an additional term by which C, should be multiplied should be included in Equation 1 to account for this. Each degree on the absolute scale represehts a change of about 0.1 SCC. in this correction in the region of 300" K. in the present apparatus. DEVELOPMENT OF IMPROVED METHOD
It is proposed to separate the dead space correction terms into those relating to that part of the system a t ambient temperature and that a t liquid nitrogen temperature. Such an equation would have the form
where d IS the density of the sample in grams per cubic centimeter, T4' is the weight of the sample in grams, T is the ambient temperature in degrees absolute, C,' is the dead space correction, in standard cubic centimeters, T o is the temperature a t which CO1is determined in " K.. and Ti, is the volume of the sample tube in cubic centimeters. While Innes used 6 inches of mercury as the end point pressure, 6.5 inches of mercury was used in the present apparatus. A relative pressure (pip.) of 0.2 was more closely approached by using the higher end point value. A is a constant A = - 273 X - X -6.5 78 29.92
1 .Z
(3)
where 2 is the apparent conipressibility factor of nitrogen gas a t 78" K. and approximately 6.5 inches of mercury. Thus, A will also account for the nonideal behavior of the nitrogen in the sample tube. It is assumed that the nitrogen a t the higher temperature is practically ideal in its behavior, that is, 2 2 7 3 = 1. A small segment of tubing between the bulk of the system, at ambient temperature, and the sample tube, a t liquid nitrogen temperature, is necessarily a t some intermediate temperature, and its dead space correction must be included in that of the total apparatus. Actual volume measurement of this small segment, a t the end point pressure used, showed that 0.5 scc. of nitrogen was contributed to the dead space correction by this segment of tubing. To determine the dead space correction of the bulk of the system, a t ambient temperature, a cap is applied to the system a t the point where the intermediate segment leading to the sample tube would
Table I.
Comparison of Surface Area Values by BET and lnnes Methods
(Square meters per gram Adsorbent Iron borings General Chemical catalyst Aero catalyst (Am. Cyanamid) Titanium dioxide-rutile Titanium dioxide-anatase Phthalocyanine Blue T
BET 0.915 0.98 1.04 1.63 8.8 9.7 5.95 6.2 28.5 74.7 79.5
;I
I11 IV V Silica-base catalyst A B C
94.8 172.4 294 0
ordinarily be attached, the flow rate is determined, and nitrogen is allowed to flow into the previously evacuated system until the end point pressure is attained. The number of standard cubic centimeters required to attain the end point pressure is then added to the 0.5 scc. contributed by the intermediate segment. This gives Col. It was found that 28.0 SCC. of nitrogen were required for the bulk of the warm system, and upon addition of the 0.5 SCC. contributed by the intermediate segment, one obtains 28.5 scc. for Col. A is determined by attaching an empty sample tube of known volume to the system, immersing it in liquid nitrogen, and noting the number of standard cubic centimeters required to attain the end point pressure in the previously evacuated system. In one instance, this required, for one sample tube, 7.8 SCC. more than previously determined to be required for the remainder of t.he system, including the intermediate segment of tubing. As W = 0 (empty sample tube), the ratio of the number of standard cubic centimeters of nitrogen required to attain the end point pressure in the sample tube, to the actual volume of this tube in cubic centimeters is, in fact, the value of A . I n this particular case, a sample tube of volume 9.2 cc. was employed, thus giving a value for A , for this measurement, of 7.8
= 0.85 =
Innes DSC Modified Original 0 . 796 0.986 1.78 1.33 9.2 10.2
5 20 5.9 32,l 79 9 72.0
3.07 3.67 3.46 $1 . 6
6,6 32.7 80 2
96.8 172.0 291 . 0
versatile expression because it takes explicit account of: nonideal behavior of nitrogen gas a t 78" K.; variations in the volume of the sample tube (any size tube may now be used without any loss of validity) : and the dependence of the dead ?pace correction on the ambient temperature, which represents a change of approximately 0.1 scc. per degree absolute at 300" K. Recently, Clayton and Rulll (2) applied this improved dead space correction equation to surface areas of uranium and titanium oxides with good agreement shown when compared with values obtained by the conventional BET volumetric method. RESULTS
A comparison of the results obtained by the original and modified Inncs methods with those given by the BET method is made in Table I. The data show wbstantial agreement between all three procedures when the area being measured is not less than about .5 square meters per gram. For materials with lower areas, the modified treatment of the data clearly gives results in much closer agreement with the BET method than does the original Innes equation.
-4 ACKNOWLEDGMENT
These measurements may be repeated to obtain an average value of Col and of A to insert into the dead space correction equation. Inserting the constants into the revised equation, one obtains the following expression in the present apparatus :
The analogy of this expression to that of Innes' equation (Equation 1) is readily apparent. I t is felt that this is a more
The authors gratefully acknoKledge the helpful suggestions and assistance of Charles Maresh in the preparation of this paper. LITERATURE CITED
1) Brunauer, S., Emmett, P. H., Teller. E., J.Am. Chem. SOC.60,309 (1938). (2) Clayton, J. C., Rulli, J. E., Chemist Analyst 47, 62 (1958). (3) Innes, W. B., ANAL. CHEM.23, i 5 9 (1951).
RECEIVEDfor review July 11. 1958. Accepted April 13, 1959. VOL. 31, N O . 8, AUGUST 1959
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