V O L U M E 23, NO. 5, M A Y 1 9 5 1
759
to destroy organic matter, and recovered by one of the standard procedures, the bmic tartrate method ( 3 ) being preferred. ACKNOWLEDGMENT
The work described herein waa supported by a Frederick Gardner Cottrell Grant from the Research Corp., whose financial assistance is gratefully acknowledged. LITERATURE CITED
(1) Berg, Richard, “Die analytische Verwendung von o-Oxychinolin
(‘Oxin’) und seiner Derivate,” 2nd ed., Stuttgart, F. Enke,
1938. ( 2 ) Frere, F. J.,
J. Am. Chem.
Soc., 55, 4362 (1933).
Fresenius, R., and Jander, G., ”Handbuch der arialytischen Chemie,” Part 111, p. 734, Berlin, Julius Springer, 1940-42. Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” rev. ed., p. 638, New York, htaomillan Co., 1948. XIcMpine, R. K., and Soule, B. A., ”Qualitative Chemical Analysis,” p. 374, New York, D. Van Nostrand Co., 1933. Meyer. R. J., 2. anorg. Chem., 60, 144 (1908). Ibid.,p. 147. Ostroumov, E. 8., Zhur. anal. Khim.,3, 153 (1948). Pokras, L., and Bernays. P. M., J . Am. Chem. Soc., 73, 7 (1951). Sterba-Bohm, Johann, 2. Elektrochem., 20, 292 (1914). RECEIVED August 19, 1950. Paper I1 in a series, the chemistry of scandium. Paper I (9). Part of this paper was presented before the Division of Physical and Inorganic Chemistry a t the 117th Meeting of the AYEHICAN CHEMICAL SOCIETY. Detroit, 1Iich.
Apparatus and Procedure for Rapid Automatic Adsorption, Surface Area, and Pore Volume Measurement W. B. INYES American Cyanamid C o . , Stamford, Conn. Knowledge of surface area and pore structure is of primary importance in understanding the behavior of catalysts and other finely divided materials. The conventional Brunauer-Emmett-Teller method is laborious and time-consuming. Apparatus for continuous adsorption measurement has been developed. The method involves ‘a constant slow flow of gas into the adsorbent section, so that equilibrium pressure is approximated and the amount adsorbed (uncorrected for dead space) is proportional to time. Surface area is calculated from the time required to reach a relative pressure end point of 0.2, using nitrogen as the adsorbate a t liquid nitrogen temperature.
I
S COSTriCT catalyst development work i t is highly desirable to have rapid methods for the determination of surface area and pore volume. Catalytic activity defined by relative space velocity for given conversion is usually proportional to surface area when diffusion is not rate controlling, and the nature of the surface is constant. Porosity is important because of its effect on the diffusion of reactanta and products to and from the
1
I
ggI 80
500
BET"^"
1500
1000 VALUE
Figure 1. Correction Factor for Calculating BET Surface Area from Innes Surface Areaa a
calculated from aorption at P / P ~= 0.2.
(:1
= 3.5~0.2)
This time is indicated automatically by a Bourdon gage with contact points or a vacuum recorder. The apparatus is very flexible and can also be used to equilibrate the sample at any desired pressure. This technique using a relative pressure of 0.97, followed by desorption through a wet-test meter, is chosen for pore volume measurement. The simplicity of procedure, short working time (15 minutes per test), ruggedness of apparatus, etc., place surface area determination in the same class a s other routine physical tests. Surface area results can be expected to be within 5% of those of the Brunauer-EmmettTeller method and t o be reproducible within 3970.
internal w f a c e and its effect on area stability. Higher pore volume for a given surface area would be expected to correlate with higher catalyst stability because of greater resistance to sintering. Porosity-area data may also be used in some cases to determine the cause of catalyst deactivation-for example, steam deactivation of silica-alumina cracking catalyst does not cause appreciable change in pore volume whereas thermal deactivation does. Therefore, pore volume data on deactivated cracking catalyst may give an idea as to the relative importance of these factors. The work of Emmett and coworkers (3,4)has established that low temperature nitrogen-sorption measurements provide the best data for surface area determination. A number of methods have been proposed for the calculation of area from these data (6). Because of simplicity, the author prefers to use the relation, surface area = 3.5V0.2where V0.Z is the volume in milliliters at standard pressure and temperature sorbed a t a relative pressure of 0.2. The theoretical agreement of this procedure with the Brunauer, Emmett, and Teller method (15.4 A . 2 per molecule) as a function of the BET C value is shown in Figure 1 (obtained by substitution in the BET equation (4)of P / P , = 0.2). The value, 15.4 -4.2 per molecule, was recommended by Livingston for the nitrogen cross section ( 5 ) ; Emmett prefers the value of 16.2 .4.2 per molecule. This is more generally used and would correspond to the relation, surface area = 3.68Vo.o.~. Silicaalumina cracking catalysts ranging in area from 50 to 700 square meters per gram had on the average (25 samples) a C value of 70. The factor 3.5 was chosen so that perfect agreement would
ANALYTICAL CHEMISTRY
760 be obtained on the average. The average deviation between individual determinations of BET area and 3.5V0.2was 2%. Conventional adsorption measuremepts are carried out by adding gas in increments and allowing it to equilibrate with the adsorbent after each addition. This requires considerable time if several points are determined and does not readily adapt itself to automatic operation. It appeared that more rapid results with automatic operation could be obtained by a continuous, constant. and slow introduction of adsorbate.
'1
APPARATUS
The apparatus developed for continuous adsorption nieasurement is shown in Figure 2. The primary feature is that it is possible to obtain a constant, small rate of flow of gas into the adsorbent section so that the amount of gas introduced may be determined simply by a time measurement while the pressure in the adsorbent section approximates equilibrium pressure. This is accomplished by means of the Moore flow controller which operates to maintain a constant 3-pound-per-square-inch pressure drop across the capillary, or needle valve 2. A constant f o r e pressure (6.0 pounds per square inch) is realized by the action of the pressure regulator and pressure relief valve. Hence, the flow into the adsorbent section should be constant despite variations of the pressure in the adsorbent section. To obtain the low flow rate desired, small diameter capillary tubing must be employed. Fever thermometer tubing with an internal diameter of about 0.001 inch served the purpose. Standard Hoke needle valves are used throughout and have proved satisfactory.
Figure 2.
Figure 3. Determination of Constant:of Flow Rate into Small Volume with Varying Back Pressure Volume, 28 ml. Flow rate, 9.7 ml./minute, STP
volume where the pressure was measured with a mercury manonieter are given in Figure 4. The constancy of flou rate despite change in pressure is indicated by the straight-line relationship. Flow rate can be determined from the slope of these plots by application of the perfect gas law. h measure of the f l o ~rate may also be obtained from the reading of gage 1. At low back pressures in the range of primary interest (22 to 31 inches vacuum), no ELIGOID VACUUM GAGE evidence of varying flow has ever been ohELECTRIC TIMER served. €?offever, with a forepreaaure less than 5 pounds per square inch, a back pressure greater than 20 inche3 vacuum, and flow rates less than 6 ml. per minute a t standard temperature and pressure, the flow decreased slightly with increasing back pressure. Approach to Equilibrium. This i.3 naturally determined by the flov rate, size, packing, and 2''114" FLARE ADAPTER nature of the sample as well as the relative pressure involved. If flow rate is to be conCOPPER TUBING stant over the whole range, the findirigs on one Moore flow controller and information from thc Moore Pioducta Co of Philadelphia, Pa., Apparatus for Coiitinuous Adsorption, Surface h e a , and suggest that floss in esceSS of per ElinUte Pore Yolume Measurement a t standard temperature and pressurc should
/
the vacuum measurements. Periodic checks against a mercury manometer showed no change in its calibration over several months' usage. Its good features include the following: 1. No volume change occurs with pressure change
section as a function of time-for example, valves 4 and 5 may be closed with valve 3 (Figure 2) open and gage readings taken as a function of time. A typical plot of data obtained in this manner is given in Figure 3. Data on flow into a known larger
I
l
l
I
I
l
'
l
/
'
l
-
P 800-
s
Figure 4. Determination of Constancy of Flow Rate into Large Volume Volume, 3160 ml. Flow rate, 5.7 ml./minute, STP
V O L U M E 23, NO. 5, M A Y 1 9 5 1 Table I.
761 0.2 was not investigated thoroughly. ' B limited amount of work showed that equilibrium was most closely approached in the relative pressure region of 0.2, as illustrated in Figure 6. Pretreatment of Sample. If a sample is not heated prior to or during outgassing, i t may contain physically adsorbed water which might lon-er the surface area and pore volume results. High temperature heat treatment may of course cause sintering with decrease in surface area. However, most catalytic materials are used a t fairly high temperatures and, if not, it is the area after reasonable heat treatment that is of importance anyway. Raising temperature is in general more effective in eliminating absorbed water than lorrering pressure-that is, in general more m t e r would be expected to be evolved in a given time on heating a t 400" C. in a muffle open to the atmosphere (50% relative
Water Pickup during Weighing
Time on Balance Pan', MiU.
a
Sample Wt., Grams 0 3.34 6 3.36 12 3 40 25 3 45 120 3.55 60% relatire humidity, 25' C., activated alumina sample.
I
I
- CONTINUOUS
0 EQUILIBRIUM
M
20
25
VACUUM
15 GAGE
FLOW GAGE READINGS GAGE READINGS
1 5
10 READING
I
0
Figure 5 . Comparison of Continuous Flow with Equilibrium Pressure Readings on Nonporous Iron Oxide Sample, 9.7 grama F l o w rate, 10.2 ml./minute, STP
I
20
ments S ~ O U ~beL ~specified in ordering, as stock controllers, Type 63BD, are not normally designed for such a low flow.) Holyever, it I S possible to vary sample size over wide limits and, hence, approach equilibrium about as closely as desired. I
I
I
I
I
I
60
-
,
I
I
'
I
,
Figure 6. Comparison of Continuous Flow with Equilibrium Pressure Readings on Small Pore Size Alumina Sample, 1.90 grams Flow rate, 10.5 ml./minute, STP
1Iany oherviitione on the nearness of approach to equilibrium with fairly small samples (5 to 10 grams) and a flow rate of 10 ml. per minute, shewed that Kith nonporous or large pore (over 200 -4.) materials, negligible pressure changes occurred on closing off, as shorn in Figure 5 . For small pore materials such as fresh silica gel-base catalyst using a 2-gram sample, equilibrium adsorption a t a relative pressure of 0.2 was realized within 4% a t a flow rate of 10 ml. per minute and within 2% a t a flow rate of 7 ml. per minute (standard temperature and pressure). As the author was primarily interested in measuring area, the closeness of approach to equilibrium except near a relative pressure of
humidity) than on evacuating at 200" C. and 0.01 mm. of mercury pressure because of the great dependence of adsorhate vapor pressure on temperature and the slowness of desorption a t low adsorbate presswes. For these reasons as n-ell as greater convenience, this laboratory pretreats the samples a t an elevated temperature in a muffle to remove physically adsorbed mater. The sample might be n-eighed before heat treatment but as specific area on a dry basis is usually more significant, i t appears preferable to weigh the sample afterward. Weighing and transfer to the adsorbent tube must result in some water pickup unless it is carried out in a dry gas atmosphere. However, this precaution is not believed generally necessary if the weighing is carried out rapidly (less than 1 minute) with the possible excFption of low area samples. .In illustration of the negligible effect of brief atmospheric exposure is given in Table I ; it can also be shown by calculating the possible water pickup for a 1-minute period if every molecule colliding with the external surface is adsorbed. The surface 31 e,i value obtained with 1-minute atmospheric exposure m-as 260 square meters per gram: after the 2-hour exposure it was 258 squaIe meters per gram. Both measurements were made on the initial weight basis. Thus, in this case, negligible water adsorption occurred in 1 minute and the adsorption of large amounts of water did not effect the specific area results appreciably except for its effect on sample weight. PROCEDURE
Surface Area Determination. Customar procedure is to place the samples in a temperature-controlledv muffle (usually a t 590" C.) and transfer t o a desiccator after 1 hour until needed. A 2- to 20-gram sample is then rapidly weighed t o + 1% and transferred t o the adsorbent tube with the aid of a powder funnel and brush. It is then connected t o the adsorbent tube and outgassed
762
ANALYTICAL CHEMISTRY
for a few minutes to an absblute pressure of 0.3 mm. of mercury or less. The system is tested for tightness by measuring the leak rate on closing the valve to the pump and again measuring the pressure after a 30-second time interval. Valve 4 is then shut and the adsorbent tube is immersed in liquid nitrogen. Valve 3 is opened allowing nitrogen to flow through the controller section and into the vacuum pump for about 30 seconds until gage I shows that steady flow has been attained. Flow rate is then measured by determining with a stopwatch the time required for the vacuum to drop from 29 to 24 inches gage on closing valve 5. Flow rate is then calculated from the relation, r =
AP X V , X 273
BOURDON GAGE 0-25 LBS.
Y-
/HOKE
ti X 29.92 X T
.---4/4"sS
where r = flow rate, milliliters per minute, standard t e m p e r a t u r e and pressure ti = time f o r i n d i c a t e d pressure change AP = pressure c h a n g e i n inches Of mercury T = room temperature in degrees, absolute T', = volume of the gage section in milliliters
NEEDLE VALVE PIPE WALL S.S TUBING
~i~~~~ 8. Nitrogen \ram por Pressure Thermometer for l~easuring Saturation Pressure at Bath Temperature
This flow rate is normally constant within 1%, and only daily measurements are necessary if the room temperature is constant within =t2' C., and the barometric pressure constant within +0.2 inch of mercury. The contact points are then set to break at a relative pressure of 0.2. The gage reading a t the end point ( p / p o = 0.2) is conven; iently read off a graph as a function of barometric pressure. ( I n calculating po,,it must be remembered, for accurate results, that commercial liquid nitrogen usually contains a per cent or more of oxygen, etc., and, hence, p, is slightly greater than barometric pressure. With the author's source which is uniform and 2.5, where rontains about 4% oxygen, the expression p , = p b p b = barometric pressure in inches of mercury, agrees with p , values measured with the nitrogen-vapor pressure thermometer shown in Figure 8, except after long usage of the liquid nitrogen which results in higher p , values, probably owing to selectivr vaporization of the nitrogen.) The run is then started by opening valve 5 briefly before closing it, starting the timer and opening valve 4 to the adsorbent tube The timer automatically shuts off when a relative pressure of 0.2 is reached.
It' = sample weight in grams d = skeletal density of sample in grams per milliliter A p = pressure change in inches of mercury corresponding to a change of gage reading from 29 to 24 inches The skeletal or material density can normally be estimated accurately enough from handbook values if the chemical nature of the adsorbent is known. Pore Volume Determination. When pore volume is to be determined, nitrogen is rapidly introduced by partially opening valve 2 until the sample is fairly well saturated a t a pressure approaching saturation. Valve 4 t o the sample i then closed. If a noticeable pressure drop occurs in 1 minute, more nitrogen is added (approaching equilibrium by desorption was more rapid, and hence is the preferred procedure). Valve 7 is then opened to the at'mosphere. Valve 5 is adjusted so that a reading corresponding to 0 . 9 7 is ~ realized ~ on the vacuum gage. The p , value is determined with the nitrogen vapor pressure thermomet,er shown in Figure 8 together with the vacuum gage reading on evacuation. A relative pressure of 0.97 fills cylindrical pores smaller in diameter than 556 A. according to the Kelvin equation, or smaller than about 660 A. if multilayer adsorption is also considered ( 2 ) . Use of a higher relative pressure might result in considerable interparticle condensation but would be necessary to get the total pore volume of large pore materials ( 1 ). Valve 4 is then opened rvhich raises the gage reading until equilibrium is reached, at which point it will return to the initial setting of 0 . 9 7 ~ . because no gas will then be flowing in or out of the adsorbent section. .Ifter 30 minutes to 3 hours depending on the nat'ure of the sample (large pore samples are slow in reaching equilibrium), valves 3 and 5 are simultaneously closed and if no measurable change in pressure occurs in a 1-minute period, valve 4 is closed, valves 5 and 7 are opened, and the wet test meter set a t zero reading is connected. The adsorbent tube is then temporarily immersed in warm water in order to rapidly bring it to room temperaturc, and valve 1 is partially opened. From the wet test meter reading, R in cubic feet, after the adsorbent tube has come to room temperature, the specific pore volume, V p ,in milliliters per gram, is calculated as follows:
+
2:
-
3.5 (rto.2 W
Z = specific area in square meters per gram of heat-treated
sample
to 2 = time in minutes for the relative pressure to reach a value
W C1
A W E GN RUN 0 2ND CHECK RUN
loti si
a-here 0.2
L
Y
+ C,)
of 0.2 = sample weight in grams = dead space correction in milliliters, standard temperatui
X F I R S DElERMVIAllON
20
Y
4
The specific area is then calculated using the expression, 0.2
I
30
! 28
I I , 29 e? 2s e5 z4 e3 VbCUuM GbGE READING (IN.HQ)
,
I
22
Silica gel base catalyst Sample, 2.0 g r a m s Flow rate, 9.7 ml./minute, STP
(8
The dead space correction is read off a graph such as the one shown in Figure 7 as a function of sample skeletal volume ( W / d ) and saturation preesure. The graph is based on the expression, (3) = a constant quantity = rt' p o = saturation pressure in inches of mercury t' = time in minutes for the gage reading t o change from 29to 24-inch gage when a run is made with the adsorption tube empty and immersed in liquid nitrogen r = flow rate in milliliters per minute, standard temperature and pressure
V2
,
Figure 9. Typical Example of Reproducibility
and pressure
where
1
DEAD SPbOE CDRRECTION
where F = pressure a n d tempera t u r e factor to convert to O0C.,760mm of m e r c u r y , dry gas W = sample weight in grams A = conversion factor to convert cubic feet at standard temperature and pressure of gaseous nitrogen to milliliters of liquid nitrogen at -196" C., taken as
28 28,316 = 43.8 22,414 X 0.808 The dead space correction, Cz (in milliliters), is normally smal I and varies only slightly with skeletal volume and saturation pressure so that a correction (0.06 ml.) based on average conditions ordinarily gives results accurate within 1%. The dead space correction is calculated by considering both gas remaining after desorption and gas in the dead space during sorption. The full expression used is
cz = 22,4142;
0.808
pg
+
''
273
0.97~.
29.92
+
V O L U M E 23, NO. 5, M A Y 1 9 5 1
763
where pb
= barometric pressure in inches of mercury
V , = volume of the adsorbent section when empty (27.5 ml. for the author's apparatus) DISCUSSION OF METHOD
The method has been applied to a variety of materials having surface areas ranging from 2 to 700 square meters per gram. This method is particularly advantageous for mat,erials having specific areas in the range from 10 to 200 square meters per gram because of short time requirements. For accurate ineasurernent of materials less than 10 square meters per grain, it xould be advisatile to reduce the dead space. BET C' values (estiniat'ed from the pressure a t 1 = '/zk.J were found in all cases to be greater than 30. -1check run was carried out on many samples inmediately after the first run. Excellent reproducibility (within 2%) was invariably obtained, as illustrated in Figure 9. When check runs were oarried out a t a later date, reproducibility was within 4%. The average deviation from the mean of check determinations was 1.2%. The amount of error in area measurement result,ing from various causes on a 2OO-square-nleter sample is believed to be within t.he following limits: Measurement of flow rate Hydration of sample from exposure t o atmosphere during weighing Calibration of volumes Dead m w.-~ a c _e correotion .. _..~ ..~~.~ ~ eighing and transfer of sample to adsorbent tube Measurement of flow time during adsorption to end point Temperature fluctuations
1% 1%
1% 1V"
2%
1%
1%
Less than 15 minutes of working time is generally required for a surface area determination. ALTERNATIVE PROCEDURES
Although this laboratory has adopted the procedures detailed herein, there are other alternative procedures and usages as follows:
1. The increment method of gas addition can be used if desired and the amount of gas introduced measured by the time of aow ,
2. The gas can be allowed to equilibrate with the adsorbent at any desired pressure in the range from 0 to 33 inches of mercury by proper setting of valves 2, 5, 6, and 7; the desorption can be measured Fith a wet test, meter or by the pressure developed on gage 3. 3. The continuous flow method can be used for pore volume determination as well as area measurement. The Helicoid gage has both high and low vacuum contact points. However, equilibrium cannot, be approximated closely enough for accurate results (Figure 6). 4. Other more convenient gases such as n-butane a t 0" C. might be employed for area and pore volume measurement. However, this has not been tried as yet. 5. B vacuum recorder with a short time cycle can be used in place of the Helicoid gage to record the adsorption isotherm directly (uncorrected for dead space). 6. A manifold with several adsorption tubes might be employed t,o increase the capacity of t'he apparatus, or the same 6 pound-per-square-inch nitrogen pressure sources may be used for several units of the type described. ADDENDUM!
Improved operation has now been realized by use of a special low-flow lfoore flow controller; by use of a low pressure, pressure regulator (Conoflow Corp., Philadelphia, Pa.) in series with the cylinder regulator, and replacement of the relief valve with a needle valve. A Precision Scientific Time It timer is now used. The new Moore controller gives a constant flow rate up to saturation pressure with flows less than 5 ml. (STP) per minute. ACKNOWLEDGMENT
The author wishes to t,hank K. D. Ashley for his interest and encouragement, and the American Cyanamid Co. for permission to publish this work. LITERATURE CITED
(1) Barrett, E. P., and Joyner, L. G., ANAL.CHEY.,23,791 (1951). (2) Barrett, E. P., and Joyner, L. G., .J. Am. Cliem. Soo., 73, 373 (1951). (3) Brunauer, S., Emmett, P. H., and Teller, E., I b i d . , 60, 309 (1938). (4) Emmett, P. H., Ibid., 68, 1784 (1946). ( 5 ) Livingston, H. K., J . Colloid Sci., 4, 478 (1949). (6) Ross, E., J . Am. Chem. Soc., 70,3830 (1948). RECEIVED June 8, 1950. Presented before the Division of Colloid Chemistry a t the 118th hleeting of the AXERICAS CHEMICAL SOCIETY, Houston, Tex.
Polarographic Investigations of Reactions in Aqueous Solutions Containing Copper and Cysteine (Cystine) Amperometric Titration of Traces of Cysteine and Cystine with Cupric Copper as Reagent I. M. KOLTHOFF
AND
WALTER STRICKS, Vniuersity of Minnesota, Minneapolis, Minn.
From a polarographic study of the reaction between cupric copper and cysteine in ammoniacal medium in the presence of sulfite, i t was inferred that this reaction could be made the basis of a rapid amperometric titration of traces of cysteine and cystine, using the rotating platinum electrode as the indicator electrode. Procedures are given for the rapid and accurate determination of traces of cysteine and cys-
A
MPEROMETRIC titrations of sulfhydryl groups in amino acids and proteins with silver nitrate have been reported (1,2 ) . No analytical use has been made of the reaction between cupric copper and cysteine in the presence of sulfite. From a polarographic study (3) it was inferred that it should be possible t o make this reaction the basis of a rapid amperometric titration. It is shown in this paper that with the rotating platinum electrode as indicator electrode traces of cysteine can be titrated
tine at concentrations between 4 X and M. Cadmium, zinc, and iodide do not interfere. Cobalt interferes. A n accurate and rapid determination of traces of cysteine and cystine is of great significance in the investigation of biological materials like normal and pathological blood sera. The new method is more specific and precise than methods that ha\-e been previously used.
simply, rapidly, and accurately. The end point can be detected more sharply with cupric copper than with silver as a reagent. The diffusion current involved in the titrations with cupric copper corresponds t o the reduction of cupric to cuprous copper, and not t o a reduction to the metal. Copper salts catalyze air oxidation of cysteine ( 5 ) , but it is possible t o obtain accurate results with copper solutions containing air if titration mixtures of the proper composition are used.
