rigid glass at 77 OK (cf. Table I). Mirror symmetry was still conserved in all these solvents. There are several centers in the bicyclic ring system where interaction with a polar solvent can be envisaged, with a resultant reduction in the fine structure. This is in accord with a general trend for polar solute molecules in polar and nonpolar solvents (6). The polarity of the solvent influenced the fluorescence quantum yield to a more significant extent than the spectral solvent shifts. The variations extend from a 2-fold increase in Compound 1 to an approximately 10-fold increase in Compound 8. The quantum yields were greatest in benzene and least in D M F or ethanol. The competing excited state processes in operation in these molecules are apparently fluorescence emission (Equation 2), intersystem crossing plus deactivation (Equation 3) and internal conversion from the singlet excited state (Equation 4). ‘A % ! 1A* ‘A* -C ]A hv’ ‘A* + 3A* + ‘A ‘A* + lA
+
(1) (2) (3) (4)
Permanent photochemical reactions were not observed in these molecules even upon 2-3 weeks of continuous irradiation. The only part triplet states apparently play in these compounds is to provide a possible means of deactivation. Triplet states apparently deactivate quite rapidly as no phosphorescence was observed at 77 OK and the triplet quenchers biacetyl and piperylene were not effective in the transfer of triplet energy. In the case of biacetyl quencher, no biacetyl phosphorescence was observed in thoroughly outgassed solutions of mixtures of a representative series of Compounds 1-12 and biacetyl. In the case of piperylene no cis-trans photostationary state was obtained in Compound 1-piperylene mixture in benzene. The increased solvent polarity enhances the efficiency of internal conversion and/or intersystem crossing relative to fluorescence. The reasons for the increase in relative efficiencies of excited state processes is largely unknown and is an area under intense investigation.
ACKNOWLEDGMENT
~~~
(6) H. H. Jaffe and M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy,” John Wiley & Sons, New York, N. Y.,1962, p 186.
RECEIVED for review January 25, 1971. Accepted April 22, 1971. Work supported in part by U. S. Public Health Service Research Grant CA 08495, National Cancer Institute, and by a National Dairy Fellowship to R.A., 1969-70.
Determination of Surface Areas by an Improved Continuous Flow Method Michael G . Farey and Bernard G . Tucker Ministry of Aviation Supply, Explosives Research and Development Establishment, Waltham Abbey, Essex, U. K . THECONTINUOUS FLOW METHOD of surface area determination normally requires premixed gases and frequent detector calibration. A system is described in which careful attention has been paid to flow control. These precautions have eliminated the usual problems associated with in situ mixing of gases, and the effects of flow fluctuation on the detector performance. The system is stable enough for the routine determination of surface areas of 0.1 m2/g or less and the measurement of adsorption isotherms. The flexibility of the system is demonstrated by nitrogen adsorption on ammonium perchlorate, titanium dioxide, and alumina. The surface areas of these samples range from 0.1 m*/g to 70 mz/g. An adsorption isotherm for titanium dioxide is presented. In the formulation of solid propellants, a knowledge of the surface areas of solid ingredients is very important. This is particularly true for burning rate catalysts such as titanium dioxide and the oxidant which is usually ammonium perchlorate. Static gas adsorption methods are accurate but time consuming. More convenient methods such as air permeability have given nonreproducible results with finelyground ammonium perchlorate because of pellet compression problems. The continuous flow method was introduced by Nelsen and Eggertsen ( I ) . These authors stated that their method ~
~~~~
(1) F. M. Nelsen and F. T. Eggertsen, ANAL. CHEM., 30, 1387
(1958).
of flow control, which utilized capillaries, did not have long term stability. They assumed that the detector response was constant over the range of carrier gas compositions used. Daeschner and Stross (2) improved the method and extended its range to lower surface areas by utilizing a stopped flow technique during desorption. They also investigated the variation in detector response with change in carrier gas compositions. Later publications (3, 4 ) have described the use of premixed gases and frequent calibrations in order to overcome problems engendered by poor flow control and in situ gas mixing. The use of premixed gases involves a large number of gas cylinders if adsorptions are required over a n extensive range of relative pressures, or a number of different adsorbent gases are to be studied. Modern gas chromatographic practice involving temperature programming has successfully overcome difficult flow problems and it was felt that similar solutions could be adopted here. Careful flow control and the adoption of an independent gas supply for the reference and measuring cells of the katharometer has improved stability and decreased the time required to re-establish equilibrium after a change in carrier gas composition. Results obtained with nitrogen gas are presented but it (2) H. W. Daeschner and F. H. Stross, ANAL.CHEM.,34, 1150 (1962). (3) J. F. Roth and R. J. Ellwood, ibid., 31, 1739 (1959). (4) R. M. Cahen and J. Marechal, ibid., 35, 259 (1963).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10. AUGUST 1971
1307
DETECTOR
m
ADSORPTION BLANK
6ooT __
400 RESPONSE
300
I
Figure 3. Blank and typical adsorptiondesorption peaks
Figure 1. Block diagram of apparatus
DETECTOR
DE50 R PT ION
t
2ol 100
instruments, a minimum pressure drop of 10 psi was required. N was a fixed flow resistance giving 36 ml/min of Nz at a head of 40 psi. The detector was a semidiffusion type, thermal conductivity cell, Model 10-285 (Gow-Mac Instrument Co., Madison, N. J.) The detector current was 60 mA supplied by a Model 10-201 katharometer power supply (Gow-Mac). The output was monitored by a 1-mV Speedomax W recorder (Leeds & Northrup, Birmingham, England) fitted with a Model 224 integrator (Disc Instruments, Hemel Hempstead, England). A liquid nitrogen trap F was provided to ensure complete removal of condensible impurities before the gases entered the sample tube. A similar tube G was placed in the reference gas stream in order to preserve the thermal identity and geometry of the two limbs. When the sample tube H, a U-shaped gas tube 4-rnm i.d. and 300-mm total length was cooled or heated, tube Z in the reference stream was similarly treated. The gases were passed through lengths of copper tubing (0.2-cm i.d. X 100 cm) immersed in a water bath at 30 “C before entering the katharometer block; the block was also immersed in the water bath. Gas flow measurements were made with a range of soap film flowmeters. Tubing connections were made with a range of captive seal unions (Drallim Couplings Ltd., Whyteleafe, Surrey). Procedure. Nitrogen flow rate was set by adjusting flow controller C with valve J closed. The flow through the fixed resistance N was measured. The normal range required was 2 to 20 ml/min. Valve J was now opened and M closed. The combined He/Nz flow was measured across N . Flow controller B was adjusted to give a He flow between 50 and 75 ml/min. Valve L was closed and M opened to admit the gas mixture to the measurement section. Pressure regulator A was set to give a downstream pressure of 35 psi and flow controllers D and E were adjusted to give flows of about 25 ml/min measured at the detector outlets. After this initial procedure, it was normally necessary only to adjust flow controller C to give the required partial pressures of NZ in the gas stream. The flow ratios of He and Nz determined in the mixing section, were confirmed by both gas chromatographic and mass spectrometric analysis of samples taken from the katharometer outlet. It was necessary to purge the system for ten minutes with each new gas mixture prior to carrying out a determination. The gas connections were l/c-inch bore tubing and a reduction to inch might well decrease the time required for stabilization. Calibration. The detector was calibrated by the injection introduced by means of a gas samof known aliquots of Nz, pling valve (Loenco Model 206/6) which replaced the normal sample tube. Calibrations were performed using 0-250 mm partial pressure of Nz in He. A plot of detector response against partial pressure of N2 in the carrier gas is shown in Figure 2. Within the range of Nz samples, 0 to 7.4 ml at STP, the detector response was linear.
i
0
100
50
P
N,
150
200
250
in carrier gas
Figure 2. Variation of detector response with nitrogen content of carrier gas would be a simple matter to extend the method to other gases. Surface areas are normally calculated by use of the Brunauer, Emmett, and Teller (BET) equation (5) but for titanium dioxide the “point B” method (6)has also been used in order to verify the adsorption isotherms. Substrates examined in this work included ammonium perchlorate at five different particle sizes, titanium dioxide, and alumina. These materials cover the range of surface areas 0.1 mz/g to 70mZ/g. EXPERIMENTAL
Apparatus. Figure 1 shows a block diagram of the apparatus and indicates the approximate pressures at various points. Gases were supplied from cylinders equipped with two-stage regulators adjusted to give an output of 100 psi Diu drying tubes packed with 4A molecular sieve. Pressure controller A was Model 8601, 0-60 psi output (Brooks Instrument Division of Emerson Electric Co., Hatfield, Pa). Massflow controllers B, C, D,and E are Model 8744, 0 to 750 ml/rnin (Brooks). For the correct functioning of these ~
(5) S. Brunauer, P. H. Emmett and E. Teller, J. Amer. Chem. SOC., 60, 309 (1938). (6) S, J. Gregg and K. S. W. Sing “Adsorption, Surface Area and Porosity,” Academic Press, London, 1967, p 54.
1308
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
80
7.0
6.0
“ads.
5.0
ml. at
STP
4.c
3.0
2.0
g x lo2
1 so
Figure 4. BET plot obtained for degassed TiOz C
I
0.1
Table I. Values of Surface Area and Constant C Obtained from BET Plots Surface area, BET m2/g constant, C Sample Alumina 67.0 20.4 157.5 Titanium dioxide 9.91,9.82 Ammonium perchlorate (Sample 1) 3 . 0 42.5 Ammonium perchlorate (Sample 2) 0.85 13.2 26.6 Ammonium perchlorate (Sample 3) 0.24 Ammonium perchlorate (Sample 4) 0.114, 0.114, 116 (mean) 0.118 Ammonium perchlorate (Sample 5 ) 0.107, 0.105, 128 (mean) 0.107,0.104, 0.106
Std dev
= 0.001
Surface Area Determination. Samples ranging from 0.1 gram t o 5 grams were placed in the sample tube and the bed located with plugs of glass wool. The samples were usually degassed for one and a half hours a t 150 “C while purging with He. Figure 3 shows typical adsorption and desorption peaks and also the record obtained if the procedure was followed with the sample tube empty. As in previous work ( I ) , the desorption peak was used for quantitative measurements. The temperature of the liquid nitrogen was measured with a n oxygen gas thermometer. RESULTS AND DISCUSSION
The volumes of gas adsorbed a t a number of partial pressures of N P were used t o calculate the surface area and constant C, by the normal BET method (5). Table I lists results obtained for samples of A1203, Ti02, and five samples of NH4C104milled in different ways t o give different particle sizes. As few as three to four points were necessary to establish unequivocally the BET plot. A detailed investigation of the method was made with TiO2, determinations being made a t ten different partial pressures of nitrogen. An excellent BET plot with little scatter (correlation coefficient = 0.999) was obtained (Figure 4); these data are plotted in Figure 5 as a n adsorption isotherm.
I
02
I
0.3
0.4
0‘5
% Figure 5. Plot of volume of gas adsorbed us. relative pressure of nitrogen in carrier gas for Ti02
For this sample the volume of gas necessary t o form a monolayer (xm),as determined by the BET method, was 5.30 ml (at STP). The corresponding value obtained by the point B method (5) ( x B ) was 5.84 ml (at STP). The ratio X ~ : X B is 0.91 and this agrees well with values quoted by Young and Crowell (7). At low partial pressures the adsorption peak was broad and flat-topped. As the NB content of the carrier gas increased, the peak sharpened and eventually lost its plateau. It is probable that a t low N2 levels equilibrium cannot occur until a considerable volume of gas has passed over the sample. We have noted that desorption peaks equivalent to volumes of gas which are insufficient to give a monolayer have a slight shoulder high on the leading edge. This disappears when the volume of gas adsorbed exceeds that required for monolayer formation. This has been observed in a rather different system by ‘Kuge and Yoshikawa (8) who suggest that the shoulder may be related to the change from monolayer t o multilayer adsorption. Flow rates through the detector momentarily change during the rapid temperature changes encountered in a n adsorption/ desorption cycle. However, the flow controllers rapidly compensate and during the production of peaks, the flow is a t its previous value. If the sample tubes were too large, the flow could actually reverse during cooling. In extreme cases it was possible t o draw air back into the detector. However, large tubes have been used successfully. This was achieved by providing 100-ml baffle volumes a t the detector outlets. (7) D. M. Young and A. D. Crowell, “Physical Adsorption of Gases,” Butterworth, London, 1962, p 198. (8) Y. Kuge and Y. Yoshikawa, Bull. Cliem. SOC.Juputi, 38, 948, (1965).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
1309
The apparatus described was a prototype and certain modifications are now being made to give still greater stability and sensitivity. These include replacement of the two-stage cylinder regulators by pressure regulators, 0-200 psi output (Model 8601, Brooks). A model 10-460 flow-through katharometer (Gow-Mac), having a lower internal volume, has been shown to give a n increase in sensitivity and an improved base line at the same filament current. It is hoped that these improvements will enable surface areas of 0.02 mz/g to be measured. At present the total time required to determine a surface area is three and a half hours of which one and a half hours
is taken up by degassing the sample. For the examination of large numbers of samples, it should be possible to operate a number of sample tubes in series, as suggested by Daeschner and Stross (2). This would allow simultaneous degassing, and following the establishment of a given relative pressure of adsorbate gas, a rapid adsorption/desorption could be performed on each sample in succession.
RECEIVED for review February 1, 1971. Accepted April 29, 1971. Crown Copyright. Reproduced by the permission of the Controller of Her Majesty's Stationery Office.
Hydrochloric Acid and Ammonia Gaseous Standard Solutions Elio Scarano, Michele Forina, and Giovanni Gay Institute of Analytical Chemistry, Faculty of Pharmacy, University of Genoa, Italy RECENTLY it has been shown ( I , 2) that concentrated lithium chloride or sulfuric acid aqueous solutions, containing small amounts of hydrochloric acid, possess a hydrochloric acid partial pressure (PHCI) higher than that of hydrochloric acid aqueous solutions at the same concentrations; a nitrogen stream, passing through these solutions in a wide range of flow rates, becomes saturated by hydrochloric acid at the corresponding equilibrium partial pressure, thus allowing the determination of the PIXIfrom the amount of hydrochloric acid contained in a given volume of gaseous mixture; the very low water content, in both the liquid and the gaseous phases, reduces or nullifies the condensation of water in tubing and the solubilization in this water of the hydrochloric acid of the gaseous phase (these phenomena take place when the water content is high, giving anomalous results in the P H Cdeterminations). ~ These results allow the possibility of preparation and use of gaseous standard solutions. A gaseous standard solution (GSS) can be defined as a mixture of known and constant composition of a gaseous reagent and an inert gas (really, also small amounts of aqueous vapor are present). The concentration of the gaseous reagent is expressed in this paper in molarity, i t . , in moles of reagent per liter of gaseous mixture at given temperature and pressure. A GSS is prepared at the time of use, allowing a stream of the inert gas to flow through a suitable liquid solution, the mother solution, where the reagent is dissolved. Then the GSS passes through the sample solution, leaving the reagent in it. Finally, the inert gas volume is measured. In this paper we discuss hydrochloric acid and ammonia gaseous standard solutions (HC1-GSS and "3-GSS). EXPERIMENTAL
Mother solutions were prepared by gravimetric and volumetric procedures in amounts of about 100 ml for each series of experiments. Titrations were carried out potentiometrically with the glass-saturated calomel electrode system, with 5-ml portions of standard Na2B40ior HC1 aqueous solutions. (1) E. Scarano, G. Gay, and M. Forina, ANAL.CHEM.,43, 206 ( 197 1). (2) E. Scarano, M. Mascini, and G. Gay, ibid., p 442. 1310
The apparatus and the procedure have been already described ( I ) . The mother solution was kept in a cell (the A cell) connected by means of glass tubing to another cell (the B cell), containing the Na2B40ior the HC1 standard solution (these latter solutions substituted for the acidic 1 M KC1 solution described in reference I). A nitrogen stream passed continuously through the B cell. Prefixed nitrogen volumes (accurately measured with a soap bubble flowmeter downstream) could also be passed through the A cell, thus transferring known amounts of the gaseous reagent (hydrochloric acid or ammonia) in the B cell. These amounts of reagent were small compared to the reagent content of the mother solution. The composition of the mother solution and that of the GSS were practically unaffected, even after many titrations, provided that the following condition was fulfilled : 4
- n 100 < E
Q
where q = moles of reagent consumed for each titration; Q = moles of reagent in the mother solution; n = number of titrations; E = per cent error in titrations. The concentration C of the GSS was calculated by means of the following formulas (I):
(3)
P,, = C R T,,
(4)
where meq = milliequivalent of the standard substance (Na2B40ior HC1); V = volume (ml) of the GSS delivered from the A cell; V N 2= water saturated stripping nitrogen volume (ml) measured downstream, at the temperature T,; Tms= temperature ( O K ) of the mother solution; T, = room temperature ("K); P = atmospheric pressure; P H ~ O= water partial pressure, at T,; AP = bubbling overpressure in the A cell, due to the gas bubbling in the B cell; Psx0 = water partial pressure of the mother solution, at Tm8;Pgr = gaseous reagent partial pressure of the mother solution (all pressures in mm Hg); R = gas constant. When C > lO-4M (Pgr>, 2 mm Hg), C was calculated by means of Equations 5 and 6, obtained from Equations 2, 3, and 4:
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10,AUGUST 1971
