PHOTOELECTRIC lMETHODS FOR FOLLOWING FAST GAS-PHASE REACTIONS BY HAROLD S . JOHNSTON Department of Chemistry, Stanford University, Stanford, California Received 26th January, 1954 Several similar systems are described which, by photoelectric and oscilloscopic methods, directly follow fast gas-phase reactions over a wide range of pressure and temperature. The guiding principle behind the development of these methods was that reaction should be followed at constant temperature and at constant volume, so that results may be compared directly with those obtained by regular means. The fact that a reaction is fast makes difficult the requirement of constant temperature for gases. For this and other reasons, rates’ successfully measured by these methods have had half-lives no shorter than 0.1 sec and usually from 0.1 to 1 or more sec.Oxides of nitrogen, ozone, and fluorine are typical of the substances whose rates have been followed by these methods. Fast gas-phase reactions are interesting in providing extreme cases for testing theories of reaction rates against experiment. For unimolecular reactions it is of great interest to study the effect of foreign gas pressure on the first-order rate constant from the high-pressure limit to the low-pressure limit. Since this takes a range of 105 or 106 in total pressure and since there is a range of 103 or more in the rate constant at one temperature, one needs a fast reaction method at high pressures in order not to have to wait unduly for the reaction to go at low pressures. With these purposes behind our studies, we have attempted to retain conditions such that results can be compared directly with rates measured by regular, slower means, that is, the reaction must be carried out at constant temperature and constant volume. For the sake of studying unimolecular reactions we have been interested in measuring rates over wide ranges of pressure and temperature.In the study of fast gas-phase reactions at constant volume and constant tem- perature, there are two easily differentiated tasks. First, one must rapidly prepare the reaction system, that is, bring up (or down) to temperature, mix, and isolate the reactants. Secondly, one needs a fast sensitive method of following the reaction after the system is prepared. The first task requires a different treatment for different pressure regions, and several systems will be described later.The second task is easily solved by a light absorption method which is essentially the same for each kinetic system, and it will be described first. A PHOTOELECTRIC METHOD FOR FOLLOWING REACTANT OR PRODUCT A schematic diagram of the photoelectric method is given in fig. 1. A light source suitably filtered and chopped, is focused through a reaction tube or bulb on to an electron- multiplying photoelectric tube (RCA 931A, 1P22, or 1P28). The output of the photo- electric tube is put across the vertical deflection plate of an oscilloscope (Dumont 208, 247, 304 or 322). The oscilloscope is set to give a single sweep, and the beam is focused off the screen. The operator opens the shutter of a camera (35 mm or 5 by 7 in view camera) focused on the oscilloscope screen, and at the same time he makes an action which simultaneously prepares the reaction mixture and initiates a single sweep of the oscilloscope.This action is to close or open a stopcock of some sort (stainless steel plunger, coupled stainless steel needle valves, 4- or 6-way glass stopcock) and to make an electrical contact in the sweep circuit of the oscilloscope. The camera records the path of the beam as it moves once across the screen, the vertical amplitude giving the 14HAROLD S. JOHNSTON 15 concentration of some reactant or product and interruption of the beam by the rotating sector giving a time scale along the horizontal axis. The procedure is carried out on an evacuated cell to give an incident-light intensity (corrections must be applied for non- linearity of the oscilloscope screen), and the cell is calibrated by measuring light intensity when it is filled with a known pressure of a given substance.The important characteristic of a light source for this method is that it does not vary in intensity at a rate comparable to the chopping rate, 0.01 to 1 sec usually. A slow C I I ~ H O D E UAY OSCILLOSCOPE S\R'CI- ,,UP',13 HrACl10N MOTOR AND LlGKi CELL BULB CHOPPER fly& GLASS FIUERS OR QUARTZ YONOCHROMATOR ! CAMERA t 1 JF I"" INITIATES I"". BY OPERAT'JR FOR TIME EXPOSURE ACTION -------9 - FIG. 1.-Schematic diagram of optical system for recording rate of fast reaction. steady drift over a long period of time is much less serious than short-term fluctuations. At present we prefer a tungsten bulb operated from a large storage battery for visible light, and for ultra-violet radiation the hydrogen arc or mercury arc used with the Beckman DU spectrophotometer.We have always used photomultiplier tubes because they are both very fast in response and sensitive to small signals. The sensitivity is important in reducing the danger of photo-chemical action by the measuring light. If the half-life is as long as 3 sec, the output of the phototube is sent to a fast-responding electronic voltmeter, and readings are taken visually with the services of two operators when necessary. The Voltmeter has much better resolution and linearity than the oscilloscope, and when it can be used it is preferred.This general system has been used by us for several different reactions, and the components differed from one study to another. Table 1 gives a summary of the various components used and references. TABLE COMPONENTS OF OPTICAL SYSTEM USED FOR DIFFERENT REACTIONS light source filter substance followed reaction ref. 120 V d.c. glass NO2 from tungsten 4300 to 4700A 2N02 + 0 3 = N205 -t 0 2 2 tungsten 3389 and 51 13 4100 to 4600 A 2N02Cl= 2N02 + Cl2 4 AH4 mercury Corning NO2 at 4358 8, NO + N205 = 3N02 5 6 V d.c. Corning : NO2 from 2HN03 = 2N02 + H20 + 4 0 2 3 arc 3389 add 5113 2N0 + 0 2 = 2N02 6 7 8 9 2N02C1= 2N02 + Cl2 10 low-pressure chemical 0 3 at 2537 A NO + 0 3 = NO2 3- 0 2 11 mercury arc solutions hydrogen arc Beckman quartz HNO3 at 2HN03 = 2NO2 + H20 + Q02 3 monochromator 2100 A N204 at N2O4 = 2N02 12 2100 A N2O5 = 2N02 + 4-02 2N02 + F2 = 2N02F N02Cl+ NO = NO2 + NOCl REACTION CELLS WHICH TRAP A SAMPLE OF FLOWING GAS.-FlOWing gases may be rapidly mixed in a Hartridge and Roughton 1 type mixing chanber.For the liquid phase one can measure the rate of a fast reaction from the steady-state structure of a continuously flowing system. For gas-phase reactions the pressure drop due to flow, the mole number change (if any) due to the reaction, and the thermal effects due to the heat of reaction make it difficult to follow reactions in this way and to have the results strictly comparable to the conditions of constant volume and constant temperature. However, if a sample16 PHOTOELECTRIC METHODS of mixed, but not completely reacted, gases is isolated in a fixed volume and if the subse- quent changes with time are followed, one gets away from some of the difficulties inherent in the flow method.If a positive closure is made on both sides of a reaction cell, the condition of constant volume is easily met. If both reactants are pre-heated (or pre-cooled) before mixing, uncertainties in temperature are reduced. If the reaction is highly exothermic, the reactants must be diluted with a great excess of inert gas in order to main- tain constant temperature. If the reaction is a decomposition at high temperature, it may be rapidly and sharply brought up to temperature by mixing with a great excess of pre-heated inert gas. An example of this type of system, suitable for use from 50 to 800 mm, is shown in fig.2. The two reactants and the diluent gas are stored in 3-1. flasks, and their partial I2 LITER VACUUM RESERVOIR FIG. 2.-Reaction system for intermediate pressures and near room temperature. pressures are known by synthesis. TO carry out a run, one evacuates the reservoir, re- action cell, through the second 4-way stopcock up to the first 4-way stopcock. A reading of lo, the initial light intensity, is taken. Then the first 4-way stopcock is opened, and the reactants flow through the heating coils, mixing chamber, reaction cell, and the constricting capillary into the vacuum reservoir. After 2 or 3 sec of such flow, during which a steady state is attained, stopcock 2 is rapidly closed, and this makes an electrical contact leading to a single sweep of the oscilloscope and a recording of a run, The total pressure in the 3-1.flasks is read before and after each run. The pressure drop due to flow from the storage bulbs to the reaction cell must be calibrated ; by use of large-bore stopcocks this pressure drop can be held to 1 % or less. If one channel of the second STEEL SLIPRIFJ G, GLASS ,,GOLD SOLDER STAINLESS STEEL FIG. 3.-Reaction cell for high pressures, temperatures up to 180" C. 4-way stopcock cuts off slightly before the other, it is essential that the first line to close be on the exit side, otherwise serious error can arise from partial evacuation of the reaction cell during the closing of the stopcock. The reaction cell of a high pressure apparatus is shown in fig. 3. The body is nickel, the windows are glass fused to a Covar rim (specially made by Stupakopf Ceramic Co., Latrobe, Pennsylvania), and the Covar rim is clamped tightly against a gold gasket.The windows are coated with magnesium fluoride for protection against fluorine. Teflon- packed needle-valves of nickel or stainless steel are used. Up to this time, this system has been used only up to 180" C ; in the past we have had highly unsatisfactory service from such systems when made of stainless steel and used at 400" C . At 200" C nickel seems to be far superior to stainless steel for the handling of corrosive substances.HAROLD S . JOHNSTON 17 Temperature uniformity in these systems is attained by enclosing them in massive castings of aluminium. Various modifications of this system have been made for different reactions, and different combinations of components are listed in table 2.The system which can measure the shortest half-life is that using a very short cell and a steel plunger as a stop-gate. Wayne and Yost 13 measured half-lives as low as 0.01 sec by this method. The system using the steel plunger gives results with a rather high experimental error, and the other systems with longer light paths, leak-proof stopcocks, and reduced trouble from vibra- tions give more precise results. By virtue of the longer light path these systems can measure an equally large second-order rate constant as the plunger device, even though it follows a longer half-life in doing so. TABLE 2.-cOMPONENTS OF FLOW SYSTEM USED FOR VARIOUS REACTIONS reaction cells ref. cut-off device total dimensions mm -- reaction carrier pressure mm material gas dilasr$.length body window NO + N2O5 I NO2Cl (decomp.) HNO3 (decomp .) N02Clf NO 0 2 760 N2 200to 700 {arious 50 to 700 7000 40,000 N2 2000 to N2 1000 to Nz 760 N2 300-700 2 37 glass glass st. st. plunger 2 8 100 st. st. quartz coupledneedle 1 1 8 105 glass glass 4-way stopcock 15 valves 10 100 st. st. glass coupledneedle 15 20 100 nickel glass- needle valves 4 Covar rim 20 48 glass quartz 6-way stopcock 3 valves 8 100 quartz quartz 4-way stopcock 9 BIG BULB METHOD OF STUDYING FAST REACTIONS A separate, though prior, development of a photoelectric method for following fast gas-phase reactions was carried out by Smith and Daniels,*4 who initiated reaction by shooting reactants into a 3-1. bulb or into cylinders of different sizes.Big bulbs have a great advantage in that surface effects are minimized, and they are ideal for studying reactions at low pressure. In the systems we have used, a half-life of 1 sec is about the fastest rate which can be followed. However, the long light path available in the large bulbs makes it possible to measure second-order rate constants almost as big as those found by the flow method. One example of a big bulb reaction system will be described in detail. It is based on a 144-1. Vycor bulb (Vycor is the trade name of the special Corning glass which is 96 % silica; it is made over a period of several months by leaching out substances other than silica after the glass is formed, and then it is heat-treated again).In the autumn of 1950 we asked Corning Glass Works if they could make a Vycor flask 22-1. or larger. For moxe than a year they tried and repeatedly failed to make a 33-1. bulb. Finally they tried the next smaller size utilizing standard moulds, and after 6 months’ of work and one failure, they completed and delivered a 144-1. Vycor flask equipped with outlet tube 1 cm in internal diameter and with a graded seal to Pyrex. The entire reaction system is shown in fig. 4. The flask is enclosed in two close-fitting silver hemispheres of 0.040 in. thickness, and this is wrapped with asbestos. This unit sits in sand inside a welded aluminium can 1 ft. 3 in. diameter, 1 ft. 3 in. high, & in. wall thickness ; the metal is 2 S alloy, that is, virtually pure aluminium.The can is wrapped with asbestos and wound closely and uniformly on all sides with a 14-ohm Chrome1 A heating coil, controlled by a Powerstat variable tiansfoxmer operated from 208-V, 60-cycle a.c. supplied by a Stabiline Electro- mechanical Voltage Regulator (Superior Electric Co., Bristol, Conn.) The can is placed on fire bricks inside an outer welded aluminium can 2 ft. 4 in. high, 2 ft. 4 in. diameter, and fi in. wall thickness. The space between the inner and outer cans is filled with 6in. of Santocel-A insulation (Monsanto Chemical Co., Merrimac Division, Boston,18 PHOTOELECTRIC METHODS Mass.). The Vycor bulb could be taken up to 800" C, but the present system is limited by the melting point of aluminium and we have not gone above 625" C.Temperature is measured by one Pt-Ir thermocouple and four chromel-alumel thermocouples. The e.m.f. is read on a Leeds and Northrup type K potentiometer. At 600" C temperature varies IF from point to point over the silver shell and at any one point it drifts 4" from time to time depending on room temperature. Lengths of quartz tubing lead through lin. holes in the side of the cans, ending with quartz windows inside the inner can. Holes are provided on the silver shell for the light beams. Two of these light paths are arranged at right-angles to each other, so that it is possible to follow the reaction by two different wavelengths at once. The Vycor flask mounted in this way transmits ultra-violet radiation above 2350 A. The Pyrex system of gas pipettes a and b in fig.4, Bourdon gauge, storage bulb d, and vacuum lines are all of the usual sort. The optical system differs from the general one described above only in that it has a second phototube which measures the incident light. The output of the two beams is presented at once on the Dumont 322 dual-beam oscilloscope. n MERCURY MANOME T€R DRY AIR VACUUM LIGHT L-- IMSULATlOk i l__ll__-_l_- PHOT~TUEE FIG. 4.-Reaction system for low pressures, temperature up to 600" C. a and b, calibrated gas pipettes ; c, 14h-1. Vycor flask ; d, storage bulb ; 1-7, high vacuum stopcocks greased with polychloro-trifluoro ethylene ; 3, stopcock with 8-mm opening. A second example of the big bulb system will be mentioned briefly. It consists of a 50-1. Pyrex bulb with two quartz windows protruding 7 cm inside it.These windows are fused to a 12-mm quartz tube which connects through a graded seal to the body of the Pyrex flask. It was necessary to have these windows protrude inside the flask instead of outside in order not to have 14 cm of optical path through the reaction mixture with a very non-representative surface to volume ratio. This flask is transparent to radiation above 2000 A. It is mounted inside two large concentric steel cans with 5 in. of Santocel-A insulation between them. Heating wires are uniformly spaced inside the inner can and control is obtained by a second small on-and-off heater at the bottom. The air is vigorously stirred by a large blower mounted inside the inner can and connected by a long shaft to a motor outside. The top of this furnace is removable and replaceable by a large copper can which can be filled with a coolant.This system has been operated from Several other big bulb systems have been used, some successfully and some un- successfully. A 2-1. Vycor bulb is set up very much the same as the 144-1. bulb, except it has only one light path. A 22-1. reaction bulb and its pipette system has been described eIsewhere.5 A 34-1. stainless steel bulb was rejected as useless after it was found that it could not be protected against fluorine at moderately high temperatures, and that the fluorinated surface adsorbed nitrogen dioxide very strongly. A summary of rate studies of fast reactions which we have made by the big bulb method is given in table 3. The study of the decomposition of nitrogen tetroxide, mentioned in table 3, is very frag- mentary, representing so far only a couple of rate measurements at - 50" C and at 0.1 mm total pressure.- 50" C UP to 250" C.HAROLD S. JOHNSTON 19 TABLE 3.-D"ACTIONS STUDIED IN VARIOUS BIG BULBS useful temp. range reactions O C bulb material Pyrex 25 to 100 NO 3- NzOs NO + N02Cl 2N02 + F2 Pyrex with -50 to 250 NO2Cl (decomp.) N2O4 (decomp. at quartz windows - 50") Vycor 25 to 600 HNO3 (decomp.) Vycor 25 to 600 HNO3 (decomp.) volume of bulb, 1. 22 50 2 14.Q ref. 5 9 8 10 12 16 3 RATE OF MIXING, HEATING, AND ISOLATING REACTION MIXTURE If the integrated rate expression is known for a reaction, a suitable plot can be made against time such that extrapolation to zero gives the effective time of mixing and flowing into the cell.Plots of this sort have been published for nitrogen dioxide and ozone,2 delay time 0.03 sec, and for nitric oxide and ozone,ll 0-07 sec. The steady-state temper- ature of the reaction cell of the flow method has been measured by small thermocouples. Based on these studies we use 70 cm of 8 mm glass tubing for a pre-heat coil for each re- actant, and it appears that highly exothermic reactions in 8-mm tubes gives a temperature rise of only about + of that expected from adiabatic conditions. For the big bulbs a direct measure can be made between the time the stopcock starts to turn and the time for a stable gas, such as bromine, to reach a steady concentration inside the flask ; for the 14i-1. flask these times are 0.2 to 0.4 sec. The time for a gas IN THE PRESENCE OF 37 MM OF NITROGEN .I .2 .3 R .S .6 7 .8 .S 1 .0 1.1 I2 13 1.4 15 1.6 I7 IS 19 SECONDS FIG. 5.Time to flow in and heat up to 603" C. Straight line is plot of integrated rate expression for decomposition of nitrogen dioxide. to heat up in the big bulb can be estimated by measuring the initial rate constant for the well-known bimolecular decomposition of nitrogen dioxide.17 The apparent decrease in the rate of this reaction with increase in the pressure of inert gas is interpreted as a failure of the gas to reach the bulb temperature as fast as it flows in. Fig. 5 gives an example of a calibration of this sort in the 14i-1. flask, and it also indicates a time of about 0.2 sec for the gas to flow in. These calibrations are not completed, but it appears that 2 mm of nitrogen dioxide and 20 mm of argon reaches bulb temperature at 450' C as fast as it can flow in.Twice this amount of each gas appears to reach a temperature about 4" C too low upon flowing in, and the gas does not pick up the remaining few degrees in the 1 to 10 sec during which a fast reaction might be followed. This feature of a steady but incorrect temperature over the course of a reaction can be a serious and easily over- looked source of error. In using big bulbs for fast reactions one must take care to go to20 PHOTOELECTRIC METHODS low enough pressures or go to small enough bulbs so that serious temperature gradients are not set up because of the heat of reaction18 HANDLING CORROSIVE GASES Corrosive gases can be handled rapidly and conveniently by means of high-vacuum hollow-bore glass stopcocks greased with polychloro-trifluoro-ethylene stopcock grease.Though corrosive gases do not seem to react with this grease, the less volatile ones such as nitric acid and nitrogen dioxide arc quite noticeably soluble in it. Also the grease itself has a considerable vapour pressure. Some of these gases are best handled in a metal vacuum system with Teffon-packed needle valves, and junction to glass is made by Covar- to-glass graded seals. For certain cases it is desirable to protect the Covar with a film of polychloro-triff uoro-ethylene (Kel-F dispcrsion, The M. W. Kellogg Co., Jersey City, New Jersey). FUTURE WORK As these methods have been used by us in the past, the precision of the rate constants measured has varied from fair to poor.Some progress has been made recently in separating, identifying, and removing sources of error, and this pro- gramme is our principal goal so far as methods are concerned. As a summary of the work done so far on these methods and as a map for future extensions, a graphical outline is given in fig. 6 of the regions of pressure I I I I I I 1 FIG. 6.-Temperature and regions within which these have been used to study actions. pressure methods fast re- TEMPERATURE *C and temperature at which rate measurements or calibrations have been made. Rate studies under way at present make interesting the closing of the gap between 10 mm and 1 atm below 500" C. It seems that apparatus could readily be designed and built to fill in most of the area in fig. 6, perhaps excepting the upper right-hand corner.We know of no method to measure fast reactions at very high pressure and very high temperature. Another line of development under way at present is to modify the materials so that these methods can be used with fluorine and its compounds. It is a pleasure to acknowledge the contribution of Prof. Don M. Yost to the conception of these methods and this field of research. Especial thanks are expressed to Dr. Robert L. Mills who designed and built systems for all pressures, to Mr. Daniel Devor who designed and put together most of our recent apparatus for low pressures, and to all the co-workers who have used and improved on these systems. Much of this work could not have been done without the support of The M. W. Kellogg Co., Special Projects Department; the United States Office of Naval Research; and the Naval Ordnance Test Station, Inyokern. We are grateful to Corning Glass Works for the development of the big Vycor bulb, to the Reynolds Metal Co. for generous samples of aluminium, and to the Shell Oil Co. and American Cyanamid Co. for fellowships.HAROLD S. JOHNSTON 21 1 Hartridge and Roughton, Proc. Camb. Phil. Soc., 1926, 3, 450. 2 Johnston and Yost, J. Chem. Physics, 1949, 17, 386. 3 Johnston, Foering and Thompson, J. Physic. Chem., 1953, 57, 390. 4 Casalleto, work in progress. 5 Wilson and Johnston, J. Amer. Chem. Soc., 1953, 75, 5763; also 1953, 75, 1567; 6 Johnston and Slentz, J . Amer. Chem. SOC., 1951, 73, 2947. 7 Johnston and Tao, J. Amer. Chem. SOC., 1951, 73, 2948. 8 Perrine and Johnston, J. Chem. Physics, 1953, 21, 2202. 9 Freiling, Johnston and Ogg, J. Chem. Physics, 1952, 20, 327. 10 Cordes, work in progress. 11 Johnston and Crosby, J. Chem. Physics, 1951, 19, 799 ; 1954, to be published. 12 Herschbach, work in progress. 13 Wayne and Yost, J . Chem. Physics, 1951, 19, 41. 14 Smith and Daniels, J. Amer. Chem. Soc., 1947, 69, 1735. 15 Mills and Johnston, J. Arner. Chem. SOC., 1951, 73, 938. 16 White, Thesis (Stanford Univ., 1953). 17 Bodenstein and Ramstetter, 2. physik. Chem., 1922, 100, 106. 18 Benson, J. Chem. Physics, 1954, 22, 46. 1951,73,4782.