Genera
The vacuum linc
General techniques for high vacuum systems have been covered in detail in various reviews22, in particular conventional high vacuum taps and their lubri-
(el cut-offs From ref 22c
Ramsperger’s greaseless valve offers a solution for gas isolation without the issues associated with lubricants, such as vapor absorption and the inability to "bake" taps The Echols type tap utilizes graphite as a lubricant and mercury for sealing, effectively addressing these concerns Greaseless valve alternatives include mercury cut-offs and taps made with polythene or teflon keys, although these can face degassing and vapor absorption challenges Verdin has achieved rapid operation of elastomer diaphragm taps, while Ramsperger’s modification of the Bodenstein all-glass tap features a plug of AgCl seated in a Pyrex tube, ensuring effective closure without chemical reactions between components and gases.
To H.V To$asstorage manifold (GSM.)
To MC Leod gauge etc
A high-vacuum system for kinetic studies, developed by Judge and Luckey, utilizes commercially available all-metal taps that are bakeable, operate rapidly, and offer effective sealing However, these taps come with a higher price point and the potential for chemical reactions.
A typical vacuum line for gas phase kinetic studies, as detailed by Mac-coll, features a large-diameter glass tubing manifold connected to the pumping system This setup efficiently links various components, including storage and introduction vessels, mixing and reaction vessels, and the analytical section, ensuring optimal functionality for experimental processes.
This manifold system has the advantage that each section may be evacuated in- dependently of the others.
Temperature control
There are three general methods for maintaining a RV at a particular tempera- ture: thermostats, furnaces and vapour baths29
At temperatures close to 20+20", water is a suitable liquid for a thermostat
For high-temperature applications, using fluids like silicone oil or molten metals above 2W is more effective A typical furnace design features a silica former wrapped with nichrome wire, where the pitch decreases towards the center and then increases again, helping to mitigate heat loss at the ends Additionally, an Inconel tube is utilized to maintain a consistent temperature throughout the system.
Platinum Terminal resistance block t herrnometer
Loose Inconel asbestos tube silica tube
A typical furnace features tappings at regular intervals along its winding, with wire insulated using asbestos paper and cement, encased in an asbestos tube and packed with insulating materials like kieselguhr To achieve a consistent temperature profile, shunts are connected at specific taps and adjusted to maintain a temperature variation of ±0.5°C at 200°C The furnace's total resistance should support a maximum output of ±1 kW, tailored to the desired temperature range For photochemical or radiochemical applications, the design requires modification to allow for an unobstructed radiation path, often achieved by splitting the furnace either longitudinally or laterally This design is detailed in Calvert and Pithg's work on photochemistry and can involve using aluminium half-tubes that fit together, facilitating entry tubes at the junction Such configurations are particularly advantageous for conducting flash photolysis studies.
A temperature control circuit for thermostats and furnaces utilizes a constant voltage transformer and a variac transformer to supply the appropriate current to the furnace This setup includes a small variable resistance connected in series with a relay The relay is activated by a toluene/mercury or mercury thermo-regulator based on the temperature for thermostats, while electric furnaces use platinum resistance thermometers or thermocouples integrated into a Wheatstone bridge network to control the relay Commercially available regulators ensure precise current control for optimal operation.
Fig 6 Circuit for heating and controlling the temperature of a furnace possible to control at 200" C to 5 0.2" and at 600" C to & 2" Variations of this type of control are described in ref 22c
Temperature measurement is almost invariably made using thermocouplesz2"
The latter must be constructed from fine wire30 and have a fast response3' such that even a very small temperature change may be measured precisely3'.
Thermal systems
Static method
Reaction vessels are usually spherically or cylindrically shaped, vary in size from
Thermocouple wells, typically ranging from 200 to 1000 cm³, feature a thin-walled design at the tip, often utilizing a drop of silicone oil to enhance thermal contact Pyrex glass, with a maximum temperature of 600°C, and fused silica, which can withstand up to 1200°C, are the primary materials used for these wells At room temperature, both materials are nearly impermeable to all gases except helium; however, at elevated temperatures, hydrogen can diffuse through the glasses, particularly through silica As temperatures rise further, oxygen and nitrogen may also permeate silica, likely through a mechanism involving adsorption at the glass surface followed by diffusion It is crucial to avoid heating silica reaction vessels to high temperatures in contact with metals like iron or nickel, as this can lead to diffusion into the quartz and cause heterogeneous reactions Additionally, many gases are strongly adsorbed on glass surfaces, making it essential to thoroughly degas all glass apparatus, especially reaction vessels, at elevated temperatures.
Pyrex glass are resistant to most gases at room temperature except HF However,
At high temperatures, HCl, HBr, and possibly HI are adsorbed onto glass surfaces, reacting with them through a thin layer of water that remains even at 400°C Atomic hydrogen and chlorine, as well as potentially atomic iodine, can also react with glass and silica To mitigate heterogeneous processes and reduce the adsorption and diffusion of gases like hydrogen and water, the surfaces of reaction vessels (RVs) are often coated These coatings may include films of KCl or H3BO3 from rinsing solutions, carbonaceous films formed by the pyrolysis of alkyl halides or nitrites, or polymeric coatings from olefin polymerization or polymer solutions Additionally, the "aging" of RVs is frequently achieved through preliminary experiments, which can lead to the formation of carbonaceous films.
The extent of a partly heterogeneous reaction can be assessed by varying the surface-to-volume ratio (S/V) using different vessel shapes, such as spherical or cylindrical The homogeneous rate (WL) is determined by plotting the overall rate (W) against S/V and extrapolating to zero S/V More significant changes in S/V, achieved by using thin-walled glass tubing, may effectively isolate the heterogeneous reaction, although the impact of S/V variation can be complex For example, in free radical reactions, an increase in S/V may simultaneously enhance surface initiation and termination, resulting in no net effect In contrast, for reactions like H2 + O2, a moderate increase in S/V can decrease the rate due to heightened surface termination The packing method used can also influence results; powdered glass may increase the reaction rate due to enhanced heterogeneous interactions A more reliable approach for examining surface effects is differential calorimetry, where fine-gauge thermocouples measure temperature changes at the center and wall of the reaction vessel, allowing for the calculation of heterogeneous contributions to the overall reaction rate.
In highly exothermic or endothermic reactions, thermal gradients can develop within the system, influenced by the reactor's geometry The temperature distribution over time, represented as aT/at, is crucial for understanding these dynamics It is essential to maintain a sufficiently thick film to prevent the exposure of the original surface, which may occur due to the volatilization of the salt.
++ This is discussed more fully in the analysis section
In studying slow reactions, various experimental methods can be employed to analyze the process effectively Key parameters include the coefficient of thermal conductivity (h), density (p), and the heat capacity at constant volume (cv) of the gas involved Additionally, understanding the specific rate of the reaction (Q) is crucial for accurate assessments By focusing on these elements, researchers can gain insights into the dynamics of slow chemical reactions.
H is the molar heat of reaction If concentration and density gradients are small, this equation reduces to aT/at = hV2T/pc,.+QH/pcv (J)
In a spherical vessel with radius r, it is assumed that the parameters h and c remain relatively constant over time and across different positions within the vessel, with negligible convection effects Consequently, the maximum temperature difference, denoted as ΔT_max, can be determined using the specified formula.
Averaged over the vessel, this becomes
If r is 5 cm and we consider the decomposition of di-tert.-butyl peroxide at
~ m - ~ sec-l and h = l o F 4 cal cm.-' sec-I), we find a significant temperature difference given by
The time required to establish a thermal gradient is determined by the formula t = r²pCv/n²RTh(N), where p represents pressure at temperature T, Cv is the molar heat capacity at constant volume, and R is the gas constant (6 cal mole⁻¹ deg⁻¹) When using Cv = 6 cal mole⁻¹ deg⁻¹, a thermal gradient is achieved in one second However, reducing the radius (r) to 0.5 cm results in a negligible decrease in ATav, which is two orders of magnitude lower This phenomenon was explored by Batt and Benson, who demonstrated a notable difference in Arrhenius parameters derived from spherical versus octopus-shaped reaction vessels (RVs).
Direct pressure measurements can be conducted using traditional manometers filled with various fluids such as mercury, silicone oils, hydrocarbon oils, and butyl phthalate However, these fluids may interfere with the reaction being studied through chemical reactions, vapor adsorption, or surface modifications of the reaction vessel Additionally, they can create a fluctuating "dead space," which should ideally be maintained between 1% and 4% to ensure accurate measurements.
Mechanical gauges, including spiral, Bourdon, and diaphragm types, are essential for measuring RV volume These gauges, depicted in Fig 7, function as null instruments, utilizing pointer movement or sound indicators at balance points Bourdon and spiral gauges can also measure direct pressure, often employing optical levers Additionally, Bourdon gauges find applications in capacitance and photoelectric measurements, enhancing their versatility in various measurement tasks.
For a detailed discussion see ref 22c, p 81
The spiral gauge is more robust but less sensitive compared to other gauges; its sensitivity can be enhanced by etching in a 10 H F solution without significantly compromising its durability This gauge can be mounted either horizontally or vertically, with vertical setups benefiting from a float attached to the spindle immersed in silicone oil to mitigate vibrations All three gauges are thoroughly discussed in references 22c and 56b Additionally, all-metal diaphragm gauges detect pressure changes through capacitance measurement, while pressure transducers, available in various commercial forms, utilize a Wheatstone bridge network to measure pressure differences, providing voltage output that can be recorded For applications involving corrosive gases, the resistance spirals of transducers can be safeguarded using silicone oils or diaphragms, though sealing spirals in glass or coating them with Teflon or silicone is often a more effective solution to avoid vacuum issues.
Changes in the number of moles during a reaction, along with known stoichiometry, allow for the determination of reaction order through pressure measurements Letort3 identified that the decomposition of AcH exhibits a reaction order of 3 with respect to initial concentration and 2 concerning time However, such direct conclusions are often challenging to make in oxidation reactions using pressure measurements To address this, Du gleux and Frehling developed a differential system that provides direct information This system includes two reservoirs (V1 and V2) connected to a Bourdon gauge, enabling simultaneous introduction of mixtures while compensating for temperature fluctuations in the furnace This setup facilitates the study of rapid reactions and the impact of promoters and inhibitors in oxidation processes and may be applicable to other systems as well.
Fig 8 All-metal diaphragm gauge From ref 22c
Fig 9 Apparatus for differential pressure measurement From ref 59
Pressure effects are observed on the dissociation of diatomic molecules and small polyatomic species such as CHO and HNO since the decomposition occurs in a bimolecular process In reaction (7)
The unimolecular decomposition of a species A is influenced by pressure, as established by the principle of microscopic reversibility This principle indicates that reverse processes are also pressure-dependent In particular, unimolecular decompositions, especially during free radical reactions, exhibit pressure effects in their pressure-dependent regions According to the simple Lindemann theory, the mechanism underlying this unimolecular decomposition can be detailed through specific schemes, with more comprehensive theories available for further reference.
A * represents a molecule activated by collision The rate of decomposition of A is given by t For qualifying remarks see ref 4, pp 110 and 329
-d[A]/dt = k,[A]’ = k0[AI2 at low pressures (0)
Equation (R) suggests that plotting l/kexp against 1/[A] will yield a linear graph, where the slope is represented by Ilk and the intercept by l/km This type of plot can provide valuable data, as long as the pressures in the studied system are not near the rate coefficient k.
In systems where hot molecules or radicals are generated, pressure effects can be observed These effects involve the quenching of hot species, which competes with their spontaneous decomposition or isomerization.
If the value of k can be estimated, plotting the left-hand side (LHS) of the equation against [MI] will yield a value for k This value can then be compared to the rate coefficient for spontaneous decomposition or isomerization, as derived from classical HRRKM theory or quantum mechanical theory.
Flow method
“Straight through” system In the simplest case (Fig 15) the reactant A, main-
Fig 15 “Straight-through” flow system From ref 66
24 E X P E R I M E N T A L M E T H O D S FOR S L O W R E A C T I O N S tained at a given temperature to provide a constant vapour pressure (liquid), or a constant flow obtained by means of a needle valve (gas), passes through the
RV and condensable species are captured at points E, F, and G The flow rate is regulated by capillaries located at A and D, with measurements taken by manometers at C, which monitor the entry and exit of the RV This measurement also provides insights into the average pressure within the system.
RV to be measured Non-condensable products are passed into the storage vessel
In the experimental setup, diffusion or Topler pumps facilitate the use of a carrier gas, such as in the toluene carrier technique, where A serves as the carrier gas The reactant is introduced into the carrier gas stream from either liquid (System I) or gaseous (System II) sources For liquid reactants, the bulbs W and A are weighed before and after the experiment to assess the quantity used, while the flow rate is measured via a capillary at the exit and the surrounding bath temperature In System II, a gaseous reactant is stored in bulb Z, allowing the amount consumed to be calculated from the pressure change in manometer Y System III accommodates low-volatility reactants, which are collected from a U-tube and held at varying temperatures by the carrier gas.
A flow system was utilized to establish the explosion limits at various flow rates and pressures In this setup, the gas mixture from the gas burette is directed into the reaction vessel at controlled rates and pressures, as regulated by specific taps and measured by a manometer The water levels in the gas burette are maintained consistently through automatic adjustments Once the desired flow rate and pressure are achieved, the furnace temperature is increased until an explosion is triggered.
Circulating system In some cases a circulating system may be useda4 The car- c
Fig 16 Flow method for determining explosion limits From ref 22c
Fig 17 Circulation flow system The technique was first used by E T Butler and M Polanyi,
The carrier gas is circulated through the reaction vessel (RV) using a mercury diffusion pump, as illustrated in Fig 17 The reactant is sourced from the Warhurst double trap system, maintaining N2 at a temperature 10-15°C above that of N2 to create a supersaturated mixture This setup ensures a consistent vapor pressure of the reactant in the carrier gas stream, as long as N2 is kept at a stable temperature After passing through the RV, both the reactant and products are captured in traps F, D, and D' The flow rate can be adjusted using capillaries C1, C2, or C3 and is measured with manometers.
M, and M, The latter also give a value for the pressure in the RV Other gases may be added from the dosing system T,, T,, and kept in J, or through capillary and needle valve systems such as C4 or C,, c 6and H (needle valve) Measurements of flow and the pressure of these gases in the RV are made by manometers L1, L, and L,
Very low-pressure pyrolysis (VLPP) operates at significantly reduced pressures, typically around 10^-4 to 10^-5 torr, in flow systems compared to static systems This technique, emphasized by Benson and Spokes, facilitates energy transfer primarily through gas-wall collisions VLPP serves as an innovative kinetic tool for in-depth investigations of unimolecular reactions, energy transfer processes, bimolecular gaseous reactions, and heterogeneous reactions, with the reactant sourced from a 5-liter reservoir.
CVC Ambient chevron baffle pre-amp
The VLPP apparatus, as illustrated in Fig 18, operates by allowing gases to flow at a few torr through a variable leak valve and a short length of capillary tubing to the reaction vessel (RV) The gases that effuse from the reactor are directed to the ionization chamber of a quadrupole mass spectrometer, which is used to assess the extent of the reaction.
Typical reactor vessels (RVs) face challenges such as uncertainty in volume and residence time, preheating issues, pressure drops, and partial mixing of reactants and products, especially when assuming "plug" flow Research by Gilbert has demonstrated that partial mixing occurs during hydrazine decomposition using toluene as a carrier gas These challenges can be mitigated by utilizing stirred-turbulent or capacity-flow RVs, a system developed by Bodenstein and Wohlgast While this method has been widely applied in liquid-phase reactions, its application in gas-phase reactions has only recently gained traction, with theoretical advancements reviewed by Denbigh.
Fig 19 Typical RV's-thermal systems From ref 22c
Fig 20 Typical RV's-photochemical systems R = vessel, L = light From ref 22c
Fig 21 A stirred flow reactor From ref 95 ling has been accomplished mechanicallyg6, by diffusiong4 or by turbulent 95
Turbulent flow is achieved using a cyclone reactor, designed similarly to a cyclone separator, as demonstrated by Houser and Bernstein In this setup, reactants and carrier gas are injected radially from a pyrex tube with a "pepper-pot" head into a spherical reactor The products then exit tangentially, while the gas temperatures are monitored using thermocouples The effectiveness of mixing has been validated through gas-liquid chromatography analysis.
( c ) Control and measurement of flow, and measurement of pressure
Flow rates can be regulated using capillaries of varying diameters or needle valves, as illustrated in Figs 15-17 By measuring the pressure at both ends of the capillary and applying Poiseuille's equation, flow rates can be calculated, where p1 and p2 represent the pressures at the entry and exit points, po is the measurement pressure, L is the capillary length, and R is the radius A more reliable method involves measuring the gas volume delivered by the capillary from an aspirator at a known pressure, which can also be applied to specific, reproducible positions of the needle valve.
RV serves as a crucial means for monitoring entry and exit points in pressure measurement Conventional U-manometers, utilizing mercury or oil, are effective for measuring both high and low pressures, often protected from reactants by strategically placed cold traps For low-pressure scenarios, alternatives such as McLeod gauges with low compression ratios or oil-mercury magnifying manometers can be employed The magnification factor in these devices is influenced by the angle and the diameters of the inclined and vertical tubes, specifically those measuring -7 mm and -25 mm.
8 - 15", the magnification factor with respect to a mercury manometer is 14
Fig 22 Oil-mercury magnifying manometer From ref 22c
Comparison offlow and static systems
In a static system, a finite time is needed for reactants to fill the reaction vessel (RV) and reach the desired temperature, which can complicate initial pressure determination and rate measurements The filling and heating process typically takes 5-15 seconds, necessitating a speed greater than the reaction rate, thus limiting studies like the decomposition of dtBP to temperatures of 160-170°C Additionally, connections to and from the RV and pressure-measuring devices create a "dead space" that should ideally not exceed 4% of the RV volume, requiring corrections in pressure measurements and kinetic expressions For reactions with low extents of conversion, minimal corrections are needed, but in large diameter RVs undergoing adiabatic reactions, significant errors in rate coefficient values may arise due to thermal gradients Moreover, for partly heterogeneous reactions, isolating or determining the rate of the homogeneous process can be challenging.
Flow systems with low contact times enable the use of higher temperatures, which can minimize or eliminate heterogeneous processes This approach allows for low percentage conversion while maintaining precision in measuring the extent of reactions, as runs can continue until adequate product accumulation occurs, reducing further reactions of initial products Despite these advantages, traditional flow systems have seen limited use due to inherent errors in rate coefficient determinations However, the implementation of a stirred flow reactor (RV) addresses many of these challenges, and when combined with a static system, it facilitates the study of reactions across a broad range of pressures and temperatures Nonetheless, two challenges persist: the heat capacity effect of the gases entering the RV can lower the temperature, despite heat exchange efforts.
In an RV center, the presence of thermal gradients due to the endothermic or exothermic nature of reactions can hinder instantaneous cooling after leaving the reaction zone To minimize the heat capacity effect, low pressures are recommended; however, this may result in pressure-dependent rate coefficients, raising concerns about the accuracy of bond dissociation energies measured by techniques like the toluene carrier gas method Additionally, side reactions should not be overlooked On the other hand, flow systems are advantageous for investigating pressure-dependent decompositions or isomerizations of free radicals, as their rate coefficients approach high-pressure limits under normal conditions At elevated temperatures in flow systems, the transition pressure from second to first-order kinetics for unimolecular reactions occurs at higher pressures, with the transition pressure varying as the (s-3)th power of the absolute temperature.
3.2.4 Analytical section ($ow and static systems)
This section typically includes traps, sample bulbs, a gas burette, and an automatic Topler pump, with the latter being less common today Two types of automatic Topler pumps are illustrated, one of which integrates a McLeod gauge for precise pressure measurement of small gas quantities The system employs three tungsten contacts and mercury to operate a solenoid valve, facilitating the transfer of gases to the gas burette An alternative setup uses a photoelectric device to mitigate issues related to tungsten electrode sparking, especially with explosive gases Additionally, low-temperature distillation can effectively separate products from undecomposed reactants using various slush baths made from different liquids and liquid nitrogen.
- r i g ~ 5 Auromaric iopier pumps (01 is irom rer IUY
Automatic Topler pumps operate by maintaining constant temperature baths through the careful addition of liquid nitrogen and stirring, monitored by a copper/constantan thermocouple to avoid “carry over.” It is essential to ensure that no gas is trapped in the remaining liquid or solid during trap-to-trap distillation Despite these precautions, achieving clear separations of C and C5 hydrocarbons remains challenging.
The article discusses various chemical compounds including ethyl bromide, n-propyl chloride, n-butyl chloride, methyl cyclohexane, n-propyl alcohol, allyl alcohol, isobutyl chloride, and n-pentane It also highlights the compositions of different mixtures, such as chloroform combined with ethyl bromide, trans-1,2-dichloroethylene, and trichloroethylene in varying percentages Notable mixtures include chloroform (19.7%) with ethyl bromide (44.9%), as well as other combinations featuring methylene chloride and ethyl chloride, showcasing the diverse chemical interactions and applications of these substances in industrial processes.
Roy has constructed a variable temperature still (Fig 25)'13 A, B and C are
6 mm, 12 mm and 25 mm OD respectively where C ( -35 cm long) terminates in a
The B34 joint features a B component equipped with four copper/constantan thermocouples positioned at 0, 2, 8, and 14 cm from the base The surface is tightly wrapped with lead or silver foil up to a designated point, D This layer is then overlaid with a heating coil (10-15 52) featuring a 10 mm pitch, with the lead-in wire insulated by a glass tube The thermocouple and heater wires exit through W to a cap, which is securely sealed to the tubing using araldite By surrounding the trap with coolant at point C and adjusting the air pressure between B and C, the trap's temperature can be modified, creating a vertical temperature gradient With appropriate winding, a consistent temperature can be maintained along the trap's length, typically requiring the use of two traps in tandem For additional details, refer to sources 22c, pages 156 and 242.
Analytical section (flow and static systems
Fig 25 LeRoy still From ref 113
The different fractions obtained in this way may be analysed later by various methods Liquids, sealed in sample tubes, and gases may be analysed by GLC or
Gases like halogens, hydrogen halides, or carbon monoxide can be absorbed onto various solids and quantitatively analyzed using potentiometric titration techniques Alternatively, in situ analysis or the extraction of small samples for direct gas-liquid chromatography (GLC) or gas-solid chromatography (GSC) can be performed A detailed discussion of these techniques will follow.
Photochemical systems
Introduction
Most experimental techniques for thermal systems are applicable; however, a critical exception exists for experiments utilizing Hg 2537A radiation, which necessitates a mercury-free vacuum system This means that mercury diffusion pumps or McLeod gauges should not be employed unless the focus is on mercury-sensitized reactions Additionally, the use of iodine, gold, or similar amalgamating metals does not eliminate mercury presence in the vacuum system For further insights, refer to the comprehensive work on photochemical reactions by Calvert and Pitts.
The combined Beer-Lambert law is crucial for experimentalists as it defines the absorption of monochromatic light by a homogeneous system This relationship is mathematically represented by the equation log I₀/I = εCℓ, where I₀ is the incident light intensity, I is the transmitted light intensity, ε is the molar absorptivity, C is the concentration, and ℓ is the path length Understanding this law is essential for accurately measuring light absorption in photochemical experiments.
Here I , is the intensity of incident monochromatic radiation, I is the intensity of radiation at a distance I cm, and E is the decadic molar extinction coefficient of an absorbing species (concentration, c mole I-') This law is strictly valid only if molecular interactions are unimportant at all concentrations Deviations occur for a variety of reasons; this means that the validity of the law should be checked under the particular experimental conditions An initial determination of the absorption spectrum of the compound under investigation is obligatory This produces im- mediate qualitative information, particularly about the usefulness of the source of radiation Banded, diffuse or continuous spectra give direct information about the complexity and variety of primary processes that may occur Further information will be gained from the effect of radical traps such as 0, or NO, and of various energy transfer agents
A typical optical system is shown in Fig 26 A lens of short focal length (7-10 cm) projects a nearly parallel beam of radiation from the source A through a filter
F, to remove unwanted radiation The stop S, prevents unfiltered radiation from reaching the RV It is sometimes useful to converge the beam slightly with a sec- ond lens (focal length -40 cm) such that the beam reaches its smallest diameter in the centre of the RV The latter may be divided into two compartments, one of which contains a compound used for actinometry, or alternatively the beam is focused by the lens L, onto a photocell or thermopile P The intensity of the beam is suitably reduced by the density filter F, To provide maximum possible intensity
Fig 26 Optical set-up for photochemical experiments From ref 9
To ensure accurate experimental methods for slow reactions involving radiation, it is crucial to position the radiation source as close to the reaction vessel (RV) as possible, with all windows and lenses aligned perpendicularly to the radiation path Rough alignment can be verified using a tungsten lamp, while a more precise check can be conducted with a filter soaked in fluorescent material like anthracene Comprehensive guidelines for constructing RVs and sealing windows to the cells are provided in references 9 and 22c.
To ensure safety during photochemical experiments, operators must prevent excessive radiation exposure, particularly to the eyes, especially when using laser sources The wavelength range for photochemical reactions spans from 1200 Å to 7000 Å Converting wavelength (Å) to energy (E) in kcal is essential for studying these reactions, following Einstein's principles.
The wavelength range is divided into several sections which are shown in Table 4
As far as this discussion of sources is concerned, the range is divided into two; the vacuum uv region is treated separately
( a ) Visible to fur ultra-violet
In kinetic photochemical studies, mercury lamps serve as the primary radiation sources, with three main types available The low-pressure lamp is primarily utilized for mercury-sensitized studies, while the medium-pressure lamp is the most effective for general photochemical research, offering several strong "lines" in both the ultraviolet (UV) and visible regions Although the high-pressure lamp provides intense radiation, it emits less UV light compared to the medium-pressure variant Understanding the operation of these lamps can be enhanced by examining the electronic transitions of mercury, which has a ground state configuration of (6s2)'S,.
The selection rules for the electronic transitions of atoms (Russell-Saunders coupl- ing) are
The total spin quantum number (S) is the sum of individual spins, while the total angular momentum quantum number (L) is derived from the individual angular momenta (Z1, Z2, Z3, etc.) The total angular momentum (J) is the vector sum of S and L, with n representing the primary quantum number Although the rule ΔS = 0 is generally upheld for lighter elements, it tends to break down in heavier elements, indicating a mixing of states where singlet and triplet states exhibit partial characteristics of each other This selection rule is also evident in the lifetimes of the states, as indicated by the extinction coefficient (E), which is significantly lower for singlet-triplet transitions.
Fig 27 Lower excited states of the mercury atom From ref 9
Low-pressure mercury lamps exhibit distinct spectral lines, with resonance radiations at 1849 Å and 2537 Å corresponding to transitions between excited states and the ground state The excited states include three triplet and one singlet P states, along with higher energy states such as (6s7s)¹S₀, (6s7p)³S₁, and (6s7d)³D Additionally, double excitation from the P states results in the emission of lines at 3130 Å, 3662 Å, and 4358 Å.
Low-pressure lumps These lamps operate at or close to room temperature
The vapor pressure of mercury is measured at - torr, and approximately 6 torr of an inert gas, typically neon, is introduced to facilitate easy firing This addition may also enhance the intensity of the reaction, as suggested by evidence related to reaction (13).
(iii) High-pressure mercury lamps
(iu) Compcict mercury or lnercury-.uetzon
A H 6 (quartz jacket) 840 1.4 point source lamps
0 General Electric Company, Lamp Division, Cleveland, Ohio, U.S.A b Sylvania Electric Products, Inc., Salem, Massachusetts, U.S.A c Hanovia Lamp Division, Engelhard Industries, Newark, N.J., U.S.A
Westinghouse Electric Corporation, Lamp Division, Bloomfield, N.J., U.S.A
PEK Inc Palo Alto, California, U.S.A
87 uv Energy Eficiency Approx p e r inrh of of u v gen- useful arc length eration (%) lifetime of arc ( h )
The lifetimes indicated are approximate minimums; with proper ventilation and consistent operation of medium-pressure mercury arcs, this lifespan can potentially increase by several thousand hours.
At low pressures, pure resonance lines at 1849 and 2537A are achieved using designs made from conventional or suprasil quartz with iron electrodes, which are welded to nickel and tungsten, sealed to silica with lead or molybdenum ribbon Water cooling is essential to maintain low pressure and prevent line reversal, while thermostatted circulating water allows the main light-emitting tube to be included in the furnace for the RV, ensuring output intensity remains stable up to 600°C Suprasil or sapphire windows enhance the transmission of the 1849A line The lamps can operate in three configurations: with a heated cathode at low voltage for longer life, a cold cathode at higher starting voltages, or an electrodeless discharge excited by microwave frequencies, which yields the narrowest lines Table 5 outlines the characteristics of various commercial low-atmosphere N2 options.
Fig 29 Pressure broadening of the 2537A “line” From ref 9
The spectral profile of an emission line varies with increasing self-absorption, as illustrated in Figure 30 Low intrinsic brilliance pressure lamps, including homemade versions detailed in reference 22c, are frequently coiled around the RV or positioned parallel to it, as shown in Figure 20.
Medium-pressure lamps exhibit significant effects at elevated mercury pressures, leading to phenomena such as line broadening and line reversal, as illustrated in Figures 29 and 30 While the radiation line is ineffective for mercury sensitization studies, it proves advantageous for direct photolysis, especially in vacuum systems containing mercury, where sensitization is avoided due to the high population levels.
3 P , atoms, double excitation is possible resulting in the production of a number of “lines” (see Table 6) Some of the 3P, atoms are deactivated to the metastable
3 P , state, from where they may be collisionally reactivated or emit 2654A ra- diation
A typical lamp is shown in Fig 31 The high intrinsic brilliance of these lamps and the large number of “lines” make them particularly suitable for kinetic studies
High-pressure lamps are the most intense sources of UV radiation, operating at conditions of 800°C and 100 atm, which leads to increased pressure and temperature broadening At these extreme pressures, the emission becomes nearly continuous, producing an output that is ten times greater than medium-pressure lamps and a staggering one thousand times that of low-pressure lamps per unit length.
Uniform density filters
Uniform density filters provide the means of varying the intensity of photochem- ical radiation They are made by the vacuum deposition of thin metal films on quartz plates.
Measurement o f the intensity of radiation
A calibrated thermopile is the most effective method for measuring intensities, often used in conjunction with a bolometer or radiometer and a galvanometer system Calibration can be performed at institutions like the National Physical Laboratories or the National Bureau of Standards, or through carbon filament lamps Thermopiles consist of interconnected fine-wire thermocouples, with one circuit utilizing a photoelectric amplifier for the measuring galvanometer The tungsten filament's image is projected onto a photo-cell via a lens and galvanometer mirror, where the cell is divided into two parts sharing a common electrode When the galvanometer is balanced, the filament image is symmetrically positioned, and any deflection of the mirror disrupts this symmetry, allowing the photo-cell to register the change For more detailed information, refer to sources 9 and 22c.
Fig 47 A line thermopile; (a) arrangement of thermocouple wires, (b) close-up From ref 9
Fig 48 Photoelectric amplifier for a galvanometer From ref 158
Reaction Quantum Wavelength (A) Comments yield
(CH8)&O -+ CO+2CH3 4co = 1 3200-2500 p > 50 torr; t > 125" C; CO determined by G S C ~
(C,H,),CO -+ C0+2CaH6 4 ~ 0 = 0.93 3200-2500 As above CO only gaseous product a t liquid Ns temp.*
CH,CHO + CH,+CO 4co = 0.30 3130-2380 200 torr; 30" C; alternative to
2HI + Is+HI 4~~ = 2.0 3000-1800 0.1 torr to 3.5 atm (of Ns); 4 temp.-independent up to 175" C; must be Hg-free system.d
2HBr f Br,+HI 4~~ = 1.0 2500-1800 For conversions of 1 %, 4 invariant over a range of conditions 100 torr; 25" C.'
CO$ -+ co ++O* = 1.0 1600-1200 Fast flow system must be used, otherwise back reactions complicate the issue.'
NaO + Nz+tOs 4~~ = 1.44 1849-1470 Transparent to 2573A radiation.h 2NOC1-+ 2 N 0 + CI2 $NO = 2.0 6350-3650 Useful for Hg-free systems.' cis-2-butene 2 trans-2-butene q5t-c = 0.5 Hg (SP,)-sensitised reactions
4 independent of pressure above 30 torr.'
D S HERR AND W A NOYES, JR., J Am Chem SOC., 62 (1940) 2052
K 0 KUTSCHKE, M H J WIJNEN AND E W R STEACIE, J Am Chem Soc., 74 (1952) 714
F E BLACET AND R K BRINTON, J Am Chem SOC., 72 (1950) 4715
G K ROLLEFSON AND M BURTON, Photochemistry, Prentice-Hall, New York, 1946, p 190
G S FORBES, J E CLINE AND B C BRADSHAW, J Am Chem SOC., 60 (1938) 1431
G A CASTELLION AND W A NOYES, JR., J Am Chem SOC., 79 (1957) 290
' R B CUNDALL, in Progress in Reaction Kinetics, Vol 2, G PORTER (Ed.), Pergamon, Oxford,
* W E VAUGHAN AND W A NOYES, JR., J Am Chem Soc., 52 (1930) 559
Chemical actinometers are generally easier to use and more reliable than thermopiles, primarily due to the careful calibration required for thermopiles However, chemical actinometers must first be calibrated for their specific wavelength range and corresponding quantum yield (Cp) A variety of gaseous actinometers, along with their Cp values and wavelength details, are provided in Table 9 For optimal results, it is best to use a chemical actinometer in conjunction with a system that is fully absorbing Most of the actinometers listed are highly reliable in the vacuum-near UV range, but there is a notable scarcity of actinometers for the visible spectrum, with only one reaction identified In the realm of liquid phase actinometers, the traditional uranyl oxalate actinometer is now being superseded by the significantly more sensitive ferrioxalate actinometer, although some researchers argue that the uranyl oxalate system remains the most sensitive option when estimating CO production using GSC (FID).
Photocells, similar to chemical actinometers, require regular calibration against a thermopile-galvanometer system due to potential variations over time When properly calibrated, they can accurately measure the absolute intensity of monochromatic light The photoemissive type of cell is particularly effective for photochemical studies, utilizing the photoelectric emission of electrons from an irradiated surface These cells feature a metallic cathode housed in either a vacuum or a low-pressure environment filled with inert gases They can be designed as a single phototube or a multielement photomultiplier, with the latter achieving an impressive amplification of approximately 10^6.
Glass bulbs are effective for the visible spectrum but can be enhanced for UV use by adding quartz windows, although sensitivity decreases at shorter wavelengths A superior method involves applying a fluorescent material coating on the phototube's face, which broadens the useful range to 850A161.
The basic photocell circuit, illustrated in Fig 49, utilizes salicylate, which provides optimal film quality due to its high sensitivity to extreme UV light while minimizing interference from longer wavelengths This circuit can effectively measure light intensities of 0.01 foot-candle or higher, particularly when paired with a sensitive galvanometer For applications involving lower light intensities, refer to references 9 and 22c for amplification procedures.
The procedures employed for the determination of the fraction of light absorbed and of quantum yields with these measuring systems are clearly described by Cal- vert and Pitts’.