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    Bill

    S

    Yamanashi

    Recombination of Iodine via

    and

    Andrew

    V.

    Nowak

    Massachusetts Institute

    Flash Photolysis

    of Technology

    Cambridge,

    2139 I A chemical k inet ics exper iment in

    I

    physical chemistry

    The well-known experiment of iodine

    atom recombination in the gas phase is chosen for

    college students to acquire some understanding of the

    kinetics of chemical recombination and the techniques

    for investigating fast chemical reactions.

    The recombination proceeds via a pair of collisions

    involving a third body M.

    Phatolysis:

    1 2

    hv

    -

    1

    Recombination: I I M

    -

    n M

    It

    is hoped that the student will learn from this experi-

    ment the following points:

    (1) In order for photodissocit~ted toms with internal and ther-

    mal energy to recombine, a third body M, the chaperone, is

    necessary to remove the excess internal energy of the mole-

    cule.

    (2) The rat e of recombination depends on the nature of the third

    body.

    3) During the recombination the atom and the third body fo rm s

    transitory complex. The complex reacts with another atom

    to finally yield a stable recombined molecule and an activated

    third body.

    (4)

    The rate expression associated with the recombination is

    second order with respect to iodine atom wncentration and

    first order with respect to third body concentrntion.

    (5) The iodine molecule itself acts s a very efficient chaperone.

    The experimental techniques employed follow those

    of

    R. G.

    W. Norrish and

    G.

    Porter who were awarded

    the 1967 Nobel Prize in Chemistry for their work in

    fast reaction kinetics. The apparatus, however, has

    been modified for

    1)

    facilitating the use of standard,

    commercially available components,

    2)

    ruggedness for

    repeated use by non-experts, and 3) compactness.

    Background

    The experiment is designed for use in a junior physical

    chemistry laboratory.

    Three or four periods of

    4

    hr

    duration are needed for completing the experiment.

    The first period is used for an introductory lecture on

    flash photolysis, the theory of termolecular kinetics,

    and use of the equipment. The last two or three

    periods are used to obtain recomhination data. Al-

    ternatively, the apparatus can he built and operated by

    students as part of a term project or a senior thesis.

    During the first week a bound set of photocopies of

    pertinent literature articles arranged chronologically

    and the operational instmctions are given to each

    student. From these articles th e students are ex-

    pected to become aware of various theoretical models

    from which one can determine the rate constant.

    Included in the hackground articles is a series

    originating from Porter's laboratories. The first of

    these (1) is the most convenient place for the student t o

    begin his study of gas phase iodine recombination.

    The authors summarize the various theories of atomic

    recombination in the presence of a third hody and also

    present data on the recomhination of iodine in the pres-

    ence of all five noble gases. In their second paper 2)

    they show that a simple termolecular rate law is not

    obeyed due to the effects of iodine as a third body.

    The student is also urged to read a series of papers by

    Davidson. In the first of these 3) the authors pro-

    pose the theory that the recombination most probably

    involves a sticky collision between an

    I

    atom and the

    third body

    LI

    to form a complex IM which reacts with

    a second I atom. By this time the student is aware

    that despite any earlier thoughts on the subject, the

    recomhination does ot proceed via the association of

    two iodine atoms followed by third hody deactivation.

    In the next three papers 4-6) Davidson, t

    al.,

    discuss

    in quantitative terms various theories for the recom-

    bination.

    Some other papers in the field

    7-13)

    ound out the

    journal material suggested for the student.

    Experimental

    The apparatus is illustrated in Figure

    1.

    The mea-

    Figure

    l .

    Monitoring scheme

    l , Monitor light; 2 Rash tuber;

    3

    reoc-

    tion vessel; 4 interference filter; 5 photomultiplier;

    6

    orcillorcope

    o

    and ore the monitor light intensities

    sured quantity associated with the concentration of the

    recombining species at a given time is the photographed

    deflection of the oscilloscope trace. An external

    triggering unit causes the scope to scan once in syn-

    chronization with the beginning of the photolysis

    flash.

    Iodine molecules have a characteristic absorption in

    the wavelength region centering about 500 mp (green)

    where iodine atoms are transparent. Assuming Beer's

    law is obeyed, the concentration of iodine molecules can

    be measured by monitoring th e intensity of a spectrally

    narrow beam of light centered about 500 mp passing

    through the reaction vessel

    Volume 45 Number

    1 1

    November

    1968

    705

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    where

    lo @ ) intensity of incident light

    It(A) = intensity of transmitted light rut time

    r(A) = molar extinction coefficient

    length of reaction vessel

    4)< concentration of iodine molecules s t time

    = wavelength of incident light

    The beam intensity is monitored by a photomultiplier

    and the output fed into an oscilloscope cj., Figs.

    and

    8).

    The oscilloscope deflection x is in direct

    proportion to the monitor light intensity. Conse-

    quently, the deflection on the scope a t a given time

    after the flash, x,, is related to the iodine molecule

    concentration a t time t, (I2) , rom Beer s law as follows

    In (Tollr)= In

    zdzt) = 4 d , 1

    (2 )

    where explicit dependence on X has been omitted.

    The above relations are illustrated in Figure 2. If r id

    assumed to be a constant independent of the concentra-

    cK

    M u t .

    OFF - NO I2

    Xr.

    xo

    ON - WITH I?

    Figure

    2.

    Relotions rhowing the reope deflections for xo, x-, x s

    and

    x = x L .

    Mon = monitor light Ri n

    VL

    = re wl ion vessel. Fiorh

    Rred

    mt 0

    tions of third-body gas, then it follows from eqn. (2)

    that

    where

    I2)-

    is the concentration of iodine molecules

    before the flash.

    The atomic concentration at time t is related stoichio-

    metrically to the molecular concentration

    (11, = 2l(1 (Id ,]

    4)

    Substituting eqns.

    (3)

    and

    (4)

    into eqn. (2) and simpli-

    fying, one obtains

    To obtain absolute rate constants, (I2)- can be com-

    puted from an empirical relation in reference 14).

    In general x,/x, is only slightly different from unity.

    Rewriting x, = x,+ Ax (Fig. 2) and using the approxi-

    mation

    Equation (5) is simplified to

    ( I ) ,

    rAz

    where

    is a constant depending on the apparatus settings and

    the iodine vapor pressure.

    Photographs are taken with the time base of the

    scope set at 2 msec per cm on a 10-cm photograph

    (Fig. 3 .

    Figure 3. Typicol orcilloscope recombination curves. Photographed

    using PoloroidmType

    47

    film. M Neon

    I116 mml.

    2 mrecfhoriz.

    div.

    0.05

    V/vert. div.

    A plot of l/(I), versus t is made from the simple

    assumption that if t he sequence I + I

    +

    M = I2

    +

    M* describes the rate determining step, then the rate

    law is

    knr(I)yM)

    d t

    (7 )

    and the integrated expression is

    where

    k~

    is the rate constant for a third body 14.

    From the literature, the student will realize that the

    iodine molecule itself acts as an extremely efficient

    vhnpcrmc. Thns fnr t h e rrr~~niliinntion i t h one

    ~ d d t ~ dhird hocly, two ci~mpt,tit~gwrtion., cxidr

    where indicates an activated molecule. Hence the

    rate expression becomes

    where

    This means that k is function of (I2) and varies as

    the reaction proceeds.

    From the comparison of eqn. (7) with the linear por-

    tion of the plot the student also notices tha t the change

    in the slope as

    h I

    is changed at a given pressure (Fig.

    4), or as the pressure of a given h I is changed (Fig.

    5 , is consistent with eqn. (7).

    In the absence of a third body the student notices

    that the plot is curved but tha t the addition of 20 mm

    of He results in a straight line. The results are due to

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    Figvre 4. (top, leftl Recombinationof iodine in various third bodies at about 8 0 mm pressure. Reciprocal iodine atom pressure versus time.

    Figure 5.

    Ibottom, lef tl Recombination of iodine in helium as o third body. Reciprocal iodine ofom pressure

    verrvr

    time.

    Figure 6. (top, right) Longitudinal section of the reaction vesrel. 1,

    Gar

    inlet lball~oint); 2 capillary tube; 3 end clamp ; 4 trigger wire; 5 ratur-

    ated CuS04 solution for infrored Rltering;

    6

    Lucite protective shield (flash tube moy explodel);

    7

    Lucite mounting board for Rash tube;

    8, magnesium

    oxide reflector

    on

    aluminum foil;

    9

    xenon Rash tube.

    Figure

    7.

    [bottom; Aghtl Details of Rash tube mounting.

    A and G No. 1 0 copper wire; 8, end clamp ; C solder;

    D

    screws; E and I Lusitem mounting

    board;

    F

    Rash tube;

    H

    connection to capacitor bonk;

    J magnesium oxide reRestor; K connection to trigger box.

    the thermal gradient effect which is explained in

    reference 8).

    Description

    of

    the Apparatus

    Reaction Cell Unit

    The reaction cell, constructed of Pyrexm glass, con-

    sists of a cylinder 20 cm in length and 4 cm in diameter

    with fused-on optically flat end-pieces. It is provided

    with an outer jacket for filling with a saturated CuSOl

    solution to minimize the heating effects of the flash.

    As indicated in Figures 6 and 7 the inner unit (the

    reaction cell, the flash tubes, and the two mounting

    discs) slides into the Lucite protective shield. Mag-

    nesium oxide coated onto aluminum foil serves as a

    reflector for the flash. The flash tubes are replaceable

    by sliding out the inner unit and releasing the end

    clamps. The left of Figure shows how the end

    clamps 3 in Fig. 6), are made out of a Smith shaft ex-

    pander and fitted to the electrode terminals of the

    The authors wil l gladly supply mor detai led information

    than

    appears here.

    flash tubes. Care must be taken tha t neither trigger

    wire will arc through the aluminum foil.

    Optical Path and Monitoring Devices

    Figure 8 shows the alignment of the optical path.

    The interference filter (B in Fig. 8) is peaked at 499.5

    mp, near the center of the iodine absorption curve,

    with a full width a t half height of 7 5 mp. The width

    is sufficiently narrow so that the extinction coefficient

    r in eqn. (1) is constant over this small range of X

    An infrared filter with good transmission in the 500 mp

    region reduces the thermal effects of the monitor lamp

    on the reaction vessel. A quartz-iodine high intensity

    lamp powered by a ripple free dc source a t about 20

    V serves as the monitoring light. All components

    are mounted on an optical rail for stability and ease of

    manipulation. The apparatus is designed so that

    third body gas can be introduced without moving the

    reaction cell. Earlier investigations showed extreme

    sensitivity of oscilloscope response to the position and

    orientation of the vessel.

    Certain precautions must he taken in mounting the

    components. The photolysis flash is so much more

    Volume

    45

    Number

    1 1

    November

    1968

    7 7

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    Figure 8. Alignment of he optic01 A Photomultiplier housing;

    B

    interference fllter; C ond E collimation irises; D opticolqa th covering;

    F, infrared fllter;

    G

    biconvex lens; H quark-iodine iomp; I refiector

    mirror;

    J

    optical rail.

    vacuum stopcocks are used in regions continuously ex-

    posed to iodine vapor. Silicone high vacuum grease

    can be used elsewhere.

    Iodine crystals are held in a side arm submerged in

    intense than the monitoring light th at the photomulti-

    plier must be protected from the glare of the flash.

    Otherwise the flash can generate an out ru t voltage

    large enough to drive the CRT s electron beam onto

    one of it s electrostatic deflection plates. The result-

    ing static charge on the plate erroneously deflects t.he

    electron beam for the following 50 msec. Since the re-

    combination is essentially complete in 20 msec, all of

    t,he kinetic information is lost.. The flash may also

    saturate the photomultiplier and overload the oscillo-

    scope amplifier requiring a short but significant re-

    covery time.

    For these reasons, the irises on the left of the cell are

    open to the order of only a few millimeters and the

    interference filter is placed between the cell and the

    photomultiplier.

    Gas Handling pparatus

    The system is shown in Figure 9.

    A mechanical

    forepump is sufficient to provide a background air

    pressure of about 15p Hg. Due to the tendency of

    iodine to dissolve in stopcock grease, Teflone high

    Figure 9. Principal portr of gar-handling system. 1, Vacuum manifold;

    2 thermocouple gauge; 3 manometer for third-body pressure; 4 to

    forepump; 5 high vacuum Tefiona stopcocks; 6 third-body reservoir;

    7 lecture bottle of third body gar with a regulator; 8 thermometer; 9

    iodine reservoir rubmerged in water in a Dewar; 10, reaction vessel unit;

    11, thermocouple gau ge indicator; 12, t rop cooled with liquid nitrogen in

    o

    Dewar

    Figure 10.

    Electr icd sygtem diagram. A, Four

    xenon

    fiogh tuber; B six

    50- ~LF opacibrs;

    C

    quortz iodine lamp;

    D

    photomultiplier tube; E

    oscilloscope with external trigger rynchron irdon; F, 1 00 kohm low noire

    resistor; GI -pF blocking capocitor; Gs 6000-pF

    filtering

    capocitor; H

    dc 200-1 20 0 V photomultiplier power supply; I dc 0-5.000 V power

    supply for charging the copmitor bank;

    J trigger box; K ripple-free

    battery eliminator 10-1

    5

    V dc ;

    1

    constant voltage tron9former; M 0-10

    amp ammeter; X oc line input of the otciiioscope.

    water. The iodine vapor pressure is varied by simply

    changing the water temperature.

    The system consists of three basic units (Fig. 10).

    1) Flashlamp Circnit. An Amglo Model AC-5000

    power supply, (I), can he used to charge the capacitor

    hank to the desired voltage. Alternatively, a power

    supply can be easily constructed. 1500-3000-ohm

    limiting resistor (between I and

    B)

    is required. This

    minimizes the chances of overloading and damage from

    a back discharge through the diodes. The flash tubes

    are xenon-filled and can withstand a maximum of 2000

    V without breaking down. They are available com-

    merically from the Xenon Corp. bank of six 50-pF

    capacitors, (B), (2000

    V

    operating voltage) provide the

    energy for the flash. They should be banked with

    copper strips to promote an efficient discharge. The

    maximum voltage discharging through the tubes is

    limited by the ability of the tubes to safely dissipate

    the capacitor s stored energy (E = /LV2) . Too great

    a voltage causes the tubes to explode.

    (2) Photomultiplier and Signal Detection Circuit.

    Eithera931-A or a 1P-28(RCA or S~ lv an ia )s a suitable

    photomultiplier (D). An input of 340-520 V from the

    photomultiplier power supply (Fluke Model 409A) gave

    the best signal to noise ratio for the scope deflections.

    The trigger box, J) ,generates a spark in each flash

    tube, causing the capacitor bank to discharge through

    the tubes, while simultaneously synchronizing the scope

    trace with the firing of the flash. Any scope with an

    external trigger input, sensitivity down to 0.01 V per

    cm and provision for mounting a camera (preferably a

    Polaroidm) an be used.

    From Figure it is clear that operationally the

    recombination curve is a small ac signal x,) uperim-

    posed on a relatively large dc signal X,) . To prevent

    the dc signal from overloading the oscilloscope am-

    plifier and rendering the scope insensitive to the re-

    7 8 Journal of Chemical Educofion

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    combination curve (the ac signal) a ~ - , L Fapacitor is

    placed in series with the photomultiplier output as

    shown in Figure 10 to block the dc component but per-

    mit the ac signal to pass through. To obtain the

    values of soand x, as required from eqns. (5) or 6)

    the capacitor must be temporarily shunted.

    (3) Monitor Light Circuit. The source of moni-

    toring light, (C) in Figure 10, is a high intensity quartz-

    iodine lamp powered by two ripple-free battery elim-

    inators in series (Heathkit Model 12A). For best

    stability a constant voltage transformer (L) supplies

    power to the oscilloscope, the photomultiplier power

    supply, and the monitor lamp power supply.

    The following points should be observed in the con-

    struction of the system.

    (1) PRECAUTIONS in Flash Circuitry. The part

    of Figure 10 indicated with a bold line involves the

    charging and discharging of the capacitor bank with

    about 1500 V and sufficient energy to cause severe

    injury or death if touched. Th e use of well-insulated

    wires, proper grounding, and complete coverage of the

    capacitor bank with '/,-in. Plexiglasmare an absolute

    necessity. Especially where high voltage wires are

    connected to the "end clamps" (see G and H of Figure

    7), all conducting parts must be covered by insulation.

    High current capacity wires (such as No. 6 stranded)

    are recommended to carry the initial current of 2000

    amps through the flash tubes. The two wires coming

    out of the trigger box must be sufficiently separated

    and made short to prevent pre-arcing. These wires

    carry about 25 to 30 kV but almost no current.

    2)

    Connections between Components. For the con-

    nections in Figure 10 between H and D D and E, E

    and G D and G with F , Belden type 8885 shielded wirem

    is recommended. High voltage MHV connectorse

    are used a t the input and output of

    D.

    Procedure

    The vapor pressure of iodine at room temperature, 200

    to 300p, is sufficient for the experiment. To obtain a

    set of runs for a given third hody, first the position of

    the oscilloscope trace in the absence of iodine with the

    monitor light off (x = 0) and with the light on (x =

    xo) are recorded (Fig. 2). With the monitor light on,

    iodine vapor is then admitted into the reaction cell

    (with stopcock 6 A of Fig. 9 closed) by diffusion until

    the scope trace ceases to change (about 20 min) a t

    which point equilibrium is attained and the position of

    the trace is recorded (x = x-).

    The 2 pF blocking

    capacitor is now placed in the circuit (Fig 10). The

    triggering level on the scope is then carefully adjusted

    until the trace just disappears. If done properly,

    triggering the system (keeping the capacitor bank un-

    charged ) should cause the trace on the scope to scan

    once.

    A known pressure of third hody gas, as measured by a

    mercury U-tube manometer, is admitted into the cell

    from the reservoir (Fig. 10, No. 6) containing about

    atm of gas.

    To obtain a set of data, the following steps are

    followed in sequence:

    1) The capacitor bank is slowly charged to about 1500

    V

    2) The shutter on the scope camera is opened.

    3) The trigger button is ~ re ss e d wice, first to flash the lamps

    and

    s.

    mom ent later to obtain a trace of th e baseline,

    z,).

    4) The shutter on the camera is closed and the photograph is

    analyzed. The resulting recombination curve should be

    similar to that

    of

    Figure 3. Success in obtaining a corve such

    as in Figure 3 depends opon a systematic approach to

    optimizing all of the apparatus settings. Runs at higher

    third body pressures are oht,ained by merely add ing more gas

    to the reaction vessel. The third body gas is removed by

    pumping on the vessel while the iodine is frozen out with

    a

    dry-ice-ethanol mixture.

    Results

    Some of the better results obtained by students are

    indicated in Figure

    4.

    Recombination curves for

    various third bodies are compared directly in Figure

    5 for third body pressures in the vicinity of 80 mm.

    Clearly for these gases third body efficiency increases

    with mass and with complexity of the third body. Al-

    though expected to, the curves for a given set of runs do

    not converge to the same point a t t = 0.

    The constants obtained by one group of students are

    surprisingly close to the literature values. By varying

    the iodine pressure they were also able to obtain a value

    of the rate constant for iodine as the third body (see

    table). The rate constant for each gas is the arithmetic

    Rate Constants knr, for Various Third Bodies

    k~

    x

    1 0 ~ a ~ m l ~ m o l e c u l e ~ ~ s e c ~ lknr argon)-----

    Bunker Porter Bunker Porter

    and and and and

    This Davidson Smith This Davidson Smith

    Gas work 6) 3) work 6) 3)

    mean of a series of 6-8 kMJsobtained from the slopes

    of curves like those of Figure

    4.

    The estimated un-

    certainty limits due to random errors on

    ~

    for the

    inert gases has been estimated to he +20%, and for

    iodine to be +loo . No limits have been set on un-

    certainties resulting from systematic errors.

    The experiment is considered a success even when it

    is only possible to obtain a rough comparison of the

    effects on the rate constant of (1) variations in the

    pressure for a given third body and (2) variations of the

    third body at a given pressure.

    Acknowledgment

    The authors wish to thank Professors Kerry

    W.

    Bowers, Jeffrey I. Steinfeld, James W. Dubrin, and

    David

    M.

    Hercules for their helpful advice and sug-

    gestions as well as to the students of the physical

    chemistry lab for their patience and cooperation.

    Literature Cited

    1) CHRISTIE, . I. NORRISH, . G . W., A N D PORTER,G.,

    Proc. Roy. Soc. London), A 216, 152 1952).

    2) CHRISTIE, .

    I.

    HARRISON,

    . J.,

    NORRISK, .

    G .

    W.,

    AND

    PORTER,G. Proc.

    Roy. Soc.

    London),

    A

    231, 446 195.5).

    3) MARSHALL,., . A N D I).\VIDSON, ., J. Chem. Phys. 2 1 659

    1953).

    4 ) B U N K E R , . L., .\No I \ V I D ~ O N , N . ,

    J

    Am . Chem. Sor. 80

    5085 1958).

    5 )

    B U N K E R ,

    . L. AND

    I).\VIDSON, .,

    J

    Am . Chem. Sor . 80

    5090 1958).

    Volume

    45

    Number 1

    1

    November

    1968

    709

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    ( 6 ) E NGL E MAN,

    .,

    AND DAVIDSON,. R., J. Amer. Chem. Soc.,

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    (7) PORTER, . , A N D SMITH,

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    A., Proc. Roy Soc. (London),

    A261, (1961) .

    (8) PORTER, . , SZARO, G. , AND T OWNSE ND,

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    G., Proc.

    Roy oc. ,

    A 270,49 3 (1962) .

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    ..

    PWC.

    f

    o d SOL.A 217.

    ~ ~ ~ .

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    L., CHIEN,

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    C. W., GRA F, . E., AND WILLARD,

    J.E.,

    J .

    Chem. Phys. , 2 6 1287 (1957).

    (11) PORTER, . Techniques of Organic Chemistry, Val. 11,

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    Wiley

    Sons, Inc.) , London and New York, 1963

    p.

    1055.

    HERCULES,. M.,

    A N D

    B AIL E Y,

    .

    N . , J. CHEM.

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    GOVER,

    . A , ,

    AND PORTER, .,

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    oy

    OC., 262 ,476

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    GILLEPSIE,

    . J. AND

    FRASER,

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    H . D. J . Am

    Chem. Soc.,

    58, 2260 (1936).

    GOY,C. A, , AND PAITCHARD,.

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    71 Journal of hemical Fdumfion