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Recombination of Iodine via
2139 I A chemical k inet ics exper iment in
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.
Recombination: I I M
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-
(2) The rat e of recombination depends on the nature of the third
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
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
W. Norrish and
Porter who were awarded
the 1967 Nobel Prize in Chemistry for their work in
fast reaction kinetics. The apparatus, however, has
been modified for
facilitating the use of standard,
commercially available components,
repeated use by non-experts, and 3) compactness.
The experiment is designed for use in a junior physical
Three or four periods of
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
atom and the
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
in quantitative terms various theories for the recom-
Some other papers in the field
ound out the
journal material suggested for the student.
The apparatus is illustrated in Figure
l , Monitor light; 2 Rash tuber;
tion vessel; 4 interference filter; 5 photomultiplier;
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
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
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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.
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
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-
M u t .
OFF - NO I2
ON - WITH I?
Relotions rhowing the reope deflections for xo, x-, x s
x = x L .
Mon = monitor light Ri n
= re wl ion vessel. Fiorh
tions of third-body gas, then it follows from eqn. (2)
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 ,]
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-
Equation (5) is simplified to
( I ) ,
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
film. M Neon
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
and the integrated expression is
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
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
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
lournal of Chemicol Education
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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.
Ibottom, lef tl Recombination of iodine in helium as o third body. Reciprocal iodine ofom pressure
Figure 6. (top, right) Longitudinal section of the reaction vesrel. 1,
inlet lball~oint); 2 capillary tube; 3 end clamp ; 4 trigger wire; 5 ratur-
ated CuS04 solution for infrored Rltering;
Lucite protective shield (flash tube moy explodel);
Lucite mounting board for Rash tube;
xenon Rash tube.
[bottom; Aghtl Details of Rash tube mounting.
A and G No. 1 0 copper wire; 8, end clamp ; C solder;
screws; E and I Lusitem mounting
connection to capacitor bonk;
J magnesium oxide reRestor; K connection to trigger box.
the thermal gradient effect which is explained in
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
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
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Figure 8. Alignment of he optic01 A Photomultiplier housing;
interference fllter; C ond E collimation irises; D opticolqa th covering;
F, infrared fllter;
biconvex lens; H quark-iodine iomp; I refiector
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-
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
Gas Handling pparatus
The system is shown in Figure 9.
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
Electr icd sygtem diagram. A, Four
fiogh tuber; B six
50- ~LF opacibrs;
quortz iodine lamp;
photomultiplier tube; E
oscilloscope with external trigger rynchron irdon; F, 1 00 kohm low noire
resistor; GI -pF blocking capocitor; Gs 6000-pF
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
V dc ;
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
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
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-
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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.
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
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
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
2) The shutter on the scope camera is opened.
3) The trigger button is ~ re ss e d wice, first to flash the lamps
mom ent later to obtain a trace of th e baseline,
4) The shutter on the camera is closed and the photograph is
analyzed. The resulting recombination curve should be
similar to that
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
Some of the better results obtained by students are
indicated in Figure
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
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
The estimated un-
certainty limits due to random errors on
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.
The authors wish to thank Professors Kerry
Bowers, Jeffrey I. Steinfeld, James W. Dubrin, and
Hercules for their helpful advice and sug-
gestions as well as to the students of the physical
chemistry lab for their patience and cooperation.
1) CHRISTIE, . I. NORRISH, . G . W., A N D PORTER,G.,
Proc. Roy. Soc. London), A 216, 152 1952).
2) CHRISTIE, .
231, 446 195.5).
3) MARSHALL,., . A N D I).\VIDSON, ., J. Chem. Phys. 2 1 659
4 ) B U N K E R , . L., .\No I \ V I D ~ O N , N . ,
Am . Chem. Sor. 80
B U N K E R ,
. L. AND
Am . Chem. Sor . 80
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( 6 ) E NGL E MAN,
AND DAVIDSON,. R., J. Amer. Chem. Soc.,
82, 4770 (1960).
(7) PORTER, . , A N D SMITH,
A., Proc. Roy Soc. (London),
A261, (1961) .
(8) PORTER, . , SZARO, G. , AND T OWNSE ND,
Roy oc. ,
A 270,49 3 (1962) .
(91 RUSSEL.K. E. . .4ND BMONS.
o d SOL.A 217.
~ ~ ~ .
C. W., GRA F, . E., AND WILLARD,
Chem. Phys. , 2 6 1287 (1957).
(11) PORTER, . Techniques of Organic Chemistry, Val. 11,
Part 11 Interscience Publishers (division of John
Sons, Inc.) , London and New York, 1963
A N D
B AIL E Y,
N . , J. CHEM.
42, A 83 (1965).
. A , ,
AND PORTER, .,
OC., 262 ,476
. J. AND
H . D. J . Am
58, 2260 (1936).
GOY,C. A, , AND PAITCHARD,.
12, 38 (1964).
71 Journal of hemical Fdumfion