M. 1. Haggett, Peter Jones, and K. 8. Oldham
King's College,' University of Durham Newcastle upon Tyne, England
Flowing Clock Reactions
Textbooks of undergraduate experimental physical chemistry contain no experiments on the study of fast reactions, although it is 40 years since Hartridge and Roughton dramatically extended the accessible range of reaction velocities by the introduction of flow method^.^ I t seems probable that this omission originates in the cost and complexity of the fluid drives and measuring devices used in the conventional application of flow techniques. Our purpose is to point out that quantitative Emetic studies may be made with great simplicity on the time scale 0.01-1 see by appiying the continuous flow principle to the study of clock reactions. The fluid drive is an ordinary water jet suction pump, the measuring device is a ruler. We describe below the apparatus and its application to the study of the "iodine clock" (hydrogen peroxide iodide) and "formaldehyde clock" (formaldehyde bisulfite) reactions. Apparatus. A diagram of the apparatus is shown in Figure 1. The apparatus consists of two reactant solution reservoirs (2.50 ml measuring cylinders), each with a siphon tube connected t o the T-tap (1 mm bore) mixing chamber ( A ) . The flow tube (2 mm bore) of about 1 meter length is connected between the mixing chamber and an auxiliary tap (B), by means of which suction can be applied from a water pump. Preliminaries. Fill both cylinders with water, fully open both taps A and B, and turn on the water pump. Experiment with different settings of the water tap (C) determining the flow rate a t each setting.
The maximum flow rate with an apparatus of these dimensions is about 10 ml sec-' (5 nd sec-' from each reservoir), but any flow rate down to 3 in1 sec-I may be used. Flow a t rates of less than this minimum may cease to be t u r b ~ l e n tas , ~required for a sharp boundary. The flow rate is measured in the following way: open tap B starting a stop-clock a t the instant of opening; when a t least 50 ml has been removed from each cylinder, close B, noting the time of closure. Re-read the level in each cylinder immediately and compute the
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On August 1, 1063, King's College will become the Universit,~ Newcastle upon Tyne. Rouo~rToN,F. J . W., in "Technique of Organic Chemistry," FREISS, S. I,., AND W E ~ S B E R G E R , A,, ed8., VOI. 8, Interscience Publishers, Inc., New York, 1953, pp. 660-90. of
Figure.
1.
Diagrom of the apparatus,
separate and total flow rates. If the voluu~esleaving the two cylinders differ from each other by more than a few per cent, adjust tap A to remove this difference. After the correct setting of A has been achieved do not, disturb it. Method. For both the experiments detailed below each mixture should be prepared immediately before the run. It is convenient to make up the mixtures by measuring directly into the reservoir cylinders from stock solutions. Make sure the mixed solutions are well stirred.
Volume 40, Number 7, July 1963 / 367
Turn on C and open B. Adjust C until the endpoint (color boundary) is a t a convenient position in the flow tube. Now carry out a flow rate Cf) determination as before; but during the waiting period, measure the distance (x) between the mixing chamber and the color boundary. A white card held behind the flow tube is useful in detecting the boundary. Alter the flow rate and repeat the determinations off and z. Calculate the reaction time (7) for each set of measurements: r =
2.A
-
5
(A is the cross sectional area of the flow tube)
Two Examples
Formaldehyde Cloclc Reaction. The theory of this reaction has recently been di~cussed.~The most convenient experiment to carry out is the dilution experiment due to Barrett.' In this experiment the ratios [HCHO]: [HS03-] : [S082-] are kept constant but their total concentrations are varied. Under these conditions the reaction time (T) is related to the dilution (l/C) of any component by the equation3
, 1
=
Akr
where k is the reaction velocity constant for the addition of formaldehyde and bisulfite and A is a constant depending on the indicator used and its c~ncentration.~ Suitable recipes are as follows: buffer: 0.3 M SO3>------Reservoir
. Experimmt
Buffer solution (ml)
1 2 3 4
120 60 30 15
-I
-Reservoir
1M Form-
Students tend to divide reactions into "instantaneous," "measurable," and "slow"; measurable reaction rates are considered to be those in which concentration changes are slow enough to be followed by conventional analytical techniques, but fast enough that significant concentration changes occur within a laboratory session. Their time scale is thus limited to about 10t1O4seconds. We have recently designed kinetic experiments which may remove these arbitrary distinctions by allowing students to carry out kinetic experiments where the time scale involved in the measurements varies from to lO%econds. The fast reactions are particularly important since these studies reflect the interest of much current research and provide students with the stimulus of working on time scales much smaller than are normally within their experience.
II--
Phenol Distilled aldehyde Distilled phthalein water solution water (ml) (mli (mli (ml) 2 128 120 130 2 188 60 190 30 220 2 218 2 233 15 235
Iodine Cloclc Reaction. The reaction is carried out under conditions such that the reactant coilcentrations are maintained effectively constant, i.e., dt
Conclusions
+ 0.05 M HS08-
A set of results obtained with this reaction is shown in Figure 2-note the time scale.
- -d[H2021 =-
the iodine produced with thiosulfate, [Hf1 and [H2021 by virtue of their large excess. A suitable experiment is to determine the order with respect to each of the reactants, by changing their concentrations in turn (at the concentrations used the contribution of the acid-independent path is negligible), and hence obtain the (third-order) rate constant. Suitable recipes for 18-20°C are a t the bottom of this page.
df121 = k[H.Oz]a [I-]b [Ht]. = constant. dl
The extent of reaction which is measured is controlled by addition of thiosulfate to the reaction mixture and the detection method is starch indicator. The end point corresponds to the condition: Molarity of iodine produced by reaction = (Initial malarity of thiosulfate in reaction solution)
[I-] is kept constant by the very rapid reaction of
Figure 2.
Plot of rervltr from the formaldehydeclock reaction.
The flowing clock technique is not without flexibility. We have made preliminary studies of the sulfite hyammonia redrogen peroxide and formaldehyde actions, and it seems likely that the method offers possibilities for original kinetic investigations. The two reactions which we have described above can be studied by a conventional clock technique; we have speeded them up by concentration increases so that a flowing clock technique is applicable. However, it is probable that inherently faster reactions can be studied bv this techniaue.
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s J o ~ ~ P., s , AND OLDHAM, K. B., J. CHEM.EDUC., 40, 366 (1963). BARRETT, R. L.,J. CHEM.EDUC.,32,78-9 (1955).
-Reservoir I Experiment
4 M H20z(ml)
2 M HCI (ml)
368 / Journal of Chemical Education
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Reservoir I1 Distilled water (mlj
0.05 M X I (mlj
0.01 M NadlOl(ml)
Distilled water (mlj
Starch solution (ml)
