A. D. Baker and A. Casadevall City University of New York ~ " e e n sCollege Flushing. NY 11367
I I
A C h k Reaction with Paraquat
Clock reactions are among the most spectacular of demonstration experiments. Most well-known is the "Iodine Clock" which is a popular general chemistry demonstration and a useful means of introducing kinetics in the freshman year laboratory. Here we descrihe a new clock reaction resulting from the reaction of paraquat with base. The "Paraquat Clock Reaction" can he used to probe the mechanism of the reactions involved and to explore the role of solvents in determining reaction rates.
. Kinetics Experiments C H ~ N ~ L ~ R In beginning courses, the usual introduction to the devel-
n17-iQ-(-J-x,
CI-
CI-
a-
n I Paraquat dichloride (I) is the dimethyl quaternary salt of 1,4-bip;ridine; sometimes it is given the alternative name of "methyl viologen." Paraquat was first produced in 1932 ( I ) , and fo"nd mias a redoxindicator because of its ready conversion to the stable cation radical (11) which has a characteristic deep hlue-violet color. More recently, paraquat and related substances have found widespread use as herbicides. The herbicidal activity in part depends on the (reversible) one-electron reduction of I to I1 (2). Extensive use of paraquat in spraying Mexican marijuana led to concern tbat people could he poisoned by inhalation of the naraouat . . as a result of smokine this tm -. of marijuana, and a number of"paraquat test kits" came onto the market.'rhese were hased also on the easy reduction of 1to the hiahlv . . colored 11. Our present interest in paraquat chemistry arose from reports that in the presence of a catalyst, the radical cation I1 can reduce water to hydrogen (3),and since I1 can be produced from I by photochemical means ( 4 ) ,possible schemes for solar energy conversion hased on paraquat chemistry seem worthy of Durine these investieations. we had cause to - - investieation. - ~ examine various routes(photochem~aland'nonphotochemical) from I to 11. One nuhlished method involved the use of hydroxide, and during our investigation we discovered the clock reaction that is the subiect of this . oaoer. . On adding hydroxide solution to an aqueous alcoholic solution of paraquat dichloride. thesolution initially became . pale .yellow, then kddenly turned hlue. The induction time between initial mixing and the appearance of the blue color could he varied by altering the concentrations of the ingredients, or by changing the solvent systems. The basic explanation for the clock reaction, which we shall enlarge upon later in this paper, is tbat hydroxide ion causes the reduction of paraquat to.its radical cation 11, but as this is oxygen-sensitive, i t is initially converted back to I because of O2 present in the system. Only when the oxygen is consumed can the Pq+ concentration build up, and one then sees the hlue color. After the hlue color has anneared, viaorous shaking of the flask causes the hlue color &disappearagain, only to return almost immediately once the shaking is stopped. This sequence of ohservations can be shown, without comment to students, to stimulate interest and speculation as to the cause of the phenomena. In this respect,-there is a similarity t o the "blue bottle experiment" (51, only with the color changes reversed. (In the hlue bottle experiment, a blue color appears on shaking; an interesting contrast is to show the blue hottle experiment and the present experiments in tandem.) The effect of hubbling nitrogen through the solutions prior ~~~
-~ ~
~
to, or immediately after mixing, or alternatively, of pumping on the solutions with a water numn or laboratow vacuum, is to drastically decrease the inhuction time prio; to the first amearance of blue color. Such experiments should tip off &dents that oxygen is involved. On a more quantitative basis, a series of kinetic experiments can he designed around the paraquat clock reaction. These are the concern of the rest of this paper.
opment of a rate equation and to the concept of orders of reaction is through studies of reactions in which the initial rate is studied as a function of the initial concentrations of the different inmedienu, I, which are varied in a simple systematic fashion. For example, the student learns that if a four.fold increase in reaction rate accompanies a douhling in the initial concentration ot'a particular substance, then the order of reaction with respect to that substance is 2. For cases in which a simple relationship between the concentrations and rates is not apparent the logarithmic equation Log (Initial Rate) = log k + n log [I] is introduced. Accordingly a graph of log (Initial Rate) versus log I for arelated series of experiments should give astraight line plot of slope n. This concept can he applied to results obtained with the paraquat clock reaction. SAFETY NOTE P a r a q u a t i s toxic. Volumes of paraquat needed for this experiment should be pipetted only by means of mechanical suction devices. For the symptoms of paraquat poisoning, see reference (8).Care should he taken also to avoid splashing of paraquat solution onto t h e skin, as it may he absorbed through the skin into the bloodstream. Since the amount of paraquat used for any set of observations is quite small, however, no particular problems can he expected provided sensible operating procedures are employed. Experlmental Materink used fur the experiments were rtwk solutionsuf paraquat (accurately known, appruximarelg lo-' M I and sod~umhydroxide (accurately known, approximately .'LO All. plus methanol, ethanol, acetone. and various other sulvents. In corhexperiment,achusen volume,^ ml,ofthesodiumhydroxide solutam was mixed with r ml uf the solvent imethanol. ethanol. PIC.) and placed in a round-bottomed flask equipped with a magnetic stirrer to orovide uniform mixine. To this stirred solution. a mixture of v ml afihe stock oarsouat ~. sol;tian and z ml of water was added all ~~~~-~ at one time, the llask immediately stoppered,and asropwntch stlned. In each run, thetrml of ( a . T x T ) + 2 ) was400 ml. The tlme, 1, taken fur an initial hlue color tu appear was recorded in pnch case. For tach set of (w,x,y,r)values, three runs were carried nut to check reproducibility (good to 4% or better) and to obtain an average value for t. Since the total volume in each experiment is the same, the amount of O2present in each experiment was assumed to be thesame. Thus differentinduction times can be related directly to the initial rate of reduction of paraquat. Results and Discussion In order to explore the use of the paraquat clock reaction as a means of studvine . the kinetics of the reactions involved, a sequence of experiments was carried out using the solutions
.
~
~
~
~~
.~
Volume 57, Number 7, July 1980 1 515
Table 1.
Effect of Paraquat Concentratlon on Clock Reaction. Set a
[PO2+] (103 M)
[OH-] = 1.6 M. [ElOH] = 5.7 M Time (set). In[W2+]
Table 2. Effect of Hydroxide Ion Concentratlon on Clock Reaction. [Pq2+] = 0.0019 M [EIOH] = 6.4 M Time (sec) In [OH-]
In(l/t) [OH-], M
Set b [PO2+] (lo3 M)
[OH-] = 0.85 M [EIOH] = 4.9 M Time (set). In [POzt]
4.71 4.40 3.78 3.14
144 165 210 235
-5.35 -5.43 -5.58 -5.76
2.25 1.75 1.50 1.25 0.75
'
In(l/t) -4.97 -5.10 -5.35 -5.45
[EfOHl. M
described in the "Ex~erimental"section. The reciurocal of the time, t, measuring the interval between initial mixing and first amearance of blue color was taken as heine...~rooortioual . to the'initial rate. Thus for each set of experiments In(11f)was plotted against the lornrithm of the concentration d w h i r h kver species was having its concentration varied in a systematic fashion. In our initial set of experiments, the paraquat concentration was varied while keeping the concentrations of hase and alcohol constant. Two typical sets of results are shown in Tahle 1.In either case (i.e., for set a or set b in Tahle 1)a plot of the initial rate (i.e. In l l t ) against the logarithm of the initial ; concentration gives a straight line of slope ~ 1 . 2suggesting that the order with respect to paraquat is slightly greater than unity. Next the hase concentration was varied while keenina the alcohol and paraquat concentrations constant. 'Typic2 results (Table 2) yielded an apparent order with respect to OH- of E2.2. - -.
At this point some mention of possible mechanisms is pertinent. Based on suggestions by Ledwith et al(6), i t appears that paraquat is reduced not hy hydroxide ion itself, hut rather by alkoxide ions produced in an initial equilibrium reaction of hydroxide with, in the present case, the added alcohol (see Reaction Scheme). Any experiments with the clock reaction involving measurement of the induction time as a function of the alcohol concentration must he treated with caution since the quantity of dissolved oxygen will vary according to the alcohol concentration.
-
&action &heme
+ OH-
CHD + HC)
-2.70 -3.28 -3.62 -4.04 -5.13
[Pq2+] = 0.00275 M [OH-] = 1.25 M Time, rec ln[EtOH]
8.56 7.70 6.65 600 5.14
26 38 52 134 398
2.14 2.04 1.92 1.60 1.64
In(l/O -3.27 -3.63 -3.94 -4.90 -5 99
T h i * set of data unespondo to an apparent adtw w l h respect to ehanoi of gwtw
than 5.
Interestingly, substitution of alcohol by an equal volume of DMSO or acetone leads to a striking decrease in the induction time for the clock reaction. These systems are obviously quite complicated, and the question of what is the reducing agent in these cases is g puzzle. Further data is needed to account convincinaiv for the exuerimental observations. in ally should poiht out that we suspect that competing parallel reactions mav he involved in the clock reaction experiments. Thus, we have noted that the solution turns pale yellow before the blue color of Pqf appears, suggesting either the formation of an intermediate, or the involvement of a side-reaction. One possibility is that in addition to hvdroxide ion reacting withalcohol togive an alkoxode ion, it aisu reacts with I'q"', abstracting a proton to give the resonance-statii. lized ion 111 which may be responsible for the yellow color.
Precedent for this contention comes from the previously published observation that the diamino compound IV gives a yellow solution on addition of hase, the yellow color heing attrihuted to V (7).
(1)
As Table 3 shows, the effect of varying the alcohol concentration on the induction time is actuallv m i t e dramatic. a result we believe, based on several types o? experiments, is not solely due to changed oxygen contents, hut rather to an unusual solvent effect on the reactions. Exploration of these solvent effects makes a challenging research exercise for senior undergraduate students. In this capacity one of us (A.C.) found that not only added alcohols enable the clock reaction to work, hut also solvents such as DMSO and acetone (with just aqueous solutions, the clock reaction is extremely slow).
516 I Jownal of Chemical Eduoltion
0.810 0.560 0.405 0.223 -0.29
Table 3. Effect of Alcohol Concentratlonon Clock React1on.a
T h e time fmm the onset of mixlns reagents to appearance of ihe hlu, cola
CH,OH
15 27 37 57 168
In(l/O
The existence of parallel or competing reaction ~ a t h w a v s would of course lead to apparentordek of reaction more complex than expected on the basis of a single pathway. Literature Cited (1) Michaelis,L.andHii1.E. S.,J Gen. Physiol.. 16,S59(1933):J. Amrr Chem. Soc., 55.
..".,,am, ~.""",. ,L*,
12) Bwn. W. R.,Chemismryond Industry. 782 (1965). 13) Klasna, A. I.,Photochemislrynnd Phofubiology. 29.267 (1979). 141 (a) Reference 3: ih) Hopkin., A. S., Iadwith, A . and Stan. M. P.. Chem. Cumm., 4% 119701; 1c)Takuma.K.. Kajiwara, M.,andMnts.T.,Chom. Ldfera, 1199i19771. ( 5 ) "J. CHEM. EDUC. Tested Dernonstrationr..l6th Ed., p. 187. (6) ParringWn, J. A,, Ledwith, A , and Stam, M. F., Chem. Comm., 259 (1969). (7) Downs,J.E.,J. Chem. S o c (e). 2192 (1967). (8) Iutl, P. F., and Loth J. W., J. Chmmot. Sei.. 16,390 (1978).
