Propagation of Chemical Reactions in Space
Robert Luther
Leipzig, Germany
Translated by:
Regina Arnold and Kenneth Showalter
Department of Chemistry, West Virginia University, Morgantown, WV 26506
John J. Tyson
Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
Translators' Note
This article is an English translation of a paper by Robert Luther on his discovery.investigation. and analysis d chemical waves. The source
of the translation is a transcript of a lecture and the following discussion at the Main Meeting of the Deutsche Bunsengesellschafl fur
Angewandte Physikalische Chemie held in Dresden. Germany,from May 21 to 24. 1906.It was published by Luther in 2. fur Elektrochemie.
1906, 12[32]. 596. A discussion of this article is presented in the next paper in this issue (page 742).
Weare indebtedto M. Marek(Prague1andL.Kuhnert (E. Ber1in)forfirst
drawing our attentiontoLuther'spaper. This work was supported by
the National Science Foundation (Grants No. CHE-8613240 (KS) and DMS-8516367 (JJT)).
'
Sesslon on Tuesday, May 22, 9:00 a.m. In the Maln
Auditorium of the Chemlstrv Laboratory of the Polvtechnlc
Institute
It is known that a stimulus received on a nerve ending
propagates through the nerve with a certain finite and not
excessively large velocity of about 50 1x11s. The mode of this
propagation has not been elucidated so far. However, there
has not been a lack of experiments that attempted to provide
a clarification or, a t least, to establish a plausible picture of
the processes in the nerve on the basis of known phenomena.
Most often electrical devices were used for that nnrnose: for
example, parallel or sequentially connected self-inductances
and caoacitances. Yet I want to focus on nonelectrical hvpotheses in which the generation of different electrical phtentials is a subordinate ohenomenon. As far as I know. it
was the late "~niversalgeiehrte"Herbert Spencer who first
pointed to the analogy between the .
propagation
of a stimu- ~ u in
s the nerve a n d t h e propagating transformation of a
solid from a metastable into a stable form. Later Ostwald
used the initiation and propagation of crystallization in a
supercooled melt as an example. This phenomenon is well
kncwn: ii, tor example, a
glass tuhing rhnt contains a
iupvrcooled inelt OI rhiosuiiate is inorulnted at one end with
3 r r x e o i s o l i d rhms~rlmtr.t h r crvstnlliz;~t~on
will eradusllv
propagate through the tuhe. However, nerves are solid structures neither before nor after stimulation and the question
arises whether a similar propagation is also possible in liquid
homogeneous systems.
Several years ago Herr Schilow and myself discussed this
possibility. We also oerformed some oreliminaw
. exoeri.
ments. I would like to show you now one such experiment.
Herr cand. Meinecke has helped with the .
preparation
of the
.
experiment. emon on strati on:')
The velocity of propagation is small, only a few centimeters per hour. Based on theory, this is to he expected. In
principle, however, there is nothing to prevent any large
velocity since the velocity of propagation (V)is given by
. .
-
v=a@rTz
where a is a numeric factor, D a diffusion coefficient, C a
concentration, and K a rate constant. When K increases
sufficiently, so does the velocity of propagation.
740
Journal of Chemical Education
While the border between red and white is moving slowly,
I would like to give some further explanation. In studying
the actual cause of the propagation of crystallizations, one
soon arrives a t the conclusion that the nrocesses that nroDa.
gate in a homogeneous medium have to he autocatalytic.
Let us choose as an examole the autocatalvtic decomnosition of the salts of alkyl sulfonic acids. Suppose a tuhe is
filled with a solution of methvl or ethvl sulfate. T o prevent
convection, the solution is to be gelled.
The neutral alkvl sulfates are verv stable in aoueous solution hut slowly gi;e off sulfuric acia upon acidification. If I
introduce some acid on one end of the tuhe then, under the
catalytic influence of Ht, hydrolysis ensues and new H t ions
are formed. These ions diffuse to the right and cause the
formationof moreacid. In this way, the decomposition slowly moves through the tuhe.
The selection of useful reactions is not large a t the moment, for we do not know too many autocatalytic processes.
In addition. the reaction has to he linked to a color chanee in
order to observe the propagation with the naked eye.
In the chosen example of a salt of ethyl sulfate, one can use
any indicator, such as litmus, whose color change will show
the movement of the Ht-ion front. A different example is a
reaction studied by Eckstadt that involves nitric acid and
hvdroiodic acid. Nitrous acid or NO function as the initiating agents ("Zundmittel"). The former reacts rapidly with
hydroiodic acid and therehy is reduced to NO which,
.
I Equal volumes of the following solutions were mixed:
A. 1112 mole H,S04 + 1112 mole C204H2(oxalic acid) per liter.
6. 11125 mole KMnO, per liter.
If reducina substances are excluded durina-,the areoaration of
. the
..
SOIUIOnS, tne mixture may be kept ~nchangpdfor more than 30 mn at
room temperature After lhal tome, deco oruation sets in rather rap dly. The freshly prepared mixture was transferred to a well sterilized
oval test tube. Some drops of the already reacted mixture were
cautiouslyadded to the top of the solution. The borderline between the
lower (red)liquid and the upper (colorless)liquid was marked with a
wire ring that was pushed over the tube. The test tube was projected.
whereby the gra4ual movement of the interfaceof the color into the
,.
red region could be demonstrated.
-
~
~~~
~
~
~
~
-
~
-
through reaction with nitric acid, produces again HN02, and
SO on.
Another example is the autocatalytic reduction of permanganate. In particular the reaction scheme with oxalic
acid was largely clarified through t h e investigations of Harcourt and Esson, and Schilow and Skrabal. During the reaction Mn2+ is produced which-as was first discovered by
Hempel-accelerates the reduction of permanganate and
thus causes the propagation. T h e reaction between KMnOl
and oxalic acid was chosen for t h e demonstration.
Finally the autocatalytic reductions of halogen acids of
type H X 0 3 provide good examples. Rromic acid in particular has been studied by Schilow. Bromic acid in weakly
acidic solution is sluegish toward manv reducing agents, for
example arsenous acid. In contrast, hromic acid ;ea& rapidly with bromide ion, apparently
following the eenerallv
. ap.
plicable scheme (I):
~
+ HBr = HBrO, + HBrO.
resp. BrO? + 112 Br? + H?O.
HBrO,
As soon as one adds a trace of bromide t o the acidified
mixture of hromate and arsenite, the reaction starts. T h e
initial products which are cpickl; formed by oxidation will
he reduced bv arsenous acid t o bromide ion. Bromide enhances even more the rate of reaction, diffuses into the
neighboring layers, and thus the reaction propagates. Addition of indigo carmine makes the propagation visible.
In all these cases, a n event propagates in space, and it is
not unlikely that the propagation of a nerve impulse is similar. Although this is just one potential explanation, it appears worthwhile t o show that analogies exist for many incidental details.
First, one should note that a nerve is temporarily exhausted after propagation of a stimulus. I t takes a certain time to
renew its potential, and this is evidence t h a t a chemical
reaction has occurred. Another indication t h a t a chemical
reaction has taken place can be found in the fact that, not
only in the test tube hut also in t h e nerve, the velocity of
propagation decreases with a decrease in temperature.
An analogy for the so-called "threshold" for nerve stimuli
can also be found in the test tube. Let us take the reaction
HBrO:I As20z as a n example. T o free t h e solution from the
autocatalytically acting Br-, I have at times sterilized the
solutions by saturation with AgBrO:, so that the B r was
bound and removed as AgBr. As a result, the solution contains small amounts of Ae+. If therefore. verv little Br- is
used as a stimulus, it will keact a t once with the Ag+ t o form
AgBr and therehv is rendered inactive. I t takes a certain
minimal stirnulusto initiate the propagation. On the other
hand, several weak B r stimuli in rapid succession can use
u p the silver ions, decrease t h e sensitivity harrier, and then
initiate propagation. A similar behavior is observed for a
nerve stimulus.
Also, the nonspecificity of t h e stimulation of the nerve in
contrast t o the selective stimulation of the sensitive nerve
endings can he found in our nerve model. Our permanganate-oxalic acid nerve can be stimulated hy any reducing
agent that reacts with MnOd-. We may, however, transform
this nerve into an "ootic" nerve hv ~ l a c i n pone end of the
permanganate tube in a flask filled with fer& oxalate. Upon
exposure t o light ferrous oxalate is oroduced. which. in turn.
reacts with thk permanganate and Eauses
of the
stimulus.
Various other examples could h e mentioned; one might
even construct a chemical theorv of memorv. hut in conclusion I will point to only one more analogy. m he autocatalytic
reactions have, as can be easily seen, a striking similarity
with explosions. Here, as there, ignition proceeds from one
spot. Here, as there, we have first a small and then an everincreasing reaction rate, which, after it passes through a
+
maximum, decreases again t o a n extent as the original reactants are consumed. In a way one can talk about isothermal
explosions. T h e autocatalytic reaction between H ..7 0.9 and
permany:rnatc,. which produces 0,.may e \ w he usrd for the
w n s l r u r t i w of a n istrt hrrmal pistol. Hut that is r h i l d ' plnv.
More important from a
standpoint appear t o be
the following facts. Nernst has recently pointed out with
great emphasis that the control of ex&sive processes is
important for the theory and practical development of gas
engines. T h e same was said yesterday in Professor Will's
lecture on the propagating reactions caused by explosives.
T h e mathematical difficulties in dealing with nonisothermal
explosions are suhstantial. It is my belief t h a t the investigation of partial processes will serve as a first sten t o increase
our knowlrrlge of the tlmc drprndencv of exploiion.; in the
g3s 3nd -did stat(,. T h e inveariratim nf adinhatic reactions
has been started by Bredig. ~ o g e t h ewith
r
Herr Meinecke, I
have started t o study the propagation of isothermal reactions in homogeneous medium.
I think therefore that our experiments in progress have a
certain practical importance.
niscusslon
Dr. Brode-Ludwigshafen a. Rh.: Rust catalyzes the oxidation of
iron, a case which is very important in practical terms. Could this
also he considered as one of your reactions, Mr. Speaker?
Professor Luther-Leiprig: I excluded heterogeneous reactions
on purpose. Many such reactions are known, for example, the
dissolution of copper in nitric arid that is free of nitrous acid, the
activation of passive iron in an appropriate solution by touching it
with an active iron wire, and other examples. But for my purposes,
it is important to have totally homogeneous media.
Geheimrat Nernst (2)-Berlin: I want to allow myself one question. 1want to point to the fact that physiologists have loqked for
possibilities to explain the rapid propagation of nerve stimuli and
have pointed to the analogy with explosions. However, this is
obviously a very different process because the propagation of
explosions proceeds through a high temperature pathway, whereas the speaker has stressed the importance of isothermal propagation which is assumed to occur in the nerve. I am afraid that it is
not possible to reach high velocities of propagation, like that
nwded hy the physiologists, by this pathway. I would like to
remind yo11that the propagation of a nerve stimulus is about 30
m/s (Interjection: SO!). This results in 100 to 130 kmh, which is
fastpr than the fastest train ("Blitzzugsgeschwindigkeit").
We have, however, witnessed by looking at the beautiful experiment that you. Mr. Speaker, have demonstrated, that the velocity
of propagation is of a different order of magnitude. Let us say one
hundred thousandth or one millionth of the velocitv
needed in the
~,
nerve. I would like to ask you now Mr. Speaker-and I think this
is the crucial point-is it possible to ohtain such high velocities
through your pathway?
Professor Luther-Leipzig: In principle, yes, but hardly experimentnllv hecause in general the "Wackeligkeit", i.e., the selfdecomposition of the solution, increases with the speed of prapagation. For an autocatalytic reaction of second order the formula
that I have written down here applies. The velocity u is equal to
o
m
, where o is a numerical constant between 2 and 10 and
is dimensionless. The diffusion constant D cannot be increased
suhstantially. Also, the concentration C cannot he increased
suhstantially. However, the velocity constant K of the autocatalytir r~artioncan, in principle. assume any large value (3).
Therefore, the possibility that we have extremely rapid autocatalvtic reactions and that nerves use sueh reactions for their
purposes cannot be discounted.
Geheimrat Nernst-Berlin: Who has derived this formula?
Professor Luther-Leinzie: M w d f .
~
~
~~
the corresponding differential equation.
Grheimrat Nernst-Berlin: Frankly, I cannot understand how
one could ohtain sueh high wlocities, but one will look forward
with even greater interest to the complete publication.
Professor Luther-Leipzig: I want to add that experimentallywe
will certainly not he able to reach high velocities. Since we have
always ohserved that thp greater K, the "wackeliger" will be the
experiment and the easier it is fnrthe mixture to react ( 4 ) . We
Volume 64
Number 9
September 1987
741
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