Hattingen, Germany
Gymnasium Holthausen
June 2013
Synthesis of methyl orange in a micro reactor
By: Steffen Bötzel, Matthias Fischer, Fabian Roweda
Teacher: Mrs. Pratke
Summary:
Methyl orange which is produced in a diazotation process, is used as pH-indicator with a transition range of 3.1 to 4.4. In the
experimental procedure we produced it in a microreactor. By trying to find suitable flow rates to yield a high concentration of
methyl orange, we found that a ratio of 2:2:1 led to the highest concentrations. The best flow rates were those, which added up
to 125 µl/s, so our best flow rate ratio was 50:50:25 at 80°C. Total flow rates above 125 µl/s gave more stable results, but the
concentration decreased. At flow rates far below 125 µl/s no sufficient stability was reached and also the concentration of
methyl orange decreased. In general, temperatures between 76 and 80°C proved to give best results.
Introduction
Inquiry Question
Methyl orange is an orange, azoic dye and is used
as pH-indicator, with a transition range from 3.1
to 4.4, as well as for dyeing and printing textiles.
In the following report we will describe and
examine the synthesis of methyl orange. The
experiment is done in connection with the
“Scholierensymposium” done by the VU
Amsterdam. This competition was already done
last year and we tried to increase the amount of
methyl orange produced by Haenen et al. in
2012.
How is it possible to increase the
concentration of methyl orange achieved by
G. Haenen, M. van Harmelen and Y. Oortwijn
(0,007 mol/l) and what are the optimal
parameters to produce methyl orange?
Synthesis of methyl orange
1. The first major step in producing methyl orange
is the reaction of a nitrosonium ion with the
amino group of sulfanilic acid.(diazotation
process).
2. Now OH– ions react with the
N,N-dimethylaniline forming water and the
diazonium salt can react with N,Ndimethylaniline forming methyl orange.
Hypothesis
We assumed Haenen et al. did not yield the
highest possible concentration of methyl
orange. Long reaction times might result in a
higher concentration of methyl orange
because the reagents simply have more time
to react. Also high temperatures might
increase the concentration of methyl orange
because this means more energy for the
reaction.
Experimental procedure
For the experiment we used the VU
Amsterdam's online webexperiment. This
webexperiment includes a microreactor to
create an undisturbed environment for the
reaction.
The reactor has three inlets for the solutions
A,B,C ¹ and one outlet, the concentration of
methyl orange was measured using a
spectrometer.
The temperature can be varied from 8°C to
80°C, we mostly used temperatures between
70°C and 80°C.
After every execution we cleaned the reactor
with alcohol at a temperature of 70°C.
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Diazonium salt
Solution A:
16.7 mmol sulfanilic acid
16.7 mmol sodium carbonate
16.7 mmol sodium nitrate,
dissolved in 166.7 ml water and 83.4 ml ethanol
Solution B:
3.16 mmol N,N-dimethylaniline
6.3 ml 12 M HCl
dissolved in 238.7 ml water
Solution C:
18.8 mmol NaOH
dissolved in 62.5 ml water
and 187.5 ml ethanol
N,N
methyl orange
1
Hattingen, Germany
Gymnasium Holthausen
At a total flow rate less than 125 µl/s the
temperature was decreased to 76°C in order to
increase the stability of the reaction and to avoid
bubbles caused by boiling alcohol.
Above 125 µl/s the stability was good enough to
use the highest possible temperature (80°C).
At the a ratio of 3:3:1 (A,B,C) we also used 76°C,
because the measurements at 76°C gave better
results than those at 80°C. From these
temperatures we reduced the temperatures as
long as the concentration was increasing (80°C to
76°C to 70°C to 65°C).
We mostly used a ratio of 2:2:1. This ratio already
showed good results in the experiment of Haenen
et al. We varied the flow rates for the 2,2,1 ratio
from 200,200,100 µl/s to 25,25,12 µl/s, but also
tried some arbitrary ratios, like 4:3:3, in order to
find more promising ratios.
100,100,50 at 80°C
concentration (mol/l)
0.004
stock
0.002
0
4
1
10
7
16
13
22
19
28
25
34
31
40
37
46
43
52
49
58
55
64
61
70
67
76
73
82
79
88
85
91
94 100 106 112 118 124 130 136 142 148 154 160 166 172
97 103 109 115 121 127 133 139 145 151 157 163 169
time (in s)
flow rate
ratios
(A:B:C)
(µl/s)
ratio 2:2:1
methyl orange
0.003
0.001
ratio 3:3:1
micro reactor
example for a typical development of the graph
0.005
stoichiometric
ratio
The experimental setup:
June 2013
stabilized
relative tempe- concentration
duration rature
of methyl
for the
(°C)
orange
reaction
(average,
(a.u.)*
mmol/l)
49,33,43
1
76
0.4
200,200,100
0.25
65
3
100,100,50
0.5
80
2.9
1
80
9
50,50,25
(reference)
25,25,12
2
70
5.5
6,6,3
8.3
70
no stabilisation
108,108,36
0.5
76
2.5
54,54,18
1
76
8.6
27,27,9
2
76
8
1
76
< 3.6
47,47,31 ; 47,31,47
21,42,63 ; 50 25 50
Theoretical assumptions
18,54,54 ; 25,50,50
We assumed that the correct stoichiometric ratio
should give the best results, i.e. the highest
concentration in methyl orange. Given the
reagents A, B and C we calculated a ratio of these
reagents as 9:6:8.
49,31,45 ; 42 42 42
21,63,42 ; 25,75,25
Observations
The following diagram shows a typical result of
the development of the concentration of methyl
orange with time.
The stable concentration after some time (cf.
diagram) was used for further evaluation.
For each flow rate ratio the measurements were
repeated four times. The table shows the average
concentration of methyl orange for the various
flow rate ratios used.
63,42,21 ; 75,25,25
56 56 14 ; 59,39,26
48,48,24 ; 52,52,26
The concentration of methyl orange for a
given ratio depends strongly on the overall
flow rate, i.e. the length of time for reaction
within the microreactor. This dependency is
also shown in the table. If the relative duration
for the reaction is too big (total flow rate too
small) no stabilisation can be achieved. The
flow rates of the arbitrary ratios were adapted
so their flow rate ratios added up to the same
total flow rate of 125 µl/s.
*
The relative duration 1 is defined by a total
flow rate of 125 µl/s.
2
Hattingen, Germany
Gymnasium Holthausen
The graphical representation below shows the
concentration at the ratios 3:3:1 (shown as '3')
and 2:2:1 (shown as '2') at various total flow rates
(relative duration for the reaction). The
bars indicate the variation of the results.
At a ratio of 2:2:1 a flow rate ratio of 50,50,25
shows the highest concentration of about
9 mmol/l .
If the total flow rate is too small resulting in a big
relative duration no stabilisation can be achieved.
Conclusions
We found that the best ratios were those with
equal flow rates of solutions A and B while the
flow rate of solution C is considerably lower
(e.g. 3:3:1 or 2:2:1) as long as the flow rate of
solution C is not too low (then not all possible
reactions can take place). If the flow rate of
solution C is too high, this only leads to diluting
the output solution (low concentration of methyl
orange).
For every flow rate ratio, we found an optimal
temperature, e.g. for 200,200,100 about 65°C, for
50,50,25 80°C (results precised to ± 3°C).
The ratio 9:6:8 (stoichiometrically calculated) did
not yield a high concentration (0.4 mmol/l).
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June 2013
A long relative duration for the reaction
increases the concentration of methyl
orange effectively, but if the relative
duration for the reaction is too long (very low
total flow rates), the reactions take long to
stabilize or do not stabilize at all, so these
flow rates can often not be repeated with
the same measurement results and thus are
not valid.
The highest concentration of methyl orange
was generally obtained at a relative duration
for the reaction of 1, i.e. a total flow rate of
125 µl/s.
At a ratio of 2:2:1 with a reaction duration
of 1 (flow rate ratio of 50,50,25), the
concentration of methyl orange was highest
(9 mmol/l at 80°C). The 3:3:1 ratio at the
same relative duration of the reaction (flow
rate ratio 54,54,18) gave almost the same
concentration, but it uses more
N,N-dimethylaniline which might be less cost
efficient.
High temperatures (80°C maximum for this
microreactor used at higher flow rates, 76°C
to get a more stable reaction) reduce the
stability of the reaction even more.
Hattingen, Germany
Gymnasium Holthausen
Discussion
June 2013
Evaluation
It is not clear why the development of the
concentrations with time show strong peaks,
even in the stabilized phase. This could be
an effect of the microreactor itself, because
it also happened at low temperatures.
We observed that sometimes the number of
bubbles at the output webcam increased
with time though not a single bubble could
be seen in the micro reactor itself. In
addition at low flow rates (total flow rate
below 125 µl/s) at the beginning big bubbles
were formed even at temperatures well
below the boiling point of ethanol. This
resulted in a lower relative duration for the
reaction than intended. The reason for this is
not clear, and it is probably due to the
experimental setup as it was only a
temporary effect. The final results were not
affected.
One of the characteristics of the micro
reactor is that it works with extremely small
amounts of chemicals. At a flow rate ratio of
50,50,25 per second only two drops of
solution A, two drops of solution B and one
drop of solution C react. This could be a
reason for the strong peaks and slow
stabilization at some flow rate ratios.
Sometimes when the stabilization took very
long, the syringes (1 ml) were emptied
before a stable concentration was reached.
It could be advantagous to use bigger
syringes.
In the course of our work there were some
problems with the calibration of the
spectrometer which needs to be checked
regularly. We assume that this calibration
was correct during our measurements.
The influence of temperature, ratio and total flow
rate on the concentration of methyl orange was
examined. Due to the lack of time we did not try
every possible ratio at every possible
temperature and every possible total flow rate.
However, we assume that we got quite close to
the best conditions to produce a high
concentration of methyl orange.
Our method of finding the best parameters could
be improved by even more systematic variations
of flow rate ratios and temperature.
Sources: Synthesis of methyl orange in a micro reactor by Haenen et al.
Cygnus Gymnasium, Amsterdam, The Netherlands; 23/04/2012
Student workbook on methyl orange online synthesis, VU 2012
Chemistry Network
Department of Research and Theory in Education
VU University Amsterdam The Netherlands, 2012
Setup at the
Department of Chemistry and Pharmaceutical Sciences
VU University Amsterdam The Netherlands, 2012
CHEMISTRY ONLINE EXPERIMENT
Online synthesis of methyl-orange
A.J. van Dijk, J.M.Mulder, P.Nieuwland; March 2012
http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/methyl-orange.shtml
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