INFLUENCE OF TEMPERATURE ON THE
SYNTHESIS OF METHYL ORANGE
TijlKruyswijk& Huck Nuchelmans
Cygnus Gymasium, June 2012
S UMMARY
Methyl orange is a substance used in both scientific research and more “hands on” applications. As the name
suggest, it is an orange substance that easily dissolves in water. This makes it useful when for example dying
clothes. The other, scientific use for the substance, however, is in chemical research. The creation or synthesis
of methyl orange is a four step process that’s easy to recreate, but difficult to control in large quantities. When
talking about amounts as small as mmols, the variables in the reaction can be controlled to synthesize methyl
orange at a lower or higher rate. The variable we have chosen to adjust is temperature, and the question we
have is therefore: is it possible to control the speed of the synthesis of methyl orange by adjusting the
temperature at miniature amounts, and if so, at what way?
Our hypotheses was already made for us when we looked at the instructions of this experiment, which clearly
state that the speed of the synthesis can be controlled by adjusting temperature or flow rate.
I NTRODUCTION
Methyl orange is an azo compound (a compound that has two double-bound nitrogen atoms), used in the
textile industry to dye materials. It also has a scientific use, mainly as a pH-indicator. The substance turns red
when subjected to a low pH value, but turns yellow when the pH is high. This can be used to determine the pH
value of unknown solutions.
The synthesis of methyl orange is a four step process that starts with the reaction between sodium carbonate
and sulfanilic acid. This makes the acid more soluble to ease later reactions.
Step two involves a strong acid, for example hydrochloric, that reacts with sodium nitrite, creating a
nitrosoniumion.
The nitrosonium ion reacts with the amino in the sulfanilic acid to create triple bound pair of nitrogen atoms.
The acid becomes a diazonium salt.
In the final part of the synthesis the salt reacts with an addes N,N-dimethylanaline and a hydroxide ion to
create methyl orange.
Note that each of these reactions also have water as a by-product.
Since the synthesis of methyl orange is an exothermic reaction, it requires large amounts of cooling on a large
scale to prevent overheating the compounds. This makes adjusting temperature difficult, but at tiny amounts
of compounds, the energy released from the reaction is so small that temperature can easily be regulated to
get different outcomes.
E XPERIMENT
The experiment was set up as such: a microreactor had three inlets, one outlet and a long channel to allow for
the reaction to take place. Each of the inlets had a reagent for the synthesis (sulfanilic acid + sodium nitrite,
N,N-dimethylanaline+ hydrochloric acid and lye). Inlet 1 and 2 could also be filled with with alcohol to clean the
reactor after a measurement cycle. Each of the inlets had an adjustable flow rate (ranging from 1 µmol/min to
1500µmol/min). The entire reactor could either be heated up or cooled down using an adjustable temperature
(ranging from 8°C to 80°C).
When each of the reagents was released at a predetermined flow rate (we chose 300µmol/min), a
spectrometer in an Erlenmeyer measured the amount of methyl orange that was being produced. This data
was made into a graph, which clearly showed the concentration of methyl orange per time unit. Eventually,
the value would become a constant as no more methyl orange was being made, and the measurement could
be stopped. The graph data gets saved and the reactor is again ready to be cleaned with alcohol.
R ESULTS
The results of our experiment, where we chose to measure the speed at which methyl orange was formed
under different temperatures, we got the graph shown below. All flow rates were the same, so no significant
changes in colour was observed during the reaction (which means, the reactor got the standard brown-yellow
colour). During the cleaning, however, the colour very briefly shifted to yellow since alcohol has base-like
components. Note that when saving the data, something went wrong, and now our measurements for 60°C
and 40°C are exactly the same.
Concentration of methyl
orange in mmol/L
Time in seconds
We added a standard logarithmic graph next to our measurements to show that the correlation between time
and concentration is in fact logarithmic.
This graph shows that the temperature had a significant impact on the speed at which methyl orange was
created. The amount created at 80°C is nearly 20 times higher than that of 20°C. The stable point in the graph
was also much less stable at higher temperatures, however, indicating that the reaction was less consistent at
the higher temperatures.
C ONCLUSION
AND DISCUS SION
Our results show that a microreaction is indeed possible, and that the speed of said reaction can be altered by
adjusting the temperature of the reactor. However, a higher temperature also appears to mean less stable, so a
safe zone in temperature must still be picked. We have no idea how much methyl orange we ended up making,
though.