How does the rate of reduction (in s-1) of methylene blue by alkaline glucose solution depends on the concentration of the glucose solution used, determined using a stop-watch?

Blue bottle reaction was perhaps one of the first Chemistry activity done in my middle school Science classes during the Science Exhibition that stimulated my interest in the subject. Though at that time, what the reaction is, how it happens and why there is a color change was not known to me properly. However, an observation that triggered me at that point was though all students in the classroom was doing the activity yet some for us the color change was faster while for others it took quite a much time for the color to change. While going through the Topic-6 in DP Chemistry course, I came to know about how concentration affects the rate of the reaction. This intrigued me and I wondered if the difference in the time taken for color change was due to the difference in the mass of glucose that we used or not. To answer this question, I have zeroed down to the research question stated above.

Methylene blue is an indicator. The chemical name of methylene blue is methylthionium chloride. This is also used as a blue colored dye. It is a heterocyclic aromatic compound where two substituted benzene rings are fused through C chains that connects through N and S atoms. It exists as an ion pair where the organic compound with a positive charge on the S atom of the ring combines with a chloride ion. In the oxidized form, the dye is blue in color whereas in the reduced form, the dye is colorless. The reduction occurs due to abstraction of a proton by the N atom present in the ring.

Methylene blue is an indicator.svg

Figure-1: Structural formula of oxidized and reduced methylene blue1

&nbsp;

Glucose is a polyhydroxy aldehyde with five OH groups and an aldehyde (CHO) group. It falls under the category of carbohydrates. It can exist in both open chain and closed chain form. This is the main product of photosynthesis by which the plants can make food for itself as well as for others.

&nbsp;

Alkaline glucose is glucose dissolved in a solution with pH of more than 7. As glucose contains OH groups which are acidic in nature and can easily reacts with alkalis to liberate a H+ ion and thus generate the alkoxide forms (RO-), glucose is readily soluble in alkaline medium. The alkaline solutions of glucose are made by dissolving glucose in a solution of either NaOH or KOH.

&nbsp;

Alkaline glucose solution can act as a reducing agent. It can reduce the dye- methylene blue to the colorless form. The reduced form-Leucomethylene blue (colorless) can be oxidized back to the blue colored form in presence of dissolved oxygen in the medium. Apparently, it is like that blue color disappears and then again reappear on shaking. This gives the reaction the name of the blue bottle reaction.

Methyleneblue.svg

The current investigation aims to study the effect of temperature on the first step of the reaction- the reduction of the blue colored dye to the colorless form using alkaline glucose solution.

&nbsp;

As temperature increases, the average kinetic energy of the glucose molecules would increase too. This means that higher the temperature to which the alkaline glucose solution is heated, more will be the fraction of glucose molecules with sufficient energy to overcome the activation barrier and that will eventually result into higher number of successful collisions between the glucose molecules and the dye molecules resulting in a faster rate of reaction, lesser time for blue color to become colorless.

&nbsp;

Thus, it can be expected that with the rise of temperature, the time taken for the blue color to change into colorless should decrease.

&nbsp;

The rate of a reaction can be measured in multiple ways. In general, it is expressed as change of concentration of the reactant or product per unit time. Here, in this investigation the rate will be calculated using the following formula:

&nbsp;

Rate = \(\frac{1}{Time\ taken \ for\ the \ blue \ color \ to \ become \ colorless \ }s^{-1}\)

As discussed in the background information, the rate of the reaction must increase with the increase in temperature. Thus, it is expected that as temperature rises, it should take less time for the blue color to become colorless and the rate of the reaction must increase.

Temperature: The reaction will be carried out at various temperatures &ndash; 30.0℃, 40.0 ℃, 50.0 ℃, 60.0 ℃, and 70.0 ℃. &nbsp;As the reaction can be performed at room temperature, high values of temperature has not been chosen. A water bath will be used to vary the temperature. A laboratory thermometer will be used to check the temperature of the solution.

Rate of reduction of methylene blue by alkaline glucose in s-1

&nbsp;

The time taken for the solution to change color from blue to colorless will be recorded using a stop-watch and then the rate of the reaction will be calculated using the formula given below:

&nbsp;

Rate =&nbsp;\(\frac{1}{Time\ taken \ for\ the\ blue \ color \ to \ become \ colorless }s^{-1}\)

<ul>
	<li>Wear a protective laboratory coat.</li>
	<li>Use safety gloves and mask.</li>
	<li>Do not expose any of the chemicals to your skin.</li>
	<li>Dispose all used chemicals into the disposal bin and dilute them using tap water before disposal.</li>
	<li>Return all unused materials to the laboratory technician for re-use.</li>
	<li>Wash and clean all used apparatus before returning them.</li>
	<li>Keep your workstation clean and dry.</li>
</ul>

List Of Materials Required.svg

<ul>
	<li>Take a clean and dry 100 cc glass beaker.</li>
	<li>Take a watch glass and place it on the digital mass balance.</li>
	<li>Adjust the reading of the mass balance to 0.00 &plusmn; 0.01 g using the Tare button.</li>
	<li>Transfer solid NaOH from the reagent bottle to the watch glass until it reads 0.40 &plusmn; 0.01 g.</li>
	<li>Transfer the weighed NaOH from the watch glass to the empty glass beaker.</li>
	<li>Add 100 cc distilled water using a graduated measuring cylinder to the glass beaker.</li>
	<li>Use a glass rod to stir the solution and dissolve the NaOH.</li>
	<li>Using a watch glass and a spatula, weigh 10.00 &plusmn; 0.01 g of glucose.</li>
	<li>Place the beaker on the water bath and set the temperature at 30.0 ℃. Use a thermometer to check the temperature.</li>
	<li>Add the weighed glucose to the beaker as soon as the temperature is reached.</li>
	<li>Using a spatula, a watch glass and the digital mass balance weigh 3.19 &plusmn; 0.01 g of methylene blue indicator solid.</li>
	<li>Transfer the weighed solid into the beaker and start the stop-watch.</li>
	<li>Record the time taken by the solution to change color from blue to colorless using the stop-watch.</li>
	<li>Repeat same steps for 4 more times.</li>
	<li>Repeat all of the above steps at 40.0 ℃, 50.0 ℃, 60.0 ℃ and 70.0 ℃.</li>
</ul>

<ul>
	<li>The initial color of the solution was colorless which immediately became deep blue on adding the methylene blue indicator.</li>
	<li>The beaker appeared hot when touched on dissolving NaOH in water.</li>
	<li>The time taken to change the color from blue to colorless reduced on increasing the temperature.</li>
</ul>

Raw data for time taken for color change at various temperature.svg

For Row-1

&nbsp;

Mean time taken =&nbsp;\(\frac{Trial^{-1}\ +\ Trial^{-2}\ +\ Trial^{-3}\ +\ Trial^{-4}\ +\ Trial^{-5} }{5}\)

&nbsp;

=&nbsp;\(\frac{343.06\ +\ 344.06\ +\ 343.08\ +\ 343.09\ +\ 341.06}{5} = 342.87\)

&nbsp;

Standard deviation (SD)

&nbsp;

\(=\frac{\sum (trial \ value\ -\ mean \ value )^2}{5}\)

&nbsp;

\(\frac{ =\ (343.06\ -\ 342.87)^2\ +\ (344.06\ -\ 342.87)^2\ +\ (343.08\ -\ 342.87)^2\ +\ (343.09\ -\ 342.87)^2\ +\ (341.06\ -\ 342.87)^2 }{5}\)

&nbsp;

= 1.10

Rate of the reaction =&nbsp;\(\frac{1}{Time \ taken\ for\ the \ color\ to \ change}=\frac{1}{342.87} = 2.92 × 10-3 s-1\)

Absolute error in rate = absolute error in time recorded = &plusmn; 0.01 s

&nbsp;

Thus, percentage error in rate&nbsp;\(=\frac{absolute\ error \ in \ rate }{rate}× 100= \frac{± 0.01}{ 2.92 × 10^{-3}}× 100 = 342.87 %\)

&nbsp;

The magnitude of percentage error is abnormally high as the value of rate is extremely low.