The Iodine Clock Reaction Practical Report

Abstract

Clock reactions have been a method of studying chemical kinetics over the years and are an example of an initial rates method (Braid and Train, 2015). The rate can be measured by the time taken for an observable endpoint to occur and in the iodine clock reaction, this is a colour change of blue/black. Varying the concentrations of a reactant whilst keeping others constant will result in different times for colour changes to occur. Based on this particular experiment, varying the volume/concentration of ascorbic acid has resulted in varying times for colour changes to occur. This experiment indicated that decreasing the volume of ascorbic acid being added to the solution results in a shorter time taken for the colour change to occur and this matches the expectation of the hypothesis.

Introduction

“Clock reactions contain a complex mixture of chemicals that react to cause a physical change after a certain amount of time (induction period)” (Faculty.sites.uci.edu, 2015). In a clock reaction, the time taken for an amount of product to form changes when the concentration of one of the reactants is varied which is why they are referred to as ‘clock reactions’ – due to different physical changes occurring at different time intervals. There is usually an obvious observable endpoint, such as a colour change which indicates when the desired product has been formed.

In this iodine clock reaction, the reaction being monitored is:

I₃⁻ + starch  blue-black complex

However, in order to form the triiodide ion, the following reaction has to occur:

                                3 I⁻ + 2 H⁺ + H₂O₂ I₃⁻ + 2 H₂O   (The iodine was from a potassium iodide solution).

To convert the triiodide ion back to the iodide ion which does not form a complex

when reacted with starch, ascorbic acid must be added:

Ascorbic acid + I₃⁻ Dehydroascorbic acid + 3 I⁻ + 2 H⁺

It is only when all of the ascorbic acid is used up that the reaction will take place because there will be enough triiodide ion to react with the starch (Ferrier, C. 2018).

Aim

The aim of this practical was to determine an experimental design for the iodine clock reaction to occur using a group researched method such as varying ascorbic acid volumes to convert the triiodide ion back into the non-complex forming iodide ion in a specified amount of time (47 seconds) and this was to be indicated by a colour change from colourless to blue/black.

Hypothesis

Decreasing the volume of ascorbic acid added to the solution will reduce the time taken for the colour change to reoccur from colourless to blue/black.

Method

First and foremost, three test tubes were labelled 30μL and were then placed into a test tube rack. Using the P200 micropipette and the matching blue tips, 200μL of potassium iodide, 200μL of hydrogen peroxide and 100μL of starch solution were mixed together into each test tube (ensuring that a different blue tip was used for each solution). This resulted in a blue/black colouration. The P200 micropipette and a new blue tip was then used to add 30μL of ascorbic acid to one test tube, starting the stopwatch at the same time as well. Upon adding the ascorbic acid, the solution was colourless. The time taken for the blue/black colour to return was then recorded in a table. This was then repeated 2 more times for the other 30μL test tubes. The volume of potassium iodide, hydrogen peroxide and starch solution stayed constant when carrying out the above steps for the other volumes which were 50μL, 70μL and 100μL. The results were then recorded as a graph and a line of best fit was drawn. This would have allowed for the volume of ascorbic acid to be calculated for the given time which was 47 seconds.

Results

The following results consist of model data for the experiment provided by the module leader. This is due to the fact that the data collected during the experiment was insufficient to conduct a lab report from and therefore, model data has been used to allow for accurate data analysis and conclusions to be drawn.

Table 1:  This contains the iodine clock practical model data (including repeats) of various volumes of ascorbic acid against the time taken for colour change to occur in seconds.

Volume of ascorbic acid (μL)

Time taken for colour change (s)

30

15

30

35

30

22

50

40

50

43

50

45

70

71

70

66

70

76

100

88

100

100

100

120

Table 1 indicated that the greater the volume of ascorbic acid added to the solution, the longer it took for the colour change to occur from colourless back to blue/black. Whilst the results did show a few anomalies, for example when 35 seconds was achieved for 30μL of ascorbic acid compared to values of 15 and 22 seconds, an overall increase of time taken for colour change can be seen when the volume is greater.

Figure 1: This is a scatter graph which (includes a line of best fit) shows the volume of ascorbic acid in microlitres against the time taken for colour change to occur in seconds. The graph also contains an equation which allows the approximate volume of ascorbic acid for the time to take 47 seconds for a colour change to be calculated as well as the R² value.

The graph in figure 1 shows an overall increasing positive trend with the time taken for colour change to occur increasing when the volume of ascorbic acid is increased. Each volume had 3 repeats which allowed for a line of best fit to be drawn accurately. The Excel application has also calculated the equation from which the approximate volume of ascorbic acid can be calculated for 47 seconds. By making Y equal to 47, the volume of ascorbic acid can be calculated.

Discussion and Conclusion

Nonetheless, the hypothesis has been supported by the data collected during this experiment. The hypothesis was that decreasing the volume of ascorbic acid added to the solution will reduce the time taken for the colour change to reoccur from colourless to blue/black.  As shown in table 1 and figure 1, the taken for the colour change to reoccur was less when the volume of ascorbic acid was at 30μL (ranged between 15 and 35 seconds) compared to when the volume of ascorbic acid was 100μL (88-120 seconds). The graph also calculated that for 47 seconds to be the time for the blue/black colour to return, the volume of ascorbic acid had to be approximately 51μL and the volumes of potassium iodide(200μL), hydrogen peroxide(200μL)  and starch solution(100μL)  had to remain constant at room temperature. 

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This was anticipated because in the iodine clock reaction, the triiodide ion reacts with starch to form a blue/black complex. But in order to form the triiodide ion, iodine must react with hydrogen peroxide. The triiodide ion cannot form the iodine blue/black complex whilst this reaction is occurring and therefore, ascorbic acid is added to the solution to prevent this from happening and is indicated by a colourless solution. As long as there is any ascorbic acid present, the triiodide ions will be reduced back to the iodide ions (Wright, S. 2002). Only when the ascorbic acid is all used up, the triiodide ion accumulates and shows a visible colour change to blue/black (Shakhashiri, B. 1992). This is the reason for less time being taken for a colour change to occur, when the volume of ascorbic acid is lower, due to there being less ascorbic acid to reduce the triiodide ions to iodine to form a blue/black complex.

The resources chosen to be referenced in this practical report were done so after critically analysing each bit of information that was considered to be of good use for the background knowledge of this experiment. The fact that the information provided linked to the results collected in this experiment indicates the reliability and accuracy of the resources referenced in this report.

Additionally, there are some changes that could have been applied to the experimental method for improvements. This includes ensuring there is enough time so that no data is left incomplete which was the main issue during this practical. Due to lack of timing, the graph was not completed, and many aspects of the practical had been rushed. This meant that the results were inaccurate and prone to human error, and so model data provided by the module leader had to be used in this report to form a basis for the conclusion. Moreover, if this experiment was repeated, perhaps conducting some more background research on the actual theory behind iodine clock reactions would have been beneficial to provide greater context.

Reflection

Overall, I have benefitted notably from this experimental design practical and the Critical Skills module on a whole. During this experimental design practical, although it was in a group setting, I have gained cognitive skills that have bettered my individual knowledge of experiments and scientific research. For example, there were times during the experiment where I had to accurately measure out specific volumes of solutions using the micropipette which I had yet to use before this experiment. This has not only increased my confidence in measuring accurate volumes but has allowed me to experience an apparatus I had not used before and would have been prominent in other practicals throughout the course. As an aspiring biomedical scientist, it is very important to carry out experiments appropriately, in regard to the correct usage of apparatus, the accuracy of measurements and health and safety considerations. This is a skill I have acquired from Critical Skills as I had to thoroughly read through the proformas beforehand as well as pay attention to the in-class demonstrations. A weakness that I hope to refine using what I have learnt during the Critical Skills module is my affective skills due to the fact that I was not very open with the other members of my group. I was very hesitant in contributing and giving ideas when it came to the experimental design and looking back, I feel this was the flaw that prevented us from finishing the practical on time and achieving accurate, reproducible results. In future, I hope to communicate my thoughts and ideas more with my peers and shall do so by writing a list of aspects I would like to speak on. Another weakness I hope to refine using the knowledge I have gained from Critical Skills is working independently, as this is very common in my future career and whilst I can seek advice and help from others, it is more beneficial for me to tackle the task myself at first. In conclusion, the overall success behind this experimental design experiment was down to the fact that it was conducted in a group setting and that many skills that I applied during the practical, I had recently acquired from attending Critical Skills lectures and workshops.

Bibliography

Braid, K. and Train, B. (2015). A-Level Year 2 Chemistry. Newcastle Upon Tyne: Coordination Group Public, p.58. [Accessed 04 Dec. 2018].

Faculty.sites.uci.edu. (2015). Clock Reactions [online] Available at: http://faculty.sites.uci.edu/chem2l/files/2011/04/A01MANClockRxn.pdf [Accessed 5 Dec. 2018].

Ferrier, C. (2018). Week 8: Experimental Design [lecture notes].Critical Skills. Available from: https://learning.westminster.ac.uk/ [Accessed 05 Dec. 2018].

Wright, S. (2002). The Vitamin C Clock Reaction. Journal of Chemical Education, [online] 70(1), pp.41-43. Available at: https://pubs.acs.org [Accessed 5 Dec. 2018].

Shakhashiri, B. (1992). Chemical demonstrations. Volume 4. Madison (Wis.): University of Wisconsin Press, p.22. [Accessed 05 Dec. 2018].