Clock group of four people: William w1620499, Therese w1651229,

Clock reaction report


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Understanding the mains thing of the reaction we going to dilute the ascorbic acid in several dilution, to reduce the molarity of it, and delay the colour appear. Has been given 5ml per 50mM of ascorbic acid, moreover reducing the molarity of our acid we can make the reaction quicker and so will permit us to reach our time, that was 22seconds.

The lower molarity of ascorbic acid the faster the blue colour will appear.



The clock reaction is an experiment often done in elementary chemistry laboratory courses to illustrate solution kinetics. The simple iodine clock reaction involves a reaction sequence between some solutions and manage the time to have a precise reaction in a specific time using Hydrogen Peroxide, Potassium Iodine, ascorbic acid and starch.(Lambert, 1984)

Some of these components do not react with each other while are mixed together some of those do.

Because when starch binds to triiodide (I3-) anion the resulting complex is a characteristic blue-black colours, the triiodide ions I¬¬¬¬- from potassium iodide and hydrogen peroxide ( H2 O2.).

This reaction is 3 I- + 2 H+ + H2O2 ? I3- + 2 H2O

However, ascorbic acid (vitamin C) rapidly scavenges the I3- ion and converts it back to I- in the following reaction:

Ascorbic acid + I3- ? Dehydroascorbic acid + 3 I- + 2 H+

(Vitz, 2007)


This means that; If ascorbic acid is added, the appearance of the blue colour can be delayed until it is consumed the I3- ion until the colour will appear again.

What we going to understand is how to manage the time, what cause appear the colour and what make it delay to make appear the colour blue.


Materials and methods:

The material the will be using during the experiment are: 10 ml 3% Hydrogen Peroxide Solution,10 ml 1% Potassium Iodide Solution, 5 ml 50 mM Ascorbic Acid, 5 ml Starch Water, Distilled Water, Glass Tubes, A Stopwatch, A P1000 Micropipette and several blue tips, A P200 Micropipette and several yellow tips

In the experiment was a group of four people: William w1620499, Therese w1651229, Agatac w1669185, Angel w1660608.

 All the job has been divided between us, I was in charge of the pipetting, Angel took the time with the stopwatch, Agatac took the entire note an write it down and Therese in charge of directing  all the steps an make sure we fallowing all the procedure.

In the first step, we did several dilutions to sketch a graph. Using those different dilutions, we started to mix all of those to make appear the colour and understanding the time that it could take. We used these data to plot the graph using the lower and high point.

How to do:

1.      Label 5-glass tube 1-5

2.      Using the P1000 transferred 2000 µl of 50 mM ascorbic acid into tube 1

3.      Transfer 1000 µl distillate water into tube 2-3-4-5.

4.      Transfer 1000 µl of Ascorbic Acid from tube 1 to 2.

5.      Transfer 1000 µl of Ascorbic Acid from tube 2 to 4.

6.      Transfer 1000 µl of Ascorbic Acid from tube 4 to 5.


For each tube we Have different molarity of ascorbic acid in tube one is 50mM the tube two is 25mM tube three 12.5mM, tube four 6.25mM, tube five 3.125mM

Once we had all the dilutions, we started to mix the different components using the micropipette P200 following always the same order 1potassium iodine 100µl, 2starch 20µl, 3 Ascorbic acid 100µl, 4 hydrogen peroxide 100µl.

We repeated the experiment for each dilution 3 times. Reference point are on table 1.

Table 1 shows the different dilution of ascorbic acid and the time in second when the colour blue appear.

Assuming the fact that, dilution 3 was the closest one to our time, we decide to use this one to make several experiment; changing the volume of the ascorbic acid and we calculate the molarity of each time that we change the volume, because changing the volume of ascorbic acid, will change the molarity. Refer to table 2

 We calculate the molarity using the formula C1·V1 = C2·V2. (Concentration (start) x Volume (start) = Concentration (final) x Volume (final)).


Table  2 the table below shows the µl that has been used for our experiment and the result for each of calculation that we have done to calculate the molarity using the final volume, acid volume and the initial molarity of the vitamin C that was 12.5mM.



As reported previously the molarity of ascorbic Acid delay the colour appear from the reaction.

 After some experiment, we are now able to control the time and to obtain the time that we looking for; it is equivalent to 22second. So on using 80µl of ascorbic acid in 12.5mM that is equal to 3.33mM from the amount that we use.

Understanding the experiment, changing the molarity and volume of the ascorbic acid we were able to control the time and make the colour appear as fast and slow as we wanted.

We were given a second time +/- 10 sec it give us only a couple of minutes to sort it out  the amount of the same ascorbic acid that we should use to reach the time. We decided to take 32 sec  and our way it works we only miss for one sec because we forgot to mix the solution properly it cause some problem with the time. I plotted a graph below for better understand


Figure 1the graph above shows the time when the colours appear using different molarity of ascorbic acid.



Clearly our hypothesis has been prove the “The lower molarity of ascorbic acid the faster the blue colour will appear”.

This experiment permit us to work as team and to collaborate with each other and increase our knowledge in basic of chemistry. we spend a couple of hours before the practical to do some research and develop an idea into a hypothesis and create a plan to follow during our practical that could permit us to approve our hypothesis.



Optional additional experiment:

Ascorbic acid (vitamin C) is a water-soluble vitamin, which can be found in many biological systems and foodstuffs (fresh vegetables and fruits, namely, citrus).

 Knowing that the lemon contain ascorbic acid we decide to buy some of it and to squeeze and take some juice. We definitely had some reaction because of the vitamin C but the amount of acid was very low that the colour appear in very little time we try to repeat the experiment more time with a large amount of lemon juice but we did not filter the juice which had some bits and it compromise the pipetting.

We had 4 different time when we used 260µl of ascorbic acid we had :22-24-24-21 sec this is the closest that we could do with that lemon juice, we found very hard to pipet that juice it’s that take us very long time.

To success on it we should have more knowledge about the lemon juice doing some research about the amount of ascorbic acid that are contain in a litter and from there with the molecular weight we could calculate the molarity of the lemon juice and then with a filtered lemon juice we could have a perfect result.



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