Abstract
The purpose of this experiment was to find out the amount of Copper in a random weight of Copper(II) Sulfate given to us in a volumetric flask. I proceed to carry out the experiment by doing iodometric titration 3 times, using Sodium Thiosulfate as the titrator. 5 color changes were observed during this experiment. The last step to the titration was the most important as the color changed from dark blue to whitish pink. This tells us that the iodine was fully reacted with Sodium Thiosulfate. I used the average values of the 3 titrations to find out the unknown mass of the Copper in the Copper(II) Sulfate.
Introduction
Iodometry is use in the Pharmaceutical industry widely, mainly for development of drugs and quality control. Iodometry which is also known as iodometric titration is the process by which iodine is used in a solution and it is a redox (reducing and oxidising) reaction. This was based on the reduction of Iodine and oxidation of Iodide. The solution used in this experiment was Copper (II) Sulfate and it was the oxidising agent. Copper (II) Sulfate oxidized the excess Potassium Iodide (KI) added to an aqueous solution of Copper (II) Sulfate. KI was added to reduce the Cu (II) to Cu(I). Sodium Thiosulfate (Na2S2O3) was used as the reducing agent and it was used to titrate Iodine. A starch solution was used as the indicator to detect the endpoint of the titration.
Methods
Instruments used:
- Beakers
- Burette
- Conical flask
- Bulb pipette filler
- 25ml volumetric pipette
- 100ml volumetric flask
- Retort stand
- Burette clamps
- Precision Balance
- Weighing boats
- Pasteur pipette
- Displacement micropipette
Chemicals:
- Deionized water
- Concentrated Sulfuric Acid (H2S04)
- Sodium Thiosulfate (Na2S2O3)
- Potassium Iodide (KI)
- Potassium Thiocyanate (KSCN)
- Starch solution
- Copper (II) Sulfate( CuS04)
Procedures
- Ensure that lab coats, goggles and hand gloves are worn.
- Spray work desk with ethanol and do a wipe down
- Take all the necessary equipment and set up
- Weigh 0.20g of KI and KSCN using a precision balance
- Top up primed burette with 0.1M Sodium Thiosulfate
- Top up the given sample Copper (II) Sulfate solution to the 100ml mark in the volumetric flask
- Transfer 25ml from the volumetric flask to a conical flask and further dilute it with 25ml of deionised water. (50ml)
- Add 5 ml of concentrated Sulfuric acid by using a displacement micropipette.
- Add in the 0.20g of Potassium Iodide and immediately titrate with Thiosulfate solution of 5ml.
- Add in 3 drops of starch solution using Pasteur pipette.
- Mix the solution and continue to titrate with another 1ml of Thiosulfate solution and add 0.20g of KSCN
- Continue to titrate with constant swirling until the colour changes to whitish pink.
- Repeat the experiment for 3 times and record down the average of 3 values.
Results
At the end of the experiment, I got an average of 19.1ml of Sodium Thiosulfate. I took the average of 3 values 19ml, 19.3ml and 20ml. After doing the stoichiometry calculations the unknown mass of Copper in Copper (II) Sulfate (Sample B32) was found to be 0.25g.
Supplementary Questions
1. Compare iodometry and iodimetry
Iodometry is an indirect titration method while Iodimetry is a direct titration method. In Iodometry, iodine was produced due to a prior redox reaction. The iodine was measured through a separate titration and the concentration of the analyte that produced the iodine is determined. This technique is commonly used in experiments where the amount of oxidizing agent in a water body needs to be determined. On the other hand, in Iodimetry, the analyte is directly titrated with an iodine solution at the presence of a suitable indicator. By determining the end point of the reaction, equations can be derived, and we can use that to obtain information of the stoichiometry and other important relationships between iodine and reducing agent.
2. Explain each colour change or observation made throughout the experiment.
Throughout this experiment, there are 6 visible colour changes. Firstly, after diluting the Copper (II) Sulfate solution with 25ml of water it was blue in colour. After adding Sulfuric acid, it turns to light blue. It turns to brown when Potassium iodide (KI) was added and brown yellow when starch was added. Finally, when Potassium Thiocyanate was added it will turn to dark blue and with further titrating, the solution will turn whitish pink.
3. What applications of iodometric titrations can be seen in the pharmaceutical industry?
In the pharmaceutical industry, iodometric titrations can be used for determining dihydroartemisinin in tablets and in bulk powders.
Another application is using iodometric titration to determine exact amount of caffeine present in food and pharmaceutical samples.
In short iodometric titrations are used in the pharmaceutical industry for developing of drugs and quality control.
Discussion
In this experiment itself, the mass of Copper was determined by iodometric titration method. Iodide was a strong reducing agent while Copper (II) was a strong oxidizing agent. It is prevalent to use Iodometry to determine amount of copper in a solution.
Mistakes-
In this experiment there are a few common mistakes people tend to make including me myself. The first mistake that many of us did was misjudging the colour change near to the end point of the titration. It was hard for us to determine whether the colour was whitish pink. As a result, we titrated more Sodium Thiosulfate resulting in an inaccurate reading. The other common mistake was titrating too fast and some of us titrated with too much Sodium Thiosulfate. We should let the solution sit and settle down for a few seconds to observe the colour change.
Error-
An error in this experiment was the inconsistent starch indicator we took. The starch solution was shared among all the students doing the lab, so as time goes by the starch solute sink to the bottom of the container and was no longer fresh. As a result, the indicator did not work properly towards the end point. A solution to this is to let students prepare their own starch solution for each titration so that the experiment can be more accurate. Another error was human error and in this experiment it was totally unavoidable as the experimenter have no idea when the end point reaches exactly.
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Alternative ways to find Copper in a solution -
Another way to determine the amount of Copper in a solution is to use spectrophotometry.
Conclusion
In short, my goal was to find out the mass of unknown Copper present in the given sample (B32) Copper(II) Sulfate solution. Titration with Sodium Thiosulfate allowed me to find out the mass by writing out the chemicals equations involved and using stoichiometric calculations. The calculated mass of Copper was 0.25g
Appendix
Before the addition of Potassium Thiocyanate, there was already 6ml of titrate in the solution. After addition of 0.2g KSCN, the solution turned dark blue and the volume of titrate was carefully observed and titrated until the solution turns whitish pink. The table below show my results of each of the 3 titrations.
Table A – Titration values of Sodium Thiosulfate
- 1st titration (5+1+13)ml= 19ml
- 2nd titration (5+1+13.3)ml=19.3ml
- 3rd titration (5+1+14)ml=20ml
Average value
(20+19.3+19)ml/3 = 19.4
Chemical equations
2CuSo4 + 4KI 2CuI + I2 + 2K2S04
CuI + KSCN CuSCN + KI
2NaS2o3 +I2 Na2S4o6 +2NaI
Stoichiometric calculations
Concentration of Sodium Thiosulfate (0.1M) = 0.1mol/1000ml
Moles of Na2S2o3= (0.1/1000) x 19.4 = 0.00194
From the mole mole ratio of 2Na2S2o3 and 2Cuso4
2 moles of Na2S2o3 is needed to react with 2moles of CuSo4
Hence mole of CuS04 is 0.00194
Mole= mass/ molar mass
Molar mass of CuS04 = (63.546+32.065)+(16x2)=127.611
Mass of CuSo4= 127.611 x 0.00194 = 0.2475 ~ 0.25g
Risk Assessment
- High risk 12 and above
- Medium risk Between 5 to 11
- Low risk 5 and below
- Activity: Iodometeric titration of copper / Hazard / Risk / Likelihood of hazard / Hazard severity / Risk level (RL) / Control measure
- Copper (II) Sulfate, CuSo4 / Spillage / Possible- copper (II) sulfate is corrosive to skin and eyes and it is an irritant. / As copper (II) sulfate is corrosive to the eyes and skin. There will be an injury for over 3 days. / RL 9 / Medium risk / Appropriate PPE to be worn. Lab coat, gloves and lab goggles.
- Sulfuric Acid H2S04 / Spillage / Possible- there is a possibility of spilling the sulfuric acid while handling the chemical / Sulfuric acid is a highly corrosive chemical and it can burn through organic materials easily. For instance human skin. / RL 12 / High risk / Handling of acid should be done in a fume hood. Appropriate corrosion resistance gloves should be worn and extra PPE to be worn.
- Glass Apparatus / Breaking of glass / Unlikely- all the glassware are handled on table top surfaces. / Minor injury. May only get minor cuts. / RL 4 / Low risk / Make sure lab coat is worn at all times, to prevent accidental cuts.
- Sodium Thiosulfate (KSCN) / Spillage / Possible, KSCN causes irritation to eyes and skin. / When in contact with KSCN, there may be a risk of having temporary irritation on skin or eyes. / RL 5 / Low risk / PPE- wear lab coat and lab goggles to prevent excessive skin contact.
