The purpose of this laboratory was to determine the amount of chlorine and iodine in a sample of water by titration using a starch indicator and to standardize a sodium thiosulfate solution. Chlorine is added to municipal water supplies to purify it enough to become safe to drink. Iodine is also added to water when people camp or go hiking in the back country where they cannot bring purified water along. Chlorine and iodine are added to kill microorganisms in water. Oxidation reactions occurred in this experiment. The Chlorine was oxidized because it lost electrons in the reaction. The iodine was reduced because it gained electrons. The solutions turned a yellow color because of the iodine which disappears once all of the iodine has reacted in the titration. Sodium thiosulfate was the titrant in the process of titration. It was added to react with the iodine in the solution.
Starch was added to give the solution a blue color near the endpoint of the titration. Potassium iodate (KIO₃) was used to standardize the sodium thiosulfate solution. Practical applications would include testing unknown samples of water and municipal water supplies for the concentration of chlorine present because too much can cause health problems and not taste well. Not enough added, wouldn’t kill the microorganisms in the water to make it safe to drink.
First the standardization of Sodium Thiosulfate was completed. A 50mL buret was obtained and rinsed twice with the sodium thiosulfate solution. It was then filled with the solution. The tip of the buret was checked to make sure there weren’t any bubbles in it. Then a 250mL beaker was obtained. A 25mL pipette was used to add exactly 25mL of the KIO₃ solution. Then 50mL of deionized water and about .25g of solid KI was added. The solution was stirred until the solid was completely dissolved. 2mL of glacial acetic acid was then added. Then, the beaker was placed under the buret and was swirled as the sodium thiosulfate was added. The buret was slowed as the color became lighter. When the color was almost gone, 1mL of the starch solution was added to turn the solution blue. Then it was slowly titrated until the color disappeared. The final volume of the solution in the buret was recorded and the amount of titrant was calculated. This titration was repeated two more times to standardize the concentration of the thiosulfate. Then the concentration of Chlorine in Tap water was found. It was repeated the almost exactly the same way as before.
The buret was filled with the sodium thiosulfate solution and the initial value was recorded. Then a 50mL graduated cylinder was rinsed with tap water two to three times. The graduated cylinder was then filled with cold tap water. It was then poured into a clean beaker and .25g of KI and 2mL glacial acetic acid were added and stirred until the solid was dissolved. Then the solution was titrated. 1mL of starch was added then the yellow color almost disappeared. It was then titrated slowly until the blue color disappeared. The final volume was recorded and the volume of titrant used was determined. This was repeated two more times.
Then the concentration of iodine was determined in the iodine purified water. The buret was refilled and the initial volume was recorded. Then the 50mL graduated cylinder was rinsed with the iodine solution. 50mL of the iodine solution was then obtained and poured into a clean beaker. Then .25g of KI and 2mL of acetic acid were added and the solution was stirred until it was dissolved. Then it was titrated until the yellow color almost disappeared. 1mL of starch was then added and the solution was titrated slowly until the blue color disappeared. The final volume was recorded and the volume of titrant used was then determined. This was repeated two more times. Then everything was cleaned and put away.
III. Data and Results
Table 1: Standardization of Sodium Thiosulfate Solution
| Initial Buret Volume (mL)| Final Buret Volume (mL)| Volume Used (mL)| 1st Standardization| 50| 25.64| 24.36|
2nd Standardization| 50| 25.99| 24.01|
3rd Standardization| 50| 26.21| 23.79|
Table 2: Titration of Tap Water
| Initial Buret Volume (mL)| Final Buret Volume (mL)| Volume Used (mL)| 1st Titration| 50| 48.29| 1.17|
2nd Titration| 50| 48.25| 1.75|
3rd Titration| 50| 48.20| 1.80|
Table 3: Titration of Iodine Solution
| Initial Buret Volume (mL)| Final Buret Volume (mL)| Volume Used (mL)| 1st Titration| 50| 45.61| 4.39|
2nd Titration| 50| 45.59| 4.41|
3rd Titration| 50| 45.60| 4.40|
Table 4: Average Concentrations
| Molarity (moles/Liter)|
Sodium Thiosulfate Solution| 1.06×10⁻³|
Chlorine in Tap Water| 1.86×10⁻⁵|
Iodine in Solution| 4.66×10⁻⁵|
Sample Calculation- Concentration of Chlorine
(1.06×10⁻³)*(0.00175L)=1.86×10⁻⁶ moles thiosulfate
(1.86×10⁻⁶mol thiosulfate)*(1 mol Cl/2 mol thiosulfate)=9.28×10⁻⁷ mol Cl (9.28×10⁻⁷mol Cl)/.05L=1.86×10⁻³ moles Cl/Liter
In the first standardization of the sodium thiosulfate solution, 24.36mL of the titrant was used. 24.01mL and 23.79mL were used for the second and third standardizations. With the concentration of the sodium thiosulfate solution divided the average volume of those gave the average thiosulfate concentration which was 1.06×10⁻³ moles per liter. For the titration of tap water, an average of 1.75mL of the sodium thiosulfate solution was used. After the calculations, the concentration of chlorine in tap water was 1.86×10⁻⁵ moles of chlorine per liter.
The average of the titrations of the iodine solution was 4.40mL of titrant used. After the calculations, 4.66×10⁻⁵ moles of iodine were present per liter. Some useful observations were conducted. The iodine in the solution made it turn a yellow color when the glacial acetic acid was added. It started to disappear as the iodine was reacting with the titrant. When the starch solution was added, the solution turned blue. As the endpoint neared, the blue color started to dissipate. When the endpoint was reached, the solution was clear with no color left in the solution. Sources of error could include adding too much titrant from the buret into the solution.
The purpose of this lab was to identify the concentration of chlorine and iodine in different samples of water. The results fulfilled the purpose of this lab because the concentrations of chlorine and iodine were found. The concentration of chlorine in the tap water was 1.86×10⁻⁵ moles of chlorine per liter. The molarity of iodine in the iodine water solution was 4.66 x10⁻⁵ moles per liter. The concentration of iodine was significantly higher than the amount of chlorine in tap water. There was more iodine in the water because of the solid KI being added to the solution. Tap water contains small amounts of chlorine because that’s all that is needed to kill the microorganisms living in the water that are harmful to people. The results that were obtained were unexpected because there wasn’t a standard value to go off of. The percent error wasn’t able to be obtained.
Possible sources of error would include adding too much of the titrant to the solution. This would affect the results by having more than enough titrant used. This would affect the concentration values and make them higher than they really were. Chemical concepts used were titrations. When the Iodide ions, from the potassium iodide, react with the chlorine in the tap water, chloride ions and I₂ were formed. Then the I₂ was titrated with the sodium thiosulfate solution which created the iodide ions. Before the endpoint of the titration, the starch was added.
Once all of the iodine reacted with the thiosulfate, the blue color disappeared because the endpoint was reached and the iodide ions were formed. Also the standardization of the sodium thiosulfate solution was performed. When the potassium iodate reacted with the solid KI and the acetic acid, it created I₂ and water. The I₂ was titrated and formed with the thiosulfate and created iodide ions also. The concentration of the thiosulfate was 1.06×10⁻³ moles per liter. That result was expected because it should have been around .001 M and it was .00106 M.
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