Sodium hypochlorite in bleach lab write up: experiment 4 PDF

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Lab write up for experiment 4...

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Sodium Hypochlorite in Bleach Experiment 4

Abstract In this experiment, the molarity of sodium hypochlorite in Clorox bleach was determined through titration with thiosulfate solution. Diluted bleach was mixed with the reducing agent I- in excess solution, and a redox reaction occurred that formed Iodine, a brown liquid. This solution was titrated with thiosulfate until it turned clear, indicating that the colorless products were formed. From the volume of thiosulfate it took to reach the equivalence point, where the solution was clear, and the molarity of the thiosulfate used, it was possible to determine the molarity of the original undiluted sodium hypochlorite in bleach using balanced equations for the reactions. The molarity of the sodium hypochlorite in bleach was determined to be .109 M, with a percent precision of .445% for the volume of thiosulfate solution to titrate the iodine solution. Introduction The active ingredient in bleach is sodium hypochlorite, which is produced form the combination of chlorine gas and aqueous sodium hydroxide. Cl2(g) + 2NaOH(aq)  NaOCl(aq) + NaCl(aq) + H2O(l) In the aqueous solution, sodium hydroxide dissociates into Na+ and OCl ions, where OCl acts as an oxidizing agent, such as in laundry when it oxidizes the stains in clothing. Through this oxidizing ability, it is possible to determine the molarity of the hypochlorite ion in bleach. Mixing diluted bleach with a solution containing excess I- ion, a reducing agent, reduces the hypochlorite ion to a chloride ion and oxidizes the iodide ion to iodine. OCl-(aq) + 2I-(aq) + 2H3O+(aq) ⎯→ I2(aq) + Cl-(aq) + 3H2O(l) It is possible to measure the amount of iodine produced by this reaction through a redox titration with a solution of thiosulfate ion. I2(aq) + 2S2O32–(aq)  2I-(aq) + S4O62–(aq) brown colorless colorless colorless A color change occurs when the thiosulfate combines with the iodine solution as the solution begins a deep brown color, and then changes to clear as the reaction products, I-(aq) and tetrathionate ion, are both clear. Starch can be added to see this color change more clearly. The number of moles of thiosulfate can be determined through the titration, which leads to the number of moles of iodine, which allows the determination of the moles of hypochlorite in the diluted bleach, and finally the number of moles in the undiluted bleach solution.

Experimental Section

Procedure First, 40 mL of Clorox bleach was measured in a 100 mL beaker and labeled. A 10 mL pipet was used to transfer 10.00 mL bleach into a clean 100 mL volumetric flask. The pipet was rinsed with 5 mL bleach twice before transferring the 10.00 mL bleach. Distilled water was added to the flask until it was about two-thirds full, and after putting a stopper in, the contents of the flask were mixed by inverting the flask a few times. Distilled water was added again until it reached the ring mark on the flask, and the contents mixed again. The contents were then poured into a clean and dry 400 mL beaker. About 13 mL of diluted bleach was poured into a 50 mL beaker and the pipet was rinsed twice with 4 mL of the diluted bleach. About 50 mL distilled water was poured into a clean 250 mL Erlenmeyer flask, and with a graduated cylinder, 5.0 mL of 10.0% KI solution was added to the flask. Next, 10.00 mL of the diluted bleach was added from the 400 mL beaker to the Erlenmeyer flask. With a graduated cylinder, about 2 mL of 6 M HCl was added to the Erlenmeyer flask, and the contexts where swirled to mix. Then, 75 mL of .1215 M thiosulfate solution was poured into a 200 mL beaker. A buret was rinsed with distilled water and then with the thiosulfate solution. A dry funnel was used to fill the buret to exactly the 0.23 mL mark, making sure that the bottom was filled as well. The Erlenmeyer flask was placed on a sheet of white paper on a stirrer-hotplate below the buret, and a clean spin bar was placed in the flast. The spinning mechanism was turned on so that the solution in the flask was stirred fairly briskly. The thiosulfate was titrated fairly quickly into the Erlenmeyer flask containing the KI, diluted bleach, and HCl until the solution turned from a deep brown to a orange-yellow. The level of thiosulfate in the buret was measured and recorded to two decimal places, and then, using a scoopula, a small amount (about half the amount that could fit on a dime) of solid VitexTM soluble starch was added to the Erlenmeyer flask. Using a washbottle, distilled water was used to rinse the side of the flask into the solution. The thiosulfate was titrated slowly until the solution in the flask became colorless. The final buret reading was recorded. The stirrer was turned off, the contents of the flask were poured out, and the flask was rinsed with distilled water. Beginning from adding 50 mL distilled water in the Erlenmeyer flask with 5.0 10.0% KI solution, and then adding 10.00 mL diluted bleach and 2 mL 6 M HCL to the flask, the previous procedure of titration was repeated three more times. For each trial, enough thiosulfate was added to the buret for the titration, and the initial level measured and recorded to the nearest two decimals in order to final the difference after the final level was recorded. Note: Three trials were completed after the first trial because the first trial was very far off of a precise measurement due to errors in the procedure, which is described in the Error Analysis section. Trials 2,3, and 4 were within 1% precision of each other, so the data from those trials were used as reliable data in the calculations.

Results and Observations Results: Table 1 Initial reading on buret (mL)

Trial 1 .23

Trial 2 7.83

Trial 3 5.30

Trial 4 7.71

Reading near equivalence point: starch added (mL)

7.30

24.79

20.45

24.62

Reading at equivalence point (mL)

7.83

25.80

23.24

25.73

Difference in buret reading (mL)

7.60

17.97

17.94

18.02

Sample Calculation to find Difference in buret reading for Trial 2: 25.80 mL – 7.83 mL = 17.97 mL thiosulfate (aq) Table 2 Trial 1

Trial 2

Trial 3

Trial 4

Thiosulfate (aq) used in reaction (mL)

7.60

17.97

17.94

18.02

Molarity of NaOCl in undiluted bleach (M)

.462

.109

.109

.109

Sample Calculation to find molarity of NaOCl in undiluted bleach for Trial 2: 17.97 mL S2O32 x 1 Liter x .1215 mol S2O32 = .002183 mol S2O32 1000 mL 1 Liter 2 .002183 mol S2O3 x 1 mol I2 x 1 mol OCl = .001092 mol OCl 2 mol S2O32 1 mol I2 .001092 mol OCl x 1 mol NaOCl x 1000 mL = .109 M NaOCl 10 mL 1 mol OCl 1 Liter Observations: 10 mL diluted bleach added to KI and H2O HCl added to 10 mL diluted bleach, KI, and H2O solution Titration

Immediately when the 10 mL of diluted bleach added to KI solution, liquid turns to a semi-transparent deep orange-brown color. Upon adding the HCl, the brown solution instantly turned to a cloudy deep brown-black, as a precipitate was formed. Before being stirred, black precipitate settled to bottom of flask. During titration when solution was stirred constantly, the solution turned from deep brown to orange, to a deep lemon-yellow when it was close to its equivalence point. When the starch was added, the solution turned to a deep brown-purple, and slowly lightened to a light purple as more thiosulfate was added. At the drop before the equivalence point, the solution was a very, very light purple. At the equivalence drop, the solution became clear.

Additional Calculations: Average thiosulfate used in titration (mL): 17.98 Average molarity of NaOCl in bleach (M): .109 Percent precision of titration trials: (18.02 – 17.94) / (17.98) = .445% *Note that these calculations exclude trial 1 as the procedure was done incorrectly and thus the molarity calculated using this data was very far from being within 1% precision of the other three trials. During the procedure, an additional trial was done to substantiate the precision of trial 2 and 3. Discussion & Error Analysis All three of the molarities calculations from the titrations are approximately equal to the 3rd decimal place, so according to this procedure the molarity of sodium hypochlorite in Clorox bleach is close to .109 M. Since the exact molarity is unknown, this experimental molarity cannot be deemed accurate, but can be described as very precise, as all values of titrated thiosulfate solution (excluding Trial 1) are within . 445% of each other. If Trial 1 were included, the precision would be much more off and the average molarity would be off as well. The major error that took place in this experiment was that during Trial 1, the procedure was misread and the KI and distilled water were added in one beaker, and the diluted bleach and HCl were added in another beaker. Then, the contents of these beakers were combined. This was not the correct order that the reactants were supposed to be combined, so the resulting titration was very off. The titration was followed through with after the error to observe whether it would cause a major difference, which it did. This is why four trials were completed instead of three. Another source of error could be that the thiosulfate was titrated past the equivalence point. As it was very difficult to only titrate the thiosulfate one drop at a time, it is possible that too many drops were added to the solution, which would cause the measured molarity to be off....