Experiment 2: Analysis of Commercial Bleach Solutions

Transcription

1 Experiment 2: Analysis of Commercial Bleach Solutions I. Introduction The ability of household bleach to remove stains is related to the amount of oxidizing agent in it. The oxidizing agent in bleach is sodium hypochlorite, NaOCl, which dissociates in water as follows: NaOCl (aq) Na + (aq) + OCl - (aq) (1) In this experiment we will determine, by titration, the % NaOCl in commercial bleach solutions. We will compare the concentration of sodium hypochlorite available in a name-brand vs a generic bleach. The method by which this will be accomplished is to first add an excess of potassium iodide, KI, to an acidified sample of diluted bleach. The following reaction will occur: OCl - (aq) + 2H + (aq) + 2I - (aq) I 2(aq) + Cl - (aq)+ H 2 O (l (2) The produced product, I 2(aq), is yellow when dilute and red-brown when concentrated. The amount of I 2(aq) formed by this reaction will then be measured by titrating it with a standardized solution of sodium thiosulfate, Na 2 S 2 O 3, which reacts with I 2 (aq) as follows: 2S 2 O 3 2- (aq) + I 2(aq) S 4 O 6 2- (aq) + 2I - (aq) (3) From the volume of standardized Na 2 S 2 O 3 solution used, the % NaOCl in the sample can be calculated. The instructions for this experiment are written in the traditional, compact form. This form does not include reminders to clean/zero the balance before use, or reminders to rinse pipet/buret first with deionized water, then with the solution to be measured. These are considered to be basic laboratory techniques, which you would know to do without being reminded. All Chem 401 experiments will be written this way. Primary Standards A primary standard is a material that can be quantitatively used to determine the concentration of other substances. In order to be a good primary standard, a substance must be a) of high purity b) stable c) no waters of hydration (not hydroscopic or hygroscopic) d) have a high molar mass e) be soluble in the solvent of interest. In this experiment, we need to know the concentration of the sodium thiosulfate very accurately, because it will be used to stoichiometrically determine the concentration of hypochlorite ion in our bleach. However, it forms a hydrate, with 5 waters of hydration. This means that it cannot be weighed out accurately as a primary standard. We must prepare it at an approximate concentration and then standardize it against a primary standard. The primary standard that we will be using is potassium iodate, KIO 3. Potassium iodate can be used to standardize the sodium thiosulfate by the following reactions. First, a known amount of KIO 3 (limited reactant) is reacted with an excess of KI in an acidified solution: IO 3 - (aq) + 5I - (aq) + 6H + (aq) 3I 2(aq) + 3H 2 O (aq) (4) Rev:

2 This produces a known amount of I 2 in solution. (You may be asking yourself, why didn t we just weigh out the right amount of iodine and dissolve it? The reason is that it cannot act as a primary standard either. Iodine is not stable enough. It sublimes at a fairly low temperature!) This known iodine concentration can then be titrated against the unknown (or only approximately known) thiosulfate concentration, based on the reaction: 2S 2 O 3 2- (aq) + I 2(aq) S 4 O 6 2- (aq) + 2I - (aq) (5) Note this is the same reaction as shown for the analysis of the bleach above (3). During the first half of the experiment, the concentration of the thiosulfate solution is the unknown that we are standardizing against a primary standard. Once its concentration is known, it becomes a secondary standard, which can be used to determine the concentration of an unknown concentration of iodine. This time the iodine will come from the reaction with the hypochlorite we are trying to determine. II. Experimental Procedures A. Equipment needed Set-up in lab: 25mL burette, funnel Chemicals in lab: Na 2 S 2 O 3 5H 2 O, KIO 3, KI, 1M HCl, 1% starch solution, name-brand (Clorox) bleach, generic bleach B. Waste Disposal Pour all titrated solutions and excess left-over reagents into the large bottles labeled "Used I -, S 4 O 6 2-, Starch, Bleach Waste". C. Experimental considerations: 1. The concentrated bleach solutions can cause bleach spots if splashed on clothing. Be sure to use with caution and to keep goggles on at all times. You may want to wear a protective coat or apron during this experiment. D. Before Starting Experimental Work (Before Class) 1. In your notebook, enter the experiment title, date, your name and name of partner. Number each page in your entire notebook if you have not already done so. 2. Write the purposes of the lab. 3. Write out all reactions that are involved in the analysis. 4. Write an executive summary of the procedures described in this lab. Include all major procedural steps that you will need to follow. E. Procedure 1. Preparation and standardization of 0.05 M Na 2 S 2 O 3. a. To prepare the ~0.05M sodium thiosulfate solution: Add approximately 1.9 g of Na 2 S 2 O 3 5H 2 O into a large (250 or 500mL) beaker or Florence flask. Record the weight in your lab notebook. Add approximately 150 ml of deionized water (with your large graduated cylinder) and stir or swirl the mixture until the Na 2 S 2 O 3 5H 2 O has completely dissolved. Condition and fill a ml buret with the sodium thiosulfate solution. b. To prepare the potassium iodate (primary standard) solution: Accurately weigh out 0.24 g of KIO 3 (using the most sensitive balance available) into a weighing boat. Record the weight in your lab notebook. Transfer to a 100mL volumetric flask. Add enough deionized water to half fill it. Rev:

3 Swirl the mixture until the KIO 3 has completely dissolved. Carefully add more DI water until the bottom of the meniscus reaches the mark on the neck of the flask. Cap the flask and invert it with shaking times to insure complete mixing. c. Preparation of a solution of known iodine concentration: Pipet ml of the KIO 3 solution into a clean 125 ml or 250 ml Erlenmeyer flask. Add 30 ml deionized water and 0.5 g KI (about 1 small scoop). Swirl the mixture until the KI completely dissolves. Add 10 ml of 1 M HCl (using your small graduated cylinder) and swirl for 1 minute. A brownish red color will appear due to the presence of aqueous iodine. d. Standardization of the sodium thiosulfate (secondary standard) solution: While swirling the solution from step c, add the Na 2 S 2 O 3 solution from the buret until the brown fades to yellow. Add ~2mL of starch suspension (using your small graduated cylinder). The solution in the flask should turn dark blue because of a complex that starch forms with iodine. (You do not need to record the volume at which you added the starch!) Continue adding the Na 2 S 2 O 3 solution drop-wise with swirling until the dark blue color disappears. (A little blue adhered to the outside of the starch particles may still be present.) Record the total volume of Na 2 S 2 O 3 solution delivered (from the start of the titration to where the indicator color has disappeared) to 2 decimal places in Table 1. Pour this titrated solution into the large bottles labeled "Used I -, S 4 O 6 2-, Starch, Bleach. Additional rinses do NOT need to go into the waste container. Repeat Steps c and d two more times, for a total of 3 titrations. If all volumes delivered agree within 0.2 ml, continue to Procedure 2. If not, show your results to your instructor. You may be required to perform a 4 th titration. F. Procedure 2. Titration of the Liquid Bleach a) To a clean 125 ml or 250 ml Erlenmeyer flask add 30mL deionized water and 0.5 g KI (about 1 small scoop). Swirl until the KI has dissolved. Add 10 ml 1M HCl using your small graduated cylinder and swirl to mix. Add 0.500mL of the name brand (e.g. Clorox) liquid bleach. Swirl for 30 seconds, and then titrate with the Na 2 S 2 O 3 solution until solution turns from red-brown to yellow. Add ~2mL starch suspension and titrate until the dark blue color disappears. (A little blue adhered to the outside of the starch particles may still be present.) b) Repeat this procedure with 1 more sample of the name-brand bleach. Average the results of your two titrations. c) Repeat steps a and b with a sample of generic bleach. d) Review your results with your instructor. If your titrant volumes vary too greatly for a given bleach, you may be asked to do one more titration if time permits. e) After your instructor has approved the values, pour the unused KIO 3, 1M HCl, and Na 2 S 2 O 3 solutions into the large plastic bottles labeled "Used I -, S 4 O 6 2-, Starch, Bleach". f) When finished, rinse your buret several times with DI water, fill it with DI water, stopper it, and return it to its original location. Also refill your DI wash bottle for use in the next experiment. Rev:

4 III. Further Instructions A. Required Calculations 1. Q-test: The Q-test is a statistical test to evaluate potential outlier points in a data set. The calculations for the Q-test are explained in detail in the introduction to this lab manual. Apply the Q-test to see if any of the 3 or 4 volumes from your standardization of sodium thiosulfate (Procedure 1d/e) should be eliminated from the average volume calculation. Be sure to list the suspect value, calculated value of Q, the critical value of Q from the table, and your decision to accept or reject the suspect data point. 2. Determination of average volume of sodium thiosulfate solution used: Calculate the average volume of sodium thiosulfate used to titrate the 10.00mL portion of the potassium iodate primary standard. In determining the average, only use those values that the Q-test told you to retain. 3. Determination of the concentration of the sodium thiosulfate solution. Use the average volume of sodium thiosulfate to calculate the molarity of the Na 2 S 2 O 3 solution. Use the stoichiometry from the reactions shown in the introduction section. Be sure to use good dimensional analysis and show all of your units! 3a) First calculate the molar mass of KIO 3. 3b) Next calculate the molarity of the KIO 3 solution. 3c) Calculate the moles of KIO 3 (or IO 3 - ) in your 10.00mL aliquot. 3d) Use stoichiometric factors (from equations 4 & 5) to convert mole IO 3 - to mole S 2 O f) Calculate the molarity of the S 2 O 3 2- solution from the average titration volume determined in calculation Determination of the % NaOCl in the bleach: From the average volume of Na 2 S 2 O 3 used in Procedure 2, calculate the % NaOCl in each bleach solution. 4a) Calculate the mol S 2 O 3 2- used based on the average titration volume for the name-brand bleach from procedure 2, and the molarity of Na 2 S 2 O 3 from calculation 3. 4b) Calculate the mol of OCl - in the aliquot of name-brand bleach using stoichiometric factors from equations 3 & 2. 4c) Convert mol OCl - to g NaOCl using the molar mass of NaOCl. 4d) Determine % (mass/volume) bleach (NaOCl) in the name-brand bleach. (Remember this is the mass of solute in grams per ml of solution.) Record your result in Table 2. 4e) Repeat calculations 4a to 4d for the generic bleach. B. In-Class Work 1. All data collection must be completed during class time. 2. Perform calculations A1, A2 & A3. C. Results: at home 1. Complete all remaining calculations in Section A 2. Place a copy of the Q-table critical values (from the Lab Manual Introduction) inside the back cover of your laboratory notebook. Rev:

5 D. Discussion and Experimental Summary: In your notebook, after completion of all experimental work and calculations, create the heading Discussion and Experimental Summary. 1. What percent stronger (or weaker) was the name-brand bleach versus the generic bleach? (% NaOCl in name brand bleach)-(%naocl in generic bleach) % difference= x100 (%NaOCl in generic bleach) 2. Write an executive summary that addresses the purpose of this lab, describing what was done and what was learned in this experiment, including any issues that were encountered and dealt with. Include critical values determined during the experiment. 3. Name 5 qualities of a good primary standard. 4. Why can t we use I 2 as a primary standard? 5. Place a copy of the Q-Table Critical values in your notebook for future reference. 6. Perform a Q-test on the following data. Draw a proper conclusion from your Q-test. Remember to state the calculated value of Q, the decision criteria, and the decision made. Data: 10.25mL 10.37mL 10.16mL 10.83mL 7. Most reactions that occurred during this experiment are oxidation/reduction reactions (Redox). For reactions 2, 3, and 4, fill in the following table. (Hint: decide for the iodine/iodide pair first and then fill in the oxyanions.) Reaction Substance Oxidized Substance Reduced Oxidizing Agent Reducing Agent Rev:

6 Rev:

7 Table 1: Standardization of Sodium Thiosulfate Solution. Mass of KIO 3 KIO 3 dissolved in ml H 2 O Mass of Na 2 S 2 O 3 5H 2 O Na 2 S 2 O 3 5H 2 O dissolved in ml H 2 O Run # (4) Aliquot Size (ml) Burette Ending Volume (ml) Burette Starting Volume (ml) Volume Delivered (ml) Note: All titrations also contained about: g KI and ml M HCl Table 2: Analysis of Bleach Name-brand Bleach Generic Bleach Run # 1 2 3* 1 2 3* Aliquot Size (ml) Burette Ending Volume (ml) Burette Starting Volume (ml) Volume Delivered (ml) %NaOCl Note: All titrations also contained about: g KI and ml M HCl * Run 3 is only required if assigned by instructor after review of runs 1 & 2 are completed for BOTH bleaches. Rev:

8 Rev: