BLEACH- THE ACTIVE INGREDIENT

Transcription

1 1 BLEACH- THE ACTIVE INGREDIENT INTRODUCTION How does one measure the concentration of something in solution? It is easy enough to weigh a solid prior to dissolving it, but what if the solution is already made up? There are two methods that can be used. One involves using an instrument to make the measurement. This is usually a spectrometer of some description and we will meet more of this later in the course and in subsequent laboratory work. The other method requires a precise measurement of a chemical reaction and that will be the focus of this experiment. The stoichiometric equation of a reaction describes the exact ratio of reagents that must be added together to obtain a complete reaction. If we have a method of accurately determining when complete reaction has taken place, then we can use the stoichiometry to tell us something about the quantity of one of the reagents present. For example, consider a compound A in solution. If we know that reagent B will react with A to give C according to the equation: A + B 6 C then by knowing when there is an excess of B present (and no A left), the reaction may be assumed to be complete. Doing this carefully will allow an exact determination of the quantity of A present and subsequently its concentration may be calculated. In this experiment, the amount of the active ingredient, sodium hypochlorite (NaClO), in household Bleach will be determined. The oxidizing action of the hypochlorite ion, ClO -, kills germs and also decolorizes many stains and dyes. Being a strong oxidizing agent, the hypochlorite ion reacts with iodide ion in acid solution to release free iodine as below. ClO - + 2H + + 2I - 6 I 2 + Cl - + H 2 O Equation 1. The iodine released then may be titrated with a standard (known concentration) solution of sodium thiosulphate (Na 2 S 2 O 3 ). 2S 2 O 3 + I 2 6 2I - + S 4 O 6 Equation 2. In this reaction, thiosulphate ion is reduced to the tetrathionate ion. The quantity of hypochlorite ion in a sample of household bleach may be determined by measuring the amount of thiosulphate required to react with the iodine released by the reaction of the hypochlorite with the iodide ions. The easiest way to determine the exact amount of thiosulphate is by titration. Titration is a process of adding small increments of one reagent solution (titrant) slowly to a known quantity of another chemical solution, until the reaction is complete. An indicator is added to show when the end-point has been reached.

2 2 Starch is used as the indicator in this titration. Starch forms a deep blue complex with iodine. If no iodine is present, there is no blue colour. So, the endpoint of the titration is indicated by a colourless solution. There is one important point about using starch as an indicator. It tends to form an irreversible (or very slowly reversible) complex if it is added when lots of iodine is present. To circumvent this, the starch is not added until the titration is partly done, when the natural colour of iodine has started to lighten from dark red-brown to amber. Sodium thiosulphate cannot be weighed out accurately as it is not considered a primary standard. Consequently, the sodium thiosulphate solution has to be standardised to determine its concentration. Standardisation involves reacting (in this case, titrating) the sodium thiosulphate solution with a dry and pure reagent that can be weighed out accurately. This latter reagent is known as the primary standard. Primary standards are reagents that are >99% pure and do not readily pick up water, i.e. dry and are easily weighed. The sodium thiosulphate will be titrated against a solution of iodine. As iodine is a volatile red/brown solid which is difficult to weigh out accurately, it will actually be generated in solution and then will react with the thiosulphate. Iodine will be generated from potassium iodate which can be weighed accurately as it is a primary standard. The following equation describes the generation in situ of iodine from iodate: IO I - + 6H + 6 3I 2 + 3H 2 O Equation 3. The iodine thus formed then reacts with the thiosulphate as described below: 2S 2 O 3 + I 2 6 S 4 O 6 +2I - Equation 4. EXPERIMENTAL PROCEDURE Throughout the experiment, document all observations and data. The copy of your in-lab notes will constitute a part of your report. Before starting the titration, check the Appendix for information as to how to set up a table to summarize titration data. Part 1. Standardisation of Na 2 S 2 O 3 Solution Preparation of starch Please see the Chem101 lab website for the preparation of the starch solution. This information is to be included on your flowchart. If it is not there, you will not be able to write the quiz. Preparation of sodium thiosulfate solution In a 400 ml beaker, dissolve sodium thiosulfate pentahydrate, Na 2 S 2 O 3 C5H 2 O, (5.6 g) in about 250 ml distilled water. Stir well to make sure the solution is homogeneous.

3 3 Standardisation of thiosulphate with iodate Weigh accurately, to the nearest 0.1 mg, about 0.1 g potassium iodate (AnalaR grade) into a clean 125 ml Erlenmeyer flask and dissolve in about 25 ml distilled water. Add potassium iodide (2 g) and 3 M sulfuric acid (5 ml). As necessary, see the Appendix for the use of a buret and doing a titration. Read the initial volume and titrate the iodate solution until a reddish-orange (amber) colour appears. Add starch (2 ml) and continue the titration until the solution goes colourless. Don t forget to document the final buret reading. Repeat the standardisation a second time. Part 2. Analysis of Household Bleach Collect about 5 ml of Bleach solution provided in the lab. Note all the appropriate information about the Bleach solution in your notebook (See Appendix). Pipette (See Appendix) accurately 1.00 ml of the Bleach solution into a 125 ml conical flask. Add water (50 ml), 3M H 2 SO 4 (10 ml), and KI (2g). Swirl the solution to ensure a homogeneous solution. Titrate (see Appendix) this solution with the thiosulphate solution until the solution in the flask becomes a pale amber (yellow) colour. Add 2 ml of starch indicator to the flask and continue the titration until the blue starch colour has disappeared. Don t forget to record the final buret reading. Repeat the titration a second time. DATA MANIPULATION Part 1. Standardization of Na 2 S 2 O 3 Solution 1. Determine the number of moles of KIO 3 that was used in the titration. 2. It can be seen in Equation 3 that for every mole of IO 3 - used 3 moles of I 2 are generated. It can also be seen in Equation 4, that 2 moles of thiosulphate react with 1 mole of iodine. 3. In order to relate these two equations, Equation 4 needs to be multiplied by 3. This gives 6S 2 O 3 + 3I 2 6 3S 4 O 6 +6I - Equation 5. Now it can be determined that the relationship between thiosulphate and iodate is 6 to 1. In short, 1 mole of iodate generates 3 moles of iodine and 6 moles of thiosulphate reacts with three moles of iodine. Calculate the number of moles of thiosulphate used in the titration. 4. Knowing the volume of thiosulphate added and the number of moles (calculated above) calculate the concentration of the thiosulphate solution. 5. Repeat these calculations for the second titration. Determine the average concentration in

4 4 moles per liter of the thiosulphate solution. 6. See the Appendix and calculate the standard deviation and the relative precision, %RSD. 7. It will be the average concentration that will be used in the calculations below. Part. 2 Analysis of Household Bleach 1. For each trial: a. Calculate the number of moles of thiosulphate added in the titration, using the concentration of the S 2 O 3 solution (See Part 1 above) and the volume of thiosulphate solution added. b. Examine Equation 2 and determine the stoichiometric (molar) relationship between thiosulphate and iodine. Using this value, calculate the number of moles of I 2 that had reacted. c. From Equation 1, it can be seen that for every one mole of hypochlorite present, one mole of I 2 is generated. Note the number of moles of sodium hypochlorite and the reasoning behind how the value was obtained. d. Calculate the grams of sodium hypochlorite present in the sample that was titrated. 2. Determine the average amount (g) of sodium hypochlorite present in the sample. 3. Determine the % weight/volume (weight(g) of NaClO / volume(ml) of solution expressed as a percentage) of NaClO solution. 4. Calculate the standard deviation and the %RSD. REPORT As this is an analytical experiment, the emphasis in the report is on the data and calculated results. The report is to contain an Abstract, Data and results (see below), Calculations, Discussion, Conclusion and References sections. Use the Appendix as guideline for the report and also include the following. For the Data and Results section: Part 1.Standardization of NaS 2 O 3 solution Present all data in a table. Include weight of potassium iodate, final buret reading (ml), initial buret reading (ml), volume of thiosulphate solution used (ml), moles of KIO 3, moles of thiosulphate, [S 2 O 3-2 ] in moles per litre, average [S 2 O 3 ], standard deviation, %RSD. Part 2. Analysis of the Bleach solution

5 Present data in a table. Include final buret reading (ml), initial buret reading (ml), volume of thiosulphate added (ml), average [S 2 O 3 ], moles of S 2 O 3, moles of I 2, moles of NaClO, grams of NaClO, average grams of NaClO, % weight/vol. of NaClO, s, %RSD. For the Calculations, give all the calculations used to determine each of the values given in the tables above in your report. Ensure that the calculations are clearly labeled. If no calculations are presented then no marks will be given. For the Discussion, follow the guidelines as outlined in the Appendix. In addition, give the equations that describe the reactions that occur during the standardization. Give the equations using the symbols of the components and the whole words. For example : Na + + Cl - 6 NaCl, one mole of sodium ions plus one mole of chloride ions combine to give one mole of sodium chloride. As well, give the equations for the reactions that occur during the titration of the bleach. Again give both word and symbol representation as above. Clearly indicate which reactions belong to which part of the experiment. A Conclusion for an analytical experimental report typically only contains the details of what was analysed, how much of whatever was being anaylsed is present and the standard deviation and %RSD. 5 ADDITIONAL SOURCES T.L. Brown, H.E LeMay, B.E. Bursten, J.R. Burdge, Chemistry, The Central Science, 9 th ed. pp. 20, 140, 890. (Pearson Education Inc.: Upper Saddle River, NJ) T.L. Brown, H.E LeMay, B.E. Bursten, J.R. Burdge, Chemistry, The Central Science, 10 th ed. pp. 20, 152, (Pearson Education Inc.: Upper Saddle River, NJ) K. Hartmann, Chemistry 121 lab notes. University of Victoria, B.C., University of Victoria B.C.: last accessed Oct PRE-LABORATORY QUIZ QUESTIONS A closed book laboratory quiz of 10 minutes of similar questions to these will be given at the start of the laboratory period. 1. What is the object of this experiment? 2. In one sentence each, explain the terms, titrant and titration. 3. Give the number of significant digits in the following measurements. a) grams b) 0.26 ml c) 4.03 ml d) grams e) metres 4. Explain why all volumetric glassware must be read at eye level. 5. What glassware is used to measure out the solution of starch during the titration and why that piece of glassware?

6 6 6. What does a solution look like when all solids have been dissolved? 7. During the preparation of some solutions a kimwipe is used, when and why is it used? 8. In the standardization of the thiosulphate solution, part 1 of the experiment, why does the flask to which the KIO 3 is added not need to be dry at first? 9. After carefully cleaning a buret or pipet, why is it necessary to prerinse it with small amounts of the solution to be delivered? 10. In this experiment, which solution is the titrant and which solution is put into the buret? 11. What is the purpose of the white tile placed under the conical flask during the titration? 12. Why is the inside wall of the conical flask rinsed with a small spray of distilled water just before the end point? 13. What does it mean to standardise a solution? 14. Discuss what a primary standard is and what properties it has. 15. What compound is used as the primary standard in this experiment? 16. What is the molecular weight of Na 2 S 2 O 3 C5H 2 O? 17. A student did the titration for the standradisation as outlined in this experiment and got the following data. Volume of sodium thiosuphate added in titration : ml Moles of sodium thiosuphate present in flask: X 10-3 What is the concentration of the sodium thiosulphate? 18. A student did the titration for the analysis of the bleach following the directions in this experiment and got the following data. weight of NaClO in trial 1: 4.47 x10-2 g What is the % weight/volume of NaClO?