hoi io i h
TRANSCRIPT
Experiment 10: Analysis of Iodine Sterilized Drinking Water
Iodine is an effective, simple and cost effective means of disinfecting contaminated water to provide clean drinking water in regions of extreme poverty or those devastated by natural disasters. Considering some recent natural disasters where clean water has become scarce, iodine purification is a practical application to demonstrate the use of chemistry in a real-‐world situation. At neutral to slightly acidic pH, three major iodine species exist in solution: free iodine, triiodiode and hypoiodous acid, with the amount of each species being pH dependent. The species that have the largest sterilizing effect are aqueous elemental iodine and hypoiodous acid, with the former having the greatest effect. Although the recommended dose may vary depending on the microorganisms present in the water supply, a dose of 2mg/L of aqueous iodine is generally considered effective. There are many commercial aqueous iodine sources but the two most common forms are an iodine tincture solution and a tablet containing tetraglycine hydroperiodide (Figure 1). The tincture solution is a mixture of 2–7% elemental iodine, with sodium or potassium iodide, dissolved in a mixture of ethanol and water. The iodine solution does not use a buffer to control the amount of aqueous iodine released but rather relies on ethanol to lower the solubility of triiodide to increase the amount of aqueous iodine. In the case of the tablet, tetraglycine buffers the water to pH 5.5 to ensure a high level of aqueous iodine is formed. As a consequence, an equal dose of the tincture solution only generates about half as much aqueous iodine as the tablet. Due to the lack of buffering, the tincture solution is not as effective as the tablet; hence the experiment will focus on he iodine sterilization tablet. The tablets that will be used in this experiment are Coghlan's Emergency Drinking Water Germicidal Tablets. According to the included documentation, one tablet is effective in the disinfecting 0.5L of water and each tablet contains 20mg of tetraglycine hydroperiodide, which produces 8mg of aqueous iodine.
Figure 1: structure of Tetraglycine Hydroperiodide
In Brief… Under the conditions relevant to this experiment, the chemistry of aqueous iodine
solutions can be described by the following chemical equations:
I2 + H2O HOI + I − + H +
I2 + I− I3
−
HOI OI − + H +
3HOI IO3− + 2I − + 3H +
IO3− + 5I − + 6H + 3I2 + 3H2O
During this experiment each lab section will be working together to analyze a water sample treated with a Coghlan's Emergency Drinking Water Germicidal Tablet. Each group will analyze the water sample using UV-‐visible spectroscopy, two redox titrations and an Iodide selective electrode. Using the listed instrumental techniques and the iodine equilibrium equations above it is possible to determine the amounts I2, I⁻, I3⁻ and HOI present in the tablet-‐sterilized drinking water. Before the Lab Period: 1. To use ascorbic acid and iodate as primary standards they must first be dried for at
least two hours. The week before you perform experiment 10 make sure you prepare dried potassium iodate and ascorbic acid.
• Use a top loading balance to dispense ~5% more than the calculated amount of potassium iodate and ascorbic acid into separate clean, dry, labelled weighing bottles. Dry the bottles in a 100°C oven for at least 2 hours with the lid open. After the 2 hours transfer the weighing bottles into a desiccator to cool
2. Determine how to make all stock solutions and calibration standards required in the
experiment and record this information in your laboratory notebook.
3. Determine the physical and hazardous properties of the chemicals used in this experiment and record this information in your laboratory notebook.
4. Answer the pre-‐lab questions and submit these one business day before your lab
period. Chemicals and Supplemental Equipment:
• 500mL of Coghlan's Emergency Drinking Water Germicidal Tablet sterilized water
o One tablet dissolved in 500mL of water • 0.4mM Glycine Buffer (pH=5.5) • 0.4mM Glycine – 50g/L Potassium Iodide Buffer (pH=5.5) • Iodine-‐TISAB Solution
o 5.5M NaNO3 • Deionised water • 2 matched pairs of 10.0 mm pathlength 1mL UV cuvettes (TA) • A variety of volumetric transfer pipettes in volumes ranging from 1 – 50mL • In addition to the volumetric flasks located in the drawers you will also be provided
with 10mL and 25mL volumetric flasks Pre-‐Lab Questions: 1. Give balanced chemical equations for the preparation of the triiodide and iodine
standards, as well as the two redox reactions performed in the titration experiments.
2. At what UV-‐visible wavelengths do triiodide and iodide absorb in aqueous solution? 3. Why must the ascorbic acid standard solution be used immediately, and not made
ahead of time? UV-‐VIS Spectrophotometer Operating Instructions: 1. Handle the UV cuvettes with care, handling them in such a way as to not touch the
front and rear clear windows. Pick up and move the cuvettes by touching only the opaque surfaces.
When filling a cell, first rinse it at least three times with small portions (~0.5 mL) of
the solution using a Pasteur pipette: position the tip of the pipette near the bottom of the cell against one side and slowly expel the pipette contents. This helps avoid air bubble formation, which can affect the reading if the bubbles stick to the faces of the cell in the light path.
Wipe the outside surfaces carefully, as not to scratch, with Kimwipe™ before placing
the cell in the spectrophotometer with the clear windows in the light path. 2. When performing multiple measurements, it is usually a good idea to retain one
Pasteur pipette for each solution, just in case you have to repeat some measurements with fresh portions of your samples. Do not mix the pipettes up or use the same pipette for different samples! Note which side of the cell is the front, it will be marked by an arrow. Always place the cell in the instrument the same way, and make sure it is fully seated in the cell holder before taking a measurement.
3. When emptying a cell, always pour the contents into a waste container, never back
into your sample container. Rinse the cell thoroughly with deionised water from a wash-‐bottle, and place the cell on a Kimwipe™ in a safe place while not in use.
4. The spectrophotometers and connected computers should already be on when you
come into the lab; if not, either your demonstrator or the laboratory instructor will start them for you. The method should be loaded and the spectrophotometer ready to start the experiment, if this is the case move on to no. 5, if not continue reading.
A Perkin-‐Elmer UV/visible spectrophotometer -‐ the Lamda11 Within the main UVWinLab window should be a Methods window; if this is not
visible, select Methods from the Window menu. The desired method can be launched simply by double-‐clicking on the corresponding entry in the Methods window. Once the method file opens, you will see three tabs labelled Scan, Inst., and Sample. The scan tab should show the correct wavelength range; note that the instrument scans from long to short wavelength.
5. In the Sample tab, enter a unique Result Filename, and type in suitable Sample
Identity values in the spreadsheet-‐like area. Once satisfied, click on the Setup button in the toolbar. The software will configure the instrument; once it is ready, the Start button should be highlighted in green. When you are ready to start collecting data, click the Start button!
6. When prompted to Insert Blank, fill the cuvette with your blank solution, open the
sample compartment, and carefully slide the cuvette into the appropriate holder (see below). Make sure the cuvette is properly seated in the bottom of the holder, but do not force it into position. Gently lower the lid, then click the OK button in the dialog box.
Single beam (left) and double beam (right) spectrophotometer sample compartments. The sample cuvette is placed in the position marked ‘S’ in the photographs. The double beam instrument requires a second cuvette filled with the solvent or reagent blank to be placed
in the ‘reference’ beam. The light path is from left to right through the sample compartment.
7. Once the blank has been scanned to obtain the background reading, you will be
prompted for each solution in turn, in the order they appeared in the method’s sample list. Note that, depending on how this is set up, you may be asked to scan the blank a second time, so that a ‘blank’ spectrum appears in the graphics window. As each spectrum is obtained, it will be added to the graphics window. Once you have acquired all the data, ask your TA how to print a copy of the spectra.
During The Lab Period: During this experiment each lab section will work together to analyze iodine tablet sterilized drinking water using UV-‐VIS spectroscopy, redox titrations and an iodide selective electrode. Each student should be prepared to know and understand how to perform any part of the experiment, as the whole demo group will be working together as one team. No one may leave the lab until the entire experiment is finished. If you are finished the section of the experiment you were assigned, move on and help other members of your group who are still working. *** Be careful and pay attention to ensure the correct buffer is used in each part of the
experiment!
*** Be sure to mix the iodine tablet sterilized water sample thoroughly before dispensing into a beaker or aliquoting for analysis!
*** As with experiment 5 all solutions are to be transferred to Nalgene bottles after preparation.
*** Retain all your solutions until you have acquired all your data and plotted your calibration curves; you may need to make additional calibration solutions before you are finished!
*** Calculate the actual concentrations of each solution. Record this information along
with all the data your group has acquired in your lab notebooks before the end of the lab period, this will contribute to your performance grade. All team members must have the data from every section recorded in their lab book, no exceptions!
*** DO NOT FORGET to provide the TA with a copy of the data as well. Use excel on one
of the lab computers to make a data table for the TA and provide him/her with the original labeled printouts from the UV-‐VIS and auto-‐titration experiments.
Part A – Determination of Triiodide Using UV-‐VIS Spectroscopy: *** In this section of the experiment be sure to rinse all your glassware and make up all
your solutions using the provided 0.4mM glycine-‐50g/L potassium iodide buffer (pH=5.5).
A.1: Verify that the UV-‐visible spectrophotometer is turned on when you arrive in the lab.
If you are unsure what instrument you will be using, consult your TA.
A.2: Prepare a ~1x10-‐3 M triiodide stock solution by combining 0.0650g (±10%) of iodine with 0.8500g (±10%) of Potassium Iodide in a 250mL volumetric flask.
NOTE: Iodine can be troublesome to dissolve in aqueous solutions however; iodine dissolves more readily in concentrated KI solutions. Therefore, first dissolve the KI in ~50mL of the glycine-‐potassium iodide buffer within the volumetric flask, then dissolve the iodine in that solution. Once the iodine is dissolved fill the volumetric flask to the mark with buffer.
A.3: Accurately prepare a series of triiodide calibration standards from the stock solution
with the approximate concentrations of 1x10-‐5M, 1x10-‐6M, 1x10-‐7M. You can use small volume volumetric flasks (i.e. 10mL) for this as the cuvettes you will be using to make your UV measurements only require 1mL of sample. Transfer all your standards and stock solution into Nalgene bottles. NOTE: Be sure to calculate the actual concentrations for your lab report
A.4: Dispense a sample of the iodine tablet disinfected drinking water into a Nalgene bottle. Be sure to mix the water sample well prior to dispensing. Consult your TA for instructions on how to run the UV-‐VIS spectrophotometer experiments to analyze your samples. Be sure to use glycine buffer as your blank (Not the glycine-‐iodide buffer you used to prepare your samples)
NOTE: Make sure to obtain the absorbance of the sterilized drinking water sample in triplicate.
A.5: After obtaining the UV-‐VIS data, plot a calibration curve in excel. Determine the
concentration of triiodide in the sterilized drinking water to then prepare another two calibration standards to bracket around your sample, so you end up with a 5-‐point calibration curve.
A.6: Acquire the UV-‐VIS data of the new standards. Part B – Determination of Iodine Using UV-‐VIS Spectroscopy: *** In this section of the experiment be sure to rinse all your glassware and make up all
your solutions using the provided 0.4mM glycine buffer (pH=5.5). B.1: Verify that the UV-‐visible spectrophotometer is turned on when you arrive in the lab.
If you are unsure what instrument you will be using, consult your TA.
B.2: Prepare a ~1x10-‐2 M Iodine stock solution by combining 0.2500g (±10%) of potassium iodate with 1.3500g (±10%) of Potassium Iodide in a 250mL volumetric flask. First dissolve the solids, within the volumetric flask with ~50mL of glycine buffer and ~5mL of 3M hydrochloric acid (use a graduated cylinder for the acid). After the solids have dissolved fill to the mark with glycine buffer.
B.3: You will need to accurately prepare a series of iodine calibration standards from the stock solution with the approximate concentrations of 1x10-‐3M, 1x10-‐4M, 1x10-‐5M. However prepare them one at a time, and run them immediately on the UV-‐VIS spectrophotometer. Before you start preparing the calibration standards consult your TA for instructions on how to set up and run the UV-‐VIS spectrophotometer experiments. Set up the method to include all your samples, and run the sample blank. What should the blank be?
B.4: Using small volume volumetric flasks (i.e. 10mL, you will only need 1mL of sample to
take a measurement) prepare your calibration standards one at a time. To prepare a calibration standard add the correct volume aliquot of stock or higher concentration standard into your 10mL volumetric flask, then add ~1mL of 3M HCl using a graduated cylinder, mix and fill to the mark with glycine buffer. After the standard is prepared, take the UV-‐VIS measurement of that standard. Repeat until you have a measurement for all your standards.
B.5: Dispense a sample of the iodine tablet disinfected drinking water into a Nalgene
bottle. Be sure to mix the water sample well prior to dispensing. Analyze the water sample using the UV-‐VIS spectrophotometer in triplicate.
B.4: After obtaining the UV-‐VIS data, plot a calibration curve in excel. Determine the
concentration of triiodide in the sterilized drinking water to then prepare another two calibration standards to bracket your sample. Prepare the standards as above, if using a different size volumetric flask be sure to maintain the same ratio of 3M HCl. Acquire the UV-‐VIS data immediately after each standard is prepared.
Part C – Iodometric Titration Using Ascorbic Acid: *** The group members performing this part of the experiment should also
simultaneously prepare the samples and standard solutions required for part D. *** In this section of the experiment be sure to rinse all your glassware and make up all
your solutions using the provided 0.4mM glycine buffer (pH=5.5).
C.1: Obtain three 100mL beakers and rinse thoroughly with glycine buffer. To each beaker accurately pipette 25mL of the sterilized drinking water then add a mini magnetic stir-‐bar to and fill approximately to the 60mL mark of the beaker the with glycine buffer. NOTE: Be sure to thoroughly mix the sterilized drinking water prior to dispensing into the beaker from which you will take your 25mL aliquots.
C.2: Accurately prepare a ~5x10-‐4M ascorbic acid standard solution in a 500mL
volumetric flask using the ascorbic acid that was dried during the previous lab period. You may opt to prepare this via serial dilution of a more concentrated stock solution;
either method is fine. Transfer the solution to the provided bottle that can mount to the auto titrator. If you cannot find the bottle consult your TA. NOTE: The titrations must be performed immediately after preparation of the
ascorbic acid standard solution. Why?
C.3: After the ascorbic acid standard solution is prepared, consult your TA on how to set up and run the auto-‐titrator. Then titrate each water sample with the ascorbic acid standard solution, be sure to allow enough time for your sample to mix thoroughly before starting the titration.
Part D – Iodometric Back Titration: *** In this section of the experiment be sure to rinse all your glassware and make up all
your solutions using deionized water. D.1: Accurately prepare a ~4x10-‐5M potassium iodate standard solution in a 500mL
volumetric flask using the potassium iodate that was dried during the previous lab period. You may opt to prepare this via serial dilution of a more concentrated stock solution; either method is fine. Transfer the solution to the provided bottle that can mount to the auto titrator. If you cannot find the bottle consult your TA.
D.2: Accurately prepare a ~1x10-‐4 sodium thiosulfate solution in a 500mL volumetric
flask. You may opt to prepare this via serial dilution of a more concentrate stock solution; the choice is yours.
NOTE: You will be using sodium thiosulfate pentahydrate make sure you use the right molecular weight.
D.3: To a 100mL beaker accurately dispense 10mL of the sterilized water sample and
10mL of the sodium thiosulfate solution. The colour of the solution should turn clear, why is this? After the solution turns clear add deionized water up to the 60mL mark on the side of the beaker.
D.4: Consult your TA on how to set up and run the auto-‐titrator. D.5: Just before you titrate your samples, add 2mL of 0.5g/ml potassium iodide solution
and 2mL of 3M hydrochloric acid to your 100mL beakers using a graduated cylinder. Allow the samples to mix thoroughly on the auto-‐tirator stirring plate, and then titrate the water sample with the iodate standard solution.
NOTE: You must only add the acid and potassium iodide just before the titration and not before, if allowed to sit the acid may prevent the titration from proceeding properly. Also these components must not be added before the thiosulfate is allowed to react with the tablet sterilized water.
D.6: Repeat the titration with two more sterilized water samples to obtain triplicate results.
D.7: Once the titrations of the water sample are complete, you must standardize
thiosulfate solution (Why is this?). Prepare a thiosulfate titration sample by accurately dispensing 10mL of the sodium thiosulfate solution into a 100mL beaker then add deionized water up to the 60mL mark on the side of the beaker.
D.8: As outlined in section D.5, just before you are ready to titrate the sample, add 2mL of
0.5g/ml potassium iodide solution and 2mL of 3M hydrochloric acid to your 100mL beaker using a graduated cylinder. Allow the samples to mix thoroughly on the auto-‐tirator stirring plate, then titrate the sample with the iodate standard solution.
D.9: Repeat the thiosulfate standardization titration two more times to obtain triplicate
results.
Part E: Determination of Iodide by Ion Selective Electrode: *** In this section of the experiment be sure to rinse all your glassware and make up all
your solutions using the provided 0.4mM glycine buffer (pH=5.5). E.1: Accurately prepare a ~1x10-‐2 M potassium iodide stock solution in a 250mL
volumetric flask.
E.2: Accurately prepare a series of iodide calibration standards from the stock solution with the approximate concentrations of 1x10-‐3 M, 1x10-‐4 M, 1x10-‐5 M, 1x10-‐6 M, 1x10-‐7 M, 1x10-‐8 M. (Be sure to accurately calculate the actual concentrations for your lab report)
E.3: To prepare a sample for ISE measurement, accurately pipette 15mL of the standard to
be analyzed into a 50mL Nalgene beaker. To the same beaker accurately pipette 15mL of iodide-‐TISAB solution, and add a mini-‐magnetic stir bar.
E.4: Using the same method as in experiment 5 use the ISE to measure the calibration
standards. Preparation Sterilized Water Sample for ISE Measurement: E.5: Accurately prepare a ~1x10-‐4 sodium thiosulfate solution in a 250mL volumetric
flask. You may opt to prepare this via serial dilution of a more concentrate stock solution; the choice is yours. NOTE: You will be using sodium thiosulfate pentahydrate make sure you use the right molecular weight.
E.6: Prepare a sterilized water sample for ISE measurement by accurately dispensing 15mL of the water sample followed by 15mL of iodide-‐TISAB solution into a 50mL Nalgene beaker. Add a mini-‐magnetic stir bar to the beaker as well.
E.7: Using the same method as in experiment 5, use the ISE to measure the sterilized
water sample. E.8: After recording the value in your lab notebook DO NOT remove the ISE from the
sample, instead, dispense 15mL of the thiosulfate solution into the Nalgene beaker containing the sterilized water sample. Allow the reading to stabilize and record in your lab notebook. DO NOT FORGET to account for the dilution factor of adding the thiosulfate solution to your sample. (What is the difference between measuring the sample before and after the addition of the thiosulfate? What does the thiosulfate do to the water sample?)
E.9: Repeat two more times to obtain triplicate results. Post-‐Lab Assignment:
Although you will be using the data from the entire group, you should each submit your own written report for this experiment. There is no report form for this experiment; instead you will be preparing a formal lab report consisting of the following: Title page: This should clearly state the title of the formal report, as well as your name, student number, and demo group number. Departmental policy also requires that you include a signed academic honesty pledge: “I certify that this submitted laboratory report represents entirely my own efforts. I have read and understand the University of Toronto policies regarding, and sanctions for, plagiarism” Abstract (1 mark): This should be a ~1/2 page (50–125 words) summary of what was done in the lab and what was found, including but not limited to the concentrations and uncertainties of the different iodine species found in the iodine tablet sterilized water. Don’t include procedural details. Introduction (2 marks): This should outline the purpose of the experiment and give the reader sufficient scientific background necessary to understand what you are writing about in the report. In this case you should assume that the target audience of your report have a background in analytical chemistry, so the scientific background of the introduction should focus on aqueous iodine chemistry.
Experimental (1 marks): Include a short journal style outline of the experimental procedures used. Include sufficient detail so that another scientist can reproduce your experiments. Be concise, do not include vessel sizes or masses of the components used to make solutions, just state the final concentrations of the samples you made. You also do not have to explain the bracketing process, just list the concentrations you decided to use as brackets when you list the concentrations of the standards used. Do not include results in this section. For example, a partial summary of part A: A 9.9x10-‐4 M triiodide stock solution was prepared by dissolving iodine and potassium iodide in a 0.4mM glycine – 50g/L iodide buffer (pH=5.5). The stock triiodide solution was serially diluted into standards with concentrations of 9.9x10-‐6M, 5x10-‐6M, 9.9x10-‐7M, 5x10-‐7M and 9.9x10-‐8M. UV-‐visible spectra of the triiodide standards and neat Coghlan's Emergency Drinking Water Germicidal Tablet sterilized water were acquired. Results (8 marks): Within the report include a results section that consists of a summary table of the results obtained and sample calculations of each experiment. The results table should consist of two columns, one with a description of each experiment and the other with the results of each experiment including units and uncertainties. The experiment description should be more descriptive than, for example, “part A”. A more appropriate description would be, “Part A: determination of the triiiodide concentration of iodine tablet sterilized water”.
The sample calculations should be subdivided into each experiment with the use of sub-‐headings. Each section should first include an equation derivation. If the equation is generated from a calibration curve, show the curve and the equation of the line, and then the expression based on the curve. If the equation is based on a chemical equation or series of chemical equations, first show the balanced chemical equations and then the derived expression. At this level you should not be inserting hand written or ASCII based equations or calculations, but rather you should be generating your own. Software such as Microsoft Equation Editor (included in most versions of Microsoft office) or the freely available MathType software package make it easy to generate your own equations.
You will also be required to had in an annotated excel spreadsheet using the one handed in with experiment 5 as a template. This spreadsheet will include all the raw data from each experiment and all the calculations, statistical analysis, regression analysis and error analysis. In the cell next to a calculation clearly annotate the excel equation used to generate that result.
Attach this to the end of your report as an appendix. Always use the correct number of significant figures/decimal places. Discussion (10 marks): This should be a detailed exploration of the findings from your experiment. Your discussion should include but not be limited to discussing the following questions:
• What do the results outlined in the preceding section indicate?
• Can you determine if any HOI is present and estimate a concentration? • Part C and D determine the concentration of reducible iodine species, are the results
of each section statistically different? Which experimental design introduces less error?
• Is the concentration of I2 determined in part B the same as part C and D, why or why not?
• Find the literature values for the equilibrium constants of the different iodine species under acidic conditions. Are the concentrations of the different species you determined in agreement with the literature equilibrium constant? Assume each tablet releases a max of 8mg of I2.
• Is there any evidence of systematic error (bias) in any of the techniques used? • If you encountered problems in performing the experiment, what were they and
what did you do to address them? Or how did they adversely affect your experiment?
Conclusion (3 marks): Provide a final summary of your findings and conclusions from your discussion, what you learned from the experiment overall, and any recommendations as to what you might do differently if you were to repeat this experiment. At the end of your conclusion it is a good idea to relate your findings back to how they relate to real world problems or issues. A real world problem related to iodine drinking water sterilization tablets are the health effects associated to long-‐term exposure to iodine. What is the maximum recommended daily ingestion level of iodine and based on this are these tablets a viable long-‐term water purification solution?