INTERDISCIPLINARY INVESTIGATION (IDI)-LAB LABORATORY HANDOUT

Transcription

1 INTERDISCIPLINARY INVESTIGATION (IDI)-LAB LABORATORY HANDOUT Research-based Interdisciplinary Science Education ION CHROMATOGRAPHY TO INVESTIGATE WATER QUALITY ISSUES: FLUORIDE IN DRINKING WATER Introduction Global Environmental & Human Health Problem The availability of clean and plentiful water is fast becoming one of the most pressing global challenges, with far-reaching consequences for the health and well-being of all living organisms. Changing climate patterns are threatening lakes and rivers, and our current drinking water sources are being overtapped and polluted. Millions of tons of sewage and industrial and agricultural waste are dumped into the world s water every day. This type of water contamination weakens or destroys natural ecosystems that support human health, food production, and biodiversity. Worldwide, infectious disease such as waterborne diseases are the number one killer of children under five years old and more people die from unsafe water annually then from all forms of violence, including war. World Health Organization, 22 Figure 1: Young girl Seeking Clean Water to Drink. While developing nations struggle with the whole spectrum of water quality, the United States and other developed nations mostly face less visible health challenges that result from the release of chemical pollutants that end up in our water supply. Chemicals such as pharmaceuticals, drycleaning solvents, gasoline, and microplastics are regularly found in ground water and streams. In addition, agricultural practices can contribute to high levels of nitrates (NO3 - ) and phosphates (PO4 3- ) in surface and ground waters, which can lead to toxic algal blooms, low oxygen levels, and blue-baby syndrome. Water treatment practices including additives, such as fluoride (F - ) to prevent tooth decay, have recently re-surfaced as a national controversy. One side argues that adding fluoride to drinking water supplies to reduce tooth decay across the population leads to large public health benefits. The other side argues that the unintended health consequences due to over-exposure to fluoride should be considered on an individual level and not forced upon an entire population. As controversy rages on, scientists continue to study the health impacts of fluoride and monitor fluoride levels in drinking water. To determine water quality, scientists measure and analyze a range of characteristics, such as temperature, ph, dissolved oxygen, ions, and bacterial load. Standards and guidelines have been established for public drinking water and are monitored on a regular basis. For example, in 215 the U. S. EPA introduced a recommended level of fluoride of.7 ppm, with the enforceable standard remaining at 4. ppm. As a result, water treatment officials have changed the amount of fluoride added to the water supply and continue to evaluate levels of fluoride in the drinking water. Page 1 of 6

2 What is Ion Chromatography? Ion chromatography (IC) is a powerful technique to quantify individual ions in aqueous samples, and thus it is used for water quality analysis. With an ion chromatograph (Figure 2) mixtures of water soluble major ions found in a variety of aqueous samples are separated, identified, and quantified. Concentrations in the parts-per-million (ppm) range can be determined of anions and cations, including fluoride (F - ) chloride (Cl - ), nitrate (NO3 - ), nitrite (NO2 - ), sulfate (SO4 2- ), and phosphate (PO4 3- ), lithium (Li + ), sodium (Na + ), ammonium (NH4 + ), potassium (K + ), calcium (Ca 2+ ), and magnesium (Mg 2+ ). Concentrations of organic acids, amino acids and even proteins can be measured through ion chromatography with appropriate columns and detectors. Figure 2: Metrohm Ion Chromatography Instrument with Auto-sampler. How Does Ion Chromatography Work? Chromatography is a general term used to describe a collection of laboratory techniques designed to separate individual components in mixtures. Ion chromatography, then, is a type of liquid chromatography designed to separate ions found in aqueous solutions. As shown in the diagram of the inner workings of the instrument (Figure 3), the sample is injected as a plug into a flowing stream of eluent that carries the sample through a stationary or solid phase packed in a column. The column is where separation occurs. The stationary phase is designed to interact electrostatically with the sample molecules to varying degrees. The ions that are less attracted to the column move more quickly through, while the ions with greater attractive forces to the column take longer to move through. Size of the analyte ion as well as the relative affinity of the eluent molecules with the stationary phase play a role in separation. Generally speaking, if the charges are equal, the smaller ions move through the column more quickly. Most IC systems have a conductivity detector that measures the conductivity of the effluent as it exits the instrument. The effluent will contain background eluent ions as well as the separated individual analyte ions that once were mixed. This signal is recorded as a function of time and then used to identify and quantify each ion. The suppressor is present to reduce the background conductivity from the ions in the eluent, so that the signal from the sample ions is more apparent. We will discuss all of these Figure 3: Schematic of an IC. parts of the IC in more detail in our class discussion. Page 2 of 6

3 What is a calibration curve? A calibration curve is a linear plot (y=mx +b) of concentration (x) versus instrument response (y) that provides a relationship between the physical measurement output from the instrument and the expected concentration of a given analyte in a sample. Thus, concentrations in an unknown sample can be determined from such a calibration curve. For more information please visit: onovmolieccxbwr You will construct a calibration curve by obtaining the instrument signal of a series of standard solutions with known concentrations. You will prepare these standard solutions in lab that will be run by the lab tech This Investigation In this investigation, you will learn how ions within aqueous samples are separated and quantified with the IC. In addition, you will prepare solutions with specific concentrations that will be evaluated using the IC, analyze and represent your results, participate in a seminar, and complete a post-lab assignment. Laboratory Outcomes: Recognize how scientists engage in research practices including communicating through dialogue and writing Identify a variety of water quality issues on the global scale and recognize how ion chromatography can contribute to water monitoring efforts Explain how ion chromatography can be used to separate and quantify ions in aqueous solutions Review and critique science writing and respond to your peers in a productive manner Prepare stock solutions from solid solute and prepare a variety of solutions using serial dilutions Use unit conversion calculations to assess and compare laboratory results to EPA regulations Represent and analyze laboratory results using Excel Quantify ions in an unknown sample using a calibration curve To accomplish this, we will progress through the following parts of this project, listed and presented schematically: Pre-Lab Assignment Part 1: Introduction to Ion Chromatography: Powerpoint and Activity (Day 1 in class) Part 2: Laboratory: Solution Preparation & Analysis using the IC (Day 1 in class) Part 3: Seminar (prepare outside class between Days 1 & 2, discussion in class on Day 2) Part 4: IDI Challenge (Day 2 in class) Post-lab Assignment (outside class) Page 3 of 6

4 Pre-Lab Assignment, due on Day 1 Add the procedure and any tables to your lab notebook. Answer the following pre-lab questions: 1. Convert 52.6 mm (millimolar) to μm (micromolar) and M (molar). 2. Convert 2. μm to mm (millimolar) and to M (molar). 3. The new U.S. Public Health Service optimal fluoride concentration in water is.7 mg/l. a. Convert.7 mg/l to M. b. Convert.7 mg/l to ppm. 4. What is the fluoride ion (F - ) concentration in M and mm of a solution made by adding 2.29 g of solid NaF to a 1. L volumetric flask, and dissolving it in ultrapure water up to the 1. L line? 5. You will be provided a stock solution (A) of 1. mg F - / L (1. ppm). What is this concentration in molarity (M) and millimolarity (mm) of fluoride ion? 6. Describe a procedure for making 5. ml of 2. μm F -. You will use this 2. μm standard solution (B) as a stock in step 2. of your procedure below. Draw a picture of the tools that you will use to create this dilute solution (i.e.: volumetric flask, micro pipet, etc.). a. What mass of NaF would you need to weigh out on the laboratory scale if you wanted to use solid NaF to create 5.mL of 2. μm F - solution? Can the scales in lab do this? 7. The dilute solutions listed in Table 1 (next page), column 1, will be needed for your calibration curve. Complete the table using solution B, the 2. μm F - solution, to make the following dilute solutions. For example: to make the diluted1. μm solution: M1= 2. μm, M2 = 1. μm, V2 = 1.mL and V1 is your unknown. M1V1 = M2V2 (2. μm)(v1) = (1. μm)(1. ml) V1 = 5. ml of 2. μm F - Thus, you would use the P1 micropipette, set it to the maximum as shown here to deliver 5. ml of 2. μm F - directly into the IC tube. Then use the same micropipette with a clean tip to deliver 5. ml of water into the same IC tube. Cap the IC tube. 1 Page 4 of 6

5 Fluoride Concentration needed Total volume in IC Tube Volume of ultrapure water to add to the IC Tube Volume of concentrated F - (2. μm) to add to the IC Tube. Which Micropipette will you use? P2 or P1 Micropipette Volumeter Setting 2. μm 1. ml. ml 1. ml None 1. μm 1. ml 5. ml 5. ml P μm 1. ml 2. μm 1. ml 1. μm 1. ml. μm 1. ml 1. ml. ml none Part 1: Introduction to Ion Chromatography: Powerpoint and Activty provided in class on Day 1 Part 2: Laboratory: Solution Preparation & Analysis using the IC, Day 1 Procedure: (WARNING: make sure to use ultrapure water for all rinsing and dilutions) 1. Prepare Solution B (5. ml of a 2. μm F - ) from Solution A (the 1. mg/l F - ). a. Write your own procedure with detailed steps. Your tools available for this procedure include a 5.mL volumetric flask and micropipettes. Be specific about which micropipette you will use and what setting you will adjust the volume on your micropipette. b. Rinse your 5. ml flask three times with ~1-2 ml of ultrapure water for each rinse. c. Use only ultrapure water to make your solutions. 2. Prepare the dilute F - solutions (Table 1) for your calibration curve (2. μm, 1. μm, 5. μm, 2. μm, 1. μm,. μm) using the Solution B and water volumes you calculated in Table 1 above. Use only ultrapure water to dilute your solutions. a. Label six IC tubes with your lab group number and F - concentration. b. Write your own detailed steps for the dilution procedure. You will have IC tubes and pipettes available for this procedure. c. Cap and shake each IC tubes to assure that the added ultrapure water and F - is well mixed. d. Give your labeled and filled IC tubes to your instructor. They will be analyzed and the data returned to you. Page 5 of 6

6 Part 3: Seminar Preparation (outside class between Days 1 and 2) Read the article: Second Thoughts about Fluoride by Dan Fagin. To prepare for the seminar, write a summary of the article, and write three questions about the article. Bring the summary and questions with you to class and be prepared to participate in an inclass seminar. You will also be asked to submit your summary and questions to your instructor after the seminar. Part 4: IDI Challenge (Day 2 in class discussion, include in postlab) Quantify the fluoride (F - ) concentration in two samples of water from different municipalities in the Puget Sound area. You will be given chromatograms from these two water samples that have been run on the IC. Here are specific steps to guide you through the process: 1. On the chromatographs of the standards that you had made, identify the retention time of fluoride (F - ). Are there any other peaks in your chromatograph? If so, discuss what these might be. 2. From your results, make a table including the known concentration of fluoride (μm) of each solution and the given area under each of the corresponding peak (μs*min). You may wish to put these in order from least concentrated to most concentrated. Be sure that your table is labeled and units are included. 3. Use Excel to create a calibration curve with the trendline option by plotting concentration of fluoride ion (μm) vs. peak area (μs*min). Your instructor will give you an example of what this looks like. Be sure to label your graph and include the best-fit line equation. Show your plot to your instructor before moving on. 4. Figure out a way to use your calibration curve to determine the concentration of fluoride in your two real water samples. 5. Use Excel or another program to represent these results on a bar graph, including in your graph, a comparison to the.7 ppm recommended level and the maximum EPA limit of 4. ppm. 6. What did the fluoride ion concentrations you determined tell you about these municipalities? 7. Discuss in your group what you think about the controversy surrounding adding fluoride to public drinking water. Post-lab Assignment (complete after Day 2 outside of class) Clearly report your results, all graphs, all tables, and answers to the following post-lab questions. 1. What is a serial dilution? What are the benefits of doing a serial dilution? 2. What is the purpose of making a μm solution that contains no NaF? Why would we want to run a μm sample in the Ion Chromatograph instrument and graph it on a calibration curve? 3. Could you have made the most dilute 1. μm solution from weighing solid NaF and adding it to the 2. ml volumetric flask with water? Calculate the mass of NaF that would be needed and explain. 4. What is the purpose of a calibration curve? How did your calibration curve allow you to determine the concentration of F - in the water samples from different communities? 5. Your city is considering adding fluoride to the water supply. You are invited to a city council meeting to give a 2-minute presentation on adding fluoride to your city water. Would you argue for or against on adding fluoride, and what key bullet points would you include in your presentation? Page 6 of 6