Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 1 Total Ion Concentration in Water Determination of water hardness and alkalinity by ion exchange chromatography and pH titration Objective Over the course of three weeks you will determine the alkalinity and total ion concentrat ion in a water sample . In Week 1 you will prepare and standardize 0.1 M NaOH and HCl solutions for later use. In Week 2 you will perform a pH titration of the Evian water and an unknown water sampl e . In Week 3 you will prepare a cation exchange column and use the column to exchange metal cations in the water sample for H+. Finally, you will titrate the column eluate to determine the total cat ion concentration and compare with the Evian water quality report. Logistics The duration is three weeks. You will work i ndividually for all parts. Week 1 : Standardization of NaOH and HCl Week 2 : Determination of alkalinity. Bring in a water sample to analyze as your unknown. Week 3 : Total cation content by cation exchange chromatography. Your notebook should be prepared with the appropriate data tables for each week before the lab period. Introduction In general chemistry you learned to determine the moles of protons in solution u sing titration. In this lab you will learn to prepare your own reagents with an accurately determined concentration of NaOH. We cannot directly determine the mass of NaOH because it is hygroscopic and also absorbs carbon dioxide from the atmosphere. Theref ore, NaOH cannot be used as the primary standard. The reagent KHP (potassium hydrogen phthalate, molar mass= 204.23 g/mole) will serve as the primary standard. KHP (Figure 1 ) is monoprotic and is nonreactive with air, making its mass determination very accurate. Titration of KHP will be performed using calorimetric titration with the indicator phenolphthalein to determine the equivalence point.
Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 2 Figure 1 . Chemical structure of KHP In week 2, you will determine the alkalinity of a sample of Evian and an unknown water sample. Alkalinity is a property that arises from the chemistry of dissolved anions that act as bases in water, such as HCO3 â€“ and CO3 2 â€“. In order to estimate the amount of CO3 2 â€“ in solution, you will perform a potentiometric titration with a Vernier pH electrode. The water sample will be titrated with your standardized HCl to convert the bicarbonate to carbonic acid, as in the equation below. Titrating to a pH of 4.5 will ensure we have reacted all OHâ€“, CO3 2 â€“, and HCO3 â€“. HCO3 â€“ (aq) + H+ (aq) H2CO3 (aq ) Alkalinity is calculated as the millimoles of HCl added to obtain a pH of 4.5 per liter of solution tested. Some sources report alkalinity as milli equivalence units (mEq) per liter, where the conversion is 1:1. The image in Figure 2 shows the alkalinity of several bodies of water around the country, and the variation is usually linked to the minerals in bedrock. For reference, water samples with zero alkalinity have no buffering power and aquatic life requires a minimum of about 0.4 mEq/L.1 Sea water has an alkalinity around 2.3 mEq/L.1 Figure 2 . Alkalinity of surface waters of the United States. Reprinted from Omernik JM, Griffith, GE, Irish JT, and Johnson CB (19 88) Total alkalinity of surface waters: a national map. Corvallis Environmental Research Laboratory, U.S. Environmental Protection Agency , Corvallis, OR.
Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 3 In week 3, you will determine the water sampleâ€™s t otal ionic content using ion exchange chromatography . The intent of ion exchange chromatography is to completely remove mineral components such as Mg2+ and Ca2+. To accomplish this , a negatively charged resin is used that has a higher affinity for Mg2+ and Ca2+ compared to prot ons, as in Figure 3 . The result is that the inorganic minerals are removed and protons are released from the resin into the bulk solution . Figure 3 . Sulfonic resin used in cation exchange chromatography to replace Ca2+ ions in water with H+. The resulting solution will be very acidic, and the moles of protons in solution will convey the moles of positive charge from the minerals. Taken together, the results of weeks 2 and 3 will inform us about the quality of your water samples in terms of buffering capacity and hardness, respectively. P relab Questions Prepare the following in your notebook for the first week of lab. 1) Calculate the volume of 2 M HCl required to make 1 L of 0.1 M HCl ( aq). 2) Calculate the mass of NaOH required to make 1 L of a 0.1 M NaOH ( aq). 3) Write the balanced chemical equation between KHP and NaOH. 4) Calculate the mass of KHP required that will consume 2 0 mL of 0.1 M NaOH ( aq). Show your work using dimensional analysis and stoichiometry.
Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 4 Procedure Week 1 Preparation and standardization of NaOH and HCl solutions. In Part A you will make secondary standards that have approximate concentrations. Then in Part B you will use the primary standard KHP to accurately determine the concentration of NaOH. The standardized NaOH will then be used in Part C to accurately determine the concentration of HCl. A. Preparation of 1 L solutions of ~0.1 M NaOH and HCl . Using the 1 L polyethylene bottles supplied, prep are 1 L solutions of each reagent using the amounts calculated in prelab questions #1 & 2 . Note that it saves work/effort to use this bottle to make an approximate solution, rather than a volumetric flask. As neither the HCl nor the NaOH can serve as a primary standard, the concentrations would be approximate regardless of the 1 L flask used. These solutions should be stored in their respective capped bottles when not being poured, as we want their concentrations to stay constant throughout the course of the experiment. Evaporation of wate r will change their concentrations if you are not vigilant on keeping containers closed. B. Standardization of NaOH with KHP. Weigh three portions of the amount of KHP that you calculated in prelab question #4, making sure to record the exact mass of each in your notebook. Place the samples into three 250 mL Erlenmeyer flasks that are labeled to keep track of the samples. Dissolv e each portion with ~25 mL of nanopure water and add 3 5 drops of phenolphthalein indicator solution. Ensure all of the KHP is dissolved before commencing. Rinse your buret using less than 10 mL of the NaOH solution you prepared in Part A, then fill the b uret. Be sure air bubbles are removed from the tip. Titrate each KHP aliquot to the end point (first appearance of pink color). Use the inform ation from the masses of KHP and volume delivered to calculate the molarity of NaOH for each trial. You should have a minimum of three data points that agree well , having a %RSD less than 1%. If your three data points do not meet this requirement, complete additional trials. The NaOH solution is now standardized, and it will be used as a volumetric reagent in further steps of the experiment. Store the NaOH solution in a tightly capped bottle when not in use, and use the solution promptly after filling the buret. C. Standardization of HCl with NaOH. Dispense three 25 mL aliquots of your ~0.1 M HCl solution into three 250 mL Erlenmeyer flasks; use a 25.00 mL Class A glass pipet. Now add 3 5 drops of phenolphthalein indicator solution, and titrate using the standardized NaOH solution. Calculate the molarity of each HCl solution. You should have a minimum of three data points that agree well , having a %RSD less than 1%. If your three data points do not meet this requirement, complete additional trials.
Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 5 Procedure Week 2 Determination of alkalinity by pH titration with HCl You should bring in a water sample to analyze as your unknown this week. You may be as creative as you like! You should collect about 0.5 L of your sample. A. Preparation of samples Prepare a sample batch for analysis that consists of one 100.00 mL sample of Evian (your QC Control ) and one 100.00 mL sample of your unknown water sam ple. Prepare a buret with HCl, ensuring that the buret has been thoroughly rinsed after being used for dispensing NaOH solution. For ease of collecting data, fill your buret to 0.00 mL. B. Calibration of the pH meter Perform titrations at the Logger Pro Vernier s tations using the pH probes and HCl filled buret. Open â€œLogger Pro Potenti ometric Analysisâ€ from the computer desktop. Before beginning a titration, calibrate the pH meter with pH 4 and pH 10 buffers. From the menu bar, select experiment, calibrate, then pH. Insert the probe into the pH 4 solution and select the â€œcalibrate nowâ€ button. Enter 4.00 as reading 1, and then select â€œkeepâ€ w hen the voltage stabilizes. Rinse the electrode with DI water and repeat for reading 2 and the pH 10 buffer. Then select â€œdoneâ€ and check the pH reading with the pH 7 buffer. If the reading is more than 0.05 pH units off, repeat the calibration. C. pH titration of samples Insert a magnetic stirring bar into the solution to be ti trated and position the pH probe so that it does not com e into contact with the stirring bar. S elect start to begin collecting data and â€œkeepâ€ to record the pH with a volume of 0.00 mL HCl added. Add HCl dropwise , and select â€œ keep â€ to record a pH value and corresponding volume . Enter the volume added to two decimal places. In your notebook, record the volume of HCl required to bring each solution to a pH of 4.5. You may need to interpolate to find this volume. Select stop when finished, then e xport the data as a *.CSV for each trial . Calculate the milli moles of HCl required to bring each solution to a pH of 4.5. Then report the alkalinity as the milli moles of HCl per L of solution tested. D. Calculation of alkalinity for Evian You will use the 2014 Evian Water Quality Report on Canvas to calculate the alkalinity of Evian. The reported result is in units of mg/L of CaCO3. Use dimensional analysis to convert this to units of mmol HCl/L of solut ion.
Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 6 Procedure Week 3 A. Calculation of total cation content in Evian water You will use the 2014 Evian Water Quality Report on Canvas to calculate the total molarity of positive charge for the major cat ions in bottled water (Na+, K+, Mg2+, Ca2+). Since results are reported in mg/L, you must convert each reported ion concentration into molarity. In this lab, we are interested in knowing the total moles of positive charge for comparison with our cation exchange chromatography results of our unknown w ater samples . In or der to calculate the total molarity of positive charge from the Water Quality Report, you will sum for all cations the molarity of each times the charge ratio as shown below. + + + = Total Molarity of positive charge B. Preparation of the cation exchange column Each student should prepare two columns containing ~ 10 mL of Dowex 50W X8 cation exchange resin. Assemble the columns as shown by the instructor. Obtain the bottle of charged resin, shake and then immediately pour into the column until almost full . Once the resin begins to settle, make sure the column is about 2/3 full. Once filled, do not let the resin bed go dry: maintain t he column covered with nanopure water when not in use, or another aqueous solution at all times. Rin se each column with about 400 mL of nanopure water to ensure that all free H+ ions have been washed out of the resin bed and will therefore not interfere with your results . You may notice some color to the rin se solution at first ; this is normal but should run clear. C. Running the samples Prepare a sample batch for ion-exchange that consists of one 100.00 mL aliquot o f your unknown water sample and one 100.00 mL aliquot of Evian (the QC control) . Place these samples in 250 mL beakers in preparation for column filtration. After the column has been washed with 400 mL of nanopure water, introduce the unknown water
Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 7 sample to the column and insert a clean beaker below to collect the eluate. Add about 10 mL of nanopure water after your sample to ensure all of the sampleâ€™s eluate has passed through. Repeat this process with the second column and the Evian. Rinse with 10 mL of nanopure water to ensure all sample has passed through. D. Titration of the samp les Prepare a buret with NaOH, ensuring that the buret has been thoroughly rinsed after being used for dispensing HCl solution. Perform titrations at the Logger Pro Vernier stations using the pH probes, NaOH filled burets, and the graphing software as dem ons trated by the instructor. Export the data generated for both titrations. E. Calculation s Use the Excel template on Canvas to ge nerate first derivative plot s for each titration, then determine the volume to the equivalence point for each sample. Calculate the cation content for each sample as the molarity of H+ ions. Clean up All titration samples may be disposed of in the acidbase container in the hood. Ion exchange resin should be place d in the designated container in the hoodâ€”do not put this in the acid -base container! Conclusion Your concl usion should be submitted electronically on Bb. Format your typed con clusion to include the purpose and following results. Tell the molarity of your standardized NaOH and HCl solutions with standard deviations and % RSD. Report the alkalinity of the Evian control and your unknown. Give context to these values by comparing to the calculated alkalinity from the Evian water quality report. Using the results f rom your first derivative plot , list the molarity of H+ ions for each sample. This represents the total cation content in each sample. How does the total cation content determined for your water sample compare to the total cation conte nt calculated for Evian? Any references should be cited at the end of your conclusion using ACS specifications. References 1. Mattson, M.D. â€œAlkalinityâ€ Encyclopedia of Inland Waters, Elsevier, 2009. P 16.