Report 9_ Shahd Ahmed

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Jan 9, 2024

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Shahd Ahmed Report 9: Gravimetric Analysis of Chloride and Sulfate Abstract : In this experiment, the concentration of Cl- in seawater was estimated to be 11939.333 mg/L, 321.364 mM, and 1.171% by mass. The concentration of SO42- in the first trial is determined to be 2966.667 mg/L, 30.884 mM, and 0.284% by mass. In the second trial, the SO42 concentration is 2360 mg/L, 26.205 mM, and 0.225% by mass. Introduction : Using Gravimetric Analysis, the concentration of chloride and sulfate ions could be determined in a seawater sample. Using techniques such as filtration, pipetting, and dehydrating the precipitate from the seawater solutions allowed the concentration of Cl- and SO42- to be determined. Experimental Methods : Part 1 - Separation of chloride and sulfate by anion exchange Weigh a clean 50 mL beaker, take a 1.5 mL sample of filtered seawater, and transfer it to the beaker, weighing it again. Using an anion exchange column, drain the liquid into a flask underneath until the liquid is 2-3 millimeters above the resin bed. Transfer 5mL of 0.5 M NaNO3 solution into the column to check for chloride ions. Adjust the solution rate to 20 drops a minute and let the column drain until 1 mL of the 5 mL added remains. Replace the collection beaker with a clear test tube and collect the last mL, leaving 2-3 millimeters above the resin bed. Test the eluate for the presence of Cl- by adding a few drops of 0.2 M AgNO3 and looking for a white precipitate. If a precipitate forms, add 5 mL of 1.0 M NaNO3 to the column and collect the last mL of eluent. Once there is no precipitate, place a 250 mL Erlenmyer flask under the column and carefully load the previously weighed 1.5 mL sample of seawater. Rinse the beaker with 1-2 mL of 0.5 M NaNO3 solution and transfer the entire solution. Allow the liquid to drain through the column until the resin is covered by 2-3 mm of liquid. Attach a plastic funnel to the column's opening and slowly add 60 mL of 0.5 M NaNO3 solution to the funnel. Allow the solution to run through the column for 60 minutes, and begin working on part 3. Once 1 mL of the eluent is left in the column, stop the flow and collect the last milliliter in a test tube. Add a few drops of AgNO3 to see if precipitate forms. Cover the test tube in foil away from sunlight if a precipitate is present, and place the Erlenmyer flask back under the column. Repeat the process and add 15 mL more of the 0.5 M NaNO3 solution to the column until there is no precipitate.

Shahd Ahmed Report 9: Gravimetric Analysis of Chloride and Sulfate Part 2 - Quantitative gravimetric determination of chloride To a sulfate-free sample, which is 50 mL of eluate, add 8.0 mL of 0.2 M AgNO3 solution. Shield the flask from sunlight, as precipitation should occur while swirling the flask, and place the flask in the oven for 20 minutes. After 20 minutes, let the flask cool down, then add a few drops of 0.2 M AgNO3 solution to test for completeness of precipitation. If additional precipitation occurs, add 1 mL of 0.2 M AgNO3 solution. Set up a filtration apparatus with a clean collecting flask underneath. Obtain and weigh a Fisher MCE membrane filter, then place it in the filtration apparatus. Add 1 mL of DI water on top of the filter, then transfer the AgCl precipitate using two 5 mL portions of 0.01 M HNO3 on the walls of the flask. Turn on the vacuum in the aspirator and filter. Once filtered, discard the waste solution properly and rinse the precipitate with 2 mL of 0.01 M HNO3 and aspirate. Add a few drops of 0.1 M NaCl solution and look for precipitate; if precipitate occurs, add 5 mL of 0.01 M HNO3. Allow precipitate on the filter to air dry for 2-3 minutes with aspiration on, then place the filter in over for 60 minutes. After 1 hour, remove the sample from the oven and weigh it. Part 3 - Separation of chloride and sulfate by selective precipitation Prepare two identical samples, weigh 50 mL, transfer 1.5 mL of filtered seawater, and weigh the beaker again. Add 4-5 mL of Di water and 2 mL of 0.06 M Ba(NO3)2 in 0.01 M HNO3 to the beaker. Swirl the flask and let the precipitate stand for 15-20 minutes for both samples. Part 4 - Gravimetric determination of sulfate Weigh a 0.45 um membrane filter and place it into an assembled filtration apparatus. Place a clean receiving flask under and transfer each seawater sample separately with 4-5 mL of 0.01 M HNO3; filter with aspiration. Rinse the precipitate with 4-5 mL of 0.01 M HNO3. Allow the filter to dry for 2-3 minutes with the aspirator on, then place into the oven for 15-20 minutes. Once dry, weigh the filter paper for each seawater sample. Results : Table 1: Mass of seawater used in the sample

Shahd Ahmed Report 9: Gravimetric Analysis of Chloride and Sulfate Table 2: Quantitative Gravimetric determination of Chloride Molar Mass(AgCl): 143.32 g Molar Mass (Cl): 35.453 g Literature value of Cl concentration is 18,900 mg/L. Sample calculation for mg/L : Sample calculation for mmol/L : Sample calculation for mass% : Comparison with literature value of Cl - concentration : The concentration of Cl- in seawater, according to Lenntech, should be 18,900 mg/L. However, my calculations determine the concentration to be 11,939.333 mg/mL. The percent error between the two concentrations is 36.095%, and my calculated concentration could be lower based on human error. During the experiment, circumstances such as not retrieving all the precipitates from the flask when weighing the filter paper or not leaving the filter paper in the oven long enough could have caused my concentration of Cl- to be lower than the Lenntechs value of Cl- in seawater.

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Shahd Ahmed Report 9: Gravimetric Analysis of Chloride and Sulfate Table 3: Separation of Chloride and Sulfate by selective precipitation Table 4: Gravimetric Determination of Sulfate MM(BaSO4): 233.38 g MM(SO4): 96.06 g Literature value of SO4 concentration is 2.469 mg/L. Sample calculation for mg/L : Sample calculation for mmol/L :

Shahd Ahmed Report 9: Gravimetric Analysis of Chloride and Sulfate Sample calculation for mass% : Comparison with literature value of SO 42- concentration. Cite source : The concentration of SO42- in seawater, according to Lenntech, should be 2,469 mg/L. However, my calculations determine the concentration to be 2,966.667 mg/mL. The percent error between the two concentrations is 7.857%. The calculated concentration in this experiment is a few hundred milligrams higher than the Lenntechs concentration, so the values are pretty close. A cross-comparison of the concentration of the sulfate ion determined by the turbidimetric analysis performed earlier and the gravimetric analysis and your comments: The concentration of sulfate ions determined in the turbidimetric analysis was calculated to be 2834.839 ppm in seawater, which is also 2,834.839 mg/L. Using Gravimetric Analysis in this experiment, the concentration of sulfate ions was determined to be 2,966.667 mg/L. The two concentrations are not far off; the difference between them could be caused by the difference in the technique used to calculate them and human error that could have occurred during the experiment. Discussion : 1. The phrase "nitrate form" means that all the sites in the resin where ion exchange occurs are occupied by nitrate ions. By what simple laboratory process can one convert a column to "nitrate form"? - In order to convert a column to "nitrate form," adding a solution that contains no ions into the column would allow them to attach themselves to the resin. Therefore, the column is in nitrate form since the solution with no ions attached itself to the positively charged resin. 2. Why can't you let the level of the liquid in the column fall too low and expose the resin to air? - The resin can not be exposed to air because once it starts to dry, it affects the entire ion exchange column, causing the silica to crack with an inaccurate and poor separation of compounds if anything were poured into the columns.

Shahd Ahmed Report 9: Gravimetric Analysis of Chloride and Sulfate 3. Why is it suggested to use the flow rate at about 1 mL/min, about 20 drops in a minute and not faster? - Using a flow rate of 1 mL/min allows the sodium nitrate to flow through the resin slowly, allowing ions to separate through the ion exchange column, or else it would wash through the column too quickly. 4. Why SO 4 2- retains on the column when CL - is washed out? - Sulfate ions remain on the column because the molecule is larger, so the Cl- ions pass through more quickly. Sulfate also has a high retention, so it stayed on the top layer of the column until the eluent concentration was high. 5. What is the chemical reaction for photodecomposition of AgCl? What color change would be observed if the decomposition occurs? - 2AgCl(s) + Sunlight → 2Ag(s) + Cl2(g) - The color of AgCl would change to black when exposed to sunlight due to the absorption of energy and the formation of metallic silver in the reaction. 6. Why should you rinse precipitates with HNO 3 , not the distilled water? Use your textbook to answer. - Since AgCl is not soluble in water, it would be ineffective to rinse precipitates with it. However, AgCl can be rinsed with HNO3 since it prevents peptization, which is the formation of colloids when a precipitate disperses, which is why we use it during this experiment. Conclusion : This experiment determines the gravimetric analysis of chloride and sulfate ions in a seawater sample using ion exchange columns. The chloride ion concentration was determined to be 11,939.333 mg/L compared to Tennechs' calculated 18,900 mg/L concentration. The drastic difference between the two concentrations could be caused by experimental and human error, such as not removing the entirety of the precipitates left in the flask to weigh, causing the concentration of chloride to be calculated lower than it should be. The sulfate concentration in this experiment is concluded to be 2,966.667 mg/L while Lennetch concentration is 2,468 mg/L, which a slight error could have caused during the experimental procedure.

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Shahd Ahmed Report 9: Gravimetric Analysis of Chloride and Sulfate Work Cited : “Major ion composition of seawater (mg/L)”, Composition of seawater, LENNTECH. P.S. Kandagal, “Impact of Royal Society of Chemistry” ResearchGate, Feb 2019. https://www.mtsu.edu/chemistry/chem2230/pdfs/Exp%202.pdf

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