4: Hard-Soft Acid Base Remediation
- Page ID
- 509300
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Lab 4: Hard-Soft Acid-Base Remediation
This lab was developed from the following articles:
Stathatou, P. M., Athanasiou, C. E., Tsezos, M., Goss, J. W., Blackburn, L. C., Tourlomousis, F., Mershin, A., Sheldon, B. W., Padture, N. P., Darling, E. M., Gao, H., & Gershenfeld, N. (2022). Lead removal at trace concentrations from water by inactive yeast cells. Communications Earth & Environment, 3, Article 132. https://doi.org/10.1038/s43247-022-00463-0
&
Sharma, H., Sharma, A., Sharma, B., & Karna, S. (2022). Green analytical approach for the determination of zinc in pharmaceutical product using natural reagent. International Journal of Analytical Chemistry, Article 8520432. https://doi.org/10.1155/2022/8520432
- Students will...
- Students will...
Procedure:
Extraction of acacia catechu phenols (groups of 2-4)
Measure out 25 g of Acacia catechu powder into a 100 of 150 mL Erlenmeyer flask. Add 5 g \(\ce{NaHCO3}\).
Background:
Lewis acids and bases can be further classified as “hard” or “soft” acids and bases. Hard acids tend to have very low electronegativities and small electron spheres. Hard bases tend to have very high electronegativities and small, nonpolarizable electrons spheres. Soft acids and bases tend to have moderate electronegativities and more polarizable electron spheres.
Questions:
1. Where in the periodic table would you expect to find a hard Lewis acid? Give 2 examples.
2. Where in the periodic table would you expect to find a hard Lewis base? Give 2 examples.
3. Where in the periodic table would you expect to find a soft Lewis acid? Give 2 examples.
4. Where in the periodic table would you expect to find a soft Lewis base? Give 2 examples.
Background:
Because orbitals need to overlap constructively to form bonds, the strongest bonds are typically formed between hard acids and hard bases, which have both have smaller orbitals, or between soft acids and soft bases, which both have larger orbitals. Metal ions in the bottom-center to the bottom right of the periodic table tend to be soft acids, with the notable exception of the lanthanides and actinides in the f-block.
Questions:
5. Look back at your notes on ionic radii. Why are the lanthanides and actinides hard acids and not soft?
6. Identify the following toxic metals or precipitating anions as hard or soft.
· U+6 · O-2 · F- · Hg+2 · Pb+2 · Cd+2 · Sn+2 · S-2
Background:
Heavy metals contamination can pose environmental and health hazards. Unlike organic contamination, however metals cannot be further degraded to a less toxic form. Instead, remediation (or treatment in the case of ingestion) often involves chelation (binding with multiple base atoms on the same molecule), adsorption, precipitation, or demobilization, all strategies that involve binding the metals by other compounds to help them be isolated or made less available.
Chelators and adsorbents often will have multiple atoms that serve as Lewis bases that are covalently linked together. For example, the biodegradable chelator NTA (nitriloacetic acid, see right), has 7 atoms that may serve as Lewis bases.
Questions:
7. What two types of atoms serve as Lewis bases in NTA?
8. Is NTA a hard base or a soft base?
9. Do you think most chelators, adsorbents, or precipitants for remediation should be hard bases or soft bases? Consider your answer to 6. Why or why not?
10. Metals like \(\ce{Ca^{+2}}\) and \(\ce{Mg^{+2}}\) are essential to all forms of life. Are they hard or soft acids?
11. What dangers can you think of for using a hard base chelator for remediating a heavy metal in a natural system?
12. Which is a harder Lewis base \(\ce{O^{\text{-}2}}\) or \(\ce{S^{\text{-}2}}\)? Why?
13. Iron occurs naturally as \(\ce{Fe^{+2}}\) and \(\ce{Fe^{+3}}\). Which form is a harder Lewis acid? Why?
14. Which is a more likely compound – \(\ce{FeS}\) or \(\ce{Fe2S3}\)? Why?
15. Which is a more likely compound – \(\ce{FeCO3}\) or \(\ce{Fe2{(CO_3)}3}\)? Why?
Background:
In lab today, we will be testing a bevy of chelating agents and adsorbents to see which may be able to remediate heavy metals. We will be using zinc for initial screenings as a model for soft Lewis acid interactions with our remediation candidates, as zinc waste is not toxic. A common method of metals analysis is by colorimetric determination – we did this last week with an iron – ferrozine complex. Sometimes the compounds combined with the metals are toxic, and can create more complex waste for disposal. We are using acacia leaf powder, rich in catechols that create colored complexes with many metals, to determine zinc concentrations without generating hazardous waste. When we add our remediation candidates, only zinc that is unbound (free zinc) can react with the catechols to form a colored solution, so we can tell how much has been “remediated”.
Procedure:
Colored Reagent Solution (groups of 2-4):
1. Filter the acacia powder extract using a Buchner funnel, filter paper, and a clean side-arm vacuum flask attached to a vacuum line. It is important to add a little water to the filter paper with the vacuum applied so the filter paper sticks to the funnel.
2. As the filtrate (the stuff on top) begins to dry out, rinse three times with 1-3 mL of water, continuing to pull the vacuum.
3. Transfer the filtered solution to a 50 mL volumetric flask using a funnel. After pouring the solution into the flask, rinse three times around the side-arm flask with about 1-3 mL of DI water, emptying the rinse water into the volumetric flask each time. Then rinse the funnel. This is called a quantitative transfer, as it attempts to get all of the material into the new container. This should be your normal practice for transferring liquids between containers.
4. Bring the volume of the solution up to exactly 50.0 mL using the pH 5 acetate buffer. This solution is your reagent solution.
Standard solutions (per table):
5. Carefully measure 12.5 mL of 100 ppm zinc sulphate with a graduated cylinder.
6. Quantitatively transfer to a 50 mL volumetric flask and bring the total volume to 50.0 mL using the pH 5 acetate buffer. The buffer is This new solution is 25 ppm.
7. Repeat steps 5 and 6 with 2.5 mL, 5.0 mL, 7.5 mL, and 10 mL. What are these solution concentrations (hint: how does 2.5 compare to 12.5)?
Standard curve (groups of 2)
8. Using a calibrated micropipette, combine 1 mL of the 25 ppm solution with 5 mL of reagent solution. Use different tips for the zinc sulphate and the reagents solution. Do not touch the mixture while adding the reagents solution. After adding the last mL of reagent solution, pipette up and down several times to mix.
9. Fill a cuvette 2/3 from the top with this mixture. Fill 1 mL at a time, and make sure you know exactly how many mL you used for the cuvette.
10. Connect a SpectroVis to a LabQuest. Make sure the units are set to “Abs”. If they aren’t, click on the red banner and select “Change Units”.
11. Click on the red banner and select “Calibrate”. Follow the instructions on screen.
12. Click on the red banner again and select “Mode”. Choose “Full Spectrum”.
13. Use the “play” button to start taking data.
14. The best place to get data is usually the wavelength where absorption is the highest, the λmax. What is the λmax?
15. Return to the home screen and click on the red banner again and select “Mode”. Choose “Events with Entry”, and set the wavelength to λmax. For the event, give the label “[Zn]” and the unit “ppm”.
16. Use the “play” button to start taking data.
17. Place your lowest concentration sample in the reader, making sure the clear sides line up with the light and arrow icons on the SpectroVis. Hit “Keep” on the bottom of the screen, and enter the concentration for the first event. Repeat for each sample, moving from lowest concentration to highest. Keep your highest concentration (25 ppm) cuvette.
18. Hit “stop” to complete data collection.
19. Use the “Table” icon to navigate to your data in table form. Download a local copy of the linked Excel spreadsheet and record the absorbance data on your spreadsheet.
20. Create a scatterplot with Target concentrations (A5-A9) as the x-axis and Absorbance (E5-E9) as the y-axis. Make sure to label axis and the graph clearly.
21. Add a linear-trendline for the displayed data, showing the equation and R2 value. What is the molar absorptivity ( ) of the acacia / zinc solution at 550 nm?
Evaluation of remediation candidates (groups of 2)
22. Ask your instructor for a remediation candidate. You will be given a concentrated solution of glutathione, cysteine, selenium yeast, garlic extract (use a hood), brewer’s yeast (which all have sulfur-containing compounds), and citric acid, alginic acid, succinic acid, and nitrilotriacetatic acid, which are all carboxylic acids.
23. Return to the home screen and click on the red banner again and select “Mode”. Choose “Events with Entry”, and set the wavelength to λmax. For the event, give the label “Remediation Compound” (use the name of your remediation compound) and the unit “drops”.
24. Use the “play” button to start taking data. Hit keep immediately and enter “0” for the number of drops.
25. Add a drop of your remediation compound. Use a separate pipette to mix the solution by pipetting up and down. Check to see if your absorbance has changed significantly. If it has, hit “Keep” on the bottom of the screen, and enter the number of drops for the first event. If it hasn’t, slowly add (while counting) drops until you’ve noted a change, then hit “Keep” and enter the number of drops. Continue adding drops in the same increment you chose between the first and second number of drops 5 more times, keeping and recording the number of drops each time.
26. Hit “stop” to complete data collection.
27. Use the “Table” icon to navigate to your data in table form, and record the absorbance data on your spreadsheet by changing the sheet to “Evaluation of Remediant”.
28. Using your calibration curve’s line of best fit equation (step 21), create a calculate column in Excel for the concentration of free zinc corresponding to each absorbance measurement (C5-C11).
29. Create a scatterplot with “drops of remediation candidate” (A5-A11) as the x-axis and concentration of zinc (C5-C11) as the y-axis. Make sure to label axis and the graph clearly.
30. You do not need to create a line of best fit for this data.
31. On the online version of the linked Excel spreadsheet, go to the “Class Data” sheet and enter your names, remediant, and data in the next available slot. It should automatically plot in the “Plot of Class Data” tab.
Post-lab questions:
16. Look at the “Plot of Class Data”. What types of remediants were most successful? Least successful? How can you tell?
17. Propose an experiment that you would like to perform to follow up now that you’ve seen some preliminary data.
See also:
El-Khishin, I. A., El-fakharany, Y. M. M., & Hamid, O. I. A. (2015). Role of garlic extract and silymarin compared to dimercaptosuccinic acid (DMSA) in treatment of lead induced nephropathy in adult male albino rats. Toxicology Reports, 2, 824-832. https://doi.org/10.1016/j.toxrep.2015.04.004
Siriangkhawut, W., Ponhong, K., & Grudpan, K. (2019). A green colorimetric method using guava leaves extract for quality control of iron content in pharmaceutical formulations. Malaysian Journal of Analytical Sciences, 23(4), 595-603. https://www.ukm.my/mjas/v23_n4/pdf/Siriangkhawut_23_4_5.pdf