1 / 38

SURVEY OF CHEMISTRY LABORATORY I CHEM 1151L SPECTROPHOTOMETRIC DETERMINATION OF CHROMIUM

SURVEY OF CHEMISTRY LABORATORY I CHEM 1151L SPECTROPHOTOMETRIC DETERMINATION OF CHROMIUM. CAUTION!!!. BE SURE TO WEAR SAFETY GLASSES AT ALL TIMES IN THE LABORATORY NO EXCEPTIONS TO THIS RULE!. SPECTROPHOTOMETRY. - To measure the absorbance of different concentrations of chromium

numbersr
Download Presentation

SURVEY OF CHEMISTRY LABORATORY I CHEM 1151L SPECTROPHOTOMETRIC DETERMINATION OF CHROMIUM

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. SURVEY OF CHEMISTRY LABORATORY I CHEM 1151L SPECTROPHOTOMETRIC DETERMINATION OF CHROMIUM

  2. CAUTION!!! BE SURE TO WEAR SAFETY GLASSES AT ALL TIMES IN THE LABORATORY NO EXCEPTIONS TO THIS RULE!

  3. SPECTROPHOTOMETRY - To measure the absorbance of different concentrations of chromium Students will - Prepare chromium-EDTA complex solutions (colored complex) - Determine the wavelength of maximum absorbance of the colored complex (λmax) - Determine the concentration of an unknown chromium solution

  4. CHROMIUM SOLUTION - The chromium ion (Cr3+) react with EDTA (ethylenediaminetetraacetic acid) to form a colored complex - Heat is required for reaction to go to completion - The complex absorbs light at particular wavelength and reflects others - Intensity of color depends on the concentration of chromium - The color intensity can be used to measure the concentration of solutions

  5. BEER’S LAW - The absorbance of radiation (light) is related to the Wavelength of radiation and Concentration of the absorbing species - The equation for the relationship between the optical absorbance and concentration is known as Beer’s Law

  6. BEER’S LAW A = kbc - A is the optical absorbance - k is a constant known as the molar absorptivity - b is the pathlength (distance through which light passes sample) - c is the concentration (molarity) of the solution

  7. BEER’S LAW A = kbc - Optical absorbance (A) is measured using the spectrophotometer (read directly from the instrument) - The spectronic 20 will be used for this experiment - The small test tube that holds the sample is known as the cuvet - The diameter of the cuvet = 1 cm - Implies the pathlength (b) for this experiment = 1 cm - Beer’s law reduces to A = kc

  8. PROCEDURE I DETERMINATION OF THE WAVELENGTH OF MAXIMUM ABSORBANCE (λmax)

  9. PREPARATION OF Cr3+-EDTA COMPLEX - Set up a hotplate at your work station - Fill a beaker with water (about half-filled) - Place the beaker of water on the hotplate to heat up

  10. PREPARATION OF Cr3+-EDTA COMPLEX - Use the buret for volume measurements - Obtain a regular large test tube (not the small test tubes near the instrument) - Measure 5.00 mL of chromium nitrate solution and 5.00 mL of EDTA solution into the test tube

  11. PREPARATION OF Cr3+-EDTA COMPLEX - Place the test tube in the boiling water bath for at least 10 minutes

  12. PREPARATION OF Cr3+-EDTA COMPLEX - Remove and cool the solution by placing the test tube in cold water

  13. SPECTRONIC 20 - Refer to the “General Procedure for Spectrometer Use” in Appendix II - Turn on the Spectronic 20 instrument to warm up for at least 5 minutes before use

  14. λmax DETERMINATION - Obtain two clean cuvets (small test tubes near the instruments) - Fill one cuvet with EDTA solution - Fill the other cuvet with the prepared colored complex - Fill each to at least three-quarters full

  15. λmax DETERMINATION - Avoid holding the sides of the cuvets (do not leave finger prints on them) - Carefully clean the sides with kim wipes Click on picture to play movie

  16. λmax DETERMINATION - Set the instrument to 350 nm - Insert the EDTA cuvet into the sample holder - Mark on the cuvet should be in line with mark on the sample holder - Make sure instrument is in the absorbance (A) mode - Zero the instrument Click on picture to play movie

  17. λmax DETERMINATION - Remove the EDTA cuvet - Insert the COMPLEX cuvet into the sample holder - Mark on the cuvet should be in line with mark on the sample holder - Read and record the abosrbance displayed Click on picture to play movie

  18. λmax DETERMINATION - Change the wavelength to 360 nm - Insert the EDTA cuvet into the sample holder - Mark on the cuvet should be in line with mark on the sample holder - Zero the instrument

  19. λmax DETERMINATION - Remove the EDTA cuvet - Insert the COMPLEX cuvet into the sample holder - Mark on the cuvet should be in line with mark on the sample holder - Read and record the absorbance displayed

  20. λmax DETERMINATION - Repeat steps at wavelength increments of 10 up to 600 nm - Zero instrument with EDTA cuvet each time wavelength is changed - Clean cuvets with kim wipes each time before inserting into the instrument - Always aline marks on the cuvets and the sample holder

  21. ABSORBANCE VS WAVELENGTH GRAPH - Use the Chart Wizard in Excel - Plot a graph of absorbance verses wavelength - This is not a straight line so do not join the points with a line - Only plot the points on scatter and print it out - Use pencil to join the points with your free hand - Determine the wavelength corresponding to the maximum absorbance This is the λmax of the Cr3+-EDTA complex

  22. PROCEDURE II DETERMINATION OF THE RELATIOSHIP BETWEEN ABSORBANCE AND CONCENTRATION OF THE Cr3+ AT THE λmax DETERMINATION OF THE CONCENTRATION OF AN UNKNOWN Cr3+ SOLUTION

  23. HOMEWORK EXERCISE Do the following calculations before coming to the next class - Calculate moles of Cr3+ for test tubes 1 through 5 - Calculate molarities (M) of Cr3+ for test tubes 1 through 5 - Complete the table with moles and molarities of Cr3+

  24. HOMEWORK EXERCISE Use the dilution method: M1V1 = M2V2 Molarity (concentration) of stock Cr3+ solution = M1 = 0.00750 mol/liter Final volume after mixing for each test tube = V2 = 10.00 mL For example Mixing 2.50 mL (V1) of stock Cr3+ solution with 7.50 mL EDTA 2.50 mL Cr3+ + 7.50 mL EDTA = 10.00 mL total solution = 0.0100 L Moles of Cr3+ = Stock concentration (Molarity) x Volume of Cr3+ (liters) = 0.00750 moles/liter x 0.00250 liter = 0.00001875 mole of Cr3+ Molarity (M2) = Moles Cr3+ / Total volume of solution (liters) = 0.00001875 mole/ 0.0100 L = 0.001875 M (or moles/liter) Cr3+ M means Molarity or moles/liter

  25. HOT WATER BATH - Set up a hotplate at your work station - Fill a beaker with water (about half filled) - Place the beaker of water on the hotplate to heat up

  26. PREPARATION OF KNOWN SOLUTIONS - Use the buret to measure volumes of solutions - Obtain five large test tubes and label them 1 through 5 - Carefully measure out the Cr3+ and EDTA solutions directly into the test tubes using volumes in the table that was designed as the homework assignment

  27. PREPARATION OF AN UNKNOWN SOLUTION - Obtain an unknown sample of 5.00 mL Cr(N03)3 of an unknown concentration - Record the unknown number on your data sheet - Add 5.00 mL of EDTA solution to give you a total of 10.00 mL

  28. HEAT SOLUTIONS - Place all the test tubes (knowns and unknown) in the boiling water bath for at least 10 minutes - Remove any caps on the tubes before heating

  29. COOL SOLUTIONS - Remove and cool the solutions by placing the test tubes in cold water

  30. SPECTRONIC 20 - Refer to the “General Procedure for Spectrometer Use” in Appendix II - Turn on the Spectronic 20 instrument to warm up for at least 5 minutes before use

  31. ABSORBANCE DETERMINATION - Obtain one clean cuvet (small test tubes near the instruments) - Fill the cuvet with EDTA solution - Fill to at least three-quarters full

  32. ABSORBANCE DETERMINATION - Avoid holding the sides of the cuvet (do not leave finger prints on them) - Carefully clean the sides with kim wipes

  33. ABSORBANCE DETERMINATION - Set the instrument to the determined λmax - Insert the cuvet into the sample holder - Mark on the cuvet should be in line with mark on the sample holder - Make sure instrument is in the absorbance (A) mode - Zero the instrument

  34. ABSORBANCE DETERMINATION - Remove the cuvet - Pour out EDTA into a waste beaker - Rinse cuvet twice with bits of solution #1 (pour rinsing solution into the waste beaker) - Fill cuvet with solution #1

  35. ABSORBANCE DETERMINATION - Wipe sides with kim wipes - Insert cuvet into the sample holder - Mark on the cuvet should be in line with mark on the sample holder - Read and record the absorbance displayed

  36. ABSORBANCE DETERMINATION - Repeat steps for the rest of the test tubes including the unknown - Use the same cuvet for all the samples - Always pour out used sample back into its tube and rinse twice with the next solution before refilling (do not rinse with water) - Clean cuvet with kim wipes each time before inserting into the instrument - Always aline marks on the cuvet and the sample holder - Wavelength remains at the λmax - Clean up when done and pour all solutions into the appropriate waste bottle

  37. ABSORBANCE VS WAVELENGTH GRAPH - Use the Chart Wizard in Excel - Plot a graph of absorbance verses wavelength - This is the Beer’s Law line graph - Line is expected to be a straight line so join the points with a trendline (the best straight line) - Include the equation of the trendline

  38. ABSORBANCE VS WAVELENGTH GRAPH - Use the equation of the trendline and the optical absorbance of the unknown to determine the concentration of the unknown - Also estimate the concentration of the unknown visually from the graph - Include the two graphs in your laboratory report

More Related