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

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Presentation on theme: "SURVEY OF CHEMISTRY LABORATORY I CHEM 1151L SPECTROPHOTOMETRIC DETERMINATION OF CHROMIUM."— 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 - 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 (λ max ) - Determine the concentration of an unknown chromium solution SPECTROPHOTOMETRY

4 - The chromium ion (Cr 3+ ) 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 CHROMIUM SOLUTION

5 - 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 Beers Law BEERS LAW

6 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 BEERS LAW

7 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 - Beers law reduces to A = kc BEERS LAW

8 PROCEDURE I DETERMINATION OF THE WAVELENGTH OF MAXIMUM ABSORBANCE (λ max )

9 - 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 PREPARATION OF Cr 3+ -EDTA COMPLEX

10 - 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 PREPARATION OF Cr 3+ -EDTA COMPLEX

11 - Place the test tube in the boiling water bath for at least 10 minutes PREPARATION OF Cr 3+ -EDTA COMPLEX

12 - Remove and cool the solution by placing the test tube in cold water PREPARATION OF Cr 3+ -EDTA COMPLEX

13 - 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 SPECTRONIC 20

14 - 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 λ max DETERMINATION

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

16 - 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 λ max DETERMINATION Click on picture to play movie

17 - 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 λ max DETERMINATION Click on picture to play movie

18 - 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 λ max DETERMINATION

19 - 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 λ max DETERMINATION

20 - 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 λ max DETERMINATION

21 - 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 λ max of the Cr 3+ -EDTA complex ABSORBANCE VS WAVELENGTH GRAPH

22 PROCEDURE II DETERMINATION OF THE RELATIOSHIP BETWEEN ABSORBANCE AND CONCENTRATION OF THE Cr 3+ AT THE λ max DETERMINATION OF THE CONCENTRATION OF AN UNKNOWN Cr 3+ SOLUTION

23 Do the following calculations before coming to the next class - Calculate moles of Cr 3+ for test tubes 1 through 5 - Calculate molarities (M) of Cr 3+ for test tubes 1 through 5 - Complete the table with moles and molarities of Cr 3+ HOMEWORK EXERCISE

24 Use the dilution method: M 1 V 1 = M 2 V 2 Molarity (concentration) of stock Cr 3+ solution = M 1 = mol/liter Final volume after mixing for each test tube = V 2 = mL For example Mixing 2.50 mL (V 1 ) of stock Cr 3+ solution with 7.50 mL EDTA 2.50 mL Cr mL EDTA = mL total solution = L Moles of Cr 3+ = Stock concentration (Molarity) x Volume of Cr 3+ (liters) = moles/liter x liter = mole of Cr 3+ Molarity (M 2 ) = Moles Cr 3+ / Total volume of solution (liters) = mole/ L = M (or moles/liter) Cr 3+ M means Molarity or moles/liter HOMEWORK EXERCISE

25 - 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 HOT WATER BATH

26 - Use the buret to measure volumes of solutions - Obtain five large test tubes and label them 1 through 5 - Carefully measure out the Cr 3+ and EDTA solutions directly into the test tubes using volumes in the table that was designed as the homework assignment PREPARATION OF KNOWN SOLUTIONS

27 - Obtain an unknown sample of 5.00 mL Cr(N0 3 ) 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 mL PREPARATION OF AN UNKNOWN SOLUTION

28 - 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 HEAT SOLUTIONS

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

30 - 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 SPECTRONIC 20

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

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

33 - Set the instrument to the determined λ max - 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 ABSORBANCE DETERMINATION

34 - 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 ABSORBANCE DETERMINATION

35 - 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 ABSORBANCE DETERMINATION

36 - 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 λ max - Clean up when done and pour all solutions into the appropriate waste bottle ABSORBANCE DETERMINATION

37 - Use the Chart Wizard in Excel - Plot a graph of absorbance verses wavelength - This is the Beers 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 ABSORBANCE VS WAVELENGTH GRAPH

38 - 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 ABSORBANCE VS WAVELENGTH GRAPH


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