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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 - PowerPoint PPT Presentation
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SURVEY OF CHEMISTRY LABORATORY I
CHEM 1151L
SPECTROPHOTOMETRIC DETERMINATIONOF CHROMIUM
CAUTION!!!CAUTION!!!BE SURE TO WEAR SAFETY GLASSES AT
ALLTIMES IN THE LABORATORY
NO EXCEPTIONS TO THIS RULE!
- 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
SPECTROPHOTOMETRY
- 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
CHROMIUM SOLUTION
- The absorbance of radiation (light) is related to the
Wavelength of radiationand
Concentration of the absorbing species
- The equation for the relationship between the optical absorbance and concentration is known as
Beer’s Law
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
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
BEER’S LAW
PROCEDURE I
DETERMINATION OF THE WAVELENGTH OF MAXIMUM ABSORBANCE
(λmax)
- 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 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
PREPARATION OF Cr3+-EDTA COMPLEX
- Place the test tube in the boiling water bath for at
least 10 minutes
PREPARATION OF Cr3+-EDTA COMPLEX
- Remove and cool the solution by placing the test tube in
cold water
PREPARATION OF Cr3+-EDTA COMPLEX
- 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
- 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
λmax DETERMINATION
- Avoid holding the sides of the cuvets (do not leave finger prints on them)
- Carefully clean the sides with kim wipes
λmax DETERMINATION
Click on picture to play movie
- 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
λmax DETERMINATION
Click on picture to play movie
- 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
λmax DETERMINATION
Click on picture to play movie
- 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
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
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
λmax DETERMINATION
- 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
ABSORBANCE VS WAVELENGTH GRAPH
PROCEDURE II
DETERMINATION OF THE RELATIOSHIP BETWEEN ABSORBANCE AND CONCENTRATION
OF THE Cr3+ AT THE λmax
DETERMINATION OF THE CONCENTRATIONOF AN UNKNOWN Cr3+ SOLUTION
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+
HOMEWORK EXERCISE
Use the dilution method: M1V1 = M2V2
Molarity (concentration) of stock Cr3+ solution = M1 = 0.00750 mol/literFinal volume after mixing for each test tube = V2 = 10.00 mL
For exampleMixing 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
HOMEWORK EXERCISE
- 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
- 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
PREPARATION OF KNOWN SOLUTIONS
- 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
PREPARATION OF AN UNKNOWN SOLUTION
- 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
- Remove and cool the solutions by placing the test tubes in cold
water
COOL SOLUTIONS
- 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
- 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
- Avoid holding the sides of the cuvet
(do not leave finger prints on them)
- Carefully clean the sides with kim wipes
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
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
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
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
ABSORBANCE DETERMINATION
- 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
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
ABSORBANCE VS WAVELENGTH GRAPH