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