3472
Determination Chemistry of manganese in a vitamin tablet by atomic
absorption
Purpose:
To determine the concentration of a common metal ion in commercial vitamin tablets by
developing an analytical method based on atomic absorption spectrophotometry.
Apparatus:
Atomic absorption spectrophotometer and accessories
Hollow-cathode lamp: Mn
Standard laboratory analytical glassware
VWR 413 filter paper
0.2 m syringe filter
Instructions:
Brief Operating Procedure for Varian SpectrAA-5 Flame Atomic Absorption
Spectrophotometer (see end of this document).
Chemicals:
Prepare 250 mL of 8 M HCl, [Concentrated HCl is about 12 M]
Manganese standard (1000 g/mL)
Vitamin sample: Each tablet contains 2 mg manganese (as manganese sulfate), 162 mg of
calcium (as calcium phosphate and calcium carbonate), 20 g of selenium (as sodium
selenate), 75 g molybdenum (as sodium molybdate), 150 g boron (as sodium borate)
NaOH pellets (for neutralization of solutions)
Procedure:
1. You will need to construct a calibration curve for Mn. Then develop dilutions of
the solution prepared from the tablet which allow you to work in the linear range
of the calibration curve
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2. Calibration curve
Manganese
Pipet 5 mL of the manganese standard (1000 g/mL) into a 50 mL flask and
dilute to the mark. Pipet 0, 1, 5, 10, and 20 mL of this Mn stock (100 g/mL) into
50 mL volumetric flasks. Add 25 mL 8 M HCl to each and dilute with deionized
water to the mark.
3. Unknown
Carefully weigh a tablet and place it in a 250 mL Erlenmyer flask with 25 mL 8 M
HCl. Boil slowly on a hot plate for 5 min. Cool and add 10 mL deionized water.
Filter through a VWR 413 filter paper. Transfer the filtrate to a 100 mL volumetric
flask and dilute to the mark with deionized water. This is the vitamin stock solution.
If the filtrate is cloudy, filter it through a 0.2 m syringe filter.
Aspirate each of the calibration standards and record the absorbances. Rezero the
instrument with the blank and repeat your readings at least 5 times for each standard.
Rezero the instrument and aspirate the sample and read the absorbance. Each group
should have at least 5 independent measurements. [Remember more independent
measurements reduce the error in the mean as an estimate of the concentration.
BE SURE TO CHECK TO SEE WHETHER DATA FOR THE UNKNOWN FALLS
IN A LINEAR PORTION OF THE CALIBRATION CURVE. This means you will
have to have some idea of what your calibration curve looks like before you measure
the unknown. If it is too concentrated, dilute (quantitatively) and remeasure.
4.
Leaving Lab
Be the sure to neutralize all your solutions and dispose of them properly. The TA;s will
guide you in this.
Data Analysis
1. Construct a calibration curve. Be sure to include y error bars. Note any nonlinearities. Try to work in the linear portion of the curve. Fit the linear portion of the
curve using linear regression.
2. Determine (1) the amount [report it as mg (or g, as appropriate) per gram of crude
sample] and (2) the 95% confidence interval.
Lab Report
A good analytical chemist is a chemist who has decided to make good
measurements
In addition to the data analysis and results, you will need to address the following:
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1. Does a 1000 ppm standard have to be exactly 1000 ppm?
2. Must one always work in the linear portion of a calibration curve?
Brief Operating Procedure for
Varian SpectrAA-5 Flame Atomic Absorption Spectrophotometer
1.
Install Mn Lamp (This will be carried out by the TA prior to lab.)
Rotate turret to loading position.
Place Mn lamp in turret by depressing white button and aligning connections.
Set slit to 0.5 nm.
Set lamp current to 3 mA:
press -->
Lamp 1 mA
press -->
3
press -->
Abs
Take brake off.
Adjust wavelength to obtain optimum signal. Refer to the AA Manual for the
proper wavelength. Observe the meter in the lamp housing, and Rotate gears
from low to high wavelength.
Optimize lamp signal using 2 knobs on left of turret (alignment of lamp)
Iterate the above two steps
Place the brake on.
Turn off current to lamp until students are ready to use AA
2.
Turn on Mn lamp
Press Lamp 1
Press3
Press Abs
3.
mA
Ignite Flame
Open acetylene tank
Turn on air knob
Turn on acetylene knob
Press ignite button
Once the flame is lit, place capillary tube in distilled water
4.
Set up integration time
Press Time sec
Press 0.5
Press Run Mean
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5.
Set Zero
Make sure capillary tube is in distilled water
Press -->
Read
Press -->
Cal. Zero
Iterate until absorbance reads zero
6.
Measurements
Place capillary tube in sample.
Take absorbance reading from sample.
press Read
After reading stabilizes, write down value of absorbance
Repeat, alternating measurement of distilled water and sample.
7.
Shut down instrument.
Aspirate 50 mL of distilled water (to clean capillary system!)
Turn off acetylene tank at main valve. Flame will extinguish when
acetylene in the line is gone.
Turn acetylene knob to off
Turn air knob to off
Turn lamp current off
press -->
Lamp 1 mA
press -->
0
press -->
Abs
Remove Mn lamp (TA will do this)
Turn off power to instrument
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