Spectrophotometric detection of iron - Chemistry for engineers | Trường Đại học Quốc tế, Đại học Quốc gia Thành phố HCM

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Môn: Chemistry for engineers (CH011IU)

Trường: Trường Đại học Quốc tế, Đại học Quốc gia Thành phố Hồ Chí Minh

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Profile Mai Nguyệt
Internaonal University, Vietnam Naonal University - HCMC
ANALYTICAL CHEMISTRY LABORATORY
1
Lecturer: Dr. Nguyen Thao Trang
Semester I: 2013-2014
EXPERIMENT 4 –
SPECTROPHOTOMETRIC DETECTION OF
IRON
1. OBJECTIVES
Familiar with the use of spectrophotometric technique to determine the concentraon of an unknown
soluon.
2. INTRODUCTION
Fe
+2
is reacted with o-phenanthroline to form a orange-red complex ion. The intensity of the colored
complex is linearly proporonal to the concentraon of Fe This compound absorbs light in the visible
+2
.
region with a maximum absorbance occurring around 515 nm.
A calibraon curve (absorbance versus concentraon) is constructed for known concentraon soluon
Fe
+2
and the concentraon of the unknown iron sample is determined.
As
Fe
3+
does not form the complex with o-phenanthroline, all of the iron must be reduced to its ferrous
state. In this experiment, the reducon is done by hydroxylamine hydrochloride ( NH
2
OHHCl).
4Fe
3+
+ 2NH HCl 4 Fe O + 4H O
2
OH
2+
+ N
2
+
+ H
2
Orange red-
Internaonal University, Vietnam Naonal University - HCMC
ANALYTICAL CHEMISTRY LABORATORY
2
Lecturer: Dr. Nguyen Thao Trang
Semester I: 2013-2014
Sodium acetate (CH COONa) is used as a buer to maintain a constant pH at 3.5. Noted that Fe will be
3
2+
oxidized to Fe if the pH is high. When the pH is low, however, H will compete with Fe for phenanthroline
3+ + +2
and form phenanthroline -H complex.
+
3. PROCEDURE
3.1
Preparaon of standard Fe
2+
soluon
Mohr’s salt (ferrous ammonium sulfate hexahydrate, Fe(NH
4
)
2
SO
4
.6H
2
O) at a concentraon of 0.07
mg/mL is used as the standard in this experiment. This standard soluon is made by dissolving 0.07 g
Fe(NH .6H
4
)
2
SO
4 2
O in 300 mL DI water, then transferring to a 1000 mL volumetric ask. 25 mL H 2 M
2
SO
4
is added into the ask and DI water is lled to the calibraon mark. This soluon is referred to the stock
soluon.
3.2
Formaon of [ -o-phenanthroline] complex Fe
2+
- Preparaon of 9 test tubes:
B: is labeled for the blank
1-5 are labeled for the standard soluons
6-8: are labeled for the unknown sample (triplicate)
-
The formaon of the -o-phenanthroline] complex is carried out by the following procedure:[Fe
2+
Tube
B
1
2
4
5
6-8
Diluon
1:100
1:50
1:10
1:5
1 mL
sample
Stock soluon [mL]
0
0.1
0.2
1
2
CH
3
COONa [ml]
1
1
1
1
1
1
NH
2
OH.HCl [mL]
1
1
1
1
1
1
Phenanthroline [mL]
1
1
1
1
1
1
DI water [mL]
7
6.9
6.8
6
5
6
Total volume [mL]
10
10
10
10
10
10
- Then cover all test tubes with aluminum foil to prevent them from air exposure for 10 minutes for
the color development.
- Performing a wavelength scan to determine the wavelength where the absorpon is maximum,
called .
max
Internaonal University, Vietnam Naonal University - HCMC
ANALYTICAL CHEMISTRY LABORATORY
3
Lecturer: Dr. Nguyen Thao Trang
Semester I: 2013-2014
- The absorbance will be measured at .
max
- Record the absorbance values of the blank, standards and unknown sample.
- Construct a calibraon curve.
- Determine the unknown concentraon.
4. EQUIPMENT AND REAGENTS
Equipment
No.
Equipment
No.
100-mL beaker
50-mL beaker
Test tube
01
01
09
1-mL pipet
5-mL pipet
10-mL pipet
Srring rod
01
01
01
01
Reagents
Locaon
Reagents
Locaon
Fe(NH .6H
4
)
2
SO
4 2
O
assigned area
NH
2
OH.HCl
assigned area
Phenanthroline
CH
3
COONa
assigned area
assigned area
Iron tablet
assigned area
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Internaonal University, Vietnam Naonal University - HCMC 1
ANALYTICAL CHEMISTRY LABORATORY EXPERIMENT 4 –
SPECTROPHOTOMETRIC DETECTION OF IRON 1. OBJECTIVES
Familiar with the use of spectrophotometric technique to determine the concentraon of an unknown soluon. 2. INTRODUCTION
Fe+2 is reacted with o-phenanthroline to form a orange-red complex ion. The intensity of the colored
complex is linearly proporonal to the concentraon of Fe+2. This compound absorbs light in the visible
region with a maximum absorbance occurring around 515 nm. Orange-red
A calibraon curve (absorbance versus concentraon) is constructed for known concentraon Fe+2 soluon
and the concentraon of the unknown iron sample is determined.
As Fe3+ does not form the complex with o-phenanthroline, all of the iron must be reduced to its ferrous
state. In this experiment, the reducon is done by hydroxylamine hydrochloride (NH2OHHCl). 4Fe3+ + 2NH 2+ + 2OHHCl  4 Fe + N2O + 4H + H2O
Lecturer: Dr. Nguyen Thao Trang Semester I: 2013-2014
Internaonal University, Vietnam Naonal University - HCMC 2
ANALYTICAL CHEMISTRY LABORATORY Sodium acetate (CH 2+
3COONa) is used as a buer to maintain a constant pH at 3.5. Noted that Fe will be
oxidized to Fe3+ if the pH is high. When the pH is low, however, H+ will compete with Fe+2 for phenanthroline
and form phenanthroline -H+ complex. 3. PROCEDURE
3.1 Preparaon of standard Fe2+ soluon
Mohr’s salt (ferrous ammonium sulfate hexahydrate, Fe(NH4)2SO4.6H2O) at a concentraon of 0.07
mg/mL is used as the standard in this experiment. This standard soluon is made by dissolving 0.07 g
Fe(NH4)2SO4.6H2O in 300 mL DI water, then transferring to a 1000 mL volumetric ask. 25 mL H2SO4 2 M
is added into the ask and DI water is lled to the calibraon mark. This soluon is referred to the stock soluon. 3.2 Formaon of [Fe2
+ -o-phenanthroline] complex - Preparaon of 9 test tubes: B: is labeled for the blank
1-5 are labeled for the standard soluons
6-8: are labeled for the unknown sample (triplicate) - The formaon of the [Fe2
+ -o-phenanthroline] complex is carried out by the following procedure: Tube B 1 2 3 4 5 6-8 Diluon 1:100 1:50 1:20 1:10 1:5 1 mL Stock soluon [mL] 0 0.1 0.2 0.5 1 2 sample CH3COONa [ml] 1 1 1 1 1 1 1 NH2OH.HCl [mL] 1 1 1 1 1 1 1 Phenanthroline [mL] 1 1 1 1 1 1 1 DI water [mL] 7 6.9 6.8 6.5 6 5 6 Total volume [mL] 10 10 10 10 10 10 10 -
Then cover all test tubes with aluminum foil to prevent them from air exposure for 10 minutes for the color development. -
Performing a wavelength scan to determine the wavelength where the absorpon is maximum, called max.
Lecturer: Dr. Nguyen Thao Trang Semester I: 2013-2014
Internaonal University, Vietnam Naonal University - HCMC 3
ANALYTICAL CHEMISTRY LABORATORY -
The absorbance will be measured at max. -
Record the absorbance values of the blank, standards and unknown sample. - Construct a calibraon curve. -
Determine the unknown concentraon.
4. EQUIPMENT AND REAGENTS Equipment No. Equipment No. 100-mL beaker 01 1-mL pipet 01 50-mL beaker 01 5-mL pipet 01 Test tube 09 10-mL pipet 01 Srring rod 01 Reagents Locaon Reagents Locaon Fe(NH4)2SO4.6H2O assigned area NH2OH.HCl assigned area Phenanthroline assigned area Iron tablet assigned area CH3COONa assigned area
Lecturer: Dr. Nguyen Thao Trang Semester I: 2013-2014