Determination of Fe3+ with UV/VIS
Spectrophotometry
By Matthias Heyner
Group 2 Pair 3
General Experiment Description:
“Fe3+ ions react with salicylic acid in aqueous solution by forming Fe(sal)3 complex. The
complex has an intensive violet color, possessing an absorption maximum between 400600nm.”
1. Create the Standard Fe3+-solution:
 Measure 2ml of the concentrated FeCl3 solution and fill it to a 100ml flask
 Add 0.1 M HCl solution till the 100ml mark
 Shake
2. Create Salicylic acid solution:
 Measure 0.5g salicylic acid and dissolve it in 50 ml ethanol
 Fill it in a 100ml flask
 Add distilled H2O till the 100ml mark
 Shake
Fill 7 Flasks with the necessary Volumes of FeCl3 solution and salicylic solution given in the
table below:
Sample FeCl3 mg / 100ml Necessary salicylic acid
Necessary standard FeCl3
solution, ml
solution, ml
Blind
0
10
1
2,5
10
2
5,0
10
3
7,5
10
4
10,0
10
5
12,5
10
6
15,0
10
Then add distilled H2O till the 100ml mark and shake and incubate the samples for 20 minutes
at room temperature.
Start measuring the Samples and measure for each absorbance at least three parallel
measurements.
1
0
1
2
3
4
5
6
Calculation:
Calculation table
Sample
1
2
3
4
5
6
Unknown
FeCl3 mg / 100 ml Abs 1 Abs 2 Abs 3 Abs Average
2,5 -0,02 -0,03 -0,03
-0,027
5,0
0,03
0,02
0,04
0,030
7,5
0,07
0,08
0,10
0,083
10,0
0,12
0,13
0,13
0,127
12,5
0,19
0,19
0,21
0,197
15,0
0,21
0,21
0,21
0,210
32,5
0,59
0,58
0,57
0,580
Graph:
Absorbtion vs Concentration
y = 49,833x + 3,6006
16
14
FeCl3 mg / 100 ml
12
10
8
6
4
2
-0,050
0
0,000
0,050
0,100
0,150
0,200
0,250
Absorbtion
From the Graph we get the best fit line with y=49.883X+3.6006 for x=0.58 => y=32.50374
which is the Concentration in the Unknown Sample.
Unknown Sample= 32.504 FeCl3 mg / 100 ml

Molar concentration: c=n/V n=m/M => c=2.018865839 moles per liter.

2