1. Experiment 23 Determination of the iron content with Spectrophotometry
Purposes
1. Learn to operate 722 Spectrophotometer.
2. Master to determine the iron content by use of
sulfosalicylic acid method.

2. Principles
In this experiment, we use Fe3+ ions to react with sulfosalicylic acid and produce brown [Fe (Ssal) 2]- ions under a condition
of HAc-NaAc buffer solution (pH=5). Determine the absorbance with λ=460nm.
The Lambert-Beer law provides the
mathematical correlation between
absorbance and concentration. It is
usually stated as

3. The relationship between A and c serves as the basis for the quantitative analysis of a great many substances.

4. Standard Curve A0 A1 A2 A3 A4 A5 ········· ··········· · cx · · · Ax ·
λ=460nm
c0 c1 c2 c3 c4 c5
A0 A1 A2 A3 A4 A5
The graph should
be a straight line

5. Prepare standard solutions and a sample solution
Procedure
Prepare standard solutions and
a sample solution
Prepare a series of standard solutions of
different accurately known concentration.
Get 6 volumetric flasks (10mL) and use the
measuring pipets to transfer solutions into the
volumetric flasks (according to the table).
Prepare a sample solution of unknown
concentration.

6. Preparation of Fe3+ standard solutions and a sample solution
solution/mL unknown
0.2mol/LHNO3 /mL
sulfosalicylic acid
solution /mL
Buffer (pH=4.7)/mL
Add distilled water
to mark /mL
Final c(Fe3+) cx
Absorbances (A)

7. 2. Determine of absorbances (A)
According to the usage of 722-type
spectrophotometer, use λ=460nm and use
blank solution (1#) as reference solution to
determine absorbance of each standard
solution and a sample solution.
Record the results.

8. 3. Draw a calibration curve
Draw a working curve. The abscissa is
the concentration of Fe3＋ of standard
solutions, the ordinate is the absorbance
of the standard solution.

9. 4. Determine the [Fe3+] of sample solution
According to the determined absorbance
of the sample solution, We can look up the
concentration of Fe3+ in sample from the
working curve drew by yourself.
Calculate the concentration of Fe3+ in
original undiluted sample.
[Fe3+] (original undiluted sample)
=[Fe3+] (diluted sample) ×10