Spectrophotometric determination of trace iron in water with heptaphenanthroline.
[Date: May 2008-12][ Font: large, medium and small]
First, the experimental purpose
1. Understand the general method for the determination of trace iron.
2. Learn to draw absorption curve and choose measuring wavelength.
3. Familiar with the experimental technique of standard curve quantification.
4. Learn to use spectrophotometer correctly.
Second, the experimental principle
Phenanthroline is a highly sensitive and selective reagent for the determination of trace iron. Phenanthroline spectrophotometry is a general method for the determination of trace iron in chemical products. In the solution of pH=2 9, phenanthroline and Fe 2+ form a stable orange complex, λ max =508nm. εmax = 1. 1× 104 l/(mol·cm)。 In the presence of reducing agent, the color can remain unchanged for several months. Phenanthroline and Fe 3+ also form a 3: 1 complex, which is light blue. Therefore, hydroxylamine hydrochloride (or ascorbic acid) is needed before color development.
2Fe 3 ++ 2NH 2 OH = 2Fe 2 ++ N2 ↑+ 2H 2o+2H+
Third, instruments and reagents
Instruments: spectrophotometer volumetric flask (25mL, 9), pipette (5mL, 1 standard solution for removing iron), pipette (10mL, 1 removing unknown test solution).
Reagent: iron standard solution (40.0 μ g/mL), accurately weigh 3.454g ammonium ferric sulfate [NH4Fe (SO4) 2. 12h2o] into a clean and dry beaker, add 30mL HCl and 30mL water, dissolve and transfer to a 1L volumetric flask. Add 300mL HCl, add water until the line is crossed, and shake well as a stock solution. Take 100.0mL stock solution in 1L volumetric flask, and dilute it with water until it is crossed) hydroxylamine hydrochloride (100g/lnH2OH HCl solution, effective within two weeks) o-phenanthroline solution (2.0g /L,
Fourth, the experimental steps
1. Draw absorption curve
Transfer 2.00 mL of iron standard solution, pour it into a volumetric flask, add 0.5mL of hydroxylamine hydrochloride solution, shake well, let it stand for 2min, add 1.0mL of phenanthroline solution and 2.0mL of sodium acetate solution, add water to draw a line, and shake well. Taking water as reference, the absorbance of different wavelengths (from 450±550nm to 65438 00 nm) was measured.
λ/nm
450
460
470
480
490
500
505
5 10
5 15
520
530
540
550
A
Draw an absorption curve with wavelength as abscissa and absorbance as ordinate on coordinate paper, and determine the appropriate determination wavelength according to the absorption curve.
In this experiment, the fourth solution in the next experiment can be used instead of preparing a separate test solution.
2. Make standard curve
Clean five volumetric flasks, add 0.50 ml, 1.00 ml, 1.50 ml, 2.00 ml and 2.50mL iron standard solutions respectively, add 0.5mL hydroxylamine hydrochloride solution respectively, and mix well. After standing for 2 minutes, add 1.0mL phenanthroline solution and 2.0mL sodium acetate solution respectively, and add water to the scribe line.
serial number
1
2
three
four
five
Pentavalent iron ion/ml
0.50
1.00
1.50
2.00
2.50
ρ (Fe 3+ )/ (μ g/ml)
A
Draw a standard curve with the mass concentration of iron ρ (Fe 3+) as the abscissa and the absorbance as the ordinate on the coordinate paper.
3. Determination of iron content in samples
Take 10.00mL sample solution, develop color and constant volume according to the same steps as making standard curve, then measure absorbance at the same wavelength, find out the mass concentration of iron from the standard curve, and then convert it into the mass concentration of trace iron in the original sample.
V. Thinking about problems
1. What are the functions of hydroxylamine hydrochloride and sodium acetate in the experiment?
2. According to our own experimental results, calculate the molar absorption coefficient at the maximum absorption wavelength.
3. What is the physical meaning of Lambert's Law? What are absorption curves and standard curves?