Colorimetric analysis method (1)

1 Overview

Many of the compounds are colored, for example, permanganic acid ion (of MnO _.

This kind of analysis method that uses the measured components to react with reagents under certain conditions to produce colored compounds, and then measure the color of the colored solution and compare it with the standard solution to determine the content of the components is called colorimetric analysis

Colorimetric analysis is a method widely used in the determination of trace and trace components, with high sensitivity

In colorimetric analysis, the influence of interfering ions can often be eliminated according to the difference in light absorption by substances, such as selecting an appropriate wavelength or adding a masking agent

2.

(1) The principle of color development of colored compound solutions

Light with the same wavelength is called monochromatic light; light with a combination of multiple wavelengths is called mixed color light

When a beam of white light passes through the solution, if the solution does not absorb any light in this wavelength range, the solution is transparent and colorless

The color and complementary light of the solution are shown in Table 3.

Table 3.

If the solution absorbs light in multiple wavelength bands, the color of the solution will also be a mixed color corresponding to the complementary colors of the absorbed light

If the solution absorbs light of various wavelengths in the white light fairly uniformly, the solution will appear dark gray

(2) Selection and dosage of display agent

1) Selection of display agent

Colored coordination compounds for colorimetric determination should meet the following three conditions as much as possible

a.

b.
The dissociation constant of the colored coordination compound should be small
.
The smaller the dissociation constant of the colored coordination compound, the more stable the coordination compound; the more stable the coordination compound, the higher the accuracy of the colorimetric determination, and the interference of other ions in the sample can be avoided or reduced
.

c.
The composition of the colored coordination compound should be constant
.
The coordination compound used for colorimetric determination preferably has a certain composition
.
Changes in composition can easily cause changes in hue.
This is because some color reactions themselves are step-by-step reactions.
Each step generates a colored compound, and the hue of these compounds is not the same
.
For example, when the concentration of SCN ^- ions increases, the composition of the coordination compound formed by Fe ³⁺ ions and SCN ^- ions is changed as follows:

However, the hue of these coordination ions is different.
For example, Fe(SCN) ^{2+ has} a yellower hue and Fe(SCN) ²⁺ has a reddish hue
.
Therefore, the composition and hue of the colored compound here are determined by the concentration of the reagent; the change in the concentration of the reagent will cause a change in the hue, which is not conducive to colorimetric determination
.

2) The amount of developer
.
When the stability of the colored coordination compound is great and there is no interfering substance that can interact with the tested component or reagent in the solution, the concentration of the display agent does not need to be strictly controlled
.

If the dissociation constant of the colored coordination compound is large, it will partially dissociate:

At this time, part of the measured component X in the solution is not coordinated and compounded.
In this way, the color depth displayed by the colored coordination compound RX cannot represent the content of the measured component
.
However, in colorimetric analysis, the color depth of the unknown solution is usually measured by comparing it with the color depth of the standard solution
.
If the dissociation degrees of the colored coordination compounds in the two solutions are different, errors will result
.
Conversely, if the dissociation degree of RX is the same in different concentrations of colored coordination compound RX solution, the correct result can still be obtained
.

Related links: Titration Analysis-Redox Titration (4)