Colorimetric analysis method (two)

(3) Other factors affecting the color reaction

1) The influence of acidity on color development

2) The influence of temperature on color rendering

3) The color development time and the stability time of the solution color

4) The influence of interfering ions

(4) Lambert-Beer law

Based on the above relationship, the mathematical expression of Lambert-Beer law is derived as

In the formula, I ₀ -the intensity of incident (before entering the solution);

I—Intensity of transmitted light (after passing through the solution);

L—The thickness of the liquid layer through which light passes through the colored solution (also known as the optical path or optical path);

C—The concentration of colored substances in the solution;

K—constant.

If L is in centimeters and C is in molar concentration, then this constant is called the molar extinction coefficient

1gI ₀ /I indicates the degree of light absorption when passing through the solution, generally called "absorbance" or "optical density", usually expressed in A, that is, A=1gI ₀ /I=KCL

Note: Lambert-Beer law only applies to monochromatic light

3.

Colorimetric analysis methods are divided into visual colorimetry and photoelectric colorimetry

(1) Visual colorimetry

In order to reduce errors, when preparing the standard solution and the unknown solution, it must be done in exactly the same conditions as much as possible.

The advantages of the standard series method are as follows:

1) The colorimetric tube is very long, and very shallow solutions are easy to observe

2) Once the standard series are prepared, it is very convenient to analyze multiple samples of the same type
.

3) The instrument used is simple and economical
.

4) Even if the solution does not strictly comply with Beer's law, it has little effect
.

The main disadvantages are as follows:

1) Time is wasted when preparing standard series
.

2) Many colored substances are often not stable, and standard series solutions cannot be stored for a long time
.

In order to make up for the above two shortcomings, various substances are commonly used to make permanent standard colors to replace the color of the standard solution
.
The permanent standard colors are divided into two types: liquid and solid
.

Liquid standard colors are generally made by mixing different amounts of stable, two or more colored inorganic salt solutions
.
The color of this set of liquids completely imitates the color of a certain series of standard solutions, so it can replace the standard solution as a colorimetric standard
.
It must be noted that the standard color is the same as the color of the series of standard solutions they replace, but the composition is completely different
.
Since the composition of one or two substances is different, the absorption properties of different light waves are different
.
Different lighting conditions will produce different results
.
Therefore, the lighting conditions during color comparison must be tested and should not be changed once it is determined
.

For the standard color of solid, various colors of glass or colored paper can be used
.

The accuracy is not high when measuring with the solid standard series, so it is only suitable for work with lower accuracy requirements
.

The main disadvantage of the visual colorimetry method is that when the human eye observes the color, it will produce a large subjective error
.
Since humans have poor resolution of color shades and the eyes are easily fatigued when observing colored solutions, the accuracy of observing color differences will be reduced
.

(2) Photoelectric colorimetry
.
Photoelectric colorimetry was a widely used analysis method in the 1960s.
With the development of the times, it was gradually replaced by spectrophotometry.
Therefore, we will not introduce it here, but we will focus on the spectrophotometry in Section 3.
2
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