The concept and principle of colorimeter

3. Colorimeter Principle Colorimeter Principle When monochromatic light passes through a solution of the same thickness and a small concentration, according to Lambert-Beer's law, the extent to which light is absorbed by the solution is called absorbance, and the concentration of the solution is Proportional, proportional to the thickness of the solution, ie A = εCL, where: A is the absorbance, C is the concentration of the solution, L is the thickness of the solution, and ε is the extinction coefficient.
According to Lambert-Beer's law, when a bundle of monochromatic light passes through a solution, the intensity of the light is attenuated because the solution absorbs a portion of the light energy. If the concentration (or thickness) of the solution does not change, the greater the thickness (concentration) of the solution, the more pronounced the decrease in light intensity.
The standard solution and the solution to be tested prepared by the same method have the concentrations C1 and C2, respectively, and the same solution ε, when the thickness is also the same:
A1=εC1L
A2=εC2L
C2=(A2/A1)*C1
In the formula, A1 and A2 can be directly read by a Lovibond colorimeter, and C1 is a known concentration of the standard solution, from which the concentration of the solution to be tested can be calculated.
Lambert-Beer's Law The solution of many chemical substances has a color (a colorless compound can also be reacted with a color developing agent to form a colored substance). When the solubility of a colored solution changes, the color depth changes accordingly. The greater the concentration, the greater the concentration. The darker the color. Therefore, the concentration of the colored solution can be determined by comparing the color depth of the solution. This method is called colorimetric analysis.
1. Lambert-Beer's Law When a bundle of monochromatic light passes through a colored solution, a portion of the incident light is reflected back by the vessel, one portion is absorbed by the solution, and the other portion is transmitted through the solution as shown. They have the following relationship:
Io=Ia+Ir+It 1-1
Where: Io - incident light intensity, Ia - absorbed light intensity, Ir - reflected light intensity, It - transmitted light intensity. The intensity of the reflected light is a certain value and does not cause measurement errors, so the influence of the reflected light can be ignored. Then the above formula can be simplified as:
Io=Ia+It 1-2
As is clear from the formula 1-2, when the incident light intensity Io is constant, the larger the absorbed light intensity Ia is, the smaller the transmitted light intensity It is. That is to say: the decrease in light intensity is only related to the absorption of light by a colored solution.
So, what are the factors related to the absorption of light by the solution? Experiments have shown that the greater the concentration C of the solution, the thicker the thickness of the liquid layer L (i.e., the longer the distance traveled by the light in the solution), the more the solution absorbs light. The relationship between them is determined by the following formula:
Lg = KCL 1-3
This formula is the Lambert---Beer law.
The K in the formula is called the absorption coefficient, which indicates the absorbance of the colored solution at unit concentration and unit thickness. K is a constant value under the condition that the wavelength of the incident light, the type of the solution, and the temperature are constant. The absorption coefficient is one of the important characteristics of colored compounds and has important significance in colorimetric analysis. The larger the K value, the stronger the absorption capacity of the substance for light, and the more obvious the change in absorbance when the concentration is changed, so the sensitivity is higher when the colorimetric measurement is performed.
Lambert-Beer's law is the degree to which a colored solution absorbs light of a certain intensity, proportional to the product of the thickness of the liquid layer and the concentration of the colored substance in the solution. The Lambert's law states the relationship between absorbed light and thickness; Beer's law states the relationship between absorbed light and concentration.
The application of Lambert-Beer law in an optoelectronic colorimeter assumes two colored solutions, one of which is a standard solution of known concentration and the other is a solution to be tested. According to the formula:
In the standard solution: As = KsCsLs 1-4
In the solution to be tested: Ax=kxCxLx 1-5
Divide Equation 1-4 by Equation 1-5 to get:
= 1-6
If the liquid layers of the above two solutions have the same thickness, the same temperature and two different concentrations of the same substance, the wavelengths of the selected monochromatic light are also the same, then:
Ls=Lx, Ks=Kx, substituting into 1-6 can be obtained:
= 1-7
Thus, under the above conditions, the absorbance is proportional to the concentration. This relationship is the design basis of the photoelectric colorimeter and one of the basic calculation formulas of colorimetric analysis. In the formula, the concentration Cs of the standard solution is known, and As and Ax can be measured by a photoelectric colorimeter, the concentration Cx of the solution to be tested can be obtained:
Cx = × Cs 1-8
Since the standard solution and the solution to be tested are diluted in the actual measurement, and when the result is reported, it is expressed in a content of 100 ml (or 1000 ml). Therefore, in the actual calculation, it is necessary to multiply the dilution factor in the above formula.
There are three methods for obtaining the concentration of the solution to be tested: direct comparison method (calculation method), factor method and standard curve method. These methods are described in the "Biochemical and Biochemical Testing Technology" course.
Wavelength selection:
Since the colored solution is selective for the absorption of light, the filter must be selected for colorimetric determination, otherwise the sensitivity is low, resulting in inaccurate measurement results. The general rule for selecting filters is that the light that the filter transmits at its maximum should be the light that is absorbed by the solution. In terms of color, the color of the filter and the color of the solution to be tested should be "complementary colors".
What is a complementary color? When two colors are added to obtain white, the two colors are called "complementary colors", and the two directly opposite colors in the figure are complementary colors.
Why do you choose the filter to match the color of the filter to the color of the solution to be tested? This is because the filter and the colored solution have similar light transmission characteristics, and the same color as their own color can be transmitted to the maximum extent. The color light complementary to their own color can be absorbed to the maximum.