As a common instrument for quantitative analysis of chemical substances, photometers are widely used in scientific research, production, medical treatment, laboratory, and other fields. In order to ensure the accuracy and reliability of the measured values of the instrument, the colorimeter must be verified according to JJG178- 2007 “Verification Regulations for Ultraviolet, Visible and Near-Infrared colorimeters”. However, in the process of use and verification, it is often encountered that the instrument transmittance indication value deviates or the verification results are different. In this regard, this paper analyzes the reasons for the deviation from the following aspects.
Analysis from instrument optical path
The working principle of the colorimeter is that the substance in the solution has a selective absorption phenomenon under the excitation of light to carry out qualitative and quantitative analysis of the substance, which conforms to the Lambert-Beer law.
T=I/I0, A=-lg(I/I0)=KCL
In the formula: T is the transmittance; I0 is the incident light intensity; I is the transmitted light intensity degree; A is the absorbance; K is the absorption coefficient; L is the optical path length of the solution degree; C is the concentration of the solution.
The conditions for the application of the Lambert-Beer law: First, monochromatic radiation must be used; second, absorption occurs in a homogeneous medium; third, in the process of absorption, the absorbing substances do not interact with each other. In practical work, the linear relationship between absorbance and concentration often deviates, and the main factors are as follows: (1) The influence of non-monochromatic light; (2) Effects of non-absorbed light; (3) The effect of scattering; (4) The influence of non-parallel light; (5) Effect of fluorescence.
Analysis from Instrument Structure
There is a slight deviation in the installation of the cuvette holder during the production of the instrument (does not affect the normal use). The cuvette rack of the instrument is slightly offset from the bottom plate of the cuvette, so that the light beam cannot completely pass through the cuvette and enter the detector, but shoots outside the cuvette, causing loss of effective light energy and making the incident light.
The monochromatic light intensity changes so that the displayed transmittance value deviates from the standard value. In addition, the poor closure of the optical door of the instrument causes light leakage, which will also deviate from the transmittance value.
Analyzing the properties of the tested samples
In the sample solution to be tested, the dissociation, association, photochemistry, etc. of the measured component, or interaction with the solvent, will significantly change the absorption curve of the measured component (such as the shape, position, intensity of the absorption peak) and fine structure changes), leading to deviations from the Lambert-Beer law.
Analysis from the standard used in the verification
The standard substances used for calibrating the transmittance of the colorimeter include neutral filters for calibrating visible light regions and standard neutral filters or standard solutions for calibrating ultraviolet light regions.
As we all know, transmittance indication is an important technical indicator of a caliber 4mm pocket colorimeter. By analyzing the various reasons that affect the deviation of the transmittance indication value of the colorimeter, it can be seen that there are many factors that affect the transmittance indication value in the process of verification, maintenance, or use of the colorimeter. Therefore, when the transmittance indication value deviates or exceeds the standard, the influence of these factors should be considered to avoid misjudgment or wrong detection conclusion. In addition, the influence of these factors can also be considered in the analysis of uncertainty sources for colorimeter transmittance deviation.
