At present, the spectrophotometric method can only measure water samples with color or absorbance. If the tested water sample has no color or the absorbance value is very small, the water quality inspector needs to add a reaction reagent that can form a stable and colored substance with the tested substance in the tested sample. This process is called a color reaction, and this reagent is called a color developer. In addition, the color reaction is related to the color-developing agent and the color-developing conditions.
The main purpose of selecting the color reaction conditions in water quality testing is to enable the components to be tested to be effectively transformed into compounds suitable for photometric determination under the selected reaction conditions. In order to improve the sensitivity and accuracy of the determination, it is necessary to select the appropriate amount of developer, solution acidity, color development time and temperature.
Color reaction conditions of spectrophotometry
1. The amount of developer
In the color reaction, a proper excess of color developer is generally added to make the tested substance as possible to convert into a colored compound, but it is not that the more color developer is added, the better. Too much color developer will cause other side effects. The reaction is unfavorable to the determination of water quality. In actual work, the amount of developer should be strictly controlled according to experimental requirements.
2. Acidity of color developing solution
Most of the organic color developers are organic weak acids, and the acidity of the solution affects the concentration of the color developer and its own color. Since most metal ions are easily hydrolyzed, acidity will also affect the existence of metal ions, and further affect the composition and stability of colored compounds. Therefore, the appropriate acidity range should be determined through experiments and strictly controlled in the water quality analysis process.
3. Color reaction time
Some color reaction can be completed quickly and the color is stable; some color reaction rate is slow, and the color can be stable after being placed for a period of time. Therefore, it should be measured within the time range when the color reaches the maximum depth (that is, the maximum absorbance) and is stable after the color reaction is completed.
4. Color temperature
In general, the color reaction is mostly carried out at room temperature, but some color reactions need to be heated to a certain temperature to complete. Therefore, the appropriate color temperature should be selected for different color reactions, and attention should be paid to temperature control.
5. Solvent
Different solvents may affect the color development time, dissociation degree and color of colored compounds, etc. The standard solution and the test solution should use the same solvent during the measurement.
The above is the color reaction conditions of the spectroscopic water quality detector
