As an instrument for quantitative analysis of the absorption characteristics of specific wavelengths based on substances, spectrophotometer is widely used in environmental monitoring, water quality analysis and other fields. Chemical oxygen demand (COD), ammonia nitrogen, and total phosphorus are the key indicators for evaluating the degree of water pollution. This article will explore the practical application of spectrophotometers in the determination of these three indicators, covering the principles, operational points and precautions of the method.
Principle of determination
Spectrophotometry determines COD, ammonia nitrogen, and total phosphorus, all based on specific chemical reactions that convert the components to be measured into colored compounds whose color depth is proportional to concentration, following Lambert-Beale's law. The mathematical expression of this law is:
A = εbc
Among them, A is the absorbance, ε is the molar absorbance coefficient, b is the length of the optical path, and c is the concentration of the solution. Quantitative analysis of unknown samples can be performed by measuring the absorbance of a standard series of solutions and plotting a working curve.
Determination of COD
In COD assays, fast digestion spectrophotometry is commonly used. The samples were oxidized by potassium dichromate at a specific temperature in a highly acidic medium, and the absorbance of trivalent or hexavalent chromium at specific wavelengths was determined after the reaction. The actual combat process mainly includes two core links: sample digestion and colorimetric determination.
During operation, the digestion temperature and time should be strictly controlled to ensure that the oxidation reaction is complete. The digested solution needs to be cooled to room temperature before colorimetric to avoid the effect of temperature on absorbance readings. Common interfering substances such as chloride ions need to be masked by adding mercury sulfate. The measurement wavelength is usually 600 nm (corresponding to Cr³⁺) or 420 nm (corresponding to Cr⁶⁺).
Determination of ammonia nitrogen
Ammonia nitrogen is often determined by Knott reagent spectrophotometry or salicylic acid spectrophotometry. The former is based on the reaction of ammonia with an alkaline solution of mercury iodide and potassium iodide to produce a yellowish-brown complex; the latter is based on ammonia, salicylic acid and hypochlorite catalyzed by sodium nitrosoferric cyanide to form a blue complex.
In practice, sample pretreatment is crucial. Filtering is required for turbid samples, and distillation pretreatment is required for samples with color interference. The pH, reaction time and temperature of the reaction system must be strictly controlled according to standard methods to ensure complete and stable color development. The wavelength determined by the Knotts reagent method is usually 420 nm, and the salicylic acid method is 697 nm.
Determination of total phosphorus
Total phosphorus determination is usually performed using ammonium molybdate spectrophotometry. The principle is that the sample is digested to convert various forms of phosphorus into orthophosphate, and under acidic conditions, orthophosphate reacts with ammonium molybdate and potassium antimony oxide tartrate to form phosphorus-molybdenum heteropolyacid, which is then reduced to blue phosphomolybdenum blue by ascorbic acid.
The key step in practice lies in the digestion process of the sample, which requires the use of oxidants such as potassium persulfate to completely convert organophosphate and polyphosphate into orthophosphate under high temperature and pressure or thermal digestion conditions. The degree of completeness of digestion directly affects the accuracy of the results. The chromogenic reaction is sensitive to acidity, and it is necessary to ensure that the sulfuric acid concentration of the reaction system meets the method requirements. The measurement wavelength is typically 700 nm or 880 nm.
Notes:
In order to ensure the accuracy and reliability of spectrophotometer measurement results, the following general points need to be paid attention to in actual combat.
Instrument performance verification is the foundation. It needs to be warmed up and stabilized every time it is turned on, and the wavelength accuracy and absorbance accuracy are regularly calibrated. Use a matching cuvette and keep it clean and scratch-free.
The drawing and verification of standard curves are indispensable. The standard series should cover the concentration range of the sample to be tested, and the correlation coefficient should meet the method requirements. Each batch of tests should be carried out with standard samples for quality control.
Sample handling needs to be standardized. Sampling is representative, and pretreatment steps (e.g., dilution, filtration, digestion) strictly follow standard operating procedures. Pay attention to the purity of the reagent, preparation date and storage conditions.
The results should be calculated and reported rigorously. Concentrations are calculated according to standard curves and corrections are made for dilution or pretreatment steps. Pay attention to the detection limit and upper limit of the method, and properly handle samples that are out of range.
| Measure items | Brief description of the core features |
| COD | Fast digestion, relatively easy to operate, suitable for large-volume water sample screening. |
| Ammonia nitrogen | The method was selective, and attention should be paid to sample pretreatment to eliminate interference. |
| Total phosphorus | The digestion steps are demanding and are key to ensuring accurate results. |
Epilogue
Spectrophotometry for the determination of COD, ammonia nitrogen and total phosphorus integrates the specificity of chemical reactions with the objectivity of instrumental measurement, and is an effective means of routine monitoring in the laboratory. Practical success depends on a thorough understanding of principles, strict control of operational details, and standardized management of instruments and reagents. Through systematic practice, analysts can obtain reliable data and provide a scientific basis for water quality evaluation and management.