1. Laboratory Environment
In the laboratory where ammonia nitrogen is analyzed, it's crucial to maintain a clean environment free from dust and ammonium salts. Avoid performing tests involving nitrate nitrogen, as ammonia can interfere with the results due to its high volatility. The Nessler reagent is sensitive to airborne ammonia, which may lead to falsely elevated readings. All reagents and glassware should be stored separately to prevent cross-contamination and ensure accurate blank values.
2. Preparation of Ammonia-Free Water
The quality of water used in experiments must be extremely high. Regular distilled water often doesn't meet the required standards, so secondary processing is necessary to produce ammonia-free water. Based on practical experience, when distilling, the initial and final portions of the distillate should be discarded, and only the middle portion should be collected in a sealed glass bottle. This method yields low blank values but is time-consuming and inefficient. An alternative is using fresh deionized water produced through a composite resin exchange column, which can replace ammonia-free water for ammonia nitrogen analysis and still meet experimental requirements in terms of blank absorbance.
3. Control of Reaction Conditions
3.1 Reaction Temperature
Temperature plays a key role in the reaction between the Nessler reagent and ammonia nitrogen, significantly affecting the color development. At 25°C, the reaction is complete, and the absorbance remains stable. Between 5°C and 15°C, the reaction is incomplete, and the color is not fully developed. At 30°C, the solution may fade, leading to a significant drop in absorbance. Therefore, maintaining the temperature between 20°C and 25°C ensures reliable results.
3.2 Reaction Time
Within the first 10 minutes, the color development is incomplete. From 10 to 30 minutes, the color stabilizes. Between 30 and 45 minutes, the color becomes deeper, and after 45 minutes, it starts to fade. To obtain accurate results, the color development time should be controlled between 10 and 30 minutes, and the measurement should be performed as soon as possible.
3.3 pH of the Reaction System
The pH of the sample has a major impact on ammonia nitrogen determination. If the pH is too low, the color development is incomplete. If it's too high, the solution may become turbid. At a pH of 13, the color develops completely without any turbidity. Therefore, the optimal pH for the reaction is around 13.
4. Other Considerations
4.1 Sample Pretreatment for Clean Surface Water and Groundwater
When analyzing clean surface water or groundwater, samples usually require flocculation and sedimentation before filtration. Ordinary filter paper may contain soluble ammonia nitrogen, especially quantitative filter paper. In practice, it's better to use low-ammonia filter paper or ultra-fine glass fiber membranes. Before filtering, rinse the filter with a small amount of ammonia-free water several times to reduce measurement errors and improve accuracy and sensitivity.
4.2 Sample Preparation for Ammonia Nitrogen Analysis
Whether or not a sample needs pretreatment depends on the level of contamination. Organic compounds like fatty amines, aromatic amines, aldehydes, acetone, and alcohols, as well as inorganic ions such as iron, manganese, magnesium, and sulfur, can cause discoloration or turbidity, which interferes with colorimetric measurements. To address this, methods such as flocculation, filtration, distillation, or heating under acidic conditions can be used to remove interfering substances. Metal ion interference can be mitigated by adding an appropriate masking agent.
4.3 Expanding the Measurement Range
If the measurement range is too large, increasing the buffer capacity can help expand the detection range and improve accuracy.
5. Summary
When using the Nessler reagent photometric method to determine ammonia nitrogen, several factors should be carefully considered: First, use qualified reagents. Second, ensure correct preparation of the reagent, especially the Nessler reagent. Third, check the test water, reagent blank, and filter paper to minimize blank values and improve precision. Fourth, control the reaction temperature, time, and pH under optimal conditions. Lastly, for large sample batches, the color development and dilution method can be directly applied, ensuring that the results meet analytical requirements.
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