Collect water samples
The commonly used methods for determining ammonia are Nessler's reagent colorimetry, phenol-hypochlorite colorimetry, electrode method and titration method. The choice of ammonia determination method should consider two main factors, namely the concentration of ammonia and the presence of interfering substances. The two direct colorimetric methods are limited to the determination of clean drinking water, natural water and highly purified wastewater effluent, so the color of these waters should be very low. Turbidity, color, and magnesium, calcium and other substances that can be precipitated by hydroxide ions can interfere with the measurement, and can be removed by predistillation: if the concentration of ammonia is high, finally use the titration method: color and turbidity counter electrode method to determine ammonia No effect. Water samples generally do not require pretreatment, and have the advantages of simplicity and wide measurement range, but the determination is affected by high concentrations of dissolved ions.
Save water samples
Use fresh samples for the most reliable results. Water samples should be analyzed as soon as possible after collection and immediately destroy residual chlorine to prevent it from reacting with ammonia (0.5mL 0.35% sodium thiosulfate can remove 0.25mg residual chlorine). If it cannot be analyzed in time after sampling, 0.8mL of concentrated sulfuric acid should be added to each liter of water sample to make the pH between 1.5 and 2.0, and store at 4 °C. Some wastewater samples require more concentrated sulfuric acid to reach this pH value. Before the determination, neutralize with alkali. There are also literatures suggesting adding 20-40 mg of HgCL2 per liter of water for storage.
Pretreatment of water samples
In order to obtain accurate results, it is recommended to pre-distill the sample. In the water sample that has been adjusted to neutral, add phosphate buffer solution to keep the pH at 7.4 and heat to distill. The ammonia is distilled out in gaseous state and absorbed in sulfuric acid (0.01~0.02mol/L) or boric acid (2%) In the solution, when using Nessler's reagent colorimetric method or titration method, it is more appropriate to use boric acid as the absorption liquid. When using phenol hypochlorite or ammonia selective electrode method, sulfuric acid should be used as the absorption liquid.
The pH of the water has a great influence on the recovery of ammonia. If the pH is too high, some organic compounds containing ammonia can be converted into ammonia: the pH is low, and the recovery of ammonia is incomplete. Generally, the sample can be added to the buffer solution to achieve the desired pH. Value, water sample is acidic or alkaline, you can add 1mol/L (1/2H2SO4) sulfuric acid solution to adjust the pH to neutral. But for some water samples, even if it is adjusted to neutral, and then added with buffer solution, the pH is still far below the expected value. For unknown water samples, measure the pH after distillation. If the pH is not in the range of 7.2~7.6, the amount of buffer should be increased. Generally, 10mL of phosphate buffer solution should be added for every 250mg of calcium. Foreign countries also recommend using borate buffer solution to buffer the water sample to pH 9.5 for distillation, which can reduce the hydrolysis of cyanate and organic nitrogen compounds (such as glycine, urea, glutamic acid and acetamide, etc.). At this time, urea hydrolyzes only about 7%, while cyanate is about 5%. It is also introduced that after the water sample is adjusted to neutral with sodium hydroxide, a small amount of powdered iron oxide is added for distillation.