1. Preparation and storage of reagents The preparation process of alkaline potassium persulfate is very important. If it is not well mastered, it will affect the digestion effect and will have a certain influence on the determination results. The preparation of alkaline potassium persulfate in GB 11894-89 is simply to say that potassium persulfate and sodium hydroxide are dissolved in water and no other requirements are made. In fact, the dissolving rate of potassium persulfate is very slow. If you want to speed up the dissolution, you must not blindly heat it. Even if it is heated, it is best to use the water bath heating method, and the water bath temperature must be lower than 60°C, or the potassium persulfate will decompose and fail. . When the solution is prepared, potassium persulfate and sodium hydroxide may be weighed separately, and the two components may be separately prepared and then mixed to a constant volume, or a sodium hydroxide solution may be prepared first, and then the temperature is lowered to room temperature and potassium persulfate is added to dissolve the solution. If the two are dissolved in water in a beaker, add water slowly while stirring to prevent the exothermic sodium hydroxide from overheating the solution causing local potassium persulfate to fail.
The storage of potassium persulfate should also be noted that it should avoid mixing with reducing substances, sulfur, phosphorus, etc. In addition, potassium persulfate tends to absorb moisture and release oxygen. Therefore, to prevent failure, it should be placed in a dry reagent cabinet. in.
2. The preparation of ammonia-free water The experimental process requires very strict water, and ordinary distilled water often fails to meet the experimental requirements. At this time, secondary processing is required to obtain ammonia-free water. In the preparation of ammonia-free water by distillation, GB 11894-89 states: "Discard the first 50 ml of distillate and then collect the distillate in a glass bottle with a glass stopper." According to the author's work experience, it is not enough to simply discard the first 50 ml of distillate. For example, if 1000 ml of ammonia-free water is distilled out, the previously distilled 200 ml of distillate should be discarded. Finally, the distilled 200 ml of distillate should be discarded, leaving only the ammonia-free distilled water to be used. Otherwise, the blank value of steamed ammonia-free water is often not as good as the blank value of ordinary distilled water before preparation.
3. The analysis of total laboratory nitrogen environment should be conducted in an ammonia-free laboratory environment. Indoors should not contain dust, petroleum, and other nitrogen compounds. Absolutely no nitrogen can be used in laboratories that analyze ammonia nitrogen and other nitrogen projects. Analysis of the project, reagents used, glassware, etc. should also be stored separately to avoid cross contamination and affect the blank value.
4. The glassware used for the washing of glassware should be soaked with (1+9) hydrochloric acid and then rinsed with ammonia-free water several times before it can be used. Otherwise, the blank value may be too high or the parallelism may be poor.
5, digestion temperature, pressure control For the use of hand-held medical vapor sterilizer laboratory, because the measured pressure is 1.1 ~ 1.4kg/cm2, the temperature is 120 °C ~ 124 °C, this time can be installed a regulator, the pressure The control is in this range, which eliminates the need for manual power-off control and is stable and labor-saving. When digesting, GB11894-89 required to reach the specified temperature pressure after the start of time, and the author's experience is that after reaching the specified temperature and pressure should be deflated first pressure indicator pointer back to zero, once again to reach the required temperature and pressure time. Or directly open the air release valve for a period of time, until the cold air in the steam sterilizer is exhausted completely, release the hot steam and then close the air release valve to digest, and the digestion temperature is controlled at 123 DEG C., so that the determination result is most ideal.
6. Precautions for colorimetric measurement The measurement of this project involves two wavelengths (220nm and 275nm). A conditional laboratory can use a dual-optical path UV spectrophotometer. The advantage is that it is convenient and fast, and it can avoid repeated adjustment of the wavelength to generate measurement error. Errors between dishes can also be automatically corrected. If you do not have a photometer with a two-optical path, it is recommended that after measuring the same wavelength of a group of samples, adjust the wavelength to another wavelength, and measure them in unison. Do not measure two absorbances of one sample and then change another sample. This adjusts the wavelength repeatedly. Will cause a certain measurement error.
7, the choice of reagents Alkaline potassium persulfate digestion UV spectrophotometric determination of total nitrogen, potassium persulfate is a critical reagent. First, the purity of the reagent is related to the blank value and the accuracy of the measurement result. The general analysis of pure potassium persulfate has a maximum total nitrogen content of no more than 0.005%. However, due to differences in reagent quality, the nitrogen content of some manufacturers and batches often fails to meet this requirement, resulting in a high blank value. In addition, although the content of nitrogen compounds used for the analysis of pure sodium hydroxide is much lower than that of potassium persulfate, it should be carefully selected. In the work of the author, I encountered the phenomenon that the potassium persulfate solution and the sodium hydroxide solution were mixed into the alkaline potassium persulfate solution, which actually emits the odor of ammonia, which indicates that the purity of the reagent is not enough. Therefore, if conditions permit, it is recommended to use superior grade or reference reagents to minimize the amount of nitrogen in the reagents, thereby reducing the experimental blank value.
8. If the blank value of the experiment is not ideal, test water and reagents need to be tested to select the water and reagents with the lowest nitrogen content to obtain the ideal blank value. The author's experience is that if every change of water, every change of a manufacturer, a batch of potassium persulfate, sodium hydroxide reagents to do a total of total nitrogen in the whole process, it will be quite troublesome, and in the end it is often difficult to draw in conclusion. The following is a brief summary of the author's work, for reference only.
8.1
Water test: Place all the experimental waters to be selected into quartz cuvettes, absorb the absorbance at 220 nm and 275 nm, and correct the absorbance at A220 nm-2A275 nm. The water with the smallest absorbance is corrected. Experimental water.
8.2
Reagent test: Potassium persulfate and sodium hydroxide to be tested are divided into solutions of corresponding concentrations according to the content of the solution after digestion and constant volume in the experiment. The solution is used as a sample to measure ammonia nitrogen and nitric acid, respectively. The absorbance of the salt nitrogen can be selected using the lowest absorbance, and if necessary, the nitrogen content can be further calculated. It is necessary here to emphasize the test of nitrate nitrogen. If the phenol disulfonic acid spectrophotometric method is used, the procedure is somewhat tedious. It is recommended to refer to the UV spectrophotometry mentioned in the fourth edition of "Water and Wastewater Monitoring and Analysis Methods", which will be more convenient. The alkaline potassium persulfate solution that was mentioned by the author as being ammonia-water-flavored was determined by a reagent test. It was determined that the potassium persulfate reagent was too high in ammonia nitrogen and it was decided to discard it.
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