The QYKDN-AS Kjeldahl analyzer is taken as an example to introduce the analysis principle and daily use and maintenance of Kjeldahl nitrogen analyzer.
First, the analysis principle of Kjeldahl nitrogen analyzer
When the Kjeldahl method is used to determine the sample, it needs to be digested, distilled, and titrated. The digestion of the three process samples is to heat and digest the nitrogen-containing organic compound together with sulfuric acid and the catalyst to decompose the organic nitrogen, and the decomposed ammonia is combined with sulfuric acid. Sulfuric acid is converted into inorganic nitrogen by digesting the sample, and the digested sample is added to a nitrogen analyzer distiller for alkali hydrolysis.
Kjeldahl nitrogen analyzer is to complete the alkaline decompression process of inorganic nitrogen, that is, the alkali solution is added to the sulfuric acid after digestion, and the alkali solution distillation is carried out under hot steam, as follows:
Kjeldahl nitrogen analyzer can automatically complete the quantitative alkali addition, acid addition and automatic distillation process. When the sample to be tested is completely digested, the ammonia gas and water vapor released in the reaction are condensed through the condensation tube and collected. The boric acid absorbing solution (containing methyl red-oxocresol green mixed nitrogen indicator) is finally titrated with a standard acid solution.
Second, the use of Kjeldahl
1. Determination of the amount of acid plus alkali
Calculate the amount of sodium hydroxide to be added according to the amount of sulfuric acid added when the sample is cooked. Calculate the amount of boric acid to be added according to the approximate content of nitrogen in the cooking liquid. The time of adding alkali and the time of adding boric acid are determined according to the calculated amount.
2. Boot
Turn on the power switch and the display shows “P†(KDY-9820 type Kjeldahl analyzer structure is shown in Figure 1.) Press [Convert] button, the boric acid status indicator lights, press [+] or press [-]. Set the time for adding boric acid and press the [Convert] button to make the alkali-added status indicator light. Press [+] or press [-] to set the alkali time (see Figure 2 for operating the keyboard).
3. Place the empty triangle bottle on the tray. At this time, the tray is at a high position. Place the cooking tube with the sample on the tray, and tighten it with the rubber plug on the upper end.
4. Press the [Start] button, the instrument will automatically add boric acid to the flask, add alkali to the digestion tube, and then enter the distillation state. When the condensate in the flask reaches the predetermined volume, the triangle tray falls and distills 12 After the distillation is stopped, the sound is emitted. During the distillation process, the absorption liquid gradually turns blue-green. If manual distillation is used, the distillate should be tested with a pH test paper. According to the acidity and alkalinity, it is judged whether the distillation is finished. If the pH test paper is neutral or acidic. That means the end of distillation. If it is alkaline, it needs to continue distillation until it is neutral or acidic.
5. Remove the triangular flask and titrate the liquid in the triangular flask with an acid burette. The color changes from blue-green to brick red, that is, the end point is calculated according to the formula of nitrogen content and crude protein content, and the measurement result is obtained.
Third, the considerations for the use of Kjeldahl nitrogen analyzer
1. If the boric acid addition time or the alkali addition time is set to zero seconds, the display window will display “E†without flashing after the automatic work process is completed, and “嘀~†will be issued.
2, the prompt tone,
2. If the [Start] button is pressed, the triangle bottle tray is still at the lower end, and the display window displays the "-" symbol flashing from bottom to top, and the prompt tone of "嘀~嘀~嘀" is issued, indicating that the triangle should be emptied. The bottle is on the tray to raise the tray
3. The mixed indicator is green in an alkaline solution, gray in a neutral solution, and red in an acidic solution.
4. Whether the ammonia is completely distilled out, the pH test paper can be used to test whether the distillate is alkaline.
5. When a concentrated alkali is added to the distillation flask, a brown precipitate tends to appear due to the decomposition-promoting alkali reacting with the added copper sulfate to form a cuprous oxide precipitate.
Fourth, the maintenance and maintenance of Kjeldahl nitrogen analyzer
1. The instrument should be installed in places where it is exposed to direct sunlight and excessive cold, overheating or humidity. The indoor temperature should not exceed 100C~300C, ventilation should be good, and there should be good heat dissipation conditions.
2. If there is liquid in the front of the instrument, it should be cleaned frequently.
3. After long-term use, there will be scale on the heater, which will affect the heating efficiency. If the scale is too thick and the power is turned off in the off state, a cock on the top of the steam generator can be unscrewed and inserted at the nozzle. A small funnel, inject detergent or glacial acetic acid to clean the scale (also can be diluted with sulfuric acid), then open the steam generator drain door in the chassis to drain the water, and add water to clean several times.
4. Add lye barrel and boric acid barrel to clean the sediment and wash it regularly.
Five, common faults and troubleshooting methods of Kjeldahl
1. The water surface of the steam generator has reached the water level, but no steam is generated, and the heating status indicator is not bright.
The main reason may be that the 15A fuse is blown, so a new 15A fuse is required.
2. Can not add acid and alkali
This is mainly due to too little acid and alkali in the barrel, which causes the straw to leave the acid and alkali level, so it is necessary to increase the acid and alkali solution.
3. Buckwheat ale barrel has no air pressure
Mainly caused by the lack of tight sealing of the gas filling pipe or the lid of the tank. It is also possible to carefully check the joints of the pipes and the lid of the pipe and seal the leaking air. The electromagnetic pump may leak. The electromagnetic pump must be replaced.
4. There is air pressure in the alkali barrel, but it can not add lye
This is because the solenoid valve can not be turned off due to power failure, and must be shut down for repair; it may also be a solenoid valve or a pipeline that is blocked, and it is necessary to clear the solenoid valve and the pipeline.
5. The automatic working state can't continue working, suddenly stop
This is because there is an electric field around the computer that interferes with the computer. Press the reset button or turn it off and then on again.
6. The measured value is unstable or too high
It may be that the steam generator is not clean, the water in the steam generator is drained, and the new water is used for the determination or the liquid in the cooking tube is too much, and the alkali liquid is flushed into the distillation system. At this time, the steam distillation system can be used. Then make the measurement.
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