Kjeldahl Nitrogen Analyzer Operation Precautions

Kjeldahl nitrogen analyzer is an automatic measurement system designed based on the classic Kjeldahl method. According to the principle of constant nitrogen content in protein, the protein content is calculated by measuring the nitrogen content in the sample.


Kjeldahl nitrogen analyzer is easy to install and operate, and is widely used to detect the content of ammonia nitrogen and protein nitrogen in grain, food, dairy products, beverages, feed, soil, water, drugs, sediments and chemicals. During use, the following items need attention to ensure the accuracy of the measurement.


I. Precautions for sample preparation

The sample should be uniform. Large solid samples should be crushed evenly with crushing equipment, and liquid samples should be shaken or stirred evenly. The general sampling range of solid samples is 0.20g~2.00g, while the general sampling range of semi-solid samples is 2.00g~5.00g, and the sampling range of liquid samples is 10.0mL~25.0mL (about 30mg~40mg of nitrogen). If a liquid sample is tested, the result is expressed in g/100mL.


When the sample is put into the nitrogen bottle, do not stick to the neck. In case of adhesion, it can be washed down with a small amount of water to avoid incomplete digestion of the tested sample and the low result, or wrap it with filter paper and put it into digestion, and the effect of filter paper is deducted by the blank.


II. Precautions during digestion

During the digestion, ensure that the amount of concentrated sulfuric acid is sufficient. If the fat content of the sample is high, the amount of sulfuric acid should be appropriately increased. When the sample is digested in the nitrogen distiller, do not use strong fire. If the sample contains high sugar or fat content, pay attention to controlling the heating temperature to avoid a large amount of foam spraying out of the flask and causing sample loss. A small amount of octanol or liquid paraffin, or silicon defoamer can be added to reduce foam. Maintain a gentle boiling during the entire digestion process, so that the fire power is concentrated at the bottom of the Kjeldahl bottle, so as to prevent the protein attached to the wall from losing nitrogen in the absence of sulfuric acid.


When digesting, remember to rotate the Kjeldahl flask to wash down the carbon particles attached to the bottle wall to completely digest the sample. If the sample is not easy to digest to become clear and transparent, cool the solution in the flask, add a few drops of hydrogen peroxide, and continue heating to achieve complete digestion.


III. Precautions for measurement

The instrument dilution water adopts neutral deionized water. The water in the steam generating bottle must remain acidic. Neutral deionized water should be used when preparing the boric acid absorption solution to avoid mixing of alkaline substances, and the container containing the boric acid absorption solution should be scrubbed clean. Alkali liquor should be configured with neutral deionized water. The standard acid used for titration must be prepared and calibrated in accordance with the standard. Before testing the samples on the machine, the instrument should be turned on to preheat, and a digestion tube should be placed for steaming once to remove the air in the distillation pipeline.


Alkali must be added during distillation. The effect of adding alkali is to neutralize sulfuric acid and to make the solution strong alkaline so that (NH4)2SO4 can be turned into NH3 and be absorbed by boric acid, usually four times the volume of concentrated sulfuric acid used for digestion. (40%NaOH). Copper sulfate can be used as a catalyst and as an indicator of alkaline reaction during distillation. Whether sodium hydroxide is sufficient or not, it can be indicated by the brown precipitate or dark blue copper ammine ion generated by copper sulfate under alkaline conditions. If the color of the solution does not change, it means that the added alkali liquor is insufficient.


Semi-automatic Kjeldahl nitrogen analyzer can use precise pH test paper to measure the condensate of the condenser tube to determine whether the distillation is complete. Fully automatic Kjeldahl nitrogen analyzer currently mainly uses distillation volume and setting time (empirical value) to ensure complete distillation.


After the distillation, the titration is mainly divided into manual titration and machine automatic titration that calculates and prints experimental results. Operators are required to operate according to actual conditions.