In the Kjeldahl method, the boric acid receiving solution is not involved in the calculation. Under normal circumstances, the concentration and volume of boric acid have no effect on the measurement results. However, correctly preparing the boric acid indicator solution can ensure more accurate and stable measurement results.

☆ Concentration of Boric AcidIn the new national standardGB5009.5-2025In the traditional manual nitrogen analyzer, the boric acid concentration is2%For the fully automatic Kjeldahl nitrogen analyzer, each manufacturer recommends a different concentration of boric acid,1%、2%and4%All are possible, please follow the manufacturer's operating procedures. For the mode of simultaneous distillation and titration,1%The concentration of boric acid is sufficient; For the titration mode after distillation, the amount of boric acid must be sufficient to absorb the distilled nitrogen, especially for samples with high nitrogen content and large sample size.

☆ Volume of Boric AcidFor traditional manual nitrogen analyzers, dilution is used for the digested sample solution10The volume of boric acid is generally measured using the double method10mlBut it is necessary to ensure that the lower end of the distillate is below the boric acid liquid level to prevent the escape of ammonia; For a fully automatic Kjeldahl nitrogen analyzer, due to the complete distillation of the digested sample solution, the volume of boric acid is generally30ml.

☆ Types of indicators: National standardGB5009.5-2025Two combination indicators are provided: methyl red/Methylene blue and methyl red/Bromocresol green, a commonly used indicator in practice is methyl red/Bromocresol green and methyl red are difficult to dissolve, and can be accelerated by ultrasound to dissolve in ethanol solution without residue; The endpoint determination of Kjeldahl nitrogen mainly relies on the color of methyl red, so it is necessary to ensure the quality of methyl red and its pure color after dissolution.

☆ Quantity of indicator：For traditional manual nitrogen meters, the new national standardGB5009.5-2025The usage of indicators has been increased in the middle3-4Drip, it is easier to manually determine the color of the endpoint, which is different from the previous national standard; For a fully automatic Kjeldahl nitrogen analyzer, the amount of indicator used is per1000mlAdd to boric acid solution10mlMixed indicator, mixed indicator:0.07Carboxymethyl red+0.1Bromocresol green with anhydrous ethanol or95%Dissolve in alcohol and dilute to volume100milliliter.

☆Preservation of Boric Acid Indicator：The prepared mixed indicator should be stored in a brown bottle for no more than three months; Boric acid solution with indicator is recommended to be prepared and used immediately, and the usage time should not exceed1A week.

☆The influence of boric acid indicator：Boric acid indicator directly affects the determination of blank value size and endpoint color, as well as the stability and repeatability of blank value; Prepare and maintain the color of boric acid absorption solution to be normal and fresh according to the instructions of the instrument.