In the accuracy check section for Total Kjeldahl Nitrogen (TKN), the instructions say that a 1 mg/L NH3-N standard should read 26-27 mg/L TKN. Why should it read 26-27 mg/L?

Document ID

Document ID TE392

Version

Version 2.0

Status

Status Published

Published Date

Published Date 27/05/2017
Question
In the accuracy check section for Total Kjeldahl Nitrogen (TKN), the instructions say that a 1 mg/L NH3-N standard should read 26-27 mg/L TKN. Why should it read 26-27 mg/L?
Summary
TKN accuracy check on a 1 mg/L standard should read 26-27 mg/L TKN
Answer
Using an ammonia standard is a good way to check whether your instrument, reagents, and technique are working correctly for the TKN test.

Two dilutions are factored into the calibration curve. In addition, the dilution of the standard needs to be taken into account. The calibration curve for TKN includes a dilution factor of 33.3. This incorporates an assumed 25 in 100 mL dilution (4-fold) from the Digesdahl digestion as well as an assumed 3 in 25 mL dilution (8.33-fold) in the analysis procedure (4 x 8.33 = 33.3). The calculation in the last step adjusts for dilutions other than these. If 25 mL of the 1 mg/L NH3-N standard solution were used without digestion, the expected displayed concentration would be 33.3. However, only 20 mL of the standard is put into the mixing cylinder, which is then diluted to 25 mL. This reduces the displayed result from 33 to 27: 1 mg/L NH3-N x 33.3 x 20 mL/25 mL = 26.7 mg/L.