Why might TNT830 provide a false low value?

Document ID

Document ID TE10997

Version

Version 1.0

Status

Status Published

Published Date

Published Date 01/17/2019
Question
Why might TNT830 provide a false low value?
Summary
TNT830 false low results
Answer
In the TNT830 chemistry ammonium ions at a pH value of 12.6 react with hypochlorite ions and salicylate ions in the presence of sodium nitroprusside as a catalyst to form the blue dye indophenol blue. Excess sodium nitroprusside gives the solution a yellow color. The combination of the blue and yellow gives reacted sample a green hue. This green color is measured spectrophotometrically at 690 nm. The amount of green color is proportional to the concentration of ammonia nitrogen in the sample.

High concentrations of ammonia nitrogen consume some of the excess sodium nitroprusside, thereby removing a portion of the yellow color complex, and changing the overall color of the sample from green to turquoise. As the yellow color is removed, the intensity of green ceases to increase in relation to ammonia nitrogen concentration and actually begins to decrease. This can explain why the results of a TNT830 and TNT831 test on the same sample might produce different results, but that are within range of each of the respective test ranges. 

DR-series Spectrophotometers can be configured with a multiple wavelength reading fo TNT830 to indicate when the quantitation range has been exceeded. Iinstructions for this update are available here: Application Instruction: Ammonium Measuring Range Exceedance (TNT830) for DR2800 / DR3800 / DR3900 / DR5000 / DR6000.
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