Evaluation of a hand held reflectometer for rapid quantitative determination of nitrate

Publisher Website
Google Scholar
Abstract
An inexpensive hand held reflectometer was used to estimate NO3 concentration with Merckoquant NO3 indicator strips. For KNO3 solutions a fourfold improvement in precision was obtained over the visual method of colour comparison. The instrument calibration was stable over several days and not affected by temperature in the range 8–34o C. The reflectometer showed quantitatively that increasing temperature (8–34oC) reduced the optimal time for reading the strips from 120s to 40s. With standard NO3 solutions a simple hyperbolic transformation of the reflectometer readings produced a linear calibration of good fit (r2=0.99). NO3 could be estimated in different soil types within half an hour. The results agreed well with values from ion chromatography (r2 =0.97, slope=1.07). Plant sap analysis was moderately precise (r2 =0.92) but inaccurate and underestimated by more than 60% values obtained by ion chromatography. This discrepancy was not caused by the reflectometer but seemed to be due to factors within the plant sap that interfered with the NO3 colour reaction in the Merckoquant strips.