Color Removal From Dried-lettuce Extracts When Nitrate is Determined

Abstract

There is an increasing trend in many countries towards the implementation of control measures to ensure low nitrate (NO3 ) levels of human food, therefore the methods used to measure nitrate concentration should be precise, practical and inexpensive. In this work two methods were evaluated to determine nitrate concentration in extracts of dry lettuce leaves, the potenciometric based on the electrode of specific activity and the colorimetric of Cataldo et al. (1975). Nitrate concentrations measured by colorimetry were 30% higher than those obtained by potenciometry, and this difference was due to the interference caused by the color of the extracts. Cataldo et al. (1975) suggested a procedure to avoid this interference which consisted in discounting the absorption of the extract before color development, using hence one blank per sample, but these authors considered that this procedure would only be needed for water extracts obtained from fresh samples. When this procedure was applied to dryleaves extracts, the difference between the colorimetric and the potenciometric methods was statistically non-significant. The other procedure evaluated was the addition of activated carbon to the extract, which also eliminated this background absorbance. Although both procedures were equally effective in eliminating the background color of the extract, the use of activated carbon is recommended because it does not require the use of one blank per sample.