Sample Temperature Affects Measurement of Nitrate with a Rapid Analysis Ion Selective Electrode System Used for N Management of Vegetable Crops

Abstract

The practical value of portable hand-held ion selective electrode sensors (ISE) for on-farm [NO3−] measurement to assist with crop N management of vegetable crops has been demonstrated in numerous previous studies. They provide rapid, in-situ measurement of the nitrate concentration ([NO3−]) in nutrient and soil solutions, and in petiole sap. Sample temperatures, for on-farm measurements, vary appreciably. This study evaluated the effects of sample temperature on [NO3−] measurement using two different models of a commonly used, commercially available, portable ISE meter. The temperatures (5, 10, 15, 20, and 25 °C) examined were in the range likely to be encountered in practical on-farm work. Aqueous solutions of 6, 12, and 18 mmol NO3− L−1 were prepared from KNO3, Ca(NO3)2 and NaNO3. [NO3−] was measured in three replicate samples of each of the three concentrations, made from each NO3− compound, at each temperature. The results consistently and clearly demonstrated a strong negative linear relationship between temperature-induced errors and sample temperatures. The temperature-induced error was considerable for cooled samples, being +50% at 5 °C and +31% at 10 °C. At sample temperatures of 17–20 °C, the temperature effects were minimal. Above this range, the temperature effect caused underestimation. At 25 °C, the temperature-induced error was −24%. These results show that care must be taken to ensure that sample temperatures do not erroneously affect the measurement of [NO3−] with ISE meters. Particular care needs to be taken with both refrigerated and warmer samples.