Articles
EVALUATION OF THE PERFORMANCE OF ION-SELECTIVE ELECTRODES IN AN AUTOMATED NFT SYSTEM
Article number
304_31
Pages
273 – 280
Language
Abstract
In the NFT system described by Heinen et al. (1991) experiments without crops were carried out to evaluate the performance of membrane ion-selective electrodes for NO3 and Ca, and a glass ion-selective electrode for K. The electrodes were cleaned and calibrated before and after 42 days of continuous use in the system.
The calibration lines for NO3 and K hardly showed drift, but the calibration line for Ca changed markedly.
For constant concentration the average drift for the NO3, K and Ca electrodes was -0.09, -0.03 and +0.51 mV·d-1, respectively.
The recorded potentials of the NO3 and K electrodes during a 14-day period showed an average change in potential of -0.9 and -2.8 mV·d-1, resulting in a gradual in- and decrease in computed concentration, respectively.
This change may be due to the algae growth on the electrodes observed at the end of the test.
It can not be ascribed to changing concentrations since these were constant, or to the change in calibration lines.
The relation between measured potential and temperature showed hysteresis loops; those of NO3 and K had a slope.
A theoretical temperature correction on the sensitivity of the NO3 electrode according to Nernst’s law did not yield better results, it even increased the NO3 concentration fluctuation.
This is ascribed to the fact that the zero-potential is temperature dependent also, for which no theoretical correction is known.
The hysteresis loops were well suppressed by placing the measuring device in a constant temperature water bath.
Depending on the range of temperature fluctuation, the change in potential measured by the NO3 electrode per unit degree increase in temperature was 0.37 (for large temperature fluctuation) and 0.27 (for small temperature fluctuation) mV·K-1. For K these changes were -0.03 and -0.19 mV·K-1, respectively.
The calibration lines for NO3 and K hardly showed drift, but the calibration line for Ca changed markedly.
For constant concentration the average drift for the NO3, K and Ca electrodes was -0.09, -0.03 and +0.51 mV·d-1, respectively.
The recorded potentials of the NO3 and K electrodes during a 14-day period showed an average change in potential of -0.9 and -2.8 mV·d-1, resulting in a gradual in- and decrease in computed concentration, respectively.
This change may be due to the algae growth on the electrodes observed at the end of the test.
It can not be ascribed to changing concentrations since these were constant, or to the change in calibration lines.
The relation between measured potential and temperature showed hysteresis loops; those of NO3 and K had a slope.
A theoretical temperature correction on the sensitivity of the NO3 electrode according to Nernst’s law did not yield better results, it even increased the NO3 concentration fluctuation.
This is ascribed to the fact that the zero-potential is temperature dependent also, for which no theoretical correction is known.
The hysteresis loops were well suppressed by placing the measuring device in a constant temperature water bath.
Depending on the range of temperature fluctuation, the change in potential measured by the NO3 electrode per unit degree increase in temperature was 0.37 (for large temperature fluctuation) and 0.27 (for small temperature fluctuation) mV·K-1. For K these changes were -0.03 and -0.19 mV·K-1, respectively.
Authors
M. Heinen, K. Harmanny
Keywords
drift, temperature dependence, hysteresis
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