3.3. Application to hydroponic solution
Fig. 9 compares NO3–N and K concentrations in hydroponic
solutions determined by NO3 and K electrodes with those obtained
by standard laboratory analysis, i.e., ion chromatography for nitrate
and ICP spectrophotometer for K measurement. On average,
hydroponic NO3–N levels determined by the ISE electrodes were
28% higher than those obtained with the standard analyzer. However,
a strong relationship (R2 = 0.85) was observed between the
two methods. Such high NO3–N estimates might be due to the fact
that the hydroponic samples with NO3–N levels of >150 mg L1
were over-estimated with the NO3 electrodes. The overall standard
error of prediction for NO3–N concentration was high at
40.5 mg L1. A strong relationship (R2 = 0.85) existed between
hydroponic K concentrations determined by ISE and those by the
ICP spectrometer, but the ISE method measured an average of
26% less K with a standard error of prediction of 74.1 mg L1. As
compared to results of soil extract tests for NO3 and K ions in
Kim et al. (2007), more variable ISE measurements were obtained.
In particular, relatively high standard deviations of 20–100 mg L1
were measured at concentrations >100 mg L1. Such reduced
repeatability might be attributed to lack of a buffer solution. The
buffer solution keeps the ionic strength of the solution constant
regardless of ions present in the solution, causing the ISEs to
exhibit a linear response to concentration. Without a buffer, the
ISEs would respond to ionic activity, requiring the use of the Debye–
Hückel formula to calculate the ion activity coefficient in order
to directly relate electrode response to the concentration.
Thus, a buffer solution such as Tris or acetate would be necessary
to obtain more accurate estimates of NO3 and K concentrations
in mixed hydroponic solutions when using the ISE sensing method.