# Research on On-Line Detection Method and System for Nitrate in Groundwater
## 3 Results and Discussion
### Performance Test  
The fundamental purpose of using newer modified microelectrodes to detect nitrate ions is to eliminate the electrode passivation effect and improve the long-term repeatability of sensor detection. Therefore, the reproducibility and consistency of the proposed renewable microelectrode for nitric acid detection were examined in detail.  
1. Repeatability The same microelectrode was used to perform nine repeated tests in the test solution (a mixture of 0.1 mol/L Na\({}_{2}\)SO\({}_{4}\), 0.05 mol/L CuSO\({}_{4}\) and 1000 mol/L NaNO\({}_{3}\)) with the new modification method, and the results are shown in Figure 7.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Detection Phase** & **Working Potential (mV)** & **Duration Time (s)** \\ \hline Cleaning & +140 & 10 \\ Modification & \(-\)400 & 4 \\ Detection & \(-\)597 & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Optimization parameter settings (with the current at T = 4.04 s as the electrode response signal).  
Figure 7: Repeated testing of samples with the same concentration nine times in a mixture of 0.1 mol/L Na\({}_{2}\)SO\({}_{4}\), 0.05 mol/L CuSO\({}_{4}\) and 1000 \(\mu\)mol/L NaNO\({}_{3}\); the resulting chronocurrent scanning curve and the corresponding response current value change the curve with the test times (tested once every 5 min and repeated nine times).  
The time-current curve obtained by nine tests basically coincides with the response curve of the detection stage, and the response current value of the electrode is \(-120.20\)\(\pm\)\(1.51\)\(\mu\)A with a maximum relative deviation of 1.26%. This indicates that the catalytic activity of the sensitive film on the surface of the modified electrode is relatively stable and not affected by the passivation effect.  
Consistency  
Five different microelectrodes were used in the test solution (a mixture of 0.1 mol/L Na\({}_{2}\)SO\({}_{4}\), 0.05 mol/L CuSO\({}_{4}\) and 500 \(\mu\)mol/L NaNO\({}_{3}\)) to perform repeated tests three times each, and the results are shown in Figure 8.  
The response curve of the detection stage is also basically coincident with the time-current curve obtained from the 15 tests. The response current value of the electrode is \(-105.11\)\(\pm\)\(2.61\)\(\mu\)A, and the maximum relative deviation is 2.48%. This indicates that the new modification method can also show better repeatability and consistency when applied to different microelectrodes.  
Detection performance of nitrate  
Under the optimal parameter settings, the electrode pair concentrations are 0, 5, 10, 40, 100, 200, 400, 700, 1000, 1500, 2000, 2500, 3000, 3500 \(\mu\)mol/L, and all 14 nitrate samples were detected. The experimental results showed that there was a good linear relationship between the electrode response current value y and NO\({}_{3}\)\({}^{-}\) concentration x. The linear fitting between the two results in the functional relationship as follows:  
\[\mathrm{y}\ [\mathrm{\mu A}/\mu\mathrm{mol}\ \mathrm{L}^{-1}]=-0.031\mathrm{x}-2.75 7,\ \mathrm{R}^{2}=0.9879\]  
These results indicate that the on-line modified copper-sensitive film modified electrode can be used for the detection of nitrate concentration in water.  
Figure 8: Five different microelectrodes were tested with the same concentration of samples three times each in a mixture of 0.1 mol/L Na\({}_{2}\)SO\({}_{4}\), 0.05 mol/L CuSO\({}_{4}\) and 500\(\mu\)mol/L NaNO\({}_{3}\); the resulting chronocurrent scanning curve and the corresponding response current value changed with the test times (tested once every 5 min and repeated three times).