# Research on On-Line Detection Method and System for Nitrate in Groundwater
## 3 Results and Discussion  
### Parameter Optimization  
(1) Dissolution potential E\({}_{1}\)  
To ensure repeatability of the test, the copper already present on the electrode surface must be completely dissolved before each remodification. In order to obtain the most efficient surface cleaning solution, the microelectrode was placed in the test solution and the optimized solution potential was investigated by examining the time-current curve. The results are shown in Figure 4; that is, E\({}_{2}\) = \(-\)300 mV and E\({}_{3}\) = \(-\)600 mV were kept unchanged while the time-current scan curve was changed in which E\({}_{1}\)= (+0.1 V, +0.2 V, +0.3 V, +0.4 V, +0.5 V). It can be seen from the figure that when E\({}_{1}\) = +0.1 V, the dissolution current gradually decreases from 30 \(\upmu\)A in the initial stage to about 100 nA in the second stage, which indicates that the copper film prepared in the previous deposition operation on the electrode surface is being effectively dissolved. When E\({}_{1}\) varies between +0.2 V and +0.5 V, the corresponding dissolution current is always about 100 nA. At the same time, the current values corresponding to the dissolution stage are also very different under different dissolution potentials, which indicates that the dissolution potentials within this range cannot effectively remove the deposited copper on the electrode surface and thus each subsequent electrodeposition process is carried out on the electrode surface in different states. Figure 4: _Cont._  
In order to explore the optimal value, in the same test environment, one of the values of E\({}_{1}\) = (+0.11 V, +0.12 V, +0.13 V, +0.14 V, +0.15 V, +0.16 V) was changed again, and the test results were shown in Figure 3b. It can be seen that the deposition current value decreases obviously under different dissolution potentials. This indicates that the dissolution potential in this range can effectively dissolve the copper on the electrode surface so that the subsequent deposition process is carried out on the electrode surface environment in a similar state. When E\({}_{1}\) = 0.14 V, the process of decreasing the dissolution current from the maximum value to 100 nA takes the shortest time. (2) Deposition potential E\({}_{2}\)  
In order to select and optimize deposition potential E\({}_{2}\), when one of the values of E\({}_{2}\) = (\(-\)0.1 V, \(-\)0.2 V, \(-\)0.3 V, \(-\)0.4 V) is changed under the above optimized parameter settings, a series of samples with different concentrations are tested and the optimal value is determined by comparing the response sensitivity under different E\({}_{2}\) values. When E\({}_{2}\) = \(-\)0.3 V is taken as an example, the optimization experiment is illustrated. When E\({}_{2}\) = \(-\)0.3 V, the electrochemical response curve of the sensing electrode is shown in Figure 5. It can be seen that in the detection stage (T = 4.0-5.0 s), the response current value of the electrode increases with the increase in NaNO\({}_{3}\) concentration in the test solution. When E\({}_{2}\) = one of (\(-\)0.1 V, \(-\)0.2 V, \(-\)0.3 V, \(-\)0.4 V), the above experiments are repeated and the linear fitting lines at T = 4.04 s are obtained, respectively. The comparison results under the four circumstances are shown in Figure 6. In order to eliminate the influence caused by background noise, the longitudinal coordinate of the curve is the difference between the response current of the test liquid and that of the blank solution. It is found that when E\({}_{2}\) = \(-\)0.1 V, the background noise of the electrode is too large, resulting in no linear relationship between the response current value and the concentration of the test liquid. When E\({}_{2}\) = \(-\)0.3 V or \(-\)0.4 V, there is a good linear relationship between the response current and the concentration of the test solution, which indicates that the modified microelectrode can be used for the detection of nitrate ions.