## 3 Results and Discussion
### Permanent and Temporary Drift Quantification and Correction  
Measurements for calcium, chloride, nitrate, and nitrite ISEs were taken in a continuous flow-through system using Nico2000 ELIT 6-Channel Flow-through Monitor (MCC-MON-6f) for 8, 16, and 24 h. Figure 1 shows the set-up of this experimental system. The purpose of this set-up was to determine what corrections are needed to account for electrode drift over time. Additionally, different methods of electrode restorations were evaluated to determine how often cleaning is needed, and how it should be done. During the usage of ISE electrodes, it was expected that the electrode potential would drift over time. For this reason, experiments were first kept short intentionally. This allowed for the electrodes to be washed and the drift'reset'. However, this would not be practical for long-term use in a wastewater treatment system. If the drift of the ISEs being used can be quantified, then washing can be delayed until it may avoid permanent damage to the electrode membrane. Figure 3 shows the reading of a calcium and chloride ISE over an 8 h period at relatively benign conditions (ionic strength below 0.001 M and at 20 \({}^{\circ}\)C). During this time the drift experienced is mostly linear with respect to time with spikes occurring randomly. Longer experimental trials (up to 24 h) did not give consistent or quantifiable trends. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Ionic Strength (moles/L)**} & \multicolumn{4}{c}{**Calculated Potential (mV)**} \\ \cline{2-5}  & **Ca\({}^{2+}\)** & **Cl\({}^{-}\)** & **NO\({}_{2}\)\({}^{-}\)** & **NO\({}_{3}\)\({}^{-}\)** \\ \hline
0.001 & 382.7 & 161.4 & 156.2 & 345.18 \\
0.01 & 382 & 170.4 & 181.8 & 315.33 \\
0.1 & 307.4 & 179.5 & 207.5 & 16.837 \\ \hline \multirow{2}{*}{**Concentration of Ion (moles/L)**} & \multicolumn{4}{c}{**Calculated Reading (mV)**} \\ \cline{2-5}  & **Ca\({}^{2+}\)** & **Cl\({}^{-}\)** & **NO\({}_{2}\)\({}^{-}\)** & **NO\({}_{3}\)\({}^{-}\)** \\ \hline
0.000 1 & 362.4 & 240.8 & 274.7 & 356.77 \\
0.001 & 382 & 170.4 & 181.8 & 315.33 \\
0.01 & 401.5 & 100.1 & 89 & 273.89 \\ \hline \multirow{2}{*}{**Temperature (\({}^{\circ}\)C)**} & \multicolumn{4}{c}{**Calculated Reading (mV)**} \\ \cline{2-5}  & **Ca\({}^{2+}\)** & **Cl\({}^{-}\)** & **NO\({}_{2}\)\({}^{-}\)** & **NO\({}_{3}\)** \\ \hline
30 & 384.1 & 163.9 & 180.5 & 309.26 \\
40 & 382 & 170.4 & 181.8 & 315.33 \\
50 & 379.8 & 177 & 183.1 & 321.41 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Sensitivity analysis of Linear Regression Model. A possible time dependent correction factor could be as simple as the one presented in Equation (4). \[EC_{1}X_{1}+C_{2}X_{2}+C_{3}X_{1}X_{2}+C_{4}X_{1}X_{3}+f_{1}(t)+f_{2}(t) \tag{4}\]  
where, \(f_{1}(t)\) and \(f_{2}(t)\) are simple functions of time. \(f_{1}(t)\) refers to permanent damage that occurs to the membrane. This can be found by comparing the signal in a standard solution to that predicted by the empirical model. \(f_{2}(t)\) refers to drift that occurs over time as the electrode is being used. Once the electrode is removed from solution and cleaned, \(f_{2}(t)\) value will be set back to zero. While keeping track of the permanent damage will increase the accuracy of measurements, it is still desirable to mitigate any damage. Several methods were tested to evaluate the best ways of restoring sensitivity to the electrode membrane including washing with deionized water, washing with 10% (by volume) ethanol (only for PVC membranes), and conditioning in concentrate of 1000 ppm target ion. The conditioning in high concentrate had shown the most promise in restoring sensitivity to the membrane, while washing with DI water had shown to be effective at resetting the short-term effects of drift. For example, two calcium electrodes were observed to have a 30-mV difference at the same test conditions. One electrode had been used heavily and the other was unused but conditioned previously and was ready to use.