## 2 Materials and Methods
### Experimental Methods  
Experiments where created to determine the relationship between the potential difference of the electrodes and the concentration of nitrate, nitrite, calcium and chloride in an aqueous solution. A starting solution of 250 mL deionized (DI) water and a concentrated solution (ranging from 0.001 to 1 molar to target different concentration ranges) of preferred molarity for one of the selected ions was prepared for each test. The solutions were stirred and heated when necessary. Tests where preformed at 20, 30, and 50 \({}^{\circ}\)C. 1 mL of the concentrated solution was added into the DI water and mixed for 5 min to create the starting solution. The ISE of the selected ion, reference electrode and thermocouple were rinsed with DI water before being placed in the starting solution. After 5-10 min (to allow for the electrodes to stabilize) 5 measurements of the electrodes were taken, to increase reliability of the results. Next, 1 mL of the concentrated solution was added to the starting solution. This was repeated every 5 min until there was a minimal change in the measurements, this typically resulted after 25 or so additions, which is expected for this semi-batch style additive process. The final concentration of test solutions would be approximately 1/10 th that of the concentrated solution. While the wait time for measurements is far greater than the response time, it ensured that accurate measurements were being taken. A similar method was used to evaluate the effect of ion strength on electrode measurements. A starting solution was prepared using the methods previously described. A non-interfering ion wasthen added to the solution and the change in measurement was recorded after the system gave a constant measurement. Sodium chloride, calcium chloride, and sodium nitrate were used to increase the ionic strength of the solution. This method of interference evaluation is similar to those done in studies evaluating novel membranes for use in future analytical techniques [4,5]. The range of experimental values are shown in Table 1.  
The electrodes were calibrated using standard solutions of calcium chloride for the calcium and chloride ISE's, and sodium nitrate and sodium nitrite for the nitrate and nitrite electrodes respectively. These standard solutions were verified using the previously described ICP methods. For verification of accurate measurements, standard soltuions were used that spanned the intended test range. For example, a test of the calcium ISE that ranged from 0.0001 to 0.001 M would be calibrated at 0.00001, 0.0005, and 0.002 M before the experiments. These values were checked to previous measurements to ensure that the electrodes were not damaged and had not experienced significant performance degradation since the previous experiments. Two reference electrodes were used during this calibration process to verify that the reference electrode was functioning properly as well. For the determining drift over time, measurements for calcium, chloride, nitrate, and nitrite ISEs were taken in a continuous flow-through system using Nico2000 EILT 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. \begin{table}
\begin{tabular}{c c c c} \hline
**ISE** & **Concentration [M]** & **Temperature (°C)** & **Additional Ionic Strength [M]** \\ \hline
**Calcium** & 0.00008–0.053 & 20.4–50.7 & 0.0002–0.065 \\
**Chloride** & 0.00019–0.028 & 20.3–50.7 & 0.0019–0.0655 \\
**Nitrate** & 0.00002–0.05 & 20.3–51.6 & 0.00002–0.053 \\
**Nitrite** & 0.004–0.016 & 20.8–50.9 & 0.0001–0.08 \\ \hline \end{tabular}
\end{table} 
Table 1: Experimental Range for Ion selective electrodes (ISE) Evaluation.