## 3 Results
### Sensor Stability  
Some widely available hard plastic cases (Ray Allen Manufacturing, Nanuk, and Pelican brands) used for training aid kits were used to test the stability of the sensor over 10 weeks in these containers. As time passed, the sensor color was visually observed to be altered by its surrounding environment in different degrees (Figure 5). Figure 6 shows the relationship between time and the normalized RGB data for a sensor in each case tested. As color changed, the RGB values shifted as well. From the graphs shown in Figure 6,  
Figure 3: Comparison of pH measurements for NH4 OH dilutions in headspace and in solution (n = 4).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**NH4 OH Concentration** & **Solution** & **Headspace** & **Solution vs. Headspace** & \multicolumn{2}{c}{**Headspace vs.**} \\
**(ppm):** & **pH \(\pm\) sd** & **pH \(\pm\) sd** & **p Score** & **Subsequent Headspace** \\  & & & & **p Score** \\ \hline
8760 (0.25 M) & 11.42 \(\pm\) 0.09 & 10.38 \(\pm\) 0.17 & \textless{}0.001 & \textless{}0.001 \\
500 & 10.35 \(\pm\) 0.10 & 9.53 \(\pm\) 0.04 & \textless{}0.001 & \textless{}0.001 \\
100 & 9.80 \(\pm\) 0.06 & 8.89 \(\pm\) 0.11 & \textless{}0.001 & \textless{}0.001 \\
50 & 9.52 \(\pm\) 0.02 & 8.41 \(\pm\) 0.06 & \textless{}0.001 & 0.007 \\
20 & 9.22 \(\pm\) 0.04 & 8.21 \(\pm\) 0.08 & \textless{}0.001 & \textless{}0.001 \\
10 & 9.00 \(\pm\) 0.11 & 7.89 \(\pm\) 0.06 & \textless{}0.001 & 0.010 \\
4 & 8.87 \(\pm\) 0.22 & 7.53 \(\pm\) 0.19 & \textless{}0.001 & - \\ \hline \hline \end{tabular}
\end{table}
Table 5: Measurements of pH for NH4 OH liquid solutions and their corresponding headspace (n = 4) as well as results from independent sample \(t\)-test from SPSS.  
it can be seen that the sensors in the Ray Allen Manufacturing case had the most drastic separation between the RGB values. The increasing separation between the red, blue, and green lines across time corresponded to a change in the overall sensor color. The larger the separation, the more significant the change of color. Visually, the RAM case sensors were seen to begin transitioning from green to blue between 19 and 26 days, indicating a basic environment (Figure 5). The PEL and NAN case sensors did not show as wide a separation for the RGB lines as the RAM case. However, when looking at Figures 5 and 6, it is noted that at approximately 26 days the Pelican case sensors began acquiring a blue ring of color as did the Nanuk case sensors after 30 days. This is suspected to be due to minor contamination from the ambient environment. In general, regardless of the outer blue ring color, they remained dominantly green and, thus, were still functional for detection of NH4 OH. One possible explanation for the significant color change in the RAM case is the foam lining contained within the kit. As opposed to the Pelican and Nanuk cases, Ray Allen Manufacturing uses an adhesive to affix the foam to the plastic case. This adhesive may have released VOCs that generated a basic environment within the case, causing the change in color.  
Figure 4: SEM imaging of (**A**) plain filter paper and (**B**) bromocresol green sol–gel-coated sensor.  
Figure 5: Progression of sensor color throughout the 10-week period for the stability experiment.