## 5 Discussion  
Various research groups have proposed different systems for pH sensing. Table 3 presents a comparison of sensor features, including _pH sensitivity_ and _range, application area, cost_, and _portability_, as required for global applications. This analysis serves to present the results of this work in the light of previous advances reported in the scientific literature. The features of the laboratory grade apparatus, which was employed as gold standard, are also included for completeness. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**pH Sensing** & **pH** & **pH** & **Application** & **Cost** & **Portable** & **Global** & **Ref.** \\
**Materials** & **Range** & **Sensitivity** & **Area** & **(USD)** & **Cost** & **Access** & **Ref.** \\ \hline CuO Nanorods & 5–8.5 & 0.64 \(\mathrm{\SIUnitSymbolMicro}\)/pH at 50 Hz & Biological, Food, Medicine, Agriculture & N/A & No & No & [28] \\ Graphite-polyurethane & & & & & & & \\ composite & 5–9 & \(-11.13\pm 5.8\mathrm{\SIUnitSymbolMicro}\)/pH & Health Monitoring * & N/A & N/A & N/A & [29] \\ (AgIAgCl) & & & & & & & \\ EGFET & & & & & & & \\ (ZnOI AgI ZnO) & 2–12 & 0.62 \(\mathrm{\SIUnitSymbolMicro}\)A\({}^{1/2}\)/pH & Health Monitoring & N/A & N/A & N/A & [31] \\ Glass electrode & N/A & 51 \(\mathrm{\SIUnitSymbolMicro}\)/pH & Laboratory Instruction & \$50 ** & Yes & No ** & [24] \\ (PdI PdO) & & & & & & & \\ Carbon fiber thread & & & & & & & \\ electrodes coated with & & & & & & \\ self-healing polymers & 3.89–10.09 & 58.28 \(\mathrm{\SIUnitSymbolMicro}\)/pH & Health Monitoring and & & & & \\ (AgIAgCl and Pt wire) & & & & & & & \\ Sol–gel-entrapped & 10.2–12 & N/A & Cementitious materials & N/A & N/A & N/A & [25] \\ TiN-gate ISFET & 4–10 & 53.98 \(\mathrm{\SIUnitSymbolMicro}\)/pH & Medicine and Biological & & N/A & N/A & N/A & [41] \\ Ohaus AB33M1 & & & & & & & \\ (AgIAgCl) & 1–14 & N/A & Not restricted & \$715 & No & No & [TW] \\ Glass electrode & & & & & & & \\ (AgIAgCl) & & & & & & & \\ \hline \end{tabular}  
* The device was originally reported for _sweat monitoring_ applications. ** Reported cost refers to hardware electronics, the design of which is intended for a laboratory setting and would require modularity to enable global adoption. ** TW = This Work.  
\end{table} 
Table 3: Comparison of pH sensor features as required for global applications. Figure 7: The performance of the portable instrumentation for pH measurements is established by comparing our results to those of a reference instrument (Ohaus, AB33M1). The solutions utilized for this experiment, in ascending order of pH value, commencing from the most acidic one, are given as follows: a. Lemon juice, b. Cola soda, c. Vinegar, d. Orange juice, e. Beer, f. Coffee, g. Tea, h. Milk, i. Tap water, j. Human saliva, k. Hand soap, I. Sodium hypochlorite, and m. Sodium hydroxide. As seen from the data in Table 3, most of the end applications are related to biological areas. Additionally, a crucial aspect is that measurement range are limited, and that sensors have been designed, developed, and implemented for specific tasks. In contrast, it is clear from Table 3 that the instrumentation described in this work provides the features of _accuracy_, _measurement range_, _portability_, _cost-efficiency_, and _adaptability_, necessary to address global _Vertical Farming_ and _Industry 4.0_ pH sensing challenges. Nonetheless, some systems, such as those reported by [PERSON] et al. [31] and [PERSON] et al. [24], exhibit similar characteristics in terms of measuring range or cost (i.e., specific features needed for global adoption). Having a single instrument that can offer the complete set of features needed for global adoption is a key contribution of this work. The instrumentation described in this work enables precise determination of the pH, with an expected absolute deviation of 0.06 pH units and a standard deviation of 0.03 pH units. The precision required of pH measurements is intrinsically related to the task under consideration.