## 2 Related Works
### Vitamin C Determination  
Color standardization techniques are crucial in analytical chemistry, especially in the colorimetric determination of different substances. This is particularly evident in methods where solutions of tested compounds are photographed, and concentration information is derived from these photos using various statistical tools and multivariate analysis. An illustration of such a methodology is the colorimetric determination of vitamin C.  
Ascorbic acid (AA), i.e., vitamin C, is a natural, highly water-soluble compound, which plays a dominant role in human health and is essential for a variety of metabolic reactions and biological processes, with strong antioxidant activity, limiting free radical damage. Insufficient concentration of vitamin C in the body, among others, causes scurvy and anemia and is associated with some psychological abnormalities, for example, depression. Overconsumption of vitamin C can have negative consequences, such as impeding vitamin B12 absorption and potentially causing gastrointestinal problems or kidney stones [19; 20]. As humans are unable to produce vitamin C internally, it is essential to supplement with a regulated amount of AA, typically from fresh fruits and vegetables [21; 22]. Analytical chemistry provides many strategies to determine AA in food products and vitamin C supplements. Traditionally, instrumental analysis techniques have been utilized to measure ascorbic acid levels, offering a wide concentration range with a low detection limit. These methods include direct titration [23], ultraperexome liquid chromatography (UPLC), high-performance liquid chromatography (HPLC) [24; 25], Fourier transform infrared spectrometry (FT-IR) [26], chemiluminescence [27], Raman spectroscopy [28], near-infrared (NIR), FT-Raman spectroscopy [29], and capillary electrophoresis [30]. Furthermore, a variety of electrochemical methods have been used to determine AA, offering extremely high sensitivity, selectivity, reproducibility, and reliability [31; 32; 33]. However, the mentioned methods of chemical instrumental analysis require specialized laboratory rooms, expensive instrumentation, many auxiliary reagents, time-consuming extraction steps, and complicated measurement procedures that require highly skilled staff for tasks such as preparing reagents and samples, conducting research, or interpreting results. The development of novel methodologies that either substitute or support traditional laboratory strategies with cost-effective instruments fulfills the need for highly efficient and adaptable approaches in food, pharmaceutical, and environmental analysis. Popular and widely available smartphones offer a promising analytical platform characterized by full automation, large local memory capacity, access to external cloud storage, and especially high-quality cameras, making them suitable for wide use as portable measuring devices. Compared to conventional methods, smartphone-based measurements use digital imaging, offering many applications in various fields and significant advantages. These include low cost, simple operation, the possibility of installing various dedicated sensors, precise measurements in real time, a non-destructive nature of testing in many cases, fast analysis, and portability to other similar devices. In analytical chemistry, commonly available devices, such as cameras, webcams, scanners, and smartphones, are applied to register images from colorimetric reactions in which the intensity of the resulting color is directly correlated to the concentration of the analyte. The literature provides various descriptions of the use of digital imaging methods for determining AA. The work [34] presents a straightforward, rapid, and cost-effective technique employing smartphone image analysis to quantify AA in an aqueous solution. The study was based on the evaluation of the disturbing effect of AA in the enzymatic colorimetric detection of glucose.