## 1 Introduction  
Since the oral route of administration of active pharmaceutical ingredients (API) is the most popular, conventional tablets are the most popular pharmaceutical forms. Unfortunately, these forms show slower absorption and, in turn, delayed onset of action; additionally, tablets need to be swallowed, which may be a problem for many patients, especially the elderly and pediatric ones. Effervescent tablets may overcome the disadvantages of traditional tablets [1]. They are used to administer dietary supplements used for prophylactic purposes, as well as analgesics, antipsychotics, antacids, betaines, and glucosamines [2; 3]. These tablets differ from conventional ones in the composition of their excipients, as they contain substances that disintegrate upon contact with water, resulting in the release of carbon dioxide and API [4]. Most commonly, CO\({}_{2}\) is released by the interaction of tartaric or citric acid with alkali metal carbonates or bicarbonates in an aqueous environment [1]. This form allows for large doses of API to be taken and masksits unpleasant taste [5; 6; 7]. However, the disadvantage of effervescent preparations is their sensitivity to environmental conditions, especially moisture. Hygroscopic ingredients can absorb moisture from the environment, leading to hydrolysis processes, especially when the storage temperature is above 25 \({}^{\circ}\)C.  
The therapeutic value of a medicinal preparation is ensured, among other things, by its durability, which means that it should be produced, stored, and used under appropriate conditions. Each ingredient in the drug formulation should be selected in such a way as to ensure the stability of the final product. The assessment of the stability of pharmaceutical products is carried out to ensure that appropriate standards of quality, efficacy, and safety of drugs are maintained throughout their shelf life. The guidelines for stability tests are set out by the International Conference on Harmonization (ICH) Stability QIA-Q1F [8]. These guidelines require conducting organoleptic, physicochemical, and microbiological tests relevant to the stability of the medicinal product. Currently, new analytical techniques are being sought to allow for the quick and effective detection of unfavorable changes in medicinal products. Previously, several spectroscopic and spectrometric methods were used to evaluate effervescent tablets in the context of their stability [9; 10; 11]. The API content was analyzed using methods such as near-infrared spectroscopy [9; 10] or by ratio spectra-continuous wavelet transform and ratio spectra-derivative spectrophotometry [11]. In turn, the HPLC method was used to quantify the degradants and process-related impurities of effervescent formulations with N-acetyl-L-cysteine [12]. In another study of hydrogen-producing effervescent tablets, the total amount of generated hydrogen and the weight gain of the samples were assessed in a stability test [13]. The change in moisture content, as determined by the dynamic vapor sorption method, was used as a criterion for the stability of the effervescent granulate formulation [13]. Water sorption tests were also used to study the physical stability of effervescent tablets containing benzonidazole complexed with cyclodextrin [14]. The tablets were stored at 25 \({}^{\circ}\)C under 0 or 80% relative humidity (RH) in hermetic systems for 7 days. [PERSON] et al. [15], in accordance with ICH guidelines, determined the physical stability of imatinib mesylate effervescent floating sustained-release tablets, stored for 6 months at 40 \({}^{\circ}\)C and 75% RH, using parameters such as drug content, hardness value, floating behavior, and visual observations of matrix integrity. One of the novel, non-invasive methods of analysis of pharmaceutical products is the measurement of their reflectance. Reflectance is a quantity describing the ratio of the light beam reflected from the surface of the tested object to the beam incident on the object.