Parameters of the colorimetric signal of bromothymol blue and bromophenol red solutions

Background . The analysis of colored solutions is a promising direction for using digital colorimetry. Bromothymol blue is a reagent for cationic surface-active substances, which are actually objects of study, as they are often used in the composition of medicines, cosmetic chemistry, and the chemistry of detergents. The addition of surface-active substances to analytical systems in molecular spectroscopy methods improves the determination parameters and stabilizes the colloidal-chemical state of the solutions. The work aimed to investigate the parameters of the colorimetric signal of aqueous solutions of bromothymol blue (BTB) and bromophenol red (BPR) in the presence of surface-active substances and to evaluate the further possibility of developing methods for colorimetric determination of the content of ionic surfactants with BTB or BPR in medicinal products.
Methods . The research was conducted using digital colorimetry with a scanner and smartphone. The RGB characteristic was used to quantitatively assess the color intensity of the solutions.
Results . It was established that within 90 minutes, the change in the value of the R-, G-, and B-components does not exceed ±1 % of the average value of the signal of the measured samples. An increase in the light temperature from 3000 to 5000 K practically does not affect the value of the R- and G-components of dye solutions, while the signal intensity of the B-component increases. The study of the influence of lighting intensity on RGB characteristics showed that when lighting dye solutions of 500 Lux and higher, the R-, G-, and B-component signal values are constant. With an increase in the height of the layer of the reagent solution in the range of 0,5 – 5 cm, an increase in the intensity of the color of the solution is observed, which causes a decrease in the values of R-, G-, and B-components. The possibility of smart colorimetric determination of the dissociation constant of organic reagents is shown. The introduction of Triton X-100 into the system improves the linearity of the graduation dependence of BTB determination, lowers the detection limit of the dye, improves the sensitivity of the determination, and leads to additional stabilization of the colloidal
chemical state of the system, which prevents turbidity when using high concentrations of reagents.
Conclusions . The influence of the main parameters on the colorimetric signal of sulfophthalein dyes was studied. The data obtained in the work can be used to develop methods for colorimetric determination of the content of ionic surfactants in medicinal products.

Корж Л., Титович В., Лелюшок С., Куліченко С. Параметри кольорометричного сигналу розчинів бромтимолового синього та бромфенолового червоного. Вісник Київського національного університету імені Тараса Шевченка. Хімія. 2023. Вип. 1(58). С. 42-48.