Euclea natalensis Root Extract as a Green Acid-Base Indicator: pK_a Determination and Application in Acid-Base Titration

This work presents Euclea natalensis root extracts as a Green acid-base indicator, focusing on the application in titration and determination of pK_a. The Euclea natalensis root extracts were obtained by heating the biomass in water at 60 °C for 1 h. The obtained extracts were applied for acid-base titration, and the pK_a value was determined and compared to bromothymol blue and methyl orange, conventional acid-base indicators. Furthermore, the pK_a of some naphthoquinones that occur in Euclea natalensis root were predicted using MolG pKa web server and compared with experimental pK_a value. The results showed that the Euclea natalensis extract presented a titration performance comparable to common synthetic acid-based indicators (bromothymol blue-NaOH/HCl, required volume-11.8 ± 0.1 mL and Euclea natalensis, NaOH/HCl, required volume 11,6 ± 0.1 mL). Euclea natalensis root extracts presented a pK_a of 7.01, corroborating the predicted pK_a values of two main candidate compounds in Euclea natalensis root. These two compounds may cause color changes in Euclea natalensis root extracts in acidic media and basic solutions. These results showed that the Euclea natalensis root extracts are potential green and locally available acid-base indicators. However, additional work is still necessary to understand the stability and which compound or compounds are the core for color change.