Indicators used in titration are generally chemical dyes of synthetic origin that provide distinct color transitions of pH. Nature has abundant source of plants that need to be identified for their indicator property there by promoting SDG 12 (Responsible Consumption and Production) and SDG 15 (Life on Land) goal. The present study focuses on the applicability of vegetables’ and fruits’ extracts as natural indicators. A comparative study of red cabbage (Brassica oleracea), dragon fruit (Selenicereus undatus), pomegranate (Punica granatum), beetroot (Beta vulgaris) and turmeric (Curcuma longa) extracts with synthetic indicator viz. phenolphthalein was carried out to evaluate their potential as acid-base indicator. The findings revealed that all plant extracts tested were effective as acid–base indicators in strong acid–strong base titrations. Additionally, red cabbage (Brassica oleracea) and dragon fruit (Selenicereus undatus) demonstrated indicator activity even in strong acid–weak base titrations. The study detected the presence of anthocyanins in all plant extracts which were analysed by determining their λmax by UV spectrophotometry and Rf by Thin Layer Chromatography. The choice of such natural indicators can be explained by their wide occurrence, non-toxicity, simple methods of extraction, and low cost, which makes these indicators a sustainable alternative to synthetic ones in the context of pharmaceutical use.
Regarding the consistency of results, it is true that natural indicators, being plant-derived, may show minor variations due to factors such as seasonal changes, geographical source, maturity stage of the plant material, storage conditions, and extraction method. These factors can influence the concentration of anthocyanins or curcuminoids, which are responsible for color change and pH sensitivity.
To minimize this variability, we standardized certain parameters such as the solvent used, extraction time, and temperature, and ensured fresh plant material from the same source and batch was used during experimentation. The color transition ranges observed were reproducible within acceptable limits, confirming satisfactory consistency.
However, as rightly pointed out, the source and condition of vegetables/fruits can influence the intensity of color change, and this aspect indeed warrants further investigation and standardization if such natural indicators are to be used routinely in pharmaceutical analysis.
In the present preliminary study, our main focus was to evaluate the feasibility of using plant-derived extracts as natural indicators in acid–base titrations. Although a detailed stability study was not the primary objective, we did conduct observations over a six-month period, which revealed that the burette readings changed when the indicator extracts were stored at room temperature without refrigeration. This variation was attributed to the degradation of color intensity and reduced pH responsiveness over time.
Based on these findings, we conclude that refrigeration is necessary to maintain the stability and reliability of the natural indicators. Storage at low temperature helps in preserving the color pigments (such as anthocyanins and curcuminoids) and minimizes oxidative degradation.
In our future work, we plan to perform a systematic stability study under different storage conditions—varying temperature, light exposure, and pH—to determine the optimum storage conditions and practical shelf life of these natural indicators for routine analytical applications.
Reepa Ughreja
Thank you very much for your encouraging and constructive feedback. I completely agree that including validation parameters is essential to establish the analytical reliability of natural indicators. In the present preliminary study, my focus was mainly on identifying and comparing the qualitative performance and color transition behavior of plant-based extracts with synthetic indicators.
However, as you rightly suggested, in my future work, I plan to extend this research to include method validation studies in accordance with ICH and pharmacopeial guidelines. Parameters such as accuracy, precision, reproducibility, linearity, and robustness will be assessed to confirm that these natural indicators can provide results comparable to conventional synthetic indicators. This will help demonstrate their practical suitability for routine analytical use in pharmaceutical laboratories.
