Performance of Ipomoea batatas L. Pigment as a pH Strip and Acid–Base Titration Indicator

Purple sweet potato tubers (PSPT) are rich in anthocyanins, pigments that exhibit visible colour changes across varying pH levels. This study aims to evaluate the performance of PSPT pigment (PSPTP) as both a pH strip and a titration indicator for strong acid–strong base (SASB), strong acid–weak base (SAWB), weak acid–strong base (WASB), and weak acid–weak base (WAWB) titrations. PSPTP was extracted from dried tubers using medical-grade (MG) and analytical-grade (AG) ethanol, resulting in PSPTP-MG and PSPTP-AG, respectively. Strips were fabricated by immobilizing the extracts on cellulose paper, producing PSPTPIP-MG and PSPTPIP-AG. The strips were unchanged at pH 1–7, pale blue at pH 8–9, green at pH 10–12, and yellow at pH 13–14. PSPTPIP-MG was stable for 14 days, while PSPTPIP-AG lasted 21 days; both gradually faded but retained acceptable stability for up to 180 days. They demonstrated excellent sensitivity and produced clear results when applied to household chemicals. As titration indicators, PSPTP-MG and PSPTP-AG showed good precision (CV 0.09%–1.17%) across all titration types but were accurate only for SASB and SAWB titrations (recovery 102.4%–102.5%). In the analysis of pharmaceutical samples, SASB and SAWB titrations yielded acid concentrations of 0.042 M for PSPTP-MG and PSPTP-AG, and 0.041 M for phenolphthalein (PP) in unspiked samples, while spiked samples yielded 0.165 M with all three indicators. Overall, this study demonstrates that PSPTP performs effective as both a pH strip and a titration indicator, particularly for SASB and SAWB titrations.