Senna flower extract as an indicator for acid-base titration

Abstract

Senna (Khi-Lek in Thai) is a plant that produces yellow flowers, and which grows naturally throughout Thailand. This study investigated the use in the acid-base titration of extracts from the flowers of Senna siamea (Lam.) H.S. Irwin & Barneby, Senna garrettiana (Craib) H.S. Irwin & Barneby, and Senna surattensis (Burm.f.) H.S. Irwin & Barneby. The dried flower extracts were added to buffer solutions, HCl/NaOH solutions, and CH3COOH/NaOH solutions, and any color change was observed. All three extracts caused the solutions to become darker brown as the pH increased. Senna surattensis Burm. f. the extract produced the most easily observed change. This extract was therefore shown to be an effective indicator in acid-base titration of 0.1 M HCl and 0.5 M NaOH. The solution changed visibly from colorless to brown. Its effectiveness compared favorably with that of phenolphthalein, a standard laboratory indicator. Senna extract is particularly appropriate for use in chemistry demonstrations at Thai schools as it is widely available, economical, easy to prepare, safe in use, and environmentally friendly.