Phenolphthalein pink: the base is in excessBurette of NaOH, read at the meniscus to 0.05 mlOxalic acid in the conical flaskTitrating sodium hydroxide against 0.1 M oxalic acid. Each drop of base makes a pink flash where it lands, and swirling clears it as the base meets the acid. Near the end point the flashes linger, because less acid is left to use them up. At the end point the acid is exactly used up, and the next drop leaves the whole flask pale pink: a titre of about 20.4 ml here. Past it the pink deepens and the titre is too big. (COOH)2 + 2NaOH → (COONa)2 + 2H2O, so a1M1V1 = a2M2V2 with a1 = 2 and a2 = 1, and the strength in g/L is M2 × 40. CBSE asks for three concordant readings (within 0.1 ml); ISC for two consecutive titres exactly equal.