= KHP standardization protocol
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Columbia University's institutional laboratory manual, Experiment 1, Glassware Calibration, Primary and Secondary Standards, and Manual Titrations, Part 3, specifies two weighed $0.2$–$0.3\,\mathrm g$ portions of <potassium hydrogen phthalate>, transferred completely into separate $250\,\mathrm{mL}$ <Erlenmeyer flasks>. Dissolve each in approximately $100\,\mathrm{mL}$ distilled <water>; any warmed sample cools to room <temperature> before titration. Add two or three drops of <phenolphthalein>. Rinse and condition a clamped <burette> with the diluted <sodium hydroxide>, then titrate while swirling to faint pink lasting about $20$ seconds. Calculate <concentration> from each portion's own <mass> and delivered <titre>; repeat if results scatter. Part 1 identifies a Mettler balance, and Part 2 includes paired weighings of exposed <sodium hydroxide> and <potassium hydrogen phthalate>. These are prescribed procedures, not reported measurements.<Glassware Calibration, Primary and Secondary Standards, and Manual Titrations, Columbia University Experiment 1>[Experiment 1; Part 1, balance capacity note; Part 2, steps 1–3; Part 3, steps 1–6 and results table]{ref}
The same manual directs keeping the diluted <sodium hydroxide> covered and limiting air exposure after <titrant standardization>.<Glassware Calibration, Primary and Secondary Standards, and Manual Titrations, Columbia University Experiment 1>[Experiment 1, Part 3, introductory paragraph and step 1]{ref} The MIT handout explains the associated <carbon dioxide> problem: carbonate contamination of <sodium hydroxide> is controlled during preparation, bottles are promptly capped, and atmospheric <carbon dioxide> makes the pink <phenolphthalein> endpoint fade.<The Potentiometric Titration of an Acid Mixture (MIT 5.310 laboratory handout)>[Printed Titration-5, §IV.1 and footnote 3; Titration-6, footnote 6]{ref} Consequently, a standardized <concentration> applies to the stored <chemical solution> under its handling conditions; continued uptake of <carbon dioxide> can change the reacting inventory per delivered <volume (thermodynamics)>.
The color is the readout, and <stoichiometry> supplies the inference. <Phenolphthalein> indicates a <pH> range, so its first reproducible pale pink is an operational <titration endpoint>, rather than a direct observation of equal reacting inventories. A <potentiometric titration> can locate an endpoint from a <pH> versus <volume (thermodynamics)> curve, but <glass electrode> calibration alone still does not determine <sodium hydroxide> <concentration>. Repeat portions test scatter; incomplete dissolution, an unmatched <titration blank>, or a shared endpoint error can bias all repeats together.
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