page_title: Acid–base titration
section_title: Indicator choice
hierarchical_section_title: Acid–base titration / Indicator choice
caption_attribution_description: English: An acid-base titration using phenolphthalein as the indicator. The conical flask contained solution that just reached the endpoint. 中文: 採用酚酞指示劑進行酸鹼滴定，達到滴定終點時的情形。
context_page_description: An acid–base titration is a method of quantitative analysis for determining the concentration of an acid or base by exactly neutralizing it with a standard solution of base or acid having known concentration. A pH indicator is used to monitor the progress of the acid–base reaction. If the acid dissociation constant of the acid or base dissociation constant of base in the analyte solution is known, its solution concentration can be determined. Alternately, the pKa can be determined if the analyte solution has a known solution concentration by constructing a titration curve.
context_section_description: A suitable pH indicator must be chosen in order to detect the end point of the titration. The color change or other effect should occur close to the equivalence point of the reaction so that the experimenter can accurately determine when that point is reached. The pH of the equivalence point can be estimated using the following rules:
  A strong acid will react with a strong base to form a neutral (pH = 7) solution.
  A strong acid will react with a weak base to form an acidic (pH < 7) solution.
  A weak acid will react with a strong base to form a basic (pH > 7) solution.
  When a weak acid reacts with a weak base, the equivalence point solution will be basic if the base is stronger and acidic if the acid is stronger. If both are of equal strength, then the equivalence pH will be neutral. However, weak acids are not often titrated against weak bases because the colour change shown with the indicator is often quick, and therefore very difficult for the observer to see the change of colour.
  The point at which the indicator changes colour is called the end point.  A suitable indicator should be chosen, preferably one that will experience a change in colour (an end point) close to the equivalence point of the reaction.
caption: An acid-base titration using phenolphthalein as the indicator. The conical flask contained solution that just reached the endpoint.
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