Why phenolphthalein is used in acid base titration?
Phenolphthalein is another commonly used indicator for titrations, and is another weak acid. In this case, the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless.
A strong acid- strong base titration is performed using a phenolphthalein indicator. Phenolphtalein is chosen because it changes color in a pH range between 8.3 – 10. It will appear pink in basic solutions and clear in acidic solutions.
- Phenolphthalein is an indicator of acids (colorless) and bases (pink). Sodium hydroxide is a base, and it was in the pitcher at the beginning, so when added to the phenolphthalein in beakers 2 and 4, it turned pink (top half of the graphic).
- Universal indicator is unsuitable for titrations because it has a range of colours. Phenolphthalein is often used instead. It changes from pink in alkali to colourless in acid.
- The pH range of phenolphthalein is about 8.3 to 10.0, but the titration curve is so steep at the equivalence point that phenolphthalein makes a good indicator.
Indicator: A substance that changes color in response to a chemical change. An acid–base indicator (e.g., phenolphthalein) changes color depending on the pH. Redox indicators are also used. A drop of indicator solution is added to the titration at the beginning; the endpoint has been reached when the color changes.
- A titration is a technique where a solution of known concentration is used to determine the concentration of an unknown solution. Typically, the titrant (the know solution) is added from a buret to a known quantity of the analyte (the unknown solution) until the reaction is complete.
- Chemical indicator, any substance that gives a visible sign, usually by a colour change, of the presence or absence of a threshold concentration of a chemical species, such as an acid or an alkali in a solution. An example is the substance called methyl yellow, which imparts a yellow colour to an alkaline solution.
- Phenolphthalein's common use is as an indicator in acid-base titrations. Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols for use in experiments. It is a weak acid, which can lose H+ ions in solution. The phenolphthalein molecule is colorless, and the phenolphthalein ion is pink.
Phenolphthalein is an indicator of acids (colorless) and bases (pink). Sodium hydroxide is a base, and it was in the pitcher at the beginning, so when added to the phenolphthalein in beakers 2 and 4, it turned pink (top half of the graphic).
- The equivalence point is the point in a titration where the amount of titrant added is enough to completely neutralize the analyte solution. The moles of titrant (standard solution) equal the moles of the solution with unknown concentration. The equivalence point is not the same as the endpoint of a titration.
- Acidic and basic are two extremes that describe a chemical property chemicals. Mixing acids and bases can cancel out or neutralize their extreme effects. A substance that is neither acidic nor basic is neutral. The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14.
- Bases have a pH greater than 7 and can accept a proton or produce an OH- ion in a reaction. Now, if you had more acid than base in this reaction, not all of the acid would react, so the result would be salt, water, and leftover acid, so the solution would still be acidic (pH < 7 ).
Updated: 2nd October 2019