Why is phenolphthalein used when an acid is titrated with a strong base?
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 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.
- This compound's color change happens through a process called ionization. When exposed to alkaline solutions, it begins to block the blue colors of the spectrum, which turns the light pinkish. The stronger the alkaline solution is, the more the phenolphthalein molecule changes and the darker the pink hue will be.
- 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.
- Preparation of Phenolphthalein Indicator (Acid/Base Indicator)
- Weigh out 0.5g of phenolphthalein.
- Prepare 50% ethyl alcohol solution contained of 50mL ethonal and 50mL water.
- Now, dissolve the phenolphthalein in the 50% ethyl alcohol solution.
- Store in dropper mechanism for use.
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.
- The equivalence point in a titration is the point at which the added titrant is chemically equivalent completely to the analyte in the sample. End point is the point where the indicator changes its color. To get the same equivalent point as the end point, pH of the indicator should match the pH at the equivalence.
- 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 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.
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 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.
- They react with metals like zinc to give off hydrogen. Bases in water solutions also show certain properties or characteristics. They taste bitter and and turn litmus paper blue. They also have a slimey or slippery texture to them.
- 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