Video Transcript
A universal indicator is not normally used as an indicator when performing a titration. Which of the following statements explains why this is the case? (A) It decomposes too quickly. (B) It changes color over a very wide pH range. (C) It changes color over a very narrow pH range. (D) It only changes color in acidic solutions. (E) It does not change color once in solution.
A titration experiment can be performed to determine the concentration of an acidic or basic solution. Say we have a solution of sodium hydroxide and we don’t know the concentration of the solution. We can determine the concentration of this base if we react or titrate it with an acid that we know the concentration of, for example, hydrochloric acid. The products of this reaction are sodium chloride, a salt, and water. Notice that the products of this reaction are neutral. So, if the solution only contained these two chemical species, the solution would have a pH of seven. At the equivalence point of this reaction, the sodium hydroxide and hydrochloric acid will completely neutralize each other, so both will be used up. This means the pH at the equivalence point will be equal to seven.
At the equivalence point, the moles of hydroxide ions from the sodium hydroxide will be equal to the moles of hydrogen ions that were added from the acid. So, if we can detect the equivalence point during the titration, we can calculate the unknown concentration of the sodium hydroxide solution using this equivalency and the known concentration of the acid. We can detect the equivalence point if we monitor the pH of the reaction. When the pH is around seven, we know the equivalence point has been reached. Indicators can be used to visually detect a specific pH, as indicators are either a weak acid or a weak base that undergoes a color change over a specific pH range.
The universal indicator is one indicator. It’s used to determine the approximate pH of a solution. The universal indicator is a different color depending on the pH of the solution. For pHs that are neutral, the universal indicator is green. But it takes on warmer colors, like reds or yellows, when the solution is acidic. And the indicator is blue or purple if the solution is basic. So we can see that the universal indicator changes color over a very wide pH range. So, during a titration experiment, the universal indicator wouldn’t be very useful. This is because we need an abrupt color change near the equivalence point during a titration experiment.
In the reaction we were looking at for this question, the equivalence point is reached when the pH is seven. The indicator phenolphthalein might be a better choice for this experiment, as phenolphthalein changes from a dark pink to colorless in the pH range of about eight to 10, which is slightly higher than the equivalence point but close enough to give us the abrupt color change that we’re looking for. So we wouldn’t normally use the universal indicator in a titration experiment because it changes color over a very wide pH range.