why did the solution turn to pink at the end of the titration ?
The titration reached the end point.
The solution turning pink at the end of a titration is a common observation and typically indicates the completion of the reaction between the titrant and the analyte. The pink color is often due to the addition of an indicator, such as phenolphthalein, into the solution.
To understand why the solution turns pink, we need to consider the principles of titration. Titration is a technique used to determine the concentration of an analyte in a solution by reacting it with a standard solution (titrant) of known concentration.
In the case of acid-base titrations, the titrant is usually a strong base (e.g., sodium hydroxide) or a strong acid (e.g., hydrochloric acid), while the analyte is an acid or a base respectively. The goal is to reach the equivalence point, which is when the moles of acid equal the moles of base (or vice versa). At this point, there are no excess reactants left, and the reaction is considered to be complete.
The choice of indicator is crucial in acid-base titrations as it changes color when the pH of the solution reaches a specific range. For example, phenolphthalein is commonly used as an indicator in acid-base titrations. It is colorless in acidic solutions (pH less than 8.2) and turns pink when the solution becomes basic (pH greater than 10).
When the titrant is slowly added to the analyte, the indicator remains colorless due to the excess of acid in the solution. As the titrant nears the equivalence point, the amount of excess acid decreases, and the pH of the solution increases. Once the equivalence point is reached and there is a slight excess of the base, the pH rises rapidly, causing the solution to turn pink. This color change signifies the completion of the reaction, indicating that the titration is finished.
In summary, the solution turns pink at the end of the titration because the pH of the solution becomes basic, triggering the color change of the indicator, typically due to the addition of phenolphthalein.