Because the pH of hydrochloric acid can vary depending on its concentration, specific indicators are used to determine when it reaches a certain pH during titration. Bromocresol green is used for this purpose because it exhibits a color change within the pH range of 3.8 to 5.4.
Bromocresol green turns one of three colors depending on the pH of the solution to which it is added. Within its approximate "neutral" pH range of 3.8 to 5.4, it exhibits its namesake green color. Anything below the 3.8 pH mark causes bromocresol green to take on its acidic form and turn a yellow color. Likewise, anything above the 5.4 pH mark causes it to take on its base form and turn a blue color. The broad range of pH detection in the acid range, below the 7 pH mark, makes bromocresol green useful for testing the equivalence point of acidic solutions.
Titrations are performed to determine the overall pH and concentration of a solution. An indicator is added to a solution based on whether it is acidic or basic and what pH is being tested for. A second solution with a known concentration and of the opposing pH, whether basic or acid, is added incrementally until the equivalence point is reached and a color change is exhibited by the indicator.