Final answer:
The endpoint in an acid-alkali titration with a pH change from 3 to 10 and an equivalence point at pH 7 is reached when bromothymol blue changes from yellow to green and then to blue, indicating the transition around the equivalence point.
Step-by-step explanation:
In an acid-alkali titration where the pH changes quickly from 3 to 10 with the equivalence point at pH 7, the endpoint is reached when the indicator undergoes a distinct color change, corresponding to the pH range it is sensitive to. Bromothymol blue, an indicator with a range around pH 6.0 to 7.6, turns yellow below pH 6.0 and blue above pH 7.6. As the equivalence point is pH 7, bromothymol blue will show a green color at neutrality due to the mixture of its acidic and basic form colors and will transition from yellow to blue as the titration reaches the endpoint.
For an accurate endpoint determination, it's critical to choose an indicator that changes color sharply at or near the equivalence point. Since the titration you describe has its equivalence point at pH 7, and bromothymol blue changes color around that pH, it is suitable for your titration. At the endpoint, you will observe a shift from yellow to green and finally to blue as you reach and then surpass the equivalence point.