Bromothymol Blue has a limited pH range in which it can effectively change colour. It works best in the pH range of 6.0 to 7.6, where it transitions from yellow to blue. Outside this range, the colour change may not be as noticeable or accurate.

For instance, if the pH is below 6.0, Bromothymol Blue may remain yellow, making it difficult to distinguish acidic solutions. Similarly, if the pH exceeds 7.6, the solution might appear blue, masking the presence of basic conditions.

To ensure accurate and reliable pH measurements, it's crucial to use Bromothymol Blue within its specified pH range and consider alternative indicators for solutions with extreme acidity or basicity.