Which tool provides the exact pH of a solution?

The pH probe is the correct tool for measuring the exact pH of a solution because it provides a direct and specific electrical measurement of hydrogen ion activity in the solution. It consists of a glass electrode that responds to the concentration of hydrogen ions, allowing it to produce accurate and precise readings, typically displayed in numerical form on a digital meter. This high level of accuracy is essential for many scientific applications, where even small changes in pH can be significant.

In contrast, litmus paper provides a qualitative indication of whether a solution is acidic or basic but does not give an exact value of pH. Similarly, a universal indicator, while able to show a range of pH values through color changes, lacks the precision of a pH probe and can only give an approximate range rather than an exact measurement. A color chart, which typically accompanies universal indicators, also only allows for approximate readings based on visual comparison, making it less reliable for precise laboratory work.