What instrument is commonly used to measure pH?

A pH meter is the commonly used instrument for measuring the pH of a solution because it provides precise and accurate measurements of acidity or alkalinity. It operates by using a glass electrode that responds to the hydrogen ion concentration in the solution, allowing it to determine the pH on a scale typically from 0 to 14.

While a barometer is used to measure atmospheric pressure and a thermometer measures temperature, neither is suitable for assessing pH levels. A hydrometer measures the density of liquids, which is unrelated to pH. Therefore, the pH meter stands out as the specialized tool designed specifically for pH measurement, making it the appropriate choice among the provided options.