pH measurement requires the following:

The measurement of pH is fundamentally based on the use of electrodes that can provide accurate readings of the hydrogen ion concentration in a solution. This requires specific instruments to ensure precise and reliable results.

The voltmeter is essential because it measures the electrical potential difference generated by the glass pH electrode and the reference electrode when immersed in a solution. The glass pH electrode is the primary sensor that reacts to the hydrogen ion concentration, while the reference electrode provides a stable reference point against which measurements can be taken. A temperature compensating device is also crucial in pH measurement since the ion activity and electrode response can vary with temperature changes. Thus, incorporating temperature compensation allows for the adjustment of readings to reflect an accurate pH value at a standardized temperature, typically 25°C.

In contrast, graduated cylinders and thermometers, or pH paper, lack the specificity and accuracy required for precise pH analysis. A digital scale and volumetric flask are more suited for weighing and diluting solutions rather than directly measuring pH. Similarly, a refractometer and thermometer are instruments used for measuring refractive index and temperature, which are not relevant to pH measurement. Therefore, the first option accurately outlines the necessary equipment for pH measurement and is the correct choice