As refrigerant passes through the expansion valve, it cools down or heats up?

It cools down

Explanation:

The expansion valve lowers down the pressure of the refrigerant (in liquid form). This allows the refrigerant to expand. As the refrigerant expands the heated molecules in it get spread to a larger area and thus the oscillatory vibrations in the molecules get reduced there by reducing the kinetic energy ( KE )and thus the heat. Also sometimes spray of Aerosols are used to cool down the refrigerant.

the ratio of the conjugate base to the acid required to attain the desired ph.