Which is more acidic: venous or arterial blood?

A byproduct of tissue metabolism is carbon dioxide. When carbon dioxide enters the blood stream, it is converted to carbonic acid:

CO₂ + H₂O → H₂CO₃

The effect of this is to lower the pH of the blood. Since this carbon dioxide is produced by the tissues, it is the venous blood that picks up the extra carbon dioxide, not the arterial blood. The arterial side has already shed some of this carbon dioxide into the lungs, where it is hopefully breathed away, and it hasn't yet got to the CO₂-producing tissues yet.

So the venous blood is more acidic. However, the effect is really quite mild, since the blood's buffer system greatly limits any pH change. Thus, the difference is only around pH 0.04 between the two sides of the circulation.