The technique of sodium removal from acid solutions with Hydrated Antimony Pentoxide (HAP) has been used as a separation and preconcentration step in the determination of trace elements in many matrices. In trying to understand the mechanism of the sodium retention process the studies have revealed that although the retained sodium is difficult to elute, it can undergo an exchange process with dilute solutions of NaCl in hydrochloric acid. Hydrogen ions play a role in this exchange process.