The influence of salt purity on the interactions between Na+ ions and the carboxylate (COO-) head group of palmitic acid (PA) monolayers is studied in the COO- and OH stretching regions using broad-band vibrational sum frequency generation (VSFG) spectroscopy. Ultrapure (UP) and ACS grade NaCI salts are used for aqueous solution preparation after proper pretreatment. The time evolution of VSFG spectra of PA monolayers on solutions made from these two grades of salts is different, which reveals that the salt purity has a significant impact on the interactions between Na+ ions and the COO- group of PA. The trace metal impurities in ACS grade salt, which are more abundant than those in UP grade salt, are responsible for this difference due to their stronger affinity for the carboxylate group relative to Na+ and further affects the interfacial water structure. These results suggest that the alkali salt grade even after pretreatment is critical in the studies of alkali cation-carboxylate interactions and comparison of relative binding affinities of different cations.