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n <br />U <br />Antimony Geochemistry <br />Antimony (Sb) occurs in two oxidation states in natural waters, +3 and +5. Sb(+5) exists <br />predominantly as a negatively charged ion (anion) above a pH of about 2.7 and as a <br />neutral ion below pH 2.7, as shown in Figure 1. <br />8b(V) <br />• <br />Sb(OH)o° Sb(OH)„ - <br />0 2 4 e e 10 12 14 <br />DH <br />Figure 1 - Antimony (+5) speciation as a function of pH (alpha is the fraction of the <br />total dissolved antimony (+5) consisting of the given species). <br />1 <br />0.9 <br />0.8 <br />0.7 <br />0.6 <br />0.6 <br />0.4 <br />0.3 <br />0.2 <br />0.1 <br />0 <br />Sb (+3) exists mainly as a positively charged ion below a pH of 1.4, a neutral species <br />between pfi 1.4 and 11.8, and a negatively charged species above pH 11.8 (see Figure 2). <br />