| http://www.w3.org/ns/prov#value | - Then the sodium component is usually reduced to as small an amount as possible, generally less than about 0.30 weight percent, through ion exchange with hydrogen ions, hydrogen-precursors such as ammonium ions, or polyvalent metal ions, including calcium, strontium, barium, and the rare earths, such as cerium, lanthanum, neodymium, and naturally-occurring rare earths and their mixtures.
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