XO is a colorimetric reagent and metal indicator of various metal ions. It is readily soluble in water. Its proton dissociation constants are reported to be -1.74, -1.09, 0.76, 1.15, 2.58, 3.23, 6.40, 10.5, and 12.6 (ion strength: 0.2). The color of the aqueous XO solution is yellow at pH < 6.3 and pink at pH > 6.5, and turns reddish-violet in the presence of metal ions. XO forms complexes with various metal ions in acidic conditions. Therefore, XO can be used to determine Ca, Cd, Mg, Pb, and Zn in the presence of Zephiramine in acidic conditions. XO is also a very versatile indicator for EDTA titration in acidic solutions.
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