Chlorophosphonazo-III is a colorimetric reagent for thorium, uranium, lanthanoids, actinoids, and alkaline earth metals. This reagent is readily soluble in water and is violet at pH 4 or lower, blue in NaOH solution, and brilliant green in acids such as H2SO4, HCl and diluted HNO3 (it decomposes in concentrated HNO3). Its proton dissociation constants are reported to be -1.1, -1.1, 0.6, 0.8, 1.5, 2.5, 5.47, 7.20, 12.15 and 15.13 (μ=0.1, 20ºC). Chlorophosphonazo-III is used as a selective colorimetric reagent for calcium in a mixed Ca/Mg sample in the pH range of 2-3. It is also a useful indicator for the precipitation titration of SO42- using Ba2+ as a standard solution.
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Table 1 Metal Detection Conditions Using Chlorophosphonazo-III