PAR is a metal indicator and colorimetric reagent for heavy metal ions. The solubility of this reagent in water (5 mg per 100 ml at 10ºC) is higher than that of PAN. PAR is slightly soluble in alcohol. The aqueous solution is red at pH 5.5 or less, orange at pH 6-12.5, and red at pH 13 or higher. Its proton dissociation constants are reported to be pKa1(NH+)=3.1, pKa2(p-OH)=5.6 and pKa3(o-OH)=11.9 (m=0.2). PAR forms complexes with various metal ions that can be extracted with chloroform.
Chelate titration: Al, Bi, Cd, Cu, Ga, Hg, In, Mn, Ni, Pb, Zn
Precipitation titration: MoO4(II), WO4(II)
Colorimetry: Al, Au, Bi, Co, Cr, Cu, Fe, Hf, Ga, In, Nb, Ni, Pb, Pd, Sb, Sn, Ti, Tl, U, V, Zn
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2. R. M. Dagnall, et al., Determination of Lead with 4-(2-Pyridylazo)-resorcinol-I Spectrophotometry and Solvent Extraction. Talanta. 1965;12:583-588.
3. G. Pilloni, et al., Spectrophotometric Determination of Diethyllead and Diethyltin Ions with 4-(2-Pyridylazo)-resorcinol. Anal Chim Acta. 1966;35:325-329.
4. L. Sommer, et al., Complexation of Aluminium, Yttrium, Lanthanum and Lanthanides with 4-(2-Pyridylazo)Resorcinol (PAR). Talanta. 1967;14:457-471.
5. R. Sawyer, Determination of Dialkyltin Stabilisers in Aqueous Extracts from PVC and Other Plastics. Analyst. 1967;92:569-574.
6. Z. Kleckova, et al., Spectrophotometric Study of Complex Equilibria and Determination of Lead(II) with 4-(2-Pyridylazo)resorcinol. Collect Czech Chem Commun. 1978;43:3163-3178.
7. A. Rios, et al., New Configuration for Construction of pH Grandients in Flow Injection Analysis. Anal Chem. 1986;58:663-664.
Table 1 Photometric Detection Conditions Using PAR
M=metal ion, L=PAR, X=antipyrine, TPAC, Crystalviolet or Diphenylguanidine