A reevaluation of the Fe(II), Ca(II), Zn(II), and proton formation constants of 4,7-diphenyl-1, 10-phenanthrolinedisulfonate

The compound 4,7-diphenyl-1,10-phenanthrolinedisulfonic acid (BPDS) has been found to be very useful in studying Fe uptake by plants, because it forms a large charged complex that is not absorbed. The quantity of BPDS, bound to metals in hydroponic solutions can be estimated from calculations using formation constants of BPDS complexes. These formation constants were used in an earlier experiment to predict the availability of Cu to corn plants. In the experiment, bioassays indicated that Cu was not as phytoavailable in the BPDS-added solutions as predicted by chemical equilibrium calculations. To determine sources of error in this prediction, metal and proton BPDS formation constants were reevaluated at 25°C and 0.10 M ionic strength. The CaBPDS formation constant was determined by direct measurement of CaBPDS₃ formation and was shown to be ∼1.0; a value much less than that reported before. Formation constants for the HBPDS, H(BPDS)₂ and H(BPDS)₃, β₁, β₂, and β₃ complexes were, respectively, 5.05±0.044, 7.44±0.019, and 9.33±0.28. The BPDS sulfonic acid group p Ks were <1.0, not 2.8 as has been reported. The FeBPDS₃ complex determined by ligand competition with EDTA (ethylenediaminetetraacetate) was 20.24±0.08. Copper and Zn constants were determined using the method of corresponding solutions. The CuBPDS, CuBPDS₂, and CuBPDS₃, β₁, β₂, and β₃ constants were, respectively, 9.76±0.08, 15.9, and 20.9. The ZnBPDS, ZnBPDS₂, and ZnBPDS₃ β₁, β₂, and β₃ constants were, respectively, 6.43±0.07, 10.7±5.4, and 17.3±0.8. Results indicated that, BPDS affinity to metals was similar to that of its parent compound, phenanthroline, and that errors in published formation constants caused erroneous predictions of Cu phytoavailability used in an earlier experiment.