Stability constants of aminophosphonic acid

Protonation constants of three aminophosphonic acids are measured: 1,6-hexamethylenediamine-N,N,N$spprime$,N$spprime$-tetra(methylenephosphonic acid), HDTMP, ethylenediamine-N,N,N$spprime$,N$spprime$-tetra(methylenephosphonic acid), EDTMP, and diethylenetriamine-N,N,N$spprime$,N$sp{primeprime}$,N$sp{primeprime}$-penta(methylenephosphonic acid), DTPMP. Potentiometric methods are used primarily and supplemented with information from Phosphorus-31 NMR. The NMR proved to be a powerful tool in studying protonation of the amine groups, especially those on HDTMP, and may prove to be useful in studies of complexation. For the first protonation constants of all three compounds, log K $geq$ 13 at 25$spcirc$C in 0.1 M KNO$sb3$. These had to be measured indirectly. In addition, complexation constants of these three compounds with Mg(II), Ca(II), Ba(II), Mn(II), Cu(II), and Zn(II) are measured at 25$spcirc$C in 0.1 M KNO$sb3$. It is found that all three phosphonates are good chelating agents over a wide range of pH. The order of chelation among the phosphonates is HDTMP $<$ EDTMP $<$ DTPMP. The order of affinity among the metals is Ba $<$ Ca $approx$ Mg $<$ Mn $<$ Zn $<$ Cu. Comparison with analogous carboxylic acids is made with respect to protonation, complexation, and conditional stability constants. Protonation and complexation constants of aminophosphonic acids are higher than the analogous carboxylic acids. Conditional stability constants show that the analogous carboxylic acids tend to be better chelating agents than EDTMP and DTPMP over the range of pH from 2 to 12, whereas HDTMP is better than its analogous carboxylic acid. Phosphonic acids tend to chelate better at pH above 12.