The Dissociation Constants of some Phosphorus(V) Acids

The dissociation constants of ortho-, monothio-, monoamido- and diamido-phosphoric acids have been measured at a series of temperatures using a potentiometric titration method involving com­puter fitting of the theoretical titration curve to the experimental one, the constants being used as adjustable parameters. Less accurate values for diamidothio- and tetrathio-phosphoric acids were also obtained. The results are discussed with respect to the site of protonation. For the monothio acid only the first dissociation (K1) appears to involve the sulphur atom. The monoamido compound may be protonated first on nitrogen (K2). The diamido- and diamidothio-phosphoric acids show a remark­able effect in that, although monoanionic, two p Ka values may be measured, the first protonation (K2) being probably onto nitrogen, the second (K1) involving the oxygen atom more.