INFLUENCES OF PH AND IONIC-STRENGTH ON AQUEOUS VANADATE EQUILIBRIA

V-51 NMR spectroscopy is used to investigate the thermodynamics of the reaction HVO42- + H+ reversible arrow H2VO4-. The enthalpy and entrop y of this reaction are found to be very close to those of analogous re actions with phosphate and arsenate. This provides firm evidence that incorporation of water into the vanadate ion does not accompany the pr otonation of the dianion. Therefore, the tri-, di-, and monoanionic va nadates are all four-coordinate. The oligomerization reactions of vana date are studied as a function of ionic strength and of pH and the res ults displayed graphically. The formation of the vanadate trimer and p entamer is examined in some detail, the identities of these materials are confirmed, and the apparent disappearance of the trimer at near-ne utral pH is discussed.