Electrometric studies on the system acid-vanadate and the formation of heavy metal vanadates

The stoichiometry of the polyanions formed by the action of nitric acid on sodium ortho-vanadate solution has been studied by means of electrometric techniques involving glass electrode and conductometric titrations as well as Job’s method of continuous variation. The well defined inflections and breaks in the titration curves and maxima from continuous variation study provide cogent evidence for the existence of the anions, pyro-V₂O₇^4?, meta-VO₃^? and poly-H₂V₁₀O₂₈^4? corresponding to the ratios of VO₄^3?:H⁺ as 1:1, 1:2 and 1:2.6 in the pH ranges (10.1–10.7), (7.0–7.5) and (3.4–3.8), respectively. Similar titrations between alkali and V₂O₅ solutions also confirm the existence of the same anions. The interaction of lead nitrate with sodium vanadate solutions, at specific pH levels 12.4, 10.5, 7.4 and 3.6 was also studied by pH and conductometric titrations. The end-points obtained from the sharp inflections in the titration curves provide definite evidence for the formation of lead ortho-3PbO·V₂O₅, pyro-2PbO·V₂O₅ and meta-PbO·V₂O₅ vanadates in the vicinity of pH 8.2, 6.5 and 5.6, respectively.