Vincenzo Romano

Hydrolytic equilibria of lead(II) at 100 °C in 3.6 mol kg–1 LiClO4

An in situ method, which allows the generation, analysis and measurement of the free hydrogen-ion concentration of aqueous solutions at high temperature (100 °C), has been elaborated. It is based on the use of a galvanic cell without a liquid junction containing a glass electrode and of a cell where controlled external and/or internal electrolysis processes take place. The reliability of the method has been checked by carrying out coulometric acid–base titrations of standard dilute HClO4 solutions in 3 mol dm–3 LiClO4 as ionic medium. The high accuracy so obtained has made it possible to study lead(II) hydrolysis at 100 °C. In the whole range of lead(II) concentration investigated (2 × 10–3–0.1 m) the collected data (Z→ log h) can be explained by assuming the formation of a mononuclear [Pb(OH)]+(log β11=–6.05 ± 0.01) and a tetranuclear species [Pb4(OH)4]4+(log β44=–15.94 ± 0.03). An increase in temperature from 25 to 100 °C has the effect of increasing the acidic properties of lead(II) solutions and of decreasing the number of hydrolysed species.

Redox potentials of vanadium-containing couples. Part 3. The formal redox potential of the V3+–V2+ couple

A method for the chemical and electrochemical preparation of mixtures of vanadium-(III) and -(II) in 1 mol dm–3(H+, Na+) Cl– aqueous solutions has been elaborated. The concentration of each ion can be determined under nitrogen in a completely closed system comprised essentially of a spectrophotometric cuvette and a junction-free cell containing mercury and glass electrodes. By changing the vanadium ion concentrations in ‘titrations’ of different kinds and by measuring the electromotive force of the galvanic cell the formal redox potential of the VIII–VII couple has been deduced. Its value at 298 K against the normal hydrogen half cell and in 1 mol dm–3(H+, Na+) Cl– is –242 ± 2 mV over a large range of V3+/V2+ ratio and of total vanadium concentration. Using the specific interaction theory (s.i.t.) approach the standard redox potential is estimated to be –196 mV. The methodology reported seems to be appropriate for studying the solution chemistry of the rather oxidizable V2+ ion.

The determination of mineral acidity excess in solutions containing Ga(III), Al(III), Bi(III)

Abstract A new procedure is proposed to carry out volumetric determinations of the strong acid content of a solution in the presence of inorganic cations such as Ga 3+ , Al 3+ , Bi 3+ which, showing in aqueous solution an acidic behaviour, introduce non negligible interferences in these determinations. Before the acid–base titration, an equivalent amount of a ligand, such as dihydrogen ethylendiamino tetraacetate is added, in order to strongly complex the cation masking it toward the successive addition of a strong base. Interferences are thus removed, and the volumetric determination of the strong acid excess plus the hydrogen ions set free from the ligand, as a consequence of the complex forming reaction, can be accurately carried out, in the presence of a colorimetric indicator.