Reagents. All solutions were prepared with ultrapure
water with a specific resistivity of 18 M¿âcm, obtained by
filtering double-distilled water through a Milli-Q purifier
system (Millipore) immediately before use. The reagents were
of analytical reagent grade or higher purity. The Pb standard
solution (1.00 ( 0.002 g) was Titrisol from Merck (Darmstadt,
Germany). Nitric acid (65%, Merck), hydrogen peroxide
(Merck), and vanadium pentaoxide (Merck) were used. Sodium
tetrahydroborate solution was preparing by dissolving
sodium tetrahydroborate(III) powder (Merck) in deionized
Milli-Q water and stabilizing in 1% sodium hydroxide (Merck).