PURIFICATION OF ANALYTICAL REAGENTS BY CONSTANT-CURRENT ELECTRODEPOSITION OF HEAVY-METALS AT ULTRA-TRACE LEVELS FROM HIGHLY CONCENTRATED SALT-SOLUTIONS

The constant-current electrodeposition of trace elements (Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn) onto a graphite tube cathode from highly concen trated NH4F solutions was studied. Element concentrations in the ng ml (-1) range were decreased to values of less than or equal to 3 pg ml(- 1) (Co-60) within 60-120 min of electrolysis except for manganese. A r ecrystallized solid ultra-high purity product with an impurity level i n the low pg g(-1) range was obtained by partly evaporating the solven t of these high-purity NH4F solutions in a specially designed evaporat ion device, in order to minimize contamination and decomposition of th e reagent. The element concentrations at the ng ml(-1) and pg ml(-1) l evel were determined using a ''direct'' electrothermal atomic absorpti on spectrometric procedure for NH4F solutions up to 20%. To improve th e detection limits and to verify the accuracy a multi-step ion-exchang e procedure was used to separate and preconcentrate the trace elements from the matrix (up to 40% NH4F solutions).