Instrumental Analysis- Chem 466 Exam 3

The oxidation number of As in:

H3AsO3

+3

The oxidation number of As in

H2AsO4

+5

Now, how many electrons for each As in H3AsO3 to H2AsO4– ?

2

What is a supporting electrolyte? Give two reasons for using a supporting electrolyte in electrochemical experiments.

The supporting electrolyte is an inert ionic compound such as KCl or NH4ClO4 that is added to an electrochemical solution to:

1) Reduce charging current

2) Decrease electrical resistance

Supporting electrolyte is typically added at 50 to 100 mM concentrations.

Overpotential

Potential required to overcome the activation energy of an electrode reaction

Ohmnic potential

voltage necessary to overcome the internal resistance of the cell

Concentration polarization

Occurs when concentration near electrode is different from the bulk solution

Controlled-potential electrolysis

using a three electrode cell, the working electrode potential is measured vs the reference

coulometry

moles of electrons needed for a chemical reaction are measured

amperometry

current at the working electrode is proportional to analyte concentration

voltammetry

The dependence of current on the applied potential of the working electrode is observed

sample current polarography

Uses a staircase voltage profile for measurements with

successive, static drops of Hg

square wave polarography

Increased sensitivity and derivative peak shape by applying a

square wave on a staircase voltage ramp

Anodic Stripping Voltammetry

Analyte is concentrated in a single drop or thin film of Hg by reduction, then current is measured as the voltage is made more positive and the analyte is reoxidized, appearing as peaks