Complex-Ion Forming Reactions In Titrimetric Analysis

Introduction

Titrimetric methods of analysis are capable of rapid and convenient analyte determinations with high accuracy and precision. Titrimetric analysis is based on the complete reaction between the analyte and a reagent, the titrant: (Rosenfeld: 72-75)

aA + tT t products

where A and T represent the analyte and titrant, respectively, and a and t are the stoichiometric coefficients. Titrations are often classified by the nature of this titration reaction: acid-base, redox, precipitation and complexation reactions are the most common reaction types.

Redox Titrations

Redox reactions are the most diverse of the four main classes of inorganic aqueous reactions (acid-base, pptn, complexation and redox). In principle, then, redox titrations can be used to analyze for any oxidizing or reducing agent. However, many redox reactions are either too slow or have inconsistent stiochiometry. The stability of titrant and analyte solutions can also be a problem. Nevertheless, a wide variety of analytes can be conveniently determined by redox titrations. (Harris: 111-112)

Consider a generic redox half-reaction (charges omitted for clarity):

A chemical (i.e., ox in this equation) that pulls electrons from another substance is an oxidizing agent, while a chemical (red) that forces another substance to accept electrons is a reducing agent. Together, ox/red form a redox couple; redox couples are analogous to acid/base conjugate pairs. And just like acid-base reactions, the “conjugate” of a strong oxidizing agent is a weak reducing agent.

The strength of oxidizing/reducing agents can be deduced by the standard reduction potential: a very positive standard potential indicates a strong oxidizing agent, while a low positive or a negative potential is characteristic of a strong reducing agent. The strength of an oxidizing or reducing agent is very often dependent on pH. There is a general rule of thumb: acidic conditions tend to make oxidizing agents more powerful and render reducing agents ...