Gravimetric Analysis

General Principles

In gravimetric analysis measures the mass of a material formed in the reaction of the analyte with the reagent. A chemical reaction for gravimetric analysis is

where a moles of analyte A contained in the sample reacts with r moles of the reagent R to form the precipitate AaRr, noted as solid phase (s) in the reaction. Since one does not know the concentration of the analyte before hand, an excess of reagent is used. In electrogravimtery, the reagent is the electron, which is added to the solution bay passage of electrical current at a potential appropriate for oxidation or reduction of the analyte.

There are several factors to keep in mind when designing the chemistry:

  1. There must be a phase change so that the product can be separated from the reaction solution. This is satisfied when the product is a solid precipitated from solution.
  2. The reaction must be significantly complete. This is satisfied when the Ksp of the product is small. It also means that there are no competing equilibria that might re-dissolve the product.
  3. The final product must be pure and must be of a definite chemical composition.
  4. The rate of reaction must be fast enough to be practical.

The reaction is generally carried out in a flask containing (liquid-phase) dissolved sample. The reagent solution is added to the reaction flask until the reaction apparently stops. This is detected by noting that there is no longer any precipitate formed upon adding reagent.


Separation

When the reaction is complete, we are left with precipitate containing all, or most of the analyte that was initially in our solution. This precipitate product is subsequently from the reaction solution using filtration. This is done using one of the several techniques discussed in class. Repetitive rinsing of the reaction flask with an electrolyte solution insures quantitative transfer of the precipitate to the filter paper or crucible. The filtrate must also be pure. To insure purity, the filtrate is rinsed with an appropriate electrolyte solution.

An electrolyte solution is used to avoid breaking up the colloids that may be glued together with counter ions (peptization). The electrolyte ions replace ions from the reagent or that might have been present in the analyte solution. The electrolyte should be volatile and the ions of the electrolyte should not be part of the product complex. For example, use of ammonium nitrate (NH4NO3) or ammonium hydroxide (NH4OH) in distilled water results in the volatile species; NH3, HNO3, and H2O.


Digestion

The precipitate often contains ions that where trapped when the precipitate was formed. This is mostly a problem for crystalline precipitates. If the trapped ions are not volatile, then their presence will corrupt the weighing step. Concentration of interfering species may be reduced by digestion.

Digestion is a process where the precipitate is re-dissolved and precipitated out of a cleaner environment (solution). The precipitate obtained in the separation step is placed into a volatile electrolyte solution and heated. Large particles are broken up to speed up digestion. This "solution" is often heated to increase the kinetic rates of dissolution and precipitation. Since the solid is in dynamic equilibrium with the solution, in time, all material will cycle from solid to solution and back. Observation does not speed up the chemical kinetics, so we take a cola break during digestion. The solution is cooled after digesting for an hour or more. The precipitate is now refiltered.


Preparation for Weighing

After separation, the material must be prepared for weighing. This is accomplished by heating up the precipitate to drive off excess solvent and volatile electrolytes. Low temperature drying may be used for some lyophobic (solvent phobic) precipitates. High temperature drying, called ignition, is used for the lyophilic (solvent liking) precipitates. The thing to remember is that drying or ignition may change the chemical composition of the precipitate.


Weighing

The dried or ignited is solid cooled prior to weighing. This avoids convection currents from altering the mass measurement. The hot sample is placed into a desiccator during the cooling stage to reduce adsorption of water from the air. Since weighing is performed in a vessel, e.g, a weighing bottle or a crucible, the dry weight of the vessel must be determined before hand. The solid weight is determined by difference.


More Rules for a Successful Gravimetric Analysis

A few additional rules of thumb for a successful quantitative precipitation are:


Example of a Gravimetric Analysis

A 0.5962 g sample of iron ore is dissolved in perchloric acid (HClO4). All iron present is oxidized to Fe3+. The solution is filtered to remove solid matrix materials and made basic with addition of ammonium hydroxide. The iron precipitates as the Fe(OH)3 .xH2O gel. The precipitate is collected in a cistern crucible and ignited to produce Fe2O3. What is the wt. % of iron in the sample if the analysis produced 0.3210 g Fe2O3?

First, examine the chemistry

The overall reaction is

From this we derive the gravimetric factor relating weight of final material to the weight of iron analyte


Second, calculate the mass of iron

The mass of iron in the ignited precipitate is


Last, calculate weight %

The weight % of iron in the ore is obtained from the mass of iron and the sample weight,


Back to Chemistry 3600 Home


This page was created by Professor Stephen Bialkowski, Utah State University.

Last Updated Tuesday, August 03, 2004