Gravimetric Analysis

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Gravimetric Analysis

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Gravimetric analysis is the quantitative determination of analyte concentration through a process of precipitation of the analyte, isolation of the precipitate, and weighing the isolated product.

Uses of gravimetric analysis…– Chemical analysis of ores and

industrial materials– Calibration of instrumentation– Elemental analysis of

inorganic compounds

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1. A weighed sample is dissolved

2. An excess of a precipitating agent is added to this solution

3. The resulting precipitate is filtered, dried (or ignited) and weighed

4. From the mass and known composition of the precipitate, the amount of the original ion can be determined

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Criteria for Gravimetric Analysis

1. The desired substance must completely precipitate from solution

• In most determinations the precipitate is of such low solubility that dissolution of the analyte is negligible

• An additional factor is the "common ion" effect, further reducing the solubility of the precipitate

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When Ag+ is precipitated from solution through the addition of Cl-

the (low) solubility of AgCl is further reduced by the excess of Cl- that is added, pushing the equilibrium to the right (Le Chatelier’s Principle).

)(sAgClClAg

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2. The weighed form of the product should be of known composition.

3. The product should be "pure" and easily filtered.

• It is usually difficult to obtain a product that is "pure“ (i.e., one that is free from impurities)

• Careful precipitation and sufficient washing may reduce the level of impurities

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Mechanism of Precipitation

• Induction period– The time before nucleation occurs after the addition

of the precipitating agent to the solution– May range from milliseconds to several minutes

• Nucleation– Formation of small, stable aggregates or nuclei of

precipitate – Nuclei have sizes down to ~1 nm, composed of a

few atoms, and there may be up to 1010 nuclei per mole of analyte

– Excess ions from solution collect around the nuclei

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Silver nitrate is added very slowly to an acidic solution containing chloride. Silver chloride nuclei form with a surface layer of ions. The “charged” AgCl particles (or colloidal particles) repel each other.

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Mechanism of Precipitation• In addition to the primary adsorbed silver ions, some

nitrate ions form an electrostatic layer around the nucleus.

– These counter ions tend to aggregate around the [AgCl:Ag]+ center because these centers have a net positive charge (excess Ag+) and additional negative charge is required to maintain electrical neutrality.

– Counter ions are less tightly held than the primary adsorbed ions and the counter ion layer is somewhat diffuse and contains ions other than those of the counter ions.

– These layers of charged ions associated with the surface of the nuclei are known as the electric double layer.

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More Terminology

• Adsorption is a process in which a substance (gas, liquid, or solid) condenses onto the surface of a solid

• The electric double layer of a colloid consists of a layer of charge associated with the surface of the particles and a layer with a net opposite charge in the solution surrounding the particles

• A colloid is a finely divided particle (typically with diameters from 10 nm to 1 m) that forms a stable dispersion within a medium (air or liquid)

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Conditions for Analytical Precipitation

• Von Weimarn showed that particle size of precipitates is inversely proportional to the relative supersaturation of the solution during precipitation

• Relative supersaturation = (Q-S)/S

– Where Q is the molar concentration of the mixed reagents before any precipitation occurs and S is the molar solubility of the product (precipitate) when the system has reached equilibrium.

– For the best possible results, conditions need to be adjusted such that Q will be as low as possible and S will be relatively large.

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• Precipitation from hot solution – The molar solubility (S) of precipitates increases

with an increase in temperature

– An increase in S decreases the supersaturation and increases the size of the particle.

• Precipitation from dilute solution – This keeps the molar concentration of the mixed

reagents low. Slow addition of precipitating reagent and thorough stirring keeps Q low. (Uniform stirring prevents high local concentrations of the precipitating agent.)

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• Precipitation at a pH near the acidic end of the pH range in which the precipitate is quantitative. – Many precipitates are more soluble at the lower

(more acidic) pH values and so the rate of precipitation is slower.

• Digestion of the precipitate. – The digestion period can lead to improvements in

the organization of atoms within the crystalline nuclei, such as expulsion of foreign atoms (or other impurities).

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Heating the precipitate within the mother liquor (or solution from which it precipitated) for a certain period of time to encourage densification of nuclei.

– During digestion, small particles dissolve and larger ones grow. This process helps produce larger crystals that are more easily filtered from solution

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Homogeneous PrecipitationWhat?

Precipitating agent generated slowly by chemical reaction in analyte solution

Why?Precipitant appears gradually throughoutKeeps relative supersaturation lowLarger, less-contaminated particles

How?(OH-) by urea decomposition(NH2)2CO 2 OH- + CO2 + 2 NH4

+

i.e Precipitation of Barium chromate

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Impurities in Precipitates

• Coprecipitation…

…is the precipitation of an unwanted species along with your analyte of interest;

… occurs to some degree in every gravimetric analysis;

• A major factor for precipitations of barium sulfate and those involving hydrous oxides

… and cannot be avoided, but can be minimized by careful precipitation and a thorough washing of the precipitate.

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Impurities in Precipitates• Surface adsorption

– Unwanted material is adsorbed onto the surface of the precipitate

– Digestion of a precipitate reduces the relative surface area and, therefore, the area available for adsorption of impurities

– Washing can remove impurities bound to the surface

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Impurities in Precipitates

• Occlusion– A type of coprecipitation

in which impurities are trapped within the growing crystal

• Post-precipitation– Sometimes a precipitate in contact with the

mother liquor is contaminated by the precipitation of an impurity

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• Inclusion

– A type of coprecipitation in which the impurities occupy the crystal lattice sites

Isomorphous Inclusion

Compounds with the same type of formula crystallizing in similar geometric forms

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Gathering

Precipitation of low concentration substances (i.e trace metals) by co-precipitation onto a macro quantity of another precipitate.

AgCl (s) → AgCl (colloid)

• Peptidization

– A procedure where the precipitate is washed and filtered, but part of the precipitate reverts to the colloidal form because supporting electrolyte is gone.

Gravimetric Analysis

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