Gravimetric Analysis

Inorganic and Analytical ChemistryPE-214

Dr. Riham Hazzaa

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Gravimetric Analysis: is based upon the measurement of mass

Gravimetric Analysis generalized into two types: precipitation and volatilization (i ) A technique in which the amount of an analyte in a sample is

determined by converting the analyte to some product Mass of product can be easily measured

(ii) Analyte: the compound or species to be analyzed in a sampleAdvantages - requires minimal equipment Disadvantage – requires skilled operator, slow.

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• The quantitative determination of a substance by the precipitation method of gravimetric analysis involves isolation of an ion in solution by:

1.precipitation reaction,

2. filtering,

3.washing the precipitate free of contaminants, conversion of the precipitate to a product of known composition,

4.drying

5.weighing the precipitate and determining its mass by difference. 

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Precipitation

Reaction of potassium iodide solution and lead (II) nitrate solution.

Determination of lead (Pb+2) in water

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Determination of lead (Pb+2) in water

Pb+ + 2Cl- PbCl2(s)

By adding excess Cl- to the sample, essentially all of the Pb+2 will precipitate as PbCl2.

Mass of PbCl2 is then determined.used to calculate the amount of Pb+2 in original solution

ReagentAnalyte Solid Product

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Mechanism of precipitation1. Induction period time between mixing and visual

appearance of a precipitate called the induction period

2. Nucleation is the formation, in a super saturation solution, of the smallest aggregate of molecules capable of growing into a large precipitate particle.

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3. Crystal growth Once a nucleation aggregate has formed, it begins to grow as ions or molecules from the solution deposit on the surface in a regular, geometric pattern.

4. Aggregate growth Natural cohesive forces exist between particles having the same composition and, as a result, most precipitate to consist of a relatively few large aggregate of crystals.

Crystal Growth

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Why colloids occur??

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How colloidal suspension is prevented????

• Keep the volume of the counter ion layer small:• Keep the charge of the primary adsorption layer small by

avoiding an excess of the precipitating ion.• Increase the ionic strength by adding soluble electrolyte. • Heating and stirring reduces the volume of the electrical

double layer. This will allow the ions in the counter ion layer be closer to the opposite charged ions of the primary layer.

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Desired Properties of Solid Product Should be very insoluble Easily filterable (i.e., large crystals) Very Pure Known and constant composition

Few precipitates have all of these properties, but in most cases appropriate techniques can help optimize these qualities

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Solubility:The solubility of a precipitate can be

decreased by: Decreasing temperature of solution Using a different solvent

- usually a less polar or organic solvent (likes dissolves likes)

Solubility vs. pH Solubility vs. TemperatureSolubility vs. Common Ion Effect

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Filterability:product be large enough to collect on filter:• Doesn’t clog filter• Doesn’t pass through filterBest Case: Pure Crystals

Worst Case: Colloidal suspensionDifficult to filter due to small sizeTend to stay in solution indefinitely suspended by

Brownian motionusually 1-100 nm in size

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Conditions for analytical precipitationAn analytical precipitate for gravimetric analysis should consist of perfect crystals large enough to be easily washed and filtered. The perfect crystal would be large and free from impurities. The precipitate should also be "insoluble".

Colloidal suspension

Crystal formation

Want to Convert to

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• It has been shown that the particle size of precipitates is inversely proportional to the relative supersaturation of the solution during precipitation;

  Relative supersaturation = S)SQ(R

S = concentration of solute in solution at equilibriumQ = actual concentration of solute added to solution

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Methods used to improve particle size and filterability

1. Precipitation from hot solution The solubility S of precipitates increases with temperature and so an increase in S decreases the supersaturation.

2. Precipitation from dilute solution  This keeps Q low. Slow addition of precipitating reagent with effective stirring. This also keeps Q low; stirring prevents local high concentrations of the precipitating agent.

3. Precipitation at a pH near the acidic end of the pH range Many precipitates are more soluble at the lower (more acidic) pH values and so the rate of precipitation is slower.

4. Digestion of the precipitate. Heating the precipitate in the precipitating solution, a process called digestion, results in larger and purer particles by giving the crystal a chance to dissolve and reprecipitate.

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Impurities in Precipitates

Impurities can be incorporated into a precipitate during its formation, called co-precipitation, or after its formation while

still in contact with the precipitating solution, called postpricipitation

Co-precipitation

a) Surface adsorption

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• b) Occlusion Impurities absorbed or trapped within pockets in the crystal

• c) Inclusion Impurities placed in the crystal instead of analyte

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There are several requirements that must be met to make precipitation reliable:

• The precipitate must have a very low solubility in water; i.e. its Ksp must be very small number

• It must precipitate in a high state of purity or be capable of reprecipitation for further purification.

• It must be capable of drying or of ignition.• It should not be hydroscopic at room temperature.

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Ageing &digestion

• The precipitate should be in contact with the solution from which the precipitate is formed.

• Warm the solution that contains the precipitate for some time to obtain complete precipitation in a form which can be readily filtered.

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During the process of ageing and digestion, two changes occur:

• After precipitation has occurred, the very small particles, which have a greater solubility than the greater ones, tend to pass into solution and will redeposit upon the larger particles. Thus co precipitation on the minute particles is eliminated.

• The rapidly formed crystals are irregular. Thus on ageing they will become regular and the surface area is reduced, so adsorption will be reduced. The net result of digestion is usually to reduce the extent of co precipitation and to increase the size of the particles, rendering filtration easier.

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Filtration

• A precipitate may be separated by filtering it through• paper, • sintered glass, • or sintered porcelain. • The choice depends on the nature of the precipitate

and on the temperature to which it will be heated after filtering.

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Washing• The precipitate and filter must be washed with

suitable electrolyte to remove dissolved solids that remain in the precipitate and wetted filter.

• Problems with coprecipitation and surface adsorption may be reduced by careful washing of the precipitate.

• With many precipitates, peptization occurs during washing.

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Precipitates from ionic compounds- need electrolyte in wash solution TO keep precipitate from breaking up and redissolving (peptization)

Electrolyte should be volatile removed by drying- HNO3, HCl, NH4, NO3, etc.

Example:AgCl(s) should not be washed with H2O, instead wash with dilute HNO3

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Drying/Igniting Precipitates Precipitates are dried at about 120oC for

accurate, stable mass measurements

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