Gravimetric analysis

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Transcript of Gravimetric analysis

  • GRAVIMETRIC ANALYSISBy Dr Mark Selby(from lecture slides developed byD. Sharma, Department of Chemistry, Simon Fraser University, British Columbia, Canada)

    2/20/2015

    PQB313 Analytical Chemistry for Industry

    1

  • GRAVIMETRICANALYSIS Chapter 10 in Christian (7th Ed.) pages 342f.

    2/20/2015 PQB313 Analytical Chemistry for Industry 2

  • Gravimetric AnalysisGravimetric 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.

    CVB212 Industrial Analytical Chemistry 3

    Uses of gravimetric analysis Chemical analysis of ores and

    industrial materials Calibration of instrumentation Elemental analysis of

    inorganic compounds

  • Gravimetric Analysis1. 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

    5. Stoichiometry is important (write down the chemical equation!)

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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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  • Criteria for Gravimetric Analysis

    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 Chateliers Principle).

    )(sAgClClAg

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  • Criteria for Gravimetric Analysis

    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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  • Some Organic Precipitants

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    Practical 1: Determination of Nickel in Steel

    Christian 7th Ed., Table 20.2, pg 354.

    CVB212 Industrial Analytical Chemistry

  • Example: Ni in Steel To measure Ni in steel, the alloy is dissolved in 12 M HCl and

    neutralised in the presence of citrate ion, which maintains iron in solution.

    The slightly basic solution is warmed and dimethylglyoxime (DMG) is added to precipitate the red DMG-nickel complex quantitatively.

    The product is filtered, washed with cold water, and dried at 110 C.

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    Harris 8th ed., pg 681.

    CVB212 Industrial Analytical Chemistry

  • 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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  • Mechanism of Precipitation

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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.

    Harris 8th Ed., Figure 26-2, pg 678.

    Nucleus of AgCl(s) colloid

    Primary adsorbed Ag+

    Loosely associatedcounter ion

    Illustration of an ElectricalDouble Layer

    Homogeneous solution (charges balanced)

    CVB212 Industrial Analytical Chemistry

  • 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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  • Mechanism of PrecipitationDigestion 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 (Ostwald ripening). This process helps produce larger crystals that are more easily filtered from solution

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    T

    CVB212 Industrial Analytical Chemistry

  • Ideal Analytical Precipitation

    In an ideal world, an analytical precipitate for gravimetric analysis should consist of perfect crystals large enough to be easily washed and filtered.

    The perfect crystal would be free from impurities and be large enough so that it presented a minimum surface area onto which foreign ions could be adsorbed.

    The precipitate should also be "insoluble" (i.e., low solubility such that loses from dissolution would be minimal).

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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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  • Conditions for Analytical Precipitation 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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  • Conditions for Analytical Precipitation 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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  • 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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    0 2 4 6 8 10 12 14

    Particle Surface Area = 4(r2)Particle Volume = 4/3(r3)

    Particle Surface Area

    Particle Volume

    Particle Radius (A.U.)

    Scaling per Particle

    CVB212 Industrial Analytical Chemistry

  • Impurities in Precipitates

    Occlusion A type of coprecipitation

    in which impurities are trapped within the growing crystal

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    Post-precipitation Sometimes a precipitate in contact with the mother liquor is

    contaminated by the precipitation of an impurity

    CVB212 Industrial Analytical Chemistry

  • Impurities in Precipitates Inclusion

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

    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.

    Cooling the system with an ice-water bath minimi