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!)

4CVB212 Industrial Analytical Chemistry

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



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



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


Some Organic Precipitants

8

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.

9

Harris 8th ed., pg 681.


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



11

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)



• 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.


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)


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

14

T


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).


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.


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.)

17PQB313 Analytical Chemistry for Industry

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).

18PQB313 Analytical Chemistry for Industry

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.


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

20

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


Impurities in Precipitates

• Occlusion• A type of coprecipitation

in which impurities are trapped within the growing crystal

21

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

contaminated by the precipitation of an impurity


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 minimizes loss of precipitate due to dissolution

22

AgCl (s) → AgCl (colloid)


Increasing Purity• Re-precipitation

• a procedure including washing away the mother liquor, redissolving the precipitate, and precipitating the product again

• Drying the solid• Generally the solids are dried at

~120 oC, but conditions fordrying can vary considerably.


Increasing Purity• Precipitation in the presence of electrolyte

• Coulombic repulsion is diminished in the presence of electrolyte because of a compression of the volume of the ionic atmosphere

• Digestion • Raising the temperature will increase the collision

energy for colloidal particles and overcome Coulombic repulsion, leading to formation of larger particles (coalescence)


Gravimetric Analysis

• For example: determination of silver or chloride by the formation of AgCl (s)

• Precipitation occurs when the value of [Ag+][Cl-] exceeds the solubility product Ksp of AgCl(1.810-10).

)(sAgClClAg


Gravimetric Analysis

26

A 10.0 mL solution containing Cl- was treated with excess AgNO3 to precipitate 0.4368 g of AgCl. What was the concentration of Cl- in the unknown? (AgCl = 143.321 g/mol)

Number of moles of Cl- = number of moles of AgCl

mol103.048ol143.321g/m

0.4368g 3-

Concentration of Cl- M0.3048 L0.01000 mol103.048 3

Harris 8th Ed., pg 674.


Other Analytes

27

Why is the form weighed different from the precipitate?

Christian 7th Ed., Table 10.1, page 353


Gravimetric FactorIn general the precipitate we weigh is usually in a different form than the analyte whose weight we wish to report.

The gravimetric factor (GF), represents the weight of analyteper unit weight of precipitate. It is obtained from the ratio of the formula weight of the analyte to that of the precipitate, multiplied by the moles of analyte per mole of precipitate obtained from each mole of analyte, that is:

2/20/2015 Footer Text 28

analyte g/molprecipitate g/

FW ( ) (mol analyte/mol precipitate)FW ( m )ol

aGFb

Gravimetric Factor - ExampleQuestion: Calculate the grams of analyte per gram of precipitate for the following conversion:

Answer:

2/20/2015 Footer Text 29

Analyte PrecipitateBi2S3 BaSO4

2 32 3 4 2 3 4

4

2 32 3 4

4

FW Bi S (g/mol) 1g Bi S /g BaSO = (mol Bi S / mol BaSO )FW BaSO (g/mol) 3

514.15(g Bi S /mol) 1GF = 0.73429 g Bi S /g BaSO233.40(g BaSO /mol) 3

Sample Calculation

30

A certain barium halide exists as the hydrated salt BaX2.2H2O, where X is

the halogen.

The barium content of the salt can be determined by gravimetric methods. A sample of the halide (0.2650 g) was dissolved in water (200 cm3) and excess sulphamic acid added. The mixture was then heated and held at boiling for 45 minutes. The precipitate (barium sulfate) was filtered off, washed and dried. Mass of precipitate obtained = 0.2533 g.

Determine the identity of X.


Sample Calculation

31

The precipitate is barium sulfate. The first stage is to determine the number of moles of barium sulfate produced, this will, in turn give us the number of moles of barium in the original sample.

Relative Molecular Mass (Mr) of barium sulfate Mr = 137.34 (Ba) + 32.06 (S) + (4 x 16.00) (4 x O) = 233.40 g/mol

Number of moles = mass / Mr = 0.2533 (g) / 233.40 (g/mol)= 1.09 x 10 –3 (mol)


Sample Calculation

32

This is the number of moles of barium present in the precipitate and, therefore, the number of moles of barium in the original sample. Given the formula of the halide, (i.e., it contains one barium per formula unit), this must also be the number of moles of the halide. From this information we can deduce the relative molecular mass of the original halide salt:

Mr = mass / number of moles = 0.2650 (g) / 1.09 x 10-3 (mol)= 244.18 (g/mol)


33

Sample CalculationThe atomic mass (Ar) of 2 X will be given by the Ar of the whole salt – that of the

remaining components:

Ar of 2 X = 244.18 (g/mol) – 137.34 (g/mol Ba) – 2 x 18.02 (g/mol H2O) = 70.81 (g/mol)

2 X = 70.81, so X = 35.41 (g/mol)

The Ar of chlorine is 35.45 (g/mol), which is in good agreement with the result obtained and hence the halide salt is hydrated barium chloride and X = Chlorine

Final formula is BaCl2.2H2O

Compare this example with the worked solution for RaCl2, in Harris 8th Ed., 674.

PQB313 Analytical Chemistry for Industry

Gravimetric analysis

Education

Transcript of Gravimetric analysis