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Quantitative AnalysisClassical• Gravimetric – mass of analyte• volumetric (or titrimetric) – volume of solution

containing sufficient reagent to react completely with analyte

Instrumental• Electroanalytical – properties resulting from Ox./Red.

behavior of analyte• Spectroscopic – measures electromagnetic radiation

absorbed or emitted by analyte• Chromatographic – separates a mixture into its

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Ch 27 Gravimetric Analysis

Chemical analysis based on the determination of weight of a substance of known composition (the final product) that is chemically related to the analyte.

gravi – metric(weighing - measure)

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Gravimetric Analysis

• precipitation method: Dissolved analyteconverted to sparingly soluble precipitate

Ag+ + Cl-à AgCl(s)

• volatilization method: Analyte is volatilized at suitable temperature; the volatile product is collected and weightedNaHCO3(aq)+H2SO4(aq)àCO2(g)+H2O(l)+NaHSO4(aq)CO2(g)+2NaOH(s)àNa2CO3(s)+H2O(l)

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Precipitation Method

(1) The desired substance: completely precipitated."common ion" effect can be utilized: Ag+ + Cl- AgCl(s)excess of Cl- which is added

(2) The weighed form: known composition.

(3) The product: "pure", easily filtered.

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Steps in Gravimetric Analysis (precipitation)

1) Dry and weigh sample2) Dissolve sample 3) Add precipitating reagent in excess4) Coagulate precipitate usually by heating & wait for

some time (Aging) 5) Filtration-separate precipitate from mother liquor6) Wash precipitate 7) Dry and weigh to constant weight (0.2-0.3 mg)

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Filtration

Mother liquor

avoid colloidal suspension, ideally, produce crystals

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Particle Size & Filterability - Precipitates

Colloids – (d = 10-6 to 10-4 mm)-invisible to naked eye-don’t settle out of solution -difficult or impossible to filter

Particles – (d = 0.10 mm or greater)-spontaneously settle out of solution-readily filtered and washed free of impurities-more desirable (typically of higher purity than colloids) 8

Mechanisms of PrecipitationTwo competing processes:(1) Nucleation

When a small number of ions, atoms, molecules initially unite.

(2) Particle growthThe 3-D growth of a particle nucleus into a larger crystal

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Particle Size & Filterability - ControlRelative supersaturation (RSS)

RSS = (Q-S)/Q

Where Q = concentration of solute; S = equilibrium solubility of solute

RSS can be used estimate/control the type of precipitate that is formed:

large: nucleation, small particles (colloids)small: particle growth, crystalline solid likely

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Particle Size & Filterability - Control

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The goal is to form crystalline precipitates so RSS must be minimized.

This can be done by:Increasing SDecreasing Q

Recall: RSS = (Q-S)/Q = 1-S/QQ = [solute]S = solute’s Equil. Sol.

Particle Size & Filterability - Control

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(1) Raising the temperature (increase S).(2) Adding precipitant slowly with vigorous

mixing (decrease Q). (3) Keeping the volume of solution large

(decrease Q).

Techniques to promote crystal growth

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pH control of precipitationCa2+ + C2O4

2-D CaC2O4 (s)H2C2O4D 2 H+ + C2O4

2-

Homogeneous PrecipitationThe precipitant is generated slowly by a chemical

reaction.Fe3+ + 3 HCO2

-D Fe(HCO2)3⋅nH2O(s)HCOOH+OH-D HCO2

-+H2O(NH2)CO + 3 H2O + heat D OH- + CO2(g)+ 2NH4

+

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Treatment of colloidal Precipitates(1) Increasing the electrolyte concentration

Boundary of ionicatmosphere

Colloidal Particle of AgCl

•Decreasing the vol. of the counter-ion layer•Increasing the chance for coagulation

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Treatment of colloidal Precipitates(2) Using a volatile electrolyte

Avoid peptization

Ex. AgCl, wash with HCl. Drying precipitate at 110°C will remove HCl.

This displace the less volatile, excess counter ion.

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Treatment of colloidal Precipitates

Digestion: Heating the solution for about an hour after precipirate formation. This helps to remove weakly bound waterAging: Storing the solution, unheated, overnight. This allows trapped contaminates time to “work their way out”.Both can result in a denser precipitate that is easier to filter.

(3) Digestion and aging

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Representative Gravimetric Analyses

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Advantages/Disadvantages

• Experimentally simple and elegant• Accurate• Precise (0.1-0.3 %)• Macroscopic technique-requires at least

10 mg ppt to collect and weigh properly• Time-consuming (1/2 day?)

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Calculation

• Design of experiment• Content Calculation• Evaluation of the results

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Calculation

• % of analyte, % A

• %A = weight of analyte x 100weight of sample

• weight of ppt directly obtainedà%A

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How Do We Get %A from ppt?

• % A = weight of ppt x gravimetric factor (G.F.) x 100weight of sample

G.F. = a (FW of analyte)b (FW of precipitate)

• G.F. = # gms of analyte per 1 gm ppt

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Gravimetric Factor

• G.F. = a (FW of analyte)b (FW of precipitate)

• Analyte ppt G.F.CaO CaCO3FeS BaSO4UO2(NO3)2

.6H2O U3O8Cr2O3 Ag2CrO4

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Gravimetric Factor

• Analyte ppt G.F.CaO CaCO3 CaO/CaCO3FeS BaSO4 FeS/BaSO4UO2(NO3)2 U3O8 3UO2(NO3)2/U3O8Cr2O3 Ag2CrO4 Cr2O3/2Ag2CrO4

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Exercise

• Consider a 1.0000 g sample containing 75% potassium sulfate (FW 174.25) and 25% MSO4. The sample is dissolved and the sulfate is precipitated as BaSO4 (FW 233.39). If the BaSO4 ppt weighs 1.4900 g, what is the atomic weight of M2+ in MSO4?

• ANS: Mg2+