• Chapter 8 Gravimetric Analysis

– Skoog Book

• Page 179-198 Do Problems:

1,2,4,9,10,11,14,16,21,27,30,33

• Chapter 9 Electrolyte Effects Activities

– effective concentration and equilibrium

• Chapter 10 Complex Equilibria

– similar but a few steps we have not looked at

Gravimetric Methods of Analysis

Gravimetric Analysis• Gravimetric Analysis--quantitative technique based on

the determination of the mass of a precipitated or volatized compound which the analyte is stoichiometrically related

• Analyte: the substance determined in the procedure.

– converted to an insoluble form, collected and massed on

an analytical balance.

• Reaction: aA + rR ===> AaRr (s)

Analyte PrecipitatingAgent

Precipitate that we will dry, mass and relate to analyte

Gravimetry

Vacuum Filteration Set Up

Buchner Funnel

Filter Flask

Filter PaperFilter Adaptor Rubber

tubing

Aspirator

Mother Liquor

Mechanicism of Precipitation

• Two Competing Processes

– Nucleation vs Particle Growth

Representative Gravimetry

Representative Gravimetry

Mechanicism of Precipitation

• Two Competing Processes

– Nucleation vs Particle Growth

Desirable!

Creating “Ideal” Precipitates

AgNO3(aq) + NaCl(aq)!"# AgCl(s) + NaNO3(aq)

• Now let’s learn more about precipitate formation in

particular colloids and crystalline precipitates

precipitating agent

Colloidal Double Layer Model

• If we add Cl- we run out of Cl and NO3- “fills” in

Precipitates Vary And Complicate! Particle Size of the precipitate is determined

by a number of factors, such as

! Relative supersaturation (RSS)

! Mechanism of formation: nucleation vs particle growth

! Experimental control: pH, temperature, mixing,

! Supersaturation occurs when precip. agent is added to analyte.

• System reacts to reach equilibrium (Q > Ksp). It does this by “nucleation”.

• Nucleation is not desirable as the particles are small.

• More nuclei bring about entrapment or unwanted ions.

• Study shown that particle size is inversely proportional.

• Nucleation

– Individual ions/atoms/molecules coalesce to form “nuclei”

• Particle Growth

– Condensation of ions/atoms/molecules with existing “nuclei” forming larger particles which settle out

• Colloidal Suspension

– Colloidal particles remain suspended due to adsorbed ions around a small particle yielding a net + or - charge

• Coagulation, agglomeration

– Suspended particles coalesce (lump together) to form

larger filterable particles

• Peptization

– Re-dissolution and dispersion of coagulated colloids by

washing and removing inert electrolyte (not good)

Terms Used In Gravimetry

• Studies show that Particle Size is inversely proportional to Relative Supersaturation

How Can We Create “Ideal” Precipitates

Relative Supersaturation =Q! S

S

where: S = solubilty of precipitate at equilibrium,

and Q = concentration of mixed reagents before precipitation.

• Large Q gives undesirable colloidal precipitates.

• Small Q gives large crystalline filterable particles

How To Keep RSS Low “Best Practice”

• Precipitate from dilute solution with good stirring, this keeps Q low

• Add dilute precipitating reagents slowly with good stirring (prevents local excess or saturation)

• Precipitate from “hot” solutions. This normally increases S (solubility) ....followed by cooling for complete quantitative precipitation.

• Precipitate at lower pH as generally precipitates are more soluble in acid medium--this slows precipitation.

Important Factors for Gravimetric AnalysisSomtimes We Get Colloids (not fun)

• When colloidal particles form we need to aggregate or “coagulate” them to form easily filterable particles.

• Two Common Approaches To Accomplish This

– “Digest” the colloidal sample by heating (1-hr)

– Let the heated precipitate “age overnight”

– Add an electrolyte (a non-interfering salt)

– Both reduce the number of stabilizing counter-ions and allow for particles to approach one another

Fig. 10.2. Representation of silver chloride colloidal particleand adsorptive layers when Cl- is in excess.

Cl- adsorbs on the particles when in excess (primary layer).

A counter layer of cations forms. The neutral double layer causes the colloidal particles to coagulate.

Washing with water will dilute the counter layer and the primary layer charge causes the particles to revert to the colloidal state (peptization). So we wash with an electrolyte that can be volatilized on heating (HNO3).

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

Important Factors for Gravimetric AnalysisAddressing Peptization

• Peptization occurs when one washes the precipitate to remove the “counter-ions” and as a result some of the colloid redissolves.

• Two Common Approaches To Accomplish This

– “Digest” the colloidal sample by heating (1-hr)

– Let the heated precipitate “age overnight”

– Add an electrolyte (a non-interfering salt)

– Both reduce the number of stabilizing counter-ions and allow for particles to approach one another

Important Factors for Gravimetric AnalysisCrystalline Precipitate

• Much larger particle size

– Optimize by keeping RSS << 1

– Easy to filter

– Ideal to work with

– Best done with dilute precipitation then digest

Important Factors for Gravimetric AnalysisOther Problems: Co-precipitation

• Co-precipitation occurs when another soluble compound in the mix precipitates with your analyte unwantingly.

• 4-Types of Co-precipitation

1. Surface Absorption

2. Occlusion

3. Mechanical entrapment

4.Mixed crystal formation

Important Factors for Gravimetric AnalysisCoprecipitation: Surface Adsorption

• Surface Adsorption: surface of the precipitate will contain some absorbed ions.

– Example: AgCl(s) + NO3- adsorbed (mass will be high)

– Occurs mostly with colloidal precipitates as the surface

area is larger

– alters the mass and alters the accuracy of the analysis

• How To Deal With It

– Digest for longer periods

– Washing--usually doesn’t work as counter ion is strong

– Washing With Volatile Electrolyte (HCl followed by

drying)

Important Factors for Gravimetric AnalysisOcclusion Traps Counter-Ions

Important Factors for Gravimetric AnalysisMechanical Entrapment

Important Factors for Gravimetric AnalysisOcclusion and Entrapment

Important Factors for Gravimetric AnalysisMixed Crystalline Precipitate

Difficult to deal with. Alternate analysis?

Worked Example

A 0.4500 g sample of impure potassium chloride was dissolved in

water and treated with an excess of silver nitrate. A 0.8402 g of silver

chloride was massed after digesting, collecting, washing and drying

the precipitate. Calculate the percentage KCl in the original sample.

(9712)