Topic III: Gravimetric Analysis

Precipitation methods

Volatilization methods

2 methods of Gravimetric Analysis

precipitation methods

analyte is converted to a sparingly soluble precipitate which then filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment, and weighed.

volatilization methods

analyte is separated from other constituents by conversion to a gas of known chemical composition.

1. Weighing the sample

2. Moisture determination

3. Conversionoftheanalyteto
   a sparingly soluble salt (precipitate)

4. Filtrationoftheprecipitate

5. Washing the precipitate

6. Drying or ignition of the precipitate (conversion to a form suitable for weighing)

   1. Weighing of the converted form of the precipitate.

7 steps of Precipitation Methods

precipitate

A ________ is formed when an aggregate of atoms, ions or molecules is greater than 10–4 cm.

Colloidal particles

are electrically charged and resist combination to form larger particles which will settle down the solution.

10-8 cm

size of ions in soln

10-7 to 10-4 cm

size of colloidal particles

>10-4

size of precipitate

1. filtered and washed free

2. low solubility

3. unreactive

   1. known composition after it is dried

Ideally, a gravimetric precipitating agent should react specifically or if not selectively with the analyte, and would give a product that is:

1. colloidal suspension

2. crystalline suspension

Factors that Affect the Particle Size of a Precipitate

colloidal suspension

are tiny particles that are invisible to the naked eye (10-7 to 10-4 cm in diameter); shows no tendency to settle from the solution nor are they easily filtered.

crystalline suspension

the temporary dispersion of such particles in the liquid phase, dimensions are in order of tenths of a millimeter or greater.

precipitate solubility, temperature, reactant concentration, and rate at which reactants are mixed.

The particle size of a precipitate is influenced by such experimental variables such as? (4ptrr)

relative supersaturation or Von Weimarn’s ratio.

The net effect of these variables (4-ptrr) can be accounted by assuming that the particle size is related to a single property of a system called ____ or ____.

rs=Q-S/S

relative supersaturation formula

Q

in the formula of relative supersaturation, this represents the concentration of a species at any instant

S

in the formula of relative supersaturation, this represents the equilibrium solubility

supersaturation solution

an unstable solution that contains a higher solute concentration than a saturated solution. With time, this is relieved by precipitation of the excess solute.

large

when relative supersaturation is ____, precipitate tends to be colloidal.

small

when relative supersaturation is _____ (Q is low and S is high), a crystalline solid is more likely to form.

1. Nucleation

2. Particle Growth

Precipitates form by two different pathways. What are these two?

nucleation

a few ions, atoms or molecules come together to form a stable solid. Often, these nuclei form on the surface of suspended solid contaminants such as dust particles.

particle growth

Further precipitation then involves a competition between additional nucleation and growth on existing nuclei

nucleation

when a precipitate is formed at high relative supersaturation, ______ is the major precipitation mechanisms, and a large number of small particles are formed.

particle growth

At low relative supersaturation, the rate of _______ tends to predominate, and deposition of solid on existing particles occur to the exclusion of further nucleation; a crystalline suspension results.

colloidal suspensions

The stability of most ________ can be decreased by heating, by stirring and by adding an electrolyte

coagulation or agglomeration or flocculation

The process of converting the colloidal suspension into filterable solid is called (CAF)

Colloidal suspensions

are stable because all of the particles of the colloid are either positively or negatively charged.

Paneth-Fajans-Hahn Rule

According to ________________(rule), the ions that are preferentially adsorbed on the surface of the crystal lattice are those which are common to the lattice and are in excess.

primary adsorbed layer

In Paneth-Fajans-Hahn Rule, Attached directly to the solid surface is the?

counter-ion layer

In Paneth-Fajans-Hahn Rule, surrounding the charged particle is a layer of solution called ________ which contains sufficient excess of ions to just balance the charge on the surface of the particle.

electric double layer

The primary adsorbed layer and the counter-ion layer constitute an ____________ that imparts stability to the colloidal suspension.

Coagulation of a colloidal suspension

can be brought about by a short period of heating particularly if accompanied by stirring.

peptization of colloids

• refers to the process by which a coagulated colloid reverts to its original dispersed state.

Colloids

________ are best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation.

digestion

the process in which a precipitate is heated for an hour or more in the solution from which it was formed (the mother liquor).

crystalline precipitates

are generally more easily filtered and purified than coagulated colloids, and the size of individual crystalline particles, and their filterability can be controlled to a degree.

Digestion of crystalline precipitates

for some time after formation frequently yields a purer, more filterable product resulting from the dissolution and recrystallization that occur continuously and at a enhanced rate at elevated temperature.

dilute solutions

For crystalline solids, it can be often improved by minimizing Q and/or maximizing S.

Minimizing Q is accomplished by using _______ and adding the precipitating reagent slowly and with good mixing.

precipitating

For crystalline solids, it can be often improved by minimizing Q and/or maximizing S.

Maximizing S can be achieved by ______ from hot solution or by adjusting the pH of the precipitation medium.

coprecipitation

a phenomenon in which otherwise soluble compounds are removed from solution during precipitate formation.

1. surface adsorption

2. mixed-crystal formation

3. occlusion

4. mechanical entrapment

4 types of coprecipitation (smom)

mechanical entrapment

4 types of Coprecipitation

occurs when crystals lie close together during growth, here, several crystals grow together and in so doing trap a portion of the solution in a tiny pocket.

occlusion

4 types of Coprecipitation

when a crystal is growing rapidly during precipitate formation, and foreign ions in the counter-ion layer may become trapped or occluded with the growing crystal.

mixed-crystal formation

4 types of Coprecipitation

here, one of the ions in the crystal lattice of a solid is replaced by an ion of another element. For exchange to occur, the two ions must have the same charge and their sizes differ no more than about 5 %. The two salts must belong to the same crystal class. Examples include PbSO4 in BaSO4, MgKPO4 in MgNH4PO4, SrSO4 in BaSO4 and MnS in CdS.

a. digestion

b. washing

c. reprecipitation

Methods for Minimizing Adsorbed Impurities on Colloids under Surface Adsorption (dwr)

surface adsorption

4 types of Coprecipitation

can cause significant contamination of precipitates with large specific surface areas, that is coagulated colloids. The net effect of surface adsorption is therefore the carrying down of an otherwise soluble compound as a surface contaminant.

washing

Methods for Minimizing Adsorbed Impurities on Colloids

_______ a coagulated colloid with a solution that contains volatile electrolyte – exchange between existing counter-ions and ions in the wash liquid.

digestion

Methods for Minimizing Adsorbed Impurities on Colloids

duringthisprocess,waterisexpelledfrom the solid to give a denser mass that has a smaller specific surface area for adsorption.

Reprecipitation

Methods for Minimizing Adsorbed Impurities on Colloids

a drastic but effective way to minimize the effects of adsorption also known as double precipitaton. The filtered solid is redissolved and reprecipitated.

digestion

At low supersaturation (rate of precipitation formation is low), both occlusion and mechanical entrapment are at a minimum and _____ is helpful in reducing these types of coprecipitation.

Surface adsorption and mixed crystal formation

occlusion and mechanical entrapment

___________ & _____________ are equilibrium processes whereas ______________ & _____________ arise from the kinetics of crystal growth.

positive errors

Coprecipitation impurities may cause either negative or positive errors in an analysis.

If a contaminant is not a compound of the ion being determined, __________ always result.

positive

A ______ error is observed whenever colloidal silver chloride adsorbs silver nitrate during a chloride analysis.

positive or negative errors

Coprecipitation impurities may cause either negative or positive errors in an analysis.

When the contaminant does not contain the ion being determined, either ______ or ________ may be observed.

positive

In the determination of barium by precipitation as barium sulfate, occlusion of other barium salts occurs. If the occluded contaminant is barium nitrate, a ______ error is observed because this compound has a larger mass than barium sulfate.

negative

If barium chloride is the contaminant, the error is ___ because its molar mass is less than that of the sulfate salt.

direct addition

In general, homogeneously formed precipitates, both colloidal and crystalline are better suited to analysis that are solids formed by ______ of a precipitating reagent.

heated

After filtration, a gravimetric precipitate is ____ until its mass becomes constant.

Heating

removes the solvent and any volatile species carried down with the precipitate.

high

If the particle size is colloidal, the relative supersaturation is

nucleation

If the particle size is colloidal, the mechanism is

high q, low s

If the particle size is colloidal, the condition should be

low

If the particle size is crystalline, the relative supersaturation is

particle growth

If the particle size is crystalline, the mechanism is

low q, high s

If the particle size is crystalline, the condition should be

molar mass

the mass in grams of 1 mol of a substance calculated by summing the atomic masses of all atoms appearing in the chemical formula.

6.022 × 10²³

a mole is equal to ____ particles

Stoichiometry

the quantitative relationship between among reacting chemical species.

Empirical formula

gives the simplest whole number ratio of atoms in a chemical compound.

Molecular formula

specifies the number of atoms in a molecule. Two or more substances may have the same empirical formula but different molecular formulas.

balanced

A ______ chemical equation gives the combining ratios, or stoichiometry – in units of moles of reacting substances and their products.