gravimetric methods of analysis methods based on measurement of weight of an analyte or a compound...

Gravimetric Methods of AnalysisGravimetric Methods of Analysis

• methods based on measurement of weight of an analyte or a compound containing the analyte

Precipitation methodsPrecipitation methods

• based on isolation of an insoluble precipitate of known composition

Volatilization methodsVolatilization methods

• analyte is volatized, weighed and the loss is determined

Properties of precipitatesProperties of precipitates

To obtain good results, you must be able to produce a“pure” precipitate that can be covered with highefficiency.

We want the precipitate to:

• have low solubility, so that no significant loss of the analyte occurs during filtration and washing• easily filtered and washed free of contaminants• unreactive with the air, water, …• very pure• know composition after after it is dried or, if necessary, ignited.

Particle size and filterability of Particle size and filterability of precipitatesprecipitates

Colloidal suspensions

• size range : 10-6 – 10-4 mm• no tendency to settle• difficult or impossible to filter

Colloidal suspensions

• size range : 10-1 – 10 mm• will settle spontaneously• readily filtered• typically of higher purity than colloids


We have some control on particle size based on how weadd our reagents ( precipitating agent or precipitant).

Relative supersaturation (RSS)Relative supersaturation (RSS)

S

SQRSS

where

Q = the concentration of the solute at any timeS = equilibrium solubility of solute

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


S

SQRSS

If RSS is large, the precipitate tends to be colloidal.If RSS is large, the precipitate tends to be colloidal.If RSS is If RSS is smallsmall, the precipitate tends to be , the precipitate tends to be crystalline.crystalline.

We can keep RSS small by:

• using dilute solution and reagent• slowly adding precipitant• stirring the solution• warming the solution

Mechanism of precipitation formationMechanism of precipitation formation

Precipitates form in two way:Precipitates form in two way:

Nucleation

Particle Growth

• a process in which a minimum number of atoms, ions, or molecules join together to give a stable solid (nuclei).• spontaneous• induced

• the three dimensional growth of a particle nucleus into a lager crystal.


nucleation

spontaneous

induced

Spontaneous nucleation will occur on its own.Induced nucleation required a ‘seed’ particle to get things started (dust, another crystal, glass fragment, …)


Particle GrowthParticle Growth

Once nucleation site has formed, other ions are attractedto the site. This will result in the information of large,filterable particles.

If done properly, it also reduces contaminates since theydon’t ‘fit’ in the crystal structure.


Nucleation and particle growth are competing processesalthough at least some nucleation must initially occur.

solution

nucleation particle growth

nucleation particle growth

We want the particle growth to be the predominate factorin precipitate formation.


Rate of nucleation is eRSS

Rate of particle growth is RSS

If RSS is large, nucleation is favored and colloidalsuspensions tend to be formed.

If RSS is small, particle growth will predominateresulting in crystalline in precipitates.


Rate of nucleationRate of particle growth

crystal crystal formationformation

colloidalcolloidalformationformation

RSSRSS

KKspsp


The goal is to form crystalline precipitates soRSS must be minimised.

This can be done by:

Increasing SIncreasing S Decrease QDecrease Q

• Increase temperature• Control pH

• Use dilute solution• Slowly add reagents• Stirring the solution


Example: we will use the following precipitate formationthrough out:

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

precipitant colloidalprecipitate

Colloidal precipitatesColloidal precipitates

gCl)n

Ag+

Ag+

Ag+

Ag+

Ag+

Ag+

Ag+

silver chloride particlesilver chloride particle

primary absorbed layerprimary absorbed layer

counter ion layercounter ion layer

waterwater

Start of precipitationStart of precipitation

Start of precipitationStart of precipitationColloidal precipitatesColloidal precipitates

• initially, there is little free chloride in the solution due to the excess of Ag+

• the outer layer of the precipitate contains both Ag+

and Cl- which tend to attract additional Ag+ to the surface – primary absorbed layerprimary absorbed layer

• this primary layer is simply an extension of the precipitate but we have run out of Cl-

• the counter ion in excess (nitrate) is attracted to this layer by electrostatic forces – maintains electrical neutrality


• since the overall structure tends to appear negative to the solution, it attracts water molecules that move with the particle.

High [Ag+] Low [Ag+]

• at high silver concentrations, we have a higher charge on the precipitate. This results in the larger particle.• the problem is even worse when the counter ion is large.


Net resultNet result

• at high silver concentrations and with large counter ions, we tend to form stable colloids.

• the charged structures have difficultly approaching each other since there are similarly charged.

NoteNote

The same type of problem will occur when chloride isin excess - towards end of precipitation


It would seem that the solution is to keep the [Ag+] low at all times.

• this is impossible to do - as Ag+ is added, we create locally high concentrations even when using dilute solutions.

• since we can’t eliminate the problem, we need ways to minimise its effect

Obtaining a good precipitatesObtaining a good precipitates

Several steps can be taken to help precipitate the colloidand reduce impurities.

Any steps taken must address two competing processtwo competing process..

Coagulation

Peptization

a process by which a coagulated colloid is reconvertedto a colloidal suspension.

a process where colloidal particles ‘lump’ together into larger particles.

Two common approaches for causing a colloid to coagulate:

• heatingheating to the solution• addingadding an electrolyte to the solution.

Heating the colloid while stirringHeating the colloid while stirring

• significantly decrease the number of absorbed ions per particle• reduce the size of the counter ion layer make it easier for the particles to approach each other

Coagulation of colloidsCoagulation of colloids

Increasing the electrolyte concentration of the solutionIncreasing the electrolyte concentration of the solution (HNO(HNO33 for AgCl) for AgCl)

• reduce the volume of the ionic atmosphere and allow particles to come closer together before electrostatic repulsion becomes significant.

Addressing peptizationAddressing peptization

Washing a colloid to remove excess counter ion ortrapped impurities can result in peptization.

There are several common approaches that can be used toaddress this problem.

Treatment of colloidal precipitatesTreatment of colloidal precipitates

Use a volatile electrolyte

• a relative volatile salt can be used to wash the precipitate

• this replaces the less volatile, excess counter ion

• heating precipitate (during drying) will remove the volatile electrolyte

Treatment of colloidal precipitatesTreatment of colloidal precipitates

Digestion and agingDigestion and aging

Digestion

heating the solution for about an hour after precipitateformation help to remove weakly bound water

Aging

storing the solution, unheated, overnight.allow trapped contaminates time to ‘work their way out’

Both can result in a denser precipitatea denser precipitate that is easier tofilter.

Crystalline precipitatesCrystalline precipitates

This type of precipitates is much easier to work with.

• easy to filter and purify

In general, the best approach is :

• slowly form precipitate using warm and dilute solution. Then, digest without stirring.

CoprecipitationCoprecipitation

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

Sources of coprecipitation

• surface absorption• occlusion• mixed crystal formation• mechanical entrapment


Surface adsorption

• the surface of precipitate will contain a primary absorbed ions. as a result, counter ions will be present.

example

AgCl, the precipitate will commonly contain nitrate.

Occurs with colloidal and crystalline ppts.


AgCl precipitate

• if nitrate is co-precipitated, our result will be too high nitrate weights more than chloride

• the results will be variable since the surface area may differ sample to sample.

• a lower weight counter ion will result in our results being too low.


Dealing with surface adsorptionDealing with surface adsorption

washing

washing with volatile electrolyte

• help but not too much the attraction of counter ion is usually too strong

• better approach the excess can be removed during drying ex: For AgCl, washing with HCl is a reasonable approach.


Dealing with surface adsorptionDealing with surface adsorption

reprecipitation

• dissolve filtered precipitate and precipitate a second time• only a small portion of the contaminate makes it into the second solution• the second precipitate will be purer

Mix-Crystal FormationMix-Crystal Formation

‘a type of co-precipitation in which a contaminate ion(similar ion) replaces the analyte ion in the lattice of a crystalline precipitate’

example

Determination of SO42- as BaSO4

The presence of Pb will cause a mixed crystal containing PbSO4.

Mix-Crystal FormationMix-Crystal Formation

• this is a problem with both colloidal and crystalline ppts

• there is no easy solution to minimise the problem

• when encountered, your only choices are to remove the interferences prior to precipitation or to select a different reagent

Occlusion and EntrapmentOcclusion and Entrapment

OcclusionOcclusion

If the crystal growth is too rapid, some counter ionsdon’t have time to escape from the surface.

counter ions

rapidprecipitateformation


Mechanical Entrapment

• occur when crystals lie close together during growth• several crystal grow together and in so doing trap a portion of the solution in a tiny pocket

rapid growth and merging particles


The best way to deal with these problem is to slow things down

• using dilute, warm solution during precipitate give counter ions time to leave and help break up pocket

• digestion and aging precipitate provides additional time for this to occur as well

Precipitation from Homogeneous SolutionPrecipitation from Homogeneous Solution

BasisBasis

• reagent is added to in an unreactive form to sample solution

• this permits of complete mixing of all materials

• some properties of solution is changed to slowly convert to the reagent to a reactive form

• the solution is slowly and uniformly changes – no locally high concentration

Precipitation from Homogeneous SolutionPrecipitation from Homogeneous Solution

example

Dimethyl sulphate – (CH3O)2SO4 is used to for thehomogeneous generation of sulphate ion.

(CH3O)2SO4 + 4H2O SO42- + 2CH3OH + 2H3O+

Eliments precipitated: Ba, Ca, Sr and Pb and sulphate.

Precipitate DryingPrecipitate Drying

• after filtration, the precipitate must be dried to constant weight

- remove excess solvent- drive off any volatile species

• in some cases, the precipitate is heated to a point where it decomposes to a stable form for weighing

• thermobalances can be used to determine optimum drying time and temperature

> 1000 oC

> 110-120 oC

Summary of methodSummary of method

• a relative slow method of analysis - most time is spent waiting

• minimal requirements- major equipment is a good balance and an oven

• no calibration is required- results are based on formula weight

• accuracy- 1-2 ppt

Summary of methodSummary of method

• sensitivity- analyte concentration should be over 1%. Below

that, you may encounter problem to solubility.

• selectivity- not very specific but can be make reasonably selective.

gravimetric methods of analysis methods based on measurement of weight of an analyte or a compound...

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