BKF1243 ANALYTICAL CHEMISTRY

Chapter 5

Gravimetric methods of

analysis

COURSE LECTURER:

DR. MUHAMMAD AMIN

mdamin@ump.edu.my

h/p: 0169625780

SEM 2 SESSION 2009/2010

Outlines

1.Overview of Gravimetry

2.Precipitation Gravimetry

3.Volatilization Gravimetry3.Volatilization Gravimetry

4.Particulate Gravimetry

5.Electrogravimetry

- Explain and differentiate the theory and practice

of precipitate, volatilization and particulate

IT IS EXPECTED THAT STUDENT WILL BE ABLE TO:

of precipitate, volatilization and particulate

gravimetry

- Determine the quantitative, qualitative application

of gravimetric method

Overview

• Measuring mass is the most

fundamental of all analytical

measurements

• Gravimetric is the oldest analytical

technique

Outlines

1.Overview of Gravimetry

2.Precipitation Gravimetry

3.Volatilization Gravimetry3.Volatilization Gravimetry

4.Particulate Gravimetry

5.Electrogravimetry

Using Mass as a Signal

• Determine analyte gravimetrically by :

- Directly:

1. determining its mass eg: weighting a solid waste1. determining its mass eg: weighting a solid waste

2. the mass of a compound containing analyte. eg: aqueousion, Pb2+, chemically converting it to a solid form.Suspend a pair of Pt electrodes in solution, Pb2+ ion insolution oxidizes to PbO2 and deposits on the Ptelectrode

Cont’

- Indirectly

1. by measuring a change in mass due to its loss, eg : heat the sample

2. The mass of a compound formed as the result of a 2. The mass of a compound formed as the result of a reaction involving the analyte, eg: using HgCl2 to measure the mass of PO3

3- (HgCl2 is added, one mole of PO3

3- produces one mole Hg2Cl2

Types of Gravimetric Methods

• Precipitation gravimetry – which the signal is the mass of a precipitate

• Electrogravimetry – which the signal is the mass of an electrodeposit on the cathode or anode in an elecrochemical cellcell

• Volatilization gravimetry – which the loss of a volatile species gives rise to the signal

• Particulate gravimetry – which the mass of a particulate analyte is determined following its separation from its matrix

• Mass of presenting analyte in sample must be proportional to

mass or change in mass serving as the analytical signal –

involves conservation of mass.

• For chemical reaction involved in gravimetric methods,

Conservation of Mass

•

stoichiometry that relates the precipitate’s mass and analyte’s

mass must be followed accurately.

• General determination techniques following conservation of

mass:

1. Suspended solid mass = filter’s final mass – filter’s initial mass

2. Fluid mass content = sample’s initial mass – sample’s final mass

Why Gravimetry is Important

• Is one of only a small number of techniques whose

measurements require only base SI units.

• Estimating the composition of these materials often involves a • Estimating the composition of these materials often involves a

gravimetric analysis

Precipitation Gravimetry

• Precipitant – a reagent that causes the precipitation of a

soluble species

• The precipitate is the product of a simple metathesis reaction • The precipitate is the product of a simple metathesis reaction

between the analyte and precipitant

• Any reaction generating a precipitate can potentially serve as

a gravimetric method

Theory and Practice (TnP)

Important attributes:

• First – the precipitate must be of low solubility, high purity

and of known composition

• Second – the precipitate must be in a form that is easy to

separate from the reaction mixture

TnP – Solubility Considerations

• An accurate precipitation requires precipitate’s solubility be

minimal (accuracy > ±0.1%)

• Isolated precipitate must account for at least 99.9%of the analyte

• Eg: Ag+ can be determined gravimetrically by adding Cl- as a • Eg: Ag+ can be determined gravimetrically by adding Cl- as a precipitant forming a precipitate of AgCl

)()()( sAgClaqClaqAg−+ + Eq 8.1

Cont’

• If this the only reaction considered, falsely conclude that the precipitate’s solubility, SAgCl, is

[ ] [ ]−

+ ==Cl

KAgS

sp

AgClEq 8.2

• AgCl shows a more complex solubility relationship than that suggested by eq 8.2. Ag+ also forms a series of soluble chloro-complexes

Eq 8.3

Eq 8.4

Eq 8.5

Ag+ (aq) + Cl- (aq) AgCl (aq)K1

Ag+ (aq) + 2Cl- (aq) AgCl2- (aq)

β2

Ag+ (aq) + 3Cl- (aq) AgCl32- (aq)

β3

Cont’

• The solubility of AgCl, is the sum of the equilibrium concentrations for all soluble forms of Ag+

Eq 8.6

Solubility of AgCl in terms of the equilibrium

concentration of Cl-

SAgCl = [Ag+] + [AgCl(aq)] + [AgCl2-] + [AgCl3

2-]

[ ] [ ] [ ]2321

−−

−+++= ClKClKKK

Cl

KS spspsp

sp

AgCl ββ

concentration of Cl

Eq 8.7

As Cl- is added to a solution of Ag+, the solubility of AgCl

initially decreases because of reaction 8.1. increasing the

concentration of chloride, however leads to an increase

in the solubility of AgCl due to the soluble chloro-

complexes formed in reaction 8.3 – 8.5.

Cont’• pH also may affects the precipitate’s solubility. eg: hydroxide

precipitate (such as Fe(OH)3), more soluble at lower pH, which

the concentration of OH- is small.

• The effect of pH on solubility also affects precipitates

containing basic or acidic ions.

• Eg : Ca3(PO4)2 is pH dependent, consists of 4 reactions,

Eq 8.8Ca3(PO4)2(s) 3Ca2+ (aq) + 2PO43- (aq)

Ksp

PO43-(aq) + H2O (l) HPO4

2-(aq) + OH-(aq)Kb1

HPO42-(aq) + H2O (l) H2PO4

-(aq) + OH-(aq)Kb2

H2PO4-(aq) + H2O (l) H3PO4(aq) + OH-(aq)

Kb3

Eq 8.9

Eq 8.10

Eq 8.11

Cont’

• The ladder diagram for phosphate, provides a convenient way

to evaluate the pH-dependent solubility of phosphate

precipitates

Cont’

• Solubility can often be decreased by using a non-aqueous

solvent

• Precipitate’s solubility greater in aqueous solutions because of

the ability of water molecules to stabilize ions through the ability of water molecules to stabilize ions through

solvation

• Poorer solvating ability of non-aqueous solvents, even those

that are polar

Avoiding Impurities

• Precipitate must be free from impurities

• Precipitate typically occurs in a solution rich in dissolved solids

• A precipitate is generally crystalline, even if only on a microscopic scale, with a well-defined lattice structure of microscopic scale, with a well-defined lattice structure of cations and anions

• Precipitate particles grow in size because of the electrostatic attraction between charged ions on the surface of the precipitate and oppositely charged ions in solution

Cont’• One common type of impurity is an inclusion

– a co-precipitated impurity n which the interfering ion occupies a lattice site in the precipitate

• Probability of forming an inclusion is greatest when the interfering ion is present at higher concentrations than the dissolved lattice ionlattice ion

• However, inclusion does not decrease the amount of analyte that precipitates and precipitate’s mass is always larger than expected

• Can be removed through reprecipitation

Cont’

• Second type of co-precipitated impurity, Occlusions –

impurity trapped within a precipitate as it forms

• Form in two ways –

1. occurs when physically adsorbed ions are surrounded by

additional precipitate before they can be desorbed or additional precipitate before they can be desorbed or

displaced- mass is always greater than expected

2. occurs when rapid precipitation traps a pocket of solution

within the growing precipitate-mass may be less than

expected

Cont’

• Occlusions are minimized by maintaining the precipitate in equilibrium with its supernatant solution for an extended time – the process called digestion

• Digestion – the process by which a precipitate is given time to form larger, purer particles

• Precipitate dissolves and reforms, ensures that occluded material is eventually exposed to the supernatant solutionmaterial is eventually exposed to the supernatant solution

• After precipitate complete, the surface adsorbates, chemically or physically adsorbed, constitute a third type of coprecipitated impurity.

• Adsorbates – a co-precipitated impurty that adsorbs to the surface of a precipitate

Cont’

• Surface adsorption is minimized by decreasing the precipitate’s available surface area

• Another source of impurities occurs when other species in solution precipitate under the conditions of the analysissolution precipitate under the conditions of the analysis

• Solution conditions necessary to minimize the solubility of a desired precipitate may lead to the formation of an additional precipitate that interferes in the analysis

Cont’

• Can be minimized by carefully controlling solution conditions,

either removed by filtration or masked using a suitable

complexing agent

• Also can separate an analyte from a potential interferent or • Also can separate an analyte from a potential interferent or

isolate and weigh both the analyte and the interferent

Controlling Particle Size

• Precipitation consists of two distinct events, nucleation and the

subsequent growth of these particles

• Relative supersaturation – a measure of the extent to which a

solution, or a localized region of solution, contains more dissolved

solute than that expected at equilibrium (RSS)

• A solute relative supersaturation, RSS

S

SQRSS

−= Eq 8.12

Q = solute’s actual concentration

S = solute’s expected concentration at equilibrium

Q – S = a measure of the solute’s supersaturation when precipitation begins

Cont’

Large value – indicates that a solution is highly

supersaturated, solution are unstable nd high rates of

nucleation-small particle

Small value – precipitation is more likely to occur by

particle growth than by nucleation

• Homogeneous precipitation – a precipitation in which the precipitant is generated in situ by a chemical reaction

• Two general methods, if pH dependent then the analyte and precipitant can be mixed under conditions in which precipitation does not occur. The pH is then raised or lowered as needed by chemically generating OH- or H3O+

Cont’

• Second method – the precipitant itself is generated by a chemical reaction.

• Eg : Ba2+ can be homogeneously precipitated as BaSO4 by hydrolyzing sulphamic acid to produce SO4

2-

• Advantage homogeneous precipitation – produce large particle of precipitate that are relatively free from impurities, however the process need time

• A precipitate’s particles are electrically neutral, they tend to coagulate into larger particles.

Cont’

• Coagulation – the process of smaller particles of precipitate

clumping together to form larger particles

• Cannot occur if the secondary adsorption layer is too thick

• Can be induced in two ways:

– Increasing the concentration of the ions responsible for

the secondary adsorption layer

– By heating the solution

Cont’

• After precipitation and digestion are complete, separate by

filtration

Proper procedure to

transferring the

supernatant* to the

filter paper cone

Supernatant – the

solution that remains

after a precipitate

forms

The steps required in a gravimetry

analysis

Preparation of the solution

Precipitation

Digestion

Filtration

Washing

Drying or Igniting

Weighing

Calculation

Determination of Mg2+ in water and wastewater

Quantitative Applications

• Inorganic analysis – most important precipitants for inorganic

cations are chromate, the halides, hydroxide, oxalate, sulfate,

sulfide and phosphate.

• Inorganic anions can be determined using the same reactions

by reversing the analyte and precipitant. by reversing the analyte and precipitant.

• Eg : chromate can be determined by adding BaCl2 and

precipitatingBaCrO4.

• Eg : sulfate is analyzed by precipitating BaSO4, using BaCl2 as

the precipitant

Quantitative Application – cont’

• Organic analysis – several organic functional groups or

heteroatoms can be determined using gravimetric

precipitation methods

• Examples are outlined in Table 8.5• Examples are outlined in Table 8.5

Gravimetry Calculations

� The aim of a gravimetry analysis is the

determination of the concentration, wA of an analyte in

a given sample using the laws of stochiometry.

100100e

af

e

mw A

A ==

Where:Where:

wA = the content of the component sought (%)

mA = the mass of the component sought (mg)

e = the mass of the sample before gravimetric treatment (mg)

a = the mass o the substance weighed after precipitation, drying, etc

(mg)

f = the stochiometric factor

(g/mol) weghedsubstance theof weight formula

(g/mol)sought substance theof weight formula=f

f 100 is the percentage of the substance sought in the substance

weight

Example:

A practical sample is given for the gravimetric

determination of Ba2+ after precipitation with H2SO4 as

BaSO4, filtration, and through drying in a quartz crucible:

Sample weight: 0.6537 g

Weight of BaSO4: 0.4288 g

wA = ???

Example:

A practical sample is given for the gravimetric

determination of Ba2+ after precipitation with H2SO4 as

BaSO4, filtration, and through drying in a quartz crucible:

Sample weight: 0.6537 g

Weight of BaSO4: 0.4288 g

wA = ???

%6.38100

5884.0/40.233

/34.137

)BaSO(

)Ba(

4

==

===

xe

afw

molg

molg

M

Mf

Ba

Solution:

Calculate the gram of analyte per gram of precipitate for the following conversions:

Analyte : Precipitate:

P Ag3PO4

S BaSO4

K2HPO4 Ag3PO4

Solution:

g 0.07399 1

1 x

/mol)POAg (g 58.418

P/mol) (g 30.97

)POAg P/mol mol(1

1

POAg wt g

(g/mol) Pat wt POAg P/g

43

3 43

43

4

==

==g

g 0.0457 3

1 x

/mol)BaSO (g 40.233

S/mol) (g 32

)BaSO S/mol mol(1

1

BaSO wt g

(g/mol) Sat wt BaSO S/g

4

4

4

4

==

==g

What weight of pyrite ore (impure FeS2) must be taken for analysis so that the BaSO4

precipitate weight is obtained will be equal to one-half that of the percent S in the samples?

Solution:

100% x sample g

)BaSO S/g (g BaSO

S x)BaSO (gA

2

1

S A%

4

4

4

=

100 x

g 6.869 sample g

sample g

233.40

32.064 x

2

1

S 1%

sample g

=

=

Evaluating Precipitation Gravimetry

• Scale of operation – governed by the sensitivity of the balance

and the availability of sample

• Accuracy – macro-major samples, relative errors of 0.1-0.2%

are routinely achieved. Limitations – solubility losses, are routinely achieved. Limitations – solubility losses,

impurities in the precipitate and the loss of precipitate during

handling

• Precision – depends on the amount of sample and precipitate

involved. Smaller amounts, relative precisions 1-2ppt, larger

amounts, extended to several ppm

Cont’

• Sensitivity – relate the signal (g of precipitate) to the absolute

amount of analyte in the sample

• g precipitate = k x g of analyte

k, method’s sensitivity, determined by the

stoichiometry between the precipitate and the

analyte

Cont’

• Method’s sensitivity can be improved in two ways: increase

the ratio of the precipitate’s molar mass to that of the analyte;

indicated by term of ½ in eq 8.14, which accounts for the

stoichiometry between the analyte and precipitate.

• Also can be improved by forming precipitates containing

fewer units of the analyte

• Selectivity : due to chemical nature, precipitants are usually

not selective for a single analyte

Cont’

• Time, cost and equipment – are time-intensive and rarely

practical when analyzing a large number of samples.

• Does not require an analysts’s immediate supervision,

practical for a few samplepractical for a few sample

• Few equipments needed, inexpensive, routinely available in

laboratories, and easy to maintain.

Volatilization Gravimetry

• Second approach to gravimetry – thermally or chemically

decompose a solid sample

• Volatile products of the decomposition reaction may be

trapped and weighed to provide quantitative informationtrapped and weighed to provide quantitative information

• Thermogravimetry – a form of volatilization gravimetry in

which the change in a sample’s mass is monitored while it is

heated

Volatilization Gravimetry

• Theory and practice

• Quantitative applications• Quantitative applications

Theory and Practice

• Requires the products of the decomposition reaction be

known

• For organic compounds, volatilization is accomplished by

combustion, and the products are gases such, CO2, H2O, N2combustion, and the products are gases such, CO2, H2O, N2

• Inorganic compounds, identification of volatilization products

depend on the temperature at which the decomposition is

conducted

Cont’

• The loss of a volatile gas on thermal decomposition is

indicated by a step in the thermogram

• Thermogram – a graph showing change in mass as a function • Thermogram – a graph showing change in mass as a function

of applied temperature

•The products of a thermal

decomposition can be deduced by

monitoring the sample’s mass as a

function of applied temperature

Thermogram for

CaC2O4.H2O

Example 8.4

• The thermogram for CaC2O4.H2O shows the change in mass.

The original sample weighed 24.60 mg and was heated from

room temperature to 1000 0C at a rate of 5 0C min. the

following changes in mass and corresponding temperature

ranges were observed:ranges were observed:

• Loss of 3.03 mg from 100 – 250 0C

• Loss of 4.72 mg from 400 – 500 0C

• Loss of 7.41 mg from 700 – 850 0C

• Determine the identities of the volatilization products and the

solid residue at each step of the thermal decomposition.

Equipment

• The weight of a solid residue is determined following either thermal decomposition at a fixed temperature or combustion

• Accomplished using a Bunsen or Meker burner, a laboratory oven or a muffle furnace, with the volatile products vented to the atmosphere

• Weight of the sample and the solid residue are determined • Weight of the sample and the solid residue are determined using an analytical balance

• Decomposition of the sample is conducted in a closed container, and the volatilized gases are carried by a purge-gas stream through one or more selective absorbent traps

Quantitative Applications

• Inorganic analysis – determining the inorganic ash content of

organic materials, such as polymers and paper, is an example

of a direct volatilization gravimetric analysis.

• Indirect analyses based on the weight of the residue

remaining after volatilization are commonly used in

determining moisturedetermining moisture

• Organic analysis –the combustion products are passed

through preweighed tubes containing appropriate absorbents

– provides a direct indication

• Also used to determine biomass in water and wastewater

Cont’

• Also used to determine biomass in water and wastewater

Cont’

• Quantitative calculations – by applying the principle of

conservation of mass to carbon

• See example 8.5 for direct volatilization gravimetric analysis

(determination of a compound’s elemental composition)(determination of a compound’s elemental composition)

• See example 8.6 for indirect volatilization gravimetric analysis

(the change in the sample’s weight is proportional to the

amount of analyte)

Evaluating Volatilization Gravimetry

• The scale of operation, accuracy and precision of gravimetric

volatilization methods are similar to that described

precipitation gravimetry

• Direct analysis (DA) - Sensitivity is fixed by the analyte’s • Direct analysis (DA) - Sensitivity is fixed by the analyte’s

chemical form following combustion or volatilization.

• DA – based on the residue’s weight following combustion or

volatilization is possible when the residue only contains the

analyte of interest

Cont’

• Indirect analysis, (IA)– sensitivity can be improved by carefully

choosing the conditions for combustion or volatilization so

that the change in mass is as large as possible.

• IA – only feasible when the residue’s change in mass results

from the loss of a single volatile product containing the from the loss of a single volatile product containing the

analyte

Particulate Gravimetry

• Two approaches :

• filtration – solid particulates are separated from their gas,

liquid or solid matrix, and

Extraction – can be extracted from its matrix with a suitable • Extraction – can be extracted from its matrix with a suitable

solvent

Quantitative Applications

• Microbiological testing of water

• Total airborne particulates are determined using a high-

volume air sampler

• Grain size distributions for sediments and soils are used to

determine the amount of sand, silt and clay present in a determine the amount of sand, silt and clay present in a

sample

Evaluating Particulate Gravimetry

• Uses the same balances as other gravimetric methods

• Also capable to achieve similar levels of accuracy and

precision