Gravimetric methods of analysis [compatibility mode]

Dr. Jehad M Diab
Faculty of pharmacy
Damascus University
Gravimetric Analysis
‫اﻟوزﻧﻲ‬ ‫اﻟﺗﺣﻠﯾل‬
Pharma.analytical chemistry II

Gravimetric Analysis
Gravimetry is the Quantitative measurement of the
analyte by weighing a pure solid form of the
precipitate.
Gravimetric Analysis is one of the most accurate
and precise methods of macro-quantitative analysis.
Often required for high precision
Obtaining pure solids from solutions containing
an unknown amount of a metal ion is done by
precipitation. Dr.Jehad Diab

Electrogravimetric methods. analyte is
precipitated on cathode as metal or on anode
as metal oxide Dr.Jehad Diab
‫اﻟﺗرﺳﯾب‬ ‫طراﺋق‬
‫اﻟﺗطﺎﯾر‬ ‫طراﺋق‬
‫اﻟﻛﮭرﺑﺎﺋﻲ‬ ‫اﻟوزﻧﻲ‬ ‫اﻟﺗﺣﻠﯾل‬ ‫طراﺋق‬

and wash
and calculate
Dry and ignite
precipitating
Precipitation steps in gravimetry
Dr.Jehad Diab
‫اﻟوزﻧﻲ‬ ‫اﻟﺗﺣﻠﯾل‬ ‫ﻣراﺣل‬

Dr.Jehad Diab
‫اﻟرواﺳب‬ ‫ﺧواص‬

Mechanisms of precipitation
1.Saturation: the amount of salt dissolved is
maximum under certain condition of pressure and
temperature
2.Supersaturation: the amount of salt is larger than at
saturation step, turbid solution
3.nucleation: when a small number of ions ,atoms,
molecules initially unite either spontaneous or
induced resulting in very small aggregates of a
solid during precipitation.
4.Particle growth: the three dimensional growth of
particle nucleus converted into a larger crystal
Dr.Jehad Diab
‫اﻟﺗرﺳﯾب‬ ‫آﻟﯾﺎت‬
‫اﻹﺷﺑﺎع‬
‫اﻹﺷﺑﺎع‬ ‫ﻓوق‬
‫اﻟﺗﻧوي‬
‫اﻟﺟزﯾﺊ‬ ‫اﻟﻧﻣو‬

Particle size and filterability
Von Weimarn described an equation that can control
the particle size of ppt; named Von Weimarn ratio
• RSS= Q-S/S (Rss = Relative supersaturation, Q is
concentration of the solute at any instant. S is its
equilibrium solubility)
Dr.Jehad Diab

Proportional to RSS
Inversely Proportional to RSS
Dr.Jehad Diab

pH control of precipitation
Ca2+ + C2O4
2-  CaC2O4 (s)
H2C2O4  2 H+ + C2O4
2-
Feeder Reaction:
Dr.Jehad Diab
Weak basic medium

10-6 - 10-4 mm
10-1 – 10 mm
Dr.Jehad Diab

Primary adsorbed layer
AgNO3 + NaCl →AgCl↓ + NaNO3
reagent analyte
(Secondary
adsorbed
layer)
Dr.Jehad Diab

Dr.Jehad Diab
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coagulation
peptization
Dr.Jehad Diab

adsorbed
adsorbed
The result is coagulation of colloidal precipitate
of adsorbed
Dr.Jehad Diab
Colloids particles must collide with one another to
coalesce. However ,the negatively charged ionic
atmospheres of particles repel one another. the particles
,therefore ,must have enough kinetics energy to overcome
electrostatic repulsion before the can coalesce. Heat
promotes coalescence by increasing the kinetic energy .
And coagulation of colloidal precipitate is resulted in.

And coagulation of colloidal precipitate is resulted in.
Dr.Jehad Diab
increasing electrolyte concentration (HNO3 for AgCl) decreases
the volume of the ionic atmosphere and allows particles to
come closer together before electrostatic repulsion become
significant. For this reason .most gravimetric precipitations are
done in the presence of an electrolyte.

High Electrolyte Concentration
to Aid Precipitation
Excess charge on colloid creates ionic
atmosphere around particle
D.C. Harris, Quantitative Chemical Analysis, 6th Ed., p686

Dr.Jehad Diab
Washing a colloid with water to remove excess counter
ion or trapped impurities can result in peptization.
‫اﻟﺗﺷﺑﻐر‬ ‫او‬ ‫اﻟﺑﺑﺗزة‬ ‫ﻣﻌﺎﻟﺟﺔ‬

Use a volatile electrolyte
Dr.Jehad Diab

Digestion and aging
,to reduce impurities present and larger
particles obtained
Or more
Dr.Jehad Diab
‫واﻟﺗﻌﺗﯾق‬ ‫اﻟﺗﮭﺿﯾم‬

Fig. 10.1. Ostwald ripening.
During digestion at elevated temperature:
Small particles tend to dissolve and reprecipitate on larger ones.
Individual particles agglomerate.
Adsorbed impurities tend to go into solution.
During digestion at elevated temperature:
Small particles tend to dissolve and reprecipitate on larger ones.
Individual particles agglomerate.
Adsorbed impurities tend to go into solution.
©Gary Christian,
Analytical Chemistry,
6th Ed. (Wiley)
‫اﺳﺗواﻟد‬ ‫ﻧﺿوج‬ ‫أو‬ ‫اﻟﺗﮭﺿﯾم‬

Dr.Jehad Diab
‫اﻟﺑﻠورﯾﺔ‬ ‫اﻟرواﺳب‬
-----------------------------
-----------------------------------------------------------------------
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(inclusion)
Sources of Coprecipitation
(trapped impurities)
adsorption
(interferences)
Post precipitation ‫اﻟﺗﺎﻟﻲ‬ ‫:اﻟﺗرﺳﯾب‬ Sometimes a precipitate standing
in contact with the mother liquor becomes contaminated by the
precipitation of an impurity on top of the desired precipitate.
Dr.Jehad Diab
‫اﻟﻣﺷﺗرك‬ ‫اﻟﺗرﺳﯾب‬
‫اﻟﺳطﺣﻲ‬ ‫اﻹﻣﺗزاز‬
‫اﻹﺣﺗﺑﺎس‬
‫اﻹﺣﺗواء‬

adsorbed
Dr.Jehad Diab
‫اﻟﺳطﺣﻲ‬ ‫اﻹﻣﺗزاز‬
()l
silver and nitrate ions, so
AgNO3 is coprecipitated with the AgCl.

Dr.Jehad Diab
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Coprecipitation error: negative or positive errors
Ex: (a). in Cl analysis, colloidal AgCl + AgNO3 → (+) error
(b). in Ba2+ → BaSO4 analysis
if Ba(NO3)2 (larger FW than BaSO4) → (+) error
if BaCl2 ( FW: BaCl2 < BaSO4) → (-) error

Dealing with surface adsorption
Dr.Jehad Diab

Dealing with surface adsorption
Dr.Jehad Diab
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Occlusion
Dr.Jehad Diab
‫اﻹﺣﺗﺑﺎس‬

Occlusion
Dr.Jehad Diab
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dDetermination of NH4
+ as NH4MgPO4 , K+ will cause a mixed
crystal containing KMgPO4
(inclusion ‫)اﻹﺣﺗواء‬
Dr.Jehad Diab

Dr.Jehad Diab
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Homogeneous precipitation.
 The best precipitates are obtained with
homogeneous precipitation, in which the
precipitating reagent is gradually generated in the
sample solution, through a slow chemical reaction.
 In this way there is never a large excess of
reagent, so that nucleation is slow, giving all
existing nuclei plenty of time to grow.
The precipitating reagent is usually formed by
the slow hydrolysis of an organic compound at
elevated temperature, but even synthesis is
possible, as in the generation of dimethylglyoxime
Dr.Jehad Diab
‫اﻟﻣﺗﺟﺎﻧس‬ ‫اﻟﺗرﺳﯾب‬

Methods have been worked out to generate a
variety of precipitants, such as hydroxide, sulfide,
sulfate, phosphate, oxalate, 8-hydroxyquinoline,
and chromate.
Both the temperature and the pH must be
controlled, because both usually affect the rates of
the hydrolysis reaction. Moreover, as we saw in the
pH is often crucial in the formation of the
precipitate.
Homogeneous precipitation
Dr.Jehad Diab
Solid formed by homogeneous precipitation are generally purer and more
easily filtered than precipitate generated by direct addition of a reagent to
the analyte solution.

3
Preparation of NH4OH as precipitant by hydrolysis of urea
Al(OH)3,Fe(OH)3,Sn(OH)4,Bi(OH)3,
Th(OH)4
Dr.Jehad Diab
∆

thioacetamide CH2·CS.NH2 + H20 → CH2·CO.NH2 + H2S Cd, Cu, Mo, Sb
urea (NH2)2CO + 3H20 → CO2 + 2NH4
+ + 20H- AI, Bi, Ga, Fe, Sn, Th
Sulfamic acid NH2S03H + H20 → NH4
+ + H + + SO4
2+ Ba, Ca, Pb, Sr
Trimethyle phosphate (CH30)3PO + 3H20 → 3CH30H + 3H+ + PO4
3- Zr
Dimethyl oxalate CH30.CO.CO.OCH3 + 2H20 → 2CH30H + 2H + + C204
2- Ca, Mg, Zn
8-acetoxyquinoline + H20 → CH3C02H + 8-hydroxyquinoline AI, Mg, U, Zn
2Cr3+ + BrO3- + 5H20 → Br- + l0H+ + 2CrO4
2- Pb
Biacetyl plus hydroxylamine CH3·CO.CO.CH3 + NH20H → dimethylglyoxime + 2H20 Ni
Table. Some common reactions for homogeneous precipitation
reagent generating reaction used to precipitate
Dr.Jehad Diab

Dr.Jehad Diab
‫اﻟراﺳب‬ ‫ﺗﺟﻔﯾف‬
--------------------------------------------------------

Dr.Jehad Diab
‫اﻟوزﻧﻲ‬ ‫اﻟﺗﺣﻠﯾل‬ ‫ﻓﻲ‬ ‫اﻟﺣﺳﺎﺑﺎت‬
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Gravimetric methods of analysis [compatibility mode]

Gravimetric calculations
f.wt analyte(g/mol) a (mol analyte)
gf = ------------------------- × ------------------------
f.wt ppt(g/mol) b(mol ppt)
gf = g analyte / g ppt
g analyte = g ppt × gf
% analyte =( g analyte / g sample) × 100
% analyte = (g ppt × GF) / g sample) ×100% analyte = (g ppt × GF) / g sample) ×100
Dr.Jehad Diab

 Cl2 → AgCl(s)
GF=1/2 × Cl2 /AgCl = Cl2 / 2AgCl
=35.5*2/2(108+35.5)= 0.25
 AlCl3 → AgCl(s)
GF=1/3 × AlCl3 / AgCl = AlCl3 / 3AgCl
 I → Hg5(IO6)2
GF=2I/ Hg5(IO6)2
Determine GF:
Dr.Jehad Diab

Problems: Calculate the mass of analyte in
term of grams to each gram of ppt for the
following analytes:
Analyte ppt
 P(31g) → Ag3po4 (711g )
 K2HPO4(136g) → Ag3po4 (711g )
 Bi2S3 (514 g) → BaSO4 (233 g)
g analyte = g ppt × gf
g p =g Ag3PO4 ×GF= 1 ×31 / 711=0.044 g p/1g ppt
Dr.Jehad Diab

Problem : Determine the gravimetric
factors in term of symbols for the
determination of:
AnalyteAnalyte pptppt G.F.G.F.
CaCaOO CaCaCOCO33 CaOCaO/CaCO/CaCO33
FeFeSS BaBaSSOO44 FeSFeS/BaSO/BaSO44
UUOO22(NO(NO33))22 UU33OO88 33UOUO22(NO(NO33))22/U/U33OO88
CrCr22OO33 AgAg22CrCrOO44 CrCr22OO33//22AgAg22CrOCrO44
Dr.Jehad Diab

Problem : Determine the gravimetric factors in
term of symbols for the determination of:
(a) Aluminum as its hydroxyquinolate,
AI(C9H60N)3 (Al AI(C9H60N)3)
(b) Phosphorus as phosphomolybdic anhydride,
P2Mo24078 (P P2Mo24078 )
(c) Potassium as its chloroplatinate
K → K2PtCl6
(d) Sulfur as barium sulfate,( S → BaS04)
(e) Nickel as nickel dimethylglyoxime,
( Ni → Ni(C4H702N2)2
Dr.Jehad Diab

Problem : Determine the gravimetric factors for the
determination of:
1. In -> In2O3 GF
2. HgO -> Hg5(IO6)2 GF
3. K3PO4 -> K2PtCl6 GF
4. K3PO4 -> Mg2P2O7 GF
5. AgIO3 -> Ba(IO3)2 GF
6. AgNO3 -> AgIO3 GF
7. (NH4)2SO4 -> BaSO4 GF
8. Mn3O4 -> MnO2 GF
9. Cu2HgI4 -> Cu GF
Cr -> Cr203 GF
Dr.Jehad Diab
10.

Example: in an organic sample (0.352g) phosphorous
was dissolved and converted to Mg2P2O7 precipitate
(0.223 g). Calculate the percentage %P in the original
sample.
Dr.Jehad Diab
2P(gfw 31 g) → Mg2P2O7 (gfw 222.6)
GF=2× 31 /222.6 =0.2783
% analyte =(g ppt × GF / g sample) × 100
%P =( 0.223 × 0.2783 / 0.352) ×100 = 17.1 %

Example: When an sample of impure potassium
chloride (0.4500g) was dissolved in water and
treated with an excess of silver nitrate, 0.8402 g of
silver chloride was precipitated. Calculate the
percentage KCl in the original sample.
Answer:
KCl ( gfw =74.50) => AgCl ( gfw =143.50)
GF= 74.50/143.50=0.519
%KCl = (mass of AgCl *GF/mass of KCl) *100=
=( 0.8402*0.519/0.4500 )*100 = 96.90%
Dr.Jehad Diab

Mainly Al and Mg
Dr.Jehad Diab

Dr.Jehad Diab
Organic reagents

Table: some of inorganic precipitating agents
precipitants analyte and (formed precipitate, weighed)*
 NH3(aq) Be (BeO),Al (Al2O3),Cr(Cr2O3), Fe(Fe2O3)
(NH4OH) Sn (SnO2),Zr (ZrO2)
 H2S Zn (ZnS→ZnO),As (As2S3 →As2O3 Or
 As2O5),Bi (Bi2S3
 (NH4)2HPO4 Mg(Mg2P2o7),Zn(Zn2P2O7),Cd(Cd2P2O7)
 H2SO4 Sr,Cd,Pb,Ba (all as sulphate)
 H2PtCl6 K (K2PtCl6)
 HCl Ag (AgCl), Hg(Hg2Cl2)
AgNO3 C l(AgCl), Br (AgBr), I (AgI)
Dr.Jehad Diab

Table: some of inorganic precipitating agents
precipitants analyte ( formed precipitate, weighed)*
 BaCl2 SO4
2- (BaSO4)
( NH4)2S Hg (HgS)
 HNO3 Sn4+ (SnO2)
 H5IO6 Hg (Hg5(IO6)2
 NaCl, Pb(NO3)2 F (PbClF)
 MgCl2,NH4Cl PO4
3- (Mg2P2O7)
Ca2+ H2C2O4 CaCO3 or CaO
Dr.Jehad Diab
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* After drying or ignition
Fe3+ + OH-→Fe(OH)3 → Fe2O3
weighed formppt

Mainly Mg and AL
‫ﺟدول‬:‫اﻟﺗرﺳﯾب‬ ‫ﻋواﻣل‬ ‫ﺑﻌض‬
‫اﻟﻌﺿوﯾﺔ‬
Cs+
Mn+ +nHR→ MRn + nH+
M2+ +2HR →MR2 + 2H+
2
, Ag+ , Cu+
M+ +NaR →MR + Na+
Dr.Jehad Diab
interfere

Mn+ +nNH4R -> MRn + nNH4
+
M2+ +H2R →MR + 2H+
An- + nRCl -> RnA + nCl
Dr.Jehad Diab

Dr.Jehad Diab
‫اﻟﻛﮭرﺑﺎﺋﻲ‬ ‫اﻟوزﻧﻲ‬ ‫اﻟﻘﯾﺎس‬
•used for electroplating, extraction and purification of
of the analyte

‫اﻟﺗﻔﻛك‬ ‫ﻛﻣون‬
current-voltage relation for electrolysis .

Cu2+ +2e => Cu
Ag(CN)2
- + e => Ag +2CN-
Pb2+ +2H2O => PbO2 + 4H+ +2e
Dr.Jehad Diab

Dr.Jehad Diab
96500=1‫ﺑﺎﻟﻛوﻟوم‬ ‫وﯾﻘدر‬ ‫ﻓﺎراداي‬

CU2+
Determine the number of grams of Cu2+ that
could be deposited on cathode as Cu,if current
of 6 amperes is applied for 5 minutes
Dr.Jehad Diab

Revision
Diminishing importance of gravimetry, as a result
of the development of much faster (though usually
less precise) instrumental methods.
Consequently, gravimetry is used only when its
superior precision is really needed. In that case,
great care must be exercised to avoid
coprecipitation of other sample components,
occlusion of solvent in the precipitate, and
adsorption of excess reagent, otherwise the extra
effort is negated by an impure or otherwise poorly
defined precipitate
Dr.Jehad Diab

We have emphasized the factors that facilitate the
formation of coarse, pure, easily filterable
precipitates, explained why precipitates are
preferably generated homogeneously, and why they
are usually washed with electrolyte solutions rather
than with water.
 Ammonium salts are often used for this purpose,
because they will readily volatilize upon subsequent
heating of the precipitate.
Dr.Jehad Diab

Gravimetric methods of analysis [compatibility mode]

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