Gravimetric methods of analysis [compatibility mode]
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Transcript of Gravimetric methods of analysis [compatibility mode]
Dr. Jehad M DiabFaculty of pharmacyDamascus University
Gravimetric Analysisالتحلیل الوزني
Pharma.analytical chemistry II
Gravimetric Analysis
Gravimetry is the Quantitative measurement of theanalyte by weighing a pure solid form of theprecipitate.
Gravimetric Analysis is one of the most accurateand precise methods of macro-quantitative analysis.
Often required for high precision
Obtaining pure solids from solutions containingan unknown amount of a metal ion is done byprecipitation. Dr.Jehad Diab
Electrogravimetric methods. analyte isprecipitated on cathode as metal or on anodeas metal oxide Dr.Jehad Diab
طرائق الترسیب
طرائق التطایر
طرائق التحلیل الوزني الكھربائي
and wash
and calculateDry and ignite
precipitating
Precipitation steps in gravimetry
Dr.Jehad Diab
مراحل التحلیل الوزني
Dr.Jehad Diab
خواص الرواسب
Mechanisms of precipitation1.Saturation: the amount of salt dissolved ismaximum under certain condition of pressure andtemperature
2.Supersaturation: the amount of salt is larger than atsaturation step, turbid solution
3.nucleation: when a small number of ions ,atoms,molecules initially unite either spontaneous orinduced resulting in very small aggregates of asolid during precipitation.
4.Particle growth: the three dimensional growth ofparticle nucleus converted into a larger crystal
Dr.Jehad Diab آلیات الترسیب
اإلشباع
فوق اإلشباع
التنوي
النمو الجزیئ
Dr.Jehad Diab
Dr.Jehad Diab
Particle size and filterabilityVon 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 itsequilibrium solubility)
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Proportional to RSS
Inversely Proportional to RSS
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pH control of precipitation
Ca2+ + C2O42- CaC2O4 (s)
H2C2O4 2 H+ + C2O42-
Feeder Reaction:
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Weak basic medium
10-6 - 10-4 mm
10-1 – 10 mm
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Primary adsorbed layer
AgNO3 + NaCl →AgCl↓ + NaNO3
reagent analyte
(Secondaryadsorbedlayer)
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Dr.Jehad Diab
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coagulationpeptization
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adsorbed
adsorbed
The result is coagulation of colloidal precipitate
of adsorbed
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Colloids particles must collide with one another tocoalesce. However ,the negatively charged ionicatmospheres of particles repel one another. the particles,therefore ,must have enough kinetics energy to overcomeelectrostatic repulsion before the can coalesce. Heatpromotes 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) decreasesthe volume of the ionic atmosphere and allows particles tocome closer together before electrostatic repulsion becomesignificant. For this reason .most gravimetric precipitations aredone in the presence of an electrolyte.
High Electrolyte Concentrationto Aid Precipitation
Excess charge on colloid creates ionicatmosphere around particle
D.C. Harris, Quantitative Chemical Analysis, 6th Ed., p686
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Washing a colloid with water to remove excess counterion or trapped impurities can result in peptization.
معالجة الببتزة او التشبغر
Use a volatile electrolyte
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Digestion and aging
,to reduce impurities present and largerparticles obtained
Or more
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التھضیم والتعتیق
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)التھضیم أو نضوج استوالد
Dr.Jehad Diab
الرواسب البلوریة
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(inclusion)
Sources of Coprecipitation
(trapped impurities)
adsorption
(interferences)
Post precipitation الترسیب التالي: Sometimes a precipitate standingin contact with the mother liquor becomes contaminated by theprecipitation of an impurity on top of the desired precipitate.
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الترسیب المشترك
اإلمتزاز السطحياإلحتباس
اإلحتواء
adsorbed
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اإلمتزاز السطحي
()l
silver and nitrate ions, soAgNO3 is coprecipitated with the AgCl.
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Coprecipitation error: negative or positive errorsEx: (a). in Cl analysis, colloidal AgCl + AgNO3 → (+) error(b). in Ba2+ → BaSO4 analysisif Ba(NO3)2 (larger FW than BaSO4) → (+) errorif BaCl2 ( FW: BaCl2 < BaSO4) → (-) error
Dealing with surface adsorption
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Dealing with surface adsorption
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Occlusion
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اإلحتباس
Occlusion
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Occlusion
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dDetermination of NH4+ as NH4MgPO4 , K+ will cause a mixed
crystal containing KMgPO4
(inclusion اإلحتواء)
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Dr.Jehad Diab
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Homogeneous precipitation. The best precipitates are obtained withhomogeneous precipitation, in which theprecipitating reagent is gradually generated in thesample solution, through a slow chemical reaction.
In this way there is never a large excess ofreagent, so that nucleation is slow, giving allexisting nuclei plenty of time to grow.
The precipitating reagent is usually formed bythe slow hydrolysis of an organic compound atelevated temperature, but even synthesis ispossible, as in the generation of dimethylglyoxime
Dr.Jehad Diab
الترسیب المتجانس
Methods have been worked out to generate avariety of precipitants, such as hydroxide, sulfide,sulfate, phosphate, oxalate, 8-hydroxyquinoline,and chromate.
Both the temperature and the pH must becontrolled, because both usually affect the rates ofthe hydrolysis reaction. Moreover, as we saw in thepH is often crucial in the formation of theprecipitate.
Homogeneous precipitation
Dr.Jehad Diab
Solid formed by homogeneous precipitation are generally purer and moreeasily filtered than precipitate generated by direct addition of a reagent tothe 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
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∆
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+ + PO43- Zr
Dimethyl oxalate CH30.CO.CO.OCH3 + 2H20 → 2CH30H + 2H + + C2042- Ca, Mg, Zn
8-acetoxyquinoline + H20 → CH3C02H + 8-hydroxyquinoline AI, Mg, U, Zn
2Cr3+ + BrO3- + 5H20 → Br- + l0H+ + 2CrO42- Pb
Biacetyl plus hydroxylamine CH3·CO.CO.CH3 + NH20H → dimethylglyoxime + 2H20 Ni
Table. Some common reactions for homogeneous precipitationreagent generating reaction used to precipitate
Dr.Jehad Diab
Dr.Jehad Diab
تجفیف الراسب
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250 oC 600 oC
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Dr.Jehad Diab
الحسابات في التحلیل الوزني
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Dr.Jehad Diab
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 interm 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 gravimetricfactors in term of symbols for thedetermination 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 interm 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)2Dr.Jehad Diab
Problem : Determine the gravimetric factors for thedetermination 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 GFDr.Jehad Diab
10.
Example: in an organic sample (0.352g) phosphorouswas 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 potassiumchloride (0.4500g) was dissolved in water andtreated with an excess of silver nitrate, 0.8402 g ofsilver chloride was precipitated. Calculate thepercentage 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
Dr.Jehad Diab
Mainly Al and Mg
Dr.Jehad Diab
Dr.Jehad Diab
Organic reagents
Table: some of inorganic precipitating agentsprecipitants 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 agentsprecipitants 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
-------------------------------------------------------------------------------------------------------* After drying or ignitionFe3+ + OH-→Fe(OH)3 → Fe2O3
weighed formppt
Mainly Mg and AL
بعض عوامل الترسیب : جدولالعضویة
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 + nClDr.Jehad Diab
Ca2+→ CaC2O4 → CaO
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Dr.Jehad Diab
Dr.Jehad Diab
القیاس الوزني الكھربائي
•used for electroplating, extraction and purification ofof the analyte
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كمون التفكك
current-voltage relation for electrolysis .
Cu2+ +2e => Cu
Ag(CN)2- + e => Ag +2CN-
Pb2+ +2H2O => PbO2 + 4H+ +2e
Dr.Jehad Diab
Dr.Jehad Diabفاراداي ویقدر بالكولوم1= 96500
CU2+Determine the number of grams of Cu2+ thatcould be deposited on cathode as Cu,if currentof 6 amperes is applied for 5 minutes
Dr.Jehad Diab
Dr.Jehad Diab
Dr.Jehad Diab
RevisionDiminishing importance of gravimetry, as a resultof the development of much faster (though usuallyless precise) instrumental methods.
Consequently, gravimetry is used only when itssuperior precision is really needed. In that case,great care must be exercised to avoidcoprecipitation of other sample components,occlusion of solvent in the precipitate, andadsorption of excess reagent, otherwise the extraeffort is negated by an impure or otherwise poorlydefined precipitate
Dr.Jehad Diab
We have emphasized the factors that facilitate theformation of coarse, pure, easily filterableprecipitates, explained why precipitates arepreferably generated homogeneously, and why theyare usually washed with electrolyte solutions ratherthan with water.
Ammonium salts are often used for this purpose,because they will readily volatilize upon subsequentheating of the precipitate.
Dr.Jehad Diab
The End
Dr.Jehad Diab