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Gravimetric and Combustion Gravimetric and Combustion Gravimetric and Combustion Gravimetric and Combustion AnalysisAnalysisCh 7Chapter 7
Department of Chemistry, Ateneo de Manila University 1
GRAVIMETRIC ANALYSISGRAVIMETRIC ANALYSISGRAVIMETRIC ANALYSISGRAVIMETRIC ANALYSISQuantitative technique based on the determination of the mass of a precipitated or volatized of the mass of a precipitated or volatized compound which the analyte is stoichiometricallyrelatedANALYTE: the substance determined in the procedure. It is converted to an insoluble form, collected and weighed on an analytical balance
Department of Chemistry, Ateneo de Manila University 2
Department of Chemistry, Ateneo de Manila University 3
Department of Chemistry, Ateneo de Manila University 4
Department of Chemistry, Ateneo de Manila University 5
Types of Gravimetric Analysis
Precipitation Volatilization
Acids react with carbonates bicarbonates Acids react with carbonates, bicarbonates, sulfites and bisulfites to give CO2 and SO2 gas respectively
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Precipitation is based on solubility rules and more importantly Solubility Product Constants (Ksp)importantly Solubility Product Constants (Ksp)
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Solubility is the maximum amount of solute one can dissolved in a given solvent at a specific temperature
• A saturated solution contains the maximum am nt f s l te that ill diss l e in a i en amount of solute that will dissolve in a given solvent at a specific temperature
i • An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a
f specific temperature• A supersatured solution contains more
solute than is present in a saturated solution at a specific temperature
Solubility is the maximum amount of solute one can dissolved in a given solvent at a specific temperature
• Some ionic salts are NOT COMPLETELY SOLUBLE• Some ionic salts are NOT COMPLETELY SOLUBLE
in water
• These are still STRONG ELECTROLYTES
• Every fraction of the ionic solid that does y
dissolve forms FREE IONS in solution
Solubility is an equilibrium process between an undissolvedsolute and its dissolved form
solute (undissolved) solute (dissolved)
We describe the solubility process in terms of the Ion Product Quotient and the Solubility-Product Constant
K2CrO4(s) 2K+(aq) + CrO4
2–(aq)
Ag2S(s) 2Ag+(aq) + S2–
(aq)
PbI2(s) Pb2+(aq) + 2I– (aq)
Some salts are very soluble in water that it is absurd to talk about Ksp. Some salts are very insoluble that Ksp is too low. Ksp is
important to ionic compounds that are slightly soluble in water.
Some salts are very soluble in water that it is absurd to talk about Ksp. Ksp is important to ionic compounds that are slightly soluble
in water (appears insoluble in our solubility table).
K2CrO4(s) 2K+(aq) + CrO4
–(aq)2 4(s) (aq) 4 (aq)
Ag2S(s) 2Ag+(aq) + S2–
(aq)
PbI2(s) Pb2+(aq) + 2I– (aq)
The magnitude of Ksp is a measure of how far to the right the dissolution proceeds at equilibrium
Name and Formula KspName and Formula sp
aluminum hydroxide, Al(OH)3
cobalt(II) carbonate, CoCO3
3 x 10-34
1.0 x 10-10( ) , 3
iron(II) hydroxide, Fe(OH)2
lead(II) fluoride PbF2
4.1 x 10-15
3 6 x 10-8lead(II) fluoride, PbF2
lead(II) sulfate, PbSO4
3.6 x 10
1.6 x 10-8
4 7 x 10-29mercury(I) iodide Hg I
silver sulfide, Ag2S
i i d Z (IO )
4.7 x 10 29
8 x 10-48
mercury(I) iodide, Hg2I2
3 9 10-6zinc iodate, Zn(IO3)2 3.9 x 10-6
Department of Chemistry, Ateneo de Manila University 16
We can expand the notion of solubility to:
Molar solubility (s) (mol/L) is the number of moles of solute dissolved in 1 L of a saturated solution.
Solubility (g/L) is the number of grams of solute dissolved in 1 L of a saturated solution. Usually volume is 100 mL.
We can expand the notion of solubility to:
Molar solubility (s) (mol/L) is the number of moles of solute dissolved in 1 L of a saturated solution.
Solubility (g/L) is the number of grams of solute dissolved in 1 L of a saturated solution. Usually volume is 100 mL.
Ksp = 1.6 x 10-10
What is the solubility of silver chloride in g/L ?
AgCl (s) Ag+ (aq) + Cl- (aq) sp
We can expand the notion of solubility to:
Molar solubility (s) (mol/L) is the number of moles of solute dissolved in 1 L of a saturated solution.
Solubility (g/L) is the number of grams of solute dissolved in 1 L of a saturated solution. Usually volume is 100 mL.
(a) Lead(II) sulfate is a key component in car batteries. Its solubility in water at 25 oC is 4 25 x 10-3 g/100 mL solution solubility in water at 25 C is 4.25 x 10 g/100 mL solution. What is the Ksp of PbSO4?
(b) When lead(II) fluoride (PbF2) is shaken with pure water at 25 oC, the solubility is found to be 0.64 g/L. Calculate the Ksp of PbF2.
Execises:
• The solubility of lead(II) chloride is 0.45 g/100 mL of solution. What is the Ksp of PbCl2?p
• The solubility of silver chromate is 0.0287 g/1.0 L of solution. What is the Ksp for Ag2CrO4?
• Calculate the molar solubility of silver oxalate, Ag2C2O4, in pure water. Ksp = 1.0 × 10-11
Ksp is important in determining in predicting whether a certain salt solution is unsaturated/saturated/supersaturated
f Dissolution of an ionic solid in aqueous solution:
Compare Q with K
N
Compare Qsp with Ksp
Q = Ksp Saturated solution
Q < Ksp Unsaturated solution No precipitate
No changeQ sp
Q > Ksp Supersaturated solution Precipitate will form
g
Ksp is important in determining whether precipitation will occur if you mix 2 salts at given concentrations
If 2.00 mL of 0.200 M NaOH are added to 1.00 L of 0.100 M CaCl2, will a precipitate form?
Calculate the minimum concentration of Cr3+ that must be added to 0.095 M NaF in order to initiate a precipitate of chromium(III) fluoride. (For CrF3 , Ksp = 6.6 x 10-11.)
The presence of a common ion decreases the solubility of the salt
What is the molar solubility of AgBr in (a) pure water and (b) 0.0010 M NaBr?
What is the common ion effect?
Th ff h This effect occurs when a reactant containing a given ion is added to an equilibrium mixture that already contains that ion, and the position of equilibrium shifts away from forming p f q f y f f gmore of it
The presence of a common ion decreases the solubility of the salt
2+ 2 2+ 2PbCrO4(s) Pb2+(aq) + CrO42-(aq) PbCrO4(s) Pb2+(aq) + CrO4
2-
(aq)
The presence of a common ion decreases the solubility of the salt
Lead(II) iodide, PbI2, is an ionic compound with a ( ) , 2, psolubility product constant Ksp of 7.9 × 10-9. Calculate the solubility of this compound in a. pure water.b. 0.50 mol L-1 KI solution.
Solubility of some salts are pH dependent
Presence of a common ion decreases the solubilityPresence of a common ion decreases the solubility.
Insoluble bases dissolve in acidic solutions.
N t l lt ff t d b H
Mg(OH)2 (s) Mg2+ (aq) + 2OH- (aq)removeadd
Neutral salts are unaffected by pH.
Ksp = [Mg2+][OH-]2 = 1.2 x 10-11At pH less than 10.45
L [OH ] sp [ g ][ ]Ksp = (s)(2s)2 = 4s3
4s3 = 1.2 x 10-11
Lower [OH-]
OH- (aq) + H+ (aq) H2O (l)
s = 1.4 x 10-4 M[OH-] = 2s = 2.8 x 10-4 M
OH 3 55 H 10 45
At pH greater than 10.45
pOH = 3.55 pH = 10.45 Raise [OH-]
Decrease solubility of Mg(OH)2
Solubility (and Ksp) is important in qualitative analysis and selective precipitation
Solubility (and Ksp) is important in qualitative analysis and selective precipitation
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis1. Sample dried, triplicate portions, weighed2 Preparation of the solution
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis
2. Preparation of the solution3. Precipitation4. Digestion4. Digestion5. Filtration6. Washing7. Drying or igniting8. Weighing9 C l l ti9. Calculating
Department of Chemistry, Ateneo de Manila University 30
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis
Department of Chemistry, Ateneo de Manila University 31
Precipitates should be insoluble, be easily filtered, and possess a known, constant compositionand possess a known, constant composition
Colloidal suspensions (not desirable, too small)p ( , )- 1-100 microns in diameter with large surface area- Normally remain suspended, wont settle out
Difficult to near impossible to filter - Difficult to near impossible to filter
Crystalline suspensions (not desirable, too small)- >0.1 mm in diameter
N ll ttl t t l- Normally settle out spontaneously- Readily Filterable
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Precipitate formation is achieved when you have a supersaturated solutionsupersaturated solution
Department of Chemistry, Ateneo de Manila University 33
solute (undissolved) solute (dissolved)
Precipitate formation is achieved when you have a supersaturated solutionsupersaturated solution
A saturated solution contains the maximum amount of solute that will dissolve in a given solvent at a specific temperatureAn unsaturated solution contains less solute than the solvent has the capacity to di l t ifi t tdissolve at a specific temperatureA supersatured solution contains more solute than is present in a saturated solutionsolute than is present in a saturated solutionat a specific temperature
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Precipitate formation is governed by two competing processes: Nucleation and Particle Growthprocesses: Nucleation and Particle Growth
Department of Chemistry, Ateneo de Manila University 35
Precipitate formation is governed by two competing processes: Nucleation and Particle Growthprocesses: Nucleation and Particle Growth
Department of Chemistry, Ateneo de Manila University 36
One parameter that can predict the effect of the two mechanism is Relative Supersaturation (RSS)mechanism is Relative Supersaturation (RSS)
Q = concentration of Q = concentration of precipitate in solution
S = solubility of precipitate
RSS > 1 nucleation is high gives colloidal particlesRSS > 1, nucleation is high, gives colloidal particlesRSS < 1, nucleation is low, gives crystalline suspension
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There are experimental suggestions (“best practices”) to keep RSS lowpractices ) to keep RSS low
Increasing temperature (increases S)Increasing temperature (increases S)Dilute solutions (to minimize Q)Slow addition of precipitating agent with good stiffring (prevents localization of precipitates)pH can also affect solubilityE l i H P i it tiEmploying Homogeneous Precipitation
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There are experimental suggestions (“best practices”) to keep RSS lowpractices ) to keep RSS low
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There are experimental suggestions (“best practices”) to keep RSS lowpractices ) to keep RSS low
Department of Chemistry, Ateneo de Manila University 40
There are experimental suggestions (“best practices”) to keep RSS lowpractices ) to keep RSS low
Increasing temperature (increases S)Increasing temperature (increases S)Dilute solutions (to minimize Q)Slow addition of precipitating agent with good stiffring (prevents localization of precipitates)pH can also affect solubilityE l i H P i it tiEmploying Homogeneous Precipitation
However! Most of the time we usually get colloids However! Most of the time, we usually get colloids… So what do we do then?
Department of Chemistry, Ateneo de Manila University 41
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis1. Sample dried, triplicate portions, weighed2 Preparation of the solution
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis
2. Preparation of the solution3. Precipitation4. Digestion4. Digestion5. Filtration6. Washing7. Drying or igniting8. Weighing9 C l l ti9. Calculating
Department of Chemistry, Ateneo de Manila University 42
Colloidal suspensions can be coagulated and agglomerated to give filterable, amorphous massagglomerated to give filterable, amorphous mass.
Colloids are stable suspensions – thus we must Colloids are stable suspensions thus we must break the stability!
Department of Chemistry, Ateneo de Manila University 43
Colloidal suspensions can be coagulated and agglomerated to give filterable, amorphous massagglomerated to give filterable, amorphous mass.
Colloids are stable suspensions – thus we must Colloids are stable suspensions thus we must break the stability!
Department of Chemistry, Ateneo de Manila University 44
Colloidal suspensions can be coagulated and agglomerated to give filterable, amorphous massagglomerated to give filterable, amorphous mass.
Colloids are stable suspensions – thus we must Colloids are stable suspensions thus we must break the stability!
Heating and stirring dislodges the ions from the double layerEl l i l i l h i i h Electrolytes in solution lower the ionic atmosphere radius.
We call this process: Digestion
Department of Chemistry, Ateneo de Manila University 45
Co-precipitation is also another problem.
Coprecipitation – where other compounds are Coprecipitation where other compounds are removed from the solution during precipitation.
Surface AdsorptionOcclusionM h i l E t tMechanical EntrapmentMixed-crystal formation
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Particles can adsorb to the surface of crystals, and more so in coagulated colloidsmore so in coagulated colloids
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Ions become trapped in the crystal if growth is too rapidrapid.
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Ions become trapped in the crystal if growth is too rapidrapid.
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Compounds with similar Ksps may also precipitate out with our desired compoundout with our desired compound
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Co-precipitation is also another problem.
Coprecipitation – where other compounds are removed from the solution during precipitation.removed from the solution during precipitation.
Surface AdsorptionOcclusionMechanical EntrapmentMi d t l f tiMixed-crystal formation
SLOW THINGS DOWN! SLOW THINGS DOWN! Use dilute solutions, with heatingDigestion and Ageing may also help
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g g g y p
Effects of other interferents can be reduced by using MASKING AGENTSMASKING AGENTS
Solid precipitates out…
Complex ion stays in solution…
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Steps in Gravimetric AnalysisSteps in Gravimetric Analysis1. Sample dried, triplicate portions, weighed2 Preparation of the solution
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis
2. Preparation of the solution3. Precipitation4. Digestion4. Digestion5. Filtration6. Washing7. Drying or igniting8. Weighing9 C l l ti9. Calculating
Department of Chemistry, Ateneo de Manila University 53
Filtration is carried out using Vacuum Filtration.
Coagulated colloids dissolve when washed with water (called Peptization).
Department of Chemistry, Ateneo de Manila University 54
( p )
Coagulated colloids dissolve when washed with water (called Peptization)water (called Peptization).
Peptization can be minimized by washing with dilute p y gsolution of a volatile solvent (ex. HNO3, HCl)
Wash with HNO3 instead of NaNO3
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Steps in Gravimetric AnalysisSteps in Gravimetric Analysis1. Sample dried, triplicate portions, weighed2 Preparation of the solution
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis
2. Preparation of the solution3. Precipitation4. Digestion4. Digestion5. Filtration6. Washing7. Drying or igniting8. Weighing9 C l l ti9. Calculating
Department of Chemistry, Ateneo de Manila University 56
Precipitates must have a known stable composition However, some are reactive in aircomposition… However, some are reactive in air.
Heating at high temp = IGNITION…
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Getting the Mass of the precipitate is the last step!
Department of Chemistry, Ateneo de Manila University 58
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis1. Sample dried, triplicate portions, weighed2 Preparation of the solution
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis
2. Preparation of the solution3. Precipitation4. Digestion4. Digestion5. Filtration6. Washing7. Drying or igniting8. Weighing9 C l l ti9. Calculating
Department of Chemistry, Ateneo de Manila University 59
Calculations in Calculations in GravimetryGravimetryIn an analysis Cl2 gas in a sample was converted to
Cl- and precipitated as AgCl The mass of the
Calculations in Calculations in GravimetryGravimetry
Cl and precipitated as AgCl. The mass of the precipitate is 1.00 g. How many grams of Cl2 was in the sample?in the sample?
Department of Chemistry, Ateneo de Manila University 60
Calculations in Calculations in GravimetryGravimetryIn one experiment, 0.3126 g of the sample was
dissolved in 25 mL of acetone and 1 mL of acetic
Calculations in Calculations in GravimetryGravimetry
dissolved in 25 mL of acetone and 1 mL of acetic acid was added. After 5 min, the precipitate was filtered, washed with acetone, dried at 110 degC, filtered, washed with acetone, dried at 110 degC, and found to weight 0.7121 g. What is the mass % of piperazine in the sample?
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Gravimetric Factor (GF)Gravimetric Factor (GF)Grams A (analyte) = Grams B (precipitate) x GF
Gravimetric Factor (GF)Gravimetric Factor (GF)
Department of Chemistry, Ateneo de Manila University 62
Calculations in Calculations in GravimetryGravimetryThe calcium in a 200.0-mL sample of a natural water
was determined by precipitating the cation as
Calculations in Calculations in GravimetryGravimetry
was determined by precipitating the cation as CaC2O4. The precipitate was filtered, washed and ignited in a crucible with an empty mass of and ignited in a crucible with an empty mass of 26.6002 g. The mass of the crucible plus CaO(molar mass = 56.077g/mol) was 26.7134 g. Calculate the concentration of Ca (molar mass = 40.078 g/mol) in the water in units of grams per 100 L100 mL.
Department of Chemistry, Ateneo de Manila University 63
Calculations in Calculations in GravimetryGravimetryA 0.7406 g sample of impure MgCO3 was
decomposed with HCl releasing 0 1881 g of
Calculations in Calculations in GravimetryGravimetry
decomposed with HCl, releasing 0.1881 g of carbon dioxide. Calculate the % Mg in the original sample.original sample.
How much CaO (in grams) can be produced from 1.500 g of calcium carbonate?
Department of Chemistry, Ateneo de Manila University 64
Calculations in Calculations in GravimetryGravimetryAn iron ore was analyzed by dissolving a 1.1324-g
sample in concentrated HCl The resulting
Calculations in Calculations in GravimetryGravimetry
sample in concentrated HCl. The resulting solution was diluted with water, and the iron(III) was precipitated as the hydrous oxide was precipitated as the hydrous oxide Fe2O3•xH2O by the addition of NH3.
After filtration and washing, the residue was ignited at a high temperature to give 0.5394 g of pure Fe2O3 (molar mass =159.69 g/mol). Calculate (a) th %F ( l = 55 847 / l) d (b) th the %Fe (molar mass = 55.847 g/mol) and (b) the % Fe3O4 (231.54 g/mol) in the sample
Department of Chemistry, Ateneo de Manila University 65
Calculations in Calculations in GravimetryGravimetryA certain barium halide exists as the hydrated salt
BaX 2H O where X is an unknown halogen A
Calculations in Calculations in GravimetryGravimetry
BaX2.2H2O, where X is an unknown halogen. A sample of the halide hydrate (0.2650 g) was dissolved in water in 200 cm3 and excess sulfuric dissolved in water in 200 cm and excess sulfuric acid added. The mixture was then digested and held at boiling for 45 minutes. The precipitate was filtered off, washed and dried. The mass of precipitate obtained = 0.2533 g. Determine the id tit f Xidentity of X.
Department of Chemistry, Ateneo de Manila University 66
Calculations in Calculations in GravimetryGravimetryA 0.3516 g sample of a commercial phosphate
detergent was ignited at a red heat to destroy the
Calculations in Calculations in GravimetryGravimetry
g g yorganic matter. The residue was then taken up in hot HCl, which converted the P to H3PO4. The phosphate was precipitated as MgNH4PO46H2O phosphate was precipitated as MgNH4PO46H2O by addition of Mg2+ followed by aqueous NH3. After being filtered and washed, the precipitate was converted to Mg2P2O7 (FW= 222 57) by was converted to Mg2P2O7 (FW 222.57) by ignition at 1000 degree Celsius. This residue weighted 0.2161 g. Calculate the percent P (FW = 30 974) in the sample 30.974) in the sample.
Department of Chemistry, Ateneo de Manila University 67
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis1. Sample dried, triplicate portions, weighed2 Preparation of the solution
Steps in Gravimetric AnalysisSteps in Gravimetric Analysis
2. Preparation of the solution3. Precipitation4. Digestion4. Digestion5. Filtration6. Washing7. Drying or igniting8. Weighing9 C l l ti9. Calculating
Department of Chemistry, Ateneo de Manila University 68
Summary of MethodSummary of Method1. Gravimetry is slow – Lots of waiting but
little effort
Summary of MethodSummary of Method
little effort2. Inexpensive – you just need a good balance
and an ovenand an oven3. Accurate and Precise4. No calibrations needed… moles is related
to MW.
Department of Chemistry, Ateneo de Manila University 69