Gravimetric and Combustion Analysis - Kimika · yQuantitative technique based on the determination ... lead(II) sulfate, PbSO 4 3.6 x 10 ... Steps in Gravimetric Analysis 1. Sample

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





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


Precipitation is based on solubility rules and more importantly Solubility Product Constants (Ksp)importantly Solubility Product Constants (Ksp)


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


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




(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


2+ 2 2+ 2PbCrO4(s) Pb2+(aq) + CrO42-(aq) PbCrO4(s) Pb2+(aq) + CrO4

2-

(aq)


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




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


Precipitate formation is achieved when you have a supersaturated solutionsupersaturated solution


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


Precipitate formation is governed by two competing processes: Nucleation and Particle Growthprocesses: Nucleation and Particle Growth


Precipitate formation is governed by two competing processes: Nucleation and Particle Growthprocesses: Nucleation and Particle Growth


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


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







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?






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


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


Particles can adsorb to the surface of crystals, and more so in coagulated colloidsmore so in coagulated colloids


Ions become trapped in the crystal if growth is too rapidrapid.


Ions become trapped in the crystal if growth is too rapidrapid.


Compounds with similar Ksps may also precipitate out with our desired compoundout with our desired compound


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


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…






Filtration is carried out using Vacuum Filtration.

Coagulated colloids dissolve when washed with water (called Peptization).


( 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






Precipitates must have a known stable composition However, some are reactive in aircomposition… However, some are reactive in air.

Heating at high temp = IGNITION…


Getting the Mass of the precipitate is the last step!






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?


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


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?


Gravimetric Factor (GF)Gravimetric Factor (GF)Grams A (analyte) = Grams B (precipitate) x GF

Gravimetric Factor (GF)Gravimetric Factor (GF)


Calculations in Calculations in GravimetryGravimetryThe calcium in a 200.0-mL sample of a natural water

was determined by precipitating the cation as


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.


Calculations in Calculations in GravimetryGravimetryA 0.7406 g sample of impure MgCO3 was

decomposed with HCl releasing 0 1881 g of


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?


Calculations in Calculations in GravimetryGravimetryAn iron ore was analyzed by dissolving a 1.1324-g

sample in concentrated HCl The resulting


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


Calculations in Calculations in GravimetryGravimetryA certain barium halide exists as the hydrated salt

BaX 2H O where X is an unknown halogen A


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.


Calculations in Calculations in GravimetryGravimetryA 0.3516 g sample of a commercial phosphate

detergent was ignited at a red heat to destroy the


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.






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.


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Gravimetric and Combustion Analysis - Kimika · yQuantitative technique based on the determination ... lead(II) sulfate, PbSO 4 3.6 x 10 ... Steps in Gravimetric Analysis 1. Sample