Gravimetric AnalysisBy: Dr. O. Rajabi (Pharm.D.- Ph.D.)By: Dr. O. Rajabi (Pharm.D.- Ph.D.)

Associate Professor of ChemistryAssociate Professor of ChemistryDepartment of Medicinal Department of Medicinal ChemistryChemistryMashad University of Medical Mashad University of Medical SciencesSciences

gravi – metricgravi – metric (weighing - (weighing -

measure)measure)►Definition:Definition:

a a precipitationprecipitation or or volatilization volatilization method based on the determination method based on the determination of weight of a substance of known of weight of a substance of known composition that is chemically related composition that is chemically related to the to the analyteanalyte

►analyte - chemical element or analyte - chemical element or compound of interestcompound of interest

►Reaction:Reaction:aA + rR -----> AaA + rR -----> AaaRRr pptr ppt

where:where: a is # of moles of analyte Aa is # of moles of analyte A r is # of moles of reagent Rr is # of moles of reagent R AAaaRRrr is a pure, insoluble is a pure, insoluble

precipitateprecipitatewhich we can dry and weigh or which we can dry and weigh or ignite to convert to something ignite to convert to something we can weighwe can weigh

ppt=precipitateppt=precipitate

►1914 Nobel Prize to 1914 Nobel Prize to T.W.Richards (Harvard T.W.Richards (Harvard University) for the University) for the atomic weights of Ag, atomic weights of Ag, Cl, and NCl, and N

►Richards and his group Richards and his group determined atomic determined atomic weights of 55 of the 92 weights of 55 of the 92 known elements using known elements using gravimetrygravimetry

T.W.Richards:

►““Every substance must be Every substance must be assumed to be impure, every assumed to be impure, every reaction must be assumed to be reaction must be assumed to be incomplete, every method of incomplete, every method of measurement must be assumed to measurement must be assumed to contain some constant error, until contain some constant error, until proof to the contrary can be proof to the contrary can be obtained.”obtained.”

7 Steps in Gravimetric 7 Steps in Gravimetric AnalysisAnalysis

►Dry and weigh sampleDry and weigh sample►Dissolve sample Dissolve sample ► Add Add precipitating reagentprecipitating reagent in excess in excess►CoagulateCoagulate precipitate usually by precipitate usually by

heatingheating► Filtration-separate ppt from Filtration-separate ppt from mother mother

liquorliquor►Wash precipitate (Wash precipitate (peptizationpeptization))►Dry and weigh to Dry and weigh to constant weightconstant weight

Precipitation:Precipitation:

precipitating agent

sample

dissolvedcomponents

►Dissolve Dissolve samplesample

►Add ppt’ing Add ppt’ing reagentreagent

►FilterFilter►DryDry►WeighWeigh

Suction FiltrationSuction Filtration► Filter flaskFilter flask► Buchner funnelBuchner funnel► Filter paperFilter paper►Glass fritGlass frit► Filter adapterFilter adapter►Heavy-walled Heavy-walled

rubber tubingrubber tubing►Water aspiratorWater aspirator

►Mother liquorMother liquor

Identify insoluble formIdentify insoluble form

Two considerations:Two considerations: Minimize errors due to limited precipitate Minimize errors due to limited precipitate

solubilitysolubility Minimize errors due to precipitation Minimize errors due to precipitation

processprocess

a.a. Finite solubility of precipitateFinite solubility of precipitate

• ideally, Kideally, Kspsp = 0 (i.e., completely = 0 (i.e., completely insoluble)insoluble)

o Some come close:Some come close: ~10~10-38-38 for Fe(OH) for Fe(OH)33~10~10-50-50 for for

AgAg22SS

o For AgCl, KFor AgCl, Kspsp = 1.78 x 10 = 1.78 x 10-10-10

For example:For example:what would be the what would be the % error% error introduced in introduced in gravimetric analysis by the solubility of AgCl?gravimetric analysis by the solubility of AgCl?

For a 0.1000 g AgCl precipitate in 200 ml HFor a 0.1000 g AgCl precipitate in 200 ml H22O:O:

NoteNote: Error is : Error is independentindependent of of massmass of precipitate, of precipitate, relative error will decrease as precipitate relative error will decrease as precipitate mass increases (i.e., 0.038% error for 1.000 g mass increases (i.e., 0.038% error for 1.000 g AgCl)AgCl)

+ -sp

+ -

K =[Ag ][Cl ]

S =[Ag ] =[Cl ]-5spS = K =1.33x10 M

-5 -4

-4

1.33x10 molAgCl 143.32gAgCl0.200L =3.824x10 gAgCl1L 1molAgCl3.824x10 gSo, %error : x100 =0.38%0.1000g

b.b. Precipitation processPrecipitation process

ideally, we’d like a precipitate that ideally, we’d like a precipitate that forms forms quicklyquickly. This implies:. This implies:

Large, pure crystalsLarge, pure crystals Low solubilityLow solubility Easily filteredEasily filtered Easily washedEasily washed

How does precipitation occur?How does precipitation occur?

1.1. As KAs Kspsp is exceeded, solution becomes is exceeded, solution becomes “supersaturated”“supersaturated”

2.2. At some point At some point nucleationnucleation begins begins3.3. At the At the samesame time, time, crystal growthcrystal growth begins begins

Two points to remember:Two points to remember:1.1. Crystal growth is Crystal growth is independentindependent of degree of of degree of

supersaturationsupersaturation2.2. Nucleation Nucleation increasesincreases with degree of supersaturation with degree of supersaturation

Minimization Minimization of supersaturation will produce the largest of supersaturation will produce the largest particlesparticles

► Two particle size classesTwo particle size classes

ColloidsColloids ►very smallvery small►difficult to handle experimentallydifficult to handle experimentally

CrystalsCrystals►““large” (~ 10large” (~ 10-1-1 mm) mm)►easily and rapidly filteredeasily and rapidly filtered►high purityhigh purity

►What affects degree of supersaturation?What affects degree of supersaturation?

KKspsp Temperature Temperature

►solubility solubility as Tas T

Reagent addition speedReagent addition speed►slower addition givesslower addition gives

precipitation a chanceprecipitation a chanceto begin at lowerto begin at lowersupersaturation levelssupersaturation levels

Solution concentrationSolution concentration►low reagent concentrationlow reagent concentration

equalsequals low supersaturation low supersaturation

ButBut,, even with the above precautions, we will even with the above precautions, we will oftenoften obtain obtain colloidcolloid instead of a crystal! instead of a crystal!

KeysKeys to successful colloid precipitation: to successful colloid precipitation:1.1. Add precipitant Add precipitant slowlyslowly and in and in slightslight excess excess2.2. Digest precipitate (Heat, stir, sit)Digest precipitate (Heat, stir, sit)

What about crystalline precipitate?What about crystalline precipitate?Similar to colloids:Similar to colloids:1.1. Dilute solutionDilute solution2.2. Slow precipitant additionSlow precipitant addition3.3. Elevated temperatureElevated temperature4.4. HeatHeat unstirred unstirred

Contaminants can escape from crystal latticeContaminants can escape from crystal lattice

Increase crystal bridgesIncrease crystal bridges

►Particle Size / FilterabilityParticle Size / Filterability produce particles large enough to be produce particles large enough to be

'caught‘'caught‘ ideally, produce crystalsideally, produce crystals avoid colloidal suspensionavoid colloidal suspension

particle size = 1 - 100 nmparticle size = 1 - 100 nm

Precipitate FormationPrecipitate Formation► crystallizationcrystallization

nucleation:nucleation:particles join to produce aggregatesparticles join to produce aggregates

crystal growthcrystal growthaggregate grows and 'fall out' of solutionaggregate grows and 'fall out' of solution  

We want a few big chunks of precipitate!We want a few big chunks of precipitate!supersaturation:supersaturation:

more solute than should be present in more solute than should be present in solutionsolutionrelative supersaturation:relative supersaturation:

a measure of supersaturation, (Q-S)/Sa measure of supersaturation, (Q-S)/SQ = actual solute concentrationQ = actual solute concentrationS = equilibrium solute S = equilibrium solute

concentrationconcentration

Controlling PrecipitationControlling Precipitation

►Increase SIncrease S Increase temperatureIncrease temperature

►Decrease QDecrease Q Dilute solutionDilute solution Well mixed (stirring)Well mixed (stirring)

What Do We Get Out of What Do We Get Out of Gravimetry?Gravimetry?

►% of analyte, % A% of analyte, % A

►%A = %A = weight of analyte weight of analyte x x 100100 weight of sample weight of sample

How Do We Get %A?How Do We Get %A?

► % A = % A = weight of ppt x gravimetric factor (G.F.)weight of ppt x gravimetric factor (G.F.) x 100 x 100 weight of sampleweight of sample

► G.F. = G.F. = a FW[analyte]a FW[analyte] b FW[precipitate] b FW[precipitate]

►G.F. = # gms of analyte per 1 gm G.F. = # gms of analyte per 1 gm pptppt

Gravimetric FactorGravimetric Factor

►X apples + Y sugar = Z apple piesX apples + Y sugar = Z apple pies

►What is this relationship in What is this relationship in chemistry?chemistry?

The Gravimetric FactorThe Gravimetric Factor

►G.F. = G.F. = a FW[analyte]a FW[analyte] b FW[precipitate] b FW[precipitate]

►AnalyteAnalyte pptppt G.F.G.F.CaOCaO CaCOCaCO33FeSFeS BaSOBaSO44UOUO22(NO(NO33))22

..6H6H22OO UU33OO88CrCr22OO33 AgAg22CrOCrO44

►AnalyteAnalyte pptppt G.F.G.F.CaCaOO CaCaCOCO33 CaO/CaCOCaO/CaCO33FeFeSS BaBaSSOO44 FeS/BaSOFeS/BaSO44UUOO22(NO(NO33))22 UU33OO8833UOUO22(NO(NO33))22/U/U33OO88CrCr22OO33 AgAg22CrCrOO44 CrCr22OO33//22AgAg22CrOCrO44

ProblemProblem

►Consider a 1.0000 g sample Consider a 1.0000 g sample containing 75% potassium sulfate containing 75% potassium sulfate (FW 174.25) and 25% MSO(FW 174.25) and 25% MSO44. The . The sample is dissolved and the sulfate sample is dissolved and the sulfate is precipated as BaSOis precipated as BaSO44 (FW (FW 233.39). If the BaSO233.39). If the BaSO44 ppt weighs ppt weighs 1.4900, what is the atomic weight 1.4900, what is the atomic weight of Mof M2+2+ in MSO in MSO44? ?

►ANS: MgANS: Mg2+2+

AnswerAnswer►The hard part is setting up the The hard part is setting up the

correct equation (good correct equation (good stoichiometry skills are essential stoichiometry skills are essential here!):here!):

►Rearranging and solving:Rearranging and solving:

06.9639.233*25.0

25.17439.233*75.04900.1

x

)(12.24;06.96

3475.584855.02Mgx

x

ProblemProblem► A mixture of mercurous chloride (FW A mixture of mercurous chloride (FW

472.09) and mercurous bromide (FW 472.09) and mercurous bromide (FW 560.99) weighs 2.00 g. The mixture is 560.99) weighs 2.00 g. The mixture is quantitatively reduced to mercury metal quantitatively reduced to mercury metal (At wt 200.59) which weighs 1.50 g. (At wt 200.59) which weighs 1.50 g. Calculate the % mercurous chloride and Calculate the % mercurous chloride and mercurous bromide in the original mercurous bromide in the original mixture.mixture.

► ANS: 0.5182 g ANS: 0.5182 g

AnswerAnswer

►Again, important to set up correct Again, important to set up correct equation:equation:

►Rearranging and solving:Rearranging and solving:

99.560

259.200*209.472

*59.200*250.1 xx

gx

x5182.0

50.127151.08498.0

Homogeneous PrecipitationHomogeneous Precipitation

(NH(NH22)CO + 3 H)CO + 3 H22O + heat O + heat

HCOOH + OHHCOOH + OH- - + CO+ CO22 + 2 NH + 2 NH44++

High Electrolyte High Electrolyte ConcentrationConcentration

to Aid Precipitation to Aid PrecipitationExcess charge on colloid creates ionic atmosphere around particle

Composition by Composition by Gravimetric AnalysisGravimetric Analysis

NiNi2+2+ (aq) + H (aq) + H22DMG DMG Ni(DMG) Ni(DMG)22 + 2 + 2 HH++

►A 0.8234 g org sample produced A 0.8234 g org sample produced 0.1397 g of bis(dimethylglyoximate) 0.1397 g of bis(dimethylglyoximate) nickel (II) (FW = 288.91 g/mol). nickel (II) (FW = 288.91 g/mol). Find the nickel content.Find the nickel content.

►Explain how to create a large, Explain how to create a large, filterable precipitate.filterable precipitate.

Combustion AnalysisCombustion Analysis

Find the empirical formula for a 13.72 mg organic sample that produced 6.97 mg of water and 28.44 mg of carbon dioxide

Gravimetric OverviewGravimetric Overview► SimpleSimple►CheapCheap

GlasswareGlassware ReagentsReagents ovens, etc.ovens, etc. BalancesBalances

► SpecificSpecific► Timely Timely (1/2 day)(1/2 day)► AccurateAccurate► Precise Precise (0.1-0.3 %)(0.1-0.3 %)► SensitiveSensitive