Gravimetric Analysis and Gravimetric Analysis and Precipitation EquilibriaPrecipitation Equilibria

Dr. A.K.M. Shafiqul IslamDr. A.K.M. Shafiqul Islam

&&

Dr. Zarina ZakariaDr. Zarina Zakaria

IntroductionIntroduction The term gravimetric pertains to a Weight The term gravimetric pertains to a Weight

Measurement.Measurement. Gravimetric method is one in which the Gravimetric method is one in which the

analysis is completed by a weighing analysis is completed by a weighing operation. operation.

Gravimetric Analysis is a group of analytical Gravimetric Analysis is a group of analytical methods in which the amount of analyte is methods in which the amount of analyte is determined by the measurement of the mass determined by the measurement of the mass of a pure substance containing the analyte.of a pure substance containing the analyte.

Gravimetric Methods can also be defined as Gravimetric Methods can also be defined as quantitative methods based on the quantitative methods based on the determining the mass of a pure compound to determining the mass of a pure compound to which the analyte is chemically related.which the analyte is chemically related.

There are two main types of gravimetric analyses

A) Precipitation

B) Volatilisation

A chemical reaction causes the formation of a sparingly soluble substance that precipitates from solution is filtered, washed, purified (if necessary) and weighed.

In this method the analyte or its decomposition products are volatilised and then collected and weighed, or alternatively, the mass of the volatilised product is determined indirectly by the loss of mass of the sample.

Example for Precipitation:-Example for Precipitation:-

Calcium can be determined gravimetrically Calcium can be determined gravimetrically by precipitation of calcium oxalate and by precipitation of calcium oxalate and ignition of the oxalate ion to calcium oxide.ignition of the oxalate ion to calcium oxide.

CaCa2+2+ + C + C22OO442-2- →CaC →CaC22OO44

CaCCaC22OO44 → CaO + CO → CaO + CO22 + CO + CO

The precipitate thus obtained are weighed The precipitate thus obtained are weighed and the mass of calcium oxide is determined.and the mass of calcium oxide is determined.

Example for Volatilisation:-Example for Volatilisation:-

The analyte or its decomposition products are The analyte or its decomposition products are volatilised at a suitable temperature. The volatile volatilised at a suitable temperature. The volatile product is then collected and weighed, i.e. the mass product is then collected and weighed, i.e. the mass of the product is indirectly determined from the loss of the product is indirectly determined from the loss in mass of the sample.in mass of the sample.

Example Example Water can be separated from most inorganic Water can be separated from most inorganic compounds by ignition, the evolved water can then compounds by ignition, the evolved water can then be absorbed on any one of several solid desiccants. be absorbed on any one of several solid desiccants. The weight of water evolved may be calculated The weight of water evolved may be calculated from the gain in weight of the absorbent.from the gain in weight of the absorbent.

Not all insoluble precipitates are well suited for gravimetric analysis.

It is important to consider what properties are required in order that a precipitate be applicable for a quantitative precipitation method:-

Solubility

Filterability

Chemical Composition

Other Desirable Properties

Solubility

Filterability

Chemical Composition

Other Desirable Properties

The product must be sufficiently insoluble to prevent the loss of weight.

Precipitate formed should be adoptable to simple and rapid filteration methods.

The product must be of known chemical composition.

Other factors effecting the stability and purity of the precipitate.

For a successful determination in gravimetric analysis the following criteria should be met :-

(1)The desired substance must be completely precipitated. In most determination the precipitate is of such low solubility that losses from dissolution are negligible. An additional factor is the common ion effect, this further decrease the solubility of the precipitate.

E.g. When Ag+ is precipitated out by addition of Cl-

Ag+ + Cl- = AgCl

The low solubility of AgCl is reduced further by the excess of Cl- which is added force to the reaction to proceed towards right side.

(2)The weighed form of the product should be of known composition.

(3)The product should be pure and easily filtered. It is usually difficult to obtain a product which is pure or which is free from impurities. This could be reduced by careful precipitation and sufficient washing.

For a successful determination in gravimetric analysis the following criteria should be met :-

Gravimetric AnalysisGravimetric Analysis

Gravimetric analysis is potentially more Gravimetric analysis is potentially more accurate and more precise than volumetric accurate and more precise than volumetric analysis.analysis. Gravimetric analysis avoids problems with Gravimetric analysis avoids problems with

temperature fluctuations, calibration errors, and temperature fluctuations, calibration errors, and other problems associated with volumetric other problems associated with volumetric analysis.analysis.

But there are potential problems with gravimetric But there are potential problems with gravimetric analysis that must be avoided to get good analysis that must be avoided to get good results.results.

Proper lab technique is criticalProper lab technique is critical

Steps in a Gravimetric AnalysisSteps in a Gravimetric Analysis

1.1. Preparation of the solutionPreparation of the solution2.2. PrecipitationPrecipitation3.3. DigestionDigestion4.4. FiltrationFiltration5.5. WashingWashing6.6. Drying or ignitionDrying or ignition7.7. WeighingWeighing8.8. CalculationCalculation

1.1. Preparation of analyte solutionPreparation of analyte solution

Gravimetric analysis usually involves Gravimetric analysis usually involves precipitation of analyte from solution.precipitation of analyte from solution.

May need to separate potential May need to separate potential interferences before precipitating analyteinterferences before precipitating analyte

May require pH and/or other adjustments May require pH and/or other adjustments to maximize precipitate formationto maximize precipitate formation

When possible, select precipitating When possible, select precipitating agents that are selective (or specific, if agents that are selective (or specific, if possible).possible).

2. Precipitation 2. Precipitation The precipitate shouldThe precipitate should

Be insoluble, but not too insolubleBe insoluble, but not too insoluble Have large crystalsHave large crystals

Easier to filter large crystalsEasier to filter large crystals Be free of contaminantsBe free of contaminants

The smaller the surface area the betterThe smaller the surface area the better

The smaller the solubility, S, the greater the The smaller the solubility, S, the greater the tendency to form lots of small crystals that trap tendency to form lots of small crystals that trap contaminants and are difficult to filtercontaminants and are difficult to filter

The greater the solubility, the more analyte is left The greater the solubility, the more analyte is left unprecipitatedunprecipitated We need to strike a balanceWe need to strike a balance

(but not the analytical balance, please)(but not the analytical balance, please)

The von Weimarn RatioThe von Weimarn Ratio von Weimarn ratio = (von Weimarn ratio = (QQ – – SS)/)/SS

A measure of relative supersaturationA measure of relative supersaturation The lower the betterThe lower the better

If high, get excessive nucleation, lots of small crystals, large If high, get excessive nucleation, lots of small crystals, large surface areasurface area

If low, get larger, fewer crystals, small surface areaIf low, get larger, fewer crystals, small surface area QQ = concentration of mixed reactants before = concentration of mixed reactants before

precipitationprecipitation Keep it low by using dilute solutions, stir mixture well, add Keep it low by using dilute solutions, stir mixture well, add

reactants slowlyreactants slowly SS = solubility of precipitate at equilibrium = solubility of precipitate at equilibrium

Keep it high with high temperatures, adjusting pHKeep it high with high temperatures, adjusting pH Can lower Can lower SS later by cooling mixture after crystals have later by cooling mixture after crystals have

formedformed

Digestion of the PrecipitateDigestion of the Precipitate Very small crystal with a large specific Very small crystal with a large specific

surface area have a higher surface energy surface area have a higher surface energy and a higher apparent solubility than large and a higher apparent solubility than large crystals. This is an initial rate phenomenon crystals. This is an initial rate phenomenon and does not represent the equilibrium and does not represent the equilibrium condition, and it is one consequence of condition, and it is one consequence of heterogeneous equilibria. When the heterogeneous equilibria. When the precipitate is allowed to stand in the precipitate is allowed to stand in the presence of mother liquor (the solution presence of mother liquor (the solution from which it was precipitated), the larger from which it was precipitated), the larger crystal grow at the expanse of the small crystal grow at the expanse of the small ones. This process is called ones. This process is called digestiondigestion, or , or OstwaldOstwald ripeningripening. The small particles . The small particles tend to dissolve and precipitate on the tend to dissolve and precipitate on the surfaces of the larger crystals surfaces of the larger crystals

When a crystalline precipitate is allowed to stand in contact with the solution from which it was precipitated (mother liquor), it is more easily filtered and requires a less dense filtering medium than the freshly formed precipitate.

This improvement in filterability is accelarated by heating, common practice is to treat crystalline precipitates in this manner prior to filtration. The process is called DIGESTION.

Considerable experimental evidence indicates that the digestion process consists of a cementing together of crystals. The resulting aggregates are appreciably larger than the individual crystals and are therefore more easily retained.

Digestion of Crystalline Precipitates –(1)

Ions making up a precipitate are in dynamic equilibrium with their counterparts in the solution. As a result, solution and reprecipitation of the solid take place constantly.

The latter process can occur in such a way as to form bridges between adjacent solid particles thus leading to crystalline aggregates.

At elevated temperatures, where the solubility is greater, the process would be expected to take place at an accelerated rate, this is in keeping with the experimental observation that the effectiveness of digestion is greater at higher temperatures.

Digestion of Crystalline Precipitates –(2)

The improved filterability of precipitates after digestion has also been attributed wholly or in part to growth of large crystals at the expense of small ones. Such process might occur since theoretically small crystals should be more soluble than large ones.

Therefore, digestion of a mixture of crystals results in the disappearance of the more soluble smaller particles and growth of the larger and more insoluble ones.

There was however some experimental evidence that the very smallest crystals of the solid may have disappeared during the digestion.

Digestion of Crystalline Precipitates –(3)

Impurities in PrecipitationImpurities in Precipitation

Precipitates tend to carry down from Precipitates tend to carry down from the solution constituents that are not the solution constituents that are not normally soluble, causing the normally soluble, causing the precipitate to become contaminated. precipitate to become contaminated. This process is called co-This process is called co-precipitation. precipitation.

Co-precipitationCo-precipitation

At least four types of co-precipitation At least four types of co-precipitation can be recognizedcan be recognized

a.a. Occlusion and InclusionOcclusion and Inclusion

b.b. Surface AdsorptionSurface Adsorption

c.c. Isomorphous ReplacementIsomorphous Replacement

d.d. PostprecipitationPostprecipitation

Occlusion is a type of coprecipitation in which the soluble impurity is enclosed or entrapped within the crystal structure of the solid. As such it acts as a seat of imperfection within the crystal.

Generally, occluded substances are distributed in a non homogeneous fashion throughout the crystal.

In addition, the amount of occluded substance is greatly influenced by the conditions and manner of precipitate formation.

a. Occlusion and Inclusiona. Occlusion and Inclusion

Inclusion occurs when ions, generally of large size or charge, are trapped within the crystal lattice (isomorphorous inclusion, as with K+ in NH4MgPO4

precipitation)

Occlusion and inclusion impurities are difficult to remove. Digestion is effective in reducing the quantity of entrapped contaminant.

a. Occlusion and Inclusiona. Occlusion and Inclusion

All precipitates carry down soluble impurities by adsorption on their surfaces.

However, only where the specific surface areas are great does coprecipitation of this type have an appreciable effect on the weight of the solid.

Therefore contamination by surface adsorption will be important only when dealing with colloidal precipitates.

Colloidal particles formed under analytical conditions will always have adsorbed on their surfaces one of the constituent ions of the precipitate.

b. Surface Adsorptionb. Surface Adsorption

b. Surface Adsorptionb. Surface Adsorption Let’s use AgCl as an example Let’s use AgCl as an example

Primary adsorption layerPrimary adsorption layer Ion in excess adsorbs on the surface of the precipitateIon in excess adsorbs on the surface of the precipitate

Crystal becomes positive if AgCrystal becomes positive if Ag++ is in excess is in excess Crystal becomes negative if ClCrystal becomes negative if Cl-- is in excess is in excess

Counter-ion layer (counterlayer)Counter-ion layer (counterlayer) Charged crystal attracts ions of opposite chargeCharged crystal attracts ions of opposite charge

For example, NOFor example, NO33-- attracted to positive crystal, results in a attracted to positive crystal, results in a

layer of silver nitrate around the AgCllayer of silver nitrate around the AgCl For example, NaFor example, Na++ attracted to negative crystal, results in a attracted to negative crystal, results in a

layer of NaCl around the AgCllayer of NaCl around the AgCl The more dilute the solution, the thicker is the counter-The more dilute the solution, the thicker is the counter-

ion layerion layer

Colloidal Precipitates (Such as Colloidal Precipitates (Such as AgClAgCl

When When SS is very small (very insoluble is very small (very insoluble precipitates), cannot avoid high degree of precipitates), cannot avoid high degree of nucleation and very small crystalsnucleation and very small crystals Colloidal particles 1 to 100 Colloidal particles 1 to 100 μμm in diameterm in diameter Surface area as high as 3,000 ftSurface area as high as 3,000 ft22/g/g

High potential for surface adsorptionHigh potential for surface adsorption

Other problems besides surface adsorptionOther problems besides surface adsorption Colloidal precipitates cause difficulties in Colloidal precipitates cause difficulties in

filtering and washing as wellfiltering and washing as well

Colloidal Particle in SolutionColloidal Particle in Solution

Cl−Ag+ Ag+ Cl− Ag+ Cl−

Cl−

Ag+

Cl−

Ag+

Cl−

Ag+

Cl− Ag+ Cl− Ag+

Ag+ Cl− Ag+ Cl−

Cl− Ag+ Cl− Ag+

Ag+

Ag+

Ag+

H+

H+

H+

NO3−

NO3−

NO3−

NO3−

NO3−

Ag+

Ag+

NO3−

NO3−

NO3− H+

NO3−

Ag+

NO3−

Counter-ionlayer of solutionwith excess anions

Homogeneous solution(charges balanced)

Colloidal solid

Positively charged primary adsorptionlayer on colloidal particle

Coagulation: aggregation of colloidal particles into larger filterable masses

b. Surface Adsorptionb. Surface Adsorption

Several techniques are adopted to reduce the magnitude of error arising from adsorption of contaminants on a colloidal precipitate.

Among these are the use of elevated temperatures and dilute solutions, digestion, washing and reprecipitation.

c. Isomorphous Replacementc. Isomorphous Replacement Two compound are said to be isomorphous if they Two compound are said to be isomorphous if they

have the same type of formula and crystallize in have the same type of formula and crystallize in similar geometric forms. When their lattice similar geometric forms. When their lattice dimension are about same, one ion can replace dimension are about same, one ion can replace another in a crystal, a resulting in a mixed crystal. another in a crystal, a resulting in a mixed crystal. This process is called isomorphous replacement. This process is called isomorphous replacement.

For example, in the precipitation of MgFor example, in the precipitation of Mg+2+2 as as magnesium ammonium phosphate, Kmagnesium ammonium phosphate, K++ has nearly has nearly the same ionic size as NHthe same ionic size as NH44

++ and can be replaces it and can be replaces it to form magnesium potassium phosphate. to form magnesium potassium phosphate.

Isomorphous replacement is very serious and very Isomorphous replacement is very serious and very little can be done about it.little can be done about it.

Post precipitation arises only when the attempt is made to separate two ions on the basis of the rate at which they precipitate.

This will come down if the solution is allowed to stand too long before being filtered.

A type of contamination wherein the impurity is an insoluble compound that precipitates after all or a major part of the analytical precipitate has formed.

d. Post Precipitationd. Post Precipitation

3. Digestion (Ostwald Ripening)3. Digestion (Ostwald Ripening) Let precipitate stand in contact with Let precipitate stand in contact with

solution, usually at high temperaturesolution, usually at high temperature Large crystals (small surface area) have Large crystals (small surface area) have

lower free energy than small crystals lower free energy than small crystals (large surface area)(large surface area) Digestion makes larger crystals, reduces Digestion makes larger crystals, reduces

surface contamination, reduces crystal surface contamination, reduces crystal imperfectionsimperfections

Digestion coagulates a colloid (causes the Digestion coagulates a colloid (causes the particles to agglomerate, or stick together), but particles to agglomerate, or stick together), but surface area does not decrease as much as if surface area does not decrease as much as if larger crystals actually grewlarger crystals actually grew

Hydrophilic vs. Hydrophobic Hydrophilic vs. Hydrophobic ColloidsColloids

Hydrophilic (water-loving) colloids form Hydrophilic (water-loving) colloids form gels and do not coagulate wellgels and do not coagulate well Hydroxides (Al(OH)Hydroxides (Al(OH)33, Fe(OH), Fe(OH)33, for example) , for example)

often form gels and are very difficult to filteroften form gels and are very difficult to filter Hydrophobic (water-fearing) colloids Hydrophobic (water-fearing) colloids

coagulate readilycoagulate readily Luckily, AgCl is hydrophobicLuckily, AgCl is hydrophobic Gravimetric analysis of silver or chloride by Gravimetric analysis of silver or chloride by

AgCl precipitation is effective in spite of AgCl precipitation is effective in spite of colloidal nature of precipitatecolloidal nature of precipitate

4. Filtration4. Filtration Sintered glass crucibles are used to filter the Sintered glass crucibles are used to filter the

precipitates. These are marked as fine, medium and precipitates. These are marked as fine, medium and coarse porosities (marked coarse porosities (marked ff, , mm and and cc).).

The crucibles first cleaned thoroughly and then The crucibles first cleaned thoroughly and then subjected to the same regimen of heating and cooling subjected to the same regimen of heating and cooling as that required for the precipitate. This process is as that required for the precipitate. This process is repeated until constant mass has been achieved, that repeated until constant mass has been achieved, that is, until consecutive weighings differ by 0.3 mg or less.is, until consecutive weighings differ by 0.3 mg or less.

Colloidal precipitates present filtering problems if Colloidal precipitates present filtering problems if particles are too smallparticles are too small Can plug filter paper or glass or pass right through the filter if Can plug filter paper or glass or pass right through the filter if

not coagulated wellnot coagulated well Hydrophobic colloids generally filter better than hydrophilic Hydrophobic colloids generally filter better than hydrophilic

colloidscolloids

5. Washing5. Washing

Removes the mother liquorRemoves the mother liquor May also remove some of the May also remove some of the

coprecipitated compoundscoprecipitated compounds Can cause peptization of colloidsCan cause peptization of colloids

Diluting the counter-ion layer causes it to get Diluting the counter-ion layer causes it to get larger, forcing coagulated colloidal particles larger, forcing coagulated colloidal particles apartapart

And THERE THEY GO right through the filterAnd THERE THEY GO right through the filter Wash with a solution of a volatile electrolyte Wash with a solution of a volatile electrolyte

that will be removed in drying stepthat will be removed in drying step

6. Drying or Igniting6. Drying or Igniting Dry the precipitate to remove water and Dry the precipitate to remove water and

volatile electrolytes from wash solutionvolatile electrolytes from wash solution

Ignition (very high-temperature drying) Ignition (very high-temperature drying) converts precipitates to compounds more converts precipitates to compounds more suitable for weighingsuitable for weighing Removes water of hydrationRemoves water of hydration Converts hygroscopic compound to non-Converts hygroscopic compound to non-

hygroscopic compoundhygroscopic compound

e.g., MgNHe.g., MgNH44POPO44 is decomposed to the is decomposed to the pyrophosphate Mgpyrophosphate Mg22PP22OO77 by heating at 900 by heating at 900 ooCC Sometimes it even converts the filter paper to Sometimes it even converts the filter paper to

ashash

IgnitionIgnition is often used to drive off water or is often used to drive off water or give an oxide formgive an oxide form

ThermogravimetryThermogravimetry is a form of is a form of gravimetric analysis where mass is gravimetric analysis where mass is measured as a function of temperature:measured as a function of temperature:

Obtaining a Final Known Obtaining a Final Known CompositionComposition

(g)(g)s(s) OH CO)(OFeOH )Fe(HCO 22321hr C850

23-2

o

OH

CO2

OH

CO2Ca

OCa

O

O

CaOCaCO C0073

C005 oo C003 o

ThermogravimetryThermogravimetry

Combustion AnalysisCombustion Analysis

Absorbs water

Absorbs CO2

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

7. Weighing7. Weighing

Properly calibrated analytical balanceProperly calibrated analytical balance Good weighing techniqueGood weighing technique Avoid static electricityAvoid static electricity Review important points on p. 51 Review important points on p. 51

periodicallyperiodically And now, step 8, the calculationsAnd now, step 8, the calculations

Chem 105 stoichiometry, anyone?Chem 105 stoichiometry, anyone?

Organic Precipitates Organic Precipitates

Organic precipitating agents have the advantages Organic precipitating agents have the advantages of giving precipitates with very solubility in water of giving precipitates with very solubility in water and a favorable gravimetric factor.and a favorable gravimetric factor.

Most of them are chelating agents that forms Most of them are chelating agents that forms slightly insoluble, uncharged chelates with the slightly insoluble, uncharged chelates with the metal ions.metal ions.

Organic Precipitates Organic Precipitates

From the weight of the precipitate formed in a gravimetric analysis, we can calculate the weight of the analyte from the weight relationship of the precipitate and analyte.

The weight of the analyte is calculated from the weight of the precipitate.

Gravimetric Analysis: Weight Gravimetric Analysis: Weight RelationshipRelationship

Cl2--------> 2AgCl (we get 2 moles of AgCl from 1 mol Cl2)

Calculation of the corresponding weight of Cl2 that would be contained in the AgCl sample will be:-

gm Cl2 / gm AgCl = [F. Wt. Cl2 ] / [F. Wt. AgCl x [1 mol Cl2 / 2 mol AgCl]

gm Cl2 = [gm AgCl] x [70.906 gm Cl2 / 1 mol Cl2] x [1 mol AgCl / 143.32 gm AgCl] x [1 mol Cl2 / 2 mol AgCl]

E.g. If we consider calculating the percentage of Cl2 by weighing the AgCl precipitate, then

We can consider Gravimetric Factor (GF) as: -

Then,

gm Cl2 = gm AgCl x GF(gm Cl2/gm AgCl)

GF = f. wt. of substance sought / f. wt. of substance weighed x a/b (mol sought / mole weighed)

e)precipitat lanalyte/mo (molb

ax

(g/mol) eprecipitat wt.f

(g/mol) analyte wt.fGF

#1 A 0.3516 gm sample of commercial phosphate detergent was ignited at a red heat to destroy the organic matter. The residue was then taken up in hot HCl which converted P to H3PO4. The phosphate was precipitated with Mg2+ followed by aqueous NH3 to form MgNH4PO4.6H2O. After being filtered and washed, the precipitate was converted to Mg2P2O7 (FW=222.57) by ignition at 1000º C. This residue weighed 0.2161 gm. Calculate the percent P (FW = 30.974) in the sample.

Gravimetric factor, GF = f. wt. of substance sought / f. wt. of substance substance weighed x a/b (mol sought / mol weighed)

GF = 30.974 gm / 222.57 gm x 2/1 = 0.278

Amount of P = GF (0.278) x Wt. of precipitate (0.2161) = 0.0601gm

% P = 0.0601 gm P / 0.3516 gm sample x 100 = 17.11%

#2 A 0.4960 gm sample of a CaCO3 is dissolved in an acidic solution. The calcium is precipitated as CaC2O4.H2O and the dry precipitate is found to weight 0.6186 gm. What is the percentage of CaO in the sample? (FW CaO = 56.08; FW CaC2O4.H2O = 146.2)

GF = 56.08 gm / 146.12 gm X 1/1 = 0.3838

Wt of CaO = Wt. of ppt. X GF = 0.6186 gm X 0.3838 = 0.2374 gm

% of CaO in CaCO3 = 0.2347 gm / 0.4960 gm x 100 = 47.87%

Precipitation ReactionsPrecipitation Reactions

Two fully dissolved and dissociated Two fully dissolved and dissociated compounds are mixed and a compounds are mixed and a “precipitate” (solid product) forms“precipitate” (solid product) forms

The mixture becomes opaque due to The mixture becomes opaque due to the formation of solidthe formation of solid

Precipitation ReactionsPrecipitation Reactions

Barium Iodate in Water:Barium Iodate in Water:

At Equilibrium:At Equilibrium:

Solubility Product (in general):Solubility Product (in general):

saturated solution contains excess solid

Ksp = [Ba2+][IO3−]2

for salt MxAy: Ksp = [M]x[A]y

Ba(IO3)2(s) Ba2+(aq) + 2 IO3 (aq)

Precipitation ReactionsPrecipitation Reactions

Ba(IO3)2(s) Ba2+(aq) + 2 IO3 (aq)

How many grams of Ba(IO3)2 dissolve in 500 mL of waterAt 25 C if Ksp = 1.57 × 10−9

S S 2S

Ksp = [Ba2+][IO3−]2 = [S][2S]2 = 1.57 × 10−9

4S3 = 1.57 × 10−9

S (molar solubility of Ba(IO3)2) = 7.32 x 10−4 mol L−1

You must learn how to use your calculator!

Precipitation ReactionsPrecipitation Reactions

S = 7.32 × 10−4

How many grams of Ba(IO3)2 dissolve in 500 mL of waterAt 25 C if Ksp = 1.57 × 10−9

mmol Ba(IO3)2 = S × Vml = (7.32 × 10−4)(500 mL) = 0.366 mmol

Wt. Ba(IO3)2 = 0.366 mmol × 487 mg mmol−1 = 178 mg 0.178 g

This could cause a significant error in a gravimetric analysis

Common Ion EffectCommon Ion EffectThe presence in solution of an excess of one ion The presence in solution of an excess of one ion from a slightly soluble ionic substance depresses from a slightly soluble ionic substance depresses its solubility (more complete precipitation).its solubility (more complete precipitation).

How many grams of Ba(IO3)2 dissolve in 500 mL of 0.0200 M Ba(NO3)2 at 25 C if Ksp = 1.57 × 10−9

Ba(IO3)2(s) Ba2+(aq) + 2 IO3 (aq)

S S + 0.0200 2S

[Ba2+][IO3−]2 = (S + 0.0200)(2S)2 = 1.57 × 10−9

Assumption: S << 0.0200 M

(0.0200)(2S)2 = 1.57 × 10−9 S = 1.40 × 10−4 M

Wt. Ba(IO3)2 = 1.40 × 10−4 M × 500 mL × 487 mg mmol−1 = 34.0 mg

Theory of PrecipitationTheory of Precipitation Precipitation occurs when the ion product Precipitation occurs when the ion product

exceeds the solubility productexceeds the solubility product

Two Steps in PrecipitationTwo Steps in Precipitation

Nucleation: small groups of ions form Nucleation: small groups of ions form stable solid masses (nuclei)stable solid masses (nuclei)

Particle Growth: ions from solution add on Particle Growth: ions from solution add on to nuclei to form particlesto nuclei to form particles

For MxAy: CMx × CA

y > Ksp

Theory of PrecipitationTheory of Precipitation The amount by which the ion product exceeds the The amount by which the ion product exceeds the

solubility product is expressed qualitatively as solubility product is expressed qualitatively as supersaturation.supersaturation.

Large supersaturation favors nucleationLarge supersaturation favors nucleation

Small supersaturation favors particle growthSmall supersaturation favors particle growth

Supersaturation = Q − S

Q: instantaneous concentration

S: equilibrium concentration (molar solubility)

Minimizing SupersaturationMinimizing Supersaturation

Supersaturation = Q − SSupersaturation = Q − S

1.1. Elevated Temperature (increase S)Elevated Temperature (increase S)

2.2. Dilute Solution (decrease Q)Dilute Solution (decrease Q)

3.3. Slow Addition, good stirring Slow Addition, good stirring (decrease Q)(decrease Q)

TUTORIALTUTORIAL

Read Gary text book on 314 to 320 Read Gary text book on 314 to 320 and understand the topics. Extract and understand the topics. Extract the objective/purpose of steps in the objective/purpose of steps in gravimetrics analysis.gravimetrics analysis.

Do questions number 16, 22, 27 and Do questions number 16, 22, 27 and 32 complete with the calculations.32 complete with the calculations.