Precipitation Reactions and Titrations Dr AKM Shafiqul Islam & Dr Zarina Zakaria.

Precipitation Reactions Precipitation Reactions and Titrationsand TitrationsDr AKM Shafiqul IslamDr AKM Shafiqul Islam

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Dr Zarina Zakaria Dr Zarina Zakaria

Precipitation ReactionsPrecipitation Reactions

A A precipitation reactionprecipitation reaction occurs when occurs when water solutions of two different ionic water solutions of two different ionic compounds are mixed and an insoluble compounds are mixed and an insoluble solid separates out of solution.solid separates out of solution.

The precipitate is itself ionic; the cation The precipitate is itself ionic; the cation comes from one solution and the anion comes from one solution and the anion from another. To predict the occurrence from another. To predict the occurrence of these reactions, we must know which of these reactions, we must know which ionic substances are insoluble in water.ionic substances are insoluble in water.

Precipitation ReactionA reaction which forms a solid (precipitate)

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

- AgCl is classified as an insoluble substance

Solubility – amount of substance that can be dissolved in a specific amount of water (g/L or mg/L)

- Any substance with a solubility less than 0.01mol/L is considered insoluble.

Metathesis ReactionsMetathesis Reactions

1. Write the balanced molecular equation.

2. Write the ionic equation showing the strong electrolytes completely dissociated into cations and anions.

3. Cancel the spectator ions on both sides of the ionic equation

Write the net ionic equation for the reaction of silver nitrate with sodium chloride.

AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)

Ag+ + NO3- + Na+ + Cl- AgCl (s) + Na+ + NO3

-

Ag+ + Cl- AgCl (s)

Writing Net Ionic EquationsWriting Net Ionic Equations

Precipitation ReactionsPrecipitation Reactions

Precipitate – insoluble solid that separates from solution

molecular equation

ionic equation

net ionic equation

Pb2+ + 2NO3- + 2Na+ + 2I- PbI2 (s) + 2Na+ + 2NO3

-

Na+ and NO3- are spectator ions

Pb(NO3)2 (aq) + 2NaI (aq) PbI2 (s) + 2NaNO3 (aq)

precipitate

Pb2+ + 2I- PbI2 (s)

Solubility EquilibriaSolubility Equilibria

Solubility Product Constant KSolubility Product Constant Kspsp

The equilibrium constant for the dissolution The equilibrium constant for the dissolution of a solid.of a solid.

MMmmXXxx(s) (s) mM mMn+n+(aq) + xX(aq) + xXy-y-(aq)(aq)

Ksp = [MKsp = [Mn+n+]]mm[X[Xy-y-]]xx

Solution must be saturated in order to use or Solution must be saturated in order to use or determine the constant.determine the constant.

Measuring KMeasuring Kspsp and Calculating Solubilty for and Calculating Solubilty for

KKspsp

Table from CRC handbook.Table from CRC handbook.Discussion.Discussion.

Factors That Affect SolubilityFactors That Affect Solubility

The Common-Ion EffectThe Common-Ion EffectHere it is again.Here it is again.

The pH of the SolutionThe pH of the Solution Formation of Complex IonsFormation of Complex Ions AmphoterismAmphoterism

All can be classified under the Le All can be classified under the Le

ChChââtelier principle.telier principle.

Factors That Affect SolubilityFactors That Affect Solubility

Common Ion.Common Ion.

MgFMgF22(s) (s) Mg Mg2+2+(aq) + 2F(aq) + 2F--(aq)(aq)

Adding NaF to the solution will reduce Adding NaF to the solution will reduce the solubility of MgFthe solubility of MgF22

Ksp = [MgKsp = [Mg2+2+] [F] [F--]]22

Since the equilibrium constant stays the Since the equilibrium constant stays the same increasing [Fsame increasing [F--] reduces [Mg] reduces [Mg2+2+] .] .

The shift in equilibrium that The shift in equilibrium that occurs because of the addition of an occurs because of the addition of an ion already involved in the ion already involved in the equilibrium reaction.equilibrium reaction.

AgCl(AgCl(ss) ) Ag Ag++((aqaq) + Cl) + Cl((aqaq))

The “Common Ion Effect”The “Common Ion Effect”

adding NaCl( ) shifts equilibrium positionaq

Concept test: What happens if you add NaF(s) into a solution of HF?

Factors That Affect SolubilityFactors That Affect Solubility

The pH of the SolutionThe pH of the SolutionAs you’ve often been told there are very few As you’ve often been told there are very few

neutral solutions. Dissolved ions impart neutral solutions. Dissolved ions impart some acidic or basic character to the some acidic or basic character to the solution.solution.

Adding an acid or base to a specific solution Adding an acid or base to a specific solution will certainly effect the solubility. (depending will certainly effect the solubility. (depending on the ions basic or acidic nature)on the ions basic or acidic nature)

CaCOCaCO33(s) (s) Ca Ca2+2+(aq) + CO(aq) + CO332-2-(aq)(aq)

Factors That Affect SolubilityFactors That Affect Solubility

Formation of Complex IonsFormation of Complex Ions If you add another matterial which would form a If you add another matterial which would form a

more soluble species in solution (complex ion) the more soluble species in solution (complex ion) the solubility of the initial material will increase. solubility of the initial material will increase.

AgCl AgCl Ag Ag++(aq) + Cl(aq) + Cl--(aq)(aq)

If you add something that reacts with Silver ions you If you add something that reacts with Silver ions you are effectively removing Agare effectively removing Ag++ from the right side of from the right side of the equation.the equation.

Almost the reverse of the common ion effect.Almost the reverse of the common ion effect.

Ksp for AgCl = 1.8 x10Ksp for AgCl = 1.8 x10-10-10

Assuming the complex ion is much more soluble.Assuming the complex ion is much more soluble.

Precipitation of Ionic CompoundsPrecipitation of Ionic Compounds

CaFCaF22(s) (s) Ca Ca2+2+(aq) + 2 F(aq) + 2 F--(aq)(aq)

KKspsp = [Ca = [Ca2+2+] [F] [F--]]2 2 at equilibriumat equilibrium

IP = [CaIP = [Ca2+2+] [F] [F--]]2 2 initially. initially.

If IP is greater than KIf IP is greater than Kspsp a precipitate will a precipitate will form.form.

Equilibrium

Ksp

Separation of Ions by Selective PrecipitationSeparation of Ions by Selective Precipitation

Addition of certain counter ions to a Addition of certain counter ions to a solution can (if chosen correctly) solution can (if chosen correctly) precipitate one of the initial ions precipitate one of the initial ions selectively.selectively.

E.g.E.g.Since AgNOSince AgNO3 3 is “relatively soluble” and AgCl is “relatively soluble” and AgCl

is “relatively insoluble” one can add HCl to a is “relatively insoluble” one can add HCl to a solution of AgNOsolution of AgNO33 and precipitate the Ag and precipitate the Ag++ ion ion from solution.from solution.

Separation of Ions by Selective PrecipitationSeparation of Ions by Selective Precipitation

Sulfide PrecipitationsSulfide Precipitations In this case the solubility of a metal sulfide is In this case the solubility of a metal sulfide is

related to the hydronium ion concentration related to the hydronium ion concentration in solution.in solution.

MS(s) + 2 HMS(s) + 2 H33OO++(aq) (aq) M M2+2+(aq) + H(aq) + H22S(aq) + 2 HS(aq) + 2 H22O(l)O(l)

Ksp = [MKsp = [M2+2+][H][H22S] / [HS] / [H33OO++]]By adding acid to the solution ….By adding acid to the solution ….Bring back QBring back Qcc

Equilibrium

Ksp

Precipitate FormationPrecipitate Formation

crystallizationcrystallization– nucleation:nucleation:

particles join to produce aggregatesparticles join to produce aggregates– crystal growthcrystal growth

aggregate 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 concentrationS = equilibrium solute concentration

Controlling PrecipitationControlling Precipitation Increase SIncrease S

– Increase temperatureIncrease temperature Decrease QDecrease Q

– Dilute solutionDilute solution– Well mixed (stirring)Well mixed (stirring)

pH control of precipitationpH control of precipitation

CaCa2+2+ + C + C22OO442-2- CaC CaC22OO44 (s) (s)

HH22CC22OO44 2 H 2 H+ + + C+ C22OO442-2-

Feeder Reaction:Feeder Reaction:

Homogeneous PrecipitationHomogeneous Precipitation

FeFe3+3+ + 3 HCO + 3 HCO22- - Fe(HCO Fe(HCO22))33nHnH22OO

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

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

Feeder Reaction:Feeder Reaction:

High Electrolyte High Electrolyte ConcentrationConcentration

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

Qualitative AnalysisQualitative Analysis

This is not quantitative. Simply This is not quantitative. Simply identification of ions present.identification of ions present.

Methods (some can be quantitative)Methods (some can be quantitative) PrecipitationPrecipitation Flame Test (color)Flame Test (color) Solution (color)Solution (color) Sodium HydroxideSodium Hydroxide AmmoniaAmmonia Sulfuric and Hydrochloric acid tests.Sulfuric and Hydrochloric acid tests. ChromatographyChromatography Flame ionization emission spectroscopy.Flame ionization emission spectroscopy.

Qualitative AnalysisQualitative Analysis Group I: AgGroup I: Ag++, Hg, Hg2+2+, Pb, Pb2+2+

Precipitated in 1 M HCl Precipitated in 1 M HCl

Group II: BiGroup II: Bi3+3+, Cd, Cd2+2+, Cu, Cu2+2+, Hg, Hg2+2+, (Pb, (Pb2+2+), Sb), Sb3+3+ and Sb and Sb5+5+, , SnSn2+2+ and Sn and Sn4+4+

Precipitated in 0.1 M HPrecipitated in 0.1 M H22S solution at pH 0.5 S solution at pH 0.5

Group III: AlGroup III: Al3+3+, (Cd, (Cd2+2+), Co), Co2+2+, Cr, Cr3+3+, Fe, Fe2+2+ and Fe and Fe3+3+, , MnMn2+2+, Ni, Ni2+2+, Zn, Zn2+2+

Precipitated in 0.1 M HPrecipitated in 0.1 M H22S solution at pH 9 S solution at pH 9

Group IV: BaGroup IV: Ba2+2+, Ca, Ca2+2+, K, K++, Mg, Mg2+2+, Na, Na++, NH, NH44++

BaBa2+2+, Ca, Ca2+2+, and Mg, and Mg2+2+

Precipitated in 0.2 M (NHPrecipitated in 0.2 M (NH44))22COCO33 solution at pH 10 solution at pH 10

Precipitation Titration Curve Precipitation Titration Curve EquationsEquations

At Equivalence PointAt Equivalence Point

[A][T] = K[A][T] = Kspsp

After the Equivalence PointAfter the Equivalence Point

[T] = (Original Concentration of Titrant)(Volume of excess [T] = (Original Concentration of Titrant)(Volume of excess Titrant / Total Volume of Solution)Titrant / Total Volume of Solution)

pT = -log[T]pT = -log[T]

Precipitation Titration Curve Precipitation Titration Curve ExampleExample

25.00 mL of 0.1000 M I25.00 mL of 0.1000 M I-- was titrated with was titrated with 0.05000M Ag0.05000M Ag++..

AgAg++ + I + I-- AgI(s) AgI(s)

The solubility product for AgI is 8.3 The solubility product for AgI is 8.3 xx 10 10-17-17. . Calculate the concentration of ICalculate the concentration of I-- ion in ion in solution solution

(a) after addition of 10.00 mL of Ag(a) after addition of 10.00 mL of Ag++; ; (b) after addition of 52.00 mL of Ag(b) after addition of 52.00 mL of Ag++; ; (c) at the equivalence point.(c) at the equivalence point.

Precipitation Titration Curve Precipitation Titration Curve ExampleExample

25.00 mL of 0.04132 M Hg25.00 mL of 0.04132 M Hg22(NO(NO33))22 was was titrated with 0.05789 M KIOtitrated with 0.05789 M KIO33..

HgHg222+2+ + 2IO + 2IO33

-- Hg Hg22(IO(IO33))22(s)(s)

The solubility product for HgThe solubility product for Hg22(IO(IO33))22 is 1.3 x is 1.3 x 1010-18-18. Calculate the concentration of Hg. Calculate the concentration of Hg22

2+2+ ion in solution ion in solution

(a) after addition of 34.00 mL of KIO3; (a) after addition of 34.00 mL of KIO3; (b) after addition of 36.00 mL of KIO3; (b) after addition of 36.00 mL of KIO3; (c) at the equivalence point.(c) at the equivalence point.

Precipitation Titration Curve Precipitation Titration Curve ExampleExample

Consider the titration of 50.00 mL of Consider the titration of 50.00 mL of 0.0246 M Hg(NO0.0246 M Hg(NO33))22 with 0.104 KSCN. with 0.104 KSCN. Calculate the value of pHgCalculate the value of pHg22

2+2+ at each of at each of the following points and sketch the the following points and sketch the titration curve: titration curve:

0.25V0.25VTT

0.5V0.5VTT

0.75V0.75VTT

1.05V1.05VTT

1.25V1.25VTT

Titration Curve ShapeTitration Curve Shape

1:1 Stoichiometry of Reagents1:1 Stoichiometry of Reagents– Equivalence point is the steepest point of a curveEquivalence point is the steepest point of a curve

Maximum slopeMaximum slope An inflection pointAn inflection point

Other StoichiometriesOther Stoichiometries– The curve is not symmetric about the equivalence point The curve is not symmetric about the equivalence point – The equivalence point is not at the center of the The equivalence point is not at the center of the

steepest section of the curvesteepest section of the curve

The less soluble the product, the sharper the The less soluble the product, the sharper the curve around the equivalence pointcurve around the equivalence point

Titration Curve ShapeTitration Curve Shape

Titration of a MixtureTitration of a Mixture

The less soluble product forms firstThe less soluble product forms first If there is sufficient difference in If there is sufficient difference in

solubility of productssolubility of products– First precipitation is nearly complete First precipitation is nearly complete

before the second one beginsbefore the second one beginsSeparation by precipitation Separation by precipitation

– CoprecipitationCoprecipitationAlters the expected endpoints Alters the expected endpoints

Titration of a Mixture ExampleTitration of a Mixture Example

Detection of End PointDetection of End Point

Methods for determining the end point for Methods for determining the end point for precipitation titrationsprecipitation titrations

– IndicatorsIndicators– ElectrodesElectrodes– Light scatteringLight scattering

We’ll discuss indicators used for the AgWe’ll discuss indicators used for the Ag++ Cl Cl-- titration equilibriumtitration equilibrium– Titrations with AgTitrations with Ag++ are called argentometric are called argentometric

titrationstitrations

IndicatorsIndicators

Volhard titrationVolhard titration– Uses the formation of a soluble colored Uses the formation of a soluble colored

complex at the end pointcomplex at the end point

Fajans titrationFajans titration– A colored indicator is adsorbed onto the A colored indicator is adsorbed onto the

precipitate at the end pointprecipitate at the end point

Used as a procedure for titrating AgUsed as a procedure for titrating Ag++; determination ; determination of Clof Cl-- requires a back-titration requires a back-titration– First, ClFirst, Cl-- is precipitated by excess AgNO is precipitated by excess AgNO33

– Excess AgExcess Ag++ is titrated with KSCN in the presence of Fe is titrated with KSCN in the presence of Fe3+3+

– When AgWhen Ag++ has been consumed, a red complex forms has been consumed, a red complex forms

The Volhard titration can be used for any anion that The Volhard titration can be used for any anion that forms an insoluble salt with silverforms an insoluble salt with silver

Volhard TitrationVolhard Titration

(s)(aq)(aq) AgCl Cl Ag -

(s)(aq)(aq) AgSCN SCN Ag -

(aq)(aq)(aq) 2-3 FeSCN SCN Fered

Fajans TitrationFajans Titration The technique uses an adsorption indicator. Prior to the equivalence point, there is excess Cl- in solution. Some is adsorbed on the surface of the crystal, giving a partial negative charge. After the equivalence point, there is excess Ag+ in solution. Some adsorbs to the surface imparting a partial positive charge to the precipitate. Choosing an indicator with a partial negative charge will cause it to adsorb to the surface.

O-O

Cl

O

Cl

CO2-Dichlorofluorescein is green in solution but pink when absorbed on AgCl