kimia analisa 33 sks sks -...

KIMIA ANALISAKIMIA ANALISA33 SKSSKS

Dr. Heru SetyawanDr. Heru SetyawanJurusan Teknik Kimia FTI Jurusan Teknik Kimia FTI –– ITSITS

Titrimetric Methods of Titrimetric Methods of AnalysisAnalysis

Equivalence Points and End PointsEquivalence Points and End Points

• For a titration to be accurate a stoichiometrically equivalent amount of titrant must be added to a solution containing the analyte.

• The product of the equivalence point volume, Veq, and the titrant’s concentration, CT, gives the moles of titrant reacting with the analyte

Teq titrant of Moles CV ×=

Dr. Heru Setyawan, Jurusan Teknik Kimia FTI Dr. Heru Setyawan, Jurusan Teknik Kimia FTI -- ITSITS

• Unfortunately, there is usually no obvious indication that the equivalent point has been reached.

• Instead, adding titrant is stopped when end point is reached.• The difference between the end point volume and the equivalence point

volume is called titration error.

Volume as a SignalVolume as a Signal

Almost any chemical reaction can serve as a titrimetric method provided that three conditions are met:

1. All reactions involving the titrant and analyte must be of known stoichiometry.

2. The titration reaction must occur rapidly.3. A suitable method must be available for determining the end point with

an acceptable level of accuracy.


Volume as a Signal: Example 1Volume as a Signal: Example 1

Analysis Ag+ using thiocyanate, SCN-, as a titrant

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

• This reaction occurs quickly and is of known stoichiometry.• A titrant of SCN- is easily prepared using KSCN.• The titration’s end point can be indicated by adding a small amount of

3+


Fe3+ to the solution containing the analyte to form a red-colored Fe(SCN)2+ complex.

DIRECT TITRATION


Determination the concentration of formaldehyde, H2CO, in an aqueous solution by taking advantage of oxidation of H2CO by I3

-

H2CO(aq) + 3OH-(aq) + I3-(aq) HCO2

-(aq) + 3I-(aq) + 2H2O(l)

• This reaction is a useful reaction, except too slow for a direct titration.• If a known amount of I3

- is added in excess, the reaction is allowed to go to completion.


go to completion.• The I3

- remaining can then be titrated with thiosulfate, S2O32-.

I3-(aq) + S2O3

2- S4O62-(aq) + 3I-(aq)

BACK TITRATION


Ca2+(aq) + Y4-(aq) CaY2-(aq)

• It often happens that there is no suitable indicator for this direct titration.• Reacting Ca2+ with an excess of the Mg2+−EDTA complex releases an

equivalent amount of Mg2+.

Determination of Ca2+ in an aqueous solution by taking advantage its reaction with the ligand ethylenediaminetetraacetic acid (EDTA), which will be represented as Y4-.


equivalent amount of Mg .

DISPLACEMENT TITRATION

Ca2+(aq) + MgY2-(aq) CaY2-(aq) + Mg2+(aq)

• Titrating the released Mg2+ with EDTA

Mg2+(aq) + Y4-(aq) MgY2-(aq)

• The amount of Mg2+ titrated provides an indirect measure of the amount of Ca2+ in the original sample.


When a suitable reaction involving the analyte does not exist it may be possible to generate a species that is easily titrated.

Example:

The sulfur content of coal can be determined by using a combustion reaction to convert sulfur to sulfur dioxide.

S(s) + O2(g) SO2(g)


Passing the SO2 through an aqueous solution of hydrogen peroxide, H2O2, produces sulfuric acid, which can be titrated with NaOH, providing an indirect determination of sulfur.

SO2(g) + H2O2(aq) → H2SO4(aq)

H2SO4(aq) + 2OH-(aq) SO42-(aq) + 2H2O(l)

Titration CurveTitration Curve

0.06

0.08

0.10

0.12

[H+ ] (

M)

[H+]

6

8

10

12

14

pH

pH

Equivalence point


0 10 20 30 40 50 60 70 800.00

0.02

0.04

[H

Volume NaOH (mL)

0

2

4

Overview of TitrimetryOverview of Titrimetry

�� TitrimetryTitrimetry:: Any method in which volume is the signal.Any method in which volume is the signal.�� TitrantTitrant:: The reagent added to a solution containing the The reagent added to a solution containing the analyteanalyte and and

whose volume is the signal.whose volume is the signal.�� Equivalence point:Equivalence point: The point in a titration where The point in a titration where stoichiometricallystoichiometrically

equivalent amounts of equivalent amounts of analyteanalyte and and titranttitrant react.react.�� End point:End point: The point in a titration where we stop adding The point in a titration where we stop adding titranttitrant..�� Indicator:Indicator: A colored compound where change in color signals the end A colored compound where change in color signals the end

point of a titration.point of a titration.Titration error:Titration error: The determinate error in a titration due to the difference The determinate error in a titration due to the difference


�� Titration error:Titration error: The determinate error in a titration due to the difference The determinate error in a titration due to the difference between the end point and the equivalence point.between the end point and the equivalence point.

�� Back titration:Back titration: A titration in which a reagent is added to a solution A titration in which a reagent is added to a solution containing the containing the analyteanalyte, and the excess reagent remaining after its , and the excess reagent remaining after its reaction with the reaction with the analyteanalyte is determined by a titration.is determined by a titration.

�� Displacement titration:Displacement titration: A titration in which the A titration in which the analyteanalyte displaces a displaces a species, usually form a complex, and the amount of the displaced species, usually form a complex, and the amount of the displaced species is determined by a titration.species is determined by a titration.

�� Titration curve:Titration curve: A graph showing the progress of a titration as a A graph showing the progress of a titration as a function of the volume of function of the volume of titranttitrant added.added.

Titrattion Based on AcidTitrattion Based on Acid--Base ReactionsBase Reactions

�� The earliest acidThe earliest acid--base titrations involved the determination of the acidity or base titrations involved the determination of the acidity or alkalinity of solutions, and the purity of carbonates and alkaline earth oxides.alkalinity of solutions, and the purity of carbonates and alkaline earth oxides.

�� Before 1800: conducted using HBefore 1800: conducted using H22SOSO44, HCl and HNO, HCl and HNO33 as acidic titrants and as acidic titrants and KK22COCO33 and Naand Na22COCO33 as basic titrants using visual indicators such as limus (red in as basic titrants using visual indicators such as limus (red in acid and blue in basic) acid and blue in basic) �� the accuracy was limited by the usefulness the the accuracy was limited by the usefulness the indicator and by the lack of a strong base titrant for the analysis of weak acids.indicator and by the lack of a strong base titrant for the analysis of weak acids.

�� 1846: the utility was improved when NaOH was first introduced as strong base 1846: the utility was improved when NaOH was first introduced as strong base titrant and the discovery of new indicators (Phenolphtalein was discovered in titrant and the discovery of new indicators (Phenolphtalein was discovered in 1871 by Bayer, used as indicator in 1877, followed by methyl orange). Despite 1871 by Bayer, used as indicator in 1877, followed by methyl orange). Despite the increasing availability of indicators, the absence of a theory of acidthe increasing availability of indicators, the absence of a theory of acid--base base the increasing availability of indicators, the absence of a theory of acidthe increasing availability of indicators, the absence of a theory of acid--base base reactivity made selecting a proper indicator difficult.reactivity made selecting a proper indicator difficult.

�� Late 19th century: developments in equilibrium theory led to significant Late 19th century: developments in equilibrium theory led to significant improvements in the theoretical understanding of acidimprovements in the theoretical understanding of acid--base chemistry, and in base chemistry, and in turn, of acidturn, of acid--base titrimetry.base titrimetry.�� Sorenson’s establisment of the pH scale in 1909 provided a rigorous means Sorenson’s establisment of the pH scale in 1909 provided a rigorous means

for comparing visual indicators.for comparing visual indicators.�� The determination of acidThe determination of acid--base dissociation constants made the calculation base dissociation constants made the calculation

of theoretical titration curves possible (Bjerrum, 1914) of theoretical titration curves possible (Bjerrum, 1914) �� a rational method a rational method existed for selecting visual indicators, establishing acidexisted for selecting visual indicators, establishing acid--base titrimetry as a base titrimetry as a useful alternative to gravimetry.useful alternative to gravimetry.


AcidAcid--Base Titrations CurvesBase Titrations Curves

�� Titrating strong acids and strong basesTitrating strong acids and strong bases�� Titration of 50.0 mL of 0.100 M HCl with 0.200 M NaOH. For the reaction of Titration of 50.0 mL of 0.100 M HCl with 0.200 M NaOH. For the reaction of

strong base with a strong acid the only equilibrium reaction of importance isstrong base with a strong acid the only equilibrium reaction of importance isHH33OO++((aqaq) + OH) + OH--((aqaq) ) ⇔⇔ 2H2H22O(O(ll) () (KKww))--11 = 10= 101414

�� First, calculate the volume of NaOH needed to reach the equivalence poin:First, calculate the volume of NaOH needed to reach the equivalence poin:Moles HCl = moles NaOH or Moles HCl = moles NaOH or MMaaVVaa = = MMbbVVbb

�� The volume of NaOH needed to reach the equivalence pointThe volume of NaOH needed to reach the equivalence point

( )( )( ) mL 0.25

M 200.0

mL 0.50M 100.0 ====b

aabeq

M

VMVV

�� Before the equivalence point, HCl is present in excess and the pH is Before the equivalence point, HCl is present in excess and the pH is determined by the concentration of excess HCl. Initially the solution is 0.100 determined by the concentration of excess HCl. Initially the solution is 0.100 M in HCl:M in HCl:

pH = pH = ––log [Hlog [H33OO++] = ] = ––log [HCl] = log [HCl] = ––log (0.100) = 1.00 log (0.100) = 1.00 �� Since the equilibrium constant for the reaction is large, we can treat as Since the equilibrium constant for the reaction is large, we can treat as

though it goes to completion. After adding 10.0 mL of NaOH, the though it goes to completion. After adding 10.0 mL of NaOH, the concentration of excess HCl isconcentration of excess HCl is

giving a pH of 1.30.giving a pH of 1.30.


( )M 200.0bM

[ ]

( )( ) ( )( )M 050.0

mL 0.10mL 0.50

mL 0.10M 200.0mL 0.50M 100.0

volume total

HCl excess molesHCl

=+−=

+−==

ba

bbaa

VV

VMVM


�� At the equivalence point the moles of HCl and the moles of NaOH are equal At the equivalence point the moles of HCl and the moles of NaOH are equal and the pH can be determined by the dissociation of water:and the pH can be determined by the dissociation of water:

KKww = 10= 10--1414 = [H= [H33OO++][OH][OH--] = [H] = [H33OO++]]22

[H[H33OO++] = 10] = 10--77

Thus, the pH at the equivalence point is 7.00.Thus, the pH at the equivalence point is 7.00.�� Finally, for volumes of NaOH greater than the equivalence point volume, the Finally, for volumes of NaOH greater than the equivalence point volume, the

pH is determined by the concentration of excess OHpH is determined by the concentration of excess OH--. For example, after . For example, after adding 30.0 mL of titrant the concentration of OHadding 30.0 mL of titrant the concentration of OH-- isis

[ ]volume total

NaOH excess molesOH

-

+−==

ba

aabb

VV

VMVM

�� To find the concentration of HTo find the concentration of H33OO++, we use the , we use the KwKw expressionexpression

giving a pH of 12.10.giving a pH of 12.10.


( )( ) ( )( )M 0125.0

mL 0.10mL 0.50

mL 0.50M 100.0mL 0.10M 200.0

volume total

=+−=

+ ba VV

[ ] [ ]13

14

3 1000.80125.0

10

OHOH −

−

−+ ×=== wK


Volume (mL) of titrant pH

0.00 1.00

5.00 1.14

10.00 1.30

15.00 1.51

20.00 1.85

22.00 2.08 8

10

12

14

Equivalence point

pH

Data and curve for titration of 50.0 mL of 0.100 M HCl with 0.0500 M NaOH

24.00 2.57

25.00 7.00

26.00 11.42

28.00 11.89

30.00 12.37

35.00 12.50

40.00 12.52

45.00 12.62

50.00 12.70Dr. Heru Setyawan, Jurusan Teknik Kimia FTI Dr. Heru Setyawan, Jurusan Teknik Kimia FTI -- ITSITS

0 10 20 30 40 500

2

4

6Equivalence point

pHVolume NaOH

Selecting and Evaluating the End PointSelecting and Evaluating the End Point

�� Equivalence point:Equivalence point: The point in a titration where The point in a titration where stoichiometricallystoichiometrically equivalent equivalent amounts of amounts of analyteanalyte and and titranttitrant reactreact �� the location can be calculated as the location can be calculated as discussed beforediscussed before..

�� End point:End point: The point in a titration where we stop adding The point in a titration where we stop adding titranttitrant �� where?where?�� Using visual indicator Using visual indicator �� weak acids and bases which are the weak acids and bases which are the derivatives of derivatives of

organic dyes organic dyes �� have at least one conjugate acidhave at least one conjugate acid--base species that is base species that is highly colored, their titration results in a change in both pH and color.highly colored, their titration results in a change in both pH and color.

�� The pH at which an acidThe pH at which an acid--base indicator changes color is determined by its base indicator changes color is determined by its acid dissociation constant.acid dissociation constant.

�� For an indicator that is a monoprotic weak acid, Hin, the dissociation For an indicator that is a monoprotic weak acid, Hin, the dissociation reactionreaction

HIn(HIn(aqaq) + H) + H22O(O(ll) ) ⇔⇔ HH33OO++((aqaq) + In) + In--((aqaq))HIn(HIn(aqaq) + H) + H22O(O(ll) ) ⇔⇔ HH33OO ((aqaq) + In) + In ((aqaq))for which the equilibrium constant isfor which the equilibrium constant is

andand

�� HIn and InHIn and In-- indicators have different colors. The color of a solution indicators have different colors. The color of a solution containing an indicator, continuously changes as the concentration of HIn containing an indicator, continuously changes as the concentration of HIn decreases and the concentration Indecreases and the concentration In-- increases. If both HIn and Inincreases. If both HIn and In-- can be can be detected with equal ease, the transition between the two colors reaches its detected with equal ease, the transition between the two colors reaches its midpoint when their concentrations are identical or when the pH is equal to midpoint when their concentrations are identical or when the pH is equal to the indicator’s pthe indicator’s pKKaa �� the equivalence point and the end point coincide if an the equivalence point and the end point coincide if an indicator is selected whose indicator is selected whose ppKKaa is equal to the pH at the equivalence point.is equal to the pH at the equivalence point.


[ ][ ][ ]HIn

OHIn 3- +

=aK[ ][ ]HIn

InlogpH

-

+= apK

Properties of Indicators for AcidProperties of Indicators for Acid--Base titrationsBase titrations

Indicator Acid Color Base Color pH range pKa

Cresol red Red Yellow 0.2–1.8 –

Thymol blue Red Yellow 1.2–2.8 1.7

Bromophenol blue Yellow Blue 3.0–4.6 4.1

Methyl orange Red Orange 3.1–4.4 3.7

Congo red Blue Red 3.0–5.0 –

Bromocresol green Yellow Blue 3.8–5.4 4.7

Methyl red Red Yellow 4.2–6.3 5.0

Bromocresol purple Yellow Purple 5.2–6.8 6.1

Litmus Red Blue 5.0–8.0 –

Bromothymol blue Yellow Blue 6.0–7.6 7.1

Phenol red Yellow Red 6.8–8.4 7.8

Cresol red Yellow Red 7.2–8.8 8.2

Thymol blue Yellow Blue 8.0–9.6 8.9

Phenolphtalein Colorless Red 8.3–10.0 9.6

Alizarin yellow R Yellow Orange/red 10.1–12.0 –Dr. Heru Setyawan, Jurusan Teknik Kimia FTI Dr. Heru Setyawan, Jurusan Teknik Kimia FTI -- ITSITS

Properties of Mixed Indicators for AcidProperties of Mixed Indicators for Acid--Base titrationsBase titrations

Mixed Indicator Acid Color Base Color pH range

Bromocresol green & Methyl orange Orange Blue-green 3.5–4.3

Bromocresol green & chlorophenolred

Yellow-green Blue-violet 5.4–6.2

Bromothymol blue & phenol red yellow violet 7.2–7.6

3.1–4.4

Screen Indicator Acid Color Base Color pH range

Dimethyl yellow & methylene blue Blue-violet Green 3.2–3.4Dimethyl yellow & methylene blue Blue-violet Green 3.2–3.4

Methyl red & methylene blue Red-violet Green 5.2–5.6

Neutral red & methylene blue Violet-blue green 6.8–7.3


Finding the End PointFinding the End Point

�� Monitoring pH by pH electrodeMonitoring pH by pH electrode�� Plotting the first or second derivative of the titration Plotting the first or second derivative of the titration

curve curve �� the slope of the titration curve reaches its the slope of the titration curve reaches its maximum value at its inflection point.maximum value at its inflection point.

�� Monitoring temperature Monitoring temperature �� exothermic reactionexothermic reaction


Titrations curves for a weak acid with 0.100 M Titrations curves for a weak acid with 0.100 M NaOHNaOH

0

2

4

6

8

10

12

14

0 5 10 15 20 25

pH

Volume of Titrant (mL)

0

2

4

6

8

10

0 5 10 15 20 25

∆∆ ∆∆p

H/ ∆∆ ∆∆

V

Volume of Titrant (mL)

Normal titration curve 1st derivative titration curve


-40

-30

-20

-10

0

10

20

30

40

2 4 6 8 10 12 14 16 18 20 22∆∆ ∆∆2p

H/ ∆∆ ∆∆

V2

Volume of titrant (mL)

0

40

80

120

160

10 11 12 13 14 15 16V

b[H

+]x

10

7

Volume of titrant (mL)

Normal titration curve

2nd derivative titration curve Gran plot: linearized form of tittratin curve, intercept = V titrant at end point, slope = -Ka

Representative Method: Determination of Protein in Representative Method: Determination of Protein in BreadBread�� Description of the method:Description of the method: this quantitative method of analysis for proteins is this quantitative method of analysis for proteins is

based on a determination of the %w/w N in the sample. Since different cereal based on a determination of the %w/w N in the sample. Since different cereal proteins have similar amounts of nitrogen, the experimentally determined %w/w proteins have similar amounts of nitrogen, the experimentally determined %w/w N is multiplied by a factor 0f 5.7 to give the % w/w protein in the sample (on N is multiplied by a factor 0f 5.7 to give the % w/w protein in the sample (on average there are 5.7 g of cereal protein for every gram of nitrogen). As average there are 5.7 g of cereal protein for every gram of nitrogen). As described here, nitrogen is determined by the described here, nitrogen is determined by the KjeldahlKjeldahl method. The protein in a method. The protein in a sample of bread is oxidized in hot concentrated Hsample of bread is oxidized in hot concentrated H22SOSO44, converting the nitrogen , converting the nitrogen to NHto NH44

++. After making the solution alkaline, converting NH4. After making the solution alkaline, converting NH4++ to NHto NH33, the , the ammonia is distilled into a flask containing a known amount standard strong ammonia is distilled into a flask containing a known amount standard strong acid. Finally, the excess strong acid is determined by a back titration with a acid. Finally, the excess strong acid is determined by a back titration with a standard strong base standard strong base titranttitrant..

�� Procedure: Procedure: Transfer a 2.0Transfer a 2.0--g sample of bread, which has previously been air g sample of bread, which has previously been air �� Procedure: Procedure: Transfer a 2.0Transfer a 2.0--g sample of bread, which has previously been air g sample of bread, which has previously been air dried and ground into a powder, to a suitable digestion flask, along with 0.7 g of dried and ground into a powder, to a suitable digestion flask, along with 0.7 g of HgOHgO as a catalyst, 10 g of Kas a catalyst, 10 g of K22SOSO44 and 25 and 25 mLmL of concentrated Hof concentrated H22SOSO44. Bring the . Bring the solution to a boil, and continue boiling until the solution turns clear, and for at solution to a boil, and continue boiling until the solution turns clear, and for at least an additional 30 min. After cooling to below room temperature, add 200 least an additional 30 min. After cooling to below room temperature, add 200 mLmL of Hof H22O and 25 O and 25 mLmL of 4%w/v Kof 4%w/v K22S to remove the HgS to remove the Hg2+2+ catalyst. Add a few Zn catalyst. Add a few Zn granules to serve as boiling stones, and 25 g of granules to serve as boiling stones, and 25 g of NaOHNaOH. Quickly connect the . Quickly connect the flask to a distillation apparatus, and distill the NH3 into a collecting flask flask to a distillation apparatus, and distill the NH3 into a collecting flask containing a known amount of standardized containing a known amount of standardized HClHCl. The tip of the condenser . The tip of the condenser should be placed below the surface of the strong acid. After the distillation is should be placed below the surface of the strong acid. After the distillation is complete, titrate the excess strong acid a standard solution of complete, titrate the excess strong acid a standard solution of NaOHNaOH, using , using methyl red as a visual indicator.methyl red as a visual indicator.


Representative Method: Determination of Protein in Representative Method: Determination of Protein in BreadBreadQuestionsQuestions1.1. Oxidizing the protein converts the nitrogen to NHOxidizing the protein converts the nitrogen to NH44

++. Why is the amount of . Why is the amount of nitrogen not determined by titrating the NHnitrogen not determined by titrating the NH44

++ with a strong base?with a strong base?There are two reasons for not titrating the ammonium ion. First, NHThere are two reasons for not titrating the ammonium ion. First, NH44

++ is a very is a very weak acid (weak acid (KKaa = 5.7 = 5.7 ×× 1010--1010) that yields a poorly defined ) that yields a poorly defined ebdebd point when point when titrated with a strong base. Second, even if the end point can be determined titrated with a strong base. Second, even if the end point can be determined with acceptable accuracy and precision, the procedure calls for adding a with acceptable accuracy and precision, the procedure calls for adding a substantial amount of Hsubstantial amount of H22SOSO44. After the oxidation is complete, the amount of . After the oxidation is complete, the amount of excess Hexcess H22SOSO44 will be much greater than the amount of NHwill be much greater than the amount of NH44

++ that is produced. that is produced. The presence of two acids that differ greatly in concentration makes for a The presence of two acids that differ greatly in concentration makes for a difficult analysis. If the difficult analysis. If the titrant’stitrant’s concentration is similar to that of Hconcentration is similar to that of H22SOSO44, then , then the equivalence point volume for the titration of NHthe equivalence point volume for the titration of NH ++ may be too small to may be too small to

22 44the equivalence point volume for the titration of NHthe equivalence point volume for the titration of NH44

++ may be too small to may be too small to measure reliably. On the other hand, if the concentration of the measure reliably. On the other hand, if the concentration of the titranttitrant is is similar to that of NHsimilar to that of NH44

++, the volume needed to neutralize the H, the volume needed to neutralize the H22SOSO44 will be will be unreasonably large.unreasonably large.

2.2. Ammonia is a volatile compound as evidenced by the strong smell of even Ammonia is a volatile compound as evidenced by the strong smell of even dilute solutions. This volatility presents a possible source of determinate error. dilute solutions. This volatility presents a possible source of determinate error. Will this determinate error be negative or positive?Will this determinate error be negative or positive?The conversion of N to NH3 follows the following pathwayThe conversion of N to NH3 follows the following pathway

N N �� NHNH44++

NHNH44++ �� NHNH33

Any loss of NHAny loss of NH33 is loss of is loss of analyteanalyte and a negative determinate error.and a negative determinate error.-- ContinuedContinued


Representative Method: Determination of Protein in Representative Method: Determination of Protein in BreadBreadContinued from previous slideContinued from previous slide3.3. Discuss the steps taken in this procedure to minimize this determinate error.Discuss the steps taken in this procedure to minimize this determinate error.

Three specific steps are taken to minimize the loss of ammonia: (1) the Three specific steps are taken to minimize the loss of ammonia: (1) the solution is cooled to below room temperature before adding solution is cooled to below room temperature before adding NaOHNaOH; (2) the ; (2) the digestion flask is quickly connected to the distillation apparatus after adding digestion flask is quickly connected to the distillation apparatus after adding NaOHNaOH; and (3) the condenser tip of the distillation apparatus is placed below ; and (3) the condenser tip of the distillation apparatus is placed below the surface of the the surface of the HClHCl to ensure that the ammonia will react with to ensure that the ammonia will react with HClHCl before it before it can be lost through volatilization.can be lost through volatilization.

4.4. How does K2S remove Hg2+, and why is this important?How does K2S remove Hg2+, and why is this important?4.4. How does K2S remove Hg2+, and why is this important?How does K2S remove Hg2+, and why is this important?Adding sulfide precipitates the Hg2+ as Adding sulfide precipitates the Hg2+ as HgSHgS. This is important because NH3 . This is important because NH3 forms stable complexes with many metal ions, including Hg2+. Any NH3 that forms stable complexes with many metal ions, including Hg2+. Any NH3 that is is complexedcomplexed with Hg2+ will not collected by distillation, providing another with Hg2+ will not collected by distillation, providing another source of determinate error.source of determinate error.


Quantitative ApplicationsQuantitative Applications

Primary Standard Standardization of Acidic TitrantsTitration Reaction

Comment

Na2CO3 Na2CO3 + 2H3O+ � H2CO3 + 2Na+ + 2H2O a

TRIS (HOCH2)3CNH2 + H3O+ � (HOCH2)3CNH3+ + H2O b

Na2B4O7 Na2B4O7 + 2H3O+ + 3H2O � 2Na+ + 4H3BO3

Primary Standard Standardization of Basic TitrantsTitration Reaction

Comment

Selected primary standards for the standardization of strong acid & strong base titrants

KHC8H4O4 KHC8H4O4 + OH- � K+ + C8H4O42- + H2O C

C6H5COOH C6H5COOH + OH- � C6H5COO- + H2O d

KH(IO3)2 KH(IO3)2 + OH- � K+ + 2IO3- + H2O


a) The end point is improved by titrating to the second equivalence point, boiling the solution to expel CO2, and retitrating to the second equivalence point. The reaction is Na2CO3 + 2H3O+ � CO2 +2Na+ + 3H2O

b) TRIS stands for tris-(hydroxymethyl)aminomethane.c) KHC8H4O4 is also known as potassium hydrogen phtalate or KHP.d) Due to its poor solubility in water, benzoic acid is dissolved ia a small amount of

ethanol before being diluted with water.

Quantitative Applications: InorganicQuantitative Applications: Inorganic

�� AcidAcid--base base titrimetrytitrimetry is a standard method for the quantitative analysis of many is a standard method for the quantitative analysis of many inorganic acids and bases inorganic acids and bases �� Standard solutions of Standard solutions of NaOHNaOH for inorganic acids, for inorganic acids, e.g., He.g., H33POPO44 or Hor H33AsOAsO44; ; HClHCl for inorganic bases, e.g., Nafor inorganic bases, e.g., Na22COCO33..

�� Inorganic acids and bases too weak to be analyzed by an aqueous acidInorganic acids and bases too weak to be analyzed by an aqueous acid--base base titration can be analyzed by adjusting the solvent or by an indirect analysis.titration can be analyzed by adjusting the solvent or by an indirect analysis.�� The accuracy in titrating HThe accuracy in titrating H33BOBO33 with with NaOHNaOH is limited by its small is limited by its small

dissociation constant of 5.8 dissociation constant of 5.8 ×× 1010--1010 �� the acid strength can be increased by the acid strength can be increased by adding adding mannitolmannitol to form a complex with borate ion to form a complex with borate ion (K(Kaa = 1.5 = 1.5 ×× 1010--44))..

�� The analysis of ammonium salts is limited by the small acid dissociation The analysis of ammonium salts is limited by the small acid dissociation constant of 5.7 constant of 5.7 ×× 1010--1010 for NHfor NH44

++ �� can be converted to NHcan be converted to NH33 by neutralizing by neutralizing with strong base (with strong base (KKbb = 1.8 = 1.8 ×× 1010--55).).with strong base (with strong base (KKbb = 1.8 = 1.8 ×× 1010 ).).

�� Inorganic Inorganic analytesanalytes that are neutral in aqueous solutions may still be analyzed if that are neutral in aqueous solutions may still be analyzed if they can be converted to an acid or base.they can be converted to an acid or base.�� NO3NO3-- can be quantitatively analyzed by reducing it to NH3 in a strongly can be quantitatively analyzed by reducing it to NH3 in a strongly

alkaline solution using alkaline solution using Devarda’sDevarda’s alloy, a mixture of 50% w/w Cu, 45% w/w alloy, a mixture of 50% w/w Cu, 45% w/w Al and 5% w/w Zn.Al and 5% w/w Zn.

3NO3NO33--((aqaq) + 8Al() + 8Al(ss) + 5OH) + 5OH--((aqaq) + 2H) + 2H22O(O(ll) ) �� 8AlO8AlO22

--((aqaq) + 3NH) + 3NH33((aqaq))NHNH33 is removed by distillation and titrated with is removed by distillation and titrated with HClHCl..

�� Alternatively, NOAlternatively, NO33-- can be titrated as a weak base in an acidic can be titrated as a weak base in an acidic nonaqueousnonaqueous

solvent such as anhydrous acetic acid, using HClOsolvent such as anhydrous acetic acid, using HClO44 as a as a titranttitrant..


Quantitative Applications: InorganicQuantitative Applications: Inorganic

�� AcidAcid--base base titrimetrytitrimetry continues to be listed as the standard continues to be listed as the standard method for the determination of alkalinity, acidity and free COmethod for the determination of alkalinity, acidity and free CO22 in in water and wastewater analysis.water and wastewater analysis.�� Alkalinity:Alkalinity: a measure of the acida measure of the acid--neutralizing capacity of a neutralizing capacity of a

water sample and is assumed to arise principally from OHwater sample and is assumed to arise principally from OH--, , HCOHCO33

-- and COand CO3322-- although other weak bases, e.g., phosphate, although other weak bases, e.g., phosphate,

may contribute to the total alkalinity may contribute to the total alkalinity �� determined by titrating determined by titrating with a standard solution of with a standard solution of HClHCl or Hor H22SOSO44 to a fixed end point to a fixed end point at a pH of 4.5 (at a pH of 4.5 (bromocresolbromocresol green end point).green end point).Acidity:Acidity: a measure of a water sample’s capacity for a measure of a water sample’s capacity for �� Acidity:Acidity: a measure of a water sample’s capacity for a measure of a water sample’s capacity for neutralizing base and is conveniently divided into strong acid neutralizing base and is conveniently divided into strong acid ((HClHCl, HNO, HNO33 and Hand H22SOSO44 commonly found in industrial effluents commonly found in industrial effluents and acid mine drainage) and weak acid (usually dominated by and acid mine drainage) and weak acid (usually dominated by the formation of Hthe formation of H22COCO33 from dissolved COfrom dissolved CO22, contributions , contributions from hydrolyzed metal ions, Fefrom hydrolyzed metal ions, Fe2+2+, Al, Al3+3+ and Mnand Mn2+2+ and organic and organic acids) acidity acids) acidity �� determined by titrating with a standard determined by titrating with a standard solution of solution of NaOHNaOH to fixed end points at pH 3.7 and 8.3.to fixed end points at pH 3.7 and 8.3.

�� Free COFree CO22:: determined by titrating with a standard solution of determined by titrating with a standard solution of NaOHNaOH to phenolphthalein end point (pH = 8.3).to phenolphthalein end point (pH = 8.3).


Quantitative Applications: OrganicQuantitative Applications: Organic

�� AcidAcid--base base titrimetrytitrimetry continuous to play an important role in continuous to play an important role in pharmaceutical, biochemical, agricultural and pharmaceutical, biochemical, agricultural and environmental laboratories.environmental laboratories.

�� The most widely employed is the The most widely employed is the KjeldahlKjeldahl analysisanalysis for for organic nitrogen, described earlier organic nitrogen, described earlier �� used in the analysis used in the analysis of of caffeincaffein and saccharin in pharmaceutical products, as well and saccharin in pharmaceutical products, as well as for the analysis of proteins, fertilizers, as for the analysis of proteins, fertilizers, sludgessludges and and sediments.sediments.

�� Several organic functional groups have weak acid or weak Several organic functional groups have weak acid or weak �� Several organic functional groups have weak acid or weak Several organic functional groups have weak acid or weak base properties that allow their direct determination by an base properties that allow their direct determination by an acidacid--base titration.base titration.�� Carboxylic (Carboxylic (––COOH), COOH), sulfonicsulfonic ((––SOSO33H) and H) and phenolicphenolic ((––

CC66HH55OH) functional groups are weak acids that can be OH) functional groups are weak acids that can be successfully titrated in either aqueous or successfully titrated in either aqueous or nonaqueousnonaqueoussolvents.solvents.

�� Aliphatic and aromatic amines are weak bases that be Aliphatic and aromatic amines are weak bases that be titrated using titrated using HClHCl in aqueous solution or HClOin aqueous solution or HClO44 in glacial in glacial acetic acid.acetic acid.


Selected elemental analyses based on acidSelected elemental analyses based on acid--base base titrationtitration

Element Liberated as

Reaction producing acid or base to be titrated

Titration

N NH3(g) NH3(g) + H3O+(aq) � NH4+(aq)

+ H2O(l)Excess H3O+ with strong base

S SO2(g) SO2(g) + H2O2(aq) �H2SO4O(aq)

H2SO4 with strong base

C CO2(g) CO2(g) + Ba(OH)2(aq) �BaCO (s)+ H O(l)

Excess Ba(OH)2 with strong acid


2 2BaCO3(s)+ H2O(l)

2strong acid

Cl HCl(g) HCl(g) + H2O(l) � H3O+(aq) + Cl-(aq)

H3O+ with strong base

F SiF4(g) 3SiF4(g) + 2H2O(l) �2H2SiF6(aq) +SiO2(s)

H2SiF6 with strong base

Quantitative CalculationsQuantitative Calculations

In an acid-base reaction the number of protons transferred between the acid and base is conserved; thus

base molesbase mole

accepted Hof moles acid moles

acid mole

donated Hof moles ×=×++


ExampleExample

A 50.00A 50.00--mL sample of a citrus drink requires 17.62 mL of 0.04166 M NaOH to mL sample of a citrus drink requires 17.62 mL of 0.04166 M NaOH to reach the phenolphtalein end point. Express the sample’s acidity in terms of grams reach the phenolphtalein end point. Express the sample’s acidity in terms of grams of citric acid, Cof citric acid, C66HH88OO77, per 100 mL., per 100 mL.SolutionSolutionSince citric acid is a triprotic weak acid, we must first decide to which equivalence Since citric acid is a triprotic weak acid, we must first decide to which equivalence point the titration has been carried. The three acid dissociation constants arepoint the titration has been carried. The three acid dissociation constants are

ppKKa1a1 = 3.13= 3.13 ppKKa2a2 = 4.76 = 4.76 ppKKa3a3 = 6.40= 6.40The phenolphtalein end point is basic, occuring at a pH of approximately 8.3 and The phenolphtalein end point is basic, occuring at a pH of approximately 8.3 and can be reached only if the titration proceeds to the third equivalence point; thus we can be reached only if the titration proceeds to the third equivalence point; thus we writewrite

3 3 ×× moles citric acid = moles NaOHmoles citric acid = moles NaOH3 3 ×× moles citric acid = moles NaOHmoles citric acid = moles NaOHMakingMaking appropriateappropriate substituionssubstituions forfor thethe molesmoles ofof citriccitric acidacid andand molesmoles ofof NaOHNaOHgivesgives thethe followingfollowing equationequation

whichwhich cancan bebe solvedsolved forfor thethe gramsgrams ofof citriccitric acidacid

SinceSince thisthis isis thethe gramsgrams ofof citriccitric acidacid inin anan aa 5050..0000--mLmL sample,sample, thethe concentrationconcentration ofofcitriccitric acidacid inin thethe citruscitrus drinkdrink isis 00..0940209402 g/g/100100 mLmL..


bb VM ×=×acid citric FW

acid citric g 3

( )( )( )acid citric g 04701.0

3

g/mol 13.192L 01762.0M 04166.0

3

acid citric FW ==×× bb VMH3CitH2Cit-

HCit2-

Cit3-

pH = 4.76

pH = 3.13

pH = 6.40

kimia analisa 33 sks sks -...

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