ANALYTICAL CHEMISTRY(BKF 1243)

CHAPTER 6

TITRATION METHODS:

PRECIPITATION TITRATION

LECTURER:

NURLIN ABU SAMAH

(FACULTY OF INDUSTRIAL SCIENCES & TECHNOLOGY, UMP)

ISO 9001:2000

CONTENT OUTLINE:

6.0 TITRIMETRIC METHODS: PRECIPITATION TITRATION

6.1 Terms used in titrimetric

6.2 Standard solution

6.3 Volumetric calculations

6.4 Titration curves

6.5 Precipitation titrimetry.

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INTRODUCTION:Definition of titration:

• Titrimetry: –

a) technique to measuring the quantity of a reagent of known concentration required to react with a measured quantity of sample of an unknown concentration.

b) includes a large group of analytical methods based on determining quantity of known concentration required to react completely with analyte.

INTRODUCTION:Type of titrimetry:

(1) Volumetric Titrimetry :

- involves measuring the volume of a solution of known concentration that is needed to react essentially completely with the analyte.

(2) Gravimetric Titrimetry:

- different only in that the mass of the reagent is measured instead of its volume.

6.1 Terms used in titrimetry

6.1.1 Terms used in volumetric titrimetry:

1. Standard solution:

- It is a reagent of known concentration that is used to carry out a titrimetric analysis.

2. Equivalence point:

-It is the point in a titration when the amount of added standard reagent is exactly equivalent to the amount of analyte.

3. Back titration:

- It is a process in which the excess of a standard solution used to consume an analyte is determined by titration with a second standard solution.

4. End point:

- It is the point in a titration at which an observable physical change signals the equivalence point.

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Cont…

5. Titration error ：Et ＝ Vep － Veq

- Vep ： actual volume of reagent ； Veq ： is the theoretical volume to reach the equivalence point.

6. Indicator:

It is often added to the analyte solution to produce an observable physical change (the end point ) at or near the equivalence point.

7. Primary standard :

It is an ultrapure compound that serves as the reference material for a titrimetric method of analysis. Or in other words, it is a highly purified compound that serves as a reference material in volumetric and mass titrimetric methods.

8. Secondary standard :

It is a compound whose purity has been established by chemical analysis and that serves as the reference material for a titrimetric method of analysis.

 

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Requirements for primary standard:

1. High purity (established methods for confirming purity should be available).

2. Atmospheric stability.3. Absence of hydrate water so that the composition of the

solid does not change with variations in relative humidity.

4. Ready availability at modest cost.5. Reasonable solubility in the titration medium.6. Reasonably large molar mass so that the relative error

associated with weighing the standards minimized.

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6.2 Standard Solutions

Requirements for standard solutions:

1. Be sufficiently stable so that it is only necessary to determine its concentration once.

2. React rapidly with the analyte so that the time required between additions of reagent is minimized.

3. React completely with the analyte so that satisfactory end points are realized.

4. Undergo a selective reaction with the analyte that can be described by a balanced equation.

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Figure 1: Apparatus of volumetric titrimetry

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Figure 2: Measuring the volume in the burette.

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21.50 mL

Figure 3: Titration methodISO 9001:2000

6.3 Volumetric calculations:

a) Some useful algebraic relationships:

Amount of (milimol) = mass (g)/ Milimolar mass (g/mmol)……..(1)

  Or

Amount of (mol) = mass (g) / Molar mass (g/mol) ………….(2)

 

ISO 9001:2000

Cont…

Amount of (mmol) = M (mmol / mL) X V (mL) ……(3)

Or

Amount of (mol) = M (mol / L) X V (L) ……(4)

 

• Use equation 1 and 3, when volumes are measured in milliliters (mL).

• Use equation 2 and 4, when volumes are measured in liters (L).

 

M=Molar

V= Volume

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Cont…b) Calculating the Molarity of Standard Solutions:

Example 1 ：Describe the preparation of 2.000L of 0.0500M AgNO3 (169.87g /mol ） from the primary standard-grade solid.

Solution ： Amount mole of AgNO3 ＝ V （ L ） ×MAgNO3

＝ 2.000L× 0.0500 (mol/L)

＝ 0.1000 mol AgNO3.

Therefore, mass (g) of AgNO3 = Mole X molar mass

= 0.1000 (mol) AgNO3 X 169.87 (g/mol)

= 16.98 g of AgNO3.

* So, the solution is prepared by dissolving 16.98g of AgNO3 in water and diluting to exactly 2.000 L.

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c) Treating titration data:

Calculating molarities from standardization data;

Formula involved: M1 X V1 = M2 X V2

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c) Treating titration data:

Example 2:

A 50.00mL portion of HCl solution required 29.71ml of 0.01963M Ba(OH)2 to reach an end point with bromocresol green indicator .Calculate the molarity of the HCl.

Solution:

ISO 9001:2000

2 2 2

22

2

2

( ) 2 2

( )( ) 29.71 0.01963 0.583

( )

2(29.71 0.01963) 1.166

1 ( )

(29.71 0.01963 2)0.023328

50.0HCl

Ba OH HCl BaCl H O

mmol Ba OHBa OH ml mmol

mLBa OH

mmolHClHCl mmol

mmolBa OH

C Mml

d) Calculating the quantity of Analyte from Titration Data

Example 3:

A 100.0ml sample of brackish water was made ammoniacal , and the sulfide it contained was titrated with 16.47 ml of 0.02310M AgNO3. The analytical reaction is

2Ag++S2-→Ag2S(s).

Calculate the concentration of H2S in the water in parts per million (ppm).

Solution:

ISO 9001:2000

33

3

22

3

322

2

3

2

16.47 0.02310 0.3804

1(16.47 0.02310) 0.1902

2

1(16.47 0.02310 ) 0.034802 6.620 10

2

6.620 10

100.0 1.000

mmolAgNOamountAgNO ml mmol

mLAgNO

mmolH SamountH S mmol

mmolAgNO

gH SmassH S g

mmolH S

gconcH S

gml

6210 6.62ppm ppmH S

ml

6.4 Titration curve

An end point is an observable physical change that occurs near the equivalence point. The two most widely used end points involve :

1. Change in color due to the reagent.

2. Change in potential of an electrode that responds to the concentration of the reagent or the analyte.

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6.4.1 Types of Titration Curves

Titration Curves are plots of a concentration-related variables as a function of reagent volume.

Two types of titration curves:

1. Sigmoidal curve: p-function of analyte (or sometimes the reagent) is plotted as a function of reagent volume.

2. Linear segment curve: measurement are made on both sides of but well away from the equivalence point. (advantageous for reaction that are complete only in the presence of a considerable excess of the reagent or analyte)

ISO 9001:2000

Figure 4 (13-2) : Types of titration curves

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Titration curves plot reagent volume on the horizontal axis and some function of the analyte or reagent concentration on the vertical axis.

6.5 Precipitation titrimetry

-Precipitation titrimetry, which is based on reactions that yield ionic compounds of limited solubility.

- It is one of the oldest analytical techniques.

- Titrimetric methods based on silver nitrate are sometimes called argentometric methods.

ISO 9001:2000

6.5.1 Precipitation Titration Curves involving silver ion

The most common method of determining the halide ion concentration of aqueous solution is titration with a standard solution of silver nitrate.

To construct titration curve three type of calculation are required, each of which corresponds to a distinct stage in the reaction ：

（ 1 ） preequivalence,

（ 2 ） equivalence, and

（ 3 ） postequivalence.

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Example 4 ：Perform calculations needed to generate a titration curve for 50.0mL of 0.0500M NaCl, with 0.100M AgNO3

（ for AgCl, Ksp ＝ 1.82×10-10).

Solution:

Calculate pH at:

(1) Preequivalence –point data

（ 2 ） equivalence, and

（ 3 ） postequivalence.

ISO 9001:2000

(1) Preequivalence –point data:ISO 9001:2000

109

9

10

(50 0.05 10 0.1)0.025 0.025

(50 10)

0.025

1.82 107.28 10

0.025

log(7.28 10 ) 8.14

(2) int

1.82 10

Nacl

sp

mmolC M

mL

Cl M

KAg

Cl

PAg

Equivalence po PAg

Ag Cl Ag Cl

Ag

10 5

5

3

3

1.82 10 1.349 10

log(1.34 10 ) 4.87

(3) int

26

26.0 0.1 50.0 0.051.316 10

50 26

log(1.316 10 ) 2.88

M

PAg

Postequivalence po

At ml

Ag M

PAg

a) The effect of Concentration on Titration curves:

ISO 9001:2000

Figure 5: Titration curve for (A) 50.00mL of 0.0500M NaCl with 0.100M AgNo3 and (B) 50.00mL of 0.00500M NaCl with 0.0100M AgNo3.

b) The effect of Reaction Completeness on Titration Curves

Figure 6: Effect of reaction completeness on precipitation curves . For each curve , 50.00 mL of a 0.0500M solution of the anion was titrated with 0.1000 M AgNO3 . Note that smaller values of Ksp give much sharper breaks at the end point

6.3.2 TitrationCurves for Mixtures of Anion

ISO 9001:2000

Figure 7:

Titration curves for 50.00mL of a solution 0.0800 M in Cl- and 0.0500 M in I- or Br-

6.5.3 Indicators for Argentometric Titrations

1.Three types of end points are encountered in titrations with silver nitrate:(1)chemical （ 2 ） potentiometric （ 3 ） amperometric.

2.The end point produced by a chemical indicator consists of a color change or occasionally. The requirements for an indicator for a precipitation titration are that:

（ 1 ） the color change should occur over a limited range in p- function of the reagent or the analyte .

（ 2 ） the color change should take place within the steep portion of the titration curve for the analyte.

6.5.3 Indicators for Argentometric Titrations

1.Chromate ion :The Mohr Method

-Sodium chromate can serve as an indicator for the argentometric determination of chloride, bromide , and cyanide ions by reacting with silver ion to form a brick-red silver chromate precipitate in the equivalence –point region.

2.Adsorption Indicators: The Fajans Method

-It is an organic compound that tends to be adsorbed onto the surface of the solid in a precipitation titration .The adsorption occurs near the equivalence point and results not only in a color change but also in a transfer of color from the solution to the solid. （ Fluorescein is a typical adsorption indicator that is useful for the titration of chloride ion with silver nitrate. ）

3.Iron （Ⅲ） Ion : The volhard Method

The Volhard mithod , silver ions are titrated with a standard solution of thiocyanate ion

Ag+ ＋ SCN- → AgSCN(s) Fe3++ SCN- → FeSCN2+

The titration must be carried out in acidic solution to prevent precipitation of iron as the hydrated oxide. Ag+ ＋ Cl- → AgCl(s)

Applications of Standard Silver Nitrate solutions

Table13-3: Lists some typical applications of precipitation titrations in which silver nitrate is the standard solution .

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ISO 9001:2000