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A number of anions form slightly soluble precipitates with certain metal ions and can be titrated...

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• A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated with Ag + , SO 4 2- titrated with Ba 2+ . CHAPTER 9 (Ch 5 161-168) PRECIPITATION REACTIONS AND TITRATIONS
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Page 1: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

• A number of anions form slightly soluble precipitates with certain metal ions and

can be titrated with the metal solutions.

• for example: Cl- titrated with Ag+ , SO42- titrated with Ba2+.

CHAPTER 9 (Ch 5 161-168)

PRECIPITATION REACTIONS

AND TITRATIONS

Page 2: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

5.5 PRECIPITATION EQUILIBRIA: THE SOLUBILITY PRODUCT

Solubility of Slightly Soluble Salts:

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

Solubility Product KSP = ion product

Ksp = [Ag+][Cl-]

The molar solubility depends on the stoichiometry of the salt.

Ag2CrO4(s) 2 Ag+ + CrO42-

Ksp = [Ag+]2[CrO42-]

Because the Ksp product always holds, precipitation will not take place

unless the product of precipitate ions exceeds the Ksp

• When compound is referred to as insoluble, it is not completely insoluble but is

slightly soluble.

Page 3: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

Example 5.6 (p 162): The Ksp of AgCl at 25C is 1.010-10 . Calculate the concentrations of

Ag+ and Cl- in a saturated solution of AgCl, and the molar solubility of AgCl.Solution

AgCl(s) Ag+ + Cl-

Ksp = [Ag+][Cl-]

[Ag+] = [Cl-] = S

S2 = 1.010-10

\ S = 1.010-5 M

The solubility of AgCl is 1.010-5 M.

Example 5.8 (p 163): What must be the concentration of added Ag+ to just start precipitation of

AgCl in a 1.010-3 M solution of NaCl.

Solution

[Ag+][Cl-] = Ksp

[Ag+] (1.010-3 ) = 1.010-10

[Ag+] = 1.010-7 M

The concentration of Ag+ must exceed 10-7 M to begin precipitation.

Page 4: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

Example 5.9 (p 164): What is the solubility of PbI2 in g/L, if the Ksp is 7.110-9 ?

Solution

PbI2(s) Pb2++ 2I-

Ksp = [Pb2+] [I-]2 = 7.110-9

The molar solubility of PbI2 :

[Pb2+] = S and [I-] = 2S

(S)(2S)2 = 7.110-9

MS 33

9

102.14

101.7

Therefore, the solubility, in g/L, is

1.210-3 mol/L 461.0 g/mol = 0.55 g/L

HOME WORK: 25, 26, 27, 30 P.169-170

Page 5: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

Predicted effect of excess barium ion on solubility of BaSO4.

The common ion effect is used to decrease the solubility.

Sulfate concentration is the amount in equilibrium and is equal to the BaSO4 solubility.

In absence of excess barium ion, solubility is 10-5 M.

The common ion effect is used to decrease the solubility.

Sulfate concentration is the amount in equilibrium and is equal to the BaSO4 solubility.

In absence of excess barium ion, solubility is 10-5 M.

Page 6: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

Precipitation titration, which is based on reactions that yield ionic compounds of limited

solubility, is one of the oldest analytical techniques. The slow rate of formation of most

precipitates, however, limits the number of precipitating agents that can be used in titrations to a

handful. The most widely used and important precipitating reagent, silver nitrate, which is used

for the determination of the halogens, the halogen-like anions.

Titration Curves:

• Consider the titration of Cl- with standard solution of AgNO3.

•A titration curve prepared by plotting pCl (-log[Cl-]) againest the volume of AgNO3.

• At the beginning of the titration: [Cl-] = 0.1 M pCl = 1

• As the titration continues: pCl determined by [Cl-] remaining (part of Cl- is removed as

AgCl)

• At the equivalent point: [Cl-] = Ksp = 10-5 M pCl = 5

• Beyond the equivalent point: [Cl-] determined from the concentration of excess Ag+ and Ksp

9.4 PRECIPITATION TITRATIONS

Page 7: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

AgI is least solubleSharpest change at

equiv. point

Least sharp, but steep enough for Equiv .point location

The smaller Ksp, the larger the break at the equivalent point.

Effect of Ksp on the Titration Curve

Page 8: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

1. The precipitate formation is stoichiometric.

2. To allow the titrant to be added quickly, the equilibrium between the precipitate and its ions in solution much be attained rapidly.

3. The precipitate must be of low solubility in the solution. This is indicated by small equilibrium constant (Ksp).

4. A method to detect the stoichiometric point of the titration must be available. Although a number of indicators are available, in general, the best method for detecting the end point in precipitation titration is by an instrumental technique.

Precipitation Titration Requirements

Page 9: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

DETECTION OF THE END POINT: INDICATORSThe end point can be detected by:

I. measuring either pCl or pAg with an electrode and a potentiometer.

II. using an indicator to produce a color change at or near the equivalence point.

Types of indicators used precipitation titrations:

1- Indicators Reacting with the Titrant:

The Mohr method

The Volhard method

2- Adsorption Indicators (Fajan’s method)

Comparison

Mohr titration – formation of colored precipitate at the end point.

Volhard titration – formation of a soluble, colored complex at the end point.

Fajans titration – adsorption of a colored indicator on the precipitate at the end point.

Page 10: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

The Mohr method

• In the precipitation of chloride by titration with silver ion, a soluble chromate ion

(CrO42) is used as the indicator. When the precipitation of Cl is complete, the

first excess of Ag+ react with the indicator to form Ag2CrO4, a reddish-brown

precipitate formed when is present.

 Ag+ + Cl AgCl(s)

titrant analyte white precipitate

Ksp= 1.8 x 10-10 (S = 1.34 x 10-5 M)

Mohr indicator reaction (end point),

  2Ag+ + CrO42 Ag2CrO4(s)

titrant indicator red precipitate

Ksp= 1.2 x 10-12 (s = 6.7 x 10-5 M)

Page 11: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

The Mohr titration must be performed at a pH 8 (neutral or slightly basic medium ).

Why?

to prevent silver hydroxide formation (at pH > 8).

2Ag+ + 2OH 2AgOH(s) Ag2O(s) + H2O

precipitate

To prevent the formation of chromic acid (at pH 6).

 CrO42 + H3O

+ HCrO4 + H2O

[CrO42] become lower, more Ag+ to be added to reach end point, which cause error.

The Mohr titration is useful for determining chloride in natural

solutions, such as drink water.

Page 12: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

Volhard titration

• For determination of (Cl, Br, SCN).

• A measured excess of AgNO3 is added to precipitate, and the excess of Ag+ is determined

by back titration with standard potassium thiocyanate solution (KSCN).

• The end point is detected by adding iron (III) solution, to form soluble red complex with

the first excess of titrant (SCN).

   Reactions:

Ag+ + Cl AgCl(s) Ksp = 1.82 x 10-10

titrant #1 white precipitate

(excess)

Ag+ + SCN AgSCN(s) Ksp = 1.1 x 10-12

titrant #2 white precipitate

Fe3+ + SCN FeSCN2+ Kf = 1.4 x 10+2

indicator red complex

Page 13: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

• The endpoint is routinely used for halide determinations where a known excess of

silver ion is added to precipitate the halide ion, and the excess silver ion is back

titrated using the thiocyanate/iron(III) as the indicator.

• The silver chloride precipitate is filtered, and the excess silver ion is titrated with

thiocyanate producing a white precipitate of AgSCN.

• Once the silver is consumed, the excess thiocyante reacts with the iron(III) ion

producing a red FeSCN2+ complex. Thus, the appearance of the red color at the

endpoint.

• The red FeSCN2+ complex color is detectable at 6.4 x 10-6 M concentrations and

above.

• The titration is usually done in acidic pH medium to prevent precipitation of

iron hydroxides, Fe(OH)3.

Page 14: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

The AgCl precipitate must be separated from the thiocyanate, why??

to prevent the reaction :

AgCl + SCN- AgSCN + Cl-

Since AgSCN is less soluble than AgCl, equilibrium will shift to the right causing a

negative error for the chloride analysis.

To eliminate this error, AgCl must be filtered or add nitrobenzene before titrating with

thiocyanate; nitrobenzene will form an oily layer on the surface of the AgCl

precipitate, thus preventing its reaction with thiocyanate .

Page 15: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

Fajans Method (Adsorption Indicators)

Adsorption indicators are organic compounds that tend to be adsorbed onto the surface of the solid precipitate in a precipitation titration.

 Example of Adsorption Indicators: Fluorescein

A polycyclic compound that ionizes in solution to yield yellow-green fluoresceinate ions.

Fluoresceinate adsorbs to silver ions on the surface of a precipitate when excess silver ion is present, producing a reddish-colored surface.

Page 16: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

Before the end point (Cl in excess).

1 Layer 2 Layer

)AgCl • (Cl Na+

When the equivalence point is reached, there is no longer an excess of analyte Cl, and the surface of the colloidal particles are largely neutral.

Page 17: A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions. for example: Cl - titrated.

(AgCl) • Ag+ X

 1 Layer 2 Layer

After the end point (Ag+ in excess).

• After the equivalence point, there will be an excess of titrant Ag+, some of these will adsorb to the AgCl particles, which will now be surrounded by a diffuse negative counterion layer.

• Now, negatively charged fluorescein can penetrate the counter ion layer and adsorb onto the AgCl lattice due to its affinity to Ag+.


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