Complexometric Titrations: Understanding Precipitation Reactions and AgNO3 Titrations, Slides of Biochemistry

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Precipitation Titrations

3 rd^ Year Students, General- Science Course Code: 317 Chem.

Lecture# 8 Date: April 5, 2020

Quiz on Complexometric titrations

1. Coordination compound is defined as-----------------------------------------------------
2. A ligand is defined as-------------------
3. The chelates of trivalent cations with EDTA are stable in strong ---------solution.

1. Indicators used in the titration of EDTA respond to-------------------------------

a) the change in the metal ion concentration b) the change in the EDTA concentration c) The change in the pH of buffer solution d) a, b, c

2. Calcium can be determined in the presence of Magnesium by titration with EDTA in the presence of ---------------

a) Eriochrome black T in Ammonia buffer pH 12 b) Murexide c) Ammonia buffer pH 10 d) Murexide in alkaline buffer pH=

3. To increase the selectivity of EDTA reactions, we use a) masking agent b) demasking agent c) control pH of solution d) a, b, c

Part I: Fill in the blank(s) with the most appropriate word(s). (1 pt. each)

Part II: Indicate the most appropriate answer in the blank provided (1 pt. each)

You are provided with a water sample

contaminated with chloride ion, how can you

estimate the sample purity?

Analytical Problem

• The reaction that forms a sparingly soluble compound

(precipitate).

Precipitation Reactions

• For sparingly soluble compound, there is an equilibrium

between ppt and its ions:

AgCl  AgCl  Ag ^  Cl 

Solid (^) saturated soln.

[ ][ ]

[ ]

[ ][ ]  

  Ag Cl

AgCl

Ag Cl

K

The product of ionic concentrations is constant at a given temperature. This constant is called “solubility product” “Ksp”

Ag Cl  Ksp

  [ ][ ]

A precipitate is formed in a solution of sparingly soluble salts, when the product of its ions concentrations exceed its solubility product.

Factors affecting the solubility of formed precipitate

How does the precipitate form?

Ionic product˃˃ Ksp

**1. Complexation ion

2. Solvent effect
3. Common ion effect**

1. Complexation ion

If a ppt. is placed in a solution of a ligand, a complex will be formed

with the metal ion of the ppt. and the solubility of the ppt. will be increased.

The solubility of AgCl increases and equals [Cl-]. However, CAg = [Cl-] = s

Example: By adding NH 3 to AgCl ppt. the following equilibria will take place:

 

 

 

 

 

 

3 3 3 2

3 3

( ) ( )

( )

Ag NH NH Ag NH

Ag NH Ag NH

AgCl Ag Cl

CAg = [Ag+] + [Ag(NH 3 )+] + [Ag(NH 3 ) 2 +]

A common ion is one of the ions of sparingly soluble salt in its

saturated solution such as AgCl is dissolved in NaCl or KC1.

3. Common Ion Effect (C.I.) :

• The Cl –^ is a common ion with chloride ion ionized from AgCl.
• The common ion usually reduce the solubility.

When Ag+^ is added (from AgNO 3 ) to AgCl ppt, Ksp of AgCl = [Ag+]

[Cl−]

Excess Ag+^ disturbs the equilibrium and combines with Cl−^ to form

AgCl ppt. till equilibrium is reached ([Ag+] [Cl−] = Ksp), the solubility

decreases.

 

 

 

 

NaCl Na Cl

AgCl Ag Cl  

 

 

 

AgNO 3 Ag NO 3

AgCl Ag Cl

Precipitation (Argentometric)Titration

• Used for determining halides e.g. Cl-^ by titration with AgNO 3.
• According to the indicator used, three methods can be

described.

• Formation of insoluble compound (precipitate) as a results of

titration of analyte and titrant

• Argentometric titration: titrimetric reaction that use AgNO 3 as

titrant

• Titrations of precipitates formation.

A titration curve for a precipitation titration can be constructed by plotting volume of Ag+^ versus pX (X: Cl-, Br-, I-, or SCN-).

a) Ksp of the silver salt. b) The concentration of reactants. This is true when a suitable indicator concentration is used.

The sharpness of the end point is directly proportional to:

Plot the titration curve for the following reaction: 0.1 N AgNO 3 solution with 100 mL of 0.1 N NaCl solution?

We will calculate the ionic concentration of chloride ion at various stages of titration with the knowledge of the solubility product.

Titration curves in precipitimetry

A) At the Beginning, Before Adding AgNO 3

The concentration of NaCl is 50 ml of 0.1 N solution in a dilution of 150 ml, the concentration of Cl will be

1 30 3. 3 10 2 150

50 0. (^1)    x

x pCl=-log( 3.3x10-2)=1.

B) Upon adding 50 ml AgNO 3

C) Upon adding 90 ml of AgNO 3 solution:

  x

x pCl=-log(5.3x10-2)

The concentration is 10 ml of 0.1 N in a solution of 190 ml, and the concentration will be

Cl-^ concentration = 0.1 , pCl=

If pCl-^ =1.48 , then for pAg+= 9.96-1.84= 8. If pCl-^ =2.28 , then for pAg+= 9.96-2.28= 7.

At equivalence points, pCl-^ =4. So, pAg+=9.96-4.98= 4.

The concentration of Ag+^ in all the previous calculations and its pC

is easily obtained from the following equations

[ ] [ ] log( 1. 1 10 ) 9. 96

[ ] [ ] 1. 1 10

10

10

  

  

p Ag p Cl x

Ksp Ag x Cl x

This value is constant .i.e. if p[Ag+] increases, p[Cl-] decreases.

So that the sum of both exponents remains 9.96 and hence:

F) Beyond the equivalence point

When 101 ml of 0.1 M AgNO 3 is added, so 1 ml excess pAg+^ will be

5 10 4 201

1. 1 100 101

1 0. 1 [ ^ ]    

 x

x Ag

p Ag+= 4-(log5)= 3.

So, pCl-^ = 9.96-3.3 = 6.

Mixtures can be titrated at enough difference in the solubilities of the two silver salts exists (at least 10^3 ).

Effect of Ksp on the shape of

titration curve.

• Chromate forms a ppt with Ag+^ of larger solubility than of AgCl.
• Direct titration
• Indicator: soluble chromate salt (Na 2 CrO 4 , K 2 CrO 4 )
• Endpoint: formation of colored secondary precipitate
  • Ag 2 CrO 4 (red) precipitated in neutral pH solution
  • Colour forms just after AgCl or AgI formation.
• Thus AgCl is formed first and after all Cl-^ is consumed, the first drop of Ag+^ will react with the chromate indicator giving a reddish ppt.

2 Ag+^ + CrO 4 2-^ (Yellow)  Ag 2 CrO 4 (Red) (indicator Reaction)

Ag+^ + Cl-^  AgCl (white) (Titration Reaction)

• Relies on Ksp differences for two insoluble silver salts

Formation of Colored Precipitate (Mohr’s method)