analytical chemistry review
DESCRIPTION
analytical chemistryTRANSCRIPT
-
5/28/2018 Analytical Chemistry Review
1/43
-
5/28/2018 Analytical Chemistry Review
2/43
Determination of Nickel Nickel ion is precipitated with a very selective organicprecipitating reagent called dimethylglyoxime (DMGH) andthen adding a slight excess of aqueous ammonia solution.
-
5/28/2018 Analytical Chemistry Review
3/43
Determination of Carbon and
Hydrogen
Determinations are based on direct volatilization
procedure and require only 5 to 10 mg of sample
The sample is heated in a steam of oxygen in the
presence of catalysts, causing it to decompose
into CO2and H2O
C10H8+ 12 O2 10 CO2+ 4 H2O
-
5/28/2018 Analytical Chemistry Review
4/43
Volumetric method of analysis
-
5/28/2018 Analytical Chemistry Review
5/43
Titration
process wherein a stoichiometrically equivalent
quantity of a standard solution is systematicallyadded to a known quantity of a sample
Requirements of a Titration:
Stoichiometric
Reaction must be rapid
No side reactions
Presence of marked change in solution when thereaction becomes complete
quantitative
-
5/28/2018 Analytical Chemistry Review
6/43
Equivalence point
point in which the amount of standard titrant
added is chemically equivalent to the amount of
the analyte in the sample
Endpoint
estimation of equivalence point manifested by achange in physical properties of solution
Titration error
Difference between equivalence point andendpoint; minimized by:
Using a suitable indicator
Using an indicator blank
-
5/28/2018 Analytical Chemistry Review
7/43
Standard solution
solution of known concentrationstandardization
Process of determining the
concentration of a solution of unknownconcentration
-
5/28/2018 Analytical Chemistry Review
8/43
Common Acid-Base Indicators
pKln Acid/Base
Colors
Methyl Orange 3.46 Red/yellow
Methyl Red 5.00 Red/yellow
Bromcresol
Green
4.66 Yellow/blue
Bromthymol
Blue
7.10 Yellow/blue
Phenolphthalein 9.0 Colorless/red
Methyl Violet 1.6 Yellow/violet
Most Suitable Indicator: the one with pKlnclosest to the
pH at equivalence point
-
5/28/2018 Analytical Chemistry Review
9/43
Strong Acids Strong Bases
HCl NaOHHNO3 KOH
H2SO4 Ba(OH)2
HClO4 CH3NOH
HBr C2H5NOH
HI
-
5/28/2018 Analytical Chemistry Review
10/43
Dissociation Constant, Ka/Kb
When a weak acid or weak base is
dissolved in water, partial dissociation
occurs
HA + H2O H3O++ A-
HA = weak acid; Ka = acid dissociation
Ka= [H3O+] [A-]/ [HA]
For weak base, Kb = base dissociation
constant
B + H2O BH+
+ OH-
-
-
5/28/2018 Analytical Chemistry Review
11/43
Multiplication of one equilibrium-constant
expression by the other gives
Ka x Kb = [H3O+] [A-]/ [HA] x [HA] [OH-]/ [A-]
Ka x Kb= [H3O+] [OH-]
Ka
x Kb= K
w= 1.0 x 10-14at 25C
-
5/28/2018 Analytical Chemistry Review
12/43
pH Calculation
pH = - log [H3O+]
When an acid HA dissolves in water, both acid
and water are suppliers of H3O+:HA + H2O H3O
+ + A- (a)
H2O + H2O H3O+ + OH- (b)
When a base B dissolves in water, both the
base and water are suppliers of OH-
:B + H2O BH
+ + OH- (c)
H2O + H2O H3O+ + OH- (d)
-
5/28/2018 Analytical Chemistry Review
13/43
For Acids
Case 1. The acid is the major supplier of H3O+
This means that the determination of hydronium ion concentration is based on
the equilibrium expression for reaction (a)
Case 2. Water is the major supplier of H3O+
The hydronium ion concentration can be determined by solving the equilibrium
expression for reaction (b)
Case 3. Both the acid and the water are major suppliers of H3O+
Both reactions must be considered
For Bases
Case 1. The base is the major supplier of OH-
This means that the determination of hydroxide ion concentration is based on
the equilibrium expression for reaction (c) Case 2. Water is the major supplier of OH-
The hydroxide ion concentration can be determined by solving the equilibrium
expression for reaction (d)
Case 3. Both the base and the water are major suppliers of OH-
Both reactions (c) and (d) must be considered
-
5/28/2018 Analytical Chemistry Review
14/43
Case Major Supplier of H3O+ Necessary condition
1
23
Strong acid
WaterBoth
CHX>> 10-7
CHX
-
5/28/2018 Analytical Chemistry Review
15/43
Case Major Supplier of OH- Necessary condition
1
2
3
Strong base
Water
Both
CB>> 10-7
CB
-
5/28/2018 Analytical Chemistry Review
16/43
- For weak acids:Case Major Supplier of H3O
+ Necessary condition
1
2
3
Weak acid
Water
Both
Kax CHA>> 10-7
Kax C
HA> 10-7
Kax CHA
-
5/28/2018 Analytical Chemistry Review
17/43
Calculation of pH of conjugate acid-base pairs
o There are two equilibria involved:HA + H2O H3O
+ + A
- (e)
A- + H2O HA + OH
- (f )
oThe ionization of water is neglected, and [H3O
+
] is taken from reaction (e)Ka= [H3O+][A
-]
[HA]
Where:
[HA] = CHA - [H3O+] + [OH
-]
[A-] = CNaA + [H3O
+] - [OH
-]
Ka=[H3O+]( CNaA + [H3O
+] - [OH
-])
CHA - [H3O+] + [OH-]
Solving for [H3O+],
[H3O+]
3 + (CNaA+ Ka) [H3O
+]
2- (KaCHA + Kw) [H3O
+] - KaKw = 0
o If CHA& CNaA 10-3 and Ka& Kb 10-3, then CHAand CNaAare larger compared to the differencebetween [H3O
+] and [OH
-], therefore
[HA] = CHA[A
-] = CNaA
Kabecomes,
Ka=[H3O+]CNaA
CHA
o Note also that conjugate acid-base do not react with each other
-
5/28/2018 Analytical Chemistry Review
18/43
pH buffers
- a mixture of weak acid or base and its conjugate- resists changes in pH upon dilution or addition of acids or base
o effect of dilution:pH = pKa - log [HA]/[A
-]
pH depends on the ratio of the concentrations of acids and conjugate bases rather thantheir absolute value
-
5/28/2018 Analytical Chemistry Review
19/43
Buffer Capacity, - depends on both the concentration of components and the concentration ratio- Defined as the quantity of strong acid or strong base needed to cause 1.0 L of buffer to undergo a pH change
of 1.0 unit- To determine the useful pH range of a buffer use the previous equation
[HA] / [A-] pH from the equation
1 / 10 pHmax= pKa - log 1/10 = pKa+ 1
10 / 1 pH min= pKa - log 10/1 = pKa - 1Therefore the useful pH range is pKa 1.
- In selecting a buffer for a given application there are two considerations to be considered:1. The desired pH and2. The chemical compatibility of the buffer components with the sample
- Preparing a buffer:o By combining the calculated quantities of an acid-base conjugate pairo In cases one of the conjugate pair is unavailable, combining of excess of the available weak acid or
baser with an appropriate amount of strong base or acid can be done.o Preparation of buffers uses the aid of pH meter
-
5/28/2018 Analytical Chemistry Review
20/43
Polyprotic Acids and Polyequivalent
Bases Case 1: A solution containing H2A. The stepwise ionization ofH2A provides two sources of H3O
+:
H2A + H2O H3O++ HA- Ka1=[H3O
+][HA-]
[H2A]HA-+ H2O H3O
++ A2- Ka2=[H3O+][A2-]
[HA-]
if Ka1
>>Ka2
and Ka1
/Ka2
100, the first ionization is the major
source of H3O+;the second ionization can be neglected for the
purpose of calculating the pH
-
5/28/2018 Analytical Chemistry Review
21/43
Case 2: A solution containing H2A + HA-.
Similar to case 1: if Ka1/Ka2 100, the second ionization can be
neglected and it becomes one of calculating the pH of a weak
acid in the presence of its conjugate base--- a buffer problem
Case 3: A solution containing HA-.
Substances such as HA- exhibit both acidic and basic character.
When a salt NaHA is dissolved in water, it dissociates completely
into Na+ and HA-. The HA- can undergo ionization,HA-+ H2O H3O
++ A2- Ka2=[H3O+] [A2-]
[HA-]
- And base ionization
HA-+ H2O H2A + OH- Kb2=[H2A][OH-]= Kw
[HA-] Ka1
-
5/28/2018 Analytical Chemistry Review
22/43
For case three both reactions should be considered,
[H3O+] = [H3O
+]formed[H3O+]lost
But [H3
O+]formed
= [A2-] and [H3
O+]lost
= [OH-]formed
= [H2
A]
Therefore [H3O+] = [A2-][H2A]
Or [A2-] = [H3O+] + [H2A]
Solving for Ka1,
[A2-] = [H3
O+] + [H3
O+][HA-]/Ka1
We obtain
Ka2= [H3O+]([H3O
+]+[H3O+][HA-]/Ka1)
[HA-]
[H3O+]2= K
a1K
a2[HA]
Ka1+ [HA-]
It is frequent that Ka1
-
5/28/2018 Analytical Chemistry Review
23/43
Calculation of pH of solutions of
Salts
Salts of strong acid and strong base(ex NaCl)
pH = 7.0
salts of weak acid and strong base or salts ofweak base and strong acid (ex.NaCH3COO;NH4Cl)
Reverse given dissociation constant
salts of weak acid and weak base (ex.NH4HCOO)
[H3O+] = KwKa/Kb
-
5/28/2018 Analytical Chemistry Review
24/43
Concept of Equivalence
Equivalents (n)
Number of reacting species per mole of the species
For acids and bases = replaceable H+ or OH- For ions = electrons that can be added or removed
Redox reactions = electrons lost or gained
equation of equivalence
Between two aqueous solution
NV = NV where N=M(n) between an aqueous solution and solid
NV = weight solid/EW; EW = formula mass/n
-
5/28/2018 Analytical Chemistry Review
25/43
DOUBLE-INDICATOR TITRATION
C:\Users\raissa\Documents\Neutralization
Titration.docx
http://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docxhttp://c/Users/raissa/Documents/Neutralization%20Titration.docx -
5/28/2018 Analytical Chemistry Review
26/43
DILUTION FACTOR
Simple dilution
DF = final volume/vol of aliquot added
serial dilution
final dilution factor = DF1*DF2*DF3
-
5/28/2018 Analytical Chemistry Review
27/43
Complexometry A titrimetric determination which involves the formation of a soluble
but slightly dissociated complex or complex ion
Metal ion + ligand complex(analyte) (chelate)
metal ion central atom in the complex; lewis acid electron pair acceptor
Ligand Group attached to the central atom; lewis base; can either be an anion or
neutral molecule
Complex Metal ligand
EDTA Well known complex
-
5/28/2018 Analytical Chemistry Review
28/43
Coordinate number
The number of bonds formed by the central atom
coordinate covalent bond
Type of bonding involved in the metal-ligand
complex
-
5/28/2018 Analytical Chemistry Review
29/43
Titration of metals with polydentate
ligands
Polydentate ligands and metals often react in a singlestep thereby avoiding the complications of stepwisereactions
Indicators for EDTA titrations: Eriochrome black T (EBT)
Used exclusively in the pH range 7-11 where the blueform of the indicator predominates in the absence ofmetal ions
Endpoint: red to blue
Calmagite
Stable in aqueous solution;
From red to orange
-
5/28/2018 Analytical Chemistry Review
30/43
Solubility equilibria
-
5/28/2018 Analytical Chemistry Review
31/43
Solubility EquilibriaDeals with substances whose solubility
are low
Ksp(solubility product constant) Equilibrium constant expressing the
solubility of a precipitate in water
Saturated : Ksp = IP
Unsaturated : Ksp > IP Supersaturates : Ksp < IP
-
5/28/2018 Analytical Chemistry Review
32/43
Factors that influence solubilitites
Common ion effect
A precipitate is less soluble in a solution containing an excess of
one of the ions common to the precipitate than it is in pure
form
Diverse ion effect An increase in solubility occurs when salts that contain no ions
in common with the precipitate are present in the solution
Ksp,actual= Ksp,apparent/ In such case, the actual Kspis derived by dividing apparent
solubility product constant by powers of mean ionic activity
coefficient, , which is the measure of the effectiveness with
which a chemical species influences equilibrium
-
5/28/2018 Analytical Chemistry Review
33/43
Debye-Huckel Limiting Law Expression
-log = 0.512 Z+Z-= 0.5 CiZi2
where: - mean ionic activity coefficient
Zcharge of ions
ionic strength
temperature
Generally, solubility increases with increasing temperature
pH
The lower the pH the higher the solubility
complex formation
Presence of complexing agents such as ammonia increases
solubility
-
5/28/2018 Analytical Chemistry Review
34/43
Solubility rules
All nitrates, acetates and perchlorates are
soluble
All halides (except F-) are soluble, except with
Ag+, Hg2+and Pb2+
All sulfates except Ba2+and Pb2+are soluble, butCa2+, Ag+, Hg2+, and Sr2+are only slightly soluble
All sulfides are insoluble except with IA and IIA
elements and (NH4
)2
S
All other common inorganic compounds are
insoluble except Ba(OH)2and Sr(OH)2 which are
soluble. Ca(OH)2is only slightly soluble
-
5/28/2018 Analytical Chemistry Review
35/43
VOLHARD MOHR FAJAN
Ag+determination Direct Indirect Direct
Cl-determination Indirect Direct Direct
Titrant KSCN AgNO3
Indicator Fe3+ CrO42- DCF-Acidity/basicity Acidic Neutral to basic Acidic
Precipitation rxn Ag++ Cl- Ag++ Cl-
Titration rxn Ag++ SCN- Ag++ Cl-
Indicator rxn Fe3++ SCN- Ag++ CrO42-
Color change Brick red Yellow orange Pink
-
5/28/2018 Analytical Chemistry Review
36/43
PermanganimetryUses KMnO4as oxidizing agent (titrant)
Permanganate processes where KMnO4is
used as titrant are self-indicating titrationi.e. endpoint is a permanent faint pink color
Primary standards for KMnO4:
AsO3 Na2C2O4 Pure Fe metal
-
5/28/2018 Analytical Chemistry Review
37/43
Other REDOX Titration
Dichromate Process ( using K2Cr2O7as titrant) Indicator: sodium/barium diphenylbenzidine sulfonate
Color change: colorless to red violet
iodometry (direct titration with iodine)
I2is the oxidizing agent
I2solution is the titrant
Indicator: starch solution
End point: deep blue solution
iodometry (indirect titration with I2)
Used for determining substance with oxidizing properties
Titrant: Na2S2O3(reducung agent)
Indicator: starch solution
End point: disappearance of blue color
-
5/28/2018 Analytical Chemistry Review
38/43
Electrochemistry
-
5/28/2018 Analytical Chemistry Review
39/43
Electrochemistry
Electrochemical cells
Consists mainly of 2 electrodes which areimmersed either into the same solution
or into 2 different solutions in
electrolytic contact with one another
-
5/28/2018 Analytical Chemistry Review
40/43
Galvanic/Voltaic
Cell
Electrolytic Cell
1. Energy Chemical reaction
furnishes electrical
energy
Chemical reaction
is forced to
proceed by
application ofelectrical energy
2. Electrode rxn Spontaneous Non-spontaneous
3. Anode rxn Oxidation Oxidation
4. Cathode rxn Reduction Reduction
5. Anode polarity - +
6. Cathode polarity + -
-
5/28/2018 Analytical Chemistry Review
41/43
-
5/28/2018 Analytical Chemistry Review
42/43
Schematic representation for
electrochemical cellsConvention:
Constituents comprising the cell are listed in the
order in which they would be encountered if we
begin at the anode and traveled through thecell solutions to the cathode
Use conventional symbols for ions, elements,
molecules
/ - boundary between 2 phases // - salt bridge
, - indicates that the species are at the same
phase
-
5/28/2018 Analytical Chemistry Review
43/43