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Chemical and Physical Principles

Analytical Chemistry

ANALYTICAL CHEMISTRY
Analytical Chemistry
 Branch of Chemistry that deals with the analysis, identification, separation and
composition of matter
 Involves methods used to identify the substances that are present in a sample
(qualitative analysis) and the exact amount of the identified substances
(quantitative analysis)
Quantitative Methods of Analysis
A
...


Classical methods
a
...
Volumetric method – measurement of the volume of solution necessary to react
completely with the analyte

2
...
Spectroscopic method – measurement of the electromagnetic radiationproduced
by the analyte or its interactions with it
b
...


Other methods – involves the measurement of the properties of the analyte such as
heat of reaction (calorimeter), index of refraction (refractometer), optical activity
(polarimeter) or mass-to-charge ratio (mass spectrometer)

B
...
Selection of an appropriate method
 In the selection of method of analysis, it is necessary to consider the level of
accuracy, complexity and component of the sample, availability of equipment
and trained personnel and the time of analysis
 Standard procedures are usually available from literature such as Chemical
Abstracts, Analytica Chimica Acta, Applied Spectroscopy, Journal of the
Association of Analytical Chemists, etc
...


Obtaining a representative sample
 The American Society for Testing and Materials (ASTM), National Bureau of
Standards (NBS) and Association of Official Analytical Chemists (AOAC) are
such a few organizations that impose standard sampling procedures for analysis
of some samples
 Three steps are generally followed in obtaining samples: obtaining a gross
sample, obtaining a laboratory sample and obtaining an analysis sample
 A gross sample is obtained from a bulk sample and obtained in such a manner
that it is considered a representative of the bulk sample
 A laboratory sample is a fraction of the gross sample weighing several grams
wherein further reduction to few milligrams results into an analysis sample

Chemical and Physical Principles
3
...

Samples that decompose upon heat treatment are analyzed on a wet basis or asreceived basis
 Replicate samples are taken for analysis to ensure accuracy of the method used
and quality of the results
...
Classification of analysis based on sample size
Method
Sample Mass
Sample Volume
macro
more than 100 mg
more than 0
...
050 mL to 0
...
050 mL
ultra-micro
less than 1 mg
–

4
...
Water
...

b
...
In many instances, some portion of the sample will
not dissolve in water and usually the addition of acids render the sample
soluble
...

c
...
For more stubborn samples, hot, concentrated sulfuricacid,
nitric acid and aqua regia are used
...
Hydrofluoric acid is also used for
dissolving silicate ores
...
Fluxing agents
...
Fluxing agents may be
classified as acidic (K2S2O7, KHF2 and B2O3), basic (Na2CO3, K2CO3, NaOH
or KOH) and oxidizing (Na2O2)
...

 Reagents and chemicals used in the laboratory are classified as follows:
a
...
Reagents that undergo superficial
purification and not directly used for analysis
b
...
Reagents used by pharmacists and unfit for analysis
c
...
Reagents that are more refined compared to
technical reagents
d
...
Reagents
analyzed by the manufacturer with the analysis found on the label of the
container
e
...
Chemicals with purity greater than 99
...
The following are most commonly used to express concentrations of
solutions:
a
...


Volume percent – commonly used to specify the concentration of a pure
liquid compound diluted with another liquid
vol  volume of solute
%  
100%
vol  volume of solution
For alcoholic beverages, percentage of alcohol is usually expressed in
terms of proof as follows:
proof   vol 
2 %  
 vol 

c
...


Mole fraction (x) – commonly used in unit operations to express
concentrations of solute present in a stream of gas or liquid
mole of solute
x
mole of solute  mole of solvent

e
...


Molarity (M) – most commonly used in titration and denotes the amount of
solute, in moles, dissolved in a solvent and diluting to a final volume of 1L
in a volumetric flask
mole of solute (mol)
M
volume of solution (L)
Formality (F) – concentration term identical to molarity commonly used for
solutions of ionic salts that do not exist as molecule in solid or in solution

g
...
The number of equivalents is given by thenumber
of moles multiplied by the number of reacting units per molecule or atom
...
Molar equivalents of solutes
Nature of solute
Molar equivalent
acid
number of replaceable H+
base
number of equivalent HO–
salt
net charge of an ion
oxidant
gain of electron
reductant
loss of electron
h
...


Treatment of the sample
 Some samples has to be reduced or oxidized prior to analysis or sometimes
treated to become colored or converted to a form that it can be readily volatilized
 More often, the accuracy of an analysis is affected by the presence of unwanted
components called interferences
 Interferences can be eliminated by converting it into non-interfering form by a
process called masking
...


Measurement of the analyte
 Using classical methods of analysis, results can be accurate up to a few parts per
thousand or better, requires relatively large amount of sample and usually
applied to measurement of major constituents in a sample
 Instrumental methods are generally more sensitive and selective
...


Analytical Chemistry

Chemical and Physical Principles

Table 3
...
00%
semi-micro
0
...
00%
micro
0
...
10%
ultra-micro
less than 0
...


Calculation of results and reporting of data
 Results of analysis can be expressed depending on the nature of analyte
a
...
Calculations on solid samples are based on mass
...
analyte
%  
100%
wt  wt
...
Concentrations of analyte in solid samples in trace concentrations
Unit
Definiton
Unit
mg
analyte
g analyte
wt  gram analyte
parts per
pt   gram sample 10 3
g sample or kg sample
thousand
wt 
μg analyte mg analyte
parts per
wt  gram analyte
ppm   gram sample 10 6
g sample or kg sample
million
wt 
ng analyte μg analyte
wt  gram analyte
parts per
ppb  gram sample 10 9
g sample or kg sample
billion
wt 
b
...
Similarly, concentrations of solid or liquid analytes in liquid
samples obtained from a macro analysis is usually expressed as % weight by
volume or % volume by volume defined as follows:
wt  gram analyte
vol  volume analyte
%  
100% or %  

100%
vol  mL sample

vol  volume sample

Table 5
...
Types of Gravimetric Analysis
1
...

3
...
The analyte in the sample is obtained using as appropriate
solvent and the residue from the solution, after evaporation of the solvent, is
chemically related to the analyte
Precipitation Method
...
The sample is treated to yield a gas that is passed in an
absorbing medium; the analysis is based upon the change in mass of the medium

B
...


Calculation in gravimetric analysis
To calculate the amount of analyte in the sample…
mass of final form
% analyte 
GF 100%
mass of sample
Gravimetric Factor (GF)
 x mol analyte 
molar mass of analyte

GF  molar mass of final form 
y mol final form molar ratio

2
...
Precipitating agents used in precipitation gravimetry
Species

Final Form

Precipitant

Precipitated

Cl

AgCl

Br

AgBr

I
SO4–2
As
Bi
Cd

AgI
BaSO4
As2O3
Bi2S3
CdSO4

Cu
Sn
Sb

CuO
SnO2
Sb2O3

Mg
Zn

Mg2P2O7
Zn2P2O7

C
...


Properties of precipitates

AgNO3

BaCl2

H2S

(NH4) 2HPO 4

Species

Final

Precipitant

Precipitated

Form

Al

Al2O3

Cr

Cr2O3

Fe
Sn
Ba
Cd
Sr

Fe2O3
SnO2
BaSO4
CdSO4
SrSO4

H2SO4

Ca
Mg
Zn

CaCO3
MgCO3
ZnCO3

(NH4)2C2O4

K
Hg

H2PtCl6
HgS

K2PtCl6
(NH4)2S

NH3

Chemical and Physical Principles

Analytical Chemistry

a
...
Solid particles formed from precipitation may vary accordingly:
 Colloidal – tiny particles with size ranging from 0
...
Appearance
...
Relative supersaturation (von Weimarn ratio)
QS
relative supersaturation 
S
where Q = concentration of the solute as precipitation begins and S = solubility
of the precipitate
 In order to obtain low relative supersaturation and form a crystalline
precipitate, Q must be minimized and S must be maximized
...


Mechanism of precipitation
Precipitation is assumed to occur in two ways:
a
...
Particle growth
 Prevails at low relative supersaturation
 Results in the formation of small number of large particles

3
...
Electrical nature of colloidal suspensions
 Suspensions, which are stable since these particles are either positively or
negatively charged, hence repel each other
 By heating, stirring and addition of electrolyte causes this suspension to
combine together and form a readily filterable solid
 This process of converting a colloidal suspension into a readily filterable
solid is called coagulation or agglomeration
b
...
Factors affecting adsorption
 Common Ion Effect
...
In cases that there is more than one ion
adsorbed, the one having a lower solubility is adsorbed to a greater extent
...
The degree of adsorption
increases as the ionization of the contaminant decreases
 Effect of Concentration
...
Important Terminologies
1
...
Standardization – process of determining the concentration of an unknown
solution
3
...
Secondary standard – compound whose purity was established by a chemical
analysis and serves as reference material for volumetric analysis
5
...
End point – an observable change in a titration process which estimates the
equivalence point
7
...
Conditions for a Volumetric Analysis
1
...
The reaction must be complete and no side reaction occurs
3
...
Characteristics of a Good Primary Standard
1
...
Stable towards air, high temperature and humidity
3
...
Types of Titration
1
...
Back Titration – type of titration where an excess standard solution is added and the
excess is determined by the addition of another standard solution
3
...
Acid-Base Titration
1
...


ion, protonated water or solvated proton
ion

Autoprotolysis or self-ionization reactions
 Involves spontaneous reaction of molecules producing a pair of ions
 Protic solvents have reactive H+ and undergo autoprotolysis
∘

H2O + H2O ⮀ H3O+ + HO–

25 C
pK auto
14
...
8

CH3COOH + CH3COOH ⮀ CH3COOH2+ + CH3COO–

pK 25 C 14
...
7

CH3CH2OH + CH3CH2OH ⮀ CH3CH2OH2+ + CH3CH2O–

25 C
pK auto
19
...
Ion product constants for water
T,C
KW1014
T,C
KW1014
T,C
KW1014
0
0
...
69
40
2
...
19
25
1
...
86
10
0
...
45
50
5
...
46
35
2
...
87
Concentrations are expressed in molarity using density of water at each temperature
...
L
...
U
...
Phys
...
Ref
...

295-304
...


Strength of acids and bases
Strong
Acids

Weak
Bases

Acids

Bases

Analytical Chemistry

Chemical and Physical Principles
HCl
HBr
HI
1
2

HNO3
HClO4
1
H2SO4

LiOH
NaOH
KOH

RbOH
CsOH
2
R4NOH

carboxylic acids
polyprotic acids
metal cations

only the first ionization is complete; dissociation of the second proton has an equilibrium constant of 1
...


Calculation of pH
 At 25C, the ion product constant for water, KW is equal to 1
...
Strong acids (SA) and strong bases (SB)
pH logM SA
 HA + H2O ⭢ H3O+ (aq) + A–(aq)
(SA):
 MOH ⭢ M+ (aq) + HO–(aq)
b
...
Hydrolysis of salts

(SB):

pH 14  logM SB

(WA):

pH  12 logK a M WA

(WB):

pH 14  12 logK b M WB

 As a general rule, salts coming weak acids or weak bases hydrolyze in water,
that is, only the strong conjugate hydrolyzes in water
 Acidic salt (AS) is formed from the reaction of a strong acid and weak base
HCl(aq) + NH3(aq) ⭢ NH4+(aq) +
Cl–1(aq)
SA

WB

SCA

SCB

Since +only the strong conjugate hydrolyzes in water…+
+
HO
⮀ NH
NH
+ HO
4 (aq)

2

Kh 

Kb

3(g)

[NH3 ][H3O ]

KW



3

(aq)

1

(AS):

[NH4  ]

M AS 

pH 7  2 log 

K b 

 Basic salt (BS) is formed from the reaction of a strong base and weak acid
NaOH(aq) + HCN(aq) ⭢ Na+(aq)
+ CN–1(aq) +
H2O
SB



WA

WCB

SCA

Since only the strong conjugate hydrolyzes in water…
CN–1(aq) +
H2O
⮀ HCN(g) + HO– 1(aq)
1
M BS 
K W [HCN][HO ]
1
Kh 

(BS):
pH
7

log


Ka
2
[CN1]
K a 
 Neutral salt (NS) is formed from the reaction of a strong base and strong acid
 Salts from weak acid and weak base (WAB) will have the following
hydrolytic
equilibrium
expressed by the equation
NH +
+ CN–1
+
HO
⮀ NH OH +
HCN
4 (aq)

K 
h

(aq)

KW



1

K K
a

2

[HCN][NH4OH]


[CN ][NH ][H O]
b

4

2

4

(WAB): pH  1 log
2

K W K a 



 K 
 b 

 Amphoteric salts (HA–1 or HA–2)ionize as a weak acid and also a Brønsted
base that hydrolyzes
+
H2O
⮀
H3O+
+
HA–2
H3A
–1
+
⮀
+
H2O
H3O
+
H2A
+
H2O
⮀
H3O+
+

Analytical Chemistry

Chemical and Physical Principles
H2A–
1

HA–2
A–3
1 

K
K

K
K
[H
A
]
 W a1
a1 a2
2

Ka1
Ka2
Ka3

1
 1 


1
2 log
 2 log K a1 K a2
Ka1 [H2 A ]




2 

K K  K a2 K a3 [HA ]  1
2
 1   W a2


pH of HA
 log K K
2 log

a2 a3
Ka2 [HA 2 ]
2




1

pH of H2A



Analytical Chemistry

Chemical and Physical Principles
Table 8
...
75 10–5
6
...
20 10–10
6
...
10 10–8
6
...
10 10–2
5
...
75 10–5
4
...
50 10–4
5
...
30 10–4
4
...
20 10–6
5
...
30 10–5
8
...
30 10–2

K a2

K a3

1
...
10 10–5
7
...
60 10–7
1
...
00 10–6

4
...
10 10–8
4
...
Buffer solutions
 Solution that has the ability to resist changes in hydrogen ion concentration
upon the addition of small amounts of acid or base (buffer action)
 Usually consists of a mixture of weak acid (HA) and its conjugate salt (A–1)
or of a weak base (B) and its conjugate salt (BH+)
 Henderson-Hasselbalch equation
[Macidic ]
[H ][Mbasic  H ]
K A 

[Macidic  H ]

pH pKA  log

[M basic

[HO1 ][Macidic  HO1 ]
K B 

[Mbasic  HO1]

]
[Macidic ]

pH pKW  pKB  log

[M basic

]

 Buffer capacity or buffer intensity or buffer index is the number of moles of
strong acid or strong base for a liter of solution to cause a unit change in pH
dC HA  dC B 
β 

dpH  dpH 
where CHA and CB = number of moles per liter of strong base or strong acid,
respectively to cause d[pH]
...
001 M, the buffer capacity is
estimated as
CHACA1
β 2
...
Commercial concentrated acids and bases
Acids

%wt

HAc
HF
HCl
HBr
HI
HNO3
HClO4
H2SO4
H3PO4

99
...
0
37
...
0
47
...
0
70
...
5
85
...
4
28
...
1
8
...
5
15
...
7
18
...
7

NH3
KOH
NaOH

29
...
0
51
...
05
1
...
18
1
...
50
1
...
67
1
...
70

0
...
46
1
...
3
11
...
9

6
...
Acidic substances for standardizing basic solutions
Name

Formula

Molar
Mass

Molar
equivalent

Benzoic acid
Potassium hydrogen bis(iodate)
Potassium hydrogen o-phthalate
Sulfamic Acid

C6H5COOH
KH(IO3)2
C6H4(COOH)(COOK)
HSO3NH2

122
...
915
204
...
09

1
1
1
1

b
...
989
216
...
22
121
...
Indicators for acid-base titration
Common Name

pKa

methyl orange
bromocresol green
methyl red
bromothymol blue
m-cresol purple
phenolphthalein
thymolphthalein
thymol blue

3
...
66
5
...
10
8
...
00
10
...
70
8
...
1-4
...
8-5
...
2-6
...
2-7
...
6-9
...
3-10
...
4-10
...
2-2
...
0-9
...
Applications of acid-base titration
a
...
Jones factor for protein conversion
Animal origin

Vegetable origin

Food

Factor

Eggs
Meat
Milk
In general…
Cereals
Meat Products
Dairy Products

6
...
25
6
...
70
6
...
38

Food

Barley
Corn
Oats
Rice
Rye
Sorghums
Peanuts

Factor

5
...
25
5
...
95
5
...
25
5
...
83
6
...
30
5
...
25

Source: Food and Agriculture Organization of the United Nations - http://www
...
org/docrep/006/y5022e/y5022e03
...
Double indicator method for mixture of bases – Warder Titration
 The presence of hydroxide, carbonate and bicarbonate in water is also referred
to as alkalinity which is a measure of the acid-neutralizing capacity of water
 One method requires titration of the mixture to reach the phenolphthalein

Chemical and Physical Principles

Analytical Chemistry

endpoint with the volume recorded as V0-Ph
...
One sample is treated with
phenolphthalein and the other with methyl red
...

 m-Cresol purple can also be used to detect phenolphthalein alkalinity (P) while
bromocresol green or methyl orange for the total alkalinity (T)
Table 10
...
Acid number or acid value
 Defined as the mass (mg) of KOH that will neutralize the acid produced from
water degradative reaction of one gram of fat or oil
)(56
...
Saponification number or Koettstorfer number
H2C

OOCR1

HC

OOCR2

H2C

OOCR3

H2C
+ 3 KOH

HO

CH

H2C

R1COOK

OH
+

OH

R2COOK

R3COOK

 Defined as the mass (mg) of KOH required to saponify one gram of fat or oil
 Can be used to determine the approximate molar mass of fat or oil
 The sample is refluxed with ethanolic KOH and the resulting solution is
titrated with standard HCl
mL
(V mL  Vwith
samplel )(M HCl )(56
...
Precipitation Titration
1
...
Solubility rules for ionic compounds in water at 25°C
Soluble compounds

Insoluble compounds

All nitrates, bicarbonates, chlorates
and compounds containing alkali
metal ions and ammonium ion
...
Solubility product constant (KSP)
 Consider an aqueous saturated solution of a sparingly soluble salt represented by
the equation:
AxBy (s) ⮀ x A+y (aq) + y B–x(aq)
The equilibrium constant for this reaction would be:
[Ay(aq) ]x [Bx (aq) ]y
Keq 
[Ax By(s) ]
However the concentration of the solid AxBy in the solution will be constant (the
ratio of the number moles of AxBy and the volume of the solid is constant)
...
Solubility product constants at 25C
COMPOUND

KSP

COMPOUND

KSP

AgCl
AgBr
AgI
BaCO3
Ba(IO3)2
Al(OH)3
CaCO3 (calcite)

1
...
00 10–13
8
...
00 10–9
1
...
00 10–34
4
...
10 10–12
6
...
70 10–5
7
...
20 10–18
5
...
70 10–29

4
...
Formation of a secondary colored precipitate – Mohr method
 Developed by Karl Friedrich Mohr (1806-1879) in 1865
Titrant:
AgNO3

Analytical Chemistry

Chemical and Physical Principles
Titration reaction:

Ag+ (aq) + Cl–1 (aq) ⮀ AgCl(s)
white

Analytical Chemistry

Chemical and Physical Principles
Indicator:
Indicator reaction:

K2CrO4
2Ag+(aq) + CrO4–2 (aq) ⮀ Ag2CrO 4(s)
yellow

red

 In practice, the indicator concentration is kept between 0
...
005 M
 Titration is done at a pH of 8 to avoid precipitation of silver as hydroxide
(above pH of 10) and eliminate formation of HCrO4–1 (below pH of 6) which
results to consumption of more titrant
 Usually a low concentration of chromate is desired to detect the end point
clearly since a chromate ion imparts an intense yellow color
b
...
01 M
 For the titration of chloride, the resulting precipitate is filtered off before the
back titration since it reacts with the titrant and is more soluble than AgSCN
 For the titration of iodide, the indicator is not added until all iodide is
precipitated since the dissolved iodide is oxidized by the ferric ion
c
...
DCF–1
excess

Indicator:
Indicator reaction:

white

greenish-yellow

fluorescein, dichlorofluorescein or eosin
Ag+ (aq) + Cl–1 (aq) ⮀ AgCl(s) :Ag+1:DCF–1
excess

white

pink

 Before the equivalence point, chloride anion adsorbs to the precipitate in the
primary adsorption layer and drives the adsorption dye anion away by
electrostatic repulsio
a red complex; when all the calcium is titrated, the liberated Mg+2 is
released, combines with EDTA and the endpoint is observed with the
formation of blue uncomplexed indicator
H
...
Important terminologies
 Oxidation – process which involves increase in oxidation state as a result of loss
of electron
 Reduction – process which involves decrease in oxidation state as a result of
gain of electron
 Disproportionation – process in which an element in an intermediate oxidation
state yields products in both lower and higher oxidation states
 Oxidant or oxidizing agent – substance that accepts electron and undergoes
reduction
 Reductant or reducing agent – substance that donates electron and undergoes
oxidation
2
...
Rules in assigning oxidation numbers
 An atom in its free or elemental form has oxidation equal to zero
 For monoatomic ions, the oxidation number is equal to its charge
 Metals have positive oxidation number such as alkali metals (+1), alkaline
earth metals (+2), aluminum (+3), zinc (+2) and silver (+1)
 Nonmetals usually have negative oxidation numbers:
 Oxygen is usually –2, except in peroxides (–2) and superoxides (–1)
 Hydrogen is usually +1, except in hydrides (–1)
 Fluorine has –1 oxidation state; other halogens are usually in the –1
oxidation state, except when combined with oxygen, they are positive;
when different halogens are bound to each other, –1 is assigned to the more
electronegative halogen
 The sum of oxidation number of elements in a compound is equal to zero
 The sum of oxidation number of elements in a polyatomic ion is equal to the
charge of the ion
b
...

Neutralize H+ by adding HO–1 on both sides of the reaction and simplify

3 NO2–1 + 2 MnO4–1 + 2 H+ + 2 HO–1 ⮀ 3 NO3–1 + 2 MnO2 + H2O + 2 HO–1
3 NO2–1 + 2 MnO4–1 +
2 H2O
⮀ 3 NO3–1 + 2 MnO2 + H2O + 2 HO–1
3 NO2–1 + 2 MnO4–1 + H2O ⮀ 3 NO3–1 + 2 MnO2 + 2 HO–1

3
...
314 J-mol–1-K–1, T = temperature [K], n = number of electrons that
appear in the half-cell reaction [mol], a = activity [ ] and F, Faraday’s
constant = 96485
...
05916
log Q
n

Chemical and Physical Principles
where Q = reaction quotient [ ]

Analytical Chemistry

Chemical and Physical Principles

Analytical Chemistry

 The equilibrium constant and the standard electrode potential are related as
follows:
nε
log K 
(T = 298
...
05916 V
4
...
Auxiliary oxidants and reductants
 Pre-reductants
 Jones reductor - consists of zinc metal treated with 2% solution of HgCl2
(amalgamated zinc) and used to reduce Fe +3 (⮀Fe+2), Cu+2 (⮀Cu), TiO+2
(⮀Ti+2), UO2+2 (⮀U+3 or ⮀U+4) and Cr+3(⮀Cr+2)
 Walden reductor - consists of a column filled with silver metal or an
insoluble salt of silver and does not reduce Fe+3 and TiO+2
 Na2SO3, NaHSO3, or SnCl2
 Pre-oxidants
 NaBiO3, (NH4)2S2O8, K2S2O8, Br2, Cl2, Na2O2 or H2O2
b
...
Dichromate titration
 Titration is carried out in acidic medium only
 Titrant is stable towards light and less easily reduced in the presence of
organic matter compared to permanganate
Titrant:
K2Cr2O7
Half-cell reaction:
Cr2O7–2 + 14H+ + 6e– ⮀ 2Cr+3 + 7H2O
Primary standards:
 Fe metal
Fe(s) ⮀ Fe+2 + 2e–
 FeSO 4
(en)2SO 4
4H 2O
Fe +2 ⮀ Fe +3 + e –
Indicator:
sodium diphenylamine sulfonate
N-phenylanthranilic acid
Endpoint:
first appearance of blue-violet

Analytical Chemistry

Chemical and Physical Principles

d
...
5 M or higher
 In the presence of HCl, titrant is consumed to oxidize Cl–1
Titrant:
Ce(SO 4)2 and (NH 4)4[Ce(SO 4) 4]
2H 2O
Half-cell reaction:
Ce+4 + e– ⮀ Ce+3
Primary standard:
As2O3
Half-cell reaction:
H3AsO3 + H2O ⮀ H3AsO4 + 2H+ + 2e–
Indicator:
ferroin / N-phenylanthranilic acid
End point:
orange-red to pale blue / yellowish-green to purple
e
...
Iodometry: Indirect titration with iodine
 The analyte is an oxidizing agent which reacts with I–1 added to the solution
in excess to liberate I2 equivalent to the amount of analyte present
Titrant:
Half-cell reaction:
Primary standard:
Half-cell reaction:

Na2S2O3
2S2O3–2 ⮀ S4O6–2 + 2e–
KIO3 or K2Cr2O7
2IO3–1 + 12H+ + 10e– ⮀ I2 + 6H2O
Cr2O7–2 + 14H+ + 6e– ⮀ 2Cr+3 + 7H2O
Indicator:
Starch solution
Endpoint:
Color change from blue to colorless
g
...
all of these
82
...
For solutions with high chloride content, addition of sulfamic acid is necessary
to eliminate the chloride interference
b
...
The interference caused by the nitrite ion is eliminated by the addition of
HgSO4 which oxidizes this ion to its nitrate form
d
...
A 100 mL water sample was analyzed by Winkler Method
...
52 mL of 0
...

a
...
8
b
...
5
c
...
4
d
...
5
84
...
Excess
thiocyanates ion was added and the resulting solutions were diluted up to the mark and
the following absorbance were obtained in a 1
...
095 0
...
433 0
...
806 0
...

a
...
5 ppm
b
...
3 ppm
c
...
0 ppm
d
...
5 ppm
85
...
93) and nickel (58
...
A 25 mL aliquot was treated with a complexing agent
to produce a colored complex and the volume was adjusted to 50 mL
...
00-cm cell:
Wavelength

Molar absorptivity, [M–1-cm–1]

Absorbance of

(nm)

Co

Ni

solution, A

510
656

36400
1240

5520
17500

0
...
314

Calculate the concentration of Co in the solution in ppm
a
...
1940 ppm
c
...
2343 ppm



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