NEUTRALIZATIONANALYTICAL CHEMISTRY● Area of chemistry responsible for
characterising the composition of matter, both
qualitatively and quantitatively (Classical
definition)
● Science of inventing and applying concepts,
principles and strategies for measuring the
characteristics of chemical systems and
species (Modern definition)
MAJOR DIVISIONSQUALITATIVE ANALYSIS
● WHAT is present
● Identification of components in a given
sample
● Reveals the chemical identity of species in
the sample
QUANTITATIVE ANALYSIS
● HOW MUCH is present
● Determination of amount of components in
a given sample
CLASSIFICATION OF ANALYSIS
1. Based on extent of determination
2. Based on sample size
3. Based on material used
4. Based on nature of method
ERRORS IN ANALYTICAL CHEMISTRY1. GROSS ERROR
2. RANDOM ERROR
3. SYSTEMATIC ERROR
GROSS ERROR
● Involves a major breakdown in the analytical
process
● Often the product of human error
● Or personal bias
Examples
1. Samples being spilt
2. Wrong dilutions being prepared
3. Instruments breaking down
4. Instruments being used in the wrong way
(If a gross error occurs, the results are rejected
and the analysis is repeated from the beginning.)
RANDOM X SYSTEMATIC ERROR
EXAMPLE
A batch of paracetamol tablets are stated to
contain 500 mg of paracetamol per tablet. For the
purpose of this example it is presumed that 100%
of the stated content is the correct answer.
Four students carry out a spectrophotometric
analysis of a sample from the tablets and
obtained the following percentages of stated
content for the repeat analysis of paracetamol in
the tablets:
Based on extent of determination
Based on sample size
Based on material used
Based on nature of method
RANDOM ERROR
● Indeterminate
● Affects precision
● Small differences in measurements
● Difficult to eliminate
● Quantifiable through standard deviation
SYSTEMATIC ERROR
● Determinate
● Affects accuracy
● Has definite value or assignable cause
● Can be eliminated
● Quantifiable by difference between mean
and true value
TYPES OF SYSTEMATIC ERROR1. Instrumental Error
● Caused by imperfections in measuring
devices
● Use of wrong grade of chemicals
2. Methodological Error
● Due to nonideal chemical or physical
behaviour of the reagents and reactions
3. Personal/Operative Error
● Arise from prejudice or natural tendency to
estimate scale readings
TITRIMETRIC ANALYSIS OR
VOLUMETRIC ANALYSIS●
●
Involves a solution of reagent of known
concentration (titrant) added to a solution of
analyte until reaction is judged complete
(endpoint)
The determination of the volume of a solution
of known concentration required to react with
a given amount of the substance to be
analysed
TITRATIONAn analytical method in which the volume of a
solution of known concentration consumed during
an analysis is taken as a measure of the amount
of active constituent in a sample analysed
HCl + NaOH → NaCl + H2O
CLASSIFICATION USED FOR OFFICIAL
ASSAYS1. Titrimetry
2. Gravimetry
3. Spectrometry
4. Chromatography
5. Electrometric
6. Miscellaneous
DEFINITION OF TERMSANALYTE
● Active constituent in the sample
● Titrand, sample
● Unknown concentration
TITRANT
● Standard solution, volumetric solution
● Known concentration
INDICATOR
● Chemicals capable of changing colors when
end point is reached in which equivalent
quantities of analyte and titrant have been
reached
EQUIVALENCE POINT
● Theoretical or stoichiometric endpoint
● Point at which a chemically equivalent
amount of titrant has reacted with the analyte
END POINT
● Practical
● Point at which the reaction is observed to be
complete
● A sudden change apparent by use of
indicators
TITRATION ERROR
● Equivalence point – End point
REPLACEABLE HYDROGEN-
VOLUMETRIC SOLUTIONBASED ON METHOD OF PREPARATION
1. Primary Standard VS
● obtained from a substance which
approaches 100% purity
● does not require standardization
● [0.05 M KIO3]
2. Secondary Standard VS
● not sufficiently pure and is standardized
against a primary standard or previously
standardized volumetric solution
● [0.1 N NaOH VS, 0.1 N AgNO3]
EQUIVALENT●
●
●
1 equivalent of an acid neutralizes exactly 1
equivalent of a base, and vice versa
1 equivalent of an oxidizing agent reacts with
exactly 1 equivalent of a reducing agent, and
vice versa
Quantity of a substance that is chemically
equivalent to 1.0079 g of hydrogen or
hydrogen ions
Examples
● 1 mole or 36.46 g of HCl and and 0.5 mol or
49.035 g of H2SO4 contains 1.0079 g of
hydrogen ions (1 equiv) and is capable of
neutralizing exactly 1 mol (1 equiv) or 40.00 g
of NaOH or 0.5 mol or 85.675 g of Ba(OH)2
●
0.25 mol or 7.997 g of oxygen gains electrons
equivalent to those gained by 1.0079 g of
hydrogen ions when they act as an oxidizing
agent and 1 mol or 166.00 g of KI loses
electrons equivalent to those lost by 1.0079 g
of hydrogen when it acts as a reducing agent
●
Number of gram-equivalents involved in a
quantitative procedure
STANDARDIZATION● Process of determining the exact value of a
volumetric solution
● Determining the exact concentration using
known amount of Primary standard or
Secondary standard
TYPES OF STANDARDS
1. Primary Standard
● pure solid substitute used for direct
standardization of a solution
2. Secondary Standard
● standard solution of known concentration,
these are volumetric solutions
PRIMARY STANDARD
REQUIREMENTS FOR PRIMARY STANDARD
1. It must be available in a highly pure state.
2. It must be readily soluble in the desired solvent.
3. It must not suffer decomposition in the
presence of the solvent.
4. It must be stable and unaffected by the
atmosphere.
5. It should have a large equivalent weight so that
weighing errors are minimum.
6. Its solution should not deteriorate on keeping.
1. Sodium Hydroxide (4 PRIMARY STANDARDS)
GRAM – EQUIVALENT WEIGHT (GEW)
●
●
Weight in grams which is chemically
equivalent to 1 gram-atom of hydrogen
(1.0079 g)
[Neutralization reactions] weight of a
substance in grams which contains,
furnishes, reacts with directly or indirectly or
replaces 1 gram-atom or ion of hydrogen
GRAM – MILLIEQUIVALENT WEIGHT (GmEW)
●
GEW/1000
BLANK DETERMINATION
●
●
Entire procedure is repeated except that the
analyte is omitted
Conduct for correction and to enhance
reliability of end point
2. Hydrochloric Acid
● Standardized using titrimetric methods using
sodium carbonate of known purity as a
primary standard
● Using standard NaOH as a secondary
standard
SECONDARY STANDARD
● A solution of known concentration used to
standardize another solution
VOLUMETRIC APPARATUS-
PRECAUTIONS FOR USE
● All burets should be tested for leakage before
use.
● Glass stopcocks may require a small amount
of special stopcock lubricant to prevent
leakage and permit ease of operation.
● Reading Burets: The surface of liquids in
narrow tubes is always curved because of
capillarity, the surface or meniscus is always
concave when the liquid wets the tube and
convex when it does not. Readings should
always be made at the lowest point of the
meniscus (the graduation mark that coincides
with the bottom of the curve, except when
measuring highly colored liquids. The eye
must be on the same level as the meniscus,
otherwise errors due to parallax will be
introduced.
●
In a clean instrument, the liquid drains down
uniformly, wetting the walls so that no
droplets are observable
CLEANING AGENTS
*use warm cleaning solutions
a. Sodium dichromate in sulfuric acid
b. Solution of trisodium phosphate
c. Solution of synthetic detergent
Note: Hot solutions should be avoided when
cleaning accurately calibrated apparatus, because
of the possible production of a permanent change
in volume caused by the heat and known as
thermal aftereffect.
SOURCES OF ERROR IN THE USE OF
VOLUMETRIC APPARATUS
Every precaution should be taken to avoid the
following common sources of error in the use of
volumetric apparatus
1. Rinse water adhering to walls of apparatus
The apparatus may be dried, but it is more
convenient and just as accurate to wash it out
with small successive portions of the liquid to be
used, discarding the washings.
TITER VALUE● Expresses the equivalency of a standard
solution with the amount of substance under
assay
2. Grease films and dirty apparatus
Dirt and oil cause irregularity in the delivery of
liquids and distort the meniscus
3. Parallax
Must be avoided to secure proper readings of the
level of the meniscus
4. Variations in temperature
Lead to changes in volume of vessels and liquids,
all measurements therefore should be made at a
temperature closely approximating that at which
the apparatus was calibrated
5. Air bubbles trapped beneath liquid surface
Especially below the stopcock in burets, displace
liquids
6. Heat
Hot solutions cause calibrated volumetric
apparatus to suffer change in volumes
CLEANING SOLUTION
● Volumetric apparatus should always be
cleaned before using and in use should be
cleaned as often as necessary
● The adherence of droplets to the wall of a
buret or pipet is positive evidence that the
apparatus is dirty
TYPES OF TITRATION
1. Direct Titration
2. Indirect Titration
3. Residual Titration
If exactly 3.0 g of NaHCO3 is dissolved in 25 mL
of water in the above assay, what will be the
normality of the solution? Does the normality
conform to the general rule stated in the
discussion?
If a 0.2800 g sample of sodium bicarbonate
(96.5% NaHCO3 ) is titrated with 0.9165 N
sulfuric acid, what volume of the acid should be
required to produce the endpoint?
DIRECT TITRATION● Analyte is reacted with titrant after rendering
it soluble in the titration medium
● Conducted by introducing a standard solution
gradually from a buret into the analyte
● One titrant (VS) used
EXAMPLEASSAY OF SODIUM BICARBONATE
Weigh accurately about 3 g of sodium
bicarbonate, mix with 25 mL of water, add methyl
orange TS, and titrate with 1 N sulfuric acid. Each
millilitre of 1 N sulfuric acid is equivalent to 84.01
mg of NaHCO3.
●
●
●
●
●
It is a general rule that when the substance to
be assayed is a solid, the accurately weighed
quantity to be used should be dissolved in
sufficient water to make the solution of about
the same equivalent concentration as that of
the acid to be used in the titration.
Methyl orange is used as an indicator
because phenolphthalein and most other
indicators are affected by the carbonic acid
liberated in the reaction, which causes a
change in color before the reaction is
complete.
From the equation it is obvious that 2 mol of
NaHCO3 is equivalent to 1 mol of H2SO4.
Therefore 1 mol of NaHCO3 is 1 equiv, and
the equivalent weight is equal to the gram
molecular weight, 84.01 g.
One millilitre of 1 N sulfuric acid or 1 meq is
equivalent to 84.01 mg or 1 meq of NaHCO3.
Calculate the following titer values for 1 N sulfuric
acid
a. KHCO3
b. K2CO3
c. CaCO3
INDIRECT TITRATION● Analyte undergoes preliminary treatment prior
to titration.
RESIDUAL TITRATION● A measured excess of one VS is added
which will react completely with the analyte
● The unreacted excess is then back titrated
with another VS
● “back titration”
● Adding an accurately measured quantity of
standard solution in excess and titrating the
excess with another standard solution
Two VS used
● 1st VS: added in excess
● 2nd VS: used to titrate the excess
Used when direct titration is not practicable
● Compounds which react too slowly with titrant
● Compound with poor water solubility
● Volatile substance
Blank Determination: process of repeating the
procedure but omitting the sample
TYPES OF REACTIONS IN TITRIMETRIC
ANALYSIS1. Neutralization (acid-base) in aqueous and
nonaqueous solvents
2. Oxidation-Reduction
3. Precipitation
4. Complexation
REQUIREMENT FOR A REACTION TO BE
USED FOR TITRIMETRIC ANALYSIS
1. The reaction must proceed to completion
Chemical equilibrium constant K must be equal to
8
or greater than 10
2. The reaction must proceed in a stoichiometric
manner
3. A suitable endpoint detecting device must be
available Chemical indicators, electrodes, etc
4. For direct titrimetric methods the reaction must
be rapid so a sharp end point is discernible
●
●
When an indicator is used to detect the
equivalence point, it must give a sharp
discernible color change at that point in the
titration; otherwise the color change may lead
to erroneous results.
Titration curves can show which indicator is
suitable for a given titration.
NEUTRALIZATION REACTIONSNEUTRALIZATION
● Reaction of acid and base
NEUTRALIZATION REACTIONS
● Chemical processes in which an acid (proton
donor) reacts with a base (proton acceptor)
● The products of a neutralization reaction in
aqueous solution are water and a salt (NaCl,
sodium acetate, NH4Cl, etc, depending on
the acid and base used in the reaction)
pH-
The preparation of a titration curve is not difficult if
one realizes that the calculations are based on
only four types of points, which correspond to:
1. The pH of the solution being titrated but before
any titrant is added (pH1)
2. The pH of the solution after titrant is added but
before the equivalence point is reached (pH2)
3. The pH at the equivalence point (pH3)
4. The pH beyond the equivalence point (pH4)
HENDERSON – HASSELBALCH EQUATION
PRACTISE PROBLEM - # 1
Calculate the pH of solution which contains 3.7 x
10 -2 moles of NH4Cl and 4.8 x 10 -2 moles of
ammonia (pKb = 4.74) in 1 L of solution.
PRACTISE PROBLEM - # 2
A buffer solution contains 0.1 M each of acetic
acid and sodium acetate. The acid dissociation of
acetic acid at 25 °C is 1.8 x 10-3 . What is the pH
of the solution?
TITRATION CURVEThe manner in which the pH of a solution changes
during a titration is best shown by preparing a
graph in which the pH value is plotted on the
y-axis and the volume of the titrant is plotted on
the x-axis.
● Equivalence point on the titration curve is that
portion where the slope of the curve is
steepest and is commonly referred to as the
break in the titration curve.
TITRATION OF A STRONG ACID USING
STRONG BASESuppose that 20.00 mL of exactly 0.1 N HCl were
taken, diluted with sufficient distilled water to
measure exactly 100 mL, and titrated with exactly
0.1 N NaOH.
●
●
Capable of existing in two forms of different
color that are mutually convertible, one into
the other at given hydrogen-ion
concentrations
Most of the indicators for acid-base titration
are acidic
As can be seen in the curve, either
phenolphthalein or methyl red indicators are
suitable.
SALT FORMATION 1. Strong Acid + Strong Base = ? – HCl+
NaOH.________
2. Strong Acid + Weak Base = ? – HCl +
NH3.________
3. Strong Base + Weak Acid = ? – NaOH+
CH3COOH ._______
CLASSIFICATIONBASED ON MEDIA USED
1. Aqueous Medium
2. Non – aqueous Medium
BASED ON TITRANT USED
1. Aqueous Medium
2. Non – aqueous Medium
INDICATOR● Are complex organic compounds used to:
● Determine endpoint
● Determine the pH
● Indicate that a desired change in pH has
been affected
CHARACTERISTICS
● Organic substances
● May be acids or bases themselves
VISUAL END POINTS ● The end points of titrations are most often
determined by noting some visual change in
the solution, usually color.
● These changes are always based on the fact
that pA or pT changes dramatically and
rapidly in the vicinity of the equivalence point.
● There are three ways that color changes can
occur near the equivalence point of a titration.
Regardless of its mechanism, the substance
undergoing the color change is called an
indicator.
1. Colored titrants or analytes
2. Specific compound formers
3. Nonspecific, equilibrium-dependent
compounds
Nonspecific, equilibrium-dependent
compounds
● Indicators of this types are characterized by
two fundamental properties
a. they exist in two equilibrium forms (i.e.
protonated-unprotonated or
oxidized-reduced)
b. each form has a different color
● The position of the equilibrium between the
two forms of the indicator, and therefore its
color, depends in some way on the
concentration of analyte or titrant in the
solution.
● The large relative and rapid change in analyte
or titrant concentrations in the vicinity of the
equivalence point causes a large shift in the
position of the indicator equilibrium.
● When the concentrations of the two forms of
the indicator change in order to coincide with
the new equilibrium, the observed color also
changes.
CHARACTERISTICS
● Organic substances
● May be acids or bases themselves
● Capable of existing in two forms of different
color that are mutually convertible, one into
the other at given hydrogen-ion
concentrations
● Most of the indicators for acid-base titration
are acidic
PHENOLPHTHALEIN● Approximate pH range for color change: 8.0 –
9.8
● [clear] [red violet]
●
●
Approximate pH range for color change: 4.4 –
6.2
[red] [yellow]
METHYL RED in its (a) weak acid, HIn, form, (c)
its weak base, In – , form, and (b) a mixture of HIn
and In–.
1. Aqueous Medium
a. SA + SB
b. WA + SB
c. WB + SA
d. WB + WA
2. Non-aqueous Medium
a. WB + salt
b. Relatively SB
c. Strong acid
d. Weak acid
Are complex organic compounds used to:
● Determine endpoint
● Determine the pH
● Indicate that a desired change in pH has
been affected
PRACTICE PROBLEM - #1 ASSAY OF SODIUM
CARBONATE
What is the purity of NaHCO3 if 0.29 g sample is
titrated with 0.9289 N H2 SO4 . It consumed 3.6
mL to reach the endpoint. (MW of NaHCO3 = 84
g/mole)
PRACTICE PROBLEM - #2 ASSAY OF SODIUM
BICARBONATE
If 0.28 g sample of NaHCO3 (96.5%) is titrated
with 0.9165 N H2 SO4 what volume of the acid
should be required to produce an endpoint?
RULE FOR THE USE OF INDICATORS 1. Use 3 drops of indicator test solution for a
titration unless otherwise directed.
2. When a SA is titrated with a SB, or a SB with a
SA, methyl orange, methyl red or phenolphthalein
may be used.
3. When a WA is titrated with a SB, use
phenolphthalein as the indicator.
4. When a WB is titrated with a SA, use methyl
red as the indicator.
5. A WB should never be titrated with a WA, or
vice versa, since no indicator will give a sharp end
point.
6. The appearance of a color is more easily
observable than is the disappearance. Therefore,
always titrate where possible to the appearance of
color.
REVIEW1. SA: HCl, HNO3, H2SO4, HI
2. SB: NaOH, KOH
3. WA: acetic acid, oxalic acid
4. WB: NH3
PRACTICE PROBLEM - #3 ASSAY OF SODIUM
BICARBONATE
The disodium salt of a dibasic organic substance
has a molecular weight of 215 g/mol, what weight
of a sample would be used for titration to
bromophenol blue with 0.25 N acid in order that
35 mL of the latter will be required?
PRACTICE PROBLEM - #4 ASSAY OF SODIUM
BICARBONATE
Twenty sodium salicylate tablets labeled 325 mg
were dispersed in sufficient water to make 200
mL. A 15 mL aliquot of the filtrate was titrated to
the endpoint in the usual way by 29.11 mL of
0.1000 N HCl. Calculate the amount in % of
sodium salicylate. (MW of Na salicylate = 160)
COMPUTATION – RESIDUAL TITRATION
PRACTICE PROBLEM - #1
If a sample of magnesia magma weighing 5.2430
g when dissolved in 25 mL of 0.9915 N H2 SO4
required 9.85 mL of 1.1402 N NaOH to titrate
excess acid, what is the percent of Mg(OH)2
present in the sample? –Mg = 24.31 –O = 16 –H
=1
ACIDIMETRIC ANALYSISTYPES OF ACIDIMETRIC ANALYSIS
DIRECT TITRATION
● Standard acid solution from a buret into a
solution of base being assayed
RESIDUAL TITRATION
● First volumetric solution (acid) is added in
excess and the excess acid is titrated with
standard base
PRACTICE PROBLEM - #2
If the USP requirement for magnesia magma is
not less than 7% and not more than 8.5%
Mg(OH)2 , did the sample conform to the official
requirement? (%P)
EXAMPLES
ORGANIC SUBSTANCES
urea, sodium salicylate, diphenhydramine,
emetine hydrochloride, meprobamate,
paramethadione, pyrazinamide
INORGANIC SUBSTANCES
sodium bicarbonate, milk of magnesia,
ammonium chloride, calcium hydroxide, lithium
carbonate, zinc oxide
PRACTICE PROBLEM - #1 ASSAY OF
MAGNESIUM HYDROXIDE
Calculate the Mg(OH)2 content of milk of
magnesia, 12.32 g of which was dissolved in 50
mL of 1.0340 N H2 SO4 , producing a mixture that
required 24.6 mL of 1.1255 N NaOH? (MWMg =
24.31; O=16; H=1) (%P)
PRACTICE PROBLEM - #2 ZINC OXIDE
If a 1.25 g sample of ZnO, were treated with 50
mL of 1.1230 N H2 SO4 and required 27.5 mL of
0.9765N NaOH in the back titration. Compute %
purity of ZnO. MW of ZnO= 81.41 (%P)
ALKALIMETRIC ANALYSIS1. The direct or residual titrimetric analysis of
ACIDS
2. Direct: sample is acid, titrant is base
3. Residual: sample is acid, treated with excess
standard base, titrated with acid
TYPES OF ALKALIMETRIC ANALYSIS
DIRECT
sample is acid, titrant is base
RESIDUAL
sample is acid, treated with excess standard
base, titrated with acid
PRACTICE PROBLEM - #1 DIRECT TITRATION
A 5 mL of HCl, (SpGr= 1.3), required 50 mL of 1
N NaOH in a titration. Calculate the % w/w HCl.
(MW= 36.45)
PRACTICE PROBLEM - #2 DIRECT TITRATION
Concentrated HCl (36.46 g/mol) has a purity of
36.5 % w/w (SpGr =1.18), approximately how
many mL of conc. HCl is needed to prepare 500
mL of 0.2 N solution?
NITROGEN DETERMINATION BY KJELDAHL
METHOD
● For organic compounds containing nitrogen
● Digest sample with concentrated H2SO4 until
decomposition is complete
● NH3 is treated with excess H2 SO4 with
NaOH
2 TYPES
1. METHOD 1 (MACRO)
2. METHOD 11 (SEMI MICRO)
The following hasten the digestion of ammonia
with sulfuric acid:
1. potassium sulfate
2. Selenium
3. copper sulfate
PRACTICE PROBLEM - #1 KJELDAHL
METHOD
The Kjeldahl procedure was used to analyze 5 g
of organic acid. It was distilled into 50 mL of 0.055
N HCl and the mixture was titrated with 20 mL of
0.1010 N NaOH. Calculate the %N in the sample
(at. wt of N= 14)