Download Standards&Indicators

STANDARDS & INDICATORS
PRIMARY STANDARDS


Substance of High Purity that is used prepare
the concentration of the standard solution in
volumetric analysis
Reference to determine unknown concentrations
PRIMARY STANDARDS – PROPERTIES

High Purity


Purity of at least 99.98%
Stable towards air
Not altered on exposure to air
 Not oxidized by air
 Not react with CO2 or absorb moisture from air


Absence of Water of Crystallization
Stable to drying
 Composition should not change when heated or due
to humidity

PRIMARY STANDARDS – PROPERTIES

Relatively large formula weight

Minimizes relative errors associated with weighing

Reasonable solubility in titration medium

Examples of primary standards






KHC8H4O4 (potassium hydrogen phthalate)
NaHCO3
Na2CO3
KIO3
(COOH)2 (oxalic acid)
Few meet all these requirements
SECONDARY STANDARDS



Compound whose purity is established by
chemical analysis
serves as a reference material for titrimetric
method of analysis
solutions of unknown concentrations that are
standardized in the laboratory against primary
standards
STANDARDS SOLUTIONS – PROPERTIES

Sufficiently stable


React rapidly with analyte


Concentration is determined only once
Time required between additions is minimized
React completely with the analyte

End points can be obtained

Undergo a selective reaction with the analyte

Few meet these requirements
STANDARDS SOLUTIONS

Accuracy of titration method is dependent on the
accuracy of the concentration of the standard
solution
How are the concentrations of standard solutions
determined?

Direct Method

Standardization
STANDARDS SOLUTIONS –
DIRECT METHOD



Carefully weighed quantity of a primary
standard
Dissolve weighed primary standard in suitable
solvent
Dilute to exactly known volume in a volumetric
flask
STANDARDS SOLUTIONS –
STANDARDIZATION

Titrant or the concentration of volumetric
solution is titrated against

Primary standard

Secondary standard

Known volume of a standard solution
SUITABLE PRIMARY STANDARDS –
ACIDS VS BASES

Standardization of Acids


Na2CO3
106 g mol-1
Standardization of Bases

KHC8H4O4
204.2 g mol-1
INDICATORS


weak acid or a weak base
undissociated form of the indicator (HInd) is a
different color than the iogenic form of the
indicator (Ind-)
HInd + H2O


H3O+ + Ind-
does not change color from pure acid to pure
alkaline at specific hydrogen ion concentration
color change occurs over a range of hydrogen ion
concentrations
INDICATORS

Choice of indicators depend on the pH range of
the solution
TITRIMETRIC ANALYSIS WITHOUT
INDICATORS

Titrimetric Analysis that require NO ACIDBASE INDICATORS

Conductrimetric Titration

Potentiometric Titration