A technique for determining the concentration of a solution by measuring the volume of one solution needed to completely react with another solution.
Titration process involves addition of solution of known a burette to the measured volumeconcentration from of analyte.
THE PRINCIPLE of TITRATION:
It is based on the complete chemical reaction between the analyte and the titrant of known concentration.
ANALYTE + TITRANT -> PRODUCT
Volumetric titrimetry
involves measuring the volume of a solution of known concentration that is needed to react essentially completely with the analyte
Gravimetrictitrimetry
differs only in that the mass of the reagent is measured instead of its volume
Coulometric titrimetry
the reagent is a constant direct electrical current of known magnitude that consumes the analyte
Standardsolution (aka. standardtitrant)
a reagent of known concentration that is used to carry out a titrimetric analysis
A titration is performed by adding a standardsolution from a buret or other liquid-dispensing device to a solution of the analyte until the reaction between the two is judged complete.
The volumeofreagent needed to complete the titration is determined from the difference between the initial and final volume readings.
Equivalence point
is reached when the amount of added titrant is chemically equivalent to the amount of analyte in the sample
Equivalence Point
the point in a titration when the amount of added standard reagent is exactly equivalent to the amount of analyte
End point
the point in a titration when a physical change occurs that is associated with the condition of the chemical equivalence
Indicators
are often added to the analyte solution to produce an observable physical change at or near the equivalence point
INDICATORS (Physical Change)
the appearance or disappearance of a color
the change in color
the appearance or disappearance of turbidity
Typical setup for carrying out a Titration
The apparatus consists of:
buret, buret stand, and clamp with a white porcelain base to provide an appropriate background for viewing indicator changes
wide-mouth precisely known volume of Erlenmeyer flask containing the solution to be titrated
The solution is normally delivered into the flask using a pipette
Detail of the Buret Graduations
Normally, the buret is filled with titrant solution to within 1 or 2 mL of the zero position at the top.
The initial volume of the buret is read to the nearest ± 0.01 mL.
There's a reference point on the meniscus and the proper position of the eye for reading.
Before the titration begins.
The solution to be titrated, an acid in this example, is placed in the flask and the indicator is added.
The indicator in this case which is phenolphthalein, turns pink in basic solution.
During titration.
The titrant is added to the flask with swirling until the color of the indicator persists.
In the initial region of the titration, titrant may be added rather rapidly, but as the end point is approached, increasingly smaller portions are added; at the end point, lessthanhalf a drop of titrant should cause the indicator to change color.
The end point is achieved when the barely perceptible pink color of phenolphthalein persists.
The final reading of the buret is made at the end point, and the volume of base delivered in the titration is calculated from the difference between the initial and final buret readings.
The solution turns a deep pink color, when the endpoint has been exceeded.
Titration error (Et)
the difference in volume or mass between the equivalencepoint and the endpoint
Et = Vep– Veq
Vep: the actual volume of reagent required to reach the end point
Veq: the theoretical volume to reach the equivalence point
PrimaryStandards
a highly purified compound that serves as a reference material in all volumetric and mass titrimetric methods.
The accuracy of a method is critically dependent on the properties of this compound
Important requirements for a primary standard are:
High purity
Atmospheric stability
Absence of hydratewater so that the composition of the solid does not change with variation in relative humidity
Ready availability at modest cost
Reasonable solubility in the titration medium
Reasonably largemolarmass so that the relative error associated with weighing the standard is minimized.
Secondary standard
a compound whose purity has been established by chemically analysis
serves as the reference material for a titrimetric method
ESTABLISHING THE CONCENTRATION OF STANDARD SOLUTIONS
The accuracy of a titrimetric method can be no better than the accuracy of the concentration of the standard solution used in the titration.
Two basic methods are used to establish the concentration of such solutions:
Directmethod
Standardization
Direct Method
in which a carefully weighed quantity of a primary standard is dissolved in a suitable solvent and diluted to a known volume in a volumetric flask
Standardization
in which the titrant to be standardized is used to titrate
STANDARDIZATION
(1) a weighed quantity of a primarystandard
(2) a weighed quantity of a secondarystandard
(3) a measuredvolume of another standard solution
EXPRESSING THE CONCENTRATION OF STANDARD SOLUTIONS
The concentrations of standard solutions are generally expressed in units of either molarity c or normality N.
EXPRESSING THE CONCENTRATION OF STANDARD SOLUTIONS
Molarity gives the number of moles of reagent contained in one liter of solution
EXPRESSING THE CONCENTRATION OF STANDARD SOLUTIONS
Normality gives the number of equivalents of reagent in the same volume.
EXPRESSING THE CONCENTRATION OF STANDARD SOLUTIONS
M= moles/L
N= equivalents/L
SOME USEFUL ALGEBRAIC RELATIONSHIPS (VOL)
Most volumetric calculations are based on two pairs of fundamentalequations that are derived from definitions of millimole, mole, and molar concentration.
SOME USEFUL ALGEBRAIC RELATIONSHIPS(VOL)
1st Pair
Amount (mol) = mass (grams)/molar mass (gm/mole)
Amount (mmol) = mass (grams)/mmolar mass (gm/mmole)
SOME USEFUL ALGEBRAIC RELATIONSHIPS (VOL)
2nd Pair from the definition of molar concentration:
amount (mol) = volume (L) x concentration (mole/L)
amount (mmol) = volume (mL) x concentration (mmole/mL)
SOME USEFUL ALGEBRAIC RELATIONSHIPS (MASS)
Weight or Gravimetric Titrimetry – Mass of titrant is measured
Weightmolality (MW) : the number of moles of reagent in 1 Kg Solution