CHapter 11 Flashcards
In a titration, the stoichiometric relation between analyte and titrant
species, together with molarity of the titrant, is used to determine
the
molarity or amount of analyte. Titrimetic methods are widely used for
routine analysis because they are rapid, convenient, accurate and readily
automated.
Types of titrimetry
Volumetric, gravimetric or weight, coulometric
Volumetric
is a type of titrimetry in which the standard
reagent is measured volumetrically – Direct and back titration
Gravimetric or weight titrimetry
is a type of titrimetry in which the
mass of the standard reagent is measured
Coulometric titrimetry
the quantity of charge in coulombs required to
complete a reaction with the analyte is measured.
Standard solution
a reagent of known concentration that is used to carry out a titrimetric analysis
titration
a process in which slow addition of a standard solution from a
burette or other liquid dispensing device to a solution of the analyte until the
reaction between the two is judged complete
end point
is a point in a titration when a physical change that is associated
with the condition of chemical equivalence occurs
equivalence point
is the point in a titration when the amount of added
standard reagent is chemically equal to the amount of analyte.
indicator
molecules added to the analyte solution in order to given an
observable physical change at or near the equivalence point
Primary standard
ultrapure compound that serves as a reference material
for a titrimetric method of analysis
secondary standard
a compound whose purity has been established by
chemical analysis and serves as a reference material
Properties of Standard Solutions
- Be sufficiently stable so that it is only necessary to determine its
concentration once. - Reacts rapidly with the analyte so that the time required between
additions of reagent is minimized. - React more or less completely with the analyte so that satisfactory
end point are realized. - Undergo a selective reaction with the analyte that can be described
by a simple balanced reaction
Direct method
The steps involved in making up a solution of known concentration (here, a solution
of potassium permanganate, KMnO4 ). (a) A known mass of the compound is dispensed into a
volumetric flask. (b) Some water is added to dissolve it. (c) Finally, water is added up to the mark. The
bottom of the solution’s meniscus, the curved top surface, should be level with the mark.
Standardization
using secondary standard solution
Methods of expressing the concentration of standard solutions
Molarity, normality, analytical molarity, equilibrium or species molarity
Molarity
= amount of solute (moles)/ volume of solution(liters)=number of mmol of solute/number of mL of solution
M = n / V
The molarity of a solute in a solution is the number of moles of solution
divided by the volume of the solution in liters
Normality
The normality give the number of equivalents of reagent in 1L
of solution
ANalytical molarity
total number of moles of a solute, regardless of its chemical state in one liter of solution
Equilibrium or species molarity
is the molar concentration of a
particular species in solution. Often symbolized by placing square
brackes.
Percent concentration
Weight percent (w/w) = (mass solute/mass solution) X 100%
Volume percent (v/v) = (volume solute/volume solution) X 100%
Weight/volume percent (w/v) = (mass solute, g/volume solution, mL) X 100%
Parts per million (ppm) and parts per billion (ppb)
c ppm = (mass of solute/mass of solution) X 10^6 ppm
c ppm is concentration in parts per million.
cppb = (mass of solute/mass of solution) X 10^9 ppm
p-functions
Often the concentrations are expressed in terms of p-function or p-value. The p-value is the negative logarithm (to the base 10) of the
molar concentration of that species.
pX = -log [X]
Density
mass,kg/volunme,L
Specific gravity
is the ratio of the mass of a substance to the mass
of an equal volume of water.
Dilution
Min Vin=Mfin Vfin
When a small volume of a solution is diluted to a larger volume, the
total number of moles of solute in the solution does not change, but
the concentration of solute is reduced
Gravimetric titrimetry
-mass of titrant is measured instead of volume. A balance and a
weighable solution dispenser are substituted for a buret and its
markings
The weight molarity cw(A) of a solution of a solute A is computed by the
following equation
cw(A)=(no.molA)/(no.of kg solution)=(no.mmol A)/(no.g solution)
number of moles of a reagent in one killogram of solution is determined
Advantages of weight titrations
- Calibration and cleaning of glassware is avoided.
- Temperature corrections are not necessary
- Weight measurements can be made with great precision and accuracy
than volume measurements. - Can be easily automated
indicators for argentometric (Ag+) titrations- 3 methods
Chemical, potentiometric, amperometric
Chemical A) Chromate ion: mohr method
Ag2CrO4 (brick red) serves as an indicator
Fajans method
adsorption indicator- Fluorescein dye
Volhard method
Iron(III) ion acts an indicator in titrations involving SCN-.
FeSCN2+ (red) is the indicator species.
