Primary and Secondary standards.pdf

Primary and Secondary Standards

1. Fundamentals of Volumetric Analysis
Primary and Secondary Standards
By
Dr. Jasmine Chaudhary
Associate Professor
MMCP
[email protected]

2. Based on determining the quantity of a reagent/titrant of known concentration
that is required to react completely with the analyte.
Titration
Slow addition of titrant (standard solution) to analyte solution from a volumetric
vessel (burrette) until a required reaction is completed. The point at which the
reaction is completed is called END POINT.
EndPoint
Occurrenceofanobservablephysicalchangelikechangeincolorindicatingthat
theequivalencepointisreached.

3. EquivalencePoint
•Whenamountofaddedtitrantischemicallyequivalenttoamountof
analytepresentinthesample.
•Theequivalencepointofatitrationcannotbedeterminedexperimentally.
•Itcanonlybeestimatedbyobservingsomephysicalchangeassociated
withtheconditionofchemicalequivalencecalledtheendpointforthe
titration.

4.  Reaction must be simple and stoichiometric
 Reaction should be rapid
 No side reactions
 Marked change in some property of the solution when reaction is complete
(Equivalence point )
 Reaction should be quantitative.

5. Advantages
Need a very simple apparatus.
No special training is required.
Inexpensive.
Equipments doesn’t require constant recalibration unlike instruments.
Disadvantages
Less sensitive and less selective.
When large no. of determinations are required, the instrumental methods are
more quicker and cheaper than more laborious less sensitive titrimetric
methods.

6. Solutions of known concentration are known as standard solutions.
The ideal standard solution for a titrimetric method should:
be sufficiently stable so that it is necessary to determine its
concentration only once
react rapidly with the analyte so that the time required between
additions of reagent is minimized.
undergo a selective reaction with the analyte that can be described by a
balanced equation.

7. Classification
Primary standard solutions
Secondary standard solutions
Primary Standards
Highlypurifiedcompoundsusedasareferencematerialin
titrimetryand
Solutionspreparedbyweighingapureprimarystandardfollowed
bydilutioninavolumetricflaskareknownasprimarystandard
solutions

8. Properties
High purity (Assumed to be 100% pure although 0.01-0.02% impurity
is tolerable if it is accurately known)
Easy to dry, obtain, purify and preserve in pure state.
Should be unaltered during weighing which indicated it should not be
hygroscopic, oxidised by air.
Readily available and inexpensive.
Reasonable solubility.
Large formula weight so that the relative error associated with
weighing the standard is minimized..
Capable of tested for impurities by qualitative and other tests of
known sensitivity as total impurity should not be more than 0.02%.

9. Examples of Primary Standard
Acid base titrations: Sodium carbonate, Oxalic acid, Benzoic acid
Oxidation reduction titrations: Potassium dichromate, potassium
iodate, oxalic acid, Arsenious Oxide
Precipitation titrations: Silver nitrate
Complexometric titrations: Zinc chloride, Calcium chloride

10. Asecondarystandardrequiresatitrationtocalibrateitsconcentration.It
wasmadeusinganacidorbasewithsuspectpurity.
Donotmeetrequirementsforaprimarystandardbutareavailablewith
sufficientpurityandpropertiestobegenerallyacceptablelike
Stable and Reacts rapidly and completely with analyte.
Reacts selectively with analyte.
Asecondarystandardsolutionisasolutioninwhichconcentrationof
dissolvedsolutehasnotbeendeterminedbyweightofcompound
dissolvedbutbytitrationofvolumeofsolutionagainstmeasured
volumeofprimarystandardsolution.(STANDARDIZATION )

11. Examples
Alkali hydroxides like NaOH (hygroscopic)
Deliquescent substances
Inorganic acids like HCl, H
2SO
4, HNO
3
KMnO
4, Na
2S
2O
3

12. Standardization–involvesestablishingtheconcentrationofa
“standardsolution”
Directmethod(Primarystandard):
Dissolvecarefullyweighedquantityofprimarystandard;diluteto
knownvolume
Indirectmethods(SecondaryMethod):
Titrateweighedquantityofsecondarystandardsolutionbyprimary
standardsolutionandthenobservetheconcentration.