• Analytical Standards
Introduction
• Standards are materials containing a known
concentration of an analyte. They provide a
reference to determine unknown concentrations or
to calibrate analytical instruments.
• The accuracy of an analytical measurement is how
close a result comes to the true value. Determining
the accuracy of a measurement usually requires
calibration of the analytical method with a known
standard. This is often done with standards of
several concentrations to make a calibration or
working curve
Working Curve or Calibration Curve
• A working curve is a plot of the analytical
signal (the instrument or detector response)
as a function of analyte concentration. These
working curves are obtained by measuring
the signal from a series of standards of
known concentration. The working curves
are then used to determine the concentration
of an unknown sample, or to calibrate the
linearity of an analytical instrument.
Primary standard
A primary standard in chemistry is a reliable, readily quantified substance.
Features of a primary standard include:
1. High purity
2. Stability (low reactivity)
3. Low hygroscopicity and efflorescence
4. High solubility (if used in titration)
5. modest cost
6. High equivalent weight
• Some examples of primary standards
according to the
European Pharmacopoeia
Benzoic acid for standardisation of waterless basic solutions
Potassium bromate (KBrO3) for standardisation of sodium thiosulfate solutions
Potassium hydrogen phthalate (usually called KHP) for standardisation of and perchloric
acid in acetic acid solutions
Arsenic trioxide for making sodium arsenite solution for standardisation of sodium
periodate solution
Sodium chloride for standardisation of silver nitrate solutions
Sulfanilic acid for standardisation of sodium nitrite solutions
Zinc powder, after being dissolved in sulfuric or hydrochloric acid, for standardisation of
EDTA solutions
•Sodium carbonate for standardisation of aqueous acids: hydrochloric, sulfuric
acid and nitric acid solutions (but not acetic acid)
Primary Standards
• A primary standard is a reagent
that is extremely pure, stable, has
no waters of hydration, and has a
high molecular weight
Some primary standards for
titration of acids:
• sodium carbonate: Na2CO3, mol wt. =
105.99 g/mol
• tris-(hydroxymethyl)aminomethane
(TRIS or THAM): (CH2OH)3CNH2, mol
wt. = 121.14 g/mol
Some primary standards for titration
of bases:
• potassium hydrogen phthalate (KHP):
KHC8H4O4, mol wt. = 204.23 g/mol
• potassium hydrogen iodate: KH(IO3)2,
mol wt. = 389.92 g/mol
Some primary standards for
redox titrations:
• potassium dichromate: K2Cr2O7,
mol wt. = 294.19 g/mol
Secondary Standards
• A secondary standard is a standard that
is prepared in the laboratory for a
specific analysis. It is usually
standardized against a primary standard
Precipitation titration
• Titrations with precipitating agents are
useful for determining certain analytes
e.g. Cl- can be determined when titrated
with AgNO3.
Detection of end point:
• Chemical
–Precipitation Type - Mohr’s method
–Adsorption – Fajan’s method
–For silver analyses –Volhard method
• Sensors –Potentiometric or
amperometric
• The chemical types are also
classified into:
• Indicators reacting with titrant
forming specific color.
• Adsorption indicators.
• Indicators reacting with the titrant:
• Two methods will be discussed where
this type of indicators are applied;
namely: Mohr and Volhard.
I) Mohr method for determining
chloride:
• Chloride is titrated with AgNO3 solution. A
soluble chromate salt is added as the indicator.
This produces a yellow color solution. When the
precipitation of the chloride is complete, the first
excess of Ag+ reacts with the indicator to
precipitate red silver chromate:
2 Ag+(aq) + CrO42–(aq) → Ag2CrO4(s)
Yellow
red ppt
• The Mohr method must be performed at
a pH about 8. This method is useful for
determining Cl- in neutral or unbuffered
solutions such as drinking water.
II) Volhard titration:
• This is an indirect titration procedure for the
determination of anions that precipitate with
silver like Cl-, Br-, I-, SCN-, and it is preferred
in acid (HNO3) solution. A measured excess
of AgNO3 is added to ppt the anion, and the
excess of Ag+ is determined by back titration
with standard potassium thiocyanate
solution:
Ag+(aq) + Cl–(aq) → AgCl(s) + excess Ag+
excess Ag+(aq) + SCN–(aq) → AgSCN(s)
The end point is detected by adding iron III (Fe3+) as
ferric ammonium sulfate which forms a soluble red
complex with the first excess of titrant.
Fe3+(aq) + SCN–(aq) → [FeSCN]2+(aq)
These indicators must not form a compound
with the titrant that is more stable than the
precipitate or the color reaction would occur
on addition of the first drop of titrant.
Adsorption indicators:
The indicator reaction takes place on the surface of the precipitate. The
indicator, which is a dye, exists in solution as the ionized form, usually an
anion.
Principle of adsorption:
Consider the titration of Cl- with Ag+. Before the
equivalent point, Cl- is in excess and the primary layer
is Cl-. This repulses the indicator anions; and the more
loosely held the secondary (counter) layer of adsorbed
ions is cations, such as
AgCl
:
Cl-
Beyond the equivalent point (end point as well), Ag+ is
in excess and the surface of the precipitate becomes
positively charged, with the 1 layer being Ag+. This
will now attract the indicator anion and adsorb it in the
2 (counter) layer:
AgCl : Ag+ : : indicator-
The color of the adsorbed indicator is different from
that of the un-adsorbed indicator, and this difference
signals the completion of the titration. The degree of
adsorption of the indicator can be decreased by
increasing the acidity.
• The titration of chloride using this kind
of indicator is called Fajan’s Method.
Fajan’s method is the most recent and most
accurate silverhalide method. It is based on the
adsorption of dichlorofluorescein (DCF) on the
surface of the positively charged silver chloride
particles formed in the precipitation titration when
Ag+ ion is in excess.
Precipitation Titration Curve
• Example – AgI(s) formation
–Ag+ + I-
AgI(s)
• Reverse of the Ksp reaction for the dissociation
of AgI(s) so the K for this reaction equal
1/Ksp[AgI(s)] = 1.2 X 1016
– Large K value means that the equilibrium lies to the
right for this reaction
– Each aliquot addition of Ag+ titrant reacts virtually
entirely to form AgI(s)
Precipitation Titration Curve
• Titration Curves typically exhibit 3
distinct regions for a single titrant
reacting with a single analyte.
–Before the Equivalence Point
–At the Equivalence Point
–After the Equivalence Point
Precipitation Titration Curve
• Before the Equivalence Point
–Example of AgI(s) formation
• Most of the [Ag+] reacts entirely to give AgI
• p[Ag+] =
– the amount of Ag+ left in solution
» The amount of I- present
» From Ksp determine the amount of Ag+ present
Precipitation Titration Curve
• At the equivalence point
–Stoichiometric amount of Ag+ as Iso all has precipitated out as
AgI(s)
• Regular Ksp calculations
Precipitation Titration Curve
• After Equivalence Point
–[Ag+] is determined by the Ag+
present after the equivalence
point with the dilution factor taken
into account
Precipitation Titration Curve
Example
• 25.00 mL of 0.1000 M I- was titrated with 0.05000M
Ag+.
Ag+ + I-
AgI(s)
The solubility product for AgI is 8.3 x 10-17. Calculate
the concentration of Ag+ ion in solution
(a) after addition of 10.00 mL of Ag+;
(b) after addition of 52.00 mL of Ag+;
(c) at the equivalence point.
Precipitation Titration Curve Example
• 25.00 mL of 0.04132 M Hg2(NO3)2 was titrated with
0.05789 M KIO3.
Hg22+ + 2IO3-
Hg2(IO3)2(s)
The solubility product for Hg2(IO3)2 is 1.3 x 10-18.
Calculate the concentration of Hg22+ ion in solution
(a) after addition of 34.00 mL of KIO3;
(b) after addition of 36.00 mL of KIO3;
(c) at the equivalence point.
Precipitation Titration Curve
Example
• Consider the titration of 50.00 mL of 0.0246 M
Hg(NO3)2 with 0.104 KSCN. Calculate the value of
pHg22+ at each of the following points and sketch the
titration curve:
0.25VT
0.5VT
0.75VT
1.05VT
1.25VT
Titration Curve Shape
• 1:1 Stoichiometry of Reagents
– Equivalence point is the steepest point of a curve
• Maximum slope
• An inflection point
• Other Stoichiometries
– The curve is not symmetric about the equivalence point
– The equivalence point is not at the center of the steepest
section of the curve
• The less soluble the product, the sharper the curve around the
equivalence point
Titration Curve Shape
Titration of a Mixture
• The less soluble product forms first
• If there is sufficient difference in solubility of products
– First precipitation is nearly complete before the
second one begins
• Separation by precipitation (section 6-5)
– Coprecipitation
• Alters the expected endpoints
Titration of a Mixture Example