Download Applications of titrimetry

Titrimetry
(Volumetric Methods)
OCN 633
Fall 2013
Titrimetric Methods of Analysis
• Some of the oldest classical wet methods…
• High accuracy and precision
• Analyte reacts with solution of known
composition
• Applicable to a wide variety of analytes
• Originally manual, now automated
Titration
• The controlled addition of a measured
volume of reagent of exactly known
concentration to a known volume of a
solution of analyte whose concentration is
unknown
Requirements
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•
•
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Known reaction stoichiometry
Rapid reaction
No side reactions
Large change in some solution property at
equivalence point
• Coincidence of equivalence and end points
• Quantitative reaction
Standards in Titrimetry
• Primary Standards
- materials that can be weighed out exactly
• Secondary Standards
- standardized against primary standards
• Standard Solutions
- made up approximately, then standardized
Requirements of Primary Standard
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•
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Highest purity…
High molecular/formula weight
Stable to drying
Readily available (and cheaply)
Not deliquescent (Phenomenon of a substance
absorbing so much moisture from the air that it ultimately
dissolves in it to form a solution)
or hygroscopic
Types of Titrimetric Methods
and oceanographic applications
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Acid-base  alkalinity
Precipitation  Chloride/Chlorinity
Complexometric  Ca, Ca + Mg + Sr
Redox  Winkler/DO
General Helpful Hints
• Need to know molarity and normality
-concept of equivalence
-all reactions occur on an equivalent basis!
• Use factor label system in all equations and
calculations
• Review an “Analytical Chemistry” text if
necessary…
Normality/Equivalents
• Concentration based on idea that all substances
react on an equivalency basis
• Definition of an equivalent dependent on type
of reaction
- acid/base  produces/reacts with one mole of protons
- precipitation  …one mole of univalent ion
- redox  …one mole of electrons
- complexometric  one mole of substance forming complex
• Normality of a solution is the # eq/L
Examples of Normality
• HNO3  H+ + NO3• H2SO4 2H+ + SO42-
1M = 1N
1M = 2N
• KMnO4… must define the reaction (i.e., how many
electrons involved in the redox)
• MnO4-(aq)+ 8H+ + 5e-  Mn2+(aq) + 4H2O
1M = 5N
• MnO4-(aq) + 4H+ + 3e-  MnO2 (s) + 2H2O
1M = 3N
• AgNO3 + NaCl  AgCl + NaNO3
• 2AgNO3 + K2Cr2O7  Ag2Cr2O7 + 2KNO3
1M = 1N
1M = 2N
Equivalent Weight
• Mass in grams of one equivalent of a
substance
• Equals molecular/formula weight divided
by # eq/mole
• Equivalent weight may be less, equal to or
greater (latter is rare) than molecular/formula
weight
Volumetric Relationships
• For any reaction, at equivalence…
V1N1 = V2 N2
• This is the basis of all calculations!
• For titrations we adapt…
VstdNstd = Vunknown Nunknown
Acid-Base Titrations
• Plot pH versus
volume of titrant
(base) added
• Near equivalence pH
changes rapidly with
small additions of
base
• Equivalence point at
pH = 7
• End point should be
nearest as possible to
equivalence point…
Weak Acid-Base Titrations
• Plot pH versus volume
of titrant (base) added
• Note slope of pH
change in first half of
titration
• Have situation when
part of titration curve
involves a “buffer”
• Equivalence point at
pH > 7
Effect of Ka on Titration
• Strength of acid
impacts titration curve
• Weaker acids more
difficult (buffer effect)
• Hard to “see” end
point as most
indicators are weak
acids(bases) too…
End Points
• The “flag” that goes up to indicate that
equivalence has been reached…
• Should coincide with the equivalence point
• Usually triggered by a (relatively) large
change in a property of the solution
- change in pH of solution
- Demf or Dcurrent flow in a solution
- refractive index changes
• Usually “enhanced” by use of an indicator
Types of Indicators
• Acid/base: weak acid/base organic dyes
• Precipitation: other ion that forms insoluble
substance with titrant
• Complexometric: other substance that forms
complex with titrant (and is colored)
• Redox: other substance that undergoes
redox reaction with titrant (or use
electrometric detection)
Phenolphthalein
• Weak organic acid (C20H14O4)
• Used as an acid-base indicator.
• Colorless in acid/pinkish in base
• Transition occurs around pH 9
• Does not dissolve very well in water,
• Usually prepared in alcohol solution.
Transitions of Phenolphthalein
• Acid-base transition(s)
• Involves cleavage of ring,
dissociation of two -OH
groups, then hydroxylation
• Activates/triggers
chromophore
Phenolphthalein End Point…
• Weak acid/base
• Transition near pH 9
End point
Eriochrome Black T
3-Hydroxy-4-[(1-hydroxy-2-naphthalenyl)azo]-7nitro-1-naphthalene-sulfonic Acid, Na
• Complexes Mg2+ (pink)
• When all free Mg2+ has been
titrated, Mg2+ EBT complex
dissociates to allow
formation of stronger
Mg2+EDTA complex
• Free EBT indicator is light
blue…
Reaction of Halides with Ag+
AgNO3 + NaCl  AgCl + Na+ + NO3• Basis of Mohr method for detn. of CL in seawater
CL = [Cl-] + [Br-] + [I-]
• Use standardized solution of AgNO3 as titrant
• Dissolved halides form insoluble compounds with Ag+
• Reactions are quantitative (because of low Ksp of AgCl:
1.82 X 10-10, AgBr: 5.2X 10-13, and AgI: 8.3 X 10-17)
Mohr Titration
• Visual e.p.: K2Cr2O7/K2CrO4 indicator
…forms brown Ag2CrO4 precipitate
• Accuracy and Precision ~0.5%
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Includes Cl- (558 mM in standard seawater)
…Br- (0.86 mM in standard seawater)
…and I- (at most 0.2 µM in standard seawater).
In extreme cases, Br- and I- can rise to 2 mM each
Determination of Cl- should be corrected for Brand I- (on a practical basis the corrections will always be
less than 1%, or approximately twice the level of accuracy
achievable by this method).
Mohr Titration
• Mohr titration with AgNO3 using the is the
preferred analytical method for CL, unless
equivalent but slightly more precise Mohr
titration with electrometric e.p. is used.
• Electrometric e.p. uses Ag+ ISE
• E.p. detected on basis of Ag half reaction:
Ag+ + e- = Ago
• Electronic e.p.  ~0.1% accuracy-precision
Silver ion electrode
Ag+ + 1e-  Ago
E = Eo – (0.059/n)log{ [products] /[reactants]}
E = Eo - 0.059 log 1/[Ag+]
E = Eo + 0.059 log [Ag+]
 Upon addition of excess AgNO3, E rises sharply…
Complexometric Titrations
• Derived from inorganic chemistry
• Basis is sharing of electron pair between
metal and ligand  complex formation
• Ligand(s) is(are) electron rich species…
• Complex formation is a multi-step process
Ni(H2O)42+ + 4 Cl- <===> NiCl42- + 4H2O
Step-wise Complexation
Ni2+ + ClNiCl+ + ClNiCl2 + ClNiCl3- + Cl-
 NiCl+
 NiCl2
 NiCl3 NiCl4-2
• Each step is described by a Kf
• Overall K is product of individual K’s
• ß1= K1 ß2= K1K2 ß3= K1K2K3 ß4=K1K2K3K4
Complexometric Titrations
• Based on complexation of analyte with large
multidentate molecule rather than step wise
complexation with individual ligands
• Common functional groups that serve as
individual ligands are e- rich
• Caboxylic acids, amines, imides, thio,
sulfate…
Stability of Chelates
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Complexes of multidentate ligands are more stable
due to the chelate effect.
Rings with five- or six-fold complexation are best.
Entropy effects enhance stability
M(H2O)6 + 6 L  ML6 + 6H2O
7  7
M(H2O)6 + L  ML + 6H2O
2 7
•
Greater entropy change for the latter reaction.
Electron Pair Sharing
Carbamic or Aminoformic acid, NH2COOH
Amino Polycarboxylic Acids
• EDTA - Ethylenediamine tetraacetic acid
• Has 6 Lewis base sites (4 oxygen and 2
amine electron pairs)
• It is often designated as H4Y and its anion
as Y4• All six complexation sites are able to bind
to a single metal ion for many metals.
EDTA
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Hexadentate complex
Two amine groups
Four acetate groups
Very strong K’s with
most metal ions (esp.
high + valency ions)
• Binding constants are
f(pH)
Changes in EDTA Speciation as f(pH)
Formation Constants with EDTA
Cation
KMY
Cation
KMY
K+
Na+
Li+
Tl+
Ra2+
Ag+
Ba2+
Sr2+
Mg2+
Be2+
Ca2+
V2+
Cr2+
Mn2+
Fe2+
La+
VO2+
6.31
45.7
6.17x102
3.5x106
1.3x107
2.1x107
5.8x107
4.3x108
4.9x108
1.6x109
5.0x1010
5.0x1012
4.0x1013
6.2x1013
2.1x1014
3.2x1015
3.5x1015
Ce3+
Al3+
Co2+
Cd2+
Zn2+
Gd3+
Pb2+
Y3+
Sn2+
Pd2+
Ni2+
Cu2+
Hg2+
Th4+
Fe3+
V3+
Co3+
9.5x1015
1.3x1016
2.0x1016
2.9x1016
3.2x1016
2.3x1017
1.1x1018
1.2x1018
2.0x1018
3.2x1018
4.2x1018
6.3x1018
6.3x1021
1.6x1023
1.3x1025
7.9x1025
2.5x1041
Ca & Mg Titration with EDTA
Cd Titration with EDTA
• Plot p(M) vs volume of
EDTA (titrant)
• Use colorimetric e.p.
• Use Cd2+ ISE
• E.p. is where pCd
changes sharply
Other Amino Polycarboxylic Acids
• NTA - Nitrilotriacetic acid. Tetradentate (best
suited for small metals)
• DETPA - Diethylenetriamine pentacetic acid.
(works best for 8 coordinate metals such as the
3rd transition series and lanthanides)
• TTHA - Triethylenetetramine hexacetic acid.
(for 10 coordinate metals such as the actinides)
Complexometric Titration of Ca
(old color end point method)
• Ca2+ is determined using a modification of the
method by Tsunogai, Nishimura, and Nakaya
(1968), Talanta, 15, p385-390.
• Uses ethylene bis(oxyethylene-nitrilo)-tetra acetic
acid (EGTA) as titrant
• Uses 2,2'ethane-diylidine-dinitrilo-diphenol
(GHA) as indicator
• The Ca(GHA) complex is quantitatively extracted
into a layer of n-C4H9OH (pink color).
New Ca titration with EGTA
• Same as old method wrt basis of method
• End point measured with Ca ISE
• Large change in pCa leads to large E
change
• Precision of method vastly improved
Autotitrators you will use…