titrimetry (volumetric methods) - school of ocean and …€¢ redox winkler/do general helpful...
TRANSCRIPT
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
• 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
• 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
• 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- 1M = 1N
• H2SO4 2H+ + SO42- 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 1M = 1N
• 2AgNO3 + K2Cr2O7 Ag2Cr2O7 + 2KNO3 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
Eriochrome Black T
• 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…
3-Hydroxy-4-[(1-hydroxy-2-naphthalenyl)azo]-7-
nitro-1-naphthalene-sulfonic Acid, Na
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%
• 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 Br- and 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- <===> NiCl4
2- + 4H2O
Step-wise Complexation
Ni2+ + Cl- NiCl+
NiCl+ + Cl- NiCl2
NiCl2 + Cl- NiCl3-
NiCl3- + Cl-
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
• 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.
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
• 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)
Formation Constants with EDTA
Cation KMY Cation KMY
K+ 6.31 Ce3+ 9.5x1015
Na+ 45.7 Al3+ 1.3x1016
Li+ 6.17x102 Co2+ 2.0x1016
Tl+ 3.5x106 Cd2+ 2.9x1016
Ra2+ 1.3x107 Zn2+ 3.2x1016
Ag+ 2.1x107 Gd3+ 2.3x1017
Ba2+ 5.8x107 Pb2+ 1.1x1018
Sr2+ 4.3x108 Y3+ 1.2x1018
Mg2+ 4.9x108 Sn2+ 2.0x1018
Be2+ 1.6x109 Pd2+ 3.2x1018
Ca2+ 5.0x1010 Ni2+ 4.2x1018
V2+ 5.0x1012 Cu2+ 6.3x1018
Cr2+ 4.0x1013 Hg2+ 6.3x1021
Mn2+ 6.2x1013 Th4+ 1.6x1023
Fe2+ 2.1x1014 Fe3+ 1.3x1025
La+ 3.2x1015 V3+ 7.9x1025
VO2+ 3.5x1015 Co3+ 2.5x1041
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