1. Chapter 13: Titrations in Analytical Chemistry

2. Type of Titrations based on Chemical Reactions
Acid-Base Titrations, example:
H+ + OH– → H2O K= 1/Kw
Precipitation Titrations, example:
Ag+(aq) + Cl–(aq) → AgCl(s) K=1/Ksp
Redox Titrations:
5 H2O2 + 2 MnO4– + H+ → 5 O2 + 2 Mn2+ + 8H2O
Complexometric Titrations, example:
EDTA + Ca2+ → (Ca–EDTA)2+

3. Type of Titrations based on Measuring Techniques
Volumetric titrimetry: Measuring the volume of a solution of a known concentration (e.g., mol/L) that is needed to react completely with the analyte.
Gravimetric (weight) titrimetry: Measuring the mass of a solution of a known concentration (e.g., mol/kg) that is needed to react completely with the analyte.
Coulometric titrimetry: Measuring total charge (current x time) to complete the redox reaction, then estimating analyte concentration by the moles of electron transferred.

4. Type of Titration Curves
Example.
y-axis
x-axis
Acid-base
HCl/NaOH pH
V. NaOH
Precipitation
Ag+/Cl–
pAg+
V. Ag+
Complexation
Ca2+/EDTA
pCa2+
V. EDTA
Redox
MnO4–/Fe2+
Potential
V. Fe2+
V. = volume
Type
Example
y-axis
x-axis
Spectro-photometric
apotransferrin/
Fe3+
Absorbance V.
Thermo-
metric
H3BO4/
NaOH
Temperature

5. 13A Some terms used in volumetric titrations
Standard solution: A reagent of known concentration.
Blank Titration: A titration procedure is carried out without analyte (e.g., a distilled water sample). It is used to correct titration error.
Direct titration: A titration in which the amount of analyte is determined by a standard solution directly.
Back titration: A titration in which a (known amounts) excess reagent is added to a solution to react with the analyte. The excess reagent remaining after its reaction with the analyte, is determined by a titration.
Example: To determine PO43– in a sample
3Ag+(known amounts) + PO43– → Ag3PO4(s) + Ag+(excess)
Ag+(excess) + SCN– → AgSCN(s)
amount of PO43– is the difference between added known Ag+ and the amount of Ag+ titrated by SCN–.

6. 13A-1 Equivalence Points and End Points
Equivalence point(Veq): The point (e.g., volume of titrant) in a titration where (theoretically) stoichiometrically equivalent amounts of analyte and titrant react.
Indicator: A colored compound whose change in color signals the (experimental) end point of a titration.
End point(Vep): The point (e.g., volume of titrant) in a (experimental) titration where we stop adding titrant in an experiment.
Titration error(Et): The determinate error in a titration due to the difference between the end point and the equivalence point. Et = Vep - Veq

7. Requirements of primary reagent are: Known stoichiometric composition
13A-2 Primary Standards
Primary standard: An ultrapure compound that serves as the reference material for a titration or for another type of quantitative analysis. The primary standard ready to be weighted and used to prepare a solution with known concentration (primary standard solution).
Requirements of primary reagent are:
Known stoichiometric composition
High purity.
Atmospheric stable.
Absence of hydrate water and nonhygroscopic.
Modest cost.
Reasonable solubility in the medium.
Reasonably large molar mass.
Atmospheric stability, therefore, chemically stable both in solid and solution.
Absence of hydrate water and nonhygroscopic, therefore, the composition of the solid does not change with variations in humidity.
Reasonably large molar mass, therefore, the relative error associated with weighing the standard is minimized.

8. 13B Standard solutions
The ideal standard solution for a titrimetric method will:
be sufficiently stable, therefore, we need to determine its concentration only once;
react 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 points are obtained;
undergo a selective reaction with the analyte that can be described by a balanced equation.

9. Standardization and secondary standard solution
Standardization: The process by which the concentration of a reagent is determined by titrating it against another standard solution.
Secondary standard solution: A standard solution which is standardized against a primary standard.
The conceptual flow chart:
Primary
standard
Standard-
dization
Secondary
standard
Titrating
sample
Standardization: 標定
Example:
KHP
Standard-
dization
Conc.
Of NaOH
Titration of
sample acidity

10. 13C Volumetric calculations

13. back titration

14. 13D Gravimetric titrations
(also called mass or weight titration)
Weight concentration for gravimetric titrations:
cw = no. mol A/no. kg solution
Advantages of gravimetric titrations:
Calibration of glassware and tedious cleaning to ensure proper drainage are unnecessary.
Temperature corrections for the concentrations are unnecessary.
Mass measurements are considerably greater precision and accuracy than volume measurements.
More easily automated than are volumetric titrations.

15. Types of Titration Curves
13E Titration curves
Types of Titration Curves
Figure 13-2 The two types of titration curves

16. Concentration Changes During Titrations

17. Figure 13-3 Titration curves of pH and pOH versus volume of base for the titration of M HCl with M NaOH.

18. End of Chapter 13