Page 376 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:14 PM Page 359

Chapter 9 Titrimetric Methods of Analysis 359

Experiments the rain water. Titration curves are measured solvent on the color transition range of common indicators
In this experiment the effect of a mixed aqueous–organic
gives the volume of NaOH needed to neutralize the acidity in
is investigated. One goal of the experiment is to design an
potentiometrically and the equivalence point determined
from a Gran plot.
appropriate titrimetric method for analyzing sparingly
soluble acids and bases.
Partanen, J. I.; Kärki, M. H. “Determination of the
Thermodynamic Dissociation Constant of a Weak Acid by
Tucker, S. A.; Amszi, V. L.; Acree, Jr., W. E. “Studying Acid-
Base Equilibria in Two-Phase Solvent Media,” J. Chem. Educ.
Potentiometric Acid–Base Titration,” J. Chem. Educ. 1994,
71, A120–A122.
1993, 70, 80–82.
This experiment shows how modifying the matrix
Directions are provided in this experiment for
determining the dissociation constant for a weak acid.
Potentiometric titration data are analyzed by a modified of the solution containing the analyte can dramatically
improve the shape of the titration curve.
Gran plot. The experiment is carried out at a variety of ionic Trialkylammonium salts, such as lidocaine
strengths and the thermodynamic dissociation constant hydrochloride, are titrated in an aqueous solution
determined by extrapolating to zero ionic strength. containing a surfactant. The presence of the surfactant
increases the trialkylammonium salt’s K a , giving a
Thompson, R. Q. “Identification of Weak Acids and Bases by
titration curve with a more pronounced break. The
Titration with Primary Standards,” J. Chem. Educ. 1988, 65,
effect of adding an immiscible organic solvent, such as
179–180.
methylene chloride or toluene, also is demonstrated.
Potentiometric titration curves are used to determine the
molecular weight and K a or K b for weak acid or weak base Werner, J. A.; Werner, T. C. “Multifunctional Base
analytes. The analysis is accomplished using a nonlinear least Unknowns in the Introductory Analytical Chemistry Lab,”
squares fit to the potentiometric curve. The appropriate J. Chem. Educ. 1991, 68, 600–601.
master equation can be provided, or its derivation can be left A potentiometric titration is used to determine if an
as a challenge. unknown sample is pure Na 2 CO 3 , a mixture of Na 2 CO 3 and
NaHCO 3 , pure Na 3 PO 4 , or a mixture of Na 3 PO 4 and
Tucker, S. A.; Acree, Jr., W. E. “A Student-Designed Analytical
Na 2 HPO 4 .
Laboratory Method,” J. Chem. Educ. 1994, 71, 71–74.
Three experiments involving complexometric titrations are described in this second block of experiments.
Fulton, R.; Ross, M.; Schroeder, K. “Spectrophotometric quantitative analysis for Zn in cold lozenges using EDTA
2+
Titration of a Mixture of Calcium and Magnesium,” J. Chem. as a titrant and xylenol orange as a visual indicator.
Educ. 1986, 63, 721–723.
Smith, R. L.; Popham, R. E. “The Quantitative Resolution of
2+
In this experiment the concentrations of Ca and Mg 2+
a Mixture of Group II Metal Ions by Thermometric Titration
in aqueous solutions are determined by titrating with EDTA.
with EDTA,” J. Chem. Educ. 1983, 60, 1076–1077.
The titration is followed spectrophotometrically by
2+
2+
Binary mixtures of Ca and Mg , and ternary mixtures
measuring the absorbance of a visual indicator. The effect of
2+
2+
2+
of Ca , Mg , and Ba are determined by titrating with
changing the indicator, the pH at which the titration is
2+
carried out, and the relative concentrations of Ca and Mg 2+ EDTA. The progress of the titration is followed
2+
2+
thermometrically. Complexation of Ca and Ba with
are also investigated.
2+
EDTA is exothermic, whereas complexation of Mg with
Novick, S. G. “Complexometric Titration of Zinc,” J. Chem. EDTA is endothermic. As EDTA is added, the temperature
2+
Educ. 1997, 74, 1463. initially rises due to the complexation of Ca . The
2+
Most experiments involving EDTA as a titrant use Ca 2+ temperature then falls as Mg is titrated, rising again as Ba 2+
2+
and Mg as an analyte. This experiment describes a is titrated.
The following five experiments provide examples of redox titrimetric methods.
Guenther, W. B. “Supertitrations: High-Precision Methods,” The purity of ferrous ammonium sulfate is determined by
J. Chem. Educ. 1988, 65, 1097–1098. a redox titration with K 2 Cr 2 O 7 , using the weight of the
reagents as the signal in place of volume.
—Continued