Research: Science and Education
Chemical Speciation: A Guide to Understand Titrimetric Analysis Janitha A Liyanage* and Thamara Janaratne Department of Chemistry, University of Kelaniya, Kelaniya, Sri Lanka; *
[email protected] 7–11
>11
Species:
H3In
H2In᎑
HIn2᎑
In3᎑
blue
yellow/ orange
red
red
Determination of Magnesium Magnesium can be analyzed directly and accurately by EDTA titration using EBT (H3In) indicator (2). The titration is carried out at pH 10 in the presence of NH3/NH4Cl buffer. The end point is determined by the color change from wine red to blue. At the end point of the titration of Mg2+ by EDTA MgHIn + EDTA → Mg(EDTA) + HIn2᎑ red
colorless
HIn2 −
60 40
MgIn
20
− −
H2In
0 0.040
HIn2 −
80 60 40 20
MgIn−
0 0.044
0.048
0.052
0.040
[EDTA] / (mol dm− 3 )
0.044
MgIn 80 60 40 20
HIn2 − 0.040
0.044
In3 − 0.048
0.052
at pH 10 100
−
0
0.048
[EDTA] / (mol dm− 3 )
at pH 9 100
MgIn
−
HIn2 −
80 60 40 20
In3− 0 0.040
0.052
[EDTA] / (mol dm− 3 )
0.044
0.048
− [EDTA] / (mol dm 3 )
0.052
blue
The magnesium–indicator complex is less stable than the magnesium–EDTA complex. During the titration, EDTA first reacts with free magnesium and then with magnesium– indicator complex, changing the color of the solution from wine red to blue at the end point. Simulation of magnesium–EDTA titrations (3) using the data in Table 1 provides the results in Figure 1. These results highlight the importance of pH, yet failure to control pH is one of the most common errors in complexometric titrations. Over the pH range indicated, the complexation of magnesium
at pH 11 100
% of Each Species
Color:
80
% of Each Species
5.5–7
at pH 8 100
% of Each Species
low
at pH 7 100
% of Each Species
pH:
by EDTA is largely indeTable 1. Formation Constants for Speciation Modeling (4) pendent of pH (3), whereas the change in Reaction Log β indicator speciation is H+ + EDTA4᎑ 9.96 highly dependent on H+ + HEDTA3᎑ 6.25 pH. + H2EDTA2᎑ 2.65 H+ At pH 7 there is no + H3EDTA᎑ 2.07 H+ significant amount of + + H E D T A 2.53 H ᎑ 4 red MgIn complex2+ 4᎑ + E D T A 1 2 .4 C a ation. At other pH val2+ 4᎑ + C a + E D T A + H 1 6 .0 ues (8, 9, 10, and 11) a substantial amount of Mg2+ + EDTA4᎑ 10.6 the indicator is Mg2+ + EDTA4᎑ + H+ 15.1 complexed with Mg 2+ H+ + In3᎑ 11.5 ions and the Mg2+ ions H+ + HIn2᎑ 6.3 are removed from the in+ H2In᎑ 1 H+ dicator by EDTA at the 2+ 3᎑ 7.0 end point. At pH 7 and Mg2+ + In3᎑ 2᎑ + I n 5.4 C a 8 the blue HIn species dominates throughout the reaction and there is no color change at the end point.
% of Each Species
Titrimetric analysis is an indispensable tool in the armory of the analytical chemist, taught in schools and universities and used widely in industry. Despite this, the analytical procedure is often followed with little or no consideration to the underlying chemical principles. Although it produces the correct result in most instances, people are unable to understand and correct their mistakes if things go wrong. Chemical speciation (1), which is the oxidation state, concentration and composition of each of the species present in a chemical mixture, is important in any titrimetric analysis. Computer speciation modeling has obvious advantages in the understanding of chemical titrations. This work shows how chemical speciation can be used as a guide to understanding chemical titrations and how chemical speciation modeling can be used as a quick and easy method of monitoring changes in experimental conditions. The examples chosen were the determination of magnesium and total magnesium and calcium in a mixture by direct titration with ethylenediaminetetraacetic acid (EDTA) using Eriochrome black T (EBT) as the indicator. As shown in the box below, EBT is a weak acid, which forms several differently colored protonated species in different pH ranges. It is blue within the pH range 7–11, which is important for this titrimetric analysis. In these analyses, published thermodynamic data were used together with published equilibrium simulation programs to perform the calculations.
MgIn−
80
In3 −
60 40
HIn2 −
20 0 0.040
0.044
0.048
− [EDTA] / (mol dm 3 )
0.052
Figure 1. Speciation of indicator as a function of EDTA concentration, [Mg2+]total = 0.05 mol dm᎑3.
JChemEd.chem.wisc.edu • Vol. 79 No. 5 May 2002 • Journal of Chemical Education
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Research: Science and Education at pH 7
at pH 8 100
HIn2−
% of Each Species
% of Each Species
100 80 60 40 20
MgIn− H2In
−
0 0.090
HIn2−
80 60 40 20
MgIn−
0 0.095
0.100
0.105
0.110
0.090
[EDTA] / (mol dm−3 )
0.095
0.100
0.105
0.110
− [EDTA] / (mol dm 3 )
at pH 9
100
100
HIn2−
% of Each Species
% of Each Species
However, at pH 7 the red H2In᎑ species is also present. This makes the end point more diffuse. At pH 9 and 10 there is no significant amount of blue HIn2᎑ formation prior to the end point. This takes place only at the end point in high proportions, creating a distinct color change from red to blue at the end point. At pH 11 the blue HIn2᎑ species is formed at the end point, but yellow/orange In3᎑ is also formed, making the end point unclear. The above simulation results show why the quantitative determination of magnesium by EDTA titration using Eriochrome black T as an indicator should be done between pH 9 and 10.
80
at pH 10 MgIn− HIn2−
80
Determination of Magnesium and Calcium Mixtures The total concentration of magnesium and calcium is determined by titrating with EDTA using EBT indicator at pH 10 in the presence of NH3/NH4Cl buffer and the end point is determined by the color change from wine red to blue. During the titration EDTA reacts first with the free calcium ions, then with the free magnesium ions, and finally with the magnesium–indicator complex (MgIn᎑). Then the free indicator is released to the solution. Since the MgIn᎑ complex is red and the free indicator is blue at pH 10, the color of the solution changes from red to blue at the end point as in magnesium analysis. The results of simulations of the above titration using a solution mixture of calcium and magnesium are shown in Figure 2. Speciation of indicator as a function of EDTA concentration is similar to that in magnesium analysis and also depends on pH. Over the pH range 7–11 calcium does not complex with the indicator, but only magnesium–indicator complex is formed. This complex and the monoprotonated free indicator species are responsible for the color change at the end point. At pH 7 and 8, substantial metal–indicator complexation does not take place. A significant amount of monoprotonated blue indicator species, HIn᎑, is present throughout. Hence the color change at the end point at both pH values is very diffuse. At pH 11 no significant amounts of blue indicator species are formed at or after the end point. Hence the color change is not clear. These simulation results also show why the quantitative determination of magnesium and calcium by EDTA using EBT as an indicator should be done between pH 9 and 10. They also clearly show that by using EBT as the indicator, the concentration of calcium together with magnesium can be determined at the end point.
Figure 2. Speciation of indicator as a function of EDTA concentration, [Ca2+]total = [Mg2+]total = 0.05 mol dm᎑3.
Conclusions According to the results obtained using the MINTEQA2 (5) model for the quantification by EDTA of magnesium alone and combined with calcium, if the Eriochrome black T is used as the indicator for the end point determination, pH 9 to 10 is the most suitable range for the titration and results will agree with the experimental conditions very well. Hence it is clear that this sort of model can easily be used for teaching purposes. Titrimetric analysis is a very useful analytical method in all disciplines of science. By using computer simulation programs such as MINTEQA2, it is relatively easy to calculate the chemical speciation at each step in a titration procedure and this enables a better understanding of the particular procedure. It also allows
1. Duffield, J. R.; Williams, D. R. Chem. Br. 1989, 375. 2. Vogel, A. I. Vogel’s Textbook of Quantitative Chemical Analysis, 5th ed.; revised by Jeffery, G. H., et al.; Longman: Harlow, UK, 1989. 3. Janaratne, T.; Liyanage, J. A. Proc. Sri Lanka Assoc. Adv. Sci. 1998, 56 (1), 279. 4. Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum: New York, 1989. 5. Allison, J. D.; Brown, D. S.; Novo-Gradac, K. J. MINTEQA2/ Prodefa2, a Geochemical Assessment Model for Environmental Systems; Scientific Software Group, U.S. Environmental Protection Agency: Washington, DC, 1993; these programs and the manual are government documents, available for downloading from the U.S. EPA through the EPA Web site or http://www.epa.gov/ ceampubl/minteq.htm. (accessed Mar 2002)
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60
40
20
MgIn
0 0.090
−
60
40
In3−
20
0 0.095
0.100
0.105
0.110
0.090
0.095
−3
0.100
0.105
0.110
− [EDTA] / (mol dm 3 )
[EDTA] / (mol dm )
at pH 11 % of Each Species
100
MgIn−
80
In 3 −
60 40
HIn 2 −
20 0 0.090
0.095
0.100
0.105
0.110
[EDTA] / (mol dm−3 )
checking whether that procedure works or does not work under the defined experimental conditions, allowing determination of the most appropriate experimental conditions for a given titration procedure. Speciation analysis also enables chemists to check whether an adequate buffering capacity is present in the medium to obtain the expected end point. Thus speciation could be employed to study the limits and validity of analytical methods. Acknowledgment The support from university of Kelaniya (grant No RP/ RG/98/01/03) is gratefully acknowledged. Literature Cited
Journal of Chemical Education • Vol. 79 No. 5 May 2002 • JChemEd.chem.wisc.edu
