A new Way to use Conductivity
Measurement in Analytical Chemistry
- Optimising pH to optimise reaction rates -
The pH-value of the solution is an essential parameter of your reaction rate and the right pH-value is vital for a high-yield. A pH-spectrum helps you to find out the right pH-value for your reaction. So far you have been able to record IR-spectra, NMR-spectra, UV-spectra and so on, now with theáTheoprax-Software you can record pH-spectra to study the interaction between different molecules in dependence on the degree of dissociation of their functional groups. This becomes possible with the Theoprax-Method, a new way to use conductivity measurement in analytical chemistry. The Theoprax-Method allows to characterize the interaction between molecules. Examples of the interaction between central ion and complexing agent, enzyme and inhibitor as well as antigen and antibody are shown.
The Theoprax-Method means a new method of calculation with quantities being measured. The word Theoprax means the calculation of the different between the whole reference value of the sample (ideally behaviour), which is Theoretically calculated from the single reference value and in the Praxis measured quantity data of the sample (really behaviour). The Theoprax-Software, based on the Theoprax-Method, makes it possible to calculate with conductivity measurements pH-spectra which shows the interaction between different molecules as a function of the degree of dissociation of their functional groups. In complex chemistry, the pH-spectrum makes it possible to make a statement about the concentration of the complex, complexing agent and central ion. Furthermore, it shows the preference of the complexing agent/central ion for an metal ion/complexing agent in the presence of other metal ions/complexing agents. Also you can make a statement about the stoichiometry of the complex in solution. It is no more necessary to isolate it. The pH-spectrum shows the optimum pH-value for complex formation. In pharmaceutical chemistry and biochemistry for example you can use the Theoprax-Method as a diagnostic tool to speed up your separation technique (affinity chromatography). The pH-spectrum makes it possible to show the enzyme-inhibitor-complex, enzyme-substrate-complex, the catabolism of the substrate as well as the pH-activity-curve of the enzyme. It is also possible to show the interaction between antigen and antibody.
2. Fundamental principles
The theoretical principles of the Theoprax-Conductivity-Measuring System are based on the following assumptions:
1. The solutions are ideally diluted, the concentration of the substances used are in the range of mmol/L.
2. The relaxation effect and the electrophoretic effect can be neglected.
3. The equivalent conductivities at infinite dilution of the ions are additive.
For example, it is possible to find out if there is an interaction between the complexing agent and the metal ion. If there is an interaction, the Kohlrausch Law of independent migration of ions will lose its validity. The measured specific conductivity k (kappa), the so-called sample-value, has a deviation of Dk (delta kappa) from the theoretically calculated value, which is calculated from the specific conductivities of the single components. The specific conductivities of the single components are designated as the single reference value, the value which is calculated from those single reference values is called the total reference value.
The degree of the deviation Dk (delta kappa) is a measure for the interaction between the ionic molecules. To show Coulomb interaction, the tests are carried out in an aqueous solution without the addition of a foreign electrolyte and to prove van der Waals interactions in an aqueous solution with the addition of a foreign electrolyte. If there are Coulomb interactions, the deviation Dk (delta kappa) changes depending on the pH-value of the solution. The pH-spectrum of the Theoprax-curve will then show peaks. If there are van der Waals interactions, the deviation Dk (delta kappa) doesn't change with the pH-value of the solution. The pH-spectrum of the Theoprax-curve will be linear .
3. Results and discussion
3.1. Complex chemistry
3.1.1. Interpretation of the pH-spectrum of the complex between, Disodium Malonate and copper ions, figure 1.
For the recording of the pH-spectrum of a Malonate-copper-complex 1:1, System (Cu2+M), eight measurements are necessary. The quantity of 1 mmol Disodium Malonate (CH2(COONa)2) and a quantity of 1 mmol CuCl2 x 2 H2O have been used.
- System Cu2 - (0.25 mmol CuCl2 x 2 H2O / 100 ml dist. water), figure 1
In comparison with System H2O, the System Cu2 has a higher consumption of OH--ions, therefore the curve of the System Cu2 has a strong decrease at a pH-value above 6.2. The forming of Cu(OH)2 can be shown by the consumption of OH-- ions in the pH-area of 6.2 to 11.6, a Dk value of - 1109 ÁS at pH 11.6 shows the consumption of 0.5 mmol of OH--ions, the solution is turbid (blue flocs).
- System M - (0.25 mmol CH2(COONa)2 / 100 ml dist. water), figure 1
In comparison with System H2O, the System M has a higher consumption of H+-ions, therefore the curve of the System M has a strong decrease at a pH-value below 6.6. The forming of CH2(COOH)2 can be shown by the consumption of H+- ions in the pH-area of 6.6 to 2.0, a Dk value of - 1080 ÁS at pH 3.6, the point of intersection of tangents, shows the consumption of 0.25 mmol of H+-ions (exact 0.27 mmol) and at pH 2.0 with a Dk-value of - 1613 ÁS the consumption of 0.41 mmol of H+-ions.
Fig. 1 pH-spectrum of the 1:1 Malonate-copper-complex (2.5 mmol/L) System (Cu2+M), pH 2.0 - 12.0, error range ▒ 50 ÁS/cm
- System (Cu2+M) - (0.25 mmol CuCl2 / 0.25 mmol CH2(COONa)2 / 100 ml H2O), fig. 1
Before titration the pH of the solution drift from pH 7.5 of pure Disodium Malonate down to 6.2 by the addition of 0.25 mmol CuCl2 x 2 H2O. The solution is clear and blue, the value for Dk is - 434 ÁS/cm, which indicates that there is an interaction between Disodium Malonate and copper ions before the beginning of the titration. On the basis of Coulomb interactions, there is a Malonate-copper-complex in the pH area of 2.0 to 12.0, the maximum is at pH 5.7 where Dk is - 435 ÁS/cm.
In the direction of lower pH-values, Dk becomes less and at pH 2.7 Dk is + 190 ÁS/cm. The variance from the basis line is 625 ÁS/cm, what means that 1.5 mmol H+-ions are free in compare to System M so that only 1 mmol of H+-ions have been consumed, explainable by a Malonate-copper-complex [0.5 mmol of (HOOC-CH2-COO-Cu-OOC-CH2-COO-Cu-OOC-CH2-COO-Cu+) and 0.5 mmol of (HOOC-CH2-COO-Cu-OOC-CH2-COO-Cu+) complex] at pH 2.7.
In the direction of higher pH the value of Dk becomes - 113 ÁS/cm at pH 9.5, the solution is clear. The variance for Dk is 320 ÁS/cm what means that 0.15 mmol of OH--ions are free in compare to System Cu2 and only 0.1 mmol of OH--ions have been consumed. The interaction between the copper ions and Disodium Malonate is explainable by a Malonate-copper-complex [0.5 mmol of (-OOC-CH2-COO-Cu-OOC-CH2-COO-Cu-OOC-CH2-COO-Cu-OH) and 0.5 mmol of (-OOC-CH2-COO-Cu-OOC-CH2-COO-Cu-OH) complex]. At pH 11.4 the value for Dk is 125 ÁS/cm so that the variance from the basis line is 560 ÁS/cm what means that 0.25 mmol of OH--ions are free in compare to System Cu2 and only 0.25 mmol of OH--ions have been consumed, explainable by a Malonate-Cu-OH complex at pH 11.4. At pH 12.0 the value for Dk is 39 ÁS/cm which indicates that there are no more interactions between Disodium Malonate and the copper ions, as if the ions would be in two different beakers, blue flocs (Cu(OH)2) falls out.
 G. Mermigidis (1995) Das Theoprax-Verfahren, GIT Fachz. Lab. 10: 959-963
G. Mermigidis (1995)
Das Theoprax-Verfahren, GIT Fachz. Lab. 10: 959-63
G. Mermigidis (1998)
Anwendungsgebiete des Theoprax-Me▀verfahrens, Analytica Conference 98, Abstracts p. 607
G. Mermigidis (1999)
Das Theoprax-Verfahren: Die Renaissance der Leitfńhigkeitsmessung, GIT Fachz. Lab. 1: 36-37
G. Mermigidis (1999)
The Theoprax-Method: A new Way to use Conductivity Measurement in Analytical Chemistry, GIT Lab. J. 1: 33-36
G. Mermigidis (2000)
Achema 2000, International Meeting on chemical engineering, environmental protection and biotechnology, abstracts of the lecture groups: Laboratory and analysis accredition, certification and QM, page 76 -80
Dr. G. Mermigidis
Agiou Dimitriou 74