Computers potentiometric titrator

The text listed below provides more details on how the potentiometric titration data may be used to calculate equilibrium constants. This text provides a number of examples and includes a discussion of several computer programs that have been developed to model equilibrium reactions. 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

Chloride can also be estimated by potentiometric titration using standard silver nitrate [27]. The results are recorded directly and evaluated by means of a computer program based on the Gran extrapolation method. The determinations have a precision of 0.02% and since many samples can be titrated simultaneously, the time for a single determination can be reduced to less than 5 min. 

There are two types of fluoride ion-selective electrodes available [27] Orion model 96-09-00, a combination fluoride electrode, and model 94-09-00, which requires a reference electrode The author prefers to use Orton model 94-09-00 because it has a longer operational life and is less expensive When an electrode fails, the reference electrode is usually less expensive to replace The Fisher Accumet pH meter, model 825 MP, automatically computes and corrects the electrode slope It gives a direct reading for pH, electrode potential, and concentra tion in parts per million The fluoride ion-specific electrode can be used for direct measurement [28, 29] or for potentiometric titration with Th4+ or La3+ nitrate solutions, with the electrode as an end point indicator... 

James, R. 0., Davis, J. A., and Leckie, J. 0., Computer simulation of the conductometric and potentiometric titrations of the surface groups on ionizable latexes. J. Coll. Interface Sci. 65, 331-344 (1978). 

An approximate numerical value for the dissociation constant of a weak acid or base can be estimated from potentiometric titration curves. This quantity can be computed from the pH at any point along the curve, but a very convenient point is the half-titration point. At this point on the curve... 

Motekaitis and Martell Model Contrary to the Ohman model, Motekaitis and Martell (1984) found no need to invoke polynuclear Al-citrate species to describe their potentiometric titration data. Their minimalist model, consisting of Al(Hcit)+(aqr), Alcif ( 7). and Al(H icit) (aqr) (Table 10.4), was generated from titrations of Al-citrate solutions in 0.10 mol KCl, using cUts/AIts ratios of 1 1, 2 1, and 3 1 from pH 2 to the onset of Al precipitation (approximately pH 7.5). They employed 4-hour equilibration times and included in their computations the AP+(aqr) hydrolysis for all four monomers [A10H2 (a ) to Al(OH)4 (a )], as well as Al2(OH)2 +( q) and AlTOH)4°+(6 g) [with log values obtained from the compilations of Smith and Martell... 

The acid-base equilibrium constants of the beta-blockers atenolol, oxprenolol, timolol, and labetalol were determined by automated potentiometric titrations. The pKg values were obtained in water-rich or water methanol medium (20% MeOH) to obviate the solubility problems associated with the compounds. The initial estimates of pKa values were obtained from Gran s method and then, were refined by the NYTIT and ZETA versions of the LETAGROP computer program. The resultant values were 9.4 (/ = 0.1 M KCI, 20% methanol) for atenolol, 9.6 (/ = 0.1 M KCI) for oxprenolol, 9.4 (/ = 0.1 M KCI, 20% methanol) for timolol and 7.4 and 9.4 (/ = 0.1 M KCI) for labetalol. The potentiometric method was found to be accurate and easily applicable. The operational criteria for applying the methodology are indicated. 

Potentiometric titrations can be easily automated and controlled by microprocessors or computers. The extent of automation is such that, in the same vessel, multiple titrants may be used sequentially, providing for the determination of several analytes in one sample. Sample changers carrying up to and beyond 60 samples can be attached with ease, vastly decreasing the technician time to produce multiple sample results. 

This section will cover the location of the equivalence point or endpoint by modern microprocessor- or computer-controUed potentiometric titrators. 

Binding or association constants in principle can be obtained by any spectroscopic method, which permits the analysis of solutions, i.e., H NMR (Chapter 9), UV-Vis and fluorescence spectra (Section 10.3), and CD, and also by microcalorimetric methods (Chapter 8), potentiometric titrations (Section 10.5), etc. [5,10]. Information about the supramolecular structure in solution, on the other hand, only can be gained by NMR (NOESY or ROESY) or CD spectroscopy - preferably in combination with computational methods [11-17] (see, however, Chapter 11, which details the limitations of CyD modeling). 

The large cavity of cryptand 17 allows another approach to anion coordination, the "cascade anion binding. " In a first step, the ligand binds by its tren subunits two Cu(II) cations. In a secorui step, the axial vacant hollow between the two alrecufy bound Cu(IT) cations will allow the binding of cn anion. An early example was based on computer analysis of potentiometric titrations data, and the efficientbinding of the chloride anion by the dicopper complex of 1 was postulated. Many other bis-tren-type figands were studied, and the cascade binding of anions such as Br, OH, N3, WCO. etc. was demonstrated. Many x-ray structures confirm the location of the anions between the various meted cations. ... 

The following entry defines the commonly used stability constants (stepwise, overall, conditional, association, dissociation, and pK) and relates the values to a rigorous thermodynamic definition of equilibrium constants. In addition, the article briefly outlines experimental techniques (potentiometric titration, spectroscopic methods involving ultraviolet/visible, infrared, Raman, fluorescence. and nuclear magnetic resonance spectroscopy), together with the numerical methods and computer programs that can be used to derive stability constants from such experimental data. 

A large number of amino-methylsubstituents were introduced by the Mannich reaction which works as smoothly with the lesorcinarene 1 as it had been reported for calixaienes [25]. From the eight aminoderivatives thus prepared and investigated [22] we restrict ourselves here to the L-proline compound 4 which was obtained without any racemization. The system can adopt four different protonation states (Scheme 11) which were all characterized. Computer-aided potentiometric titrations yielded the relevant pK values with an average error of +0.2 units, with excellent fits assuming the same value for the different independently treated phenyl units in the macrocycle. H-NMR titrations allowed the different steps as shown in Scheme 12 to be identified. Due to the betaine formation the pK of the... 

The energy of the N B interaction has been calculated from the stepwise formation constants of potentiometric titrations. Based on the relative stabilities of ternary phosphate complexes, it was calculated that the upper and lower limits of the N-B interaction must be bound between 15 and 25 kJ mol in A-methyl-o-(phenylboronic acid)-A-benzylamine. This value is in good agreement with computational data, which estimated the N-B interaction to be 13 kJ mol or less in the absence of solvent. To qualify this in terms of familiar... 

While speculative, this structural interpretation of the interaction between boronic acid and the proximal tertiary amine through a bound protic solvent molecule (solvent insertion into the N-B bond) corresponds well with contemporary computational and potentiometric titration data, in which the formation of intramolecular seven-membered rings should not be ignored. 

Anfalt and Jagner [57] measured total fluoride ion concentration by means of a single-crystal fluoride selective electrode (Orion, model 94-09). Samples of seawater were adjusted to pH 6.6 with hydrochloric acid and were titrated with 0.01 M sodium fluoride with use of the semi-automatic titrator described by Jagner [28]. Equations for the graphical or computer treatment of the results are given. Calibration of the electrode for single-point potentiometric measurements at different seawater salinities is discussed. 

For other cases, such as La3+ where more detail is required about the nature of the species present in solution, titration data can be computer fit to more complicated multi-equilibrium models containing Mx 1 v( OR)v forms whose stoichiometry is suggested by information gained from independent spectroscopic or kinetic techniques. One must be mindful of the pitfalls of simply fitting the potentiometric data to complex multi-component models for which there is no independent evidence for the various species. Without some evidence for the species put into the fit, the procedure simply becomes an uncritical mathematical exercise of adding and removing various real and proposed components until the goodness of fit is satisfactory. 

Polyprotic acids are fairly important and their potentiometric pH titrations are common. For 2-component systems of this kind, it is possible to turn around the computations and come up with explicit, non-iterative solutions. So far we have computed the species concentrations knowing the total component concentrations, which is an iterative process. This is the normal arrangement in titrations where volumes and total concentrations are known and the rest is computed, e.g. the [H+] and thus the pH. Turning around things in this context means that one calculates the titration volume required to reach a given (measured) pH. One knows the i/-value and computes the corresponding x-value. In this way there are explicit equations that can directly be implemented in Excel. 

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