Acid/base potentiometry

Acid/hase potentiometry enables the surface charge density to be measured. This involves comparison of the titration curves obtained for the suspension of oxide at several different ionic strengths (10 10" M) with that of the electrolyte alone, followed by calculation of the net consumption of protons or hydroxyl ions (mol g ) at each pH. The data is presented as a plot of excess of acid or base (Fh - Toh ) mol g or mol m ) vs pH (adsorption isotherm) or as a plot of surface charge, cr, (coulombs m ) vs pH (charging curve) (Figure 10.5). 

In a related application, polyelectrolyte microgels based on crosslinked cationic poly(allyl amine) and anionic polyfmethacrylic acid-co-epoxypropyl methacrylate) were studied by potentiometry, conductometry and turbidimetry [349]. In their neutralized (salt) form, the microgels fully complexed with linear polyelectrolytes (poly(acrylic acid), poly(acrylic acid-co-acrylamide), and polystyrene sulfonate)) as if the gels were themselves linear. However, if an acid/base reaction occurs between the linear polymers and the gels, it appears that only the surfaces of the gels form complexes. Previous work has addressed the fundamental characteristics of these complexes [350, 351] and has shown preferential complexation of cationic polyelectrolytes with crosslinked car-boxymethyl cellulose versus linear CMC [350], The departure from the 1 1 stoichiometry with the non-neutralized microgels may be due to the collapsed nature of these networks which prevents penetration of water soluble polyelectrolyte. 

Potentiometry is often used in determining dissociation constants, pKa, and homoconjugation constants, Kf(HA2>, for HA-type acids in aprotic solvents. Therefore, to begin with, we describe the method for obtaining the values of pKa and K HA ) in an aprotic solvent in which no data on acid-base equilibria is yet available. Procedures 1 to 4 seem to be appropriate in such a case [20, 21]. 

The redox and acid-base equilibria involving flavin semiquinone species are as shown in Scheme 1. pK values have been determined by both equilibrium (potentiometry) and kinetic methods and range from approximately 8.3 for... 

With very few exceptions, quantitative epoxide assay techniques currently in use are derived from the reeotion of ethylene oxides with halogen adds, notably hydrochloric acid and hydrobromio add, in a variety of solvents. Acid uptake may be determined by any of several reliable procedures. These include titration with standard base8 nr back-titration with standard acid.744 The end-point may be detected visually in the presence of suitable acid-base indicators, or by the more precise technique of potontionaetry.447.4 -470 A useful alternative, applicable in the presence of easily hydrolysed substances or of amines that buffer the end-point, is the technique of argentiometry. In this procedure excess of halide ion is titrated with silver nitrate in tV presence of ferric thiocyanate indicator,470 1884 or potentiometri-cally.188 ... 

In the use of potentiometry for the evaluation of stability constants for complex ions, the expressions can become extremely complicated if multiequilibria are present. For a simple one-to-one complex a direct potentiometric titration curve again provides die most satisfactory route to an accurate evaluation of the constant. The curve looks similar to that for an acid-base titration, and the appropriate point to pick is the half-equivalence point. If the complex is extremely stable, then die amount of free metal ion at this point on die dtration curve (ligand titrated with metal ion) is sufficiently low that it can be disregarded. If not, it must be handled in a way similar to the first point on the titration curve for phosphoric acid. Assuming that it is a stable complex, at the first half-equivalence point the concentration of complexed metal ion will be equivalent to that of the free ligand. The potential will give a direct measure of the free metal ion and allow the stability constant for the complex to be evaluated at the half-equivalence point ... 

More than brief discussion of the numerous ways in which end points can be taken other than by visual methods is beyond our scope. For example, end-point techniques may involve photometry, potentiometry, amperometry, conductometry, and thermal methods. In principle, many physical properties can be used to follow the course of a titration in acid-base titrations, use of the pH meter is common. In terms of speed and cost, visual indicators are usually preferred to instrumental methods when they give adequate precision and accuracy for the purposes at hand. Selected instrumental methods may be used when a suitable indicator is not available, when higher accuracy under unfavorable equilibrium conditions is required, or for the routine analysis of large numbers of samples. 

Potentiometric Titration Potentiometry may be used to follow a titration and to determine its end point. The principles have already been discussed in connection with acid-base or complex formation titrations where pH or pMe is used as a variable. Any potentiometric electrode may serve as an indicator electrode, which indicates either a reactant or a reaction product. Usually the measured potential will vary during the course of the reaction and the end point will be characterized by a jump in the curve of voltage versus amount of reactant added. 

The potentiometric titration method possesses some advantages characteristic of potentiometry. The measured e.m.f. values are dependent on the logarithm of the activity (concentration) of the potential-defining ion and this considerably widens the range of concentrations which can be detected. The experimental techniques and routine are rather simple and the obtained results are well reproducible. Owing to the stable activity coefficients of metal cations the measurements can be performed in sufficiently concentrated solutions of the corresponding metal halides. Performing the measurement does not imply any interference in the acid-base processes in the melt studied and the experiment is faster than the isothermal saturation technique discussed above. 

Use of potentiometry for pH titration allows analyses to be carried out in colored or turbid solutions. Also, it solves the problem of selecting the correct indicator for a particular acid-base titration. The endpoint can be determined more accurately by using a first or second differential curve as described earlier. It also permits pH titrations in nonaqueous solvents for the determination of organic acids and bases as described subsequently. In addition, it can be readily automated for unattended operation. 

It is an attractive feature of potentiometry that the equipment is rather inexpensive and simple one needs a reference electrode, an indicator electrode and a voltagemeasuring instrument with high input impedance. The potential measurement has to be accomplished with as low a current as possible because otherwise the potential of both electrodes would change and falsify the result. In the past, a widespread method was the use of the so-called Poggendorf compensation circuit. In most cases today, amplifier circuits with an input impedance up to 10 2 are used. The key element for potentiometry is the indicator electrode. Currently, ion-selective electrodes are commercially available for more than 20 different ions and almost all kinds of titrations (acid-base, redox, precipitation and complex titrations) can be indicated. In the following, some indicator electrodes and the origin of the electrode potentials will be described. 

The electron impact mass spectra of 29 oxazolidines derived from sugars, such as 71, have been recorded. Potentiometry has been used to study the acid-base and metal ion complexing properties of 2-benzylamino-2-deoxy-D-g/ycero-D-to/o-heptonic acid in aqueous solution. Complexation was similar to that seen with alanine and the D-g/ycero-D-g /o-analogue, with bodi... 

Titrimetric methods with potentiometric end point location can be applied when an electrode with the needed selectivity is not available. The precision and accuracy of potentiometric titrations are superior comparing it with the properties of direct potentiometry. However, the concentration range where potentiometric titration can be used effectively is narrower. A solution with analyte concentration below 1 mM seldom is determined by potentiometric titrations. Potentiometric end point location is most often employed in the case of acid-, base-, precipitate-, redox-, or complexometric titrations. 

Equilibrium constants for various acid-base reactions were determined based on inflection points of plots of absorbances at selected wavelengths on pH or acidity function, by potentiometry and from changes in shapes of Ei/2-pH plots in polarography or Ep-pH plots in differential pulse polaro-graphy (Table 1). 

Potentiometry is another useful method for determining enzyme activity in cases where the reaction Hberates or consumes protons. This is the so-called pH-stat method. pH is kept constant by countertitration, and the amount of acid or base requited is measured. An example of the use of this method is the determination of Hpase activity. The enzyme hydroly2es triglycerides and the fatty acids formed are neutralized with NaOH. The rate of consumption of NaOH is a measure of the catalytic activity. 

Saar and Weber [1] compared methods based on spectrofluorimetry and ion-selective electrode potentiometry for determining the complexes formed between fulvic acid and heavy metal ions. 

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