Direct potentiometric titration

The free maleic acid content in maleic anhydride is determined by direct potentiometric titration (166). The procedure involves the use of a tertiary amine, A/-ethylpipetidine [766-09-6J, as a titrant. A tertiary amine is chosen as a titrant since it is nonreactive with anhydrides (166,167). The titration is conducted in an anhydrous solvent system. Only one of the carboxyhc acid groups is titrated by this procedure. The second hydrogen s dissociation constant is too weak to titrate (166). This test method is not only used to determine the latent acid content in refined maleic acid, but also as a measure of the sample exposure to moisture during shipping. 

Direct potentiometric titration with alkali gave rather flat curves without distinct inflection points 26, 44). Villars 26) concluded that no chemical groups of distinct acidities were present. However, very often the potential becomes constant only several hours after the addition of alkali. Therefore, it was attempted in my laboratory 45-47) to differentiate the acid groups by neutralization with bases of different basicities. The samples were agitated for at least 16 hours with 0.06 N solutions of four bases NaHCOs, NajCO., NaOH, and Na ethoxide. The... 

The drug substance has been determined by direct potentiometric titration using chlorpromazine-sensitive electrodes [78. These consisted of an electroactive material in the membrane being either the chlorpromazine complex with either tetraphenylborate or dinonyinaphthalene sulfonate set in a PVC matrix. 2-nitrophenyl octyl ether was used as the plasticizer. 

The extremely low solubility of lead phosphate in water (about 6 x 10 15m) again suggests potentiometric analysis. Selig57,59 determined micro amounts of phosphate by precipitation with lead perchlorate in aqueous medium. The sample was buffered at pH 8.25-8.75 and a lead-selective electrode was used to establish the end-point. The detection limit is about 10 pg of phosphorus. Anions which form insoluble lead salts, such as molybdate, tungstate or chromate, interfere with the procedure. Similar direct potentiometric titrations of phosphate by precipitation as insoluble salts of lanthanum(III), copper(II) or cadmium(II) are suggested, the corresponding ion-selective electrodes being used to detect the end-point. 

Quadrivalent cerium salts may be volumetrically determined by arsenious acid in aqueous sulphuric acid solution using a manganese salt as catalyst, with potassium iodate present as a promoter.2 Excess of arsenious acid is used and back-titrated with permanganate. If nitric acid is present in place of sulphuric acid, an alkali chloride and a trace of iodine are used to promote the action of the manganese salt. Direct potentiometric titration with arsenious acid may also be employed. 

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 ... 

A cuprimetric method for the determination of ascorbic acid is based on a direct potentiometric titration with CUSO4 using a copper-based mercury film electrode as the indicator electrode. The electrochemical reaction taking place is ascorbic acid reducing Cu(n) ions quantitatively to Cu(I) ions in neutral or acidic aqueous solutions. The linearity... 

We reported the solubilities of 11 oxides of the MeO type (s2- and 3d-metals) in the molten KCl-NaCl equimolar mixture at 700 °C [175, 330, 332, 348] (see Table 3.7.5), studied by the method of direct potentiometric titration. The titration curves of Mg2+, Ni2+, Co2+, Mn2+ and Zn2+ do not contain an unsaturated solution section, and therefore, they are referred to the ions practically insoluble in the specified chloride melt. The potentiometric curve for Mg2+ does not contain two steps in the acidic section, and it is smooth enough (Fig. 3.7.4, curve 1) the calculated pPMgo values undergo small fluctuations about the average value, 9.27. It follows that there is no formation of intermediate compounds similar to Mg202+ in the chloride melt, at least, with the initial concentrations of Mg2+ (MgCl2) close to 0.05 mol kg-1. 

There were many investigations to explain the mechanism of reduction. Since direct chemical measurements are out of question at carrier-free concentrations of Tc (10 M), carrier technetium ( Tc) in hydrochloric acid was used to determine the oxidation state of technetium in diethylene triamine pentaacetate (DTPA) and in citrate solution. Polaro-graphic and iodometric techniques were used to analyze for unreacted stannous ion and to perform direct potentiometric titrations of pertechnetate-99 with stannous chloride (Mtinze 1980 Steigman et al. 1975). No quantitative kinetic studies had been made, but qualitative conclusions have been drawn for the reduction mechanism. Most probably, the first step is the reduction to Tc(V). Reduction to Tc(III) proceeds in two successive complementary reactions, both of which should be rapid in the low concentrations at radiopharmaceutical level ... 

Potentiometric Titration Direct potentiometric titration has been used to study for years the carbon surface, but until the mid-1990s the curves obtained did not show discrete endpoints useful for meaningful interpretation [200,205,209], The derivatives obtained in some studies were difficult to characterize due to the many maxima revealed when no well-defined endpoints were present [210], A first meaningful deconvolution of titration curves with theoretical description of acid-based dissociation on the surface of carbons was proposed in the mid-1990s... 

A quantitative comparison between direct potentiometric titration and the classical Boehm method for characterization of the acidity of activated carbons has been drawn by various researchers [85,215,218], Although a rather acceptable agreement is generally accomplished for both methods, when applied to a carbon snrface where oxygen is the major constituent, some discrepancies exist. Figure 2.7 shows a correlation between the total acidic groups detected on an activated carbon using both Boehm and potentiometric titrations. 

N2H4 and Derivatives. Interest in the use of hydrazine and its derivatives in quantitative redox reactions continues. Conditions have been developed for a direct potentiometric titration of Fe with hydrazine sulphate at room temperature ... 

Falls JH, Barnacascel WR, and Britt PW (1976) Direct potentiometric titration of urea with urease. Journal of Association of Official Analytical Chemists 59(5) 1045-1047. 

The generally accepted formal potential, E°, of the NAD+/NADH redox couple at pH 7.0 (25 °C) is —315 mV versus normal hydrogen electrode (NHE)(—560 mV vs. saturated calomel electrode (SCE) [23, 24). From thermal data and the equilibrium constants of the ethanol/acetaldehyde and 2-propanol/acetone reactions catalyzed by alcohol dehydrogenase, a value of —320 mV was calculated, which was later recalculated to be —315 5 mV versus NHE by Clark [23]. Through direct potentiometric titrations using several different mediators and xanthine oxidase as catalyst, Rodkey [25, 26] obtained an E° value of —311 mV versus NHE (25 °C) and a temperature variation of the E° of —1.31 mV/ C in the range of 20 to 40 °C. A variation of the E° with pH of —30.3 mV/pH (30 C) was found, which... 

A pronounced case is in the direct potentiometric titration of amino functions on polymer, since the titrant is an about 0.5% solution of 72% aqueous perchloric acid in trichloroethane/ethanol, which is added to the whole batch of polymer in a mixture of glacial acetic acid/dichloromethane (1 1) [134]. One titration lasts about 1 hour because of the slow reaction of the glass-calomel combination of electrodes on changing potentials and is usually repeated once. To neutralize free amino groups on polymer, each equivalent of perchloric acid of the titrant transports about 2 equivalents of water into the reaction mixture. These moist conditions endanger extended Merrifield syntheses with trifunctional amino acids by proton catalyzed side reactions including detachment from polymer as well as partial acid hydrolysis of the peptide as we found. Since each titration has to be followed by another deprotonation with tertiary base while all polar groups on polymer... 

There is not much to be said, regarding the analytical procedures used to make these determinations, because these are well-known. May I mention, however, that the percentage of soap is obtained by direct potentiometric titration of sucroglyceride dissolved in 2-propanol and water the free sugar is determined by the thin layer chromatographic method in comparison with a standard and the dimethylformamide content is obtained colorimetrically by the copper dimethyldithio-carbamate method. 

Some specialty cationics which are not sufficiently hydrophobic for direct potentiometric titration with tetraphenylborate can be determined indirectly. An excess of sodium tetraphenylborate is added to precipitate the surfactant, which is removed by filtration. Excess tetraphenylborate ion in the filtrate is then determined by titration with thallium(I) nitrate (110). This last procedure was used for determination of the amount of a cationic surfactant fixed to cotton fibers in a textile dying bath. 

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