G Potentiometric Titrations

Svehla, G., Potentiometric Titrations, New York, Pergamon Press, 1978. 

Potentiometric H2O Yalkowsky SH and Zografi G, Potentiometric titration of monomeric... 

Thermal analysis showed that structural water in the mineral was 12.49% by weight of die total solid. The combination of lOP-MS analysis of die dissolved Mn oxide and a charge balance of die different elements provides a formula for the Mn oxide mineral as Ko.i54Mni.7ge04 l.S5H20. This interpretation assumes that the oxidation state for all Mn inside the mineral is IV. As compared with other results (74), our interpretation seems reasonable. Four independent measurements using the BET method yielded a specific surfitce area for Mn oxide of 23.6 0.82 m /g. Potentiometric titrations of Mn oxide solid found the pzc for Mn oxide to be 3.7 0.4, which is con arable to the range reported (2.0 to 4.5) for different forms of MnOa (75). 

In 1965, after Frumkin had been transferred to hospital after a heart attack, he started to think about some unsolved problems posed in his early studies back in the 1930s. As a result, he revisited the thermodynamic theory of the perfectly polarizable electrode, and its experimental checkup then appeared [31, 32]. These studies were carried out by O. A. Petrii and coworkers at the Department of Electrochemistry MSU. This not only resulted in the development of some new experimental techniques (e.g., potentiometric titrations under isoelectric conditions) but also led... 

The potentiometric titration curve shown here was recorded on a 0.4300-g sample of a purified amino acid that was dissolved in 50.00 ml of water and titrated with 0.1036 M NaOH. Identify the amino acid from the possibilities listed in the following table. 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

C. Potentiometric methods. This is a procedure which depends upon measurement of the e.m.f. between a reference electrode and an indicator (redox) electrode at suitable intervals during the titration, i.e. a potentiometric titration is carried out. The procedure is discussed fully in Chapter 15 let it suffice at this stage to point out that the procedure is applicable not only to those cases where suitable indicators are available, but also to those cases, e.g. coloured or very dilute solutions, where the indicator method is inapplicable, or of limited accuracy. 

The indicator electrode employed in a potentiometric titration will, of course, be dependent upon the type of reaction which is under investigation. Thus, for an acid-base titration, the indicator electrode is usually a glass electrode (Section 15.6) for a precipitation titration (halide with silver nitrate, or silver with chloride) a silver electrode will be used, and for a redox titration [e.g. iron(II) with dichromate] a plain platinum wire is used as the redox electrode. 

To measure the e.m.f. the electrode system must be connected to a potentiometer or to an electronic voltmeter if the indicator electrode is a membrane electrode (e.g. a glass electrode), then a simple potentiometer is unsuitable and either a pH meter or a selective-ion meter must be employed the meter readings may give directly the varying pH (or pM) values as titration proceeds, or the meter may be used in the millivoltmeter mode, so that e.m.f. values are recorded. Used as a millivoltmeter, such meters can be used with almost any electrode assembly to record the results of many different types of potentiometric titrations, and in many cases the instruments have provision for connection to a recorder so that a continuous record of the titration results can be obtained, i.e. a titration curve is produced. 

Pyrolusite. Dissolve 1.5-2 g, accurately weighed, pyrolusite in a mixture of 25 mL of 1 1 hydrochloric acid and 6 mL concentrated sulphuric add, and dilute to 250 mL. Filtration is unnecessary. Titrate an aliquot part containing 80-100 mg manganese add 200 mL freshly prepared, saturated sodium pyrophosphate solution, adjust the pH to a value between 6 and 7, and perform the potentiometric titration as described above. 

Dissolve 20 g of tetra-n-butylammonium iodide in 100 mL of dry methanol and pass this solution through the column at a rate of about 5 mL min - L the effluent must be collected in a vessel fitted with a Carbosorb guard tube to protect it from atmospheric carbon dioxide. Then pass 200 mL of dry methanol through the column. Standardise the methanolic solution by carrying out a potentiometric titration of an accurately weighed portion (about 0.3 g) of benzoic acid. Calculate the molarity of the solution and add sufficient dry methanol to make it approximately 0.1M. 

The chlorine content can be determined by either chlorine elemental analysis or a potentiometric titration using a chloride-ion electrode. For titration, about 0.2 g. of polymer is heated in 3 ml. of pyridine at 100° for 2 hours. This suspension is then transferred to a 50-mi. beaker containing 30 ml. of aqueous 50% acetic acid and 5 ini. of concentrated nitric acid, and the resulting mixture is titrated against aqueous 0.1 N silver nitrate. 

A sample of polyester (ca. 1 g, exactly weighed) is dissolved in 20 mL toluene-ethanol mixture (1/1 vol.) and titrated by a solution of KOH in ethanol (0.05 mol/L) using a potentiometric titrator. A blank titration must be performed under the same conditions. Hardly soluble polyesters (e.g., PET) must be dissolved in an o-cresol-chloroform mixture or in hot benzyl alcohol.417 The result (acid content) is normally expressed in mmol COOH/g polyester but may also be given as the acid number, defined as the number of milligrams of KOH required to neutralize 1 g of polyester. [Acid number = (number of mmol COOH/g polyester) x 56.106.]... 

Potentiometric titrations of aqueous solutions of PGA with some alkali hydroxides LiOH,NaOH,KOH)a.ni. tetra-n-alkylammonium hydroxides, e.g. (C/Zj) AO//and (C Hg) NOH, were performed in three parallel determinations... 

In electroanalysis, the techniques are pre-eminently based on processes that take place when two separate poles, the so-called electrodes, are in contact with a liquid electrolyte, which usually is a solution of the substance to be analysed, the analyte. By means of electrometry, i.e., by measuring the electrochemical phenomena occurring or intentionally generated, one obtains signals from which chemical-analytical data can be derived through calibration. Often electrometry (e.g., potentiometry) is applied in order to follow a reaction that goes to completion (e.g., a titration), which essentially represents a stoichiometric method, so that the electrometry merely acts as an end-point indicator of the reaction (which means a potentiometric titration). The electrochemical phenomena in electroanalysis, whether they take place in the solution or at the electrodes, are often complicated and their explanation requires a systematic treatment of electroanalysis. 

This method is primarily concerned with the phenomena that occur at electrode surfaces (electrodics) in a solution from which, as an absolute method, through previous calibration a component concentration can be derived. If desirable the technique can be used to follow the progress of a chemical reaction, e.g., in kinetic analysis. Mostly, however, potentiometry is applied to reactions that go to completion (e.g. a titration) merely in order to indicate the end-point (a potentiometric titration in this instance) and so do not need calibration. The overwhelming importance of potentiometry in general and of potentiometric titration in particular is due to the selectivity of its indication, the simplicity of the technique and the ample choice of electrodes. 

Subsequently, Bos and Dahmen used in m-cresol65 (e = 12.29 at 25° C) a potentiometric titration method combined with conductometry. Essential precautions were the preparation of water-free m-cresol (<0.01% of water), the use of a genuine Bronsted base B, e.g., tetramethylguanidine (TMG), and the application of a glass electrode combined with an Ag-AgCl reference electrode filled with a saturated solution of Me4NCl in m-cresol. The ion product of the self-dissociation of m-cresol, Ks, was determined from the part beyond the equivalence point of the potentiometric titration curve of HBr with TMG comparison with titration curves calculated with various Ka values showed the best fit for Ks = 2 10 19... 

Generally, the results of the measurements indicated that, where dissociation constants were determined by conductometry and also potentiometric titration, they were in agreement with each other further, KHX is low, e.g., about 10 4-10 6moll 1 for aromatic sulphonic acids and 10 13-10 16 moll-1 for carboxylic acids, Xhx2 is high, e.g., 102-104, and KBis low again, e.g., 10 5-10 6 for aliphatic amines and 10 10 for aromatic amines. 

Conductometric titrations. Van Meurs and Dahmen25-30,31 showed that these titrations are theoretically of great value in understanding the ionics in non-aqueous solutions (see pp. 250-251) in practice they are of limited application compared with the more selective potentiometric titrations, as a consequence of the low mobilities and the mutually less different equivalent conductivities of the ions in the media concerned. The latter statement is illustrated by Table 4.7108, giving the equivalent conductivities at infinite dilution at 25° C of the H ion and of the other ions (see also Table 2.2 for aqueous solutions). However, in practice conductometric titrations can still be useful, e.g., (i) when a Lewis acid-base titration does not foresee a well defined potential jump at an indicator electrode, or (ii) when precipitations on the indicator electrode hamper its potentiometric functioning. 

Avdeef, A. Comer, J. E. A., Measurement of pKa and logP of water-insoluble substances by potentiometric titration, in Wermuth, C. G. (ed.), QSAR and Molecular Modelling, Escom, Leiden, 1993, pp. 386-387. 

Less complex techniques have been reported to be useful to study the acidic and alkaline treatment processes of biosorbents and the role of carboxyl and carboxylate groups in metal adsorption. Rakhshaee and coworkers101 used potentiometric titration curves to assess the content of such groups in L. minor biomass treated with NaOH and HC1. The results showed an increase (up to 25%) in the adsorption of Hg(II), Cr(III), Cr(VI), and Cu(II) with NaOH-treated biomass as a consequence of an increase of -COO- groups (0.92-2.42 mmol/g). On the contrary, the -COOH groups increase observed (1.50-2.41 mmol/g) due to the acidic treatment led to a decrease in the metal ions uptake (up to 33%) despite activation by the chloride salts. 

Acid properties. The acid properties of zeolites, including those of aluminum-deficient zeolites, have been described in several reviews (e.g. 33-35). The methods used to study the acidity of aluminum-deficient Y zeolites include infrared spectroscopy (primarily pyridine and ammonia sorption studies), n-butylamine titrations in the presence of Hammett or arylmethanol indicators, and to a lesser extent potentiometric titrations and calorimetric measurements. 

Procedure Dissolve 0.12 g of accurately weighed allopurinol in 50 ml of dimethylformamide, with gentle heating, if necessary. Titrate to the colour change of the indicator that corresponds to the maximum absolute value of dE/dV in a potentiometric titration (where E is the electromnotive force and V is the... 

The first electrochemical studies of Mb were reported for the horse heart protein in 1942 (94) and subsequently for sperm whale Mb (e.g., 95) through use of potentiometric titrations employing a mediator to achieve efficient equilibriation of the protein with the electrode (96). More recently, spectroelectrochemical measurements have also been employed (97, 98). The alternative methods of direct electrochemistry (99-102) that are used widely for other heme proteins (e.g., cytochrome c, cytochrome bs) have not been as readily applied to the study of myoglobin because coupling the oxidation-reduction eqiulibrium of this protein to a modified working electrode surface has been more difficult to achieve. As a result, most published electrochemical studies of wild-type and variant myoglobins have involved measurements at eqiulibrium rather than dynamic techniques. 

Fig. 16.13 Charge properties as a function of pH of an oxidic-kaolinitic Oxisol B horizon (Brazil) with ca. 300 g kg Fe oxides as determined (left) by Na and Cl adsorption from a) 0.2 b) 0.1 and c) 0.01 M NaCI solution and (right) by potentiometric titration in a) 1 b) 0.1 c) 0.01 and d) 0.001 M NaCI solution (Van Raij 8c Peech, 1972 with permission).

Zero Point Charge (ZPO measurements. Potentiometric titration of samples (3.0 g) was carried out in an aqueous suspension (500 ml electrolytic KNO3 solution) according to the procedure reported by Parks (24),... 

J. Chem. Ed. 2001, 78, 1132 G. Gran, Equivalence Volumes in Potentiometric Titrations, Anal. Chim. Acta 1988,206, 111 F. J. C. Rossotti and H. Rossotti, Potentiometric Titrations Using Gran Plots, J. Chem. Ed. 1965,42, 375 ... 

G. Papanastasiou and I. Ziogas, Simultaneous Determination of Equivalence Volumes and Acid Dissociation Constants from Potentiometric Titration Data, Talanta 1995,42, 827. 

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