Potentiometric titrations endpoint detection

Different experimental approaches are possible with the same endpoint detection method. For example, the titration curve can be plotted and the endpoint determined graphically. First and second derivative curves can be plotted or the derivatives obtained electronically. Another approach is to titrate to a predetermined endpoint signal. This technique is very useful with coulometric titrations, and many examples, especially those involving potentiometric endpoint detection, are found in the literature. The most widely applicable way... 

Although potentiometric measurements frequendy are applied as a means of endpoint detection in potentiometric titrations, the purpose of this volume is not to review the many titradon procedures that utilize potentiometric measurements. However, a brief summary of the species that can be monitored through the use of potentiometric measurements will illustrate the potential applications. 

With any endpoint detection system several practical considerations are important for reliable results. For example, the indicator electrode should be placed in close proximity to the flow pattern from the burette, so that a degree of anticipation is provided to avoid overrunning the endpoint. Another important factor is that the indicator electrode be as inert and nonreactive as possible to avoid contamination and erratic response from attack by the titration solution. A third and frequently overlooked consideration is the makeup of the reference electrode and, in particular, its salt bridge. For example, a salt-bridge system that contains potassium chloride can cause extremely erratic behavior of any electrochemical system if the titrant solution contains perchlorate ion (because of the precipitation of potassium perchlorate at the salt-bridge titrant-solution interface). Likewise, a potassium chloride salt bridge in a potentiometric titra-... 

Another specialized form of potentiometric endpoint detection is the use of dual-polarized electrodes, which consists of two metal pieces of electrode material, usually platinum, through which is imposed a small constant current, usually 2-10 /xA. The scheme of the electric circuit for this kind of titration is presented in Figure 4.1b. The differential potential created by the imposition of the ament is a function of the redox couples present in the titration solution. Examples of the resultant titration curve for three different systems are illustrated in Figure 4.3. In the case of two reversible couples, such as the titration of iron(II) with cerium(IV), curve a results in which there is little potential difference after initiation of the titration up to the equivalence point. Hie titration of arsenic(III) with iodine is representative of an irreversible couple that is titrated with a reversible system. Hence, prior to the equivalence point a large potential difference exists because the passage of current requires decomposition of the solvent for the cathode reaction (Figure 4.3b). Past the equivalence point the potential difference drops to zero because of the presence of both iodine and iodide ion. In contrast, when a reversible couple is titrated with an irreversible couple, the initial potential difference is equal to zero and the large potential difference appears after the equivalence point is reached. 

The amperometric approach to endpoint detection provides considerable latitude in the selection of the best conditions for the most specific and sensitive endpoint response. Furthermore, the response signal is directly proportional to the concentration of the observed species, whereas potentiometric responses are a logarithmic function of the concentration. Another attractive feature of amperometric endpoint detection is that the most important data are obtained prior to and after the equivalence point, whereas in potentiometric titrations the most important data occur at the equivalence point, which is the most unstable condition of the titration. With amperometric titrations an extrapolation of the straight-line portion of the curve, either prior to or after the equivalence point, to an intercept will provide an accurate measure of the equivalence point. 

Figure 4.8 Cell system for coulometric titration by a platinum generator electrode and an isolated auxiliary electrode system includes provision for potentiometric endpoint detection.

Because the generator electrodes must have a significant voltage applied across them to produce a constant current, the placement of the indicator electrodes (especially if a potentiometric detection system is to be used) is critical to avoid induced responses from the generator electrodes. Their placement should be adjusted such that both the indicator electrode and the reference electrode occupy positions on an equal potential contour. When dual-polarized amperometric electrodes are used, similar care is desirable in their placement to avoid interference from the electrolysis electrodes. These two considerations have prompted the use of visual or spectrophotometric endpoint detection in some applications of coulometric titrations. 

Assuming impurities can be satisfactorily pretitrated, the lower limit of the amount of sample that can be titrated is governed primarily by the sensitivity of the available endpoint detection system. Very small currents, such as 0.1 /xA, can be measured accurately (actually, currents smaller than 60 electrons per second have been measured and the time of electrogeneration can be measured accurately. With conventional amperometric and potentiometric endpoint indication, coulometric titrations in typical solution volumes cannot be accurately made at generating currents of less than about 100 A. 

The last type of metallic electrode is the redox indicator electrode. This electrode is made of Pt, Pd, Au, or other inert metals, and serves to measure redox reactions for species in solution (e.g., Fe /Fe, Ce /Ce" ). These electrodes are often used to detect the endpoint in potentiometric titrations. Electron transfer at inert electrodes is often not reversible, leading to nonreproducible potentials. Although not a metal electrode, it should be remembered that carbon electrodes are also used as redox indicator electrodes, because carbon is also not electroactive at low applied potentials. 

Figure 15.20 Apparatus for coulometric titration with potentiometric endpoint detection.

Analytical chemistry has found great utility in conductimetric measurements in spite of its apparent nonspecificity. Rapid quantitative accuracy of a few tenths of a percent may be quickly accomplished by direct conductimetric determination of binary electrolytic solutions such as aqueous acids, bases, or salts. A nearly linear increase in conductivity is observed for solutions containing as much as 20% of solute. The concentration of strong solutions, such as the salinity of seawater, may be determined from conductance measurements traces of electrolyte impurities, such as the impurity in ultrapure water, may be reported at the pgl level. Conductimetric titrations may increase the accuracy of endpoint detection and permit titrimet-ric analysis of weak electrolytes, such as boric acid, which is not feasible by potentiometric or colorimetric... 

Potentiometric endpoint detection is frequently used in automatic constant current coulometric titrators (coulometric titrations). 

The titration endpoint is detected potentiometrically using an Ag/AgCl electrode, preferably by an automatic method. The procedure described here is based on an execptionally precise method outlined by Hermann (1951)... 

Aieta et al. [10] worked out electrometric titrations for the sequential determination of chlorine dioxide, chlorine, chlorite, and chlorate. Phenylarsine oxide or sodium thiosulfate titrants and potentiometric or amperometric endpoint detection are used in their method. 

A simple potentiometric method for the determination of silica in solutions of alkali metal silicates is described. The precision of the determination of Na20 and SiQz in silicates was improved and the determination time decreased by developing a potentiometric titration method. Better determination results due to potentiometric detection of the endpoint pH are demonstrated by comparison with gravimetric determination of Na20 and Si02 in water glass containing 26-260 g/L. The potentiometric determination method can be easily automated. 

When titrating very weak acids, such as phenols, enols, or sulfonamides, in nonaqueous solvents, the endpoint detection is easier and more accurate when stronger bases are used as titrants. This is because both the potential jump in a potentiometric titration and the color change in a visual titration become more pronounced. 

Applications Potentiometry finds widespread use for direct and selective measurement of analyte concentrations, mainly in routine analyses, and for endpoint determinations of titrations. Direct potentiometric measurements provide a rapid and convenient method for determining the activity of a variety of cations and anions. The most frequently determined ion in water is the hydrogen ion (pH measurement). Ion chromatography combined with potentiometric detection techniques using ISEs allows the selective quantification of selected analytes, even in complex matrices. The sensitivity of the electrodes allows sub-ppm concentrations to be measured. 

The methanesulphonic acid content of bromocriptine mesilate is usually determined by titration with 0.1 N methanolic potassium hydroxide. The endpoint may be detected potentiometrically using a glass/calomel electrode system. 

A mixture consisting of 25 mL of an aqueous (0.2 to 1.0 mM) solution of chlorpromazine hydrochloride and 5 mL of 0.1 M acetate buffer (pH 3.3) was titrated to a potentiometric endpoint with 0.0 IM sodium tetraphenyl-borate [77]. The titrant was added at a rate of 0.36 mL/min with continuous stirring, and the temperature of the medium was maintained at 22 2°C. The end point was detected by a tetraphenylborate-selective electrode. 

The entire subject of amperometric titrations has been reviewed in a number of monographs on electrochemistry 4-6 a definitive work on this subject also has been published.7 Because the amperometric titration method does not depend on one or more reversible couples associated with the titration reaction, it permits electrochemical detection of the endpoint for a number of systems that are not amenable to potentiometric detection. All that is required is that electrode conditions be adjusted such that either a titrant, a reactant, or a product from the reaction gives a polarographic diffusion current. 

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