Conductance Titrations and Other Applications

One of the most frequent uses of conductance is in quantitative titrations of systems in which the conductance of the solution varies in such a manner (prior to and after the endpoint) that two intersecting lines can be drawn to indicate the endpoint. The actual shape of the curve depends on the sample, the titrant, and the reactions occurring. To maximize accuracy in all titration work, corrections to the measured resistance may have to be made for dilution by the titrant. To minimize this correction, titrants should be at least 10, and preferably 100, times stronger than the solute. While the term conductance implies that titrations require that conductance be measured, it should be pointed out that the reciprocal of resistance can be plotted and values need only be relative and not absolute. 

Volume corrections for the added titrant are made according to the equation 

In general, four to six points are taken prior to the end point and a similar number of points after the end point. 

Many applications of conductance titrations involve acid-base titrations. 

Strong Acids and Bases. Because of the high mobilities of and OH , the sharpest and most accurate endpoints are obtained when strong acids are titrated with strong bases and vice versa. Referring to Table 5.3, it is seen that the equivalent conductance (or mobility) of H+ is about 5 times that of the other cations, and that of OH is about 3 times greater than that of other anions. 

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This last group of solvents includes substances as diverse as acetone and benzene. Acid-base reactions in such solvents are complicated by extensive ion-pairing and by formation of other ionic and molecular aggregates. In acetone, which has a dielectric constant of 20.7 at 25°C, sodium perchlorate at O.OIM concentration is 80% associated to ion-pairs whereas the degree of association is only 31 % in nitromethane, 22% in acetonitrile, and 4% in sulpholane. In other respects acetone behaves like the dipolar aprotic solvents discussed in sect. 3.6. Jasiiiski and Pawlak, for example, showed that conductance titrations in acetone could be treated quantitatively in the same way as those in acetonitrile. The familiar potentiometric, conductometric, and spectrophotometric methods are applicable to the ionisation of anilines in acetone and acetone-water mixtures/ ... 

Conductance has achieved a great variety of industrial and research applications in a wide range of fields from simple conductance titrations to modern ion chromatographic detections. Kinetic measurements can also be made by using the stop-flow method or other methods. Experiments are fairly easy to carry out because of the availability of a variety of self-balancing bridges. 

One of the most useful applications of the conductivity of colloids is found in the conductivity titration of ionic constituents of the sol. The method was introduced and fruitfully applied by Pauli but has later on been used by many others. An example which is especially illuminating in this respect is the conductivity titration of the ions forming the counter ions of a negatively charged Agl-soh... 

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. 

The use of ISEs with ion-selective membranes based on plasticized PVC, as well as glass pH electrodes, is limited to the analysis of aqueous solutions. On the other hand, sensors based on conducting polymer membranes are usually insoluble in organic solvents, which extends the range of possible applications. Electrosynthesized polypyrrole doped with calcion works as a Ca2+ sensor that can be applied as indicator electrode in the titration of Ca2+ with NaF in mixed solvents, such as water-methanol (1 1) and water-ethanol (1 1) [52], Another example is the use of polyaniline as indicator electrode in order to follow the acid-base precipitation titration of trimeprazine base with tartaric acid in isopropanol solution (see Procedure 5). 

Conductance measurements are useful as aids in the solution of many physico-chemical problems. A few of the more important of these applications are (a) determination of the solubilities of certain substances, (b) estimation of the degree of hydrolysis of salts, (c) determination of speeds of reaction, (d) investigation of molecular complexes and (e) conductometric titrations. These will be considered in the order given. The discussions will be brief since the chief purpose of this chapter is to illustrate the use of conductance measurement as an analytical method in other than electrochemical fields of investigation. 

Variations of conductance may be used to follow the courses of acid-base and precipitation reactions. A drawback of the latter is the possible contamination of the electrodes by the precipitate formed. A grave disadvantage of any conductance-based titration is its non-applicability in the presence of high concentrations of electrolyte species other than those required to be determined. This is in contrast to many other electroanalytical techniques where such electrolytes not only do not interfere, but offer distinct advantages. 

Acid-base potentiometric titration is applicable to amphoterics. Typically, either an excess of acid or of base is added, and the titration is conducted with base or acid, respectively. The first inflection is due to the excess acid or base and subsequent inflections are due to end points associated with the surfactant. Hydrolysis and consequent blurring of the end point can be minimized by using a highly alcoholic solvent (114). Since other acidic or basic components will interfere, this approach is only applicable to concentrated surfactant solutions, not to formulations. In fact, impurities in the concentrated surfactant will interfere, which sometimes limits the usefulness of titration for assay. For determination of amphoterics in formulations, ion-pair titration is often used. 

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