Conductance titrations, acid-base precipitation

To detemriine the concentration of a particular solute in a solution, chemists often carry out a titration, which involves combining a sample of flie solution with a reagent solution of known concentration, called a standard solution. Titrations can be conducted using acid-base, precipitation, or oxidation-reduction reactions. Suppose we have an HQ solution of unknown concentration and an NaOH solution we know to be 0.100 M. To determine die concentration of die HQ solu-don, we take a specific volume of diat solution, say 20.00 mL. We dien slowly add die standard NaOH solution to it until die neutralization reaction between die HQ and NaOH is complete. The point at which stoichiometrically equivalent quantities are brought together is known as die equivalence point of the titration. 

In principle, any type of titration can be carried out conductometrically provided that during the titration a substantial change in conductance takes place before and/or after the equivalence point. This condition can be easily fulfilled in acid-base, precipitation and complex-formation titrations and also the corresponding displacement titrations, e.g., a salt of a weak acid reacting with a strong acid or a metal in a fairly stable complex reacting with an anion to yield a very stable complex. However, for redox titrations such a condition is rarely met. 

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). 

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. 

We now turn our attention to details of precipitation titrations as an illustration of principles that underlie all titrations. We first study how concentrations of analyte and titrant vary during a titration and then derive equations that can be used to predict titration curves. One reason to calculate titration curves is to understand the chemistry that occurs during titrations. A second reason is to learn how experimental control can be exerted to influence the quality of an analytical titration. For example, certain titrations conducted at the wrong pH could give no discernible end point. In precipitation titrations, the concentrations of analyte and titrant and the size of Ksp influence the sharpness of the end point. For acid-base titrations (Chapter 11) and oxidation-reduction titrations (Chapter 16). the theoretical titration curve enables us to choose an appropriate indicator. 

Salts are the products of the acid-base neutralisation reaction. The salts used most in textile wet processes are common salt (NaCl, sodium chloride) and Glauber s salt (Na SO, sodium sulphate). The content analysis of salts is usually conducted by using a precipitation titration method which may be followed by fdtering and weighing procedures to obtain the final results. 

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. 

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