LECTURE 2 Titration method ass. prof. Ye. B. Dmukhalska

Titrimetry, in which we measure the volume of a reagent reacting stoichiometrically with the analyte, first appeared as an analytical method in the early eighteenth century.

Overview of Titrimetry: Titrimetric methods are classified into four groups based on the type of reaction involved. These groups are acid–base titrations, in which an acidic or basic titrant reacts with an analyte that is a base or an acid; complexometric titrations involving a metal–ligand complexation reaction; redox titrations, where the titrant is an oxidizing or reducing agent; and precipitation titrations, in which the analyte and titrant react to form a precipitate..

Typical instrumentation for performing an automatic titration.

Equivalence Points and End Points For a titration to be accurate we must add a stoichiometrically equivalent amount of titrant to a solution containing the analyte. We call this stoichiometric mixture the equivalence point. Unlike precipitation gravimetry, where the precipitant is added in excess, determining the exact volume of titrant needed to reach the equivalence point is essential. The product of the equivalence point volume, V eq, and the titrant’s concentration, C T, gives the moles of titrant reacting with the analyte. Moles titrant = V eq. C T Knowing the stoichiometry of the titration reaction, we can calculate the moles of analyte. Unfortunately, in most titrations we usually have no obvious indication that the equivalence point has been reached. Instead, we stop adding titrant when we reach an end point of our choosing. Often this end point is indicated by a change in the color of a substance added to the solution containing the analyte. Such substances are known as indicators.

Equipment for Measuring Volume Analytical chemists use a variety of glassware to measure volume: beaker; graduated cylinder;volumetric flask; pipet; dropping pipet.

Beakers, dropping pipets, and graduated cylinders are used to measure volumes approximately, typically with errors of several percent. Pipets and volumetric flasks provide a more accurate means for measuring volume. Volumetric flask contains a solution, it is useful in preparing solutions with exact concentrations. The reagent is transferred to the volumetric flask, and enough solvent is added to dissolve the reagent. After the reagent is dissolved, additional solvent is added in several portions, mixing the solution after each addition. The final adjustment of volume to the flask’s calibration mark is made using a dropping pipet.

Pipets A pipet is used to deliver a specified volume of solution. Several different styles of pipets are available. Transfer pipets provide the most accurate means for delivering a known volume of solution; their volume error is similar to that from an equivalent volumetric flask

Common types of pipets and syringes: (a) transfer pipet; (b) measuring pipet; (c) digital pipet; (d) syringe. (a) (b) (c) (d)

Three important precautions are needed when working with pipets and volumetric flasks. First, the volume delivered by a pipet or contained by a volumetric flask assumes that the glassware is clean. Second, when filling a pipet or volumetric flask, set the liquid’s level exactly at the calibration mark. The liquid’s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware’s calibration mark. Before using a pipet or volumetric flask you should rinse it with several small portions of the solution whose volume is being measured.

Acid-base titrations Based on acid-base reactions The earliest acid–base titrations involved the determination of the acidity or alkalinity of solutions, and the purity of carbonates and alkaline earth oxides. Before 1800, acid–base titrations were conducted using H 2 SO 4, HCl, and HNO 3 as acidic titrants, and K 2 CO 3 and Na 2 CO 3 as basic titrants. End points were determined using visual indicators such as litmus, which is red in acidic solutions and blue in basic solutions, or by observing the cessation of CO 2 effervescence when neutralizing CO 3 2–. The accuracy of an acid-base titration was limited by the usefulness of the indicator and by the lack of a strong base titrant for the analysis of weak acids.

Titrations Based on Complexation Reactions The earliest titrimetric applications involving metal-ligand complexation The use of a monodentate ligand, such as Cl – and CN –, however, limited the utility of complexation titrations to those metals that formed only a single stable complex. The utility of complexation titrations improved following the introduction by Schwarzenbach, in 1945, of aminocarboxylic acids as multidentate ligands capable of forming stable 1:1 complexes with metal ions. The most widely used of these new ligands was ethylenediaminetetraacetic acid, EDTA, which forms strong 1:1 complexes with many metal ions. Ethylenediaminetetraacetic acid, or EDTA, is an aminocarboxylic acid. EDTA, which is a Lewis acid, has six binding sites (the four carboxylate groups and the two amino groups), providing six pairs of electrons. The resulting metal– ligand complex, in which EDTA forms a cage-like structure around the metal ion, is very stable. The actual number of coordination sites depends on the size of the metal ion; however, all metal-EDTA complexes have a 1:1 stoichiometry.

Precipitation Titrations A reaction in which the analyte and titrant form an insoluble precipitate also can form the basis for a titration. One of the earliest precipitation titrations, developed at the end of the eighteenth century, was for the analysis of K 2 CO 3 and K 2 SO 4 in potash. Calcium nitrate, Ca(NO 3 ) 2, was used as a titrant, forming a precipitate of CaCO 3 and CaSO 4. The end point was signaled by noting when the addition of titrant ceased to generate additional precipitate. The importance of precipitation titrimetry as an analytical method reached its zenith in the nineteenth century when several methods were developed for determining Ag + and halide ions. Pb 2+ (aq) + 2Cl – (aq) =PbCl 2 (s) In the equilibrium treatment of precipitation, however, the reverse reaction describing the dissolution of the precipitate is more frequently encountered. PbCl 2 (s) = Pb 2+ (aq) + 2Cl – (aq) The equilibrium constant for this reaction is called the solubility product, Ksp, and is given as K sp = [Pb 2+ ][Cl – ] 2 = –5

Titrations Based on Redox Reactions Redox titrations were introduced shortly after the development of acid–base titrimetry. Since titrants in a reduced state are susceptible to air oxidation, most redox titrations are carried out using an oxidizing agent as the titrant. The choice of which of several common oxidizing titrants is best for a particular analysis depends on the ease with which the analyte can be oxidized. Analytes that are strong reducing agents can be successfully titrated with a relatively weak oxidizing titrant, whereas a strong oxidizing titrant is required for the analysis of analytes that are weak reducing agents.

Thank you for attention