Harris Chapter 7

TITRATIONSSupplements Information

In Zumdahl’s Chapter 14 & 15

Titration Nomenclature Calculations Precipitation Titration

By regions around the Equivalence Point

Titration of Mixtures

Titration Spreadsheets End-Point Detection

Supplemental Content

Titration: Weak Acid by Strong Base

0

2

4

6

8

10

12

14

0 0.05 0.1 0.15 0.2 0.25

Volume of Base

pH

Titration Nomenclature

Analyte: the solution (soln.) being analyzed.

Titrant: soln. added Equivalence point:

completed reaction. End point: measured

or observed indication of change in soln.

Indicator: compound responsible for end point indication.

Standardization: fix titrant concentration with primary standard.

Back Titration: finds XS titrant to subtract.

Calculations

Buret dispenses mL of titrant, so we use mmol because M mol / L = mmol / mL.

Burette is a diminutive of the French buire, a vase for liquors!

[analyte] = (mmol analyte) / (mL initial volume) mmol analyte = (mmol titrant) (stoichiometric

ratio of analyte to titrant) mmol titrant = (mL titrant) [titrant]

In other words, CV = CV over and over.

Precipitation Titration

Simpler than weak acid since no Kw involved (unless precipitate is an acid or base).

Prior to equivalence, titrant is the limiting reactant & [analyte] = (mmol analyte – scaled mmol titrant) / (mL current solution)

Post equivalence, analyte is the limiting reactant & [titrant] = (mmol titrant – scaled mmol analyte) / ( mL current solution)

In either case and at equivalence: Ksp = [analyte] [titrant] (assuming 1:1)

Titration of Mixtures

Endpoints are characterized by inflection points where a curvature changes sign.

In mixtures leading to two precipitates, two inflection points are observed. The lowest Ksp solid finishes its reaction by

precipitating first. Thus the first inflection point. The more soluble solid precipitates last.

Spreadsheet Titration Curves

While pH is normally plotted against Vtitrant , the equations for Vtitrant = f(pH) are easier as seen in Harris 12-9. The same is true if pM vs. VM is sought in precipitation titrations. nMX(s) = C°M VM – [M+] ( VM + V°M ) nMX(s) = C°X VX – [M+] ( VX + V°X ) (now equate)

VM = V°X (C°X+[M+]–[X–]) / (C°M–[M+]+[X–])

Precipitate End-Point Detection

Volhard Titration Back titrate excess Ag+ (from a halide ppt.) with

SCN– in presence of Fe3+

Red FeSCN2+ appears after AgSCN precipitates.

Fajans Titration Won’t be used in lab., but involves coprecipitate

of colored indicator at end-point.