Chem 311 - Titration 2

Acid-Base Indicators

Weak organic acids or bases whose different protonated forms display distinct colors in aqueous solution.

Indicator Selection Rule

An indicator must be chosen whose transition range (color change) encompasses or lies very near the equivalence point pH of the titration.

Indicator Error

The difference between the experimentally observed end point marked by indicator color change and the true theoretical equivalence point.

Minimizing Indicator Error

Use a minimal amount of indicator because the indicator itself consumes titrant, and perform a blank titration to estimate and correct for the error.

Indicator Safety Caution

Review the Safety Data Sheet (SDS) prior to handling, as certain indicators can be mutagenic or carcinogenic.

NaOH and KOH Primary Standard Suitability

Neither is a primary standard because both solid pellets absorb atmospheric water and react with atmospheric CO2 to form carbonate impurities.

Strong Base Storage Container Requirements

Strong base solutions must be stored in capped high-density polyethylene bottles rather than volumetric glassware, because alkaline solutions chemically attack and etch glass.

p-Function Definition

A logarithmic scale defined as pX = -log10[X], used to plot concentrations that vary across multiple orders of magnitude.

Chemical Activity in Logarithmic Functions

Formally, pX = -log10(A_x), where A_x = gamma_x * [X] is the unitless chemical activity and gamma_x is the activity coefficient.

Argentometric Titrations

Precipitation titrations that employ silver ion (Ag+) as the standard titrant.

Precipitation Equivalence Volume Formula (Ve)

Ve = ([Analyte] * V_analyte) / [Titrant], representing the exact volume of titrant required to precipitate all analyte according to reaction stoichiometry.

Zones of a Precipitation Titration Curve

Zone 1: Prior to the equivalence point (excess unprecipitated analyte present); Zone 2: At the equivalence point (stoichiometric precipitation; [Ag+] determined by Ksp); Zone 3: Post-equivalence point (excess Ag+ titrant present).

Zone 1 Formula (Before Ve for Ag+ Titrating I-)

[I-] = [((Ve - V_added) / Ve) * [I-]initial] * [V_initial / (V_initial + V_added)]; then [Ag+] is calculated as Ksp / [I-], and pAg = -log10[Ag+].

Zone 2 Formula (At Ve for Ag+ Titrating I-)

At the equivalence point, [Ag+] = [I-] = sqrt(Ksp); pAg = -log10(sqrt(Ksp)).

Zone 3 Formula (After Ve for Ag+ Titrating I-)

[Ag+] = [(V_added - Ve) * [Ag+]titrant] / (V_initial + V_added); pAg = -log10[Ag+].

Mohr Method (1855)

Direct argentometric titration of chloride using potassium chromate (K2CrO4) indicator, forming a reddish-brown silver chromate (Ag2CrO4) precipitate at the end point.

Volhard Method (1874)

Indirect precipitation titration where excess Ag+ precipitates chloride, and the unreacted Ag+ is back-titrated with standard KSCN in ~0.5 M HNO3.

Volhard Method End Point Chemistry

When all excess Ag+ has precipitated as AgSCN(s), added SCN- reacts with iron(III) indicator to form a soluble, deep red complex: Fe3+ + SCN- -> FeSCN2+.

Volhard Method Precipitate Removal Rule

Precipitate removal before back-titration is unnecessary for Br-, I-, and SCN- (less soluble than AgSCN), but required for Cl-, PO4^3-, CN-, C2O4^2-, CO3^2-, S2-, and CrO4^2- to prevent AgSCN metathesis.

Fajans Method (1920)

Precipitation titration utilizing adsorption indicators (negatively charged dyes like fluorescein, dichlorofluorescein, or eosin) that adhere to precipitate surfaces.

Fajans Method End Point Mechanism

Prior to Ve, excess lattice anions (e.g., Cl-) give the precipitate a negative surface charge that repels the dye; past Ve, excess Ag+ imparts a positive charge that binds the dye, triggering a distinct color change.

Precipitation Titration of Sulfate (Fajans Application)

Titrated with Ba(OH)2 in 50 volume percent aqueous methanol using alizarin red S as the adsorption indicator.

Precipitation Titration of Zinc (Fajans Application)

Titrated with potassium ferrocyanide [K4Fe(CN)6] to precipitate K2Zn3[Fe(CN)6]2, monitored using diphenylamine indicator.

Precipitation Titration of Mercury(I) (Fajans Application)

Hg2^2+ is titrated with NaCl to precipitate calomel (Hg2Cl2), with the end point detected using bromophenol blue.