Titrimetric/Volumetric Methods

Titrimetric/Volumetric Methods

• Neutralization is a titrimetric/volumetric method
• Precipitation is a titrimetric/volumetric method
• Complexation is a titrimetric/volumetric method
• Oxidation-Reduction (REDOX) is a titrimetric/volumetric method

Outline of Topics

• Chemical reactions
• Indicators
• Acidimetric reactions
• Alkalimetric reactions
• Direct and residual titration
• Aqueous and non-aqueous titration

Chemical Reactions in Titrimetry

• Four types of chemical reactions include Neutralization, Precipitation, Complexation, and Oxidation-reduction
• Neutralization includes acidimetry and alkalimetry
• Oxidation-reduction includes permanganate, ceric sulfate, iodimetry and iodometry, bromine, potassium iodate and diazotization assay with nitrite

Requirements for a Reaction

• The reaction must proceed to completion
• The reaction must proceed in a stoichiometric manner
• A suitable endpoint detecting device, like indicators, is necessary
• The reaction must be rapid to make a sharp endpoint discernible for direct titrimetric methods

Neutralization Reactions

• Neutralization reactions involve the chemical process in which an acid reacts with a base
• This produces water and salt

Indicators

• Indicators are complex organic compounds used to determine the endpoint.
• Used to determine the pH.
• Used to indicate that a desired change in pH has been effected

Indicator Properties

• Indicators can be weakly acidic or weakly basic, existing in two mutually convertible forms of different colors
• The typical indicator solution has a concentration of 0.05%
• Between 0.1 to 0.2 mL of the indicator solution is used per 10 mL of the liquid sample
• Solutions of basic type indicators and phthaleins are prepared by dissolving in alcohol
• Solutions of acid group indicators must be neutralized with NaOH

Indicator Adjustments

• Most acid-base indicator solutions are adjusted so that adding 0.15 mL of the indicator solution to 25 mL of CO2-free water, and then adding 0.25 mL of 0.02N acid or alkali, will develop the characteristic color changes, unless otherwise stated
• Most indicators for acid-base titration are acidic

Common pH Indicators

• Malachite green exhibits a color change from yellow (acid) to green (base) within a pH range of 0.0-2.0
• Methyl Yellow exhibits a color change from red (acid) to yellow (base) within a pH range of 2.9-4.0
• Bromophenol blue exhibits a color change from yellow (acid) to blue (base) within a pH range of 3.0-4.6
• Methyl Orange exhibits a color change from pink (acid) to yellow (base) within a pH range of 3.2-4.4
• Bromocresol green exhibits a color change from yellow (acid) to blue (base) within a pH range of 4.0-5.4
• Methyl red exhibits a color change from red to yellow within a pH range of 4.2-6.2
• Bromocresol purple exhibits a color change from yellow to purple within a pH range of 5.2-6.8
• Bromothymol blue exhibits a color change from yellow to blue within a pH range of 6.0-7.6
• Phenol red exhibits a color change from yellow to red within a pH range of 6.8-8.2
• Cresol red exhibits a color change from yellow to red within a pH range of 7.2-8.8
• Thymol blue exhibits a color change from yellow to blue within a pH range of 8.0-9.2
• Phenolphthalein exhibits a color change from colorless to red within a pH range of 8.0-10.0
• Thymolphthalein exhibits a color change from colorless to blue within a pH range of 9.3-10.5

Specific Indicator Details

• Phenolphthalein's approximate pH range for color change: 8.0-9.8, clear in acid form that turns to red-violet at base form
• Methyl Red's approximate pH range for color change is 4.4-6.2, red in acid form that turns to yellow at base form

Rules for Indicator Use

• Use 3 drops of indicator TS unless otherwise directed
• Use methyl orange, methyl red, or phenolphthalein with Strong Acid (SA) + Strong Base (SB) titrations
• Use methyl red with SA + Weak Base (WB) titrations
• Use phenolphthalein with Weak Acid (WA) + SB titrations
• No indicator is used with WA + WB titrations

Examples of Acids & Bases

• Strong acids: HCl, HNO3, H2SO4, HI
• Strong bases: NaOH, KOH
• Weak acids: Acetic acid, Oxalic acid
• Weak bases: NH3

Indicator Papers

• Lead acetate test paper
• Litmus paper
• Mercuric bromide test paper
• Phenolphthalein paper
• Potassium iodate-starch paper
• Starch iodate paper
• Starch iodide paper
• Turmeric paper

Acidimetric Analysis

• Acidimetry involves direct or residual titrimetric analysis of BASES using an acid as the titrant
• Analyte = base
• Titrant = acid
• Types of Acidimetry include direct and residual titrations.

Titration Types

• Direct titration involves using a standard acid solution from a buret into a solution of base being assayed
• Residual Titration involves adding a first volume solution of acid in excess and titrating the excess acid with a standard base.

Acidimetry Examples

• Organic substances include urea, sodium salicylate, diphenhydramine, emetine hydrochloride, meprobamate, paramethadione, pyrazinamide
• Inorganic substances that can be used can include sodium bicarbonate, milk of magnesia, ammonium chloride, calcium hydroxide, lithium carbonate, zinc oxide

Direct Titration Calculation

• % Purity = (N x V x meq. wt. x 100) / wt. of sample where:
  • N = normality
  • V = volume

Residual Titration Calculation

• % Purity = (N1V1 - N2V2) X meq. wt. X 100 / wt. of sample

Alkalimetric Analysis

• Alkalimetry is the direct or residual titrimetric analysis of ACIDS
• Direct analysis the sample is acid and the titrant is base
• Residual analysis the sample is acid and treated with excess standard base before being titrated with acid

Nonaqueous Titrimetric Analysis

• Used for substances poorly soluble and weakly reactive in water
• Uses organic solvents
• Moisture should be avoided; limit of moisture is 0.05%

Acidimetry in Nonaqueous Solutions

• Solvents for acidimetry: acetonitrile, alcohol, dioxane, benzene chloroform
• Titrant: perchloric acid in glacial acetic acid or dioxane, or hydrogen bromide
• Example: assay of methacholine chloride

Acidimetry Indicators

• Example indicators used with Acidimetry include, crystal violet, quinaldine red, naphtholbenzein, malachite green, methyl red, methyl orange and thymol blue

Alkalimetry in Non-aqueous Solutions

• Titrants include: Na methoxide & Lithium methoxide.
• An example is an Assay of phenytoin (organic acid).

Alkalimetry Indicators

• Include azo violet, thymol blue, nitroaniline, thymolphthalein and hydroxyazobenzene