Analytical Chemistry Lecture 2: Titrimetric Methods

Analytical Chemistry - Lecture 2: Titrimetric Methods of Analysis

• Titration is a technique used to determine the concentration of an analyte using a known concentration of titrant.
• The titrant is added to the analyte until the reaction is complete.
• This is indicated by a color change (end point) or nearly equivalent point.
• The equivalent point is when the amount of titrant added is stoichiometrically equivalent to the amount of analyte.
• The difference between the equivalent point and the end point is called the titration error.

End Points

• End point: A physical change associated with the condition of chemical equivalence.
• Common end points include changes in color due to reagents or indicators and changes in the potential of an electrode.
• Titration error is the difference between the equivalent point and the end point.

Back-Titration

• Back-titration is a technique used when the reaction rate between the analyte and reagent is slow or when the standard solution lacks stability.
• An excess of standard solution is added to the analyte.
• The excess reagent is then titrated with a second standard solution.

Classification of Titrimetric Methods

• Titrimetric methods are categorized into four groups: acid-base, complexometric, redox, and precipitation titrations.

Standard Solutions

• Standard solutions are prepared from compounds of high purity.
• Standard solutions are classified into primary and secondary standard solutions.

Acid-Base Indicators

• Indicators are compounds added to the analyte solution to indicate the end point or near equivalent point. Acid-base indicators are weak organic acids or bases that change color over different pH ranges.
• Examples include thymol blue, methyl yellow, methyl orange, bromocresol green, methyl red, bromocresol purple, bromothymol blue, phenol red, cresol purple, phenolphthalein, thymolphthalein, and alizarin yellow GG. Each indicator has a specific pH range over which it changes color.

Types of Acid-Base Indicators

• Phthalein Indicators: Typically colourless in acidic solutions and show a colour change in basic solutions (e.g., phenolphthalein). They are suitable for titrations involving weak acids and strong bases.

• Sulfonphthalein Indicators: These indicators exhibit two distinct color changes in different pH ranges within acidic, neutral, and moderately basic solutions. An example is Neutral Red.

• Azo Indicators: Indicators like methyl orange and methyl red primarily change colour on the acid side of the pH scale. Suitable for weak base and strong acid titrations.

• Universal Indicator: A mixture of indicators that shows different colors over a wide pH range (1-14). Color change aids in determining the pH value.

Acid-Base Titration

• Acid-base titrations are based on neutralization reactions like NaOH + HCl → NaCl + H₂O, where the reaction takes place in a 1:1 stoichiometric ratio.
• Concentration determination is calculated using M₁V₁ = M₂V₂.

Titration Curve

• A plot of pH (or instrument reading) vs volume of titrant added. Two types of curves are sigmoidal and linear segment curves.

Strong Acid vs. Strong Base Titration

• Initial pH: Determined by the initial analyte concentration.
• Pre-equivalence point: pH calculated by the amount of excess acid.
• Equivalence point: pH = 7 (neutral)
• Post-equivalence point: pH determined by excess base.

Precipitation Titration

• Precipitation titrations involve a precipitation reaction where an insoluble compound is formed from the analyte and titrant. Applications in this method are limited due to the lack of suitable indicators, slow reaction rates in dilute solutions, and limited indicator availability. The most vital precipitation reaction involves AgNO₃, which is known as argentometric titration. This technique determines the concentration of halides.

End Point Detection Methods

• Mohr method: A direct method using chromate as an indicator. Chloride is determined in the presence of chromate. The reaction of Ag⁺ with chromate yields a visible coloured precipitate. Conditions include controlled pH levels and limited excess of indicator chromate.

• Volhard method: An indirect method involving back-titration. excess AgNO3 is titrated with SCN using a ferric alum indicator.

• Fajans method: A direct method based on indicator adsorption. The indicator adsorbs on the precipitate surface, causing a color change.

Complexometric Titration

• Complexation reaction between titrant and analyte.
• Important in applications like black-and-white photography.
• Detection of colored complex using spectrophotometric methods.
• Some soluble complex used in gravimetric and precipitation analysis. Using EDTA as a complexing agent is common.

EDTA (Ethylene diamine tetraacetic acid)

• An amino carboxylic acid with six binding sites (four carboxylate and two amino groups).
• Acts as a hexadentate ligand. Forms strong complexes with metal ions except for alkali metals. Widely used to extract, dissolve, and/or bind metal ions to prevent interference, to extract metals between solvents, or form complexes for spectrophotometric analysis.

Redox Titration

• Redox reactions occur due to electron transfer. Potential is determined using the Nernst equation ( E = E° - (RT/nF) ln( [oxidized]/[reduced])).
• EAox/Ared is the standard state potential for the half-reaction. At the equivalent point, potential calculation is simplified using the Nernst equation for the titrants half-reaction.