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Volumetric analysis

Learning Objectives

At the end of this lecture, the student will be able to

• List the types of volumetric analysis

• Explain the direct method with examples

• Explain the indirect method with examples

• Explain the back titration method with examples

volumetric analysis

Volumetric analysis, a crucial technique in analytical chemistry, involves the determination of the concentration of a solution through the reaction with a standardized reagent. There are various methods of volumetric analysis, each with its unique approach and utility.

Volumetric Titrations

Method of quantitative chemical analysis in which the amount of a substance is determined by measuring the volume that it occupies

Types of Volumetric Titrations

Type of titration	Substance that can be analysed	Reagent	Indicator	Chemical reactions
Acid Base	Acid or Base	Alkali or acid	pH indicator		OH^– + HA → A^– + H₂O
Precipitation	Ions that form insoluble salt	Silver nitrate, ammonium thio cyanate	conductivity		Ag⁺ + Cl^– → AgCl (s)
Redox	Oxidizing or reducing agent	Oxidizing or reducing agent	Redox indicator		MnO₄^– + 5Fe²⁺ + 8H⁺ → 5Fe³⁺ + Mn²⁺+ 4H₂O
Complexometric	Metal ions, Cations	Complexing agent like EDTA	Metal indicator		EDTA^2- + Ca²⁺ → EDTA Ca.xH₂O

Different Methods of Volumetric Titration

Methods

1. Direct

2. Indirect

3. Back

Direct titration:

• Substance is directly titrated with suitable titrant by using suitable indicator

• Direct titration is useful for:-

A- Strong acid

B- Strong base

C- Weak acid or base if K_a and K_b not less than 10^-7

Examples:

• Acid estimated by base

Potassium hydrogen phthalate by sodium hydroxide

KHC8H4O4 + NaOH —–> KNaC8H4O4 + H2O

• Base estimated by acid

Sodium hydroxide by hydrochloric acid

NaOH + Hcl → Nacl + H2O

Indirect titration

• Substance is not directly titrated but product formed has to be titrated

• Sometimes not feasible due to:

I. Reaction kinetic or the reaction rate is slow

II. No suitable indicator in the direct titration

III. The color change is slow or delay

IV. The endpoint is far from the equivalent point

Determination of ammonium salt (Formol titration)

4NH4Cl + 6HCHO à (CH2)6N4 + 4HCl + 6H2O

Hcl + NaOH à Nacl + H2O

N.B. Formaldehyde must be neutralized from any formic acid due to aerial oxidation

Indicator: Phenolphthalein

Colour change: Colorless to pink

Back titration:

• In this titration a known but excess amount of reagent is added to the sample solution and the excess/unreacted reagent is back titrated with a standard solution

When do we use back titration?

1- When the sample is volatile .eg. NH3, formic acid

2- When the sample is insoluble eg. ZnO, CaO, CaCO3, BaCO3

3- When reaction requires heat of standard solution

4- When the reaction proceeds only in the presence of an excess reagent

eg. With lactic acid

Back titration is generally a two-stage analytical technique:

• Reactant A of unknown concentration is reacted with excess reactant B of known concentration

A + B(excess) ——> C + Unreacted B

• Titration is performed to determine the amount of unreactant B in excess

B(Unreacted) + D ——-> E

Note: A blank titration is carried out similarly, without sample to know exactly how much of B has reacted with A to form C

Example:

Zno+H2SO4 → ZnSO4 + H2O + H2SO4 (unreacted)

H2SO4 + 2NaOH → Na2SO4+H2O

Note: A calculated amount of ammonium chloride is added in order to prevent interaction between zinc sulphate and sodium hydroxide, so that a sharp end point is obtained

Indicator: Methyl orange

Color Change: Yellow to pinkish-red

Summary:

• Methods of volumetric titration: Direct, Indirect, and back titration

• Direct method: Substance is directly titrated with a suitable titrant by using a suitable indicator

• Indirect titration: The substance is not directly titrated but the product formed has to be titrated

• Back titration:
In this titration, a known but excess amount of reagent is added to the sample solution and the excess/unreacted reagent is back-titrated with a standard solution

FAQs

1. What is the primary goal of volumetric analysis?

The primary goal of volumetric analysis is to determine the concentration of a substance in a solution accurately.

2. When should I use the direct method in volumetric analysis?

The direct method is suitable when the analyte’s properties allow for a straightforward titration process.

3. What is the key advantage of the back titration method?

The back titration method is highly versatile and can be used when direct titration is not feasible due to the nature of the analyte.

4. Can you provide an example of indirect volumetric analysis in real life?

One real-life example of indirect volumetric analysis is the determination of the concentration of chlorine ions in a sample by precipitating silver chloride.

5. What equipment is typically used in volumetric analysis?

Volumetric analysis often requires equipment such as burettes, pipettes, and volumetric flasks to measure and dispense precise volumes of solutions. Additionally, indicators, standardized reagents, and pH meters may be used depending on the specific analysis.

6. How do I prepare a standard solution for volumetric analysis?

To prepare a standard solution, you must weigh or measure a precise amount of a pure substance and dissolve it in a known volume of solvent. The concentration is calculated based on the amount of substance and volume of the solution.

7. What is the purpose of using indicators in volumetric analysis?

Indicators are used to signal the endpoint of a titration. They change color or undergo a detectable change in some other property when the reaction between the analyte and titrant is complete, helping the analyst determine the endpoint accurately.

8. Can volumetric analysis be used in qualitative analysis as well?

Volumetric analysis is primarily a quantitative technique, but it can be adapted for qualitative analysis. In qualitative analysis, the goal is to identify the presence or absence of a specific substance in a sample, rather than determining its exact concentration.

9. Are there any safety precautions to consider when conducting volumetric analysis?

Safety is paramount in any laboratory work. When performing volumetric analysis, ensure you are well-versed in handling chemicals, wear appropriate protective gear (lab coat, goggles, gloves), work in a well-ventilated area, and follow standard safety protocols. Additionally, be aware of the specific hazards associated with the substances you are analyzing.

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