The Chemistry Involved in the Titrimetric Exercises

AN INTRODUCTION TO TITRIMETRIC ANALYSIS

A titrimetric analysis is quantitative chemical analysis. It is carried out by determining the volume of a solution of known concentration, which reacts quantitatively with a calculated volume of a solution whose concentration is unknown and is to be determined. In Titrimetric analysis, the one with known concentration is referred to as the titrant. While on the other hand,  the substance being titrated is called the titrand or titer, or analyte.

The point at which the reaction involved in titration between titrant and analyte completes the equivalence point or theoretical endpoint. The completion of the involved reaction is either indicated by some physical change in the reaction mixture, which is produced by adding some kind of external indicator (to produce a color change) or some physical measurement like pH or sometimes by the standard solution itself. The indicator used must give a clear visual change when the reaction completes in the form of color change or turbidity. The point at which this is observed is called the endpoint. In an ideal case scenario, the theoretical equivalence point and the endpoint of titration should coincide, but in practice, there is a very small difference between the two which is known as the Titration Error. The indicator should be selected as per the titration reaction such that titration error is minimized. There are various types of titration analysis. 

STANDARD SOLUTION

A solution with accurately known strength is called the Standard solution. It must be pure, unreactive, and stable at room temperature. It must not undergo any change in concentration over some time. It is of two types: Primary Standard and Secondary standard. 

A primary standard is a solution of high purity and is prepared by a substance called primary standard. Examples for these are NaC2O4 (sodium oxalate), K2Cr2O7 (potassium dichromate), KBrO3 (potassium bromate), KIO3 (potassium iodate), and KH(IO3)2 (potassium hydrogen iodate).

A secondary standard is a solution whose concentration is determined with the help of a primary standard. Examples of the secondary standard are KMnO4 (Potassium permanganate), Na2S2O3 (sodium thiosulphate), and NaOH (Sodium Hydroxide)

REQUIREMENTS FOR A TITRIMETRIC ANALYSIS REACTION

 The Titrimetric analysis reaction must have the following properties:

1. The reaction must be simple and can be expressed by a chemical equation. The substance whose concentration is to be determined should react completely in stoichiometric proportions with the reagent. 
2. The reaction should be fast.
3. The reaction must undergo some alteration in the physical or chemical properties of the solution at the equivalence point. 
4. There must be an indicator available that can sharply define the endpoint of the reaction via any physical change. In case of no visible indicator, we can use change in absorbance or potential difference change to indicate the endpoint correctly.

TYPES OF TITRIMETRIC ANALYSIS

Based on the reactions involved, the titrimetric analysis can be classified into four main types:

1. NEUTRALIZATION TITRATIONS OR ACID-BASE TITRATIONS
2. REDOX TITRATIONS
3. PRECIPITATION TITRATIONS
4. COMPLEXOMETRIC TITRATIONS

NEUTRALIZATION TITRATIONS OR ACID-BASE TITRATIONS

Neutralization titrations include reactions between Acids and Bases, i.e., a combination of hydrogen and hydroxide ions to form water. There are two titration possibilities:

1.) The titration of free bases, or those formed from salts of weak acids by hydrolysis, with a standard acid which is also called acidimetry.

2.) The titration of free acids with a standard base is called alkalimetry.

In these types of titrations, we try to find the amount of acid or base which is exactly chemically equivalent to the amount of standard solution of acid or base present, which is of known concentration. The equivalence point is indicated by a pH change at the neutralization point, which is detected with the help of either some acid-base indicators or by using a pH meter. 

These acid-base indicator compounds change their color according to the hydrogen ion concentration of the solution. These indicators change from their ‘acid’ color to their ‘alkaline’ color within a small interval of pH (usually about two pH units), which is described as the color-change interval of the indicator. Different indicators have different positions of the color change interval in the pH scale. For acid-base titrations, we select an indicator that exhibits a distinct color change at a pH close to that of the equivalence point of the concerned reaction.

Titrimetric Analysis Examples for such titrations are 

1. HCl-NaOH (Strong Acid-Strong base titrations)
2. HCl-NH3 (Strong Acid-Weak Base)
3. CH3COOH-NaOH (Weak Acid-Strong Base)
4. CH3COOH-NH3 (Weak Acid-Weak Base)

2. REDOX TITRATIONS

These titrations involve reactions that change oxidation number (oxidation or reduction) or involve the transfer of electrons among the reacting substances. The standard solutions in such titrations are either oxidizing or reducing agents. The equivalence points in such titrations are indicated by redox indicators or a potentiometer (to detect the potential change). The standard solution often acts as the indicator, giving a sharp color change at the endpoint when the reaction completes.

The principal oxidizing agents are potassium bromate, potassium iodate, iodine, potassium permanganate, cerium(IV), and potassium dichromate.

 Reducing agents that are generally used include sulphate, mercury(I), iron(II), titanium(III) chloride, sodium thiosulphate, compounds, nitrate, arsenic(III) oxide

Examples for this type of titrimetric analysis are as follows

1. Titration of potassium permanganate (KMnO4) against oxalic acid (C2H2O4).
2. Iodometric titration is also an example of this type of titration in which a solution of iodine is treated with a reducing agent (thiosulfate S2O3-2) to produce iodide, which is indicated using a starch indicator.

3. PRECIPITATION TITRATIONS

These titrations involve the formation of the insoluble precipitates by the reaction of titrant and titrate at the endpoint. The reaction continues until the entire analyte is consumed. No change in oxidation state occurs in these reactions. The amount of precipitation is measured to calculate the amount of the precipitating agent, which in terms can be calculated stoichiometrically to give a concentration of unknown.

Titrimetric Analysis Example for these titrations is Argentometric titration with silver ions titrating with a solution of chloride to form white AgCl precipitate.

AgNO3 + NaCl → AgCl + NaNO3

4. COMPLEXOMETRIC TITRATIONS

These titrations involve the formation of a colored complex compound at the endpoint of the titration. The endpoint is determined by metal-ion indicators or electronically. Indicators that produce unambiguous color change such as  EDTA, Fast Sulphon Black, Eriochrome Black T, Eriochrome Red B, and Murexide are used in such reactions.

Titrimetric Analysis Examples for complexometric titration is titration of a solution of cyanide with silver nitrate (Ag+ + 2CN– → [Ag(CN)2]–) or chloride ion with mercury(II) nitrate solution Hg2+ + Cl– → Hg(Cl)2. Ethylenediaminetetraacetic acid (disodium salt of EDTA) is a very important reagent for complex formation titrations.