Volumetric Analysis questions and answers

1. Explain volumetric analysis.

It is a quantitative analytical method in which volume of one solution of unknown concentration is react with the volume of another solution of known concentration. This process is called volumetric analysis. It is the method which is used to determine the concentration of analyte.

The procedure of volumetric analysis –

 Firstly, prepare a solution of known volume taken in the conical flask.

 Prepare the standard solution of another substance that will react rapidly and completely with the analyte (the solution was taken in a conical flask). Place this standard solution in the burette. The solution taken in the burette is called titrant.

Note – Choosing the right substance that should completely react with the analyte is very important because choosing the wrong substance will give the wrong results.

 Then, slowly add the titrant into analyte until the amount of titrant is added is exactly the amount required to react with the analyte. This point where the amount of titrant completely neutralized the amount of unknown analyte, is called the equivalence point. To detect the equivalence point, add the indicator to the analyte before beginning the titration. This technique is known as titration (or titrimetry).

 This addition of titrant into analyte and indicator continues until the indicator changes color in the reaction. The point where the indicator changes color, is called endpoint that shows the completion of the reaction.

 Note the volume of the standard solution required before and after the titration. The number of moles of titrant can be measured, the molarity of the standard solution is known.

The volume of the unknown solution which is required to react completely with the volume of known strength, then the strength of the unknown solution is calculated by normality equation.

Number of gram equivalent of acid = Number of gram equivalent of base

N1V1 = N2V2

Where N1 and N2 are the normality of acid and base and V1 and V2 are the volume of acid and base.

Weight of substance (g/l) = Normality x gram equivalent weight of substance

Titration – Titration or titrimetry is the chemical analysis which is used to determine the unknown concentration (taken in the conical flask, called analyte) of a solution by the known concentration solution (taken in the burette, known as titrant). In this process, titrant (known solution) is slowly added to the analyte (unknown solution) until the reaction is complete which is indicated by color change.

Titration is classified into four categories based on the type of reaction involved –

 Acid-base titration

 Complexometric titration

 Redox titration

 Precipitation reaction

Acid-Base titration – An acid-base titration is a quantitative analysis of acids and bases in which acid or base of known concentration neutralize the acid or base of unknown concentration. This chemical reaction of acid and base is known as acid-base titration. This type of titration is carried by indicators or pH electrodes. This method is used to identify the purity of chemicals.

Types of acid-base titration

 Strong acid titrated with strong base –

When the strong acid titrated with a strong base, the equivalence point will be at pH = 7 because strong base neutralizes the strong acid. Startlingly, the acid solution has low pH, but as the base is added it will raise the pH.

For example –Let us consider the titration of HCl and NaOH. Phenolphthalein is used as an indicator in this titration which changes color in pH range 8.3-10. It will appear pink in basic solution and clear in acidic solution. Further, the addition of alkali (NaOH) to HCl, pH increases from 3 to 7. Now, acid is completely neutralized.

 Weak acid against strong base –

Let us consider the titration of CH3COOH (weak acid) against NaOH (strong base). Phenolphthalein is used as an indicator and having a pH between 8 to 10. So, this titration shows endpoint between 8 to 10.

CH3COOH + NaOH ——–>CH3COONa + H2O

 Strong acid against weak base –

Let us consider the titration of HCl (strong acid) against NH4OH (a weak base). In this type of titration, the hydrolysis of NH4Cl occurs and pH decreases mean lies in the acidic range. Methyl orange is used as an indicator that changes color in the pH range 6 to 4. Phenolphthalein is not a suitable indicator for this titration.

Complexometric titration (Chelatometry or EDTA titration) – Complexometric titration or chelatometric is the form of volumetric analysis in which metal ion is reacting with a complexing agent or chelating agent (ligand). Ligand is titrant or complex forming reagent or electron donor or Lewis base. So, the reaction between metal and base is called acid-base interaction. Since metal is not titrated with simple ligands such as ammonia or cyanide because of lack of sharp end point.

It is the quantitative chemical analysis where colored complex is used to determine the end point of titration.Titration is very useful for determining the mixture of different metal ions present in the solution but Complexometric titration are used to detect the exact number of metal ions in solution. It is the method where a simple ion is transformed into complex ion and equivalence point can be detected by using metal indicators.

EDTA Complexometric titrations

EDTA (Ethylenediaminetetraacetic acid) is very important reagent consists two amino groups and four carboxyl groups called as Lewis base. It acts as hexadentate ligands which denote six pair of electron due to the formation of covalent bonds. Commonly, the disodium EDTA is used as reagent to standardize the aqueous solution of transition metal.

EDTA has six binding sites that can donate six pair of electrons to metal ion. EDTA is also a weak acid and protonated form of EDTA, H6Y 2+ is hexaprotic weak acid. Commonly, EDTA titrations are carried out at pH = 10 in alkaline medium. It is complex forming ion H2Y 2- in aqueous solution. In this type of titrations, small metal ions lead to change in color which further leads to the formation of weak complex.

Note – Only metals that form stable complexes can titrated in acidic medium and metal form weak complexes can titrated in alkaline medium.

Precipitation reaction –

These are the stoichiometric type of reaction in which analyte or titrant form insoluble precipitate and it continues till the amount of analyte is consumed. In this type of reaction, the end point is detected either by the appearance of excess titrant or by the disappearance of reactant.

 This type of titration is useful in determining the halides (chlorides, bromides, iodides) and pseudohalides ( S2- , HS- , CN- , SCN- ).

 In this titration, standard solution of precipitating agent is used such as silver nitrate (AgNO3) is known as argentimetric titrations. This precipitating reagent should limited because a slow action to form precipitate.

 In this type of titrations, the end point is detected by either the appearance of excess titrant or by the disappearance of reactant.  This type of titrations are fast and reproducible (means no secondary reactions of interference). These titrations are completely depend on the amount of solubility product.

Requirement for precipitation titration –

 PPT should be insoluble

 Formation should be rapid

 End point should be detected properly.

Condition –

Precipitation occurs only when actual ionic product exceeds the solubility product.

Redox titrations –

The titration of reducing agent by an oxidizing agent. This titration involves the reductionoxidation reaction between analyte and titrant means if an atom undergoing oxidation, then there is probably another atom undergoing reduction. The equivalence point is reached after sufficient oxidizing agent has been added to react with all reducing agent. After adding indicator, the color changes between reduced and oxidized states, can be used to detect end point. Redox titration is further classified on the basis of titrant used in the reaction –

 Dichrometry redox titration – in which potassium dichromate is used as titrant.

 Iodometry or Iodometric titration – in which iodine (I2) is used as the titrant.  Permanganometry in which potassium permanganate is used as the titrant.

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