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Volumetric Analysis

Chemistry

Volumetric Analysis

Reviewed by:

Ritika Singla

Volumetric Analysis Titration in Chemistry

The analytical method wherein the concentration of a substance in a solution is estimated by adding exactly the same number of equivalents of another substance present in a solution of known concentration is called volumetric analysis.
This is the basic principle of titration. Another name for volumetric analysis is titrimetric analysis.

The substance whose solution is used to estimate the concentration of the unknown solution is called titrant. Titrate is the substance whose concentration is to be estimated.

The volumetric analysis can be classified into three types:

1. Simple titration

2. Back titration

3. Double titrations

SIMPLE TITRATION

The main aim of simple titration is to find the concentration of an unknown solution with the aid of the known concentration of another solution. Simple titration can again be classified into four different types:

• Acid-base titrations

• Redox titrations

• Precipitation titrations and

• Complexometric titrations

Acid-base titrations

This type of titration is used to find the concentration of an acid in a solution (or vice-versa). The estimated value of acid in a solution can be determined by adding a solution of standard base (or vice-versa). A suitable indicator is added drop-by-drop to the solution whose concentration is to be estimated. This helps in detecting the equivalence point.

An acid-base indicator gives different colors of different compounds. An indicator is chosen in a particular titration based on the pH-range of the indicator and the pH change near the equivalence point.

Redox Titrations

An oxidant can be estimated by adding reductant in redox titrations (or vice-versa). An example of this is; Fe2⁺ ions can be estimated by titration against acidified KMnO₄ solution when Fe²⁺ ions are oxidized to Fe³⁺ ions and KMnO4 is reduced to Mn²⁺ in an acidic medium. Also, KMnO₄ is a self-indicator. At the equivalence point, it can be noticed that it discharges a purple color.

Mn + 8H⁺ + 5e^– → Mn²⁺ + 4H₂O
Fe²⁺ → Fe³⁺ + e^–] × 5
Mn + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
(n=5) (n=1)

Acidified KCr2O7 can also be employed instead of KmnO₄. Other redox titrations are iodimetry, iodometry etc.

Iodimetry

Here, the process of titration involves free iodine. There is a direct estimation of iodine. Such a titration is called iodimetry. The process basically involves the titration of iodine solution with a known amount of sodium thiosulphate solution.

I₂+ 2Na₂S₂O₃ → 2NaI + Na₂S₄O₆

Iodometry

This is known to be an indirect method of estimation of iodine. Here, an oxidizing agent is made to react with an excess of solid KI. The oxidizing agent oxidizes I^– to I₂. The next step is to make the liberated I₂to react with Na₂S₂O₃ solution.

Precipitation Titrations

In a titration of this form, a compound of very low solubility is formed by combining cations and anions. A solid residue can be separated out.

For example,

AgNO₃ + NaCl → AgCI¯ (white) + NaNO₃
BaCI₂ + H2SO4 → BaSO₄¯ (white) + 2HCI

Complexometric Titrations

The titrate combines with the titrant which results in the formation of complex salts in this type of titration. The complex salts may or may not be soluble. For example,

CuSO₄ + 4NH₄OH → [Cu (NH₃)₄] SO4 + 4H2O
AgNO₃ + 2KCN → K [Ag (CN)₂] + KNO₃

Back Titration

Let us make an assumption that we have an impure solid substance ‘C'. It is required to calculate the percentage purity of ‘C' in the given sample. Two solutions ‘A' and ‘B', where the concentration of ‘B' is known and that of ‘A' is unknown is also given.

Certain conditions have to be satisfied for back titrations to work:

• Compounds ‘A’, ‘B’ and ‘C’ which are provided should satisfy the condition that ‘A’ and ‘B’ react with each other.

• ‘A’ and pure ‘C’ should also react with each other but the impurity present in ‘C’ is not supposed to not react with ‘A’.

• Another important condition is that the product of ‘A’ and ‘C’ should not react with ‘B’.

Now, a certain volume of ‘A' in a flask is taken out (the equivalents of ‘A' taken has to be greater than equivalents of pure ‘C' in the sample) and then a simple titration using ‘B' is performed.

Double Titrations

These type of titrations are done to determine the percentage composition of an alkali mixture or an acid mixture.

To find the percentage composition of an alkali mixture, consider a solid mixture of NaOH, Na₂CO₃ and some inert impurities, weighing ‘w’ g. The aim is to find the % composition of this alkali mixture. An acid reagent (HCI) of known concentration M1 that can react with the alkali sample is also provided. The mixture is firstly dissolved in water to make an alkaline solution and then two indicators are added, namely phenolphthalein and methyl orange to the solution. The alkaline solution is titrated with standard HCI.

Na₂CO₃ is a weak base while NaOH is a strong base. So it is obvious that NaOH reacts with HCI first completely and Na₂CO₃ reacts only after NaOH is completely neutralized.

NaOH + HCI → NaCI + H₂O …..(i)

Once NaOH has reacted completely, then Na2CO3 starts reacting with HCI in two steps, shown as

Na₂CO₃ + HCI → NaHCO₃ + NaCI ……. (ii)
NaHCO₃ + HCI → NaCI + CO₂ + H₂O …… (iii)

When HCI is added to the alkaline solution, alkali is neutralized. This results in the pH of the solution getting decreased. The pH decreases initially at a rapid pace since the strong base (NaOH) is neutralized completely. Whereas when Na₂CO₃ is converted to NaHCO₃ completely, the solution remains weakly basic. This is due to the presence of NaHCO₃(which is weaker as compared to Na₂CO₃). It is at this point, where it is noticed that phenolphthalein changes color since it requires a weakly basic solution to show the color change.

When HCI is further added, the pH decreases even more and when all the NaHCO₃reacts to form NaCI, CO₂ and H₂O the solution becomes weakly acidic since the formation of the weak acid (H₂CO₃) occurs. At this point, methyl orange changes color as it requires a weakly acidic solution to show the color change.

Thus, in conclusion, phenolphthalein changes color when the solution contains weakly basic NaHCO₃ along with other neutral substances while methyl orange changes color when the solution contains weakly acidic H₂CO₃along with other neutral substances.

PRECAUTIONS

a) To get accurate values, one must take special care to clean all the apparatus with distilled water. Even the slight presence of any other chemical will lead to mistakes in the result.

b) While adding an acid/base, it should be done drop-wise.

c) Contamination should be avoided as much as possible.

d) While taking the readings, make sure that the markings are at your eye level.

e) In the case of colorless solutions, the lower meniscus is read whereas the upper meniscus is read for colored solutions.

f) For pipetting any solution, always use the index finger.

g) Make sure that the burette is not leaking and that there are no air bubbles trapped inside it

h) The indicator should never be used in excess.

i) Take extra care to fill the pipette to get accurate readings.

FAQs on Volumetric Analysis

1. What is volumetric analysis? Who first found this method?

The method of volumetric analysis is used first to determine the concentration or amount of nitrogen in any given organic compound. This method is found by Jean-Baptiste-Andre-Dumas, a french chemist. Volumetric analysis is defined as the quantitative analytical method in which the volume of solution (titrant or reactant) of known volume and concentration is measured which is consumed in a chemical reaction. The measured volume is used to determine the amount or concentration of the unknown chemical in a solution (titrate or analyte) of known volume only. Also, measuring or analyzing the volume of a solution (of known volume and concentration) to determine or find the concentration of another solution is known as volumetric analysis.

2. How many types of volumetric analysis can be classified in Vedantu point of view?

According to Vedantu study concepts, volumetric analysis or titrimetric analysis is classified into three types. These three types which are pointed out at Vedantu are

Simple Titration: This type of volumetric analysis is the way to determine the concentration of an analyte or titrate with the help of a reactant or titrant.

Back Titration: Back titration is used to find the percentage purity of any solution with the help of two solutions, let’s say A and B (the concentration of A is known whereas the concentration of B is unknown).

Double Titration: Double titration is used to determine the percentage proportion of acid or base solution. Double titration is also an analytic method used to analyze the volume of an unknown substance (acid or base).

3. Is the principle of volumetric analysis noted at Vedantu? How to calculate volumetric analysis?

Vedantu noted and pointed out that volumetric analysis is a chemical method to determine the concentration of a solution (let’s say A, analyte) with the help of another solution whose concentration is known (let’s say B, titrant).

Add solution B dropwise (filled in burette) in solution B in the presence of an indicator.

As the indicator indicates the endpoint by changing the color to solution A stop adding solution B.

Measure the volume of solution B with the help of titration which is consumed to complete the chemical reaction.

Student can note formula to calculate the concentration of solution A from Vedantu site such as

Ca = Cb.Vb.M/Va where Ca is the concentration of solution A, Cb is the concentration of solution A, Vb is the volume of solution A consumed in the reaction, M is the mole ratio of titrant or reactant and the analyte and Va is the volume of solution A after the endpoint.

4. What is the difference between gravimetric and volumetric analysis?

Gravimetric analysis is based on the determination of mass (atomic mass) and density of the analyte solution while volumetric analysis is used to find the volume and concentration of the analyte solution with the help of titrant solution. Gravimetric analysis has some accuracy and provides precise value over volumetric analysis as the mass of solvent is independent of temperature, and fewer standards requirements reduce the risk of instrumental error. The rate of volume analysis is greater than gravimetric analysis as it requires less calculative standards and gives rapid results.

5. What are the limitations of volumetric analysis? Discuss precautions with limitations

Volumetric analysis is used to determine the volume of analysts and for accurate or precise results some error cause action should be avoided. The errors could take place during the finding endpoint (changed color indicates the equivalent point) if we add more drops of titrant after the ending point. Also, some instrumental error could occur if we do not find the least count of burette and pipette correctly. To avoid this type of error note the least count of the burette and every measuring instrument. Also do not keep the solution (analyte and titrant) openly in the surroundings as the solution can evaporate and it will lead to error in determining the result with accuracy.

Volumetric Analysis

Volumetric Analysis Titration in Chemistry

FAQs on Volumetric Analysis

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