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Chemistry

Volumetric Analysis

Volumetric Analysis

Q: 5. What is the difference between the equivalence point and the endpoint in a titration?

This is a critical distinction in volumetric analysis. The equivalence point is a theoretical concept; it is the exact point in the titration where the amount of titrant added is chemically equivalent to the amount of analyte in the sample, according to the reaction's stoichiometry. The endpoint, however, is the practical, observable point at which a physical change, such as a colour change from an indicator, signals that the reaction is complete. For an accurate analysis, the endpoint must occur as close as possible to the equivalence point.

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?

Volumetric analysis, also known as titrimetry, is a quantitative analytical method used to determine the concentration of a substance (the analyte) in a solution. This is achieved by carefully measuring the volume of a second solution (the titrant) of a precisely known concentration that is required to react completely with the analyte. The point of complete reaction is identified using an indicator.

2. What are the major types of titrations used in volumetric analysis?

Volumetric analysis primarily involves four main types of titrations, each based on a different kind of chemical reaction:

Acid-Base Titrations: Used to determine the concentration of an acidic or basic solution by neutralising it with a base or acid of known concentration.
Redox Titrations: Based on oxidation-reduction reactions between the analyte and the titrant. For example, the titration of potassium permanganate (KMnO₄) against oxalic acid.
Precipitation Titrations: Involve the formation of an insoluble precipitate when the titrant reacts with the analyte. Argentometry, which uses silver nitrate, is a common example.
Complexometric Titrations: Rely on the formation of a stable, soluble complex between the analyte (a metal ion) and the titrant (a complexing agent like EDTA). This is often used to determine the concentration of metal ions in a solution.

3. How do you perform a typical calculation in volumetric analysis?

The calculation in volumetric analysis is based on the stoichiometry of the reaction at the equivalence point. For most titrations, especially acid-base, the following formula is used:

M₁V₁/n₁ = M₂V₂/n₂

Where:

M₁ and V₁ are the molarity and volume of the first solution (e.g., the analyte).
M₂ and V₂ are the molarity and volume of the second solution (e.g., the titrant).
n₁ and n₂ are the stoichiometric coefficients (mole ratio) of the analyte and titrant from the balanced chemical equation.

By knowing three of these values, the fourth unknown value (usually the molarity of the analyte, M₁) can be calculated.

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

The fundamental difference between volumetric and gravimetric analysis lies in the property being measured. Volumetric analysis measures the volume of a solution required for a complete reaction to determine concentration. In contrast, gravimetric analysis measures mass, typically the mass of a precipitate formed from the analyte, to determine its quantity. While volumetric analysis is generally faster and requires less complex equipment, gravimetric analysis can often yield more precise and accurate results as mass measurements are not affected by temperature.

5. What is the difference between the equivalence point and the endpoint in a titration?

This is a critical distinction in volumetric analysis. The equivalence point is a theoretical concept; it is the exact point in the titration where the amount of titrant added is chemically equivalent to the amount of analyte in the sample, according to the reaction's stoichiometry. The endpoint, however, is the practical, observable point at which a physical change, such as a colour change from an indicator, signals that the reaction is complete. For an accurate analysis, the endpoint must occur as close as possible to the equivalence point.

6. Why is selecting the correct indicator so important for an accurate titration?

Selecting the correct indicator is crucial because it directly affects the accuracy of the result. An ideal indicator changes colour at a condition (e.g., pH for acid-base titrations) that precisely matches the equivalence point of the reaction. If the wrong indicator is chosen, it might change colour too early or too late. This means the observed endpoint will not correspond to the true equivalence point, leading to an under- or over-estimation of the volume of titrant used and an incorrect calculation of the analyte's concentration.

7. What are some real-world applications of volumetric analysis?

Volumetric analysis is a vital technique used in many industries beyond the school laboratory. Some key applications include:

Environmental Testing: To measure water hardness (calcium and magnesium ion concentration) and determine the concentration of pollutants like chlorine.
Food and Beverage Industry: To determine the acidity of vinegar and fruit juices, the salt content in food products, or the concentration of sulfur dioxide used as a preservative in wine.
Pharmaceuticals: To perform quality control by precisely measuring the concentration of active ingredients in medicines and supplements.
Medical Diagnostics: To analyse the concentration of various substances like glucose or chloride in blood and other bodily fluids.

Volumetric Analysis

Volumetric Analysis Titration in Chemistry

FAQs on Volumetric Analysis

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