Qualitative Analysis is a method of data analysis that seeks to understand phenomena or topics by exploring underlying meanings and patterns. It is used to gain insight into a subject or situation, and is often used to develop hypotheses or to support or disprove a theory.

Quantitative analysis is a method used to determine the quantity of elements or molecules produced during a chemical reaction. Organic compounds consist of carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur and halogens. There are various methods used to measure the percentage composition of elements in an organic compound, which are explained here.

Table of Contents

Detection of C and H

Test for Phosphorous

Liebig’s Combustion Method

Carius Method

Estimation of Sulphur

Estimation of Phosphorus

Estimation of Nitrogen by Dumas Method

Estimation of Nitrogen by Kjeldahl Method

Estimation of Oxygen by Aluise’s Method

Qualitative analysis is the examination of the species contained in a given compound. For example, when a compound is taken, the qualitative analysis would be more focused on identifying the elements and ions present in the compound, rather than on determining the amount of each present.

Detection of C and H

If the presence of C and H is to be confirmed, then the compound should be heated with CuO in a dry test tube. Upon oxidation, CO2 will turn lime water milky, and H2O will turn anhydrous CuSO4 blue.

Detection of Carbon and Hydrogen

Test for Phosphorous

The organic compound is heated with an oxidising agent to oxidise phosphorous to phosphate. The solution is then boiled with concentrated HNO3 and treated with ammonium molybdate. A yellow precipitate confirms the presence of phosphorous.

The reaction is given below:

Na3PO4 + 3HNO3 \rightarrow H3PO4 + 3NaNO3

H3PO4 + 12(NH4)2MoO4 + 21HNO3 → (NH4)3PO4.12MoO3 + 21NH4NO3 + 12H2O

Quantitative analysis is focused on determining the amount of elements that are present.

Also Read

Salt Analysis

Organic Chemistry

Inductive Effect

Purification of Organic Compounds

Lassaigne’s Test

Calculate Molecular Masses Using Victor Meyer’s Method

Estimation of C and H

Liebig’s Combustion Method

The mass of the compound is heated with CuO, resulting in the oxidation of carbon to CO2 and hydrogen to H2O. The CO2 is absorbed in KOH solution while the H2O is absorbed by anhydrous CaCl2. The absorbed substances are then weighed.

Percentage of C = (12/44) × ((Mass of CO2)/(Mass of compound)) × 100

Percentage of H = $\frac{2}{18} \times \frac{(\text{Mass of H}_2 \text{O})}{(\text{Mass of compound})} \times 100$

Liebig’s combustion method

| Also Check Out: Qualitative Analysis of Organic Compounds Viva Questions |

Estimation of Halogens

Carius Method

A known mass of the compound is heated with Conc. HNO3 in the presence of AgNO3 in a hard glass tube called Carius tube. C and H are oxidised to CO2 and H2O. The halogen forms the corresponding AgX. It is filtered, dried and weighed.

Estimation of Halogens using the Carius Method

‘Percentage of X: '

$\frac{Atomic\ Mass\ of\ X}{Molecular\ Mass\ of\ AgX} \times \frac{Mass\ of\ AgX}{Mass\ of\ the\ compound} \times 100$

Calculations:

The mass of the given organic compound is m g.

The mass of AgX formed is equal to m1 g.

1 mol of AgX consists of 1 mol of X.

(\begin{array}{l} Mass~of~halogen~in~m_1~g~of~AgX\=\frac {(Atomic~mass~of~X~\times~m_1~g)}{(Molecular~mass~of~AgX)}\end{array} )

Percentage of halogen $$=\frac {(atomic~mass~of~X~\times~m_1~\times~100)}{(molecular~mass~of~AgX~\times~m)}$$

Estimation of Sulphur

A known mass of the compound is heated with concentrated HNO3 in the presence of BaCl2 solution in a Carius tube. Sulphur is oxidised to H2SO4 and precipitated as BaSO4. It is then dried and weighed.

Percentage of S = ((Atomic mass of S) / (Molecular mass of BaSO₄)) × ((Mass of BaSO₄) / (Mass of the compound)) × 100

Calculations:

Suppose the mass of an organic compound is mg

Let the mass of barium sulphate formed = m1 g

1 mol of BaSO4 contains 32 g of Sulphur

Therefore, 233 g BaSO4 contains 32 g of Sulphur.

$$M_1 ; g ; of ; BaSO_4 ; contains ; \frac{32 ; \times ; m_1}{233} ; g ; of ; sulphur$$

Percentage of sulphur $$=\frac {32~×~m_1~×~100}{233~×~m}$$

Estimation of Phosphorus

A known mass of the compound is heated with HNO3 in a Carius tube, which oxidises phosphorous to phosphoric acid. It is then precipitated as ammonium phosphomolybdate ((NH4)3PO4.12MoO3) by adding NH3 and ammonium molybdate ((NH4)2MoO4). It is filtered, dried and weighed.

Percentage of P = %

((Atomic mass of P) * (Mass of (NH4)3PO4.12MoO3)) / ((Molecular mass of (NH4)3PO4.12MoO3) * (Mass of compound)) x 100

Estimation of Nitrogen

Dumas Method for Estimating Nitrogen

The reaction of a known mass of the compound heated with CuO in an atmosphere of CO2 yields free nitrogen, CO2, and H2O.

xC${2}$H${y}$N${z}$ + (2x + 0.5y)CuO → xCO${2}$ + 0.5yH${2}$O + 0.5z(N${2}$) + (2x + 0.5y)Cu

The nitrogen oxides are passed over a hot copper gauze to convert trace amounts to N2. The gaseous mixture is then collected over a solution of KOH which absorbs CO2, with the nitrogen being collected in the upper part of the graduated tube.

Estimation of Nitrogen by Dumas Method

The volume of N2 collected is V1 mL.

Then, volume of N2 at STP = $$\frac{P_1V_1 \times 273}{760 \times T_1} = V_{mL}$$

Where P1 and V1 are the pressure and volume, respectively, of N2.

Atmospheric Pressure vs. Aqueous Tension

22.4 L of N2 weighs 28 g

Therefore, V mL of N2 weighs

\frac{28 \times V}{22400} \text{ grams}

Percentage of N would be:

100 x (28/22400) x (V/M)

Kjeldahl Method for Estimating Nitrogen

A known mass of 0.5 g of an organic compound is mixed with 10 g of K2SO4, 1.0 g of CuSO4, and 25 mL of conc.H2SO4, and heated in a Kjeldahl’s flask.

The resulting mixture of CuSO4, K2SO4 and (NH4)2SO4 is reacted with an excess of NaOH solution. The NH3 so evolved is then passed into a known but the excess volume of standard acid. CuSO4 acts as a catalyst, while K2SO4 raises the boiling point of the sulphuric acid, and the nitrogen in the compound is quantitatively converted to (NH4)2SO4.

The percentage of nitrogen can be calculated by estimating the amount of acid left unreacted through titration with a standard alkali.

Estimation of Nitrogen by Kjeldahl Method

The reactions are listed below:

1. C + H + S \rightarrow CO_2 + H_2O + SO_2

2. N + 8H₂O → 2(NH₄)₂SO₄

2NaOH + (NH4)2SO4 → 2NH3 + Na2SO4 + 2H2O

4. 2NH3 + H2SO4 → (NH4)2SO4

Calculating the Percentage of N

The mass of the organic compound is mg.

Volume (V) of H2SO4 (Molarity M) = mL

The volume of NaOH of Molarity M used for titration excess of H2SO4 = V1 mL

mEq of excess H2SO4 = mEq of NaOH

MV1 mEq

Total mEq of H2SO4 taken = 2MV

Molar Equivalence of H2SO4 used for Neutralisation of NH3 = (2MV - MV1)

Henceforth,

M_\mathrm{eq}(\ce{NH3}) = \frac{2M_\mathrm{V} - M_\mathrm{V1}}{1}

1000 mEq or 1000 mL of NH3 solution contains = 17 g of NH3 (or) 14 g of N

Consequently,

(2MV-MV1) mEq of NH3 contains

= (14 x (2MV - MV1)) / 1000 g of N

% of N = ((14 * (2MV - MV1)) * (100 / (1000 * m)))

Qualitative Analysis of Organic Compounds Using Kjeldahl’s Method

Estimation of Oxygen Using Aluise’s Method

A known mass of the compound is decomposed by heating it in the presence of N₂ gas. This mixture of gases is then passed over red hot coke, so that all the O₂ is converted to CO. The mixture is then heated with I₂O₅, where CO is oxidised to CO₂, and I₂ is liberated.

The reactions are listed below:

Organic compound → Other gaseous products + O2

2C + O2 → 2CO

I2O5 + 5CO → 5CO2 + I2

Percentage of O = ((Molecular mass of O2 / Molecular mass of CO2) $\times$ (Mass of CO2 / Mass of the compound) $\times$ 100)

Frequently Asked Questions (FAQs)

Qualitative analysis is a method used to observe and describe the characteristics of something, such as behavior or outcomes. It is often used in the social sciences to gain a deeper understanding of underlying reasons, opinions, and motivations.

Qualitative analysis determines the percentage composition of elements present in a compound. Elemental Analysis is used to estimate the amount of carbon and hydrogen present.

Liebig’s combustion method is used to estimate carbon and hydrogen, with the hydrogen present being oxidized to water and the carbon present being oxidized to carbon dioxide. For the estimation of halogens, the Dumas Method is used.

The Carius Method is used to estimate halogen. Halogens form a precipitate when mixed with silver nitrate.

Lower reduction potential of an element implies that the element gets oxidized easily. Which method is used for the estimation of nitrogen?

The catalyst used in Kjeldahl’s method for the estimation of nitrogen is sulfuric acid.

CuSO4 is used as a catalyst in Kjeldahl’s method for the estimation of nitrogen.