Lassaigne’s Test - Test for Nitrogen, Sulphur, Halogens

Analytical techniques for solving mysteries in matter are based on a very interesting dynamic field of chemistry. Of such simple tools of analysis, Lassaigne's test occupies a foremost place among techniques for the detection of nitrogen, sulphur, and halogens in organic compounds. The test, named after French chemist Jean-Baptiste Lassaigne, has, over so many years, become one of the staple tools in academic laboratories; it is much more relevant than simply academic uses.

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This Story also Contains

1. Understanding Lassaigne's Test
2. Some Solved Examples
3. Summary

Lassaigne's test spans very wide-ranging fields on the practical aspects of applications, such as environmental monitoring and forensic science. The intricacies of Lassaigne's Test present a topic that is intellectually challenging and practically relevant. With this technique at their disposal, a chemist will be in a position to know the elemental composition of substances—very important in the identification of unknown compounds and the determination of quality for materials. Other than this, it gives implications for fast and correct detection of hazardous elements to public health and the environment.

Understanding Lassaigne's Test

Lassaigne's test is the qualitative analysis technique used in identifying some elements, whether combined or not with the organic compound, majorly nitrogen, sulfur, and halogens. The method involves the fusion of the organic compound with sodium metal, whereby these elements are readily converted to water-soluble sodium salts. After that, the mixture is treated with water to extract the sodium salts of these elements for subsequent tests that identify their presence.

The principle of Lassaigne's test is based on the fact that some elements form characteristic compounds with sodium. Nitrogen gets converted to sodium cyanide, sulphur to sodium sulphide, and halogens to sodium halides. These can be identified later on by the addition of certain reagents in an aqueous extract due to observable color change or precipitate formation. This test in organic chemistry is, therefore, of immense value since it provides a very simple and ready method for determining the elemental composition of compounds, which is of use in all fields of research and industrial applications.

Test for Nitrogen

The sodium fusion extract is boiled with iron(II) sulphate and then acidified with concentrated sulphuric acid. The formation of the Prussian blue colour confirms the presence of nitrogen. Sodium cyanide first reacts with iron(II) sulphate and forms sodium hexacyanidoferrate(II). On heating with concentrated sulphuric acid some iron(II) ions are oxidised to iron(III) ions which react with sodium hexacyanidoferrate(II) to produce iron(III) hexacyanidoferrate(II) (ferriferrocyanide) which is Prussian blue in colour.

$
\begin{aligned}
& 6 \mathrm{CN}+\mathrm{Fe}^{2+} \rightarrow\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-} \\
& 3\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}+4 \mathrm{Fe}^{3+} \xrightarrow{\mathrm{xH}_2 \mathrm{O}} \mathrm{Fe}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]_3 \cdot \times \mathrm{H}_2 \mathrm{O}
\end{aligned}
$

Prussian blue

Test for Sulphur

The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. A black precipitate of lead sulphide indicates the presence of sulphur. $\mathrm{S}^{2-}+\mathrm{Pb}^{2+} \longrightarrow \mathrm{PbS}$
On treating sodium fusion extract with sodium nitroprusside, the appearance of a violet colour further indicates the presence of sulphur. $\mathrm{S}^{2-}+\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NO}\right]^{2-} \rightarrow\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}^{4-}\right.$

In case, nitrogen and sulphur both are present in an organic compound, sodium thiocyanate is formed. It gives blood a red colour and no Prussian blue since there are no free cyanide ions.

$
\begin{aligned}
& \mathrm{Na}+\mathrm{C}+\mathrm{N}+\mathrm{S} \longrightarrow \mathrm{NaSCN} \\
& \mathrm{Fe}^{3+}+\mathrm{SCN}^{-} \longrightarrow[\mathrm{Fe}(\mathrm{SCN})]^{2+}
\end{aligned}
$

If sodium fusion is carried out with excess of sodium, the thiocyanate decomposes to yield cyanide and sulphide. These ions give their usual tests.

$
\mathrm{NaSCN}+2 \mathrm{Na} \rightarrow \mathrm{NaCN}+\mathrm{Na} 2
$

Test for Halogens

Lassaigne's test provides a very important link to the detection of the halogens in this technique of analysis. The aqueous extract is then treated with a solution of silver nitrate. This would then be followed by the formation of precipitate characteristics if halogens are present: white in the case of silver chloride, cream-coloured if silver bromide, or yellow if silver iodide. This kind of visual confirmation will allow the chemist to identify the presence of halogens in the organic compound being analyzed.

There are important applications of halogens in pharmaceuticals, agriculture, and other associated industries. For example, Lassaigne's Test identifies the halogen present in pesticides, which has to be known so that these compounds may conform to safety regulations. Testing for halogens in water samples in the case of environmental science would help trace contamination sources, thus making Lassaigne's Test one of the requirements for public health and safety.

The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate, sparingly soluble in ammonium hydroxide shows the presence of bromine and a yellow precipitate, insoluble in ammonium hydroxide shows the presence of iodine.

$\begin{gathered}\mathrm{X}^{-}+\mathrm{Ag}^{+} \rightarrow \mathrm{AgX} \\ \mathrm{X}=\mathrm{Cl}^{-}, \mathrm{Br}^{-}, \mathrm{I}^{-}\end{gathered}$

If nitrogen or sulphur is also present in the compound, the sodium fusion extract is first boiled with concentrated nitric acid to decompose cyanide or sulphide of sodium formed during Lassaigne’s test. These ions would otherwise interfere with the silver nitrate test for halogens.

Test for Phosphorus and Liebig's Test

The Lassaigne's Test is well known for nitrogen, sulphur, and halogens, but it methodology can also be used to test for phosphorus. To do this, fuse an organic compound with sodium, followed by treatment with water, in which phosphates will be extracted. On the addition of ammonium molybdate solution, a yellow precipitate is obtained which is a confirmation test for phosphorus.

Liebig's Test is complementary to Lassaigne's Test, especially for nitrogen. The test works on the principle of heating the organic compound with copper oxide, which is a strong oxidizing agent that oxidizes nitrogen in the form of nitrogen oxides. These are absorbed in a solution of sodium hydroxide and can now be estimated. This generally finds applications in agricultural chemistry where knowledge of nitrogen content in fertilizers is crucial and would lead to the yields of agricultural crops.

The compound is heated with an oxidising agent (sodium peroxide). The phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus.

$\mathrm{Na}_3 \mathrm{PO}_4+3 \mathrm{HNO}_3 \rightarrow \mathrm{H}_3 \mathrm{PO}_4+3 \mathrm{NaNO}_3$
$\mathrm{H}_3 \mathrm{PO}_4+12\left(\mathrm{NH}_4\right)_2 \mathrm{MoO}_4+21 \mathrm{HNO}_3 \longrightarrow\left(\mathrm{NH}_4\right)_3 \mathrm{PO}_4 \cdot 12 \mathrm{MoO}_3+21 \mathrm{NH}_4 \mathrm{NO}_3+12 \mathrm{H}_2 \mathrm{O}$
Ammonium molybdate
Ammonium phosphomolybdate

A known mass of organic compound is heated in the presence of pure oxygen. The carbon dioxide and water formed are collected and weighed. The percentages of carbon and hydrogen in the compound are calculated from the masses of carbon dioxide and water. Estimation of carbon and hydrogen in an organic compound is based on their conversion to $\mathrm{CO}_2$ and $\mathrm{H}_2 \mathrm{O}_{\text {respectively. The percentage of carbon and hydrogen are calculated from the masses of } \mathrm{CO}_2 \text { and } \mathrm{H}_2 \mathrm{O}}$

The apparatus consists of a long glass tube. This is called a combustion tube. To one end of this tube, a U-tube containing anhydrous calcium chloride and a bottle containing concentrated potassium hydroxide solution (caustic potash) are attached in series. These, in turn, are connected to a guard tube containing anhydrous calcium chloride.

The combustion tube is packed with cupric oxide and copper gauze. The other end of the tube has a provision for passing oxygen. The combustion tube is heated in a furnace. The U-tube and the caustic potash bottle are weighed before the start of the experiment. The combustion tube is heated strongly in a current of pure and dry oxygen to remove moisture and $\mathrm{CO}_2$ that may be present.

A known mass of the organic compound taken in a porcelain boat is placed in the combustion tube and strongly heated. The organic compound is oxidized by cupric oxide to$\mathrm{O} \mathrm{CO}_2$ and $\mathrm{H}_2 \mathrm{O}$Nitrogen, if present is also oxidized to oxides of nitrogen. These are reduced back to nitrogen by copper).

The combustion products first pass through the U-tube containing anhydrous calcium chloride (absorption of $\mathrm{H}_2 \mathrm{O}$ ) and then through the caustic potash bottle (absorption of $\mathrm{CO}_2$ ).The U-tube and the caustic potash bottle are weighed after cooling them to laboratory temperature.

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Some Solved Examples

Example 1
Question: If in a test, when sodium fusion extract of an organic compound is boiled with FeSO₄ and then treated with H₂SO₄, a Prussian blue colouration is observed. This colour is due to the presence of which compound?

1) $\mathrm{CuSO}_4$
2) $\mathrm{NO}_3^{-}$
3) $\mathrm{FeSO}_4$
4) $\mathrm{Fe}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]_3$

Solution:

As we have learnt,

Test for Nitrogen :

The sodium fusion extract is boiled with Ferrous Sulphate, and the solution is further acidified and heated with concentrated H₂SO₄. Upon reaction, the formation of Prussian blue confirms the presence of nitrogen.

$\begin{aligned} & 6 \mathrm{CN}^{-}+\mathrm{Fe}^{2+} \rightarrow\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-} \\ & 3\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}+4 \mathrm{Fe}^{3+} \xrightarrow{\mathrm{xH}_2 \mathrm{O}} \mathrm{Fe}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]_3 \cdot \mathrm{xH}_2 \mathrm{O}\end{aligned}$

It is to be noted that some Ferrous ions Fe²⁺ are oxidised to Ferric ions Fe³⁺ upon reaction with conc. H₂SO₄ which acts as the counter ion outside the complex.

Hence, the answer is the option (4).

Example 2
Question: In Lassaigne's test of a sulfur-containing compound, the appearance of violet colour is due to the presence of which compound?

1)$\mathrm{Na}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]$
2)$\mathrm{FeSO}_4$
3) $\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]^{4-}$
4)$\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NO}\right]^{2-}$

Solution:

As we have learnt,

Test for Sulphur:

In Lassaigne's test of Sulphur containing compound, the violet colour is due to the formation of $\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]^{4-}$ upon the reaction of sodium fusion extract with Nitroprusside anion.

The reaction is given below:

$\mathrm{S}^{2-}+\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NO}\right]^{2-} \longrightarrow\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]^{4-}$

Hence, the answer is an option (3).

Example 3
Question: A chemist has 4 samples of artificial sweeteners A, B, C, and D. To identify these samples, he performed certain experiments and noted the following observations:
- A and D both form a blue-violet colour with ninhydrin.
Lassaigne extract of C gives a positive AgNO₃ test and negative Fe₄[Fe(CN)₆]₃ test.
Lassaigne extract of B and D gives positive sodium nitroprusside test.

Based on these observations, identify the samples.

1. A: Alitame, B: Saccharin, C: Aspartame, D: Sucralose
2. A: Saccharin, B: Alitame, C: Sucralose, D: Aspartame
3. A: Aspartame, B: Alitame, C: Sucralose, D: Saccharin
4. A: Aspartame, B: Saccharin, C: Sucralose, D: Alitame

Solution: The correct answer is option 4: A: Aspartame, B: Saccharin, C: Sucralose, D: Alitame.

- A and D give a positive test with ninhydrin because both have free carboxylic and amine groups (characteristic of Aspartame and Alitame).
- C forms a precipitate with $(\mathrm{AgNO_3})$ in the Lassaigne extract because it has chlorine atoms (characteristic of Sucralose).
- B and D give a positive test with sodium nitroprusside because both have sulfur atoms (characteristic of Saccharin and Alitame).

Example 4
Question: Which of the following compounds is added to the sodium extract before the addition of silver nitrate for testing halogens?

1. Nitric acid
2. Ammonia
3. Hydrochloric acid
4. Sodium hydroxide

Solution: The correct answer is option 1: Nitric acid.

Nitric acid is added to the sodium extract before the addition of silver nitrate to eliminate any sulfides or cyanides that might interfere with the halogen test. Nitric acid converts these compounds into volatile forms, which are removed, allowing the accurate testing of halogens in the sample.

Summary

Lassaigne's Test is the fundamental, primary analytical technique applied to determine nitrogen, sulphur, and halogens in an organic compound. The study of methodology and applications would help the chemist in determining constituents of elements in a number of substances. The detection of halogen by the formation of silver halides gives valuable insight into the safety and regulatory aspects that go into chemical substances within industries like agriculture and pharmaceuticals.