The sodium fusion test, or Lassaigne's test, is used in

elemental analysis Elemental analysis is a process where a sample of some material (e.g., soil, waste or drinking water, bodily fluids, minerals, chemical compounds) is analyzed for its elemental and sometimes isotopic composition. Elemental analysis can be qualit ...

for the qualitative determination of the presence of foreign elements, namely

halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this grou ...

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

, and

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

, in an

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...

. It was developed by J. L. Lassaigne. The test involves heating the sample with

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

metal, "fusing" it with the sample. A variety of techniques has been described. The "fused" sample is plunged into water, and the qualitative tests are performed on the resultant solution for the respective possible constituents.

Theory

The halogens, nitrogen, and sulfur are covalently bonded to the organic compounds are converted to various sodium

salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...

formed during the fusion. Typically proposed reactions are: :Na + C + N → NaCN :Na + C + N + S → NaSCN :2Na + S → Na₂S :Na + X → NaX The fate of the

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

portion of the sample is disregarded. The aqueous extract is called sodium fusion extract or Lassaigne's extract.

Test for nitrogen

The sodium fusion extract is made alkaline by adding

NaOH Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...

. To this mixture, freshly prepared FeSO₄ solution is added and boiled for sometimes and then cooled. A few drops of FeCl₃ are added and

Prussian blue Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe CN)">Cyanide.html" ;"title="e(Cyani ...

(bluish green) color forms due to formation of ferric ferrocyanide along with

NaCl Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...

. This shows the presence of nitrogen in the organic compound.

6CN- +Fe^2+ ->
e(CN)6 E, or e, is the fifth letter and the second vowel letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''e'' (pronounced ); plur ...
4-

^ +4Fe^3+ -> cee4
e(CN)6 E, or e, is the fifth letter and the second vowel letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''e'' (pronounced ); plur ...
\cdot xH2O

Test for halogens

The sodium fusion extract is boiled with concentrated HNO₃ followed by the addition of AgNO₃ solution which yields a white ( AgCl) or yellow ( AgBr or

AgI Silver iodide is an inorganic compound with the formula Ag I. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a gray coloration. The silver contamination arises because AgI is hig ...

) precipitate if halogen is present.

NaX + AgNO3 -> AgX + NaNO3

Test for phosphorus

Sodium peroxide Sodium peroxide is an inorganic compound with the formula Na2O2. This yellowish solid is the product of sodium ignited in excess oxygen. It is a strong base. This metal peroxide exists in several hydrates and peroxyhydrates including Na2O2·2H2O ...

is added to the compound to oxidise phosphorus to sodium phosphate. It is boiled with concentrated HNO₃ and then

ammonium molybdate Ammonium molybdate can refer to: *Ammonium orthomolybdate, (NH4)2MoO4 *Ammonium heptamolybdate, (NH4)6Mo7O24, usually encountered as the tetrahydrate *Ammonium phosphomolybdate, (NH4)3PMo12O40 *Ammonium tetrathiomolybdate, (NH4)2MoS4 this chemical i ...

is added. A yellow precipitate indicates the presence of phosphorus.

Test for sulphur

Lead acetate test

The sodium fusion extract is acidified with

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

and

lead acetate Lead acetate can refer to: * Lead subacetate (Basic lead acetate), Pb3(OH)4(CH3COO)2 * Lead(IV) acetate Lead(IV) acetate or lead tetraacetate is an organometallic compound with chemical formula . It is a colorless solid that is soluble in nonpo ...

is added to it. A black precipitate of lead sulphide indicates the presence of sulfur.

Sodium nitroprusside test

Freshly prepared

sodium nitroprusside Sodium nitroprusside (SNP), sold under the brand name Nitropress among others, is a medication used to lower blood pressure. This may be done if the blood pressure is very high and resulting in symptoms, in certain types of heart failure, and d ...

solution is added to the sodium fusion extract, deep violet colouration appears due to formation of sodium thionitroprusside.

S2- +
e(CN)5NO E, or e, is the fifth Letter (alphabet), letter and the second vowel#Written vowels, vowel letter in the Latin alphabet, used in the English alphabet, modern English alphabet, the alphabets of other western European languages and others worl ...
2- -> e(CN)5NOS4-

References

External links

Lab Manual from the University of West Indies, Mona
Chemical tests Sodium Elemental analysis {{analytical-chem-stub