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 qualita ...
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 group is ...
s,
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 formula ...
, 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. The ...
. 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 iso ...
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 cha ...
formed during the fusion. Typically proposed reactions are:
:Na + C + N → NaCN
:Na + C + N + S → NaSCN
:2Na + S → Na
2S
: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 ex ...
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 t ...
. 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 ); plura ...
4-
^ +4Fe^3+ -> cee4e(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 ); plura ...
\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
Sodium phosphate is a generic term for a variety of salts of sodium (Na+) and phosphate (PO43−). Phosphate also forms families or condensed anions including di-, tri-, tetra-, and polyphosphates. Most of these salts are known in both anhydrou ...
. It is boiled with concentrated HNO
3 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 ...
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 component ...
and
lead acetate Lead acetate can refer to:
* Lead subacetate (Basic lead acetate), Pb3(OH)4(CH3COO)2
* Lead(IV) acetate (plumbic acetate), Pb(CH3COO)4
* Lead(II) acetate
Lead(II) acetate (Pb(CH3COO)2), also known as lead acetate, lead diacetate, plumbous acetat ...
is added to it. A black precipitate of
lead sulphide Lead sulfide refers to two compounds containing lead and sulfur:
*Lead(II) sulfide
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is s ...
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
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 is ...
.
S2- + e(CN)5NO2- -> e(CN)5NOS4-
References
External links
Lab Manual from the University of West Indies, Mona
Chemical tests
Sodium
Elemental analysis
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