Qualitative Organic Analysis
========================
The organic chemist is frequently faced with the problem of characterizing
and identifying unknown organic compounds. The worker in the field of
natural products, for example, has the prospect of isolating such
compounds from their sources in a pure state and then of elucidating their
structures. The preparative organic chemist may encounter new, or
unexpected, compounds in the course of investigations into the applicability
of new reagents or techniques, or as by-products of established reactions.
In each of these instances the elucidation of the structure requires the
identification of the molecular framework, the nature of the functional
groups which are present and their location within the skeletal structure,
and finally the establishment of any stereo-chemical relationships which
may exist . Introduction into this area is provided by a study of qualitative
organic analysis, which is an essential part of the training of the organic
chemist. The purpose is to suggest a possible general scheme which
might be adopted. The approach cannot be as systematic as that
frequently adopted in qualitative inorganic analysis . Each problem is
unique and any scheme of analysis may require considerable modification.
It is this balance between the structured systematic approach and the
intuition which organic chemist acquires with experience which the student
should aim to achieve.
Determination of the physical constants and
the establishment of the purity of the compound
=======================================
For a solid, the melting point is a great importance :
if re-crystallization does not alter it, the compound may
be regarded as pure. Further confirmation of purity may
be obtained by TLC analysis.
For a liquid , the boiling point is first determined :
if most of it distills over a narrow range (say, 1--2 oC ),
it is reasonably pure. It may be desirable to submit a
sample of the liquid to GLC analysis to establish
homogeneity. The refractive index and the density are
also useful constants for liquids.
Determination of Physical Constants
=============================
The most widely used physical constants in the characterization of organic
compounds are melting points and boiling points. In general , a sharp
melting point (say , within 0.5 oC) is one of the most
characteristic properties of a pure organic compound. The purity should
not, however, be assumed but must be established by observation of any
changes in the melting point (or in the melting range) when the compound
is subjected to purification by re-crystallization. If the melting point is
unaffected by at least one re-crystallization, then the purity of the
substance by sublimation at atmospheric or under reduced pressure.
The various methods available for experimental determination of the
boiling point of a liquid, for example sizes ranging down to less than
1 ml. If the liquid is shown by distillation to have a wide boiling range
(10--30 oC), it will be necessary to subject it to fractional
distillation in order to obtain a reasonably pure sample of the compound.
The determination of the refractive index is useful in assisting the
characterization of a pure liquid, particularly in the case of compounds
which do not possess functional groups suitable for conversion into
solid derivatives. The refractive index is conveniently determined with
an Abbe refractometer . An additional criterion of identity is provided
by the density of the liquid , which is determined with the aid of a
pycnometer.
Qualitative Analysis for the Elements
============================
The most commonly occurring elements in organic compounds are carbon,
hydrogen, oxygen, nitrogen, sulphur and the halogen elements; less
common elements are phosphorus, arsenic, antimony, mercury or other
metals which may be present as salts of organic acids. There is no direct
method for the detection of oxygen.
It is usually advisable to carry out the ignition test first. This will provide
useful information as to the general properties of the compound and, in
particular, the residue may be employed for the detection of any inorganic
elements which may be present.
Ignition test :
-----------------------
Place about 0.1 g of the compound in a porcelain crucible or crucible cover.
Heat it gently at first and finally to dull redness. Observe :
(a) Whether the substance melts, is explosive or
is inflammable and note the nature of the flame.
(b) Whether gases or vapor are evolved,
and their odour (Caution!).
(c) Whether the residue fuses.
If an appreciable amount of residue remains, note its color. Add a few
drops of water and test the solution (or suspension) with Universal
indicator paper. Then add a little dilute hydrochloric acid and observe
whether effervescence occurs and the residue dissolves. Apply a flame
test with a platinum wire on the hydrochloric acid solution to determine
the metal present . (In rare cases, it may be necessary to subject a
solution of the residue to the methods of qualitative inorganic analysis
to identify the metal or metals present). If the flame test indicates
sodium, repeat the ignition of the substance on platinum foil.
Heating with soda lime is often a useful preliminary test. Mix thoroughly
about 0.2 g of the substance with about 1 g of powdered soda lime.
Place the mixture in a Pyrex test-tube ; close the tube by a cork and
delivery tube . Incline the test-tube so that any liquid formed in the
reaction cannot run back on the hot part of the tube. Heat the test-tube
gently at first and then more strongly. Collect any condensate produced
in a test-tube containing 2--3 ml of water. Nitrogenous compounds will
usually evolve ammonia or vapors alkaline to indicator paper and
possessing characteristic odours; hydroxybenzoic acids yield phenols;
formates and acetates yield hydrogen; simple carboxylic acids yield
hydrocarbons (methane from acetic acid, benzene from benzoic or
phthalic acid , etc.); amine salts and aromatic amino carboxylic acids
yield aromatic amines; etc.
Carbon and Hydrogen for Qualitative Analysis
===================================
Evidence of the organic nature of the substance may be provided by the
behavior of the compound when heated on porcelain or platinum or other
comparatively inert metal (e.g., nickel) : the substance is inflammable ,
burns with a more or less smoky flame, chars and leaves a black residue
consisting largely of carbon (compare Ignition test above). In general
aromatic compounds characteristically burn with a very smoky flame.
If it is desired to test directly for the presence of carbon and hydrogen in a
compound, mix 0.1 g of the substance with 1--2 g of ignited, fine copper(II)
oxide powder in a dry test-tube , and fit the latter with a cork carrying a
tube bent at an angle so that the escaping gases can be bubbled below
the surface of lime water contained in a second test-tube. Clamp the
test-tube containing the mixture near the cork. Heat the mixture gradually.
If carbon is present, carbon dioxide will be evolved which will produce a
turbidity in the lime water. If hydrogen is present, small drops of water will
collect in the cooler part of the tube.
Nitrogen, Sulphur and Halogens for Qualitative Analysis
=========================================
In order to detect these elements in organic compounds, it is necessary
to convert them into ionisable inorganic substances so that ionic tests of
inorganic qualitative analysis may be applied. This conversion may be
accomplished by several methods, but the best procedure is to fuse the
organic compound with metallic sodium (Lassaigne's test). In this way
sodium cyanide, sodium sulphide and sodium halides are formed, which
are readily identified. Thus :
Organic compound containing :
C,H,O,N,S,Hal + Na =======>>> NaCN + Na2S + NaHal + NaOH
(Heat)
It is essential to use an excess of sodium , otherwise if sulphur and
nitrogen are both present sodium thiocyanate, NaCNS may be produced;
in the test for nitrogen it may give a red coloration with iron(III) ions but
no Prussian blue since there will be no free cyanide ions. With excess of
sodium the thiocyanate, if formed , will be decomposed :
NaCNS + 2Na ===>>> NaCN + Na2S
The filtered alkaline solution, resulting from the action of water upon the
sodium fusion, is treated with iron(II) sulphate and thus forms sodium
hexacyanoferrate(II).
FeSO4 + 6NaCN ====>>> Na4 [Fe (CN)6 ] + Na2SO4
Upon boiling the alkaline iron(II) salt solution, some iron(II) ions are
inevitably produced by the action of the air; upon the addition of dilute
sulphuric acid , thus dissolving the iron(II) and (III) hydroxides, the
hexacyanoferrate(II) reacts with the iron(III) salt producing iron(III)
hexacyanoferrate(II), Prussian blue :
SNa4 [Fe (CN)6 ] + 2Fe2 (SO4 )3 ===>>> Fe4 [Fe (CN)6 ]3 + 6Na2SO4
Hydrochloric acid should not be sued for acidifying the alkaline solution since
the yellow color, due to the iron(III) chloride formed, causes the Prussian
blue to appear greenish. For the same reason, iron(III) chloride should not
be added, as is frequently recommended : a sufficient concentration of
iron(III) ions is produced by atmospheric oxidation of the hot alkaline
solution. The addition of a little dilute potassium fluoride solution may be
advantageous in assisting the formation of Prussian blue in a readily
filterable form.
Sulphur , as sulphide ion, may be detected by precipitation as black color
lead sulphide with lead acetate solution and acetic acid or by the purple
color produced on addition of di-sodium pentacyanonitrosyl ferrate. Halogens
are detected as the characteristic silver halides by the silver nitrate solution
and dilute nitric acid : the interfering influence of sulphide and cyanide ions in
the latter tests is discussed under the individual elements.
Procedure for Qualitative Analysis on Nitrogen, Sulphur, and Halogens
===================================================
Support a small, soft glass test-tube (50 x 12 mm ) in a clamp or insert
the tube through a small hole in a piece of asbestos board (or of "uralite")
so that the tube is supported by the rim. Place a cube
(ca. t mm side = 0.04 g) of freshly cut sodium in the tube. Have in
readiness about 0.05 g of the compound on a spatula or the tip of a knife
blade; if the compound is a liquid, charge a capillary dropper or a melting
point capillary with about three drops of the liquid. Heat the ignition tube,
gently at first to prevent cracking, until the sodium melts and the vapor
rises 1--2 cm in the tube. Drop the substance, preferably portion-wise,
directly on to the molten sodium (Caution : there may be a slight
explosion, particularly with chloroform, carbon tetrachloride, nitroalkanes
and azo compounds). Remove the tube from its support and hold it by
means of a pair of tongs. Heat it carefully at first, then strongly until the
entire end of the tube is red hot and maintain it at this temperature for a
minute or two. Plunge the tube while still hot into an evaporating basin
containing about 10 ml of distilled water, and cover the dish immediately
with a clean wire gauze. The tube will be shattered and the residual
sodium will react with the water. It is advisable to carry out the operation
with the added protection provided by a partly close fume cupboard
window or a safety screen, in addition to the standard eye protection
which is worn at all times. When the reaction is over, heat to boiling ,
and filter. The filtrate should be water-clear and alkaline . If it is dark
colored, decomposition was probably incomplete : repeat the entire
sodium fusion.
The following alternative procedure is recommended and it possesses
the advantage that the same tube may be used for many sodium fusions.
Support a Pyrex test-tube (150 x 12 mm ) vertically in a clamp lined with
sheet cork. Place a cube (ca. 4 mm side = 0.04 g ) of freshly cut sodium in
the tube and heat the latter until the sodium vapor rises 4--5 cm in the
test-tube. Drop a small amount (about 0.05 g) of the substance, preferably
portion-wise, directly into the sodium vapor (Caution : there may be a
slight explosion); then heat the tube to redness for about 1 minute. Allow
the test-tube to cool, add 3--4 ml of methanol to decompose any unreacted
sodium, then half-fill the tube with distilled water and boil gently for a few
minutes. Filter and use the clear, colorless filtrate for the various tests on
Nitrogen, Sulphur, and Halogens. Keep the test-tube for sodium fusions;
it will usually become discolored and should be cleaned from time to time
with a little scouring powder.
Qualitative Analysis testing on Nitrogen
==============================
Nitrogen :
--------------
Pour 2--3 ml of the filtered fusion solution into a test-tube containing
0.1--0.2 g of powdered iron(II) sulphate crystals. Heat the mixture
gently with shaking until it boils, then, without cooling, add just
sufficient dilute sulphuric acid to dissolve the iron hydroxides and give
the solution an acid reaction. {The addition of 1 ml of 5 percent
potassium fluoride solution is beneficial (possibly owing to the
formation of potassium hexafluoroferrate(III), K3 [FeF6 ] )
and usually leads to a purer Prussian blue }. A Prussian blue
precipitate or coloration indicates that nitrogen is present. If no blue
precipitate appears at once, allow to stand for 15 minutes filter
through a small filter and wash the paper with water to remove all
traces of colored solution : any Prussian blue present will then
become perceptible in the cone of the filter-paper. If in doubt, repeat
the sodium fusion , preferably using a mixture of the compound with
pure sucrose or naphthalene. In the absence of nitrogen, the solution
should have a pale yellow color due to iron salts.
If sulphur is present , a black precipitate of iron(II) sulphide is obtained
when the iron(II) sulphate crystals dissolve. Boil the mixture for about
30 seconds, and acidify with dilute sulphuric acid; the iron(II) sulphide
dissolves and a precipitate of Prussian blue forms if nitrogen is present.
Testing for Sulphur
=================
This element may be tested for by either of the following two methods :
1) Acidify 2 ml of the fusion solution with dilute acetic acid, and add a
few drops of lead acetate solution. A black precipitate of lead sulphide
indicates the presence of sulphur.
2) To 2 ml of the fusion solution add 2--3 drops of a freshly prepared dilute
solution (ca. 0.1% ) of di-sodium pentacyanonitrosyl ferrate Na2 [Fe(CN)5NO ].
(The latter may be prepared by adding a minute crystal of the solid to
about 2 ml of water). A purple coloration indicates sulphur; the coloration
slowly fades on standing.
Qualitative Analysis testing for Halogens
=================================
(a) Nitrogen and sulphur absent :
----------------------------------------------
Test (i)
Acidify a portion of the fusion solution with dilute nitric acid and add an
excess of silver nitrate solution. A precipitate indicates the presence of
a halogen. Decant the mother-liquor and treat the precipitate with dilute
aqueous ammonia solution. If the precipitate is white and readily soluble
in the ammonia solution, chlorine is present; if it is pale yellow and difficultly
soluble, bromine is present; if it is yellow and insoluble, then iodine is
indicated. Iodine and bromine may be confirmed by tests (ii) or (iii); these
tests may also be used if it is suspected (e.g., from behavior in the silver
nitrate test) that more than one halogen is present.
Test (ii)
Acidify 1--2 ml of the fusion solution with a moderate excess of glacial
acetic acid and add 1 ml of carbon tetrachloride. Then introduce
20 percent sodium nitrite solution drop by drop with constant shaking.
A purple or violet color in the organic layer indicates the presence of
iodine. The reaction is :
2NaI + 2NaNO2 + 4CH3.CO2H ===>>> I2 + 2NO + 4CH3.CO2Na + 2H2O
Thiis solution may also be employed in the test for bromine. If iodine
has been found, add further additional quantities of sodium nitrite
solution, warm and by means of a dropper pipette remove and replace
the organic phase with fresh portions of carbon tetrachloride; repeat
until the organic phase is colorless. Boil the acid solution until no more
nitrous fumes are evolved and cool. Add a small amount of lead dioxide ,
place a strip of fluorsecein paper across the mouth of the tube and warm.
If bromine is present, it will color the test-paper rose-pink (eosin is formed).
If iodine has been found to be absent use 1 ml of the fusion solution ,
acidify strongly with glacial acetic acid, add lead dioxide and proceed as
above.
In this test for bromine, lead dioxide in acetic acid solution gives
lead tetraacetate which oxidizes hydrogen bromide (and also
hydrogen iodide), but has practically no effect under the above
experiment conditions upon hydrogen chloride :
2NaBr + PbO2 + 4CH3.CO2H ===>>> Br2 + ( CH3.CO2 )2 Pb + 2CH3.CO2Na + 2H2O
Fluorescein test paper is prepared by dipping filter-papers into a dilute solution
of fluorescein in ethanol; it dries rapidly and is then ready for use. The test paper
has a lemon yellow color.
To test for chlorine in the presence of iodine and/or bromine, acidify 1--2 ml of the
fusion solution with glacial acetic acid, add a slight excess of lead dioxide
(say, 0.5 g) and boil gently until all the iodine and bromine is liberated. Dilute ,
filter off excess lead dioxide and test for chloride ions with dilute nitric acid and
silver nitrate solution.
Test (iii)
Acidify 1--2 ml of the fusion solution with dilute sulphuric acid , cool and add 1 ml
of carbon tetrachloride. Prepare the equivalent of "chlorine water" by acidifying
10 percent sodium hypochlorite solution with one-fifth of its volume of dilute
hydrochloric acid. Add this solution drop-wise with vigorous shaking to the mixture .
If iodine is present the organic phase first becomes purple in color. As the addition
of chlorine water is continued, the purple color disappears (owing to oxidation of
iodine to iodate) and, if bromine is present, is replaced by a brown or reddish color.
If bromine is absent, the organic layer will be colorless. It is , of course, evident that
if the carbon tetrachloride layer remains uncolored and the results of test (i) were
positive, the halogen present is chlorine.
Qualitative Analysis testing for Halogens
====================================
(b) Nitrogen and/or sulphur present :
-------------------------------------------------------------------
To remove cyanide and sulphide ions, make 2--3 ml of the fusion solution
just acidic with dilute nitric acid, and evaporate to half the original volume
in order to expel hydrogen cyanide and/or hydrogen sulphide which may
be present. Dilute with an equal volume of water and proceed as in test
(i), (ii) and (iii) as above.
The presence of halogen may be further confirmed by the Beilstein test.
This test serves to detect the presence of halogen in many organic
compounds. It consists in heating the substance in contact with pure
copper oxide in the Bunsen flame : the corresponding copper halide is
formed, which, being volatile, imparts an intense green or bluish-green
color to the mantle of the flame.
Push one end of a 20-cm length of stout copper wire into a cork
(this will serve as a holder); coil the other end by making two or
three turns about a thin glass rod. Heat the coil in the outer mantle
of a Bunsen flame until it ceases to impart any color to the flame.
Allow the wire to cool somewhat and, while still warm, dip the coil
into a small portion of the substance to be tested and heat again in
the non-luminous flame. If the compound contains a halogen element,
a green or bluish-green flame will be observed (usually after the initial
smoky flame has disappeared. Before using the wire for another
compound, heat it until the material from the previous test has been
destroyed and thee flame is not colored.
It has been stated that many halogen-free compounds, e.g., certain
derivatives of pyridine and quinoline, purines, acid amides and cyano
compounds, when ignited on copper oxide impart a green color to the
flame, presumably owing to the formation of volatile copper cyanide.
The test is therefore not always trustworthy. The test is not given by
fluorides since copper fluoride is not volatile.
Qualitative Analysis testing for Fluorine
==============================
Use either of the following tests.
(a) Strongly acidify about 2 ml of the fusion filtrate with glacial acetic acid,
and boil until the volume is reduced by about one-half. Cool . Place one
drop of the solution upon zirconium-alizarin red S test paper. A yellow
color on the red paper indicates the presence of fluoride. Large amounts
of sulphates and phosphates may interfere with this test.
Prepare the zirconium-alizarin red S paper as follows. Soak dry filter-paper
in a 5 percent solution of zirconium nitrate in 5 percent hydrochloric acid
and, after draining, place it in a 2 percent aqueous solution of sodium
alizarin sulphonate (B.D.H. "Alizarin Red S "). The paper is colored
red-violet by the zirconium lake. Wash the paper until the wash water is
nearly colorless and then dry in the air.
(b) If nitrogen and/or sulphur is present , acidify 3--4 ml of the fusion
solution with dilute nitric acid and evaporate to half the original volume
in order to expel any HCN and/or H2S which may be present.
If nitrogen and sulphur are absent, proceed directly with 2 ml of the
sodium fusion filtrate. Render the solution just neutral to litmus by the
addition of dilute (5 M) aqueous ammonia solution, then add 5 drops of
5 M-acetic acid and 20 mg of lanthanum chloroanilate and shake
intermittently for 10--15 minutes. Filter . A pink-violet coloration of the
filtrate is a positive test for fluorine.
Qualitative Analysis testing for Phosphorus
==================================
The presence of phosphorus may be indicated by a smell of
phosphine during the sodium fusion and the immediate production
of a jet-black color when a piece of filter-paper moistened with
silver nitrate solution is placed over the mouth of the ignition tube
after the sample has been dropped on the hot sodium. Treat 1.0 ml
of the fusion solution with 3 ml of concentrated nitric acid and boil
for 1 minute. Cool and add an equal volume of ammonium molybdate
reagent. Warm the mixture to 40--50 oC, and allow to
stand. If phosphorus is present, a yellow crystalline precipitate of
ammonium 12-molybdophosphate, (NH4)3 [PMo12O40 ],
will separate.
It is usually preferable to oxidize the compound directly as follows.
Intimately mix 0.02--0.05 g of the compound with 3 g of sodium
peroxide and 2 g of anhydrous sodium carbonate in a nickel crucible.
Heat the crucible and its contents with a small flame , gently at first,
afterwards more strongly until the contents are fused, and continue
heating for a further 10 minutes. Allow to stand, extract the contents
of the crucible with water and filter. Add excess of concentrated nitric
acid to the filtrate and test with ammonium molybdate reagent as
above. A yellow precipitate indicates the presence of phosphorus. It
must be borne in mind that the above treatment will convert any
arsenci present into arsenate.
Qualitative Analysis testing for Ammonium molybdate reagent
=====================================================
Ammonium molybdate reagent may be prepared by dissolving 45 g of
pure ammonium molybdate in a mixture of 40 ml of aqueous ammonia
(d 0.88) and 60 ml of water and then adding 120 g of ammonium nitric
and diluting the solution to 1 litre with water.
Qualitative Analysis testing for Arsenic
==================================
Th presence of arsenic in an organic compound is generally revealed
by the formation of a dull grey mirror of arsenic on the walls of the
test-tube when the compound is fused with sodium in the Lassaigne
test. Usually sufficient arsenic is found in the fusion solution to give a
yellow precipitate of arsenic trisulphide when the solution is acidified
with hydrochloric acid and treated with hydrogen sulphide.
It is recommended that the compound be fused with a mixture of
sodium carbonate (2 parts) and sodium peroxide (1 part) as the test
for phosphorus. Extract the fused mass with water, filter and acidify
with dilute hydrochloric acid. Pass hydrogen sulphide through the hot
solution; arsenic is precipitated as yellow arsenic sulphide. If antimony
is present, it will be precipitated as orange antimony trisulphide.
Qualitative Analysis testing for Mercury
===============================
Upon heating a mixture of the compound with soda lime in a long test-tube,
a bright metallic mirror and, finally, drops of the metal will form in the upper
part of the tube if mercury is present.
The Sodium Carbonate :
Zinc method for the detection of nitrogen,
sulphur and halogens in organic compounds
========================================
The Lassaigne procedure for detecting nitrogen in organic compounds
frequently gives unsatisfactory results with explosive compounds
(diazonium salts, polynitro compounds and the like) and with certain
volatile nitrogenous substances, such as bases , their acyl derivatives
or their salts. These difficulties may often be surmounted either by
mixing the compound with pure naphthalene or sucrose, or by mixing
the substance with sodium and placing a layer of soda lime above the
mixture. Difficulties are also sometimes experienced in the sodium
fusion test with liquids of low boiling point, such as ethyl bromide.
Satisfactory results are obtained by heating the organic compound
with sodium carbonate and zinc powder (Middleton, 1935). The latter
method has been proposed for the detection of the common elements
in all organic compounds. It is doubtful, however, whether it is to be
preferred to the sodium fusion procedure in routine testing for
elements, although it may be recommended for those relatively few
cases in which the Lassaigne test is not entirely satisfactory.
When an organic compound is heated with a mixture of zinc powder
and sodium carbonate, the nitrogen and halogens are converted into
sodium cyanide and sodium halides respectively, and the sulphur into
zinc sulphide (insoluble in water) . The sodium cyanide and sodium
halides are extracted with water and detected as in Lassaigne's
method, whilst the zinc sulphide in the residue is decomposed with
dilute acid and the hydrogen sulphide is identified with lead acetate
paper. The test for nitrogen is thus not affected by the presence of
sulphur : this constitutes an advantage of the method.
Procedure :
------------------------
Prepare the zinc-powder-sodium carbonate mixture by grinding
together in a dry clean mortar 25 g of anhydrous sodium carbonate
(AnalaR) and 50 g of the purest obtainable zinc powder. The reagent
is unlikely to contain nitrogen, but traces of sulphur and halogens may
be present. It is therefore essential to carry out a blank or control test
for sulphur and halogens with every fresh batch of the mixture.
Place about 0.1 g of the powdered compound in a small dry test-tube ,
add sufficient of the reagent to give a column about 1 cm high and then
shake the closed tube until the contents are well mixed. Now add more
reagent, without mixing with the material already in the tube, until the
total height is about 3 cm. If the compound is a liquid, introduce
2--3 drops into a small dry test-tube, add sufficient of the mixture to
form a column about 1 cm long and allow the liquid to soak well into the
reagent. Then add more reagent, without mixing , until a total height of
about 3 cm is secured. Hold the tube horizontally (use tongs or a special
test-tube holder) and, by means of a small flame, heat a 1-cm length of
the mixture gently near the open end. Gradually increase the size of the
flame until the mixture is red hot at the end. Extend the heating gradually
and cautiously towards the closed end of the tube until the whole of the
mixture is red hot. (The extension of the heating towards the closed end
of the tube must be carried out with great care, otherwise, the mixture
may be projected from the tube ; if the mixture tends to be pushed out
of the tube by evolution of gas, stop the heating momentarily and rotate
the tube while still in a horizontal position in order to redistribute the
contents). Finally heat the tube to redness in a vertical position for a
minute or tow and, while the end of the tube is still red hot, plunge the
tube in about 10 ml of water in a porcelain dish. Boil the contents of the
dish gently for 1--2 minutes and filter. (If the filtrate is not colorless,
repeat the whole process). Retain the residue in the basin for the
sulphur test. Divide the clear filtrate into two portions.
Nitrogen :
-----------------
Treat one portion with 1--2 ml of 5 percent sodium hydroxide solution
and 0.1 g of powdered iron(II) sulphate. Boil for 1 minute and cool.
Cautiously acidify with dilute sulphuric acid (carbon dioxide is evolved).
A precipitate of Prussian blue indicates that nitrogen is present.
Halogens :
------------------
Proceed as described under the Lassaigne test.
If nitrogen is present, the cyanide must be first eliminated.
Sulphur :
------------------
Moisten the centre of a filter-paper with lead acetate solution.
Add about 10 ml of dilute hydrochloric acid to the residue in the
dish and immediately cover it with the prepared filter-paper.
If zinc sulphide is present in the residue, a dark brown stain,
visible on the upper surface of the paper, will be obtained :
frequently the presence of hydrogen sulphide can also
be detected by its color.
Study of the solubility behavior of the compound
======================================
A semi-quantitative study of the solubility of the substance in a limited
number of solvents (water, ether, dilute sodium hydroxide solution,
sodium hydrogen carbonate solution, dilute hydrochloric acid ,
concentrated sulphuric acid and phosphoric acid) will, if intelligently
applied, provide valuable information as to the presence or absence of
certain classes of organic compounds.
When mixture of a specified amount of a given solute and a specified
amount of a given solvent forms a homogeneous liquid, the former is
said to be soluble in the latter. The arbitrary standard employed is
0.10 g of solid or 0.20 ml of liquid to 3.00 ml of solvent. The study of
the solubility behavior of an unknown substance in various liquids, viz.,
water, ether, 5 percent sodium hydroxide solution, 5 percent sodium
hydrogen carbonate solution, 5 percent hydrochloric acid and cold
concentrated sulphuric acid, may provide useful preliminary
information about the nature of the compound.
The substance should be tested for solubility in the various solvents in
the order cited above since, for example, when solubility in dilute acid
or base is being considered, it is important to note whether the
unknown is more soluble in aqueous acid or base than it is in water :
this increased solubility is the positive test for a basic or acidic
functional group. Acidic compounds are detected by their solubility
in 5 percent sodium hydroxide solution. Strong and weak acids are
differentiated by the solubility of the former, but not the latter,
in the weakly basic 5 percent sodium hydrogen carbonate solution.
Nitrogenous bases are frequently detected by their solubility in
5 percent hydrochloric acid. Many compounds that are neutral even
in concentrated aqueous acidic solutions behave as bases in strongly
acidic solvents, such concentrated sulphuric acid; these include
compounds that are neutral in water and contain oxygen in any form.
The presence of acidic or basic functional groups in water-soluble
compounds is detected by testing their aqueous solutions with
litmus or other indicator paper.