Qualitative Organic Analysis

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.