GENERAL OUTLINE OF BLOWPIPE ANALYSIS
[ABBREVIATIONS: O. F. for oxidizing flame, R. F. for
reducing flame, Ch. for charcoal, Ct. for coating, Bp. for
blowpipe.]
In order to examine a substance before the blowpipe to determine the
presence or absence of certain elements, it becomes necessary to arrange
a systematic method. As with all branches of chemical work, one's
success is largely dependent upon neatness of manipulation and
carefulness of observation.
The following order of observation is essentially that given by
Berzelius:--
1. Examination on charcoal by itself.
2. Examination on charcoal with Na_{2}CO_{3}.
3. Examination in ignition tube with Na_{2}CO_{3} and
charcoal.
4. Examination on platinum wire.
5. Examination in borax bead.
6. Examination with Co(NO_{3})_{2}.
After having examined a body in these six different ways, we shall be
able to say what are its principal constituents.
Before describing the method of carrying out these six different
operations, it will be necessary to give a few definitions of terms
which we shall have frequent occasion to employ.
~Definitions.~--_Ignition_ is the heating of a substance to a high
temperature.
_Fusion_ is the heating of a substance to the melting-point.
_Intumescence_ is the swelling of the substance upon heating.
_Decrepitation_ is the crackling of a substance due to the sudden
expansion of combined water upon heating.
_Deflagration_ is the burning of a substance with explosive violence,
generally due to excess of oxygen.
_Incandescence_ is the white light emitted by a substance that is
infusible when subjected to a high temperature.
~Examination on Charcoal alone.~--The size of the assay should be about
that of a mustard seed. This is sufficiently large to show all of the
reactions clearly, and though a larger piece would exhibit the
characteristic phenomena, yet much more effort is required. A very
small, shallow hole should be cut in the Ch. to receive the assay. The
Bp. flame should be directed at an angle of about 30 deg. with the
surface of the Ch. Considerable care must be taken lest the hole in the
Ch. is burned too deep and the assay lost in the coal.
The force of the air from the jet must also be borne in mind for a
strong blast, or sudden puffs may blow the substance away.
The following changes are to be looked for:--
_a._ Whether the substance is volatile or non-volatile.
_Illustrations._ Examine before the Bp. on Ch. some arsenious oxide,
As_{2}O_{3}, also some alumina, Al_{2}O_{3}.
_b._ Whether the substance is fusible or infusible.
_Illustrations._ Examine before the Bp. on Ch. some silver oxide, AgO,
also some zinc oxide, ZnO.
_c._ Whether the substance is alkaline or non-alkaline when placed upon
moistened red litmus.
_Illustrations._ Ignite some calcium carbonate, CaCO_{3}, before the Bp.
on Ch., and place residue on moistened red litmus. In like manner,
examine some magnesium carbonate, MgCO_{3}.
_d._ Color of coating on Ch. caused by combination of metal and oxygen
due to heat of Bp. flame.
_Illustrations._ Examine some oxide of lead, PbO, before the Bp. on Ch.,
also some oxide of cadmium, CdO.
_e._ Decrepitation.
_Illustration._ Examine some sodium chloride, NaCl, before the Bp. on
Ch.
_f._ Deflagration.
_Illustrations._ Examine some potassium nitrate, KNO_{3}, before the Bp.
on Ch., also some ammonium nitrate, NH_{4}NO_{3}.
_g._ Intumescence.
_Illustration._ Examine some alum,
K_{2}Al_{2}(SO_{4})_{4},
before the Bp. on Ch.
_h._ Incandescence.
_Illustration._ Examine some oxide of barium, BaO, before the Bp. on Ch.
_i._ Formation of a metallic bead--color and malleability.
_Illustration._ Examine some silver oxide, AgO, before the Bp. on Ch.
~Examination on Charcoal with Na_{2}CO_{3}.~--Metallic compounds are often
difficult to reduce with the blowpipe flame alone, and hence no bead is
obtained. In order to facilitate reduction and the obtaining of a
metallic bead, the substance in a finely powdered condition is mixed
with four parts of sodium carbonate, Na_{2}CO_{3}, and ignited before
the Bp. on Ch. The metallic compound is decomposed, the metal being
transformed into the carbonate, which in turn, through the agency of the
Ch. and the heat of the flame, is reduced to the free metal. Sometimes
the reduction is made easier by adding to the substance about its own
bulk of potassium cyanide, KCN, which takes up oxygen from the compound
and is converted into potassium cyanate, KCNO.
The reactions in reducing copper sulphate, CuSO_{4}, with Na_{2}CO_{3}
and with KCN before the blowpipe, are here given:--
CuSO_{4} + Na_{2}CO_{3} = CuCO_{3} + Na_{2}SO_{4} } (1)
2CuCO_{3} + C = 3CO_{2} + 2Cu }
CuSO_{4} + Na_{2}CO_{3} = CuCO_{3} + Na_{2}SO_{4} }
CuCO_{3} = CuO + CO_{2} } (2)
CuO + KCN = Cu + KCNO }
After obtaining beads, it is well to obtain their coatings, for
oftentimes it is only in this way that we can distinguish between the
metals.
~Examination in Tube with Na_{2}CO_{3} and Charcoal.~--If the substance in
a finely pulverized condition be mixed with twelve parts, Na_{2}CO_{3},
and six parts of charcoal powder and the mixture be placed in an
ignition tube and subjected to heat, the acid of the substance combines
with the soda and the metal is set free.
If this metal is volatile, a sublimate is formed in the upper end of the
tube.
Mercury deposits in minute globules, which may be seen with the
magnifying glass. Arsenic forms a ring, which, when examined with the
magnifying glass, is seen to be made up of minute crystals. Ammonia is
recognized by its characteristic odor, and also by its turning a slip
of moistened red litmus (held over the mouth of the tube) blue.
~Examination on Platinum Wire.~--Many substances possess the property of
imparting to the colorless flame of the Bunsen burner characteristic
colors.
The chlorides of these substances exhibit these flame reactions best,
and hence before applying the flame tests we dip the wire which serves
as a support into hydrochloric acid and then into the substance. When
the substance has been taken up on the wire, it is placed in the edge of
the long colorless flame of the Bunsen burner near the apex, when
instantly the flame becomes tinged with the characteristic color of the
substance.
_Illustrations._ Sodium compounds color the flame yellow, and a crystal
of potassium dichromate appears colorless in the sodium light.
This sodium reaction is extremely delicate, it being possible to detect
with ease a quantity of a sodium salt less than 1/3000000 of a milligram
in weight.
Potassium colors the flame purplish-violet.
Barium colors the flame apple-green.
Strontium colors the flame crimson.
Calcium colors the flame orange-red, distinguished from strontium, by
appearing gray when seen through blue glass.
Boracic acid colors the flame green when the substance has been
moistened with glycerine.
~Examination in Borax Bead.~--Borax, Na_{2}B_{4}O_{7}, and microcosmic
salt,
NaNH_{4}H . PO_{4},
possess the property of dissolving many of the metallic oxides at the
temperature of the Bunsen flame.
For example, with oxide of cobalt, the following reactions take place
with the two fluxes:--
CoO + Na_{2}B_{4}O_{7} = Co(BO_{2})_{2} + 2 NaBO_{2}.
On heating, NaNH_{4}H. PO_{4}, it is decomposed into the metaphosphate
of sodium, NaPO_{3},
CoO + NaPO_{3} = CoNaPO_{4}.
Now in such cases of solution the metallic oxides impart a
characteristic color to the flux.
The platinum wire is the best support,--it is heated to incandescence in
the Bunsen flame, and then is quickly dipped into the borax, when a
small globule will adhere,--this is removed to the flame again when the
borax melts to a clear glassy bead. While the bead is still melted,
touch it to the finely pulverized substance and replace in the flame.
In a few seconds the small particles of the substance will have
dissolved, and the bead will be seen to have assumed the color
characteristic of the substance. Note the color when hot and then when
cold; often there is a wide difference. Then, too, the test should be
made in both O. F. and R. F.
Some analysts prefer to make a small loop in the end of the wire before
taking up the borax to make the bead. Care should be taken to see that
the bead is colorless before bringing it in contact with the substance.
As the depth of color produced is largely dependent upon the amount of
substance taken, some little caution should be exercised to insure
taking up about the same quantity each time.
_Illustrations._ Make several beads, and note the colors characteristic
of the following oxides: cobalt, nickel, iron, manganese, chromium, and
copper.
The microcosmic salt bead dissolves almost every oxide except silica,
SiO_{2}, and this is seen to float about in the melted mass. This is
used as a test for silica.
~Examination with Co(NO_{3})_{2}.~--If after examination on the Ch. _per
se_, a white infusible residue remains, it is moistened with a drop of
cobalt nitrate Co(NO_{3})_{2} and re-ignited before the Bp., when a
change of color will be observed. This change in color is owing to the
fact that the heat of the Bp. flame decomposes the cobalt nitrate,
nitric acid being driven off, and the remaining CoO forming with the
oxide of the residue a colored mass.
_Illustrations._ Ignite before the Bp. on Ch. the following
oxides,--allow to cool, add a drop of Co(NO_{3})_{2}, re-ignite, and
note color,--aluminum, magnesium, zinc, and calcium.
Care should be taken to thoroughly ignite before adding the cobalt
nitrate solution.
With the six methods of examination just given almost every simple
substance can be detected, but should any doubt remain, a few simple
tests in the "liquid way" will be sufficient to substantiate the
blowpipe examination.