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 Na₂CO₃.

3. Examination in ignition tube with Na₂CO₃ and charcoal.

4. Examination on platinum wire.

5. Examination in borax bead.

6. Examination with Co(NO₃)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° 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, As₂O₃, also some alumina, Al₂O₃.

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, CaCO₃, before the Bp. on Ch., and place residue on moistened red litmus. In like manner, examine some magnesium carbonate, MgCO₃.

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, KNO₃, before the Bp. on Ch., also some ammonium nitrate, NH₄NO₃.

g. Intumescence.

Illustration. Examine some alum,

K₂Al₂(SO₄)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 Na₂CO₃.—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, Na₂CO₃, 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, CuSO₄, with Na₂CO₃ and with KCN before the blowpipe, are here given:—

CuSO₄	+	Na₂CO₃	=	CuCO₃	+	Na₂SO₄	}	(1)
2CuCO₃	+	C	=	3CO₂	+	2Cu	}

CuSO₄	+	Na₂CO₃	=	CuCO₃	+	Na₂SO₄	}
CuCO₃			=	CuO	+	CO₂	}	(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 Na₂CO₃ and Charcoal.—If the substance in a finely pulverized condition be mixed with twelve parts, Na₂CO₃, 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, Na₂B₄O₇, and microcosmic salt,

NaNH₄H . PO₄,

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 + Na₂B₄O₇ = Co(BO₂)₂ + 2 NaBO₂.

On heating, NaNH₄H. PO₄, it is decomposed into the metaphosphate of sodium, NaPO₃,

CoO + NaPO₃ = CoNaPO₄.

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, SiO₂, and this is seen to float about in the melted mass. This is used as a test for silica.

Examination with Co(NO₃)₂.—If after examination on the Ch. per se, a white infusible residue remains, it is moistened with a drop of cobalt nitrate Co(NO₃)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(NO₃)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.