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VI. Colors of Microcosmic Salt Beads in reducing Flame 77

BLOWPIPE ANALYSIS

CHAPTER I

The blowpipe was first applied to mineral analysis in 1733 by Anton Swab, and its applications have since been improved and extended by various chemists, among whom may be mentioned Bergmann, Cronstedt, Gahn, Berzelius, and Plattner.

Blowpipe.—The common blowpipe of the jeweller is not particularly well suited to the operations of blowpipe analysis, since the flame has often to be kept playing upon the assay for some time, and the condensed moisture of the breath would seriously interfere with the passage of the air through the jet. One of the best and least expensive forms of blowpipe is shown in Fig. 1. This consists, as is seen from the illustration, of a conical-shaped tube of tin closed at the wide end and formed into a mouthpiece at the small end; soldered into the tube at the large end, and at right angles to its axis, is a small brass tube which terminates in a conical tip pierced with a very fine hole. With this pipe it is possible to perform all of the operations of mineral analysis.

Some little practice is necessary to keep the flame steady and to take the breath at the same time.

No rule can well be given to the beginner, but his experience becomes his best guide.

Bunsen Flame.—Any kind of flame can be used for the blowpipe, provided it be not too small; but since almost every laboratory to-day is furnished with gas and the Bunsen burner (Fig. 2), it will only be necessary to describe the use of the flame from this source. Upon examining the Bunsen flame with care, it will be seen that the flame consists of three distinct parts.

A dark inner cone which consists of gas not yet raised to the ignition point. Beyond this there is a luminous cone, where combustion is incomplete owing to lack of oxygen, and outside of this we find the non-luminous cone where the gas is completely burned.

This outer envelope is the hottest portion of the flame, and is known as the "oxidizing" flame because there is an excess of oxygen which is imparted to substances placed therein.

The luminous cone is known as the "reducing" flame, for in it metallic oxides are reduced, the oxygen being taken up by the small incandescent particles of carbon.

If the air-holes at the base of the Bunsen burner be opened, the two inner cones become elongated, and the flame appears almost colorless.

The blowpipe enables us to get an oxidizing and a reducing flame of better form and greater power. To do this we cut off the air supply at the base of the burner and turn off the gas until the flame is about 1 cm. high; then upon introducing the blowpipe, and blowing a strong continuous jet of air across the Bunsen flame, we produce an oxidizing flame about 4-5 cm. in length. If the tip of the blowpipe be held outside of the Bunsen flame, and the pressure of the stream of air be diminished, we obtain a reducing flame.

Supports.—For supports, charcoal, platinum, and glass are chiefly used. The charcoal should be made from some light wood, such as alder. It should be well burnt, and should not scintillate or smoke.

The platinum supports are generally in the form of wire and foil. Platinum-tipped forceps are frequently employed in blowpipe analysis.

Glass is used in the form of tubing.

Hard glass tubing, 3 mm. bore, is drawn off into ignition tubes 7-8 cm. in length. Several dozen of these tubes should be made before commencing the tests of the next chapter.

Apparatus.—A small agate mortar, 4-5 cm. in diameter, should be provided in which to grind the samples to be examined.

The pestle, which should also be of agate, must be adapted to the mortar in shape and size.

Two pairs of forceps will also be needed.

One pair should be of steel, and the other pair of brass, with fine points.

Of other apparatus, the most necessary is:—

Chemicals.—A list of the principal chemicals is here given:—

Any other special reagents which may be needed will be mentioned as required.

CHAPTER II

GENERAL OUTLINE OF BLOWPIPE ANALYSIS

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:—

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.