Appendix a Analysis by the Blowpipe

Appendix A Analysis by the blowpipe A.I The blowpipe The blowpipe is an invaluable instrument for examining a dry mineral. It consists essentially of a tube bent at right angles, one end having a mouthpiece and the other being terminated by a finely perforated jet. The tube should bulge out between the two extremities into a cavity, where condensed moisture from one's breath may accumulate so as not to be either carried through the jet or deposited on to the assay, which is the name given to that portion of the mineral being tested. It is important that the aperture of the nozzle of the blowpipe should be small and circular. This can be achieved by gently tapping the nozzle on an iron surface, inserting a square needle into the aperture and, by rotation, producing a hole of the required size and shape. The operator will probably experience some difficulty at first in keeping up a steady, continuous blast from the blowpipe, and practice will be needed to enable the instrument to be used easily. While blowing, the cheeks should be kept inflated and the air expelled by their action only, fresh air being drawn in through the nose. Trial and error are the best teachers, and practice should be continued until a steady and uninterrup ted blast can be kept up for several minutes. A gas flame is very convenient for blowpipe experiments, but the flame of an oil or spirit lamp, or a candle, will do. If a lamp or candle is used, the wick should be bent in the direction in which the flame is blown. A portable blowpipe lamp is of particular use in the field. A.2 The two types of flame In blowpipe analysis it is necessary to be able to produce and to recognize two types of flames, the oxidizing flame and the reducing flame. 446 APPENDIX A: ANALYSIS BY THE BLOWPIPE o Figure A.I The oxidizing flame, showing the position of the blowpipe and the points of oxidation (0) and fusion (F). The oxidizing flame An oxidizing flame is produced when the nozzle of the blowpipe is introduced into the flame so that it occupies about one-third of the breadth ofthe flame (see Fig. A.1). It is advisable to blow more strongly than in the production of the reducing flame. The oxidizing flame is blue and feebly illuminating, and complete combustion occurs since the air from the blowpipe is well mixed with the gases from the flame. There are two positions in this flame at which useful operations may be performed. These are the pointoffusion (position F in Fig. A.1) where the hottest part of the flame occurs, and the point of oxidation (position 0 in Fig. A.1) where the assay is heated surrounded by air, and hence oxidation takes place. The reducing flame The reducing flame is produced when the nozzle of the blowpipe is placed some distance from the flame (Fig. A.2). The reducing flame is bright yellow and luminous, ragged and noisy. In this flame the stream of air from the blowpipe drives the whole flame rather feebly before it, and there is little mixing of air with the gases from the flame. The result is that these gases are not completely burnt, and hence will combine readily with the oxygen of any substance introduced into their midst. The assay must, therefore, be completely surrounded by the reducing flame, but not introduced too far into the flame in case a deposit of soot is formed which Figure A.2 The reducing flame, showing the position of the blowpipe and the point ofreduction (R). SUPPORTS 447 will interfere with the heating of the assay. In Figure A.2 the point of reduction is at R. A.3 Supports The portion of the substance under examination (the assay) may be supported in various ways according to the requirements of each particular case. After each experiment, all supports must be thoroughly cleaned before further use. The most common supports are charcoal blocks, platinum-tipped forceps and platinum wire. Charcoal is a good support because of its infusibility, poor heat-conduc ting capacity, and its reducing action. The charcoal block is composed of carbon which readily combines with any oxygen which the assay may contain. Many metallic oxides may be reduced to their metals by heating on charcoal in the reducing flame. Sometimes, charcoal may be used as a support in oxidation, provided that its reducing action does not materially interfere with the results In order to achieve maximum success, the assay should be placed in a small hollow scraped in the charcoal block, and there should be a large area of cool charcoal beyond the hollow on which any encrustations may form. Should the assay crackle and fly about, a fresh assay must be made by powdering the substance being examined and mixing it with water into a thick paste. Points which should be noted include how easily the assay burns or flames, how easily it fuses and whether the fused assay is absorbed by the charcoal. The nature, colour, smell and distance from the assay of any encrustation are all important indicators about the nature of the elements present. Thus, for example, arsenic compounds give an encrustation far from the assay, whereas antimony compounds give an encrustation near the assay. White encrustations or residues, when moistened with cobalt nitrate and strongly reheated, give various colours characteristic of certain elements. Compounds containing lead or mercury or bismuth, give char acteristically coloured encrustations when heated on charcoal with potas sium iodide and sulphur. Section A. 7 gives details of these and other tests. Platinum-tipped forceps are useful for holding small splinters of minerals in the blowpipe flame. When substances are examined in this way, the colour of the flame is important, as also is the degree of fusibility of the mineral compared as far as is possible with the standard scale of fusibility, discussed in Section 2.6. A platinum wire may be used to give excellent results in nearly all the 448 APPENDIX A: ANALYSIS BY THE BLOWPIPE operations usually carried out with the forceps. When using platinum, whether as foil, wire or metal, care should be taken to avoid supporting minerals or other substances suspected of containing iron, lead, antimony or any other metals which can form alloys with platinum. Several elements give distinctive colours to the blowpipe flame, and this flame test is performed by introducing some of the finely powdered mineral, either by itself or moistened with HCl, into the flame on a platinum wire. The several important bead tests are carried out by fusing the mineral with a flux, in a small loop at the end of a platinum wire. A.4 Fluxes Certain substances are added to an assay so that fusion can be attained more rapidly than by heating the mineral on its own. Such substances are called fluxes, and are particularly useful when the constituents of the assay form a characteristic-coloured compound with these substances. The most important fluxes are borax, microcosmic salt and sodium carbonate. Borax (Na2B40s(OHk8H20) is a hydrous sodium borate. As a blow pipe reagent, the greater part of the attached water in the borax is first driven off by heating, and the reagent is then finely powdered. To make a borax bead, the end of a platinum wire is first formed into a loop, which is then heated to redness in the blowpipe flame and immediately dipped into the powdered borax, some of which adheres to the wire. When the loop is heated again in the blowpipe flame, the powder froths up or intumesces because of the disengagement of the water still remaining in it, and gradually fuses to a clear transparent globule, the borax head. The powdered substance to be examined is touched with the hot bead so that a small quantity sticks to it. The bead is then heated by a well sustained blast, and its colour and other characters noted, both when hot and cold, and in both the oxidizing and reducing flames. Some minerals should be added to the bead in very minute quantities, to enable the colour to be noted which otherwise might be masked. Borax changes substances to oxides, and the nature of the substance, to a large extent, can be identified by the colour and other properties of these oxides in the borax bead. Minerals containing arsenic and sulphur dissolve with difficulty in the borax bead, and their behaviour is different from oxides of the same metals. It is therefore advisable to roast the substance on charcoal in the oxidizing flame, before the test is carried out, so that any sulphur or arsenic is volatilized. Microcosmic salt (NaNH4HP04.4H20) is a hydrated ammonium hydro gen phosphate, which is so fluid when it is first fused that it usually drops TUBE TESTS 449 from the platinum wire. Microcosmic salt should therefore either be heated on charcoal or platinum foil until the water and ammonia are expelled, and then taken up on a small platinum loop, or added to the loop in small quantities at a time until the complete microcosmic salt bead is formed. The substance under examination is added in the same way as with the borax bead test, and the whole fused in the blowpipe flame. The action of microcosmic salt is to convert the oxides of metals into complex phosphates, imparting characteristic colours to the bead both when hot and when cold; these colours often differing depending upon whether the bead is produced in the oxidizing or reducing flame.

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