A quantitative sodium flame test - Journal of Chemical Education (ACS


Describes the modification of a simple spectrophotometer to allow a quantitative sodium flame test. Keywords (Audience):. Second-Year Undergraduate. K...
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A QUANTITATIVE SODIUM FLAME TEST C. C. KIPLINGER Lycoming College, Williamsport, Pennsylvania

A SIMPLE

spectrophotometer was improvised as follows. A square of polaroid was fastened in front of the telescope objective of a Bunsen-type spectroscope. A wedge of cork between the lens mounting and the prism table sufficed to hold it in place. A small disc of polaroid was cut to fit the eyepiece tube and was held in position in the lens mounting. An ordinary brass protractor was soldered to a split tuhe of such size as to fit snugly on the focussing tube of the telescope. A narrow sheet of sheet metal bent about the eyepiece tuhe, with one long arm extending out over the protractor scale to serve as a pointer, completed the instrument (Figure 1). The method of producing the flame was a modification of the qualitative procedure devised by A. Ehringhaus,' in which a roll of filter paper was substituted for platinum wire as a carrier for solutions of flamecoloring substances. Small squares of filter paper, 3 X 3 cm., were used in these tests. Glass tube holders made of 15cm. lengths of 4-mm. (internal diameter) tubing were bent to an obtuse angle of about 30 degrees at 5 cm. from one end. A square of filter paper was rolled about a small nail and inserted half way into the short arm of the bent tuhe. The nail was removed and a 4-cm. length of No. 2 twisted iron picture wire inserted in its place. The wire was bent so as not to slide out of place in the roll, and allowed to project about 3 mm. beyond the paper. This wire heats in the flame and

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accelerates the vaporization of the solution. A Fisher high-temperature burner was used and was protected from drafts by a sheet metal shield. In all of these .tests the same height of flame was found necessary. The tube thus prepared was filled half-full with the Catr. Mineral. Geol., 1919, 192.

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M/10 50 60 70 *.,tractor Reading

80

Figure 2

test solution and placed in a holder made of a stand and an adjustable buret clamp holding a slotted rubber stopper of such size as to firmly grip the long arm of the solution tuhe. The spectroscope slit was placed 15 cm. from the flame. A wide slit covering three divisions of the Bunsen scale was used in these experiments. The calibration curve in Figure 2 was based on the extinction angles of flames produced by solutions of sodium chloride of known molar strength. An approximately linear relationship was established as shown. These results were obtained in a laboratory with ordinary lighting. Using this method a sample of c.p. potassium iodide gave a sodium content of 0.17 + 0.03 per cent. The manufacturer's marking was "Na less than 0.03 %." An interesting test with magnesium sulfate, manufacturer's mark, "0.05 % Na," showed 0.02 per cent; hut when acidified with Na-free HCI, it tested 0.84 per cent. Precautions. The extinction readings must he made rapidly, because the flame intensity increases with the gradual increase in solution concentration due to evaporation. The appearance of crystals on the paper makes the readings useless. The should he inserted at the same height from the burner top, One-tenth molar sodium chloride solution was the strongest that could he used. Stronger solutions should he diluted. B~~~results come in the range indicated by the graph.