Frederick C. Strong Ill1 University of Bridgeport Bridgeport, Connecticut 06602
improving the Potassium Flame Test
The cIxssicaI flame test for potassium is performed by dipping a platinum wire loop into the sample solution, acidified with HCI, t,hen inserting into the colorless upper portion of a Bunsen flame and observing through (several thiclcnesses of) cobalt blue glass. A coloration, generally described as lavender and of short durat,ion, identifies potassium. Ways of optimizing the results uyith this test are listed below, including some original suggestions by the author. If followed, the test is reliable and sensitive. Increasing Dvop Size. The drop obtained by dipping the loop into the solution is flat a n d small in voiume. One improvement is to use a dropper to place a larger drop on the loop.2 To this, I would add the suggestion of a large loop, e.g., 4 mm i d . Heating to Dryness before Volatili&g. Another improvement is to hold the drop near enough to the flame to evaporate the liquid without volatilizing the salts.z Direct insert,ion of t,he drop into the flame causes spat,. tering and inefficient emission. Slow Healing. If the sample is volatilized slowly, fractional distillation of allcali and alkaline earth salts is possible, thus separating the emis~ions.~This also increases t,he duration of the pot,assium emission, which is short under the best conditions. I have found t,hat, using a heavier gauge wire, e.g., B & S No. 20 (0.81 mm diam.) increases the duration of emission because it. takes appreciably longer to bring the wire t,o white heat. Screening Cobalt Glasses with a Spectro~cope.~Most procedures recommend using several t,hiclcnesses of cobalt glass. Noting variation in the color of our supply of glasses, I checked their absorption spectra by placing a number of t.hem between a tungsten lamp and a Bunsen spectroscope and observing t,he spectrum. There was great variat,ion in t,heir absorption. Those deepest blue in color had a substantially opaque absorpt,ion band from 475 to 704 mp. One thickness of thesc selected glasses is perfectly satisfact,ory; there is no improvement. if more than one is used. Comparison of the glass absorption spectrum with the emission spcctrum of potassium shows that the potas-
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Journal o f Chemical Education
sium lines (404.4, 404.7 mp-violet, and 766, 769 mpred) arc completely transmitted, with good absorption of the sodium doublet a t 5S8.0,589.6 mp. Observation Technique. Using a heavier wire, larger loop, and vaporizing the liquid as described above, hold the selected cobalt glass in front of the cyes and look a t the flame against a black background. Insert the dry wire into the colorless upper port,iori of the flame and watch for a reddish colorat,ion of short duration, e.g., 0.5-1 secS Detection limits of the order of 10 pg I
