The Blue Glass Filter in the Flame Test for Potassium CRARLES E. WHITE U n i w r s i t y of M o y l o n d , College Pork, Maryland
manuals of general chemistry MANY mstruct . laboratory the student to observe the potassium
through the glass cannot be depended on as an identification of potassium because concentrated sodium salts give a similar effect. The source of the blue color from sodium salts is discussed a t some length by MeUor.' Potassium is easily distinguished from sodium by o b s e ~ n g ,through the blue glass, the bright red flame surrounded by a blue envelope. The blue glass may be tested by direct observation or by the spectroscope to see if i t transmits the red. In the latter case, to avoid the inconvenience of a potassium flame, a mercury vapor lamp may be used. This gives a red line in the vicinity of that of potassium. The G. E. H4 lamp with a blue bulb used for ultraviolet work is excellent for this purpose. An interesting observation on the potassium flame may be made with didymium glass such as is used in glass blowers' goggles or in the Corning filter No. 5120. This glass adsorbs all of the yeUow and part of the red and since i t is a pink glass, the student is not skeptical about the color's being due to the glass. Through this glass potassium gives a beautiful violet flame; however, since a large portion of the red is adsorbed, it does not serve as a good means of identifying potassium in the presence of sodium. I t is also worth mentioning that the carbonate of potassium seems to remain on a platinum wire longer than the chloride and hence serves better as a testing solution.
flame through a blue glass filter in order to distinguish this element from sodium. If any indications of results are given, i t is usuaUy stated that potassium gives a violet or lavender color through the glass. Experience with students indicates that very few make the correct observation and the chief fault lies in the blue glass and in the lack of explanation in the manual. The flame spectrum of potassium contains lines of two colors, the violet a t about 4044 and 4047 A. U. and the red a t 7665 and 7699 A. U. In the spectroscope the red is far easier to see and is generally used for identification. It is the red also that is useful in distinguishing the potassium flame through a blue glass. The blue glass must be of such a nature that the yellow of sodium is adsorbed and the red of potassium is transmitted. A few years ago we examined the so-called "cobalt" glasses used in our laboratory and found that only about 50 per cent of the entire stock would serve in this test; the remainder completely absorbed the red line of potassium. This finding was affirmed by a leading. laboratory supply house and we were assured that hereafter all glasses sold by the fum for this purpose would be examined spectroscopicauy. Students using the poor glass in previous years must have depended a great deal upon their imaginations. All of the blue glasses tested transmitted the violet line of potassium but unless the red line was also trans- ' MELLOR."A Comprehensive Treatise on Inorganic and mitted, they were useless for the identification in the Theoretical Chemistry." Longmans, Green and Company. New presence of sodium. The violet flame appearing York.1922, vol. 2, p. 464.
