the microscale laboratory
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An Ammonia Fountain in a Micropipet Dianne N. E m East High ~chobl 1000 So. 70th Lincoln, NE 68510
The great solubility of ammonia gas in water has proved a popular lecture demonstration in the form of a n ammonia fountain.',' A variation on this, which allows each student to prepare a microscale version of the ammonia fountain, has proved effective as a follow-up laboratory exercise to the lecture demonstration. Safetv Precautions .Safety goggles are required and students are reminded of the proper technique of wafting fumes to check for odor. Excess ammonia solution in the micropipets is disposed of in a closed container. Procedure A small amount of concentrated aqueous ammonia is placed in a covered beaker under the fume hood, or in a small bottle with a screw cap if a hood is not available. Each student fills a long-stemmed plastic pipet with an elongated tip half-full from this stock solution for use a s a gas generator. (Because of the small elongated tip of the pipet, very little ammonia odor is noted in the laboratory.) At their lab stations. students prepare a 250-mL beaker as a warm water bath 7 5 4 0 .C: as inhicated by theappearanceofbubbles collectineon thcinside wallsofthe beaker,. A 100-mL beaker is alsoprepared with cold water and set aside. The tip of the generator pipet is inserted loosely into the bulb of a short pipet..which has beencut so that as ammonia fills the bulb of the short pipet, the air can escape without huild-up of pressure. The bulb of the longer pipet is immersed in the hot water bath to warm the aqueous ammonia. As it is warmed, NHg gas is driven off as indicated by vigorous bubbling of the aqueous ammonia. The shorter pipet should be pumped several times to expel the air and allow it to fill with ammonia. After several minutes, the longer pipet is removed from the hot water and the short pipet is detached. The odor of the collected ammonia may be observed readily by wafting a small amount of it from the tip of the pipet towards the student. The solubility of the collected ammonia gas is now tested by placing the tip of the short pipet into a small beaker of cold water. If the pipet is squeezed gently to expel one bubble of ammonia, a very small amount of water enters the pipet, which immediately fills with water a s the re-
'Shakashiri. Bassam 2. Chemical Demonstrations. Vol. 2;Univers of w SCO& n. Mao son, w , 1985. %Jmmer n. Lee R Ea y, -ames R. Jr Chem!ca!Demonsfrabons VOI 12no ed.;ArnercanChemcalSociety: Wasn ngton. D.C.. 1988.
Micropipets. Generator Micropipet (above):pull out the end slightly to form a taper and cut so that the generator has tapered tip. Collecting Micropipet (below):cut at an angle so that the tip is about 1 cm in length. maining ammonia dissolves, dramatically reinforcing the highly soluble nature of ammonia in water. The experiment mav be repeated easily using the same generato;forseversl trials. ~ b n e w a ~ u e o ~ s a m m o r u a n c e d hr added and the same collcctlng plpet may hr used if i t 1s thoroughly shaken to remove ali
