An Inexpensive Schtlninger Flask J. R. Sowa Union College, Schenectady, NY 12308 I n these days of greater encouragement to have the students nerform oreanic oualitative analvsis and with ereater emphasis on saflty, it is worth recalfing that there is a n alternative to sodium fusion (Lassaime's test) as a method for elementary analysis. Besides thesodium carhonate/zinc power technique (Middleon, 1935), a n alternative is the Schoninger comhustion method for identifying common elements in organic compounds. T h e prohlem with the latter method has been the need for Schoninger flasks (Thomas Scientific 6513-C200, $345) with its platinum basket. For a good number of years, our students have been using the modified apparatus shown in the figure and have ohtained very reliable results. Reagents (Maximum per Student) The reagents required for analysis are 5 mL 1M sodium hydroxide.. %" mL conc. hvdrochloricacid. 2 mL 2%barium chloride and 0.1 mL 5%silver nitrate, a couple drops of 30%hydrogen peroxide, and one strip of KIIstareh test paper. Equlpment The setup is easily made using a 5W-mL Erlenmeyer flask, no. 10 one-hole rubber stopper, no. 10 plain rubber stopper, a 3-in. glass rod or tube--6 mm-ad. to fit snuggly in the hole of the ruhher stopper, a 10-mL beaker, piece of copper wire--gauge 20-to wrap around the beaker and hooked glass rod and give a separation of about 2 in. between the rod and beaker.. a 2% ashless .- in. hv . %-in. .filter paper, a source of oxygen, and the required reagents as given below. Experimental (J. Haslam et al., Analysf 88, 239 1961) To avoid contamination hy chloride ion, all of the glassware must be washed with distilled water. This combustion method should not be used for any unknowns having boiling points lower than 50 ' C . Intoa 500-mLErlenmeyer flask add 5 mL of 1M sodium hydroxide. Flush out the flask with oxygen by placing a rubber tuhe from the oxygen tank to ahout 1-2 in. from the bottom of flask. Place your hand firmly over the mouthof the flask allowing the tube to pass by your thumb and index finger. Pass oxygen into the flask as such a rate that the oxygen pressure is felt hy the hand and just about makes a noise as it paasees hy the tuhe and your hand. Allow the oxygen to pass inta the flask for 20 s. Turn off the oxygen and rapidly stopper the flask with the plain no. 10 rubber stopper. Preparlng the Sample for Combustion Solid Unknowns Roll 10-40 mg of the unknown into the hottam % in. of ashless filter paper that is 2% hy % in. Too large s piece of paper will consume more oxygen than desired. Make certain the solid can not drop out of the bottom. Place the filter paper in the 10-mL beaker, ignite it, remove the rubber stopper from the flask, rapidly place the beaker in the flask, and gently press the stopper in place. This should be done such that the minimum of air can exchange with the oxygen or the nitrogen test might appear to be positive. Treat the results aa given below. Liquid Unknowns Do not run the test on a liquid that hoila below 50 O C . It burns too rapidly. Place?-4 drops of the unknown in the lower pan of the LOml. beakrr. I'ur the filter paper in the beaker, quickly lighr the upper end, and place it in the flask, gently pressing the stopper into position. 974
Journal of Chemical Education
Schhingr flask. (drawingby Marie C. GuarinielM Working Up the Combusted Unknown
While the flask is cooling gently swirl the contents so that the sodium hydroxide interads with the gasses above it. After 5 min and occasional swirline. wash the sides of the flask with 20 mL of distilled water so there is now 25 mL of basic solution. Remove 5 mL of this solution, place it in a 50-mL beaker, and allow about 5 mm of a test strip of KIIstareh paper to sit in it. Place the paper in the beaker such that it touches the side of the beaker and remains vertical. A drop of water at the upper end of the strip might help it ta adhere to the aide of the heaker. The appearance of a pale blue solution indicates that some nitrogen from the atmosphere got inta the flask hefore combustion, which is a negative test result, as are spotty hlue patches on the test strip. A heavier hlue color indicates the presence of nitrogen in the unknown sample. Halogen Add 2 mL of 1M nitric acid to 5 mL of the test solution. Swirl the contents for about 10s ta lower the concentration of earhon dioxide. Add 2 mL of 5% aqueous silver nitrate. A slightly cloudy solution suggests that chloride ion from the tap water was present. A positive test is usually indicated by the appearance of some solid accumulating at the bottom of the test tube. Ascertain that any quantity of solid is not silver carbonate using the technique of dissolving the solid in ammonium hydroxide and reprecipitate with nitric acid. The color of the precipitate can he used to distinguish between the different halides. Consult a manual for the colors of the various silver halides. Sulfur Add 2 drops of 3000hydrogen peroxide to 5 mL of test solution. (Keep the hydrogen peroxide off your hands. The appearance of white portions of skin indicate that you are bleaching your skin. Wash with water about 2-4 min.) Acidify with 2 mL of 1 M hydrochloric acid until litmus paper indicates that it is just acidic. While shaking, add 2 mL of 1%aqueous barium chloride. Deep cloudiness or a precipiate indicates the presence of sulfur.
