R. 1. Neman1
Dodecyl Sodium Sulfate as a Reagent
Southwest Texas Stote College
San Marcos
for the Detection of Potassium
Of
the tests commonly performed in the freshman laboratory for the detection of potassium ion, the quickest and perhaps most popular is based upon the color and intensity of the lavender flame produced by the potassium ion. The flame is usually viewed through several thicknesses of "cobalt glass" to eliminate the interference of the sodium flame.2 Several precipitation reactions are also known, all of which involve compounds whose sodium form is soluble and whose potassium form is insoluble. Sodium ~ohaltinitrite,~ sodium tetraphenylb~rate,~ and chloroplatinic acid5will all precipitate the potassium ion. One ot,her reagent has been developed which can be used for t.he qualitative detection of the potassium ion. This proposed test involves the addition of an aqueous solution of dodecyl sodium sulfate, CHa(CH2),,OSO3Na, to precipit.ate dodecyl potassium sulfate, CH,(CH,),, OSOaK. The precipitate obtained in the above reaction is a silky, finely divided,white precipitate which decomposes a t llO°C and is similar in appearance to mercury(1) chloride. It forms slo~vlyat room temperature (25'C), hut will fnim quickly if the reaction mixture is chilled in ice. Dodecyl potassium sulfate is not insoluble enough ( K , = 1.5 X to be used as the precipitating form for the gravimetric determination of potassium, but it is suitable as the confirmatory test for the potassium ion, and is easily incorporated into the standard freshman qunlit,ative analysis scheme. The presence of K+, found in Group V, along with NH4+, Na+, and AIg2+ (and sometimes other cations such as Li+, Rb+, and Cs+) can be detected by following the procedure outlined below. 1. To 4 nr 5 drops of the Group V known oi unknown or to the original general unknown, add 4 draps of eonc. HCI and evaporate to dryness. Take up in 10 draps of conc. HCI. A yellow flame confirms Nn +. 2. To 3 d r o ~ of s the C;I.OUDV known or unknown. or to the original general unknown, adh 3 drops of NrtOH. lmmedistely place 1 drop of Nessler's reagent on the tip of a clean stirring rod and lower the rod into the test tube just above the contents. An orange precipitate in the Nessler's reagent confirms N&+. ~
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'Present address: Lmedo Junior College, Laredo, Texas. MCCOY,ROBERTE., J. CHEM.EDUC.,42, 444 (1965). 'ARTHUR, PAUL,A N D SMITH,0.IM., "Semimicro Qualitative Analysis," The McGraw-Hill Book Co., New York, 1942, p 267. 'CLIFFORD,A. F., "Inorganic Chemistry of Qualitative Analysis," Prentice-Hall, Inc., Englewood Rliffs, N.J., 1961, p. 76. ' BLAEDEL,W. J., A N D MELOCHE, V. W., "Elementary Quantitative Andgsis," Harper and Raw, Publishers, New York, 1963, p. 212.
3. If the solution to be tested is from Group IV, add 1 drop of 4 M ammonium oxalate and 1drop of 1 M smmonium sulfate. Separate and discard m y precipitate that may form. Use 1 ml of the solution from Group IV or 1 rnl of the Group V known or unknown. Add 1 drop of dil. HCI. Then add 4 M NH*OH until the solution is alkaline to litmus, and then add 1 drop in excess. Separate into two parts. (a) To part 1, add 2 drops of 1 M . NalHPOd. The formation of a white precipitate of MgNH4P04 confirms .Mg'+. ( b ) To the second part, add an equal volume of dadecyl sodium sulfate solution. Chill in sn ice bath for 2 3 min. Centrifuge. The formation of s white precipitate of dadecyl potassium sulfate confirms K+. If a white precipitate has not formed in 2-3 min do not report K+.
Several features of this scheme of analysis should be emphasized. First, the additiou of ammonium oxalate and ammonium sulfate is necessary to insure that the ions of Group IV (Ba2+,Ca2+,and Sr2+)do not remain in solution, as these three ions give a white precipitate in the presence of dodecyl sodium sulfate solution. Second, the solution must be made alkaline to insure the precipitation of dodecyl potassium sulfate, which is soluble in an acid medium. If the test tube containing the dodecyl sodium sulfate solution and the nuknown is left in the ice bath too long, the sodium form of the salt may crystallize out of solution, especially if it is present in large amounts. The solid dodecyl sodium sulfate has a more granular texture than does the potassium form, and is easily distinguishable from the potassium salt. The author has found that a solution of about 20 g dodecyl sodium sulfate per liter of water is concentrated enough to give a definite precipitate if potassium ion is present, and yet dilute enough to prevent the crystallization of dodecyl sodium sulfate within about a 5-min period of exposure to the ice bath. Barium, calcium, and strontium ions are not the only other cations that will give precipitates with the dodecyl sodium sulfate solution. In addition, the Group V elements rubidium and cesium will interfere with the test, each producing a white precipitate with the dodecyl sodium snlfate. Pbz+, Bi3+, Fe3+, A13+, and Tl+ will also react with the dodecyl sulfate anion in a neutral or slightly alkaline medium. The dodecyl sulfate of A13+ and K + will dissolve- in acidic medium; the others are ~ ~ i the t amh ~ monium ion (which will interfere with the test for K + when either sodium cohaltinitrite or chloro~latinicacid is the ureciuitating reagent) nor the G r o u ~V element, lithium, wifi affectthe &st &hen dodecyl sodium sulfate is used. Dodecyl sodium sulfate was used as the qualitative precipitating reagent for K+ in the second-semester freshman laboratory sections in the spring semester, 1966, a t Southwest Texas State College, San Marcos, Texas. I n these lahoratories, 91y0 of the students correctly determined the presence or absence of K+ in Volume 44, Number 8, August 1967
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their unknowns using dodecyl sodium sulfate. The 8% who reported K+ when they did not have it in their sample probably left their test solution in the ice bath
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long enough to crystallize the dodecyl sodium sulfate. Only 17, erroneously reported that they did not have K+ in their unknown.
