WHAT STARTS PRECIPITATION from a SUPERSATURATED SOLUTION?' An Example of Freshman Research
JOHN DUNNING AND PHILIP PRATT, WITH 0. E. LOWMAN Iowa State College,Ames, Iowa
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N THE course in qualitative analysis which oc-
cupies the time assigned to chemistry during the third quarter of the freshman year a t Iowa State College, magnesium is precipitated as magnesium ammonium phosphate. This salt is supposed to form supersaturated solutions and directions advise: shaking, scratching the inside of the test-tube, and scratching the outside of the test-tube. Crystal formation is ascribed to: seeding with particles of glass, the edges of the scratch on the tube, the vibrations of the tube, and the agitation of the solution. Two of the young men enrolled in the class during the spring quarter of the year 1933 lived in the same town, and had no job for the summer, but did have a chemistry laboratory in a basement a t home. They asked if they might try
'Condensed by F. E. Brown from a comprehensive report prepared by the late Dr. Lowman. Dr. Lowman died suddenly an February 10th last after an illness of only a few hours. A report of this work was presented at the 13th Midwest Regional Meeting of the A. C. S. at Kansas City, Mo., M a y 4, 1934.
to determine what started the precipitation of magnesium ammonium phosphate from its supersaturated solutions. The students were loaned one carton of new testtubes, some 5-cc. pipets, some test-tube racks, a supply of 0.01 M solution of MgC12.6H20,and a 0.01 M solution of (NH&HPOI which contained NHICl and NHIOH. Preliminary experiments showed that mixtures of equal volumes of these two solutions always produced an immediate precipitate, and therefore magnesium ammonium phosphate is not likely to form supersaturated solutions unless stabilized. Large excesses of ammonium chloride delayed the precipitation only momentarily, but Rochelle salts did produce a considerable delay in the formation of a precipitate. Enough of this salt was added to the solutiou of ammonium phosphate so that a precipitate appeared in an nudisturbed mixture of the two solutions after about one and one-half minutes.
NOVEMBER, 1934
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The directions given to the students were as follows: an average of 52.5 seconds. Rate of precipitation (a) clean all glassware with H C ~and rinse with vaned directly as: (1) the amplitude of the vibration, (2) the duration of the vibration, (3) the intensity distilled water. of the vibration, and (4) the frictional surface in the (6) A~~~~~~ten ,.lean test.tubes in a row in a rack. (c) Pipet into each test-tube 5 CC. of MgCl2.6&o tube. Slow precipitation ~roduced larger crystals. The precipitate always started where the string touched solution containing 0.96 mg. of MgC12 per cc. (a) pipet into each containing M g ~ ~ 2the , tube. Microscopic examination showed that all were "ystaUine. precipitates 5 cc. of the (NH&HP04 and NH,OH mixture Attempts were made to add powdered glass to the containing 1.32 mg. of (NH&HPO6 per cc. mixture and to add the mixture to powdered glass, hut (e) Record time a t instant of end of second delivery in every case and also at instant of end of any the experimenters were unable to distinguish between the powdered glass and the precipitate. No data are other operation. for this Cf) Note and record time a t first indication of formaBecause they decided that a file would scratch better, tion of precipitate (use black backthe insides of the tubes were scratched with a file inground). (g) Repeat ten experiments on each variation below: stead of with a glass When the tubes were scratched before the solutions were added, (1) Allow to remain still. than in un(2) Shake vigorollsly for 5 seconds and let stand. the precipitates formed no more scratched tubes. When the scratches were made in the (3) shake vigorously 5 seconds, observe 5 solutions the time until precipitation varied from onds, shake 5 seconds, etc. 11 to 21 seconds with an average of 14.3 seconds. An (4) Vibrate with rosined bow and let stand. (5) ~ d several d pieces of finely powdered glass anscratched, unshaken blank required 77 seconds for precipitation. The fie may have become seeded with and let stand. fine crystals. (6) Scratch new test-tube with glass rod before When a glass rod was rubbed up and down on the addition of solutions and let stand. (7) scratch inside of test-tube gently with glass outside of the test-tube, the time until precipitation was from 1 to 5 seconds, except for one sample which rerod and let stand. (8) Scratch outside of test-tube with glass rod quired 37 Collodion was not available and paraffin formed an and let stand. (9) Coat inside of test-tube with thmcoating of Opaque layer, therefore no experiments were made in coated tubes. collodion and let stand. Experiment proved that blowing the breath through (10) Coat inside of test-tube with collodion as in the solution of magnesium chloride produced no pre(9) and repeat (2), (3), (4). (5), cipitate, so this method of agitation was tried. When and (8). (In each instance close tube with cork stopper coated breath was blown through the mixture the time until precipitation varied from 5 to 26 seconds with an averwith collodion.) age of 12 seconds. The test-tube rack had a capacity for eight tubes, so eight experiments of each type were made instead EXPERIMENTS WITH SOLmIONS STABILIZED BY THE of ten. When the mixture stood still the time until a ADDITION OF TARTARIC ACID general precipitation occurred varied from 110 to 64 seconds. The average time was eighty seconds. The It was thought that slower precipitation might imexperimenters ascribe the steady decrease in time for prove the accuracy. To secure slower precipitation successive experiments to their increasing skill in de- Dunning's solution, named in honor of its suggester, tecting precip~tates. In three cases what might have was made by adding to each 100 cc. of the solution of been crystals appeared on the walls of the tube in 30 to the ammonium acid phosphate, 100 cc. of 7 M tartaric 40 seconds. acid and 100 cc. of ammonium hydroxide of speciiic When the tubes were shaken five seconds and allowed gravity. 0.9. to stand, the time until precipitation varied from 5 to 35 An undisturbed mixture of Dunning's solution with seconds with an average of 20 seconds. The variation an equal volume of the solution of magnesium chloride in time is ascribed to variation in violence of the shak- required from 96 to 341 seconds for precipitation. ing, which was done by hand. A rotary stirrer made by mounting a glass rod on a When the tubes were shaken five seconds and ob- Polar Cub motor (kitchen mixer) was used for agitating served five seconds alternately until precipitation oc- samples of this better-stabilized mixture. The stirrer curred, the time until precipitation varied from 11to 29 was mounted rigidly. A wooden block was bored to seconds with an average of 19.5 seconds. receive a test-tube and mounted in such a way that it Attempts to make the tube vibrate with a rosined could be charged with solution and then raised in a bow were unsuccessful, but a string of a contra bass was frame. The frame held the block in such a position bowed and the tube touched to the string. The time that the stirrer entered the solution but could not until precipitation varied from 4 to 117 seconds, with touch the test-tube. The stirrer was cleaned with
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hydrochloric acid and distilled water after each experiment. Five series of experiments were made. When the stirrer was run until precipitation occurred the time until precipitation was from 19 to 46 seconds with an average of 37 seconds. When the stirrer ran three seconds, the time until precipitation was from 62 to 110 seconds with an average of 73 seconds. When the stirrer ran five seconds, the time until precipitation was from 28 to 91 seconds with an average of 48 seconds. When the stirrer ran ten seconds the time until precipitation varied from 24 to 65 seconds with an average of 34 seconds. When the stirrer ran fifteen seconds the time until precipitation varied from 19 to 53 seconds with an average of 31 seconds. The students are convinced that agitation starts
precipitation of magnesium ammonium phosphate from its supersaturated solutions. No one type of agitation is more effective than another. The rate of formation of the precipitate depends on the intensity of the agitation. They also believe that the sizes of the crystals are affected by the intensity of the vibration, and that more intense vibration produces smaller aystals. This paper is not presented as a contribution to the solution of the question which constitutes its title. It is an example of what college freshmen, a hundred miles from their teachers, can do and may do in learning to apply the scientific attitude in answering their own questions.
