DEMONSTRATION OF DYNAMIC NATURE OF IONS USING 1131 RICHARD F. BLAKE Stratford High School, Stratford, Connecticut
THE
dynamic nature of ions in solution is particularly difficult for high-school chemistry students to comprehend. For this reason a demonstration was developed to present visual evidence of the ionic nature of solid salts and the dynamic equilibrium existing between dissolved and undissolved ions. The type of fundamental reaction chosen for this demonstration may be illustrated by: AgI"' + (ImAgI,Ial + (IL2')-(solid)
(solution)
-
(sol~d)
(solution)
This reaction was chosen because I'3Lis readily available' to any secondary-school teacher in 10 microcurie quantities and because Agl is considered "insoluble," having a solubility constant of 1.0 X 10WL6. Apparently this reaction has not previously been used for this purpose. However, in 1939, PolenssitskyZ studied exchange rates of radio bromide ion with AgBr and radio iodide ion with A d in aaueous solution a t room temnerature. ~ p p a r e n t l this i exchange is dependent upon the freshness of the solid silver halide. DEMONSTRAPON
The following tested procedure is recommended for secondary schools and should give good results in the time allotted to the usual laboratory period. Ten microcuries of NaILa'solution were diluted to 30 ml. and divided evenly among three small test tubes. The original solution contained 10 microcuries in 10 cc.
' Abbott Laboratories,
Oak Ridge Division, Oak Ridge, Ten-
DPRRPB.
Bio-Rad Laboratories, Rsdiochemicals Division, 800 Delaware Street, Berkeley, California. Nuclear Consultants, Ine., 3361 Crescent Street, Long Island City, New York. Isotopes Specialties Company, 3816 San Fernando Road, Glendale, California. The author has found the local hosoitals (Boston. Massaohusetts, and Bridgeport, Connecticut) to be the most convenient source of IIaL. a POLENSSITSKY, A,, Compt. rend. acad. sei. U.R. 8. S., 24,540 (1939).
of carrier-free 5.2X 10-'M sodium iodide solution. The amounts in the test tubes were checked with a radiation counter to make certain that they radiated equal counts and that the count was convenient for the capacity of the counter used. The counter was Marrion Instrument Company Model GM 9 calibrated against radium in milliroentgen/hour. Test tube No. 1 mas used as a control and was also used to demonstrate the half life of Pa'. To test tuhes No. 2 and No. 3, 200 me. of freshly prepared AgI12' was added. Test tuhe No. 2 was shaken thoroughly between each reading and No. 3 was handled carefully to prevent agitation to show the difference in the rate of ion exchange caused by mixing. Readings were taken a t the bottom, center, and top of each test tuhe every 15 minutes for a two-hour period. Readings were most selective when a shielding device was made which allowed a section of the test tube to be counted. The author used sheet lead 'hinch thick, rolled to surround the test tube, and cut a 1-cm. X 3-cm. window into the side. The background reading was taken and subtracted from all successive readings. The test tubes, shielding device, and Geiger Meuller tube were always in exactly the same position whenever a reading was made. After the initial readings test tuhe No. 2 was shaken vigorously for five minutes and the AgI allowed to settle while the other test tuhes were being read. This procedure was followed each time so that the time interval between the readings on all test tubes were constant. The readings on test tube No. 1 decreased exponentially according to the &day half-life of I13'. In test tubes No. 2 and No. 3 the count at the bottom rose as the (113')- exchanged with AgI in crystal form and the readings a t the center and top of ) removed from the tuhes decreased as the ( I L 3 ' was the solution. Typical readings for test tuhes No. 2 or No. 3 are shown in the figure. The author found that evidence of ion exchange was clearly shown within two hours. Furthermore, the tuhe that was shaken regularly exchanged about twice as rapidly as the tube that was not disturbed. i4
VOLUME 33, NO. 7, JULY, 1956
At the conclusion of this demonstration, the NaI solution was decanted. It was found that practically all the radioactivity was present in the solid AgI and practically none was in the solution. The solid AgI from this first part of the demonstration consists of AgILalwith a large amount of carrier. This was carefully washed and covered with 20 ml. of 0.5 M NaI12' solution. Readings were taken at the bottom, center, and top of the test tube as before and repeated hourly. The bottom readings gradually diminished and the upper readings increased slightly showing that some of the ( P I ) - from the solid AgI had exchanged and become part of the solution. This exchange was very much slower than previously, partly because the AgI had aged and partly because of the nearly comparable quantities of I- in the solid and in the solution. STUDENT REACTION
This demonstration was carried out before four chemistry sections a t the Stratford High School. How much it increased their understanding of the ionic nature of crystals is difficult t o evaluate, but the interest that they showed is hard to exaggerate. Their thoughtful questions were legion and their visits to the laboratory before and after school to check on the progress were numerous. ACKNOWLEDGMENT
The development of this demonstrat,ion was greatly
355
,2,
E
8
S
22 ,so I
2 2
5
u
'-
.2 '3 .IS 2
8
c .lo 0
15
30
45 60 75 Time in minutes
90
105
120
aided by the advice and cooperation of Rev. Gerald Hutchiison, S.J., Prof. John Irvine, Dr. Gordan Brownell, and J. A. Wiecksel.
