R. E. HEIN and R. H. McFARLAND Kansas State College, Manhattan, Kansas
Tmsexperiment was designed to illustrate to a class in
50-ml. volumetric flask. After adding about 10 ml. Iladioactive Tracer Techniques the potentialities of of H20 and one drop of concentrated nitric acid, the radioisotopes as an analytical tool. It requires only flask was placed in a water bath and warmed to 6Otwo hours of a student's time. Specifically, the chlo- 70°C. Ten ml. of silver solution (0.0440 N) mas then ride-ion concentration in an unknown solution is added dropwise from a buret to the contents of the determined by the addition of excess silver ion con- flask. After filling to volume following coagulation of taining radioactive silver. After filtration of the re- the precipitate, about 30 ml. of the filtrate was collected. sulting silver chloride, the excess silver ion in solution A 20-ml. sample was placed in the glass cylinder conis determined by measurement of the radioactivity in taining the dipping counter tube and the activity was the filtrate. This measurement is related to the con- measured. A standard sample was counted under the same centration of silver ion by an additional measurement on a solution of known concentration of silver ion. geometry conditons as the unknown. The standard Similar radiometric methods have been reported pre- sample contained five ml. of the radioactive silver viously by Langerl for the determination of silver, solution in a total volume of 20 ml. Background counts were taken on 20 ml. of distilled water before and phosphorus, and the halides. The measurement of radiations from the decay of after each series of counts and the counting rates on the Ag"' (0.59 m. e. v. beta particle and several gamma experimental samples were appropriately corrected. rays) is conveniently done by counting the activity in A typical set of data and calculations taken from a solution with a dipping counter tube. A dipping student report are as follows: counter tube2 with a glass-wall thirkness of 30 mg./ Background. . . . . . . . . . . . . . . . 1 0 0 counts/minute was centered in a glass cylinder with a stopcock on Sample. . . . . . . . . . . . . . . . . . . . . ,924 counts/minute the bottom for sample drainage and a side arm near the Standard . . . . . . . . . . . . . . . . . ,7555 oounta/minute top for the addition of the sample. The glass cylinder (1) Radioactivity of excess Agno held about 25 ml. of solution although only 20 ml. of 924 c.p.m. - 100 c.p.m. = 824 the active samples was added in this work. An as50 ml. Total activity = 824 c.p.m. X - = 2060 c.p.m. sociated scaler and timer were used to record the count20 ml. (2) Radioactivity of standard ing rate of the various samples. 7555 c.p.m. - 100 0.p.m. = 7455 c.p.m. Because of the radioactivity involved, transfers of 5 ml. of 0.044 M solution = 0.00022 males of Ag the active solutions were done in glass or stainless-steel Molea of Ag (ntandard) Moles of Ag (excess) -trays. All of the active wastes were collerted and (3) 0.p.m. (standard) ~ . p . m (excess) . stored in wide-mouthed glass bottles. 0.00022 z -- -. A solution of chloride ion (0.0380 N) was prepared 2060 x = 0.000061 7455 by dissolving the proper quantity of anhydrous potasadded - Moles excess = Moles consumed sium rhloride in distilled water to a volun~eof one liter. (1) Moles 0.000440 - 0.000061 = 0.000379 A liter of 0.0440 N silver solution was prepared in a like 0.000379 moles/l0 ml. = 0.0379 M chloride solution manner from silver nitrate. Radioactive silver under the code name of Ag-110 (P) was purchased from Oak Ridge National Laboratories. The sperific activity RESULTS AND DISCUSS1ON (100-300 millicuries/g.) is such that the addition of 0.3 A total of 49 determinations by class members over millicuries to the solution does not change the con- a three-year period on the chloride-ion solution (0.0380 centration of silver ion significantly and provides N) gave an average value of 0.0378 N. The per cent sufficientactivity for the analysis. standard deviation around this value was 0.3. The Ten ml. of the chloride-ion solution (concentration concentration of chloride ion was changed to 0.0350 unknown t o the student) was added from a buret to a for the most recent class. Thirteen determinations gave an average value of 0.0347 N with a per cent I LANCER, A., Anal. Chem., 22, 1288 (1950); J . P h ~ s c. h m . standard deviation of 0.2. 45, 639 (1941). An inherent error in any radiometric determination 2 The dipping counter tube Model TGC 5, made by Tracerlab, Inc., 130 High st., ~ o s t 10, o ~Massachusetts, was used in these involves counting statistics. While a minimum of 5000 counts were taken for most samples, laboratory studies.
34
periods prevented longer readings. Perhaps the most significant source of error unique to this technique iyas the presence of colloidal silver chloride in the filtrate, thus increasing the counting rate andresulting in a low value for the chloride-ion concentration. The value in this experiment is that it illustrates both a rapid method of determining chloride-ion con-
JOURNAL OF CHEMICAL EDUCATION
centration by tracer techniques, and the advantages of a dipping counter tube under these conditions. The method here described is not intended to replace the standard analytical methods for chloride-ion determination. However, the use of a technique not depending on a visual end point should suggest to the st,udent a variety of modifications and applications.
