JULY, 1951
AN ION-EXCHANGE EXPERIMENT FOR QUANTITATIVE ANALYSIS WILLIAM M. MacNEVIN, MARY G. RILEY, and THOMAS R. SWEET The Ohio State University, Columbus, Ohio
T m s experiment demonstrates how ion-exchange resins may be used to simplify the process of inorganic analysis. A sample containing inorganic sulfates, such as a mixture of ferrous sulfate and ammonium sulfate, is passed through a column of resin of the cation-exchange type. The cations of the sample exchange with hydrogen ions of the resin producing a dilute solution of sulfuric acid which is drawn off at the bottom of the column. The acid is then titrat,ed with standard sodium hydroxide and the percentage of SO2 calculated from the results. I t is assumed that all of the SO3is in combination with a positive ion, as in ferrous sulfate or ammonium sulfate, and that no other anions are present. Ion-exchange resins consist of complex organic molecules, usually insoluble, which possess ionizable atoms or groups. Those possessing replaceable cations such as sodium or hydrogen ion are called cation-exchange resins. An example of a cation-exchange resin is called "Dowex 50."' I t is a sulfonated aromatic hydro-
' Obtainable from the Dow Chemical Co., Midland, Michigan. Also sold as "Ion X" by Microchemical Specialty Co., 1834 University 4ve., Berkeley 3, California.
carbon polymer which
contains many
cso
j
I
groups on the surface. These groups attract *ositive ions. For example, after such a resin had been treated with hydrochloric acid, the group would be present as I C-S03H. If it is then exposed to a solution con!
taihing a metal ion, the following exchange reaction occurs.
The process of exchange is a rapid one and it goes to completion so that it is useful analytically. Exchange resins are used in a rertic'al tube or column, as shown in the figure. A porous plug at the bottom of the column retains the resin but allom the solution to pass through. The resins are prepared as small spheres or granules (Dowex 50 is 20 to 60 mesh) which pack well but do not prevent the flow of solution. The size of column and amount of resin needed depend upon the quantity of material to be treated and upon the
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capacity of the resin. "Dowex 50" is rated as having a capacity of 5.1 milliequivalents per gram of resin. One gram of the resin, when exchange is complete, should liberate acid equivalent to 51 ml. of 0.1 N acid. Of course, the efficiency of the exchange process decreases as the exchange reaction progresses and a considerably larger quantity of resin is necessary in order to make the process quantitative. The efficiency of exchange is also less the more rapid the flow of solution through the column. With a column of resin 15 cm. in length by 15 mm. in diameter, a rate of flow of 15 ml. of solution per minute will give efficient exchange for three or four runs in which 5.0 milliequivalent quantities are used in each run. Exchange resins often undergo changes in volume either upon wetting the dry resin or during the exchange process. For example, if acid is poured into a dry column of the sodium form of "Dowex 50" expansion will occur and columns have been known to explode dangerously as a result of the expansion pressure. This can be avoided by wetting the resin before the column is filled and secondly by packing the column whiie the resin is in its expanded hydrogen form. The swelling of the resin on wetting also causes a pronounced change in its density. With "Dowex 50", f i r example, the density decreases from 1.55 for the dry resin to 1.41 for the wet resin. Cation-exchange resins eventually become saturated with metal ions and must be regenerated or converted back to the hydrogen form. To do this a quantity of 10 per cent hydrochloric acid is poured through the column, and then washed out with distilled water. Preparation of Resin. Place about 8 g. of "Dowex 50" cation-exchange resin in a beaker, cover with water, and let it stand a t least one hour, preferably overnight. Mount the column and place a small wad of glass wool on top of the perforated plate at the base of the column. Pour the slurry of resin and water into the column. Do not allow the water level to drop belowa the top of the resin a t any time. If air bubbles are trapped in the resin, shake the column until thev are diseneaeed. The colnm= should now be about half &dl.
--
a Channeling will occur on refilline the column if the water level is allowed to drop below the top sur&ce of the resin.
Prepare and pour through the column 400 ml. of 1:10 hydrochloric acid at a rate of about 15 ml. per minute. Wash out the excess acid with distilled water until the pH of the wash water is greater than 5 as indicated by methyl red. This will require approximately 500 ml. of wash water. The column is now ready for use. Procedure. Weigh about 0.25-g. samples containing metal sulfates (ferrous ammonium sulfate, ammonium sulfate mixtures) into dry 400-ml, beakers. Dissolve the first sample in 15 to 20 ml. of distilled water and pour the solution quantitatively into the column. Rinse the beaker once with 10 ml. of distilled water. Open the stopcock a t the bottom of the column until the rate of flow is about 15 ml. per minute. Collect the eluted solution in a 400-ml. beaker. Rinse the beaker several t i e s more with distilled water and continue to wash the column with distilled water until a drop of eluate placed on a pH test paper8 shows a pH value of not less than 5. Approximately 150 ml. of wash water will be needed. Cover the beaker containing the eluate and set it aside. Treat the second and third samples in a similar way. Titrate each of the acid solutions with 0.1 N standard sodium hydroxide using phenolphthalein as indicator. Calculate the percentage of SOa in the sample assuming that sulfate was the only anion present. Results. This method has been applied to mixtures of metal sulfates similar to those4 given out as samples for analysis in undergraduate courses in quantitative analysis. After the method was developed and thoroughly tried by the authors, it was tested by Miss Barbara Grosjean, at that time a senior student at The Ohio State University. Sample data are given in the table. Determillation of Sulfate
Sample, g.
Rase,O ml.
0.2502 0.2386 0.2362
28.03 26.62 26.34
Theory 50.15
% SO,
Found 50.23 50.02 50.00
The conclusion is reached that the method is sufficiently reliable and reproducible to justify its use by students as an illustration of ion-exchange resins in quantitative analysis. The experimental work is relatively simple. The preparation of the resin and the necessary precautions emphasize the important chemistry of ion-exchange resins. Hydrion pH papers are available at moat scientific supply houses. 'Ferrous ammonium sulfate samples are supplied by Thorn Smith. 1847 North Main St.. Royal Oak. Michigan.
