JOURNAL OF CHEMICAh EDUCATION
STUDENT EXPERIMENTS ON ION EXCHANGE' SICIPRED PETERSON College of Arts and Sciences, University of Louisville, Louisville, Kentucky
SINCE ion exchange has reached importance as a tool of both fundamental and industrial chemistry, i t is desirable that some time be devoted to this subject in undergraduate courses. T o this end, an important part of the course in ndvnnced inorganic chemistry in t,his department has brrn devoted to the fundamentals and applications of ion exchange. Thecxperimentsdescrilxd in this pnper have been used in the laboratory to illustratr typical applications of ion exchange columns. Experiments on the fundamental propertirs of an ion rxchange resin have k n described by Connick and P~wrll.~ The experiments presented here demonntrate the softening of water, the recovery of a metal from dilute solution, and the separation of similar metal cations. The columns used are easily prepared from laboratory burets. The experiment on water softening, besides demonstrating the difference between untreated tap water and water which has p d through the column, illustrates the exhaustion and regeneration of a column. The recovery experiment remakes lanthanum ion from a very dilute Innthanurn nitrate ~olutionand elutes it with hydrochloric acid, drmonstrnting in addition the conversion of a nitrate to a chloride. Almost any cntion exchange resin is suitable for these experiments; Amberlite IR-100, Nalcite HCR, Amberlite IR-120, and Zeo-karb-H have been used. Amberlite IRGSO is especially suitable for the lanthanum experiment., since very little acid is needed for the elution. 1 Resented in part at the Southeastern American Cbemicd Boeiety Regional Meeting, Oak Ridac, T e n n e e , June 11. 1949. * CONNICK, R. E.,AND R. E.,I'OWELL, "Chemistry 120 Syllabus,'' University of California, Berkeley, 1948, p. 37.
Cadmium and copper were chosen ss cations to illustrate the techniques which have heen applied t o 8eparations of rare earths and other nets of similnr metal cntions.' Both cations arc readily obtained and easily distinguished from each other hy the simplc tests used in elementary qualitative analysis, yet the solubility and complrx-ion forming properti- of the two ions are qunlitntivrly quite similar. Like difcrent rare earth cations, these two cations differ sufficiently in their tendrncy to form citrate complexes that they may be selectively eluted by citrate solutions from a cation exchange column. To obtnin a good copper-eadmium separation in a small column requires a high-capacity strong acid type resin such as Amberlite IR-120 or Kalcite H C R (Dowex-50). While both these resins have been used successfully in thrse experiments, the experimental details given are time worked out with the latter. The column must be in the salt form (sodium or ammonium) rather than in the acid f o m - d s e the acid of the column destroys the citrate elunnt. Preparation of rhe Column. Above the atopcock in a 50-ml. buret plnce a mad of glass wool just large enough and packed enough t o prevent resin particles from entering the stopcock. Stir up the resin to be used with d i s tilled water and pour the suspension into the buret. Leave the stopcock open so the excess water can run out. The resin will settle to the bottom of the buret. Continue pouring resin suspension into the buret as the water runs out, until there is enough resin t o fill the buret t o about 5 inches from the top. During this pro'Sea, for e ~ m ~ p lE. e , R TOYPKIN~. J . Am. C h . Soc., 70, a520 (1948): B. IT. K ~ L L AND C G . E. Born. Ibid., W, 2800 (1947).
cedure and all subsequent work with the column the water level must not be allowed to drop below the top - of the resin bed, since removing air from the resin bed is very difficult. While the resins an? resistant t o dilute acids, it is best thnt the column never be left filled with acid. SOFl'ENlNO OF WATER
Hardness in water is caused by the presence of calrium and magnesium ions (in Louisville, largely calcium). I t can be r e m o v ~ by l passing thr water through a column of the sodium form of almast any cation exchanger (not the hydrogen form, since hydrogen ion interferes nith the sonp teat), sodium from the rrsin replacing the cnlcium or mngnesium in .solution. \Vater l~ardnrsscan be drmonstrnted qualitativrly by the p m cipitntion test with oxalntc, semiquantitatively hy test with n standard soap solution. Soap Trslfor Ilardness. Titrate 50 ml. of water with standard soap solution until nn good a lather is obtained as with 1.00 ml. of soap solution and 50 ml. of distilled water. The hardrr the water the greater the volume required; soft water rrquires 1.00 ml. The standnrd soap solution is prepared by su.spending 20 gm. of pure sonp in water, adding 500 ml. of alcohol (ethyl or isopropyl), and d i l u t i n ~to one lit.er. P r o d i r e . Prepare a column of the d i u m form of the resin to be used. Run tap water through the column discarding the first 20 ml. of effluent which was probably held up in the column a t the &art. Then collect two Wml. samples of the effluent. Test one for Cn++ with 2 ml. of saturated ammonium oxnlate; test. the other for hardness using the soap test. At the same time test similnr snmples of untrented tap nater. If an indefinitely lnrge volume of wnter is treated by the same column, eventually such a large proportion of the cations on the resin will be Ca++ and hIg++ t,hat the resin no longer is effective for removing these ions from the water. T o illustrate the depletion of the rolumn pour through the column a solution containing 3 grams of CaClt nnd follo~vn i t h sufficient water to wash off any Ca++ not picked up by the resin. Then collect two 50-1111. samples of column-trrated water and test for hardness and Ca++, comparing results with the previous results. The column can be regenerated by removing the Ca++ with concentrated XaCI solution, reversing the equilibrium: Ca++
+ 2NaR # 2Na+ + C(Ra b one equivalent of resin aoion)
T o regenerate the column, pass 200 ml. of saturated S a C l solution through the column. Test aportion of the effluent for Ca++. After the column is treated with S a c 1 solution and washed, again test samples of column-treated tap nater for hardness and Ca++. RECOVERY OF A RARE ELEMENT
Use a column which is completely in the hydrogen form. Pass t,hrough the e o l ~ ~ m200 n ml. of lanthanum
nitrate ~olution(100 mg./l). Test the effluent with litmus paper and interpret the result. Wash the co111mn with 50 ml. of water and then with 100 ml. of hydrochloric arid. Collect the effluent from the latter and evnporatc to dryness. The white solid is lanthanum rhloridr. If the residue is too bulky for t.he qunntity of lanthnnum expected, i t is probably sodium chloride present bemuse thc resin was not complrtrly in the acid form a t the start of the experiment. BEPARATION OF SIMILAR m I I L CATIONS
This experiment is bawd on the fact that citrate complexes of Cu++ and Cd++ are not equally stable, so thnt one of these cations is removed from the column morr rapidly thnn the ot.her by citrate solution.. Concentrations of diammonium citrate greater than that recommended will complex both ions to the extent that they will be eluterl together and not separaterl. The second ion should be rlutcd nith ammonium chloride rnthrr then hydrochloric arid if the column is to Ix IINA again for this experiment, even though thr arid is the morr ellective eluting ngent. Test for Copper. To a few milliliters of the emuent add a f r m drops of 0.5 .If potrrasium ferrocyanide. A wine-red precipitate indicates the presence of copper. Teslfor Cdr~tiurn.Make 1 ml. of thr effluent alkaline with ammonia nnd dilute to 10 ml. If copper is present add 0.5 'If p n t ~ s i u mcyanide until the solution is colorless. Saturate with hydrogen sulfide. :\ yellow precipitate indirntes the presence of Cndmium. Prmdure. Use a column which is filled with n'alcite IlCR (Dowex 50) in the soclium or nmmonium form. If the resin has not previously been used, i t is probably in the sodium form. If it is in the hydrogen form it mny be converted to the ammonium form by p m i n g 200 ml. of saturated ammonium chloride through the column. Pnss 50 ml. of 0.20 If diammonium citrate through the column to "pretreat" the column, that is, to make the II+-NIL+ balance in the column the same cia during. the separation. Add to the column 10 ml. of a solution 0.05 .If in cadmium nitrate and 0.05 M in copper nitrate. Follow this with 0.20 IM diammonium citrate, collecting 50-ml. samples of effluent. Test each effluent sample for Cu++ and Cd++. When the ion eluted no longer appears in the effluent (probnbly after four snmplea) substitute either 3 '11 hydrochloric acid or saturated ammonium chloride for the citrate solution and continue taking S m l . samples and testing for the ions until the second ion has heen removed from the column. ACKNOWLEWMENT
The author wishes to thank the National Aluminate Corporation and the Resinous Products Division of Rohm and Haas Company for samples of resins used in this work. Also, thanks are due the students, too numerous to mention individually, who have done the major pnrt of the development and testing of thrse experimentn.