Roberto Bedelti, Vincenzo Carunchio,' and Mauro Tornassetti University of Rome Rome, naiy
Elimination of Phosphates by Ion-Exchange in the Systematic Analysis of Cations
Qualitative analysis of inorganic cations may he considered as (1) a method to teach inorganic chemistry and (2) an experimental way to introduce the students to some problems of analytical chemistry. For these reasons we helieve ir might be of some utility ro direct attention to an accurate examination of the third analytical group elements (iron(III), chromium(III), aluminum(III), and manganese(II1) and (IV)) in the presence of phosphates. The phosphato species interferes with the successive identification of the earth-alkaline metals, precipitating them as insoluble salts. In general, such interference is overcome by using iron(II1) or zirconium(1V) salts, which form phosphates of low solubility. This procedure is, however, somewhat slow and elaborate and introduces an additional cation in the solution under investigation. Furthermore, the reagents are generally added in large excess with respect to the concentration of the cations to he analyzed, because the phosphates must he eliminated as quantitatively as possible. Then this additional cation must he removed in its turn so as not to interfere with the following tests. Ion-exchangers are therefore a useful and proper tool as an alternative to the above method, particularly when conditions can he set which make the phosphates separation occur in a selective and rapid way. Ion-exchangers have been employed in some cases to achieve the separation of phosphates from other ions (1-31, but when dealing with such an elimination in the course of systematic analysis of cations, i t is necessary to take under The authors wish to thank the Consiglio Nazion.de delle Ricerehe (Rome, Italy) for financial support and for a grant to one of them (R.B.). To whom all correspondence should be addressed.
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122 / Journal of Chemical Educatbn
Logarithmic concentration diagram for lo-' M phosphoric acid at 2S°C and ionic strength. (a) = Iog[H3PO~].(b) = log[H2P04-1,(c) = log[HP042-],and Id) = lag[POP-1. 0.1 M
consideration both the protolitic equilibria of PO& and the formation of coordination compounds, which occurs easily when dealing with the cations of the transition metals of the third and fourth group. In this note we wish to study the problem more accurately in order t o establish the optimal experimental conditions for the use of ion-exchangers in a very practical way, taking into account basic analytical considerations.
Good results have been obtained (4) with anionic-exchangers which may bind the phosphates, but we now present a suitable method for the inorganic qualitative analysis. At p H 2 (which is the acidity of the solution obtained after second group separation) about 50% of phosphate is neutralized as HzP04 (see the fig.); it is remarkable that the concentration of H3P04 is 90% wben the p H is lowered to 1.Such species cannot be retained on the anionic resin; and the elimination of the interfering ion is partial and questionable. For this reason, we have used a cat: ionic-exchanger, by which the phosphates (both neutral &Po4 and its negatively charged conjugated bases) are not retained, while the cations are absorbed. We note that, even in this case, the method is not free of criticisms because negatively charged complexes of the elements of the third and following groups may be eluted. However, with the conditions generally occurring in inorganic qualitative analysis such a problem is usually not relevant; in fact the negatively charged complexes may be obtained only when operating with large excess of ligand. At the beginning of the third cation group analysis, the solution a t p H 2 is highly concentrated in chloride ions; thus it is necessary to consider the formation of chloride complexes while the hydroxo ones may be neglected owing to a very low degree of formation. With regard to the chloride containing species, only the negatively charged complexes may cause some errors, but i t is reasonable to assume that they are present in very low amount since a very high C1- concentration is necessary to form them, which is not the condition of the cation solution during the analysis. For instance, to get species like MnCl3- it is necessary to operate in concentrated hydrochloric acid, more than 6 M ( 5 ) ,and wben dealing with aluminum(II1) the chloride complexes cannot be formed if the C1- concentration is lower than 11 M (6). The best condition for the quantitative elimination of phosphates with this procedure is the elution of the cationexchanger with distilled water; different eluting solutions seem to induce some troubles, as observed by Samuelson who used an aqueous solution of ammonium chloride and could not perform the right identification of the alkaline metals ( I ). In our case, the elution with water gave rapid and quantitative results; no error was made on cations within the limits of the identification methods used, since all the metals were absent in the eluate. Water elution separates all of the phosphates (H3PO4,-HzPOa-,-HP0d2-,-POa3-) well, and avoids the danger of negatively charged complexes forming for the cations. The choice of the suitable solution to elute the cations retained on the exchanger was made after a careful set of tests, in order to get a rapid and small volume of eluate; thus i t is not necessary to perform any long concentration
operation down to the volume of ahout 1 ml, as stated in the classical scheme of chemical qualitative analysis. The elution must be as accurate as possible, so no significant amounts of cations mav be missed. The elution was checked with hydrochloriE acid a t different concentrations, 1-12 N. When working with 1 and 2 N HCI, the elution is completed in too large a volume and the method is no longer convenient for a vew rapid analvsis: when 12 M HC1 is used the necessary voiumi is smail enough, but we have had some difficulty identifviue some cations: most likelv this is due to the-formation of chloride ~ o & ~ l e x e ~ s. h iron(II1) test in the presence of thiocvanate ions, which is usually uuequivocal,~wasnot satisfactory and even doubtful in some cases. I t should also be noted that with high chloride concentration, such as 12 N, even metals like barium(I1) (7), usually characterized by a low coordinating ahility, may give rise to an appreciable amount of complex formation. The best results were obtained with 6 N HC1; the volume of eluate is small (5-8 ml) and the identification tests are performed without any difficulties. Experimental
All the reagents wed were analytical grade products. Cation solutions under investigation Were about 0.1 M. Preparation of the Column
A column (20.0cm high and 1.8 cm in diameter) is charged with an aqueous slurry of cation-exchanger (5 gin 200 ml), Dowea 50 W X8 type, strong, 100-200 mesh, exchange capacity 1.7 meqlml wet. When it is well stratified for half height in the column, so as to get a uniform elution, a 12 N HC1 solution is passed through to regenerate the cation-exchanger, in the form H+; then the system is washed several times with distilled water until free of C!-. Phosphates Elimination
The solution obtained after the separation of the second cation group (about 2 ml) is passed through the column prepared as above; then it is eluted with water and the eluate is collected in test tubes (about 1 ml for each of them) and controlled for the presence uf the phosphates. In general the phosphates mny he cumpletely eliminated wlthin 20 rnl of elunte. Successively the retained rations must he separated from the resin to he examined and identified following the cla,siral tests of qualitatwe analysis. For this purpose t h e exchanger is eluted w i t h 5-8 ml of 6 A' HCI: this amountis sufficient to perform the complete cation elution as may be shown easily when considering that the exchanger capacity in the column is about 40 meq in saturation conditions, which never occur in the experimental conditions described above. Literature Cited (11 Samuelaon. 0..and Runeberg. G..Sumsk Kem. Tid.. 57.91 11945). (2) Helrieh, K..and RiemanIII, W . . A n d Chem., 19.651 11947) (31 Klcment, R..2.A n d C h o n 121.2 11944). 111 Hahn, R. B.,Backer,C..sndBacker,R..Anol.Chim. Aeto, 9.223 11953).
(5) Kraua, K.A.,snd Moore,G.E..J.Amer Chrm.Soc., 75,1460(19531. 16) Moore.G.E..and Kmus,K.A.. J.Amer Chzm Sac, 12.5792 (1950). (7) Kraus, K. A,. Nelsan. F.,sndSmith,G. W., J . P h w . Chsm.. 58.11 119541.
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