Tritrimetric Determination of Ruthenium in Hydrochloric Acid Solutions by Prereduction with Excess Iron(l1) Lawrence W. Potts and Peter James Lingane Department of Chemistry, Unicersity of Minnesota, Minneapolis, Minn. 55455 ALTHOUGH redox titrations have been used to verify the oxidation of ruthenium in many of its complexes ( I ) , only one such titration has been developed into a precise method for the determination of ruthenium (2, 3). However, this titration (#), which involves the oxidation of Ru(IV) to RuO? with Pb(IV), is not directly applicable to ruthenium distillates. We demonstrate in this work that ruthenium in hydrochloric acid solutions can be determined accurately and conveniently by oxidation with chlorine (which guarantees the ruthenium being in the +4 state), reduction to the + 3 state with excess Fe(II), and titration of the Fe(II1) so generated with a n appropriate reducing agent. Ti(II1) was used in this study and the end point was detected potentiometrically. Because the Ru(IV) is largely reduced before the start of the titration, this titration resembles the titration of Fe(II1) with Ti(II1) ( 5 ) ; the measured potentials are stable and reproducible, and the end-point potential break is well defined. The direct titration of either monomeric o r dimeric Ru(IV) with Ti(II1) in hydrochloric acid solutions ( I , 6) is unsatisfactory, even a t high concentrations and elevated temperatures, except as a tour d p force titrimetric method, because of the slow rate of reaction between Ru(1V) and Ti(II1). EXPERIMENTAL
The synthesis of potassium p-oxo-bis(pentach1ororuthenate (IV)), K4Ru20C110.H20,has been described (7). The analogous ammonium salt was prepared by trapping R u 0 4 in chilled 1 :1 concentrated HCl, 30 % H202. After boiling the solution t o half its volume, the salt was precipitated by adding a saturated solution of NH4CI. Because this salt is quite soluble in HC1, products from several batches were recrystallized from NH4Cl saturated 2.3M HCl. Samples of the potassium salt in small, porous bottomed, ceramic filter crucibles were placed in a stream of hydrogen in a vycor tube and ignited for about one hour a t a maximum temperature of 700 "C (above 600 "C for about 30 minutes). The samples were cooled in hydrogen t o 200 "C, then transferred t o a desiccator. The contents of the crucibles were leached with hot water to remove KCl, then reignited in hydrogen and weighed. The small amount of KC1 entrained in the residue is generally less than 0.1 % (8). A second leaching and reignition demonstrated that the samples were at constant weight. The ammonium salt was ignited in vycor boats and the residue weighed as the metal. Although neither salt corresponds closely t o theoretical stoichiometries, physical measurements demonstrate that the dimeric anion [Cl~RuORuClJ4- is the principal ruthenium moiety. For (1) W. R. Crowell and D. M. Yost, J . Amer. Chem. Soc., 50, 374 (1928). (2) F. E. Beamish, Anal. Chim. Acta, 44, 253 (1969). (3) F. E. Beamish, Talanfa, 13, 1053 (1966). (4) I. Ntmec, A. Berka, and J. Zyka, MicrccBem. J., VI, 525 (1962). (5) 0. Tomitek, Rec. Tral;. Chim., 43, 798 (1924). (6) N. K. Phenitsyn and S. I. Ginzburg, J . Zriorg. Clzem. USSR,2, 172 (1957). (7) J. A. Broomhead, F. P. Dwyer, H. A. Goodwin, L. KaneMaguire, and I. Reid, Znorg. Syn., X I , 70 (1968). (8) 3. L. Howe,J. Amer. Chem. Soc., 49,2381 (1927).
example, the IR active asymmetric stretch characteristic of the dimer (9) was observed (KBr disk) at 875 cm-l and 865 cm-1 for the potassium and ammonium salts, respectively. Powder patterns revealed a tetragonal structure for both salts, K2RuCI6being cubic ( I O ) , with interplanar spacings in agreement with the reference pattern (11). Solutions of both salts exhibited the strong absorptions characteristic of the dimer at 385 and 455 nm (6,12,1.3). The titanium titrant was prepared by diluting commercial TiC13 (Fisher Scientific Co.) with 3 M HCl. This was stored in and dispensed from a reservoir buret, graduated at 0.01 ml intervals, under positive argon pressure. The Ti(II1) solutions were standardized against ferric iron solutions prepared by dissolving suitable quantities of iron wire (Baker lot No. 11148, 9 9 . 8 z Fe) in chlorine saturated 3M hydrochloric acid. Ferrous ammonium sulfate served as the source of ferrous iron (14). The 0.022% Fe(II1) content was determined by titrating 2.5-gram portions of the salt with Ti(II1). N o other corrections were applied t o the titrimetric data because the experiments were designed so that a small blank in the titration of ruthenium would be largely compensated by a similar blank in the standardization procedure. The distilled water contained approximately 2 ppb heavy metals (15) as determined by dithizone extraction. The titrations were conducted in the absence of air in a water-jacketed cell of 200-ml capacity. The solutions were thermostated at approximately 60 "C because the reactions between Ru(IV) and Fe(II1) and between Ti(II1) and Fe(II1) are inconveniently slow at room temperature. It was shown that Ru(1V) does not oxidize chloride t e a significant extent over a two-hour period under these conditions. Accuracy. The data for comparative assays of the ammonium salt appear in Table I and demonstrate that the titrimetric method outlined above is accurate for the titration of 0.15 t o 1.5 m M Ru(1V) (3 to 30 mg R u in 200 ml). We feel that the poor agreement between the titrimetric and gravimetric assays of the potassium salt results from the entrainment of variable amounts of KC1 in the precipitate. To provide support for the hypothesis, a sample of the potassium salt was analyzed for ruthenium spectrophotometrically as the dithiooxamide complex L'S. five standard solutions prepared in the course of a previous investigation (16). The results [28.1, 27.8, 28.0, 27.8, 28.4, 27.8%; mean: (28.0 =k O.l)%] support the accuracy of the titrimetric assay. Atomic absorption analyses were performed by Galbraith Laboratories, Inc., Knoxville, Tenn., and these results (27.80, 28.12%) also support the accuracy of the titrimetric assay. (9) D. Hewkin and W. P. Griffith, J . Chem. SOC.(A), 1966,472. (10) "Standard X-Ray Diffraction Powder Patterns," National Bureau of Standards Circular 539, U. S. Government Printing Office, Washington, D. C., 10,46 (1960). (11) Ibid.,p 47. (12) C. K. Jgrgensen, Mol. Phys., 2, 309 (1959). (13) F. Pantoni, J . Less-Common Elements, 4, 116 (1962). (14) H. J. Keily, A. E!dridge, and J. 0. Hibbits, Anal. Chim. Acta, 21, 6 (1959). (15) T. Bidleman, Ph.D. Thesis, University of Minnesota, Minneapolis, Minn., 1970. (16) P. J. Lingane, Anal. Chim. Acta, 47, 529 (1969) and references
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1. (NH4)aRuiOClio*H20 0.02M Ti(II1):
Table I. Comparison of Titrimetric with Gravimetric Assay of Ruthenium Salts Titrimetrically Gravimetrically sample wt, mg Ru sample wt, mg
0.006M Ti(II1):
2. K4RuzOClio*HtO 0.02M Ti(II1):
z
86.27 80.40 74.44 63.12 61.83 98.09 10.94 10.94 10.94 43 * 74
30.09 30.09 29.91 30.09 29.89 30.00 30.00 29.99 30.10 30.01 30.02 f 0.024
103.53 108.37 119.85 89.02 130.49 89.26
29.93 30.04 29.91 30.00 30.03 29.97 29.98 f 0.020
109.65 113.91 113.46 121.10 114.68 120.60
27.96 28.13 27.91 27.88 27.94 27.91 27.96 f 0.037
101.61 148.04 131.80
28.39 27.89 28.72 28.3 f 0.42
Table 11. Accuracy of the Chlorination Step Sample wt., mg 86.28 164.58 115.88
Fraction pretitrated 0.16 0.17 0.90
2 Ru 29.98 30 04 30.01
I n order to apply this method t o the determination of ruthenium chloro complexes generally, it is necessary to be able t o quantitatively convert lower oxidation states of ruthenium t o Ru(1V). It has been generally assumed (I, 8) that ruthenium chloro complexes are oxidized to the + 4 state by chlorine. To prove this, a portion of Ti(II1) was added to a sample of the ammonium salt. After a few minutes, the mixture of Ru(IV), Ru(III), and Ti(1V) was treated with chlorine, etc., according to the procedure outlined above, The results of the final titrations appear in Table I1 and demonstrate that the ruthenium was quantitatively reoxidized under these conditions. (They also prove, incidentally, the absence of significant amounts of iron in the titanium solution and the efficacy of the dechlorination.) DISCUSSION
N o attempt was made to evaluate interferences because even a cursory examination of a table of standard potentials suggests that the presence of significant quantities of a majority of the elements would be disasterous. Consequently, this method, as do the Pb(IV) and gravimetric methods, requires the prior separation of the ruthenium. Because dimeric R u (IV) chloro complexes are the principal products when ruthenium tetroxide is trapped in HCI or HCl H20j,this method is directly applicable to these distillates. (Other oxidizing agents carried over from the distilling flask will lead to high results if they are not reduced by HzO?. This is readily checked by a blank.) The tetroxide may also be trapped in alkaline solutions because dimeric Ru(IV) chloro complexes are formed upon acidification of ruthenate solutions. U p to within one or two per cent of the equivalence point, the potential of the small platinum indicator stabilizes immediately upon addition of titrant. In the immediate vicinity of the equivalence point, it is necessary to wait 3-5 minutes for equilibrium at the 1 mMlevel and about 10 minutes at the 0.1 m M level. These times are but slightly longer than the equilibration time in the absence ofruthenium. The potential
+
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2 Ru
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
drift is to more negative potentials, indicating that the Fe(III)/ Fe(1I) couple rather than the Ti(IV)/Ti(III) couple is potential determining. Because the potential break is about 250 mV (99.8 to 100.2 titrated), it is entirely satisfactory to titrate to a n equivalence-point potential of about f0.2 V us. AgCl ( 3 M HCl)/Ag. If this is done, a titration at the 1 mMlevel requires less than thirty minutes. Titrations at lower concentrations are too time consuming to be generally recommended. This limit corresponds to about 5 mg ruthenium in 50 ml. Comparison with Other Methods. The gravimetric method is without peer for the analysis of ruthenium salts which yield only ruthenium metal as residue upon ignition in hydrogen. However, the titrimetric methods, and in particular the Fe(I1)-Ti(II1) method, are probably superior for the analysis of ruthenium distillates because they d o not require the isolation of the ruthenium as a hydrous oxide. The Pb(1V) method is superior to the Fe(I1)-Ti(II1) method in that he titration can be conducted at room temperature and the potentials stabilize more rapidly in the vicinity of the equivalence point. It is also slightly more sensitive, 0.4 mMsamples b-ing titrated with apparently no increase in potential stabilization time. However, the Fe(I1)-Ti(II1) method is superior to the Pb(IV) method in that it is directly applicable to ruthenium distillates, not requiring the removal of chloride, and because of the much larger potential break at the equivalence point which would make it more easily automatable. The Fe(I1)-Ti(II1) method is also the only one of these three which gives information on the oxidation state of the ruthenium. For example, by omitting the chlorination step, we were able to show that the potassium salt contained 1 Ru(II1). In an adaptation of the procedure of Backhouse and Dwyer (27), Ru(I1) can be determined in the presence of Ru(I1) by adding the ruthenium sample to a known excess of Fe(II1). RECEIVED for review December 12, 1969. Accepted February 9, 1970. The financial support of the National Science Foundation under GP-9422 is gratefully acknowledged. This work was presented in part as paper No. 15, to the Division of Analytical Chemistry, 158th Meeting, ACS, New York, N. Y . ,September 1969. (17) J. R. Backhouse and F. P. Dwyer, Proc. Roy. Soc. N . S . Wales, 83, 146 (1949).
