measuring the dissociation constants of the S-methyl or Se-methyl derivatives (Table 111) since in these methyl derivatives participation of the zwvjtter ion forms is excluded ( b ) . From the values of Kogas well as from K,, and K,, from use of Equa,tions 1 to 3 the other K, values are readily obtained. With this information (Table IV:l it now becomes possible to evaluate the effect of structural changes on the dissociation phenomena in detail. esaniination of the dissociation scheme shown above leads to the following predictions. The values of pKaD, expected to be sensitive to the electronegativity of X , should decrease considerably from 0 to Se. Similarly, values of pK,, which correspond t'o the loss of a proton from i;hp group-XH in the cationic species, !should be lower than those of pK,, by virtue of the presence of the positive charge, but should change in the same manner as pK,,. Aiscan be seen from Table IV values cf pK,, drop 4.51units from oxine to sele;.oxine in the same order as observed in the acid strengths of H20, H,S, and H2Se. The magnitude of these values seem to be reasonable since that for osine is very close to the pK, of a-naphthol (9.85) and l.hat for thiooxine is close to the pK, of thiophenol (6.5). I t is of interest to note the similarity of the value of pK,, of oxine with that
(6.8) of the 'V-methyl analog of the cationic oxine species (12). The relationship between pK,, and pK,, can be expressed as: pK,, = 1.5 pKaD - 8.4. The relationship shows two consequences of the presence of the positive charge. First, its presence causes a general lowering of the pK,, and second, this lowering varies linearly with X, being greatest with selenoxine. This might be due to the greater sensitivity of the more highly polarizable selenium to the presence of the neighboring positively charged proton. The effect of changing X on the values of pKaBand pKac would not be expected to be appreciable since the X atom is not directly involved in the dissociation processes nor is there any appreciable conjugative interaction between the X and S atoms. This is the case in the pK,, values observed and, were it not for oxine, in the pK,, values as well. The deviation of the value of pK,B for oxine may reflect the stabilization of the neutral species through hydrogen bonding. K , would be eypected to increase with increasing strength of -XH and with increasing dielectric constant of the solvent. Both selenoxine and thiooxine exist almost entirely in their zwitter ion forms in water. I n 50% dioxane-water (dielectric constant = 32) selenoxine is still present entirely as the zwitter ion
form but thiooxine, with its significantly lower K 1 even in water, is probably largely in its neutral form in 50% aqueous dioxane. For osine, even in aqueous solution, the concentration of the awitter ion form is negligible. LITERATURE CITED
( 1 ) Albert, A , , Barlin, G. B., J . Chem. SOC. 1959, p. 2384. ( 2 ) Albert, A., Hampton, A . , Ibad., 1954, p. 505. ( 3 ) Bankovsky, Y. A , , Chera, L. XI., Ievinish, A. F., J . Anal. Chem., C'.S.S.R. 18, 668 (1963). ( 4 ) Corsini, A., Fernando, Q., Freiser, H., ANAL.CHEM.35, 1424 (1963). ( 5 ) Ebert, L., Z . Phys. Chem. 121, 385 (1920). ( 6 ) Freiser, H., Charles, R. G., Johnston, W. I)., J . Am. ('hem. SOC. 74, 1383 (1952). 1~, 7 ) Johnston. W. D.. Freiser. H.. J . Am. Chem. S O C . '5239 ~ ~ ,(19521. ' ( 8 ) Lee, H. S., Freiser, H., J . Org. Chem. 25, 1277 (1960). (9) IIason, S . F., J . Chem. SOC. 1958, p. 678. (10) Muthmann, W.,Schroeder, E., Rer. 33. 1766 11900). ( 1 1 ) 'Phillips, J. P., Keown, R. W., J . Am. Chem. SOC.73,5483 (1951). (12) Sekido, E., Fernando, Q., Freise:r, H., ANAL.CHEM.35, 1550 ( i963).
RECEIVEDfor review March 6, 1964. Accepted May 1, 1964. Work supported, by the U. S. Atomic Energy Commission.
ion Exchange Separation of Microgram Quantities of Osmium from Large Amounts of Base Metals J. C. VAN LOON and F. E. BEAMISH Department o f Chemisfry, University o f Toronto, Toronto
b Microgram amounts of osmium can be separated quantitatively from decigram amounts of the associated base metals, copper, iron, and nickel by cation exchange. Significant losses of osmium will result from an evaporation of hydrochloric acid solutions of hexachloroosmates. These losses may be controlled by a prior treatment of the osmium solution with suifur dioxide.
C
rnethods for quantitatively separating iron, copper, and nickel from trace amounts of platinum and palladium ( 6 ) , rhodium and iridium ( b ) , and ruthenium (8) have been recorded. Early efforts to separate osmium in a similar manner resulted in persistently low values. It was as,umed that the difficulties incident to the separation of ruthenium applied also in the case of osmium ( 2 ) , as the elements behave alike analytiATJOX EXCHANGE
5, Ontario, Canada
cally. With ruthenium, low recoveries from the cation exchange separation were associated with aging solutions caused, presumably, either by the slow production of dissolved cationic species, and/or by hydrolysis to hydrated oxides. An additional source of loss in the case of osmium, relatively inapplicable in the case of ruthenium, is the readily formed volatile octavalent osmium oxide. Various investigators have reported losses of osmium during the evaporation of the hydrochloric acid solutions used to collect the distilled octavalent oxide. T o avoid this loss the collecting liquid is usually treated with a reducing reagent ( 1 ) . 'Cnfortunately, while some pertinent data have been recorded, dealing with the solution composition of osmium-hydrochloric acid solutions, none allows a final interpretation of the identities of dissolved constituents in the various receiving solutions containing the reducing constit-
uents. Obviously, identification of the osmium constituents would allow more acceptable conclusions as to the source and amount of losses in experiments designed to provide safe methods of dissolution and separation. However, an examination by absorption spectrophotometry of osmium-hydrochloric acid solutions prepared by the evaporation of various collecting liquids used to receive the octavalent osmium oxide and by the direct addition of chloroosmate to hydrochloric acid solution indicates a common pattern. Thus irrespective of the exact identity of the dissolved species, and in the absence of more satisfactory data, the chloroosmate may be used to determine the degree to which osmium is lost during evaporation. The use of this constituent is particularly advantageous because the comparable ruthenium salt was used for a similar investigation with ruthenium whose separations frequently involve a similar technique ( 2 ) . VOL. 36, NO. 9, AUGUST 1964
1771
EXPERIMENTAL
Apparatus. Glass distillation apparatus ( 7 ) ; ion exchange columns capable of holding 500 grams of resin; Unicam S P 500 spectrophotometer; Bausch & Lomb spectronic 505 visible and ultraviolet recording spectrophotometer; Beckman model A2 p H meter. Reagents. Dowex 50 X 8 N a f form cation exchange resin; double distilled 70% perchloric acid; ammonium chloroosmate; base metal solution. The resin in the 500-gram capacity columns was prepared as follows. Two liters of 3'1' hydrochloric acid were passed through the resin a t a rate of 35 ml. per minute followed by 3 liters of p H 1 wash solution a t a rate of 20 ml. per minute. The base metal solution was made by dissolving 1300 grams of FeC13.6H20, 140 grams of NiC12.6H20, and 100 grams of CuC12.2H20in 50 ml. of concentrated hydrochloric acid and 1000 ml. of water. The solution was filt'ered and diluted to 4 liters. llliquots of 200 ml. were used. A standard osmium solution was made by dissolving approximately 2.3 grams of ammonium chloroosmate in 200 ml. of water and 40 ml. of concentrated hydrochloric acid. The solution was heated gently, filtered through a sintered glass crucible, and diluted to 1 liter with distilled water. The pH was found to be 0.94. Several 5-ml. aliquots were placed in separate 50-ml. beakers together with 50 ml. of concentrated hydrobromic acid. The solutions were evaporated to about 5 ml. on the steam bath and diluted to 50 ml. with water. The osmium content was found to be 0.802 mg. per ml. by t'hionalide precipitation (4). In a confirmatory standardization the osmium from several more 5-ml. aliquot's of the same ammonium chloroosmate solut.ion were distilled from 70y0 perchloric acid into cooled concentrated hydrobromic acid solution. The resultant receiver solutions were transferred to 150-ml. beakers and treated as above. The osmium content was found to be 0.800 mg. per ml. by thionalide precipit,ation. Dilute working solutions of ammonium chloroosmate were prepared as required from the stock solution. The acidity of these solutions was adjusted to p H 1 with hydrochloric acid. Retention of Osmium by Resin. The separation of osmium from base metals by cation exchange is dependent upon maintaining osmium as an anionic or neutral species over the narrow range of acidity required for the retention of the base metals. Partial hydrolysis of the base metal solution was indicated above a pH of 1.5. Below a pH of 0.8 incomplete retention of the base metals was noted. These data dictate an acid adjustment of the osmium base met'al solutions to a pH between 0.8 and 1.5. In analogous ion exchange work with rut,henium it was postulated that partial hydrolysis occurred at p H 1, during aging of the ' chlororuthenat,e solution, resulting in the production of a cationic 1772
ANALYTICAL CHEMISTRY
0.6
0.5
1
A
B C
I
A G E D WORKINQ SOLUTION PHI
AGED AND FRESH II N HCL
t I
200
Figure 1 solutions
.
250
300 350 4 0 0 450 WAVE LENQTH mu
500
Absorbance spectra for ammonium chloroosmate
ruthenium species. Losses of 10 to 20y0 were found when the aged chlororuthenate solutions were passed through the cation column (8). Weakly acidic chlororuthenate solutions have been studied spectrophotometrically (8) and the recorded spectrophotometric curves showed a change in solution composition during various aging periods. I t was similarly assumed that any hydrolysis of the yellow chloroosmate solution might be indicated by changes in the light absorption characteristics of the solution, hence the ultraviolet visible recording spectrophotometer was used to trace any such changes. Spectra mere recorded of various chloroosmate solutions over the 200- to 700mp wave length range. Two 1-ml. samples of the stock chloroosmate were placed in separate 50-ml. beakers along with 2 grams of sodium chloride and 20 ml. of concentrated A very gentle hydrochloric acid. evaporation to near dryness was carried out on the steam bath. The samples were transferred to separate 100-ml. volumetric flasks and the acidity was adjusted to p H 1 and to l l S , respectively, by hydrochloric acid, In the case of the llAVacid sample the sodium chloride remained largely undissolved on the bottom of the flask. The solutions were filtered into cuvets and their absorption spectra were determined as soon as possible after preparation and the p H 1 solution again, 19 hours later, against blank solutions of distilled water. These spectra along with a spectrum of the aged standard solution are given in Figure 1. These graphs show no significant variations of peak position. Thus it was assumed that the phenomena of hydrolysis could be discounted as a cause of the low osmium values. These data justify the use of a chloroosmate standard and working solutions which had been left standing a t p H 1 for a considerable period of time, thus eliminating the need for an additional evaporation to reverse hydrolysis prior to the actual separation. Factors other than hydrolysis could
be responsible for low osmium values, and for the retention of osmium by the column. Phenomena such as reduction of osmium by the organic resin or the presence of some cationic form of osmium other than a hydrolysis product, would result in low osmium values in the effluent. Experiments were made with eluates from columns through which large quantities of solutions of chloroosmate had been passed. Five-milliliter aliquots of standard chloroosmate solution containing approximately 4 mg. of osmium were placed in separate 250-ml. beakers with 2 grams of sodium chloride. The salt was dissolved and the acidity adjusted to p H 1 in a final volume of 200 ml. The solutions were passed through a prepared cation exchange column. Three and one-half liters of pH 1 solution were used to wash the osmium from the resin bed. The columns were eluted overnight with 3 liters of 6.Y hydrochloric acid. The eluates were evaporated and treated for recovery of osmium as indicated below. Less than 20 pg. of osmium were recovered from the eluates in each case. This indicates that under the described conditions osmium is not retained on the column normally used for the separation of the platinum metals from cations. Loss of Osmium during Evaporations. The ease of formation of the volatile octavalent oxide of osmium in aqueous solution suggests the possibility of loss of osmium during evaporation of large volumes of effluent. Aliquots of chloroosmate working solution containing 161 and 80 pg., of osmium were placed in separate 4-liter beakers together with 100 ml. of concentrated hydrochloric acid and 3.5 liters of water. Each beaker was covered by a large watch glass, supported above the beaker on glass rod hooks. The solutions were placed on a hot plate and evaporated at a temperature below 80" C. to a 150-ml. volume. The content of each beaker was washed into 250-ml. beaker and evaporated to 25 ml. The solutions were analyzed
for osmium as indicated below; results are given in Table I (values 1-6). These data indicat,e that evaporation of hydrochloric acid solutions of chloroosmate results in significant loss of osmium. This finding is of general importance because of its implications for any analytical procedure which involves evaporations of osmium solutions. Several reagents havle been used for the reduction of osmium in hydrochloric acid solutions. One of the most suitable is sulfur dioxide which can be easily bubbled through t.he solution in a manner similar to the treatment of receiver solutions during a dist'illation of octavalent, osmium oxide. Bliquots of chloroosmate working solution containing 161 and 80 fig. of osmium were placed in separate 4-liter beakers, covered as before, together with 100 ml. of concentrated hydrochloric acid and 3.5 liters of water. The solut,ion was saturated with sulfur dioxide and allowed to stand for 6 hours. An evaporation was carried out on a hot plate and subsequently in a distillation flask as indicated below. Osmium values were obtained for the resultant solutions by the method given below and the results are shown in Table I, KO.7-11. The data indicate that the presence of sulfur dioxide during an evaporation of hydrochloric acid solution reduced the osmium losses to a n acceptable level. The prior evaporation was completed in the distillation flask in order to avoid osmium losses which might occur as the concent,rations of the constituents were increased. Procedure for Ion Exchange Separation of Osmium from Sodium Chloride and Base Metal Solutions. Aliquot's of chloroosmate solution containing the desired amount of osmium were placed in separate 250-ml. beakers. Solutions containing either 20 grams of dissolved sodium chloride or 20 grams of dissolved base metal chlorides a t about pH I. were added to the osmium solution. The pH was thcn adjusted by hydrochloric acid or sodium hydroxide to within the range 0.8 to 1.5. The solut.ion was passed through the prepared cation exchange column at' a rate of 30 ml. per minute and the resin was washed with 3 liters of a solution adjusted to pH 1 by adding hydrochloric acid to distilled water. The effluent, collected in a 4-liter beaker, was saturated with sulfur dioxide and then allowed to stand for 6 hours. Each beaker was covered with a large watch glass supported above the beaker on glass rod hooks. An evaporation a t a temperature below 80" C. to 500 ml. was then carried out slowly on a hot plate after which the solution was transferred to a distillation flask. The distillation flask was connected to the first section of the distillation apparatus. Sulfur dioxide was
bubbled through the solution in the flask during a gentle evaporation to 25 ml. The distillation flask and contents were allowed to cool to room temperature at which time the sulfur dioxide source was disconnected and the remainder of the distillation apparatus was assembled. A large excess of potassium permanganate was placed in the trap to destroy the residual sulfur dioxide and hydrochloric acid which was carried over during the distillation. The osmium was oxidized by 150 ml. of 7070 perchloric acid and distilled into cooled receivers containing 3% hydrogen peroxide. The solution in the receivers was transferred to the distillation flask and the osmium was again oxidized using 3OY0 hydrogen peroxide and distilled into cooled ethanol hydrochloric acid solution (6). This second oxidation avoids the occasional error arising from constituents which appear simultaneously with the osmium and interfere with the thiourea reaction. Furthermore, the second oxidation allows the separation of osmium from ruthenium in the event of the latter's presence (7'). Solid thiourea was added to the receiver solutions to give a final concentration of 2y0. The solution was heated to 85' C. for 10 minutes, then cooled to room temperature; absorbance was measured a t 480 mM. The results are listed in Table 11. RESULTS AND DISCUSSION
Solutions containing various amounts of osmium in sodium chloride or base metal solution were subjected to the above procedure and the osmium recoveries are listed in Table 11. The results in Table I1 indicate that trace amounts of osmium can be separated from larger proportions of base metals by cation exchange. There was some evidence to indicate that these persistent but very small losses reflect an inability to eliminate completely the loss of osmium during evaporations. These small losses may be eliminated by evaporating entirely within the distillation apparatus. The traces of volatilized osmium will be trapped in the condensed liquid in the acid trap and can then be volatilized into the receivers using solid potassium permanganate. It was essential to use low temperatures during the evaporation in the 4-liter beakers. Temperatures above 90" C. may result in excessive osmium losses. Also the presence of osidizing contaminants in the fume chamber air during the evaporation may cause osmium losses of 15y0 or more. To avoid this source of error, i t is recommended that all work with osmium be carried out in chambers used solely for these analyses.
Table I. loss of Osmium during Evaporation of 4-Liter Volume of Solution
Osmium
recovered, pg. -___
SatuOsmium With- rated Loss of added, out with osmium, so. pg. so2 so2 pg. 1 161 121 40 2 161 132 29 3 161 127 34 4 80 66 14 5 80 71 9 6 80 12 68 7 155 161 6 8 158 161 3 9 75 5 80 10 2 78 80 11 77 3 80
Table II. Recovery of Osmium from Base Metal and Sodium Chloride Solutions
Cations Osmium, pg. present in 20-gram Reamounts Added covered Loss Na 161 155 6 Na 161 158 3 Na + 161 157 4 Fe+3Cu+2Ni+2 61 156 5 F ~ + C U + ~ N ~ 61 +~ 153 8 Fe +3Cu+ZNi+ z 61 159 2 Fe +3Cu+2Ni++aZ 80 76 4 5 80 75 Fe +3CuC2Ni +?. 80 78 2 Fe +3Cu +2Xi+1 80 78 2 Fe +TU +2Ni 16 14 2 Fe +3Cu+2Ni+ 2 14 2 16 Fe +3Cu+2Ki+ 2 16 13 3 F e + T h +2Ni+ 2 16 15 1 +
+
LITERATURE CITED
(1) Alan, W. J., Beamish, F. E., ANAL
CHEW24,1608 (1952). (2) Beamish, F. E., Talanta 5, 1 (1960). (3) Forcheri, S., Lungagnani, V., Martini, S., Scibona, G., Energia Nucl. ( M i l a n ) 7,337 (1960). (4) Hoffman, I., Schweitzer, J. E., Ryan, D. E., Bpamish, F. E., ANAL. CHEM.25.1091 (1953). ( 5 ) Marks, 'A. G.,~Beamish, F. E., Ibid., 30., 1461 ~~-~ (1958). (6) Plummer, -%. E. V., Lewis, C. L., Beamish, F. E., Ibzd., 31, 254 (1959). (7) Westland, A. D., Beamish. F. E.. Ibid., 26,739 (1954). (8) Zachariasen. H.. Beamish. F. E.. Ibid., 34, 964 (1962j. RECEIVEDfor review March 5, 1964. Accepted May 12, 1964. Appreciation is expressed to the Kational Research Council of Canada for financial support given t o J. C. Van Loon.
VOL. 36, NO. 9, AUGUST 1 9 6 4
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