Determination of Metals in Mixed Hydrochloric and Perchloric Acids by Forced-Flow Anion Exchange Chromatography Mark
D. Seymour1 and James S. Fritz
Ames Laboratory-USAEC
and the Department of Chemistry, lowa State University, Ames, lowa 50070
Anion exchange behavior of ten metal ions is studied in mixtures of hydrochloric and perchloric acids. Several metal ions can be eluted with this mixture that are almost impossible to elute with hydrochloric acid alone. Using hydrochloric-perchloric acid mixtures, several separations are demonstrated with forced-flow chromatography and automatic UV detection of the eluted elements. The separations are rapid and quantitative.
Since the first use of anion exchangers for the recovery of metals in 1945, anion exchange has become a widely used chromatographic technique for achieving quantitative separation of metal ions prior to analysis ( I ) . The distribution properties of a large number of metal ions on anion exchange resins in various media have been reported (2-12). Hydrochloric acid has proved a useful medium for anion exchange chromatography because it can be used to dissolve a large number of metals which vary widely in their adsorbabilities a t various acid concentrations (13-16). The strong absorption of a few metals which resist rapid elution a t any hydrochloric acid concentration is a serious limitation to use of this system. Mixtures of hydrochloric and hydrofluoric acids have been employed to elute sequentially some of these elements (17). However, there remains a need for a more versatile separation medium that is chemically compatible with the hydrochloric acid system. Hydrochloric acid not only serves as a useful reagent for separation of metals but can also be used for their quantitative spectrophotometric determination (18-20). Recently anion exchange separation and spectrophotometric analysis have been coupled by continuous monitoring of column
effluents. This technique has been successfully applied to the rapid determination of iron(II1) and lead(I1) (21, 22). The element to be analyzed is first sorbed to a column of anion exchange resin, enabling its concentration and separation from interfering ions. The eluent is changed and the element eluted; the height of the recorded peaks in absorbance is proportional to sample concentration. This paper describes a new solvent system, a gradient mixture of 10M HC1 and 5M HC104. This not only enables elution of metals strongly retained on chloride-form anion exchange resins but also makes possible several useful separations of these ions. In addition it enables detection of these elements uia the ultraviolet absorption of their chloride complexes. Quantitative analysis of a multicomponent sample is illustrated.
EXPERIMENTAL
(3) (4)
F. Nelson, "Metal Separations by Anion Exchange," in ASTM Special Technical Publication No. 195, American Society for Testing and Materials, Philadelphia, Pa., 1958. J. P. Faris and R . F. Buchanan, Anal. Chem.. 36, 1157 (1964). F. Ishikawa, S. Uruno, and H . Imai, Bull. Chem. SOC.Jap., 34, 952
(5) (6) (7) (8) (9) (10) (11) (12) (13)
J. P. Faris, Anai. Chem.. 32, 520 (1960). W. E. Strelow and C. J . C. Bothma, Anai. Chem., 39, 595 (1967) J. S. Fritzand D. J. Pietrzyk, Taianta, 8 , 143 (1961). C. W. Walter and J. Korkisch, Mikrochim. Acta, 1971, 81 C. W . Walter and J . Korkisch, Mikrochim. Acta, 1971, 137. C. W . Walter and J. Korkisch, Mikrochim. Acta. 1971, 158. C . W . Walter and J. Korkisch, Mikrochim. Acta, 1971, 181 C. W. Walter and J . Korkisch, Mikrochim. Acta. 1971, 194. K . A. Kraus, F. Neison, and G . W . Smith, J. Phys. Chem.. 5 8 , 11
Apparatus. A liquid chromatograph was used to determine the adsorbabilities of ten metals studied as well as to achieve the separations described. The design has been modified from that previously described ( 2 1 ) .A schematic diagram is shown in Figure 1. The polyethylene caps on the eluent bottles have been replaced with ones machined of Teflon and braced with a steel plate. The plate is anchored by two wing nuts affixed to a n aluminum collar about the neck of the bottle. The Dacron tubing is admitted through specially machined Kel-F fittings threaded through the Teflon cap. Eluent escapes through the cap which acts as a coupling for the interior and exterior Chromatronix eluent tubing. Use of hydrochloric acid eluents caused severe corrosion of the brass valves in the gas pressurization manifold due to back diffusion of hydrogen chloride and water vapor. More resistant Powell Stainless Steel Globe valves have been installed. A stainless steel reservoir (capacity 500 ml) has been installed immediately ahead of the gas pressurization manifold to allow occasional purging with water and acetone to inhibit corrosion and prevent eluent contamination. The pneumatic actuation manifold was detached from the high pressure helium line regulated from 0-250 psig and pressurized using compressed air a t 90 psig. This enables regulation of the pressure applied to the manifold using a Harris Model No. 92-50 Helium Regulator and minimizes helium expenditure. This modification, combined with replacement of the eluent bottle caps. expands the pressure range to 55 psig with a tested safety factor of 3.3. T o remove bubbles which collect in the flow meter, the waste valve on the eluent selection manifold has been replaced by a three-way valve between the manifold and injection port. The other end of this loop is connected to a tee between the detector and the flow meter. This allows by-pass of the column and a sharp increase in eluent flow rate sufficient to purge the meter. It also allows rapid purging of the eluent manifold when a tank is repressurized. Two additional eluent tanks have been added t o facilitate separation of complex mixtures where a greater number of eluents are required.
(1954). (14) F. Nelson and K . A . Kraus, J. Amer. Chem. SOC.,76, 5916 (1954). (15) K. A . Kraus, F. Nelson, F. B. Clough, and R. C. Carlston, J. Amer. Chem. SOC..77, 1391 (1955). (16) K . A. Kraus and F. Neison, International Conference on the Peaceful Uses of Atomic Energy, paper No. 837, Geneva, 1955. (17) F . Nelson, R . M. Rush, and K . A. Kraus, J. Amer. Chem. Soc.. 82, 339 ( 1960). (18) M . A . Deseaand L. B. Rogers, Anal. Chim. Acta. 6, 534 (1952).
(19) C. Merritt. J r . , H . M . Hershenson, and L. B. Rogers, Anal. Chem., 25, 572 ( 1953). (20) I . L. Goodkin. M. D . Seymour, and J . S. Fritz, unpublished work, 1972. (21) M D Seymour, J P Sickafoose. and J S Fritz, Anal Chem 43, 1734 119711 (22) M D.Seymour and J S Fritz Anai Chem , in press
Present address, The Procter and Gamble Company, Miami Valley Laboratories. Box 39175, Cincinnati, Ohio 45239. (1) S. Sussman, F. C. (1945). (2) K. A. Kraus and
Nachod, and W . Wood, lnd. Eng. Chem.. 37,
618
(1961).
1394
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
HELIUM REGULATOR
GAS M A N I F O L D F L U S H I N G SOLVENT RESERVOIR
I
I
U
E
MICROVOLTMETER AMPLIFIER
v,J
0
EVENT MARKER RECORDER
r-------I I
I I
i
Figure 1. Schematic diagram of liquid chromatograph
The bleed valve on the gas manifold has been connected to a scrubbing tank, forcing the escaping gas to bubble through an aqueous caustic solution. This prevents venting of hydrogen chloride to the atmosphere. To neutralize column effluent, the waste is drained into a bed of crushed limestone 2 ft deep. A 10-ml graduated effluent collector has been added after the flow meter to enable calibration of the meter and accurate determination of small retention volumes. A fivefold increase in detector sensitivity is obtained by replacing the quartz tube with a 1.0-cm path length z-configuration flow-through cell. The cell is constructed of a Kel-F body with opaque Teflon caps holding 1-mm quartz windows. The body is threaded to accept Chromatronix tube and fittings. The cell is mounted in the spectrophotometer immediately adjacent to the phototube compartment. Use of a small aperture cell (2-mm diameter) necessitates amplification of spectrophotometer output. This is done by installing a Kaylab Model 202B microvoltmeter between the spectrophotometer and the recorder. An event marker used for direct correlation of recorder display and collected effluent volume has been installed across the recorder input terminal by connecting a t a p key and a 1000-ohm resistor in series. Reagents. Resin. Amberlyst A-26 macroreticular strong base anion exchange resin was obtained from the Rohm and Haas Co. In the chloride form, the capacity is about 4.1-4.4 mequiv/gram dry or 0.95-1.1 mequiv/ml. The resin was washed in dilute hydrochloric acid, concentrated hydrochloric acid, methanol, and water prior t o grinding water moist with mortar and pestle. The resin was sieved before drying and the 150-200 mesh fraction retained. Extreme fines were removed by methanol flotation. Before weighing, the resin used in column separations was rinsed with acetone, air dried, and finally dried under vacuum over anhydrous calcium sulfate for 24 hr. For batch distribution studies, a portion of the resin, prepared as described, was placed in a column and washed with 5M per-
chloric acid until the effluent was optically transparent in the ultraviolet. This serves to leach organic impurities as well as effect complete conversion t o the perchlorate form. The resin, after washing with deionized water until the column effluent reached p H 6, was air dried before weighing. Metal Ions. Except for platinum(1V) prepared from the metal, all metal ion solutions were prepared by dissolving reagent purity oxides or chlorides in either 6 M HC1 or a mixture of 0.2M HCl4.9M HC104. From these stock solutions, dilutions were made adding 70% perchloric acid, concentrated hydrochloric acid, and distilled-deionized water in appropriate amounts to match the eluent and sample compositions. Chromium(VI) was first dissolved in dilute perchloric acid or water. To avoid reduction to chromium(III), no hydrochloric acid was added until just before injection. Ruthenium(1V) was prepared by storing a solution of the commercial “Trichloride” in 6M HC1 under air for a year. After dilution, ruthenium(1V) and tin(1V) were allowed to stand 10 hr prior to injection. For column retention studies, the metal concentrations in the injected solution ranged from 100 /rg/ml to 0.1 Fg/ml, depending upon the sensitivity of detection. For the analytical separations concentrations from 40-10,000 pg/ml were employed. For batch equilibrations, mercuric oxide was dissolved in measured amounts of perchloric or perchloric and hydrochloric acids. The acidity was then adjusted by addition of sodium hydroxide and diluted to volume. The mercury(I1) concentration was 0.0200M. Procedure. Column Preparation. In changing from the chloride to the perchlorate-form, strong-base anion exchange resins shrink markedly (23). To prevent continual change of column length and bed density when changing eluents, a special packing procedure was used. A Chromatronix LC-6M-13 column (6.35-mm i.d.1 was packed in a vertical position with one of two outlet plungers in place. A thick aqueous slurry of chloride-form resin was added and allowed to settle while maintaining a constant flow by apply(23) Y . Marcusand J. Naveh, J . Phys. Chem., 73,591 (1969)
ANALYTICAL CHEMISTRY, VOL. 4 5 , NO. 8, JULY 1973
1395
90 W
6o
1 0
1
2
3
4
5
6
7
8
9
1
0
NUMBER of C O N V E R S I O N S from the CI- to the C l O i FORM-
Figure 2. Shrinkage data for a column of Amberlist A-26 anion exchange resin
300
r
t ---I-----
io0 -
t 0
2
30
Figure 4. Separation of 50 pg of arsenic(lll), 2.5 pg of bismuth( I l l ) , and 250pg of antirnony(ll1)
z
e=
;
IO
-
Conditions: resin, Amberlyst A-26; column, 9.15-cm X 0.63-cm i.d.; sample volume, 50 pl; flow rate of 4M HCI, 1.0 ml/min; detection wavelength, 225 n m
v1
E
'3
w
+[HCII
IOM 9M 8M 7M 6M 5M t
:
:
:
:
:
4M 3M :
:
2M IM OM :
:
4
O M O 5 M lOM15M2OM25M30M35M40M45M50M[HC1O41 + GRADIENT
COMPOSITION
Figure 3. Distribution ratios for ions displaying minima
ing suction at the column outlet. The other plunger was inserted and the column compressed. The column was then installed in the chromatograph and treated with water, 3M HClOI, water, 3 M HC1, and again water in 10-ml portions. The column was then reversed, both ends were compressed, and the procedure was repeated. This was continued for 10-15 cycles or until the bed no longer pulls away from the inlet plunger after conversion to the perchlorate form. This procedure was employed for the 9.15, 3.80, and 1.85 cm columns employed containing 1.072, 0.400, and 0.241 gram of dry chloride-form resin, respectively. Shrinkage data for the 9.15-cm (final length) column is shown in Figure 2 . Distribution Ratios. Distribution ratios of metal ions with A-26 resin were determined from recorded column elution curves. With the detector set at 225 nm, the flow rate was adjusted to between 0.5 and 1.0 ml/min for the 9.15-cm column or from 1.0 to 2.0 ml/ min for the 1.85-cm column. The sample was injected using a 50-ri sample loop and the elution curve (in absorbance) recorded. Nickel(I1) was used as the reference ion because it has a retention volume of zero at all concentrations hut has UV absorption sufficient for detection. The weight distribution ratio was then calcuiated.
D, =
(retention volume metal ion) (retention volume nickel) (weight of resin in column)
-
RESULTS Distribution ratios for metal ions in hydrochloric acidperchloric acid mixtures are given in Table I. The distribution ratio for several metal ions decreases in regular fashion with increasing perchloric and decreasing hydrochloric acid concentrations. The curves for some of the metal ions pass through minima, as illustrated by mercury(II), antimony(III), bismuth(III), and thallium(II1) in Figure 3. With the exception of gold(II1) and thallium(III), incorporation of perchloric acid in the eluent lowers the
(1)
By using the graduated eluent collector and event marker, di-
1396
rect calibration of the recorded peak was possible in 100-pl divisions allowing estimation of retention volumes to f 2 0 r l . Distribution ratios from t o lo2 can be determined using the 9.15cm column, but best results were obtained for values from 0.1 to 10. To shorten the time required to elute strongly sorbing ions, the 1.85-cm column was used to determine approximate values for distribution ratios greater than 50. Weight distribution ratios for mercury(I1) were determined after batch equilibration of 10 ml of solution with 1.000 gram of resin for 1 hr. A 5-ml aliquot of the aqueous phase was made to pH 6 with sodium hydroxide and sodium acetate, excess EDTA was added, and mercury(I1) determined by back titration with zinc(I1) using Naphthyl Azoxine S as the indicator ( 2 4 ) . Separations. All separations were monitored at 225 nm. The 50-pl sample loop was used. Separation of nickel(II), palladium(II), and platinum(1V) and separation of arsenic(III), antimony(III), and bismuth(II1) were accomplished on the 9.15-cm column with an initial flow rate of 1.0 ml/min. The 3.80-cm column with a flow rate of 0.86 ml/min was used to separate lead(II), copper(II), iron(III), mercury(II), and tin(1V). The pressure was adjusted to yield the appropriate flow rate and the sample was injected. Eluents were changed in a time sequence, compensating for the volume between eluent manifold and detector. The columns were regenerated using alternate 10-ml portions of water and 6M HC1.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
(24) J . S. Fritz, J . E . Abbink, and M. A . Payne, Anal. Chem., 33, 1381 (1961).
Table I . Anion Exchange Weight Distribution Ratios in a Gradient Mixture oflOM HCI and [HCIIM. [HCIOIIM
10.0, 0.0 9.0, 0.5 8.0, 1 .O 7.0, 1.5 6.0, 2.0 5.0, 2.5 4.0, 3.0 3.0, 3.5 2.0, 4.0 1 .o, 4.5 0.6, 4.7 0.2, 4.9 0.0, 5.0
Sb(ll1)
-71 21.6 11.0 7.23 5.68 4.50 4.38 4.54 5.42 8.30 11.1 16.3 3.64
-
EI(llI)
93 18.6 6.44 3.14 1.95 1.17 0.87 0.72 0.60 0.53 0.72 1.88 7.81
Cr(VI) 1.66 0.61 0.29 0.25 0.20 0.20 0.23 0.21 0.23 0.15 0.1 7 0.02 0.02
Au(l II)
>250 >250 >250 >250 >250 >250 >250 >250 250 200 200 200 -190
--
-
distribution ratios of all metal ions studied sufficiently to permit elution from an anion exchange column. Examination of Table I shows sufficient differences in distribution ratios in the hydrochloric-perchloric acid system to permit a number of column separations of metal ion mixtures. Also, conventional anion exchange separations based on variation in hydrochloric acid concentration alone are still possible. In either system, many metal ions form chloride complexes which absorb strongly in the UV, thus automatic detection and recording of eluted metal peaks is feasible. The separations described below were carried out to demonstrate the utility and rapidity of anion exchange separations using the hydrochloric-perchloric acid system using a forced-flow liquid chromatograph. The separation of arsenic(III), antimony(III), and bismuth(II1) was carried out and automatically recorded, as shown in Figure 4. Arsenic(II1) is eluted with 4M hydrochloric acid and the other two metal ions are strongly retained by the column. Upon switching to 1M HC1-4.5M HC104, bismuth (Ow = 0.53) is rapidly eluted and is separated from the following elution peak for antimony (D, = 8.30). The negative peak and the following positive peak in the base-line curve result from the change in eluent. The separation of nickel(II), palladium(II), and platinum(1V) is shown in Figure 5 . Although nickel(I1) is separated from the other two by elution with 6M hydrochloric acid alone, palladium(I1) and platinum(1V) must be separated from each other using the hydrochloric-perchloric acid system. Again, a base-line peak is observed which results from a change in eluent composition. More complicated separations can be achieved, as illustrated by the separation of five elements recorded in Figure 6. Here, lead(I1) is eluted with 8M hydrochloric acid and the other elements are held by the column. Before the lead(I1) is completely eluted, the eluent is changed to 4M HCl to complete elution of the lead and to elute copper (11). Iron(II1) is eluted with 1M HC1, mercury(I1) with 4M HC1-3M HC101, and finally tin(1V) is removed from the column with 0.1M HC-5.95M HC101. The entire separation is complete in only 25 min. Although the separations cited are complete, quantitative analysis of mixtures also depends on the ability to relate reproducibly peak height (or peak area) to the amount of a particular metal (21). Figure 7 shows calibration curves for four of the five metal ions separated in Figure 6. For mixtures of this type, it is necessary to separate the detector response for UV absorption of the eluted ion from the response caused by a change in eluent composition. In Figure 6, the sharp base-line peak precludes quantitative detection of iron(II1). The base-line peak near
Hg(ll)
-150 34.4 17.2 9.79 6.92 4.36 3.54 2.89 2.43 1.92 1.94 2.28 4.31
Pt(lV)
Pd(ll)
>166 >166 96 40 41 .O 24.3 17.9 13.0 11.1 9.01 7.85 7.10 7.12
-56 12.2 5.24 3.05 2.14 1.41 1.12 0.85 0.74 0.62 0.45 0.35 0.56
--
5M HC104 Ru(lV)
TI(Il1)
>zoo
-----
3.82 1.68 0.90 0.47 0.34 0.19 0.1 1 -0 -0 -0
83 69 67 75 65 62 60 97 -150 -170 -110 -170
... ... ...
Sn(lVI
>zoo >zoo >zoo >zoo -200 -110 67 47 47.6 29.3 22.4 13.2 1.66
-
S 0 LVEN T
Ly
a 0.60 v)
m
/\
Figure 5. Separation of 5 0 0 p g of nickel(ll), 2 p g of palladium(ll), and 5 0 p g of platinum(1V) Conditions: resin, Amberlyst A-26, column 9.16-cm X 0.63-cm iid.; sample volume, 50 PI: flow rate of 6M HCI, 1.0 m l i m i n ; detection wavelength, 225 nm
mercury(I1) causes no difficulty provided peak height, rather than peak area, is used for the quantitation. Similarly, the base-line solvent peaks do not interfere a t all with the lead, copper, or tin peaks. The calibration plots in Figure 7 were prepared by separating successive dilutions of two solutions containing varied proportions of these five metal ions. These plots are linear except for lead(I1). A curved plot is common for metals that are not taken up by the resin under the sample injection conditions. Lead(I1) has a linear calibration curve if it is first sorbed a t 0.5M hydrochloric acid and later stripped with 8M acid (22). The sensitivity of detection varies because of differences in molar absorptivity of various metal complexes and because a fixed wavelength (225 nm) is used for all elements. The sensitivity may be diminished in some cases where a base-line solvent peak coincides with a metal ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
1397
0.9
FoKU)
CD
SO VENT 0.8
1
0.7
W
2 0.6 m a
Table II. Anion Exchange Weight Distribution Ratios for Mercury( I I) [H+]
[C104-], M
pH1 pH1
0.9 4.9 1 5 4.9
1M 5M
K
0.5
5M
[Na+], M
[CI-1, M
D W
0.9 4.9
0.1 0.1
22.4
...
...
... ...
0.1
