produced readily detectable waves where no cyclic voltammetric responses were noted. The use of Equation 16 in estimating attached reactant concentrations for such waves, while exceedingly approximate, is not likely to produce errors greater than a factor of five to ten in the calculated values of rT.
CONCLUSIONS The primary objective of this study was to demonstrate the properties of differential pulse voltammetry as applied to reactants attached to electrode surfaces in order to allow this highly sensitive technique to be utilized in examinations of sub-monolayer quantities of attached reactants. The results have shown that it is possible to provide a semiquantitative account of the observed differential pulse and cyclic voltammetric responses by introducing concentration-dependent activity coefficients for the attached reactant molecules. Having done so, the magnitude of differential pulse peak currents can be correlated with the quantity of adsorbed reactant and the peak potentials with the standard potentials of the reactant couples. Tests of the equations and suggestions presented here are under way with a wider variety of attached reactants in order to verify more precisely the ranges of their validity.
LITERATURE CITED R. F. Lane and A. T. Hubbard, J. Phys. Chem., 7 7 , 1401, 1444 (1973). C. M. Elliott and R. W. Murray, Anal. Chem., 48, 1247 (1976). P. R. Moses and R. W. Murray, J . Am. Chem. SOC.,g8, 7435 (1976). D. 0. Davis and R. W. Murray, Anal. Chem., 49, 194 (1977). T. Kuwana and co-workers, private communlcatlon (1977). (6) A. P. Brown, C. Kovai, and F. C. Anson, J . Electroanel. Chem., 72, 379 (1976). (7) A. H. White, E. Kokot, R. Roper, H. Waterman, and R. L. Martin, Aust. J . Chem., 17, 294 (1964). (8) R. Chant, A. R. Herdrickson, R. L. Martin, and N. M. Rohde, Inorg. Chem., 14, 1894 (1975). (9) R. H. Abei, J. H. Christie, L. L. Jackson, J. G. Osteryoung, and R . A. Osteryoung, Chem. Instrum., 7, 123 (1976). 10) PAR 174 Manual, Princeton Applied Research Corp., Princeton, N.J. 11) J. H. Christie, J. G. Osteryoung, and R. A. Osteryoung, Anal. Chem., 45, 210 (1973). 12) E. Lavlron, Bull. SOC. Chim. Fr., 3717 (1967). 13) H. A. Laitlnen, C. A. Vlncent, and J. J. Bednarski, J. Electrochem. Soc., 115, 1024 (1968). 14) A. T. Hubbard and F. C. Anson In “Electroanalytical Chemistry”, Voi. 4, A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1970. (15) B. B. Damaskin, 0.A. Petri, and V. V. Batrakov, “Adsorption of Organic Compounds on Electrodes”, Plenum Press, New York, N.Y., 1971. (16) A. N. Frumkin, 2.Phys. Chem., 116, 466 (1925). (17) E. Laviron, J . Electroanal. Chem., 52, 395 (1974). (18) B. E. Conway and E. Giieadl, Trans. Fararaday Soc., 58, 2493 (1962). (19) M. Boudart, J . Am. Chem. Soc., 74, 3556(1952). (20) G. Haisey and H. S. Taylor, J . Chem. Pep., 15, 624 (1947). (21) M. Abrarnowitz and 1. A. Segun, Ed., Handbook of Mathematical Functions”, Dover Pubiicatlons, New York, N.Y., 1964. (22) K. Takahashi and F. C. Anson, unpublished results, 1976. (23) C. A. Koval and F. C. Anson. to be submitted. (1) (2) (3) (4) (5)
ACKNOWLEDGMENT Helpful suggestions and comments from Carl Koval and James Flanagan are a pleasure to acknowledge.
RECEIVED for review May 16, 1977. Accepted June 22, 1977. This work was supported by a grant from NSF-RANN.
Ion-Exchange Separation and Determination of Calcium and Magnesium Michael D. Arguello and James S. Fritz” Ames Laboratory-ERDA and Department of Chemistry, Iowa State University, Ames, Iowa 5001 1
Magnesium( 11) and calcium(I1) are separated from each other and from several other metal ions by ion-exchange chromatography on a sulfonated macroporous resln of 1.8 to 2.0 mequlv/g capacity. The eluent Is 1 M ammonlum chlorlde or 0.03 M ethylenedlammoniumchloride. The separated metal ions are detected automatically with a color-forming reagent and are quantitated with the ald of a callbratlon plot.
The separation and determination of calcium and magnesium has been a problem of continuing analytical interest because of the many kinds of materials in which significant amounts of these two elements occur together. Cation-exchange procedures for separation of calcium and magnesium have included elution with hydrochloric acid (1-3), ammonium chloride ( 4 ) , ammonium acetate (5), ammonium acetylacetonate (6),EDTA (7), ammonium lactate @), and pH-5 a-hydroxybutyric acid (9). These procedures are rather slow and are not readily adaptable to automatic detection of the eluted metal ions. Ion exchange with forced eluent flow and automatic detection of eluted species has been successfully used for separation of a number of metal ions. Fritz and Story ( I O ) used spectrophotometric detection after addition of a
color-forming reagent. Small, Stevens, and Bauman (11) employed conductance detection after removal of eluent ions on a “stripper” column. Freed (12) used flame emission for detection of calcium, strontium, and barium separated on a Zipax SCX cation exchanger. A method is now given for rapid separation of calcium and magnesium from each other and from other metal ions. The separation is done on a column containing a sulfonated macroporous resin of low capacity. The elution curves are recorded using a unique color-forming system and spectrophotometric detection.
EXPERIMENTAL Apparatus. The liquid chromatograph has been described previously (13)and is shown schematically in Figure 1. A Milton Roy minipump (Model 396) and a Chromatrix CMP-2 metering pump were used for solvent delivery as well as for delivery of buffered color-forming reagent. Columns were constructed from lengths of 4-mm i.d. Pyrex tubing onto which Altex 200-28 glass connectors had been fused. The use of Viton 0 rings between the polypropylene bushings and caps of the glass connectors resulted in columns that were both leak-tight and chemically inert. All plumbing components (tubing, tube-end fittings, couplings, plugs, tees, valves, sample loops, etc.) used were either purchased from Laboratory Data ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1595
I
WASTE
I
I
3M
4h
ELUENT
CATEX MONITOR PUMPING SYSTEM
t
U RECORDER
EXCHANGE COLUMN
A-
Flgure 1. Block diagram of forced-flow chromatograph
Control or Altex or were constructed in the Ames Laboratory shops. Reagents. Stock solutions (0.01 M) of metal ions were prepared from the best available grade of metal salts. In most cases reagent grade chloride salts were used but in a few cases it was necessary to use metal oxides or other salts. Sufficient hydrochloric acid was added to each stock solution to prevent hydrolysis. Ethylenediammonium chloride (en.2HCl) was prepared by dissolving 100 g of ethylenediamine (J.T. Baker) in 1L of distilled water. The solution was cooled in an ice bath and 330 mL of concentrated hydrochloric acid (Dupont reagent grade) was added slowly with stirring. The en.2HC1 was crystallized by the addition of 2-propanol, filtered with suction, washed with acetone, and air dried. Resin. Partially sulfonated XAD-2 was prepared by a procedure similar to that described by Fritz and Story (IO). A 5- to 10-gportion of the 250-325 mesh fraction was suspended in concentrated sulfuric acid (Dupont Reagent grade) and stirred for 30 min at 102-103 "C. Close control of temperature was maintained by means of a well regulated, electrically heated oil bath. The capacity of resin was 1.8 t o 2.0 mequiv/g. Color-Forming Reagents. To prepare Arsenazo I reagent, 121.1 g of THAM (Tris(hydroxymethy1)aminomethane)was dissolved in approximately 750 mL of distilled water. Sufficient concentrated hydrochloric acid was added to adjust the pH of the solution t o 9.0 and the volume was made up to 1 L using distilled water. Then 0.100 g of Arsenazo I was added with stirring. The resulting solution could be used immediately and was stable at room temperature for a month or longer. PAR reagent was prepared by dissolving 0.100 g of PAR (Eastman No. 7714) in 1 L of THAM-HC1 buffer (pH 9.0). A stock solution of Zn(EDTA) was prepared by reacting stoichiometric amounts of zinc and EDTA stock solutions. The correct ratio was determined by prior titration of the zinc stock solution with the EDTA stock solution using xylenol orange as indicator at pH 5. Several NaOH pellets were added to each liter of freshly prepared solution to raise the pH sufficientlyto prevent the precipitation of the free acid form of EDTA. The Zn(EDTA) M. concentration of the final solution was 7.65 X Fifty mL of this solution was added to each liter of PAR reagent and the final solution was made 0.200 M in sodium, hydroxide. The resulting solution of PAR-Zn(EDTA) reagent was 4.43 X lo4 M in PAR and 3.64 X 3.64 X M in Zn(EDTA) and the pH was approximately 12. Detection Wavelength. A wavelength setting of 590 nm was chosen for use in conjunction with Arsenazo I and a setting of 495 nm for use with PAR-ZnEDTA. Column Parameters. In general a column 7 cm X 0.4 cm was filled with 250-325 mesh sulfonated XAD-2 resin, 1.8 to 2.0 mequivlg. An eluent flow rate of 2 mL/min was usually employed. 1596
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
0
2M 1M CONCENTRATION
OF N H 4 C I
Figure 2. Calcium/magnesium separation factors on AG 50W-X8 and 1.9 mequiv/g low capacity catex as a function of ammonium chloride
concentration The flow rate of color-forming reagent was usually 0.5 mL/min. R E S U L T S AND DISCUSSION Resin. A highly cross-linked resin that does not swell or shrink appreciably is best for column separations where the eluent is under pressure. The resin used here (XAD-2) is a highly cross-linked macroporous polystyrene which has been sulfonated under mild conditions. This results in a resin with excellent exchange kinetics and with lower capacity than conventional resins. The lower capacity (1.8 to 2.0 mequiv/g of air-dried resin) permits use of more dilute eluents than would be needed with conventional resins. With commonly used eluents, the separation factor for calcium/magnesium is also more favorable with the sulfonated XAD-2 resin. Eluent. Hydrochloric acid and several ammonium salts were tested as eluents for separation of calcium and magnesium on sulfonated XAD-2 resin. Of these, ammonium chloride is the best with a separation factor for calcium/ magnesium of approximately 4.0 with 0.6 M ammonium chloride (Figure 2). Several column separations of calcium and magnesium were achieved using 0.8- to 1.0 M ammonium chloride as the eluent. Ethylenediammonium chloride, first proposed as an eluent by Fritz and Karraker (14, 151, is a 2+ cation and is more effective in eluting metal cations than the usual eluents containing 1+ cations. Thus a 0.035 M solution of ethylenediammonium chloride is roughly as effective for eluting calcium and magnesium as a 1.0 M solution of ammonium chloride. The lower concentration of ethylenediammonium chloride eluent also permits a more sensitive detection of calcium and magnesium with color-forming reagents than the ammonium chloride eluents. Distribution coefficients of several metal ions are plotted as a function of ethylenediammonium chloride concentration in Figure 3. Color-Forming Reagents. Solutions of the following indicators were checked for sensitivity of color-forming reaction with calcium and magnesium at 9 different pH values from p H 4.2 to 12.0: Arsenazo I and 111, Sulfonazo 111, Methylthymol Blue, Titan Yellow, Murexide, sodium rhodizonate, Eriochrome Black T, Chlorophosphonazo, and TAR. Of these, Arsenazo I appeared best suited for use in the
Table I. Effective Molar Absorptivities, M-' cm-l Color-Forming Reagents Metal Arsenazo I Arsenazo I ions ( 0 . 8 M NH,Cl)a (0.03 M en 2HCl)= 4 600 1700
Mg
Ca Sr Ba
a
._-
Arsenazo I
PAR-Zn( EDTA)
8 300 6 800 2 500 1400
17 000 22 000
19 000 18 000
Values in parentheses refer to background electrolyte.
Table 11. Chromatographic Separation and Determination of Calcium and Magnesium Ratio of other ion: CaorMg
Other ion Li
+
Na
+
K' NH, Sr2+ Ba2 Be2+, Cd". Co2+.Cuz+.Zn2' UOZit, Fe3;, Mn;+ +
50 50 50 50
0.1 1
+
1
1 each 1 each
Ni2+
1
Er3+.Yb"', Nd3+ H2P0,H,PO, -
0.75 each 25 2.5
Taken
Found
Taken
Found
25.0 25.0 25.0 25.0 25.0 25.0 23.8 25.0 25.0 25.0 25.0 25.0 25.0
24.0 27.2 22.6 24.6 24.6 23.8 23.8
41 41 41 41 41 41 41 41 41 41 41 41 41
41 39 40 38 42 41 41 40 44 42 41 39 41
25.6 24.0 25.5
300
200 ~
4
100-
z
w
9 LL
50-
LL
$ 0
z 0 2
m
30-
20
Ill1
lor
I1
H
K
I 0001 y
01 M
001 _M C O N C E N T R A T I O N OF e n 2HCl
Figure 3. Distribution coefficients of several divalent metal ions on 2.0 mequivlg low capacity catex as a function of ethylenediammonium chloride concentration continuous spectrophotometric detection of calcium and magnesium in column effluents. The sensitivity of Arsenazo I for metal ions is reduced by the eluent salt, especially in the case of calcium (Table I). However, Arsenazo I was used for most of the results reported. Towards the end of the work a much superior color-forming reagent, PAR-ZnEDTA, was developed. Equal-molar concentrations of PAR and zinc(I1)-EDTA are mixed together and buffered a t pH 9.0 with THAM and hydrochloric acid. The reaction with calcium(I1) or magnesium(I1) is as follows:
Ca"
+ Z n E D T A + PAR
+
CaEDTA ZnPAR The molar absorptivity of the resulting color (ZnPAR) is 17O00 for magnesium and 22000 for calcium, vs. a reagent blank (Table I). The kinetics for this reaction are favorable and no delay loop is needed in the detection system. As seen in Table I, this color-forming system is also effective for strontium and barium. -+
W
n 0.2 0.1
2 003M en 2HCI 0 en 2HCI 5
IO
003 M
15en
PHCl 2 0
25
TIME (MIN.)
Figure 4. Comparison of stepwise and single stage elution of calcium and magnesium. Column: 2.0 mequivlg low capacity catex; 7 cm X 0.4 cm. Eluents: 0.03 M em2HCI; 2 mL/min; 0.1 M em2HCI; 2 mL/min. Detection: Arsenazo I color-forming reageant; 0.5 mL/min; 590 nm. Sample: 38.1 pL; 4.63 pg Mg and 30.5 pg Ca Separations. Elution with 0.03 M ethylenediammonium chloride is compared with stepwise elution in Figure 4. The stepwise elution is clearly better but requires careful timing in the change of eluent. For this reason, a constant 0.03 M ethylenediammonium chloride or 0.8 to 1.0 M ammonium chloride was used for all calcium/magnesium separations reported. The effect of several other metal ions on the separation of calcium and magnesium was studied. Beryllium(I1) and aluminum(II1) can be eluted from the cation-exchange column ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1597
n e
-
0.7
M hydrochloric acid in 95% acetone. After 7.5 to 15 min, the recorder trace returns to baseline and calcium and magnesium can be separated. Although the distribution coefficients of both magnesium(I1) and calcium(I1) are high in 0.4 M hydrochloric acid-95% acetone, the magnesium recovery was always low in actual separations. However, results for calcium were always quantitative. Nickel(I1) was found t o be completely eluted with 0.25 M dimethylglyoxime in 0.4 M hydrochloric acid-95% acetone. By this method, nickel(I1) can be separated quantitatively from manganese and other metal ions (Figure 5). Rare earths are more tightly held than calcium and magnesium on the cation-exchange column. Following a calcium/magnesium separation, rare earths can be eluted with 4 M ammonium chloride. Separations can be quantitated by measuring the peak height and reading the amount of metal from a calibration plot, which is nearly linear, of peak height vs. amount of metal ion. Data for quantitative calcium/magnesium separations in various samples are summarized in Table 11. The column chromatographic method can be used to determine separately calcium and magnesium in hard water samples. This procedure should be advantageous when more detailed information than just “total hardness” is required. Results for the chromatographic method are compared with other common analytical methods in Table 111.
k
0.6 -
-
0.5 W
v,
z 0.4
-
0.3
-
w
E
G
+
p g g 0.2 .? 4
1
0.1
0 0
I
I
-0.25M DMG
5
0
I
IO
T I M E (MIN i
ACKNOWLEDGMENT
Figure 5. Separation of manganese(I1)and nickel. Column: 1.9 mequivlg low capacity catex; 7 cm X 0.4 cm. Eluents: 0.4 M %I-95 % acetone; 1.5 mL/rnin; 0.4 M HCI-95% acetone-0.25 M DMG; 1.5 mL/rnin. Detection: Methanolic solution of PAR saturated with THAM; 4 mL/min; 490 nm. Sample: 51.4 pL
Table 111. Analysis of Hard Water Samples Our
Sample Metal analysis, No. detnd ppm 1 Mg2+ 33.5 Ca2+ 99.5 2
Mgz+ CaZt
10.5 38.0
EDTA
tit., ppm ppm 34.0 38.8 106 98.8 AA,
Plasma emission, ppm
34.0 104
10.1
10.5
10.5
36.9
35.9
36.0
with 0.1 M sulfosalicylate (pH 3.6). After 2.5 min, the eluent is switched to 0.8 M ammonium chloride or 0.03 M ethylenediammonium chloride and the calcium and magnesium are separated from each other. Divalent metal ions that readily form anionic chloride complexes can also be eluted prior to a calcium/magnesium separation. Cadmium(II), cobalt(II), copper(II), iron(III), manganese(II), uranium(VI), and zinc(I1) are eluted with 0.4
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
The authors thank Walter Sutherland for performing several plasma emission analyses. They also thank Rohm and Haas for providing the XAD-2 resin used in this work.
LITERATURE CITED (1) 0. Samueison, “Ion Exchange Separations in Analytical Chemistry”, John Wiley and Sons, New York, N.Y., 1963. (2) F. W. E. Strelow, C. J. Liebenberg, and A. H. Victor, Anal. Chem., 46, 1409 (1974). (3) F. W. E. Strelow and C. R. van Zyi, Anal. Chim. Acta, 41, 529 (1968). (4) M. L. Abduilah and J. P. Riley, Anal. Chim. Acta, 33, 391 (1965). (5) A. K. De and A. K. Sen, Talanta, 13, 1313 (1966). (6) G. R . Greenhaigh, J. P. Riley, and M. Tongudai, Anal. Chlrn. Acta, 36, 439 (1966). (7) M. Marhol and K. L. Cheng, Anal. Chem., 42, 652 (1970). (8) G. M. Milton and W. E. Grummitt, Can. J . Chem., 35, 541 (1957). (9) F. H. Pollard, G. Nickiess, and D. Spincer, J. Chromatcgr. 13, 224 (1964). (10) J. S. Fritz and J. N. Story, Anal. Chern., 46, 825 (1974). (1 1) H. Small, T. S. Stevens, and W. C. Bauman, Anal. Chern., 47, 1801 (1975). (12) D. J. Freed, Anal. Chern., 47, 186 (1975). (13) Mark D. Seymour, Ph.D. Thesis, Iowa State University, A m s , Iowa, 1972. (14) J. S. Fritz and S. K. Karraker, Anal. Chem., 31, 921 (1959). (15) J. S. Fritz and S. K. Karraker, Anal. Chern., 32, 957 (1960).
RECEIVED for review May 16, 1977. Accepted June 27,1977. Work supported by the U.S. Energy Research and Development A’dministration, Division of Physical Research.
