588
Anal. Chem. 1986, 58,588-591
Dixanthogen Determination in Flotation Liquors by Solvent Extraction and Ultraviolet Spectrometry Michael H. Jones* a n d J a m e s T. Woodcock CSIRO Division of Mineral Chemistry, P.O. Box 124, Port Melbourne, Victoria, Australia 3207
Dixanthogens, (ROCSS),, may be deliberately added to sulflde flotation systems or can be formed In the ilquor, and there Is a need to determine the small amounts In solution. DIxanthogens can be determlned by UV spectrometry at 241 or 286 nm In an Isooctane extract. Alkyl dlxanthogens In isooctane have a molar absorptlvlty of 17750 f 350 L mol-‘ cm-’ at 241 nm and 8650 f 300 L mol-‘ cm-’ at 286 nm. The wavelength of the absorbance maxlmum at about 241 nm shlfts from 240 nm for ethyl dlxanthogen to 242 nm for n-amyl dlxanthogen. At pH 7, with an aque0us:organlc ratio of 1:1, virtually 100 % of the dlssolved and suspended dlxanthogen Is extracted from the aqueous phase In one stage and the absorbance of the Isooctane extract Is dlrectly proportlonal to dlxanthogen concentratlon In the original aqueous solution up to at least 8 mg/L. The nomlnal detectlon limit Is 0.2 mg/L. Few substances Interfere.
Dixanthogens, (ROCSS)2,are used as collectors in the flotation of sulfide minerals and coals ( 1 ) . They can also be formed by several routes, including mild oxidation of xanthate (ROCSS-), in flotation systems containing xanthate (2). They may be dissolved in the liquor, be present as precipitates in the pulp, or be attached to mineral surfaces (2). There is a need to determine, at least, the concentration in solution. The analytical chemistry of dixanthogens has been reviewed by Karchmer (3). Several techniques have been proposed for the determination of micro- and macroamounts of dixanthogen. These include decomposition by sulfite followed by iodimetry ( 4 ) ,titration with chloramine-T ( 5 ) ,decomposition with trithiocarbonate followed by potentiometry (6), decomposition with zinc amalgam followed by iodimetry or spectrometry (7),decomposition with cyanide followed by iodimetry (8) or colorimetry (9),high-performance liquid chromatography (10, I I ) , polarography (12,13),extraction/ weight (14, 15), and UV spectrometry (16-20). Most of these methods are not suitable, for a variety of reasons, for application to flotation solutions, but UV spectrometry offers the best prospects. However the low solubility of dixanthogens in water M or less) and the low molar absorptivity of dixanthogens (21) preclude direct determination in flotation liquors. Extraction into a suitable solvent is needed. Several authors have used this approach for ethyl dixanthogen without giving any details or molar absorptivities and without indicating whether the method is applicable to other dixanthogens. This paper describes the determination of alkyl dixanthogens by solvent extraction from aqueous solution and UV spectrometry of the extract. Isooctane was chosen as the most suitable extractant. EXPERIMENTAL SECTION Equipment. Spectral scans were obtained with a Hitachi Perkin-Elmer Model 330 recording spectrophotometer using stoppered 1-cm quartz cells and linear wavelength and absorbance presentation. Alkyl Dixanthogens. Pure potassium alkyl xanthates (ethyl, isopropyl, see-butyl, and n-amyl) were first prepared and were
then oxidized in aqueous solution with ammonium persulfate to the correspondingdixanthogen as described previously (22). The dixanthogenswere analyzed for C, H, 0, and S and were examined by reversed-phase liquid chromatography. Isooctane. Ajax “Spectrosol”grade isooctane was used without treatment. In some work, used isooctane was reclaimed for reuse by distillation (99-100 “C) and treatment with silica gel. Procedure. Stock isooctane solutions of each dixanthogen were prepared by dissolving 0.100 g of the reagent in 100 mL of isooctane. Appropriate aliquots were diluted with isooctane to give concentrations in the range C-25 mg/L. Spectral scans were then obtained, using pure isooctane as a reference, the wavelength of each absorbance peak was determined, and the molar absorptivities were calculated. A stock solution of ethyl dixanthogen in ethanol (0.050 g in 25.0 mL of A.R. ethanol) was prepared, and aqueous solutions or suspensions were prepared by slowly adding 20,40,60,80, and 100 MLof the stock solution to 25.0 mL of rapidly stirred 0.01 M phosphate buffer (pH 7.0). After the mixture was stirred for 1 min, the solution or suspension was then extracted with isooctane. Extractions were conducted at an aqueous:organic ratio of 1:l by shaking for 2 min at ambient temperatures. Because disengagement of the phases was not always complete within a few minutes, especially from neutral or slightly alkaline solutions, the isooctane extract was always filtered through Whatman 1PS paper (or equivalent). The UV absorption spectrum of the extract and the raffinate was determined between 205 and 400 nm. RECOMMENDED METHOD Adjust the p H of the sample to 7 immediately after sampling to minimize decomposition of dixanthogen. Take 25 mL of the aqueous solution, add 25.0 mL of spectroscopic grade isooctane, and shake for 2 min. After disengagement of the phases, filter the isooctane extract through Whatman 1PS paper (or equivalent). Measure the absorbance of the isooctane extract at 241 nm and/or 286 nm. Determine the original dixanthogen concentration from a calibration graph prepared in a similar way or from eq 1-4 (for specific dixanthogens) for 1-cm cells
CE~= X ~13.9A240
28.84286
(1)
30.24286
(2)
C s . ~ u=~1z6 . 9 4 2 4 2 = 34.14286
(3)
C n . ~ m=~ 18.1A242 z = 38.64266
(4)
Ci-prxz=
15.1A24, =
where CEtX2,etc., is the concentration of ethyl dixanthogen, etc., in milligrams per liter and A240,etc., is the absorbance at 240 nm, etc., in the 1-cm cell. Interferences. If interference is suspected, check the ratio A241:AzM; if this is not close to 2:1, then some other compound is present and the whole UV spectrum between 205 and 400 nm may need to be determined. In the presence of perxanthic acid, mercaptobenzothiazole, or cresylic acid, in the original aqueous solution, adjust the pH before extraction to about 8.3. RESULTS AND DISCUSSION The elemental analyses and melting points of the four dixanthogens prepared in this work are shown in Table I. The
0003-2700/86/0358-0588$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
589
Table I. Elemental Analysis and Melting Point of Alkyl Dixanthogens dixanthogen
mp, O C
ethyl
29.5"
isopropyl
58.5*
sec-butyl
liquid
n-amyl
liquid
found calcd found calcd found calcd found calcd
elemental analysis, % 0 S
C
H
30.09 29.73 35.48 35.53 40.59 40.23 44.35 44.13
4.30 4.16 5.38 5.22 6.28 6.08 6.71 6.79
13.1 13.20 13.0 11.83 10.8 10.72 10.2 9.80
c
sec-butyl nm
total
53.3 52.91 46.3 47.42 42.5 42.97 39.7 39.28
100.79 100.00 100.16 100.00 100.17 100.00 100.96 100.00
'Literature values 28-32.5 "C ( I ) . bLiterature value 58 "C (3). Table 11. Values of Molar Absorptivities of Alkyl Dixanthogens Amam
this workn
ethyl nm
E
Amam
isopropyl nm
,A,,
240 286
17400 8400
241 286
17850 8950
290
8050
290
9000
240 279 241 286
21880 8130 10420 4650
243 285
22910 8320
Lebedev (16)b Leonov, Bogidaev, and Baranov Kakovskii, Eliseev, and Averbukh (26)b Shankaranarayana and Patel (23)d Pomianowski and Leja
242 286 241 286 241 286 290
Amam
E
17700 8750 16840 8500 10180 7920 7100
n-amyl nm
242 286
t
18000 8450
Isooctane. *Ethanol. Hexane. dHeptane or cyclohexane. e Ethyl ether. elemental analyses were in good agreement with theoretical values. Chromatograms obtained by reversed-phase liquid chromatography showed that the dixanthogens were substantially free from other UV-absorbing species. Spectral Properties of Dixanthogens. The UV absorption spectra of isooctane solutions of the four dixanthogens were similar to those of isooctane extracts of ethyl dixanthogen shown in Figure 1. There are two absorption maxima-one a t about 2 4 1 nm and one a t 286 nm. Table I1 gives the wavelengths of the absorption maxima and the molar absorptivities in isooctane for the four dixanthogens prepared in this work; this appears to be the first time these data have been published. Literature values for ethyl and isopropyl dixanthogens in various other solvents are also shown in Table 11. There is reasonable agreement between these data except for those given by Shankaranarayana and Patel (23) a t 240-243 nm. The dixanthogens prepared in that study may have been contaminated by the iodine used in the preparations. The results obtained in this work (Table 11) show that there is a small shift in the wavelength of the absorption maximum (from 240 nm to 242 nm) of the more intense peak in the series from ethyl to n-amyl dixanthogen. This shift is thought to be outside the range of experimental error and contrasts with alkyl xanthates, which all have the same analytical wavelengths for UV spectrometry (24). The values for molar absorptivity differ slightly but are within the range of experimental error. The molar absorptivities listed in Table I1 were used to calculate eq 1-4, which relate the concentration in milligrams per liter of the individual dixanthogens to the absorbance in isooctane. Use of these equations for a single, known dixanthogen gives the correct value within a few percent. However in flotation practice, where more than one xanthate may be used, mixtures of dixanthogens are likely to occur and the application of any of the equations may not be very accurate at either wavelength. Averaging the factor at each
0
u
C
;
0.2
0 UI
n 4
0.1
0 200
250
300
350
400
Wavelength ( n m )
Figure 1. UV absorption spectra in I-cm cell of isooctane extract (1:l phase ratio) of aqueous phosphate solution (pH 7.0) containing ethyl dixanthogen: (a) 1.6 mglk (b) 3.2 mglL; (c) 4.8 mglL ethyl dixanthogen.
wavelength gives molar absorptivities of 17 750 f 350 L mol-' cm-l a t 2 4 1 nm and 8650 300 L mol-' cm-I at 286 nm. Equation 5 can then be used for individual or mixed dixanthogens
*
(5) where
Cdkm
is the concentration of dixanthogen in moles per
590
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
T a b l e 111. R e c o v e r y of Ethyl D i x a n t h o g e n f r o m A q u e o u s S o l u t i o n by S o l v e n t E x t r a c t i o n recovery,o concn, m g / L
a t 240
nm
1.6
99.1
3.2 4.8b
98.2 101.1
6.4b 8.0b
101.3 100.0
9% a t 286 nm
100.0 99.1
103.6 101.9 100.4
a Calculated from t h e r a t i o of t h e absorbance of t h e extract a n d t h e absorbance of a n isooctane solution of e t h y l dixanthogen. b P a r t solution, p a r t suspension.
liter and A241, A286 are absorbances of isooctane extract in a 1-cm cell. The molar absorptivities of asymmetrical dixanthogens (RIOCS2*S,CORJ, which may also be present when mixtures of xanthates are oxidized ( I I , 25),are unknown but are likely to be intermediate between those shown in Table 11. As such, the errors involved in the determination would be no more than those noted above. Solvent Extraction from Aqueous Solution. Table I11 shows the recovery of ethyl dixanthogen obtained from aqueous solution by extraction with isooctane at 1:l phase ratio. Virtually 100% recovery was obtained up to a t least 8 mg/L in the aqueous phase. Examination of the UV absorption spectra of the raffinates showed no sign of ethyl dixanthogen. For concentrations above 3.2 mg/L ethyl dixanthogen in the aqueous phase, part of the ethyl dixanthogen was present in solution and part was present as a suspension. All dixanthogens are only slightly soluble in water (at pH 7) and the solubility decreases with increasing chain length (26). Literature values for the solubility in water of ethyl dixanthogen range from 7.1 X lo4 M (1.7 mg/L) to 1.3 X M (3.1 mg/L) (23,27). However, as shown in Table 111,the isooctane readily extracts both the dissolved and the suspended ethyl dixanthogen. At 1:lphase ratio and using 1-cm cells the lower limit of detection of ethyl dixanthogen is about 0.2 mg/L. Long-chain dixanthogens are considerably less soluble in water than ethyl dixanthogen (26). Hence for precise determination of the small amounts of these dixanthogens present in solution, i t would be necessary to use aqueous: organic ratios greater than 1:l to increase the dixanthogen concentration in the isooctane extract. In addition, the long-chain dixanthogens are less stable in aqueous solution than ethyl dixanthogen (28), and it is even more important to conduct the analysis as soon as possible after sampling. Because dixanthogens are nonionic compounds, extraction from aqueous solution with isooctane is possible at any pH. However the self-decomposition of dixanthogens is pH-dependent with a minimum rate at about pH 7 (22).Hence it is recommended that the pH of the aqueous solution be adjusted to 7 as soon as possible after sampling. As shown in Table 111, complete extraction of ethyl dixanthogen was obtained in 2 min of shaking. Effect of Thiophilic Compounds. Thiophilic compounds, such as hydroxide, sulfite, thiosulfate, and cyanide, which are commonly present in flotation liquors, do not interfere per se in the solvent extraction of alkyl dixanthogens. Such compounds, however, decompose alkyl dixanthogens, and the rate of decomposition is determined by the relative concentrations (22,28). As noted above, the effect of hydroxide can be reduced by adjusting the pH to 7. Sulfite was previously shown to be a strong thiophilic compound (22). It is thought that its effect can be minimized by acidification and addition of formaldehyde, immediately
after sampling, to form the sulfite-addition complex (29). It is necessary to acidify the sample first to destroy any xanthate present (24),because if xanthate is present it reacts to form a xanthate-formaldehyde complex (30). The xanthate-formaldehyde complex would probably be extracted by isooctane, because it is extracted by benzene (31) in which it has an absorption maximum at about 290 nm, and therefore would interfere with dixanthogen determination. Effect of Other Compounds. There are very few commonly occurring compounds in flotation liquors that can be extracted into isooctane and have a UV absorbance at about 240 nm or 286 nm. Compounds that are in this category include mercaptobenzothiazole and cresylic acid (32) and perxanthic acid (33). Their effect can be minimized by adjusting the pH to about 8.3 to convert the acid to the corresponding salt and by extracting immediately to avoid dixanthogen decomposition. As a general rule, in order to check if an interfering compound is present in the extract, it is desirable to record the whole of the UV absorption spectrum of the extract and to compare the shape with that shown in Figure 1. In addition, the ratio A241:A286should be calculated; in pure alkyl dixanthogen solutions this ratio is 2:l (eq 1-4) and any deviation indicates that an interfering compound has been extracted. Registry No. (ROCSS)2 (R = ethyl), 502-55-6; (ROCSS)2(R = isopropyl), 105-65-7;(ROCSS)2 (R = sec-butyl), 54503-00-3; (ROCSS)2 (R = n-amyl), 869-91-0.
LITERATURE CITED Reid, E. E. "Organic Chemistry of Bivalent Sulfur"; Chemical Publishing Co.: New York, 1962; Voi. IV, Chapter 2. Jones, M. H.; Woodcock, J. T. I n "Principles of Mineral Flotation"; Jones, M. H., Woodcock, J. T., Eds.; Australasian Institute of Mining and Metallurgy: Melbourne, 1984; pp 146-183. Karchmer, J. H. I n "The Analytical Chemistry of Sulfur and Its Compounds"; Karchmer, J. H., Ed.; Wiiey-Interscience: New York, 1972; Part 11, pp 594-597. Rao, V. R. S.;Murthy, A. R. V. Chemist-Analyst 1961, 50, 30. Rao, S. R. Talanta 1961, 8 , 746-747. Verma, B. C.; Butail, J.; Sood, R. J. Analyst (London) 1982, 107, 691-695. Prasad, M. S. Talanta 1971, 18, 649-652. Shankaranarayana, M. L.; Patel, C. C. Analyst (London) 1961, 8 6 , 96-10 1. Higgins, J. G.;Mukherjee, L. M. Chem. Ind. (London) 1974, 7 ,317. Hasty, R. A. Analyst (London) 1976, 101, 828-829. Eckhardt, J. G.;Stetzenbach, K.; Burke, M. F.; Moyers, J. L. J. Chromatogr. Sci. 1978, 16, 510-513. Bond, A. M.; Sztajer, 2.; Winter, G. Anal. Chim. Acta 1976, 8 4 , 37-46. Plaksin, I.N.; Shrader, E. A. Izv. Vyssh. Uchebn. Zaved., Tsvetn. Metall. 1962, 5(1),41-43. Chem. Abstr. 1962, 57,40391. Plaksin, I.N.; Dzhakipov, T. D.; Shafeev, R. Sh. Tr. Inst. Met. Obogashch., Akad. Nauk Kaz. SSR 1968, 32, 18-23. Chem. Abstr. 1969, 71,63303n. Dzhaklpov, T. D. Met. Obogashch. 1969, No. 4 , 145-149. Chem. Abstr. 1971, 75,1 0 0 1 0 0 ~ . Lebedev, V. D. I n "Kontroi Ionnogo Sostava Rudn. Pul'py Fiotatsii, (Dokl. Vses. Nauchn. Soveshch.)"; Suvorovskaya, N. A,, Okolovich, A. M., Eds.; Nauka: Moscow, 1974; pp 131-138. Chem. Abstr. 1975, 83, 1 0 7 6 9 5 ~ . Suvarovskaya, N. A.; Shikhova, V. V. USSR Patent 156748, 1963. Chem. Abstr. 1964, 6 0 , 62221. Leonov, S. B.; Komogortsev, 8. V. I z v . Vyssh. Uchebn. Zaved., Tsvetn. Metall. 1963, 1 1 (3), 151-154. Chem. Abstr. 1969, 70, 93013q. Drugov, Yu. S. Vestn. Tekhn. Ekon. Inf. Nauchno-Issled. Inst. Tekhn .-Ekon . Issled. Gos Kom Khim . Promsti, Gosplane SSSR 1963, IO, 49. Chem. Abstr. 1965, 6 2 , 74968. Leonov, S. B.; Bogidaev, S.A,; Baranov, A. N. Obogashch. Rud(Irkutsk) 1981, 3-7. Chem. Abstr. 1982, 97, 190194q. (CSIRO Translation No. 13292). Flnkelsteln, N. P.; Poling, G. W. Miner. Sci. Eng. 1977, 9 (4), 177-197. Jones, M. H.; Woodcock, J. T. Int. J. Mlner. Process. 1963, 10, 1-24. Shankaranarayana, M. L.; Patel, C. C. Can. J. Chem. 1961. 39, 2590-2592. Jones, M. H.; Woodcock, J. T. "Ultraviolet Spectrometry of Flotation Reagents with Special Reference to the Determination of Xanthate in Flotatlon Liquors"; Institution of Mining and Metallurgy: London, 1973. Hasty, R. A. Analyst (London) 1977, 102, 519-524. Kakovskii, I. A,; Eliseev, N. I.; Averbukh, A. V. I z v . Vyssh. Uchebn. Zaved., Tsvetn. Metall. 1979, 2 , 7-10. Chem. Abstr. 1979, 9 1 , 77208k.
.
.
Anal. Chem. 1986, 58,591-594 (27) Pomianowski, A.; Leja, J. Can. J. Chem. 1963,41, 2219-2230. (28) Kirbitova, N. V.; Anurkina, N. V.; Eiiseev, N. I. I z v . Vyssh. Ucbebn. Zaved., Gorn. Zh. 1978, 7 , 134-137. Chem. Abstr. 1979, 9 1 , 143956f. (CSIRO Translation No. 13098). (29) Williams, W. J. “Handbook of Anion Determination”; Butterworths: London, 1979;p 587. (30)Thumm, B. A,; Tryon, S. J. Org. Chem. 1964,2 9 , 2999-3002.
59 1
(31) Rolia, E. Trans.-Inst. Min. Metall., Sect. C 1970, 79, C207-C214. (32) Jones, M. H.; Woodcock, J. T. Anal. Chem. 1975,47, 11-16. (33) Jones, M. H.; Woodcock, J. T. Talanta 1979,26, 815-820.
RECEIVED for review July 15,1985. Accepted October 4,1985.
Microdetermination of Phosphate in Water by Gel-Phase Colorimetry with Molybdenum Blue Kazuhisa Yoshimura* Chemistry Laboratory, College of General Education, Kyushu University, Ropponmatsu, Chuo-ku, Fukuoka 810, J a p a n Masatoshi Ishii a n d Toshikazu T a r u t a n i Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashiku, Fukuoka 812, J a p a n
The blue specles of molybdophosphate are strongly adsorbed on Sephadex gels. Almost all the blue species in a 50-cm3 sample solution are concentrated in 0.20 g d Sephadex 0 2 5 (fine) wlthln 10 mln. Direct absorptiometry of the heteropoly acid concentrated in the gel phase was developed for the determlnatlon of the phosphate at parts per billion levels In natural waters. The colored gel beads, on which the blue specles reduced by ascorbic acid In the presence of antimonyl Ions were adsorbed, are packed Into a 10mm cell; the attenuances at 836 and 416 nm are measured; and the attenuance dlfference Is used for the determination of traces of phosphate. The use of cells of other length (5, 2, and 1 mm) gave a wide concentration range for calibration from parts per billlon to parts per million levels. This method is simple In operatlon and has a high reproduclbillty of measurements.
Phosphate reacts with molybdate in a strong acidic solution to produce the yellow 12-molybdophosphate (12-MF’A) species (1). In the determination of the amount of phosphate in various samples, the yellow species derived by chemical reaction is absorptiometrically employed in many laboratories. It is, however, difficult to determine phosphate a t microgram per cubic decimeter levels in water, even though a long light-path cell is used after reduction of the yellow species. A preconcentration step such as solvent extraction (2) or coprecipitation with metal hydroxides (3) is usually necessary to prolong operation time and lower the precision of analysis. It has been shown that direct absorptiometry of an element concentrated in the solid phase leads to a simple method of high sensitivity (4-7). For phosphate determination, anionexchanger phase absorbance of the molybdenum blue has already been used, but it takes a rather long time for color development (8),and the color in the resin phase is not stable (9,10). In the previous paper, a sensitive method for silicic acid was reported that used solid-phase absorptiometry after concentration of the blue species on Sephadex G-25 (11).With this method, the amount of silicic acid at the parts per billion level or lower could precisely and simply be determined for high-purity industrial waters. This paper describes the ap-
plication of this technique to determine trace levels of phosphate. EXPERIMENTAL SECTION Reagents. All reagents were of analytical grade and deionized-distilled water. A standard phosphate solution (50 mg of P/dm3) was prepared by dissolving 0.220 g of potassium dihydrogen phosphate and diluting it to 1 dm3. A combined reagent was prepared by mixing 50 cm3 of 2.5 mol/dm3 sulfuric acid solution (70 cm3of concentrated sulfuric acid diluted to 500 om3),5 cm3of potassium antimonyl tartrate solution (0.274 g of potassium antimonyl tartrate hemihydrate in 100 cm3of water), 15 cm3of 4% (w/v) ammonium molybdate solution, and 30 cm3of 0.1 mol/dm3 ascorbic acid solution (0.76 g of ascorbic acid in 100 cm3 of water). Sephadex G-25 (fine) was purchased from Pharmacia Fine Chemicals. Apparatus. Absorbance measurements were made with a Nippon Bunko spectrophotometer, Model UVIDEC-320. An inside-mirrortube (12 mm i.d., 40 mm length) was placed between the cell and the detector window for recovering the scattered light from the gel layer (7,12).A perforated metal plate ( A = 1.0) was used in the reference beam to balance the light intensities. The sample cell was similar to that described in the previous paper (11). The thickness of the gel layer was varied by inserting quartz spacers of varying thicknesses into an ordinary 10-rnm quartz cell. An acrylic resin spacer was also used to make certain the entire light beam struck only the packed area. Procedure for Gel-Phase Colorimetry of Phosphate. Natural waters were filtered through a 0.45-pm membrane filter paper (Millipore). First, 8 cm3of the combined reagent and then 0.20 g of the gel beads were added to a 50-cm3water sample containing 0.05-5 pg of phosphate-phosphorus in a poly(ethy1ene) container. After the mixture was stirred for 10 min at 15-20 OC, the colored gel beads were allowed to settle. Cell lengths (1,2, 5, or 10 mm) were selected depending on the degree of the color intensity of gel beads. The gel was transferred to the sample cell with a pipet. The cell was set in an ordinary holder in the spectrophotometer. The attenuances at 836 nm and 416 nm were measured with air as a reference. The differences between the attenuances (AA) were used for determining the level of phosphate. A calibration graph of each cell length was obtained by taking standards throughout the procedure described above. Distribution Measurement. To a 250-cm3 water sample or 1.67 X lo4 mol/dm3 phosphate and 34.5 containing 2.78 X cm3of the combined reagent solution, an appropriate amount of the gel (0.01-0.05 g) was added. The mixture was stirred for 4 h at 20 “C. Then the concentration of the blue species in the
0003-2700/86/0358-059 1$01.50/0 @ 1986 American Chemical Society
