be seen from Figure 7, couples with rate constants larger than 0.1 cm/sec are effectively reversible while polarograms of couples with rate constants smaller than cm/sec are rather insensitive to variations in k " . This insensitivity is responsible for the large uncertainty in the values of k" found for Eu3+/Eu2+.As the modulation amplitude is decreased the uncertainty in k " also decreases, implying that peak shape is more sensitive to kinetic parameters at low modulation amplitude. DPP using the commercial PAR 174 instrument is best suited for studying reactions with rate constants in the 10-1-10-4 cm/sec range. The shorter pulse widths available with certain custom designed instruments should permit the study of faster reactions. Either the graphical or the curve fitting technique offers a means for studying the kinetics of charge transfer reactions involving species a t the 10-5-10-6 M level. This is two orders of magnitude lower than now possible. This should aid in studying the concentration dependence of kinetic parameters and in studying materials of limited solubility. Digital simulation is an effective tool for investigating the effects of less than ideal red/ox reactions on differential pulse polarographic analysis. The effects of homogeneous
chemical kinetics (e.g. preceding, following, or catalytic reaction) are currently under examination in our laboratory.
LITERATURE CITED ( 1 ) D. E. Burge, J. Chem. Educ., 47, A 8 1 (1970). (2) J. B. Flato, Anal. Chem., 44 (1 l ) , 75A (1972). (3) A . M. Bond and R. C. Boston, Rev. Anal. Chem., 11, 129 (1974). (4) N. Klein and Ch. Yarnitzky, J. Electroanal. Chem., 61,1 (1975). (5) E. P. Parry and R. A. Osteryoung, Anal. Chem., 37, 1634 (1965). (6) K. W. Hanck and J. W. Dillard, Water Resources Research Institute Re-
port 85, Raleigh, N.C., Dec. 1973. Feldberg in "Electroanalytical Chemistry", Vol. 3, A. J. Bard, Ed., Marcel Dekker. New York, N.Y., 1969, pp 199-296. (8) "Instruction Manual Polarographic Analyzer Model 174", P.A.R. Gorp., Princeton, N.J., 1971. (9) I. Ruzic and S. W. Feldberg, J. Electroanal. Chem., 63, 1 (1975). (10) J. H. Christie, J. Osteryoung, and R. A. Osteryoung, Anal. Chem., 45, 210 (1973). ( 1 1 ) J. H. Christie and R. A . Osteryoung, J. Electroanal. Chem., 49, 301 (1974). (12) T. P. Hoit, Ph.D. Thesis, University of North Carolina, Chapel Hill, N.C., 1972. (13) N. Tanaka and R. Tamamushi. Electrochim. Acta, 9,983 (1964). (14) Triangle Universities Computation Center, Rep. LSR-89-1, Research Triangle Park, N.C., 1972. (7) S.W.
RECEIVEDfor review August 13, 1975. Accepted September 29, 1975.
Polarographic Determination of Chloride, Cyanide, Fluoride, Sulfate, and Sulfite Ions by an Amplification Procedure Employing Metal Iodates Ray E. Humphrey* and Stanley W. Sharp Department of Chemistry, Sam Houston State University, Huntsville, Texas 77340
The anions CI-, CN-, F-, Sod2-, and SO02- were determlned polarographically by reaction of a 1:l ethanol-water solutlon wlth a relatively insoluble metal Iodate. Iodate ion is released and Its reduction current measured at the dropping mercury electrode. Mercuric iodate is used to determine CI-, CN-, and SO02-, barium iodate for SOP2-, and thorium Iodate for F-. Ions were determined in the range of approximately 2-50 ppm.
Certain anions have been determined chemically by reaction with an insoluble metal iodate to release iodate ion followed by reduction of the iodate to iodine and titration of the iodine with sodium thiosulfate solution. This results in a chemical amplification as six iodine atoms result for each monovalent anion and twelve iodine atoms for each divalent anion. Chloride ( I ) , fluoride (2), and sulfate ( 3 ) have been determined in this way. Polarographic measurement of the reduction current for iodate ion should also result in considerable gain in sensitivity since six electrons are involved. Fluoride ( 4 ) is apparently the only anion which has been determined by the measurement of iodate reduction current. The procedure was developed for the determination of fluorine in organic compounds. The anions investigated in this study are difficult to determine by a polarographic reduction procedure. Exchange reactions involving metal chloranilates and measurement of the reduction current of the chloranilate anion have been used for determination of several anions ( 5 ) . Ex222
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
change of these ions for iodate and measurement of its reduction current results in essentially a six-electron reduction for monovalent anions and a twelve-electron reduction for divalent anions and allows the determination of these species in the low parts-per-million range.
EXPERIMENTAL Apparatus. Polarographic d a t a were obtained with a SargentWelch Model XVI recording Polarograph. A conventional H-cell with a saturated calomel electrode was used and was mounted on a Sargent-U'elch constant head dropping mercury electrode assembly. T h e H-cell might be impractical for trace analysis in some instances because of t h e possibility of adsorption on t h e fritted glass and difficulty of cleaning. T h e dropping mercury electrode had a drop time of 4.11 sec and a flow rate of 2.10 mg of mercury per sec. Solutions were agitated with the insoluble iodate compounds on a Lab-Line Junior Orbit Shaker. Reagents. Mercuric iodate, used t o determine chloride, cyanide, and sulfite, was obtained from City Chemical Co., New York, N.Y. Barium iodate. used for sulfate ion, was precipitated by reaction of a solution of barium nitrate and potassium iodate. Thorium iodate for determination of fluoride was prepared from reaction of thorium nitrate and potassium iodate. T h e relatively insoluble iodates were washed with distilled water and dried. Stock solutions of the anions to be determined, prepared from t h e best available sodium or potassium salt, were made up in 1:1 ethanol-water solvent. T h e solutions of sodium sulfite contained 5% glycerol t o retard air oxidation. Procedure. Solutions containing the anion to be determined in the 1:l ethanol-water solvent were mixed thoroughly with the appropriate insoluble iodate compound by shaking for 20 min. After filtering, 0.30 rnl of concentrated perchloric acid was added, t h e hydrogen ion concentration being approximately 0.12 M . Solutions
Table 11. Current-Concentration Data
Table I. Current-Sensitivity Data Ion
pA/mmol/litera
PAIppm
14
0.39 0.58 0.63 0.24 0.29
c1-
CNF-
15
so,,s0,z-
12 23 23
10,-
14
RESULTS AND DISCUSSION Chloride and cyanide ions react with mercuric iodate t o form the soluble, undissociated mercury(11) compounds and release iodate ion. Fluoride ion reacts with thorium iodate to form an insoluble compound and release iodate while sulfate ion is exchanged for iodate on interaction with barium iodate. Equation 1 is a general representation for these ion exchange reactions, where MX is either insoluble or slightly dissociated
+ X-
= MX
+ 103-
(1)
Polarographic data indicate that these reactions are essentially complete. The reaction of sulfite ion with mercuric iodate appears to involve reduction of the mercury(I1) to elemental mercury as shown in Equation 2: Hg(I03)2
+ S032- + H20 = HgO + 2103-
+ 2H+ + Sod2-
(2)
Elemental mercury is evident with the excess mercuric iodate and there does not appear to be any mercury species in the reaction solution (6). This reaction also appears to be practically complete. Reduction of iodate ion a t the DME involves six electrons and requires hydrogen ion, as shown in Equation 3 (7). 103-
2.0 6.9 12 36
0.78
+ 6Hf + 6e = I- + 3H20
PPm
PA
PPm
PA
2.2 5.9
1.2 3.1
1.6 4.7 9.4 16
0.94 2.9 5.8 10
2.3
8.8
5.1 15
15
Sulfite ion
were purged vigorously with nitrogen for 2 min. The recorder pen was zeroed a t an applied potential of +0.10 volt vs. the saturated calomel electrode. The potential was then changed to -0.5 volt and the iodate reduction current measured.
MI03
PA
...
The extrapolated current for saturated solutions of the iodate compounds ranged from 0.5 FA to 1.2 PA. a
(3)
In this work in which the perchloric acid concentration was about 0.12 M , current for iodate reduction started a t zero volts vs. SCE and the half-wave potential, E1/2, was -0.17 volt vs. SCE. The plateau of the wave was reasonably level beyond about -0.3 volt vs. SCE. The six-electron reduction results in a considerable amplification of the current for the anions studied in this work and much greater sensitivity than would be achieved if electrode reactions requiring one or two electrons were involved. The sensitivity could be further increased by use of differential pulse polarography. The current for iodate reduction is from 3-8 times greater than that for chloranilate ion in the procedure employing metal chloranilates. Also, the iodate salts are more pure and probably more stable than the chloranilates, resulting in lower blanks ( 5 ) . Current-concentration data for the anions are presented in Table I. Current due to the limited solubility of the iodate compounds is in the range of 0.5 to 1.2 PA. This value, for correction of the blank current, was obtained by extrap-
ppma
5.5
9.1
Sulfate ion
-
P A
0.69 1.35 4.3 13 14 38 a Expressed as ppm SO,. 1.9 3.7
Fluoride ion
Cyanide ion
Chloride ion ppm
__
ppm
PA
3.1 9.3 19 46
0.72 2.4 4.5 12
olation of the current-concentration plots and was in agreement with experimental values measured for saturated solutions of the iodates. The current-concentration ratio for the monovalent ions is reasonably close to the value for iodate ion while the values for sulfate and sulfite are not quite twice than for iodate as might be expected. The current-concentration plots were essentially linear over the concentration ranges measured. Data are shown in Table 11. Reproducibility is reasonably good down to about 1 ppm. At the very low concentrations, it is especially important to be certain that all of the dissolved oxygen is purged. Higher concentrations could be determined but were not investigated in this study. I t is also important that the iodate compounds be thoroughly mixed with the solution containing the ion to be determined. Twenty minutes were allowed for this mixing but probably 5-10 minutes would be sufficient. The measurement of current can be accomplished in about one minute after purging. Fluoride seems to be the only anion which has been determined by a polarographic procedure based on reduction current of the iodate ion ( 4 ) . In that work, calcium iodate was used and acetone added to eliminate the excess reagent which would cause a high background current. The current-concentration ratio was not stated in the report but a calibration plot was shown over a range of 8-12 ppm fluoride. Calcium iodate is sufficiently soluble in water to result in an appreciable reduction current. The solubility of thorium iodate in the mixed ethanol-water solvent is not high enough to result in an unreasonable blank current. A few brief experiments indicated that mercurous iodate is also suitable for analysis of cyanide and sulfite ions. Determination of these ions with the mercury(1) compound showed essentially the same sensitivity as with mercury(I1) iodate and possibly slightly better linearity of the currentconcentration plot.
LITERATURE CITED (1) R. Belcher and R. Goulden, Mikrocbim. Acta, 1953, 290. ( 2 ) W. I. Awad, S. S.M. Hassan, and M. B. Elayes, Mikrocbim. Acta, 1969,
688. ( 3 ) D. A. Webb, J. Exp. Biol., 16, 438 (1939). (4) Y. A. Gawargious, A. Besada, and B. N. Faltaoos, Anal. Cbem., 47, 502 (1975). (5) R. E. Humphrey and C. E. Laird, Anal. Cbep., 43, 1895 (1971). (6)W. L. Hinze. J. E. Elliott, and R . E. Humphrey, Anal. Cbem., 44, 151 1 (1972). (7) E. F. Orlemann and I. M. Kolthoff, J. Am. Cbem. SOC.,64, 1044 (1942)
RECEIVEDfor review August 11, 1975. Accepted September 15, 1975. This research was supported in part by the Faculty Research Fund of Sam Houston State University.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
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