+
Table V. Determination of Calcium and Calcium Magnesium in Drinking Water. T i t r a n t 0.1 M EDTAD Ca mmol/l. Sample volume, mmol/l. 1 0
L
3 4
5 6
7 b
Ca
+ Mg mmol/l.
5 nil* found
Std dev
2 mlc found
5mlb found
Std dev
3.205 2 019 2.875 2.438 2.422 2.336 1.899 3.350
0.004 0.008 0.007 0.007 0.003 0.006 0.009 0.004
3.222 2.035 2.875 2.429 2.407 2.350 1.932 3.347
4.016 2.844 3.451 2.928 2.725 2.568 2.728 4.196
0.014 0.008 0.005 0.007 0.004 0.008 0.005
0.007
2 mlc
found 4.054 2.872 3.475 2.947 2.732 2.540 2.71, 4.198
I'wck-point calibration performed as in Table IV. b Mean of 4 determinations. Mean of 2 determinations. ('
calibrations on 5-ml and 2-ml samples diluted to identical total volumes. Only in a single instance did the deviation between the 5-mi samples and the 2-ml samples exceed 1%. The mean standard deviation is 0.25% for the calcium determinations (5-ml samples, 4 measurements on each) and 0.23% for the calcium magnesium determinations. I n t e r f e r e n c e s . In photometric titrations, trace elements of metal ions are often a problem because they form colored complexes with the indicators (7). In a potentiometric titration with a calcium-ion electrode, trace elements of other metals cause no problems, provided they are present in
+
quantities that lie below the detection limit of the analysis. Greater quantities of Fe3+ and A13+ will be effectively masked in 0.01 M acetylacetone (pH 8.5). Fez+,which often occurs in drinking water, will not be effectively masked by acetylacetone, but the transformation of the Fez+-acetylacetone complex into a Fez+-EDTA complex is so slow (rate constant = lo3 mol-' min-l) t h a t the interference will be less than 1% for a Fez+ content up t o 0.5 mmol/l. and a calcium and magnesium content of 1 mmol/l.
ACKNOWLEDGMENT The authors express their appreciation t o Ewa Dogonowski for valuable assistance with the experimental work, and t o Henrik Malmvig and 0.J. Jensen for stimulating discussions.
LITERATURE CITED (1) G. P. Hildebrand and C. N. Reilley, Anal. Chem., 29, 258 (1957). (2)7.P. Hadjiiannou and D. S. Papastathopoulos, Talanta, 17, 399 (1970). (3) I. E. Lichtenstein, Elia Coppola, and D. A. Aikens, Anal. Chem.. 44, 1681 (1972). (4) A . E. Martin and C. N. Reilley, Anal. Chem., 31, 992 (1959). (5) R. L. Clernents, J. I. Read, and G. A. Sergeant, Ana/yst(London), 96, 656 (1971). (6) Marco Mascini. Anal. Chim. Acta, 56, 316 (1971). (7) Hisakuni Sat0 and Kaso Mornoki, Anal. Chem., 44, 1778 (1972). (8) Anders Ringborn, "Chemical Analysis", Vol. XVI, "Complexation in Analytical Chemistry", interscience Publishers, New York/London, 1963. (9) Arthur Martell and L. G. Sillen, "Stability Constants of Metal-Ion Cornplexes", The Chemical Society, Burlington House W. 1, London 1964. (10) J. RGiEka, E. H. Hansen, and J. C. Tjell, Anal. Chim. Acta, 67, 155 (1973).
RECEIVEDfor review October 14, 1975. Accepted February 20,1976. Our thanks are due t o Radiometer A/S whose support made this work possible.
Determination of Selenium(1V) by Anodic Stripping Voltammetry in Flow System with Ion Exchange Separation Richard W. Andrews' and Dennis C. Johnson* Department of Chemistry, Iowa State University, Ames, lawa 500 1 1
Selenium in several NBS Standard Reference Materials and unknown samples was determined by liquid chromatography with IRA-200 cation-exchange resin. Detection of Se(lV) In the chromatographic effluent was by anodic strlpping voltammetry at a tubular Au electrode. Analytical results were excellent except when Si02 was present. The detection limlt of the method was approximately 0.6 ng of Se(lV) in a 0.160-ml sample ( 4 ppb).
The authors recently reported the results of a study of the voltammetric deposition and stripping of Se(1V) a t a rotating gold-disk electrode (RAuDE) in 0.1 M HC104 ( I ) . Three distinct stripping peaks are obtained following the deposition of the equivalent of several monolayers of Se. This evidence was interpreted t o be the result of the formation of three distinct activity states of the deposited Se: a monolayer of Se contacting the Au surface which is oxidized a t 0.8 V vs. SCE, bulk Se which is oxidized a t 0.6 V, and a Au-Se intermetallic compound of unknown stoichiometry which is oxidized a t 1.0 Present address, Department of Chemistry, University of Alabama in Birmingham, Birmingham, Ala. 35294. 1056
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
V. When the quantity of deposited Se does not exceed the equivalent of a monolayer, a single stripping peak, which is resolved from the anodic wave for the formation of gold oxide, is obtained. This stripping peak is suitable for the determination of Se(IV) by anodic stripping voltammetry (ASV). A detection limit of approximately 0.04 ppb Se(1V) in 0.1 M HC104 was reported by the authors for the determination of Se(1V) by linear scan ASV a t a RAuDE following a 10-min deposition period ( I ). The ASV procedure was applied to the determination of Se in NBS Standard Reference Material 1577 (bovine liver). The average of five determinations was 1.12 & 0.03 ppm Se whereas the certificate value is 1.10 f 0.10 ppm. Interference from codeposited As(III), Hg(II), Sb(III), Cd(II),and Pb(I1) in the determination of Se(1V) by ASV is severe. Separation of Se(1V) from these metal ions in conjunction with the electrochemical determination of the procedure is necessary if the method is to be applicable to complex samples containing these species. Several ion-exchange separations for complex samples containing Se(1V) have been reported utilizing cation exchange resins (2-4), anion exchange resins (5, 6),or a combination of both (2, 7). Nelson, Murase, and Kraus reported
t h e distribution coefficients for many metal ions on Dowex 50W-X8 (a strong acid cation exchange resin) in HC104 media as a function of acid concentration. The distribution coefficient of Se(1V) is less than 0.7 for 0.1-8 M HC104 (8).Several Soviet investigators have reported that Se(1V) is weakly sorbed by t h e cation exchangers KU-1 and KU-2, and is strongly sorbed from dilute HC1 solutions by t h e anion exchangers AN-1, EDE-lOP, and AV-18 ( 5 , 6 , 9 ) .T h e sorption of Se(1V) on t h e anion exchangers is weak for 0.5-4 M HCl, but quantitative for concentrations of HC1 greater than 6 M. Se(1V) and Te(1V) were separated with the cation exchanger KU-2 in 10-5-10-2 M HC1. In that separation, Te(IV) was retained on the column and Se(1V) was passed with the eluent (2). Se(1V) was separated from Fe(II1) and Co(I1) in solutions of iron sulfide ores using Dowex 50W-X8 with 0.5 M HC1 as t h e eluent (10). Flow-through electrodes have been used successfully as detectors for forced-flow liquid chromatography (11-16). In those applications t h e detectors have been operated in an amperometric mode in which the electrolysis current is monitored with t h e working electrode potentiostated at a value suitable for t h e determination of t h e analyte. In principle, stripping voltammetry can also be utilized for chromatographic detection. T h e procedure requires deposition of t h e analyte during elution and subsequent voltammetric stripping of t h e deposit. T h e voltammetric stripping process is incompatible with continuous monitoring of column effluent. Consequently, t h e stripping method of detection is probably desirable only for single-component determination. Here we describe t h e successful application of ASV a t a tubular Au electrode for the determination of Se(1V) in complex mixtures after separation by forced-flow ion-exchange chromatography. T h e convective-diffusional limited current in a tubular electrode under laminar flow is related t o t h e concentration of electroactive analyte in the fluid stream by Equation 1 ( 17). In Equation 1, I l ( t ) = limiting current (mA); D = diffusion coefficient (cm2/s); n = equiv/mol; L = electrode length (cm); Vf = volume flow rate (ml/s); and C ( t ) = concentration of electroactive analyte (mol/l.).
I l ( t ) = (5.24 X 105)D2/3L2/”V~1/3C(t)
(1)
T h e areas under t h e chromatographic peak and t h e subsequent stripping peak are equal and given by Equation 2
Q = SZi(t) dt = k r ~ S V f ’ / ~ C ( dt )t
(2)
k = (5.24 X 105)D2/3L2/3
(3)
where
T h e number of moles of electroactive analyte eluted is
N = JVfC(.t) d t
(4)
When Vf is constant, Equations 2 a n d 4 are combined t o give t h e analytical equation
Q=
(5)
EXPERIMENTAL Apparatus and Instrumentation. The tubular Au electrode and electrolysis cell were constructed by the Chemistry Shop at Iowa State University and are shown in Figure 1. A Au disk 10 mm in diameter and 3 mm thick was sandwiched between two Kel-F blocks held together by four stainless steel bolts. A Teflon wafer 1 mm thick and 10 mm in diameter was placed between each flat surface of the Au disk and the Kel-F blocks. Teflon flows under pressure, and the Teflon wafers ensured a leak-free seal between the Au disk and the Kel-F blocks. A Teflon “0” ring was placed around the Au disk in the space between the Kel-F blocks to prevent accidental contact of fluid with the outer rim of the Au electrode during cell clean-up procedures. A Au wire soldered to the edge of the Au disk provided electrical contact
SCE SCE
REFERENCE
ELECTRODE
‘2.J4i-1’ -
CONTACT
PT COUNTER
TO jiASTE I
AU ELECTRODE CONTACT
\
- 1/1
TEFLON
”0” R I N G A U ELECTRODE
C L U l D STREAM
I
T L. 0,031 1 INCY
hiCH
W C M A N L TEFLCV I E L
KEL-F
C E L L RUDY
Figure 1. Cross section of tubular Au electrode and electrolysis cell
to a banana jack mounted in the Kel-F. A 0.031 in. i.d. channel for fluid flow was drilled through the center of the Au disk and the Kel-F after fabrication of the cell. This assured smoothness of the channel surface to minimize disruption of laminar fluid flow in the channel. The larger bores for the tube-end fittings and reference electrode were drilled subsequently. The liquid chromatograph was constructed from Teflon tubing and Kel-F fittings and valves according to the design of Seymour. Sickafoose, and Fritz (18) with minor revisions as described by Johnson and Larochelle (11).The reagent reservoirs were provided with gas dispersion tubes for deaeration with He. The reservoirs were pres. surized through a Teflon tube which did not contact the solutinns; this prevented cross-contamination by fluid flow between reservoirs caused by pressure inequalities during depressurization. The potentiostat was Model RDE2 constructed from operational amplifiers by the Pine Instrument Co. of Grove City, Pa. A Model SR-2255 B strip chart recorder from Heath-Schlumberger Co. 0 1 Benton Harbor, Mich, and a Model 1131 XY plotter from Electronic Assoc., Inc., of West Lo,ng Branch, N.J., were used for the measurement of current-time and current-potential curves. Potentials were measured with a digital voltmeter from Systron-Donner, Inc., CUI.*er City, Calif. A Keuffel Esser Dual CompensatingPlanimeter were wed for the integration of all recorded current-time peaks. Electrode potentials are reported in V vs. SCE; electrical currents are reported in pA, and quantities of electrical charge are reported in p C . Reagents. Concentrated HC104 from G. Frederick Smith Chemical Co., Columbus, Ohio, and Baker Analyzed concentrated “ 0 3 from J. T. Baker Chemical Co., Phillipsburg, N.J., were purified by sub-. boiling distillation in a quartz still. The still was constructed from a design by Kuehner et al. (19).In subboiling distillation infrared radiators vaporize the surface without boiling the liquid which reduces entrainment of impurities and creeping of the unrectified liquid. The kchnique of subboiling distillation was shown to be particularly effective in reducing the concentrations of elements heavier than Fe in nitric and perchloric,acids (19). Stock solutions of Se(IV) were prepared by dissolving 99.99% pure Se metal obtained from Abbott Laboratories, North Chicago. Ill., in a minimum of concentrated ” 0 3 and diluting with water to give a 0.100 M Se(1V) solution. This stock solution was stored in a polyethylene bottle, and subsequent solutions of Se(IV) were prepared by dilution of quantities of the stock solutions which were measured with Gilmont micrometer burets. With the exception of the As(III), Sb(III), Cu(JI),and Te(IV),all stock solutions were prepared by dissolving Baker Analyzed salts in 0.1 M HC104. The stock solutions of Cu(I1)and Te(IV) were prepared by dissolving reagent grade metals in concentrated ” 0 3 and diluting with 0.1 M HC104. The As(II1) and Sb(II1) solutions were prepared by dissolving the oxides in concentrated H2S04 and diluting with 1 M HzS04. All stock solutions were 0.001 M with the exception of the M because of low soluhility. Te(IV) solution which was made 5 X Final H+concentration of stock solutions was approximately 0.7 M. All stock solutions were stored in polyethylene bottles, and the Te(1V) ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
1057
Table I. Composition [ppm ( p g / g ) ] of NBS Standard Reference Materials
Element N Ca K Mg
P Fe Mn Na Pb
B Zn As cu Rb Ni Hg Cd Se‘ (I
SRM 1571 (orchard leaves) 27 600 20 900 14 700 6 200 2 100 300 91 82 45 33 25 11 12 12 1.3 0.155 0.11 0.08 f 0.01
SRM 1577 (bovine liver)
SRM 1632 (coal)
SRM 1633 (fly ash)
106 000
SRM 1261 (low alloy steel) 37
