606
Anal. Chem. 1986, 58,608-611
Determination of Sulfate, Orthophosphate, and Triphosphate Ions by Flaw Injection Analysis with the Lead Ion Selective Electrode as Detector J. F. Coetzee* and C. W.Gardner, Jr.' Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
A flow injection analysis system was developed for the determination of varlous anaiytes With lead(I1) ion. The s y s t k was optlmlzed for t h l determinatioq of sulfate Ion over the cohcentratlon range of 1-1006 ppm. This entlre r a h p Is accessible with a single reagent solution of lo-' M lead perchioraie in ethangl, which is mixed with an aqueous carrier stream, making \h& method particularly useful for samples contalnlng a wide iange of sulfate concentrations. The maximum sampiitig rate ISa function of sulfate coribei(tr$tion; for 100 ppm sltlfate, the rate Is 20 samples/h. Main Interferences are duch muitlcharged anions as phosphates. The same flow systenl, but with an aqueous rather than ethanoilc reagent, was used for the determination of orthophosphate and triphosphate ions. While calibration curves tended to be nonllnear for these Ions, reproducibility was adequdte for many anaiytlcdi purposes.
One objective of this work was to study the apblicability ot ion selective electrodes (ISFh) as detectors in flow injection analysis (FIA), pdrticularly when potentially complicating factors, such as precipitate formation, are present. The lead(I1) ISE was chosen for this purpose because many anions forin sufficiently insoluble lead(I1) salts to allow their deterniination with lead ion (1, 2). Examples of solubility products (as pKsp) are ( 4 4 ) as follows: SO4", 7.79; CrOt-, 12.55; ,43.5 (38 "C); Fe(CN):-, 18.02; and MOO^^-, 13.0 (uncertain). In addition, lead(I1) ion forms moderately stable complexes ( 3 , 4 ) with such anions as P2074-(log = 7.3 a t I = 1 M) and Cz042 (log & = 6.54at 25 "C). Finally, lead@) ion ha5 also been used for the determination of shch analytes as catechols (5, 6),vitamin B2, and penicillins (7). A second objective of this work was to explore the potentialities of solvent optimization in FIA. In principle, the determination of such anions as sulfate that form only moderately insoluble lead salts can be improved by jydicious addition uf a cosolvent in order to lower the solubihty. In fact, this has been done frequently in classical gkavimetric and columetric determinations. It is ngcedsary, however, to address potential complications introduced by cosolvent addition, such as iaiide uate electrode response or undesirable precipitation or comp exation of analyte or reagent ions with other substances that may be present in actual samply4. Flow injection analysis is rapidly developing into a powerful analytical tool with many merits, such as broad scope, rapid sample throughput, and benefits accruing from controlled dispcrsiuii Its principles and applications have been critically discuBsedby Ruzicka (8,9). The use of ISEs as detectors in FIA ha5 been reviewed by Pungor (10). In such applications, ISEs hdve been used in a variety of ways, e.g., in direct sensing or analyte ion activities, in sensing the activities of reagent
?
l Present address: Mine Safety Appliances Co., 201 N. Braddock Avc., Pittsburgh, PA 15208.
ions after reaction with analytes, or in enzyme-catalyzed reactions producing ions that ctin be sensed with ISEs (11-13). In this work, we developed a simple FIA system with lead(f1) ion as reagent. We studied its applicability to the determination of sulfate ion over a wide concentration range with high sample throughput and made exploratory measurements with orthophosphate and triphosphate ions as well. The system should be adaptable to various other analytes. Several approaches already have been described for the determination of sulfate ion in flow systems. These are usually based on precipitation with barium ion followed by either turbidimetric determination of barium sulfate or spectrophotometric determination of remaining barium ion with methylthymol blue (14). Both continuous flow (15) and FIA (16) systems have been employed for this purpose. Finally, Trojanowicz (17) used letid(I1) ion as the reagent in a differential method utilizing two lead ISEs, one in a reference stream of lead ion and the other in a second stream containing the same concentration of reagent plus the sample. Useful results were dbtained in the range 30-400 ppm sulfate. We describe here a somewhat different approach by which sulfate ion can be determined in the range 1-1000 (or higher) ppm. We used a two-channel FIA system in which the reagent stream contains lead(I1) perchlorate in ethanol (or another organic solvent) with sodium perchlorate as ionic strength adjustor, while the carrier stream is water into which the sample containing sulfate ion is injected. (Direct injection of aqueous samples into the reagent stream introduces changes in peak height due solely to the changes in solvent composition.) The lead ISE and the reference electrode are located downstream after mixing of the two channels. The potential of the lead ISE a t constant ionic strength is given by
Eb = K
+ S log apb2+= K + S log (fpbZ+CPbZ+) = K'
+ S log c p b
(1)
omitting for simplicity of notation the charge of lead ion. In the absence of sulfate ion, the base-line potential of the lead ISE is therefore given by Eb = K' S log (mcpb") (2)
+
where Cpb" is the reagent concentration and the constant m is the "mixirig ratio" of the two channels, while S is the theoretical response slope of the lead ISE (0.059/2 V per decade in concentration of lead ion at constant ionic strength). The constant K' contains the electrode constant as well as the reference electrode and liquid junction potentials and also the activity coefficient, f , of lead ion. If now a sample containing sulfate ion a t a sufficiently high concentration to cause precipitation of lead sulfate is injected into the carrier stream, the lead ion concentration will be lowered to a new value. If dCso," >> mCPbo,where d is a constant representing the dilution and dispersion of the sample, the new lead ion concentration at equilibrium will be given by CPb = KSP/dCSO: (3) where KsP is the (concentration) solubility product of lead
0003-2700/86/0358-0608$01.50/00 1986 Amerk8n Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986 609 sulfate a t constant ionic strength. The potential of the lead electrode as a function of flow time will pass through a minimum given by Emi,
= K'
+
log
cpb
K'
=
+ S log KSp - S log (dCs0,")
(4)
Sbbtracting eq 4 from eq 2, the maximum deviation of the potential from its base-line value (commonly referred to as the "peak height") is given by H, = log KSp log (md) log CPb" log
+s
-s
+s
+s
cm:
(5) Hence, if m, d , and Cpb' are kept constant, and dCso," >> mCPbo, a plot of Hpvs. log Cso: should be linear with a slope equal to S and an intercept from which Ksp can be calculated if CPbo, m, and d are known. At lower sulfate ion concentrations for which dCsoaois no longer much greater than mCPbo, c p b and CSO,at equilibrium are equal to (mCpbo - x ) and (dCS0: - x ) , respectively, where x is the molar concentration of lead and sulfate ions reacted. Hence, eq 4 must be modified as in eq 6 (6) Emin K' S log Ksp - S log (dCso," - X )
+
so that eq 5 becomes Hp = log Ksp s log
-s
(mcpb")
s log (dCso," - X ) (7)
The value of KsP is known from eq 5 applied t o higher concentrations of sulfate ion, so x can be evaluated from eq 8. Ksp
=
(mcpb" -
x)(dCso," - X )
(8)
For the precipitation of phosphate ion with lead ion, eq 5 becomes
Hp = -(s/3)log Ksp + s log (rnd2l3)+ s log Cpb" -k (2S/3) log Cpo4" (9) I t is to be noted that, in general, the stoichiometry of the reaction product can be determined from the slope of the plot of Hpvs. log analyte concentration. EXPERIMENTAL SECTION Flow Injection Apparatus. Reagent and carrier solutions were contained in linear polyethylene bottles and were propelled into the flow injection analysis manifold with compressed helium at 3-6 psig, which produced less noise in the measurements than a peristaltic pump did. All tubing was made of Teflon, and connections were made with standard 1/4-in.low-pressure liquid chromatography fittings. Relative flow rates of the two channels were adjusted by changing flow paths. Other parameters of the flow system were also varied. The final system had the following components. The outlet of the reagent bottle was connected to a Kel-F mixing tee by an 11cm length of 0.05-cm4.d. tubing, while the outlet of the carrier bottle was attached through a 3 cm length of 0.08-cm-i.d. tubing to a 1/16-in.injection valve made of Teflon (Type 5020, Rheodyne, Inc., Cotati, CA), equipped with 0.5-, 0.2-, or 0.07-mL sample loops. The output of this valve was fed into the tee and mixed with the reagent stream in a 300 cm length of 0.05-cm4.d. coil of tubing wound on a 1.25-in.-diameter polypropylene rod and immersed in a beaker of water at room temperature (20-23 "C). The system gave overall flow rates of 1.0-2.0 mL min-' (determined volumetrically) and a carrier-to-reagent mixing ratio of 0.27/0.73 (determined spectrophotometricallyusing toluene as a tracer). Finally, the mixed solution flowed through the indicator electrode-reference electrode cell, which is shown in Figure 1. The body of this cell was constructed of Kel-F and was held together by two aluminum blocks clamped with six 1/4-in. bolts. The inner diameter of the inlet and outlet ports were 0.05 and 0.15 cm, respectively. The spacer (Teflon) was 0.015 in, thick (Dielectrix, Farmingdale, NY) with a 0.1-cm-wide slot defining the flow path between the inlet and outlet ports. Contact to the reference electrode was made through a Vycor glass plug and salt
Figure 1. Flow-through electrochemical detector: SBT, salt bridge tube; CF, ceramic or Vycor frit; TC, tubing connections; TS, Teflon spacer; ISE, ion selective electrode membrane; CTM, contact to membrane.
bridge tube sealed into the Kel-F block, as shown in Figure 1. A 10-mL disposable syringe body was attached to the tube and a silver-silver chloride or calomel reference electrode was placed into it. The salt bridge solution was 1M sodium perchlorate (as). The lead ISE was made by the method of Heijne et al. (18)and had a composition of 30 mol % PbS + 70 mol % AgzS. A pellet was made by pressing 750 mg of precipitate in a 3/s-in. steel die for 14 h under vacuum at a temperature of 140 "C and a pressure of 750 MPa. The pellet was sealed in the Kel-F block by using Epotek 353ND epoxy (Epoxy Technology, Billerica, MA), and electrical contact was made with a silver wire and silver-filled epoxy (Epotek 410E, Epoxy Technology). Both halves of the cell were periodically polished, first with 1000-grit silicon carbide on a nylon cloth and then with 0.5-pm alumina on LeCloth (Leco Corp., St. Joseph, MI) at 100 rpm on a Varipol (Leco) polisher. Potentials were measured with a Keithley Model 616 electrometer (Keithley Instruments, Cleveland, OH); its ground lead was connected to the aluminum block on the flow-through cell to minimize noise. The output of the electrometer was fed to a strip-chart recorder. The input impedance of the instrument is greater than 2 X 1014Q . Reagents. The reagent was lead perchlorate, typically at a concentraton of M with M sodium perchlorate as supporting electrolyte. In the determination of sulfate ion, improved results were obtained on controlling the pH of the reagent by M acetic acid and M sodium acetate. In the adding determination of sulfate ion, the solvent for the reagent was absolute ethanol that had been degassed for 15 min with stirring under aspirator vacuum. In exploratory determinations of other anions, the solvent for the reagent was water. The carrier stream in all cases was water into which 0.07-mL, or larger, samples were injected. R E S U L T S A N D DISCUSSION Optimization of Flow Injection Analysis System. Parameters of the FIA system were varied as shown in Table I in order to maximize the dynamic range, the precision, and the sampling rate. This optimization process involved the usual trade-offs. For example, the lowest permissible operating pressure (3 psig) was that which allowed proper functioning of the loop-type injection valve. At higher pressures, the precision was somewhat poorer, but the sampling rate was higher. The optimum pressure of 4 psig was chosen as a compromise. The mixing coil dimensions are critical. With somewhat thicker tubing (0.08 cm i.d.) bubble formation occurred during mixing of the ethanol and water streams, and this caused noisy base-line potentials. A minimum length of 100 cm of tubing was necessary for adequate mixing of the two streams, but even greater lengths were required for adequate approach to equilibrium of precipitation reactions, especially a t lower analyte concentrations. With these greater lengths a slight decrease in peak height was observed, probably as a result of increased dispersion of the sample, as documented by Ruzicka and Hansen (8). Optimization of the solvent for the reagent involved batch titrations of sulfate ion with lead ion in mixtures of water with several organic solvents, including ethanol, 1-propanol, 2propanol, acetonitrile, and dioxane. Details of the response
610
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
Table I. Allowable Ranges for Flow Injection Analysis Parameters and Desired Response Characteristics parameter
range
operating pressure overall flow rate, 200-cm mixing coil mixing coil length, 0.05 cm i.d. carrier/reagent ratio reagent solution ionic strength reagent (lead ion) concentration sample volume
90.0
4
100-300 cm
300
0.01-0.1 M
'18 14
112.7 0.Q1
104-10-4 M
10-5
0.5, 0.2, 0.07 mL
0.07
1
/,
tt * \
optimum value
3-6 psig 1-2 mL min-1
r
i
1
desired response 1.0
2.0
3.0
4.0
5.0
6.0
parameter Nernstian range total range precision of peak height sampling rate I 2
X
Reagent
IO-'M
,1
-log[Sulfatel
10-3-10-5 M 10-3-104 M 51 mV 120 h-l
-
Flgure 3. Calibration curve for determination of sulfate ion in unbuf-
+ lo-'
10-5M Pb(C10412 IO-'M
fered solution. Conditions are as follows: reagent, M Pb(CIO,), M NaCIO, in ethanol; carrier, water; mixing coil, 200 cm X 0.05 cm 1.d.; flow rate, 1.4 mL min-'; Injection volume, 0.5 mL. The solid line represents the calculated response for S = 25 mV/decade and "effective" pKsp = 11.0;see text.
NaCI04
I O - ~ MH O A ~ I 2 x
90.0
M NQOAC
M
(in E t O H
5 8 x IO 7 M
Corrier
-
\
1
Woter h
>
E
60.0
1
I
\
f,,
+ 1
m
. 3
W
I Y
a W
30.0
1
Figure 2. Typical recorder traces for determination of sulfate ion. Experimental conditions are as follows: mixing coil, 300 cm X 0.05 cm i.d.; flow rate, 0.9 mL min-I; injection volume, 0.07 mL.
of the lead ISE in these solvents are given elsewhere (19). It is necessary to devote special attention to the purity of the solvents, since typical reagent grades contain impurities affecting the response of potentiometric sensors (20). While the solubility of lead sulfate can be lowered much more with such cosolvents as 2-propanol, acetonitrile, and dioxane than with ethanol, undesirable reactions are also promoted to a greater extent, e.g., precipitation or complexation of lead ion with other ions or uncharged substances that may be present in practical samples. We have therefore chosen ethanol as the cosolvent, but in certain applications one of the other solvents may be more appropriate. A lead ion concentration of M was chosen as the best compromise between dynamic range and lower detection limit attainable in 73 vol % ethanol. Results for Sulfate Ion. A typical set of recorder traces is shown in Figure 2. When the reagent solution contained only lead perchlorate and sodium perchlorate, the calibration curve shown in Figure 3 was obtained. The solid line is the response calculated from eq 7, with S = 25 mV/decade, log KSP= -11.0 (calculated from the higher concentrations of sulfate ion), m = 0.73, and d = 0.27. This line agrees well with experimental data points, except at the lowest concentrations, but the sub-Nernstian slope may indicate that lead ion also reacts with impurities, perhaps carbonate ion. (For lead carbonate and lead sulfate in water, log Ksp = -13.1 and -7.79. It would be useful to determine the solubility product of lead carbonate, as well as the acid dissociation constants of carbonic acid, in ethanol-water mixtures.) Injection of bicarbonate ion a t constant sulfate concentration did, in fact, cause a relatively
2.0
3.0
4.0
5.0
6.0
-log[Suifetel
Flgure 4. Calibration curve for determination of sulfate ion in buffered solution. Conditions are as for Figure 2. Solid line represents calculated response for S = 30 mV/decade and pKsp = 11.1; see text.
Table 11. Observed Response of Flow Injection Analysis System in Determination of Sulfate Ion sulfate
peak
concn, M
height," mV
std dev in peak height," mV
error," '70
81.6, 57.8, 73.1 51.3, 64.0 34.2, 42.8 25.0,33.7 8.8, 10.8 -, 4.2
0.59, 0.29, 0.23 0.22, 1.00 0.22, 0.36 0.38, 0.74 0.28, 0.30 -, 0.26
5.4, 2.7, 1.7 2.0, 7.6 2.0, 2.7 3.5, 5.6 2.6, 2.3 -, 2.0
1.2 X 1.2 X 5.9 X 1.2 X 5.9 x 1.2 X 4.7 X
10" lo4 10-5 lo4
re1 concn
M Pb(C104)Z+ First number refers to unbuffered reagent lo-* M NaC104 in ethanol); second number refers t o same reagent M CH3COONa. buffered with lom3M CH3COOH+
large increase in peak height, while even a 1000-fold excess of chloride, nitrate, or acetate ion had no effect. Furthermore, on addition of an acetic acid buffer virtually Nernstian response was obtained, as shown in Figure 4. Closer agreement with experimental data a t the lowest sulfate concentrations can be obtained with slightly different values of the adjustable parameters of eq 7. In Table I1 results obtained with un-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986 Reagent
9.7 x 1 0 ' M~
-
M IO-'M
I\
somewhat poorer. Other details are given elsewhere (19). These results should be considered in conjunction with those obtained by Alexander (23) who determined triphosphate (and pyrophosphate) ions in the presence of a large excess of orthophosphate ion by using a copper metal indicator electrode in an FIA system. Chromate Ion. The lead ISE became passivated in aqueous solutions of chromate ion at all pH values over the range 3-8. Some of the mixture of lead and silver sulfides from which the ISE was originally pressed was immersed in M potassium chromate solution. Within 1 min the color of the solution changed from yellow to a cloudy green, indicating formation of chromium(II1) ion. Similar oxidation of the lead ISE was observed in aqueous solutions of iodine.
Pb(C104)Z
NH~OAC
p H = 8.0 ( i n water)
Carrier
-
Watet
97~10-~M
Figure 5. Typical recorder traces for determination of orthophosphate ion. Experimental conditions are as follows: mixing coil, 300 cm X 0.05 cm Ld.; flow rate, 2.7 mL min-'; injection volume, 0.07 mL.
buffered and buffered reagents are compared. A more serious interference is caused by phosphate ion, even when the buffered reagent is used. For example, for a solution otd phosphate, containing IO4 M sulfate ion as well as lo4 M t the selectivity coefficient, KAI,as defined by Hansen et al. (21) in eq 10 has a value of 2.2. Here, A refers to the analyte and
Hp(A + I)
- H,(A) = S log
[(C,
+ KAICI)/CAI
611
(10)
I to the interferent, while the selectivity coefficient also takes into account the difference in stoichiometry of the two reaction products. Hence, if the increase in Hpis not to exceed 1mV (a precision easily attained in these measurements), the phosphate concentration must not exceed 4% of that of sulfate ion. It may be possible, however, to eliminate the phosphate interference as suggested by White (22) by including ammonium and magnesium ions in the lead ion reagent so that phosphate will preciptate preferentially as magnesium ammonium phosphate. Preliminary Results for Orthophosphate and Tripolyphosphate Ions. The same FIA system as before was used for exploratory determinations of these ions, except that the lead ion reagent was present in aqueous solution containing M ammonium acetate and adjusted to a pH of 8.0 with ammonia as recommended by Selig (5, 6) for batch titrations. Under these conditions the predominant phosphate ion is HPO?-. Typical recorder traces obtained with orthophosphate ion are shown in Figure 5. The plot of Hpvs. log Cpo: is linear over the range of 10-2-10-4 M phosphate, but the slope of the line is near 33 mV/decade in concentration, whereas eq 9 predicts a slope of 20 mV. At lower concentrations the slope of the FIA plot becomes even greater. Batch titration gave the proper stoichiometry of 3:2 for lead orthophosphate. I t is possible that the anomalously high slope of the FIA plot is caused by slow precipitation of lead phosphate, especially at lower concentrations. However, the reproducibility of the data is similar to that for sulfate and it is adequate for analytical purposes. Batch titration of sodium triphosphate at a pH of 8.0, where the predominant analyte species are P3OlOEr and HP3O1OP (4), gave a 1:l stoichiometry for the reaction product. The formation of PbP3012- was also inferred from other studies (3). FIA peak heights are greater for triphosphate than for phosphate at equal concentration, but reproducibility is
CONCLUSIONS The main virtues of the approach described here are rapid sample throughput and the fact that a wide range of analyte concentrations is accessible with a single reagent solution. Lower detection limits for sulfate ion are similar to those obtained with the well-established methods based on precipitation of barium sulfate, which already have been referred to, but are inferior to those attainable by ion chromatography (24). The chromatographic technique has the added advantage that sulfate and phosphate ions are easily resolved. For certain applications it may be advantageous to use the FIA system described here as a moderately selective detector for ion chromatography, thereby minimizing the demands on the chromatographic process. Certain problems would have to be overcome in such applications, however. Problems include the fact that the response of the particular flow system described here, at least for relatively dilute analyte solutions, is somewhat slow for application as a detector. It will also be necessary to optimize the mobile-phase composition for this particular detector. LITERATURE CITED (1) Hansen, E. H.; Ruzicka, J. Anal. Chim. Acta 1974, 72, 365-373. (2) Chao, E. E.; Cheng, K. L. Talanta 1977, 2 4 , 247-250. (3) Sillh, L. G.; Martell, A. E. "Stabillty Constants of Metal-Ion Complexes"; The Chemical Society: London, 1964. (4) Smith, R. M.; Mattell, A. E. "Critical Stability Constants"; Plenum: New York, 1976; Vol. 4. ( 5 ) Selig, W. Mikrochlm. Acta 1982, 141-147. (6) Selig, W. Mikrochim. Acta 1983, 141-145. (7) Fricke, G. H. Anal. Chem. 1980, 52, 269R-275R. (6) Ruzicka, J.; Hansen, E. H. "Flow Injection Analysis"; Wiley-Interscience: New York, 1981. (9) Ruzicka, J. Anal. Chem. 1983, 55, 1040A-1053A. (10) Pungor, E.; Feher, 2.: Nagy, G.: Toth, K. CRC Crit. Rev. Anal. Chem. 1983, 14, 175-230. (11) Llendo, R. A.; Rechnitz, G. A. Anal. Chem. 1973, 4 5 , 826-833. (12) Liendo, R. A.; Rechnltz, G. A. Anal. Chem. 1973, 4 5 , 2165-2170. (13) Arnold, M. A.; Meyerhoff, M. E. Anal. Chem. 1984, 56, 20R-48R. (14) American Public Health Association "Standard Methods for the Examlnation of Water and Wastewater", 14th ed.; Washington, DC, 1976. (15) Technlcon Industrial Systems "Technlcon Auto Analyzer-I1 Industrial Method No. 118-71 WIB"; Tarrytown, NY, 1977. (16) Krug, F. J.; Zagatto, E. A. G.; Reis, B. F.; Bahia, 0. F.; Jacintho, A. 0.; Jorgensen, S. S. Anal. Chim. Acta 1983, 145, 179-187. (17) Trolanowicz, M. Anal. Chim. Acta 1980, 1 1 4 , 293-301. (18) Heijne, G. J. M.; van der Linden, W. E.; den Boef, G. Anal. Chim. Acta 1978, 100, 193-205. (19) Gardner, C. W., Jr. Ph.D. Thesis, University of Pittsburgh, Pittsburgh, PA, 1984. (20) Deshmukh, B. K.; Coetzee, J. F. Anal. Chem. 1984, 56, 2373-2378. (21) Hansen, E. H.; Ruzicka, J.; Krug, F. J.; Zagatto, E. A. G. Anal. Chim. Acta 1983, 148, 111-125. (22) White, D. C. Microchim. Acta 1959, 254-269. (23) Alexander, P. W.; Haddad, P. R. Anal. Chem. 1984, 56, 2417-2422. (24) Smith, F. C., Jr.; Chang, R. C. CRC Crit. Rev. Anal. Chem. 1980, 9 , 197-217.
RECEIVED for reiew June 27, 1985. Accepted November 14, 1985. This work was supported by the National Science Foundation under Grant CHE-8106778 and CHE-8408411.
