Anal. Chem. 1995,67,2299-2303
Flow Injection Analysis of Sulfate with a Potentiometry=BasedLinear Calibration Graph -ang
Tangt and Hsuan=JungHuang*s*
Deparlment of Chemistry,National Sun Yat-Sen University,Kaohsiung 80424, Taiwan, and Department of Industrial Safety and Hygiene, Chia-Nan Junior College of Phannacy, Tainan 71710, Taiwan
Based on the behavior of lead ion selective electrode (Pb ISE) and the interaction between a suhte-containing solution and solid lead sulfate, equations describing the explicit linear relationships between the measured potential of Pb ISE and the sulfate concentration in sample solutions were derived and applied to the determination of s&te by the flow injection analysis technique. These proposed procedures improved the sensitivity of the potentiometric method and the reliability of the analytical results. W& a mixed solvent system (35 vol % methanol) a detection limit as low as 1.O x M can be obtained optimistically. Sulfate determination is one of the routine analyses encountered frequently in analytical laboratories. Besides the technique of ion methods applicable to the analysis of sulfate are indirect. The analytical procedures usually include the precipitation of sulfate ions with barium or lead ions, followed by either turbidimetric, spectrophotometric, or potentiometric d e termination of the precipitates formed or the remaining barium or lead ions in solution.3-16 Among these methods, the potentiometric method has attracted continuous attention due to its convenience and ease of operation. Recently, the flow injection technique has been coupled with the lead ion selective electrode and used for the determination of sulfate. Although various procedures have been developed, a nonlinear or semilinear plot is usually obtained and used as the calibration graph, or the equilibrium constant for the reaction of sulfate with lead has to be known or determined in order to linearize the calibration graph.l4-I6 This introduces either unpredictable uncertainties or
’ Chia-Nan Junior College of Phannacy. * National Sun Yat-Sen University. (1) Small, H.; Stevens, T. S.; Bauman, W. C. A d . Chem. 1975,47, 1801. (2) Mulik. J.; Puckett, D.; W i U i s , D.; Sawicki, E. Anal. Left. 1976,9,653. (3) Greenberg, A E.; Connors, J. J.; Jenkins, D. Stundnrd Methods for the Eramination of Water and Waste Water, 15th ed., American Public Health Association: Washington, DC, 1981; pp 439-440. (4) &pel, B. R; Kothny, E. L;Hoffer, E. M.; Buell. G. C.; Wall, S. M.; Wesolowski J. J. Resented at a meeting of the Division of Environmental Chemistry, American Chemical Society, New Orleans, March 20-25.1977. (5) Dunk, R;Mostyn, R A; Hoare, H. C. At. Absosorpt. N e d . 1969,8, 79. (6) Forbes, E. A. Analyst 1973,98, 506. (7) Galle, 0.IC; Hathaway, L R Appl. Spectrox. 1971,29, 518. (8) Eagner, G. H.; Steele, K F. Int. Lub. 1982,(Sept. 29), 12. (9) Levins, R J. Anal. Chem. 1971,43, 1045. (10) Ross, J. W., Jr.; Frant, M. S. Anal. Chem. 1969,41, 967. (11) Analytical Methods Cui&, 9th ed.;Orion Research, Cambridge, MA, 1978. (12) 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. (13) Trojanowicz, M. Anal. Chim. Acta 1980,114, 293. (14) Coetzee, J. F.; Gardner, C. W.. Jr. Anal. Chem. 1986,58,608. (15) Hara, H.; Horvai, G.; Pungor, E. Analyst 1988,113, 1817. (16) Hara, H.; Mori, S., Analyst 1990,115, 1077. OOO3-2700/95/0367-2299$9.00/0 Q 1995 American Chemical Society
tedious procedures. In order to overcome these drawbacks, equations based on the behavior of the lead ion electrode and the interaction between a sulfatecontainingsolution and solid lead sulfate have been derived and applied to the potentiometric determination of sulfate in solutions. With the proposed proce dures, a linear calibration graph can be obtained from a plot of measured potential-related variables vs the concentration of sulfate in solutions. S i a r l y , by following the same procedures, a standard addition method can also be applied to the potentiometric determination of sulfate. In order to lower the detection limit, experiments are run with a mixed-solvent system containing 35 vol % methanol. A detection limit as low as 1.0 x M which is about 1 order lower than that of the reported potentiometric methods can be obtained optimistically. PRINCIPLE
The potential of the lead ion electrode in a lead ioncontaining solution can be represented by the extended Nikolsky equation
E = L + 0.0128 h ( a ,
+ xki,u?)
where E is the measured potential of the lead ion electrode (with respect to the reference electrode), L is a constant, 0.0128 is the explicit value of RT/nF, upb is the activity of lead ion in solution, uj represents the activity of interfering ions, 5 is the charge ratio of the lead ion to the interfering ion j , and kij is the selectivity ratio characteristic of Pb ISE. A background potential Eb can be obtained when the lead ion electrode is immersed in the blank solution:
Eb = L + 0.0128 h(xkipp) In a flow system, if the blank solution is allowed to flow through a solid lead sulfatecontaining column, the measured potential ,?& will be equal to
E, = L + 0.0128 h(w,”
+ xkipi”’,
(3)
where amo is the activity of lead ion that enters the solution from solid lead sulfate under equilibrium conditions, a representing the ratio of lead ion entering the solution from solid lead sulfate under the experimental conditions to those under the equilibrium conditions incorporating the dispersion effect of the flow system and is a constant at a given experimental condition. The potential difference, A&, caused by the dissolution of lead ion from solid Analytical Chemistry, Vol. 67, No. 13, July 1, 1995 2299
lead sulfate can be obtained by subtracting eq 2 from eq 3:
/$////I Pump1
Sample
,:A
PbSOc Column I ,
Sample
With the substitution of mpboy for @bo in eq 4, the concentration of the dissolved lead ion, a m p b o can be expressed as
(5) where E O= AEd0.0128 and y is the activity coefficient of lead ion in solution. When a sulfate-containing solution is allowed to flow through the solid lead sulfatecontainingcolumn, an equation similar to eq 5 can be derived for the concentration of lead ion dissolved from solid lead sulfate,ampb.
Carrier
Recorder
pH-Meter
Waste
Figure 1. Scheme of the FIA system.
By substituting eq 5 into eq 10, then where E ’,, = (E,, - Eb)/O.O128 and E,, is the measured potential for the sulfate-containing solution flowing through the column. With a flow system, equations describing the relationships between the sulfate-containiig solution and the solid lead sulfate can be derived by following the same procedures developed previou~ly.’~ When a sulfatecontainingsolution is allowed to flow through and react with solid lead sulfate, the dissolved lead and sulfate ions and the originally present sulfate ion will have the following relationship: (am,,
+ msol)amPb= Kspl
(7)
where ampb and am,, are the concentrations of lead and sulfate ions, respectively, that have entered the solution from the solid lead sulfate (assuming that no lead ion was present in the original sample solution), msol is the concentration of sulfate present in the original sample solution, amso ms: is the concentration of sulfate present in solution after flowing through the column, and K,; is the apparent solubility product of lead sulfate under the experimental conditions. As ampb = am,,, eq 7 can be expressed as
+
(amyb
+ ms’)amPb = Kspl
(8)
or
where K‘ = y/(Xki,@)(exp(E0? - 1) is constant at the given experimental conditions. A straight line with an intercept at the origin and a slope equal to IC could be obtained when values of (P- 1/p) were plotted against m’=. If an unknown concentration of sulfate present originally in the sample solution, a standard addithn method can be derived (following the same procedures as described above) to find the unknown concentration m, in solution. With the addition of standard sulfate solution (with a concentration of mW”) to the sample solution, the sulfate concentrationin the sample solution will become m, mW”. After interaction with solid lead sulfate, a relationship similar to that in eq 8 can be obtained:
+
(m, + ampb+ mso)’)ampb= Ks;
(14)
Following the same treatment as above for eqs 14 and 9, a new equation similar to eq 13 can be obtained
If a solution containing no sulfate (blank solution) is allowed to interact with solid lead sulfate, eq 8 becomes
amPboamPbo= Ks;
(9)
where ammo represents the concentration of lead or sulfate dissolved from solid lead sulfate. Equating eqs 8 and 9 and putting mpbo = Pmpb, the following equation is obtained: ampbo(P- 1/p) = m,;
From eq 5 and 6, P can be expressed as
(10)
From eq 15 it can be seen that a plot of (P- 1/p) vs m”= should yield a straightline with a slope equal to IC that intercepts the abscissa at -mx. Hence the unknown concentration of sulfate in solution can be obtained. EXPERIMENTAL SECTION Flow Injection System. The flow system consists of two
peristaltic pumps, one solid lead sulfate reaction column, and one injection valve (Rheodyne 5020) with a sample loop of 0.10 mL ( F i r e 1). Pump 1 was used for the delivery of sample or (17) Tang, T.C.;Huang, H-J, Anol. Chim.Acta 1990,238,431.
2300 Analytical Chemistry, Vol. 67,No. 73,July 7, 7995
standard solutions to the lead sulfate column. Pump 2 was used for the delivery of sample solution from the sample loop to the potentiometric detector. The lead sulfate column was prepared by packing lead sulfate powder (particle size 80-120 mesh) into a piece of PTFE tubing (300 mm x 3 mm i.d.). A 0.45 pm poresize filter was placed at the outlet of column to prevent the flowthrough of solid lead sulfate into the detection system. The potentiometric detector was made from a piece of PTFE block which contained a commercial Pb ISE (Orion, Model 94-82), and a homemade reference electrode (Ag/&l in 3 M NaC1). A WDF spacer of 0.015 cm in thickness with a 0.1 cm wide slot debing the flow path between the inlet and outlet ports was used. The potential of the Pb ISE was measured with a Coming pH meter-140. The output of the pH meter was recorded with a Yokogawa 3025 recorder. All of the sample and standard solutions were analyzed in triplicate. The optimum flow rate for the delivery of sample or standard solutions and the reacted solution (by carrier) was 0.3 and 0.9 mL/min, respectively. In order to improve the sensitivity of the studied method, 35 vol % methanol was added to all the sample or standard sulfate solutions. In order to minimize the possible contamination resulting from the residue of the previous sample, 4 min was allowed for the blank solution to flush and clean the solid lead sulfate column before new sample solution was introduced. It took about 3 min for each sample to go through the injection and detection processes. The ionic strength of the sample and carrier solutions was adjusted to 0.10 by the addition of 0.10 M NaC104 to the solutions. Ion Chromatographic Analysis. A Dionex series 45OOi ion chromatograph with a conductivity detector was used for the direct analysis of Sod2- content in rainwater sample. An IonPac AG4A (P/N 37042) column and an Anion Micro Membrane Suppressor (AMMS, P/N 037072) were equipped for the IC, and 0.1 M NaOAc/O.M M HOAc solution was used as the buffer solution. Reagents. All chemicals used were of GR grade (Merck) or better. Standard sulfate solutions were prepared from the dilution of Merck Titrisol solution. All the solutions were prepared from the deionized/distilled water.
00
05
10
m'so(~x104)
RESULTS AND DISCUSSION
Wect of Flow Rate on the Sulfate Determination. This method is based upon the steady state interaction that existed between the sulfate ion in the sample solutions flowing through solid lead sulfate and the amount of lead ion dissolved thus, the sensitivity of the Pb ISE determination is affected by the steady state obtained. The flow rate of the sample solution will thus affect the sensitivity of this method. With a higher flow rate, sample solution takes a shorter time to pass through the column and introduces less lead ions into the sample solution. A smaller a value in eq 7 will result. Conversely, if a lower flow rate is used, a larger amount of lead ions will dissolve and a larger a value will result to give a better sensitivity, although it takes a longer analysis time. The responses of potential difference a,, at various M sulfate flow rates are studied by the injection of 6.0 x solution into the system. It is found that the measured potential difference decreases as the flow rate increases. A compromise value of 0.3 mWmin is chosen as the optimal flow rate of sample solution for the following studies. Calibration Graphs for Sulfate Determination. Figure 2a shows the conventional calibration graph of a plot of M,,vs ms;. The concentration of standard sulfate solution, mW',is varied in
5
m"so(Mx10) Figure 2. Calibration graphs obtained (a) from the plot of potential difference A€,, vs sulfate concentration in solution (b) according to eq 13 and (c) according to eq 15.
the range 4.0 x 10+j-1.2 x M. the curve in Figure 2a is nonlinear in character. The same data are plotted according to eq 13 and shown in F i i r e 2b. A straight line with an intercept (at the 90%coddence level) and a linear of (0.05 f 0.01) x correlation coefficient of 0.9997 is obtained. In another experiM) is added ment, a known concentration of sulfate (6.0 x to the standard sulfate solutions. The obtained data are plotted according to eq 15 and shown in Figure 2c. A straight line with a linear correlation coefficient of 0.9998 and an intercept of -(6.14 f 0.20) x (at the 90%contidence level) at the abscissa is obtained. The determined sulfate concentration agrees with the known value very well. Analytical Chemistty, Vol. 67,No. 13, July 7, 1995
2301
Effect of Methanol on the Detection Limit of S a t e Determination. As the solubility of lead sulfate decreases with the decrement of the dielectric in solution, the Kspl value of lead sulfate and therefore the detection limit of sulfate can be lowered by using a methanol-containing solution in the system. Calibration graphs for solutions.contained 25 and 35 vol % of methanol were constructed. Straight lines with linear correlation coefficients better than 0.999 are obtained for these two plots. The slopes of the linear calibration graph obtained are 5.2 f 0.1 and 45.2 f 1.0, respectively. The variation of slope is due to the change in the exp (E
4.0
1
3.0
-
2.0
-
1.0
-
I
P
00
0 5
10
1 5
20
2 5
30
m'SO(~x1~5) Figure 3. Calibration graphs obtained from standard sulfate solutions in the presence of 1 .O x M oxalate.
curvature found in Figure 3 thus shows the innuence of the oxalate (at a concentration of 1.0 x M) on the calibration graph. The calibration graph shown in Figure 3 is useful for diagnosing whether any interferent is present in the sample solutions or not. From the curvature obtained and the level of interferent present, the effect of interferent on the calibration graph can be understood. It assures the reliability of the analytical results obtained. Different from anions mentioned above, cations which form more insoluble compounds with sulfate ion than lead ion do induce an overdissolution of lead ions from solid lead sulfate and introduce negative errors (cause a downward shift of the calibration graph) at higher sulfate concentrations. Barium ion is a typical example showing this type of interference. The second type of interference comes from the interference inherent with the Pb ISE. From the literature, cations such as Cd2+ CuZf, Hg2+,and Ag+ interfere with the lead ion determination. The presence of these ions causes an enhancement of potentiometric response which introduces a constant additive error. The effect of this constant error can be calculated by taking the differential of mso' or (m, m,,") with respect to mpb in eq 13 or 15.
+
dm,,'/dmpb = - (P
+ 1)
(16)
From eq 16, if dmpb is the constant additive error introduced by the presence of Cd2+,CuZt,Ag2', and Cd2+in sample solutions, dm,; will be the resulting deviation in the determination of mS; and dms; is equal to - (P +1) dmpb. At low sulfate concentrations, P 2 approaches 1 so dms,,' = -2 dmpb. The effect of the constant error on sulfate determination is insignificant if dmpb is small compared with mpb. However, at high sulfate concentrations, P 2 becomes much larger than 1 so that a small error in dmpb will result a significant error in dmso'. Besides the heavy metal ions mentioned, chloride, nitrate, and bicarbonate when present in 1Wfold excess over the concentration of SO42- might seriously interfer with the lead ion determination.I6 The possible interference of C1- is studied as it is the most common anion existing in various sample solutions. With the addition of 2.0 x 10-4 M C1- to the standard sulfate solutions, a calibration graph similar to that of Figure 3 was obtained.
Adequate linearity is found for solutions with sulfate concentration higher than 7.5 x 10-5 M. The upward shift of the calibration graph occurred when the sulfate concentration was lower than M. The deviation may be due to the complexhg effect 4.0 x of C1- with the dissolved lead ion. To test the validity of this method, a rainwater sample collected at Kaohsiung was measured by the proposed standard addition procedures. The sulfate content in the collected rainwater sample was found to be (1.36It 0.10) x M. Another portion of the rainwater sample was analyzed by the ion chromatography method and a result of (1.46 f 0.06) x M was obtained. Comparing these two results, the discrepancyis less than 10%and is thought to be tolerable from the viewpoint of potentiometric analysis. COWCLUSIONS With the proposed method, linear calibration graphs are obtainable for the potentiometric determination of sulfate. The precision and sensitivity of the proposed method are improved compared with the potentiometric methods reported by Coetzee and by P~ng0r14-16as the calibration graphs used by them were obtained by a series of tedious procedures and deviations occurred at a low SOf- concentration range. In this experiment, a parameter such as Kw is eliminated during the process of equation derivation; the error possibly inherent with the determination of Kspis thus eliminated. The other superior point of this method ~
~~~
~~
~
(18) Kumamaru, T., Tao, E., Okamoto, N.,; Yamamoto, Y.BuU.Chem. SOC.Jpn. 1966,38,2204. (19) Collinson, W.J., Boltz, D.F.Anal Chem. 1968.40,1896. (20) Hara, H.;Kuzu, S. Anal. 0 t h . Acta 1992,26, 411.
isthat whenever interferences are present in the determination system, a curvature of the linear calibration graph will be found. This waming curvahtrewill assure the reliability of the determined results. With the proposed flow injection method, as the solid lead sulfate inside the column dissolves gradually as the sample or carrier solution flows through the column continuously, the degree of partial equilibrium between the sulfate ions in the sample solution and the lead sulfate in the column may change gradually and thus induce errors. This type of error can be minimized by using a column of larger capacity or by decreasing the volume of sample solutions flowing through the lead sulfate column. Due to the passivation of the lead ion electrode, the sensitivity of a Pb ISE may be lowered and thus the slope of the linear plots may by varied, but the linearity of the plots will not be affected as long as the experiment is run for the same continuous period of time. The same procedures should be applicable to the other indirect potentiometric determination systems that are based on precipitation or complex formation processes, e.g., the determination of nitrate,18perchlorate,lg and various pho~phates.'~B ACKNOWL~WME" The authors thank the National Science Council of R0.C. for financial support. Received for review November 28, 1994. Accepted March 21, 1995.@ AC941155Q e Abstract published
in Advance ACS Abstmcts, May 15,1995.
Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
2303
