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mandrels of the peristaltic pump contacting the manifold tubing. CONCLUSIONS The results of this study have shown that thermal lens calorimetry is tolerant of flowing samples at flow velocities which would be suitable for applications in liquid chromatography or automated analysis. Although some reduction in sensitivity is observed, the major loss in performance is the increase in detection limits due to flow pulsations adding to the proportional noise. Several improvements in the system could reduce this problem. The peristaltic pump provides a worst-case example of pulsations; a pulse-damped,dual-piston pump or a syringe pump would greatly improve the situation. Furthermore, a differential thermal lens calorimeter (24), where reference and sample cells are placed on opposite sides of the beam waist, has been shown to exhibit excellent immunity to enhancement fluctuations and could be applied to a flowing sample experiment. One possible advantage of flowing the sample could be expected when the solvent composition is changed or otherwise unknown. Under static conditions, the sensitivity (enhancement) of a thermal lens measurement is governed by the thermal conductivity, K, and (dn/dT) of the solvent. When the mixing process induced by flow dominates heat transport, however, the enhancement should be less significantly effected by the thermal conductivity of the solvent. This could, therefore, reduce the dependence of sensitivity on solvent composition.
LITERATURE CITED (1) Freeman, N. K.; Upham, F. T.; Wlndsor, A. A. Anal. Lett. 1973, 6 , 943-950.
(2) Deiboid. G. J.; Zare, R. N. Sclence 1977, 196, 1439-1441. (3) Yeung, E. S.; Steenhock, L. E.; Woodruff, S. D.; Kuo, J. C. Anal. Chem. 1980, 52, 1399-1402. (4) Hu, C.; Whlnnery, J. R. Appl. Opt. 1973, 12, 72-79. (5) Hordvlk, A. Appl. Opt. 1977, 76, 2827-2833. (6) Kreuzer, L. B. US. Patent 4048499, 1977. (7) Gordon, J. P.; L e k , R. C. C.; Moore, R. S.; Porto, S. P. S.; Whinnery, J. R. J. Appl. PhyS. 1965, 36,3-8. (8) Whinnery, J. R. Acc. Chem. Res. 1974, 7 , 225-231. (9) Harris, J. M.; Dovlchl, N. J. Anal. Chem. 1980, 52, 695A-706A. ( I O ) Hunt, J. N. "Incompressible Fluid Dynamlcs"; American Elsevkr: New York, 1964. (11) Carslaw, H. S.; Jaeger, J. C. "Operatlonal Methods In Applled Mathematics", 2nd ed.;Oxford University Press: London, 1948; Chapter VI. (12) Dabby, F. W.; Gustafson, T. K.; Whinnery, J. R.; Kohanzandeh, Y.; Kelley, P. L. Appl. Phys. Lett. 1970, 16, 362-388. (13) Akhamanov, S. A.; Krindach, D. P.; Migulin, A. V.; Sukhorukov, A. P.; Khokhlov, R. V. I€€€ J . Quantum Electron. 1966, QE-4, 568-575. (14) Gebhardt, F. G.; Smlth, D. C. Appl. Phys. Lett. 1969, 74, 52-54. (15) Smith, D. C.; Gebhardt, F. G. Appl. Phys. Lett. 1970, 16, 275-278. (16) Gebhardt, F. G.; Smlth, D. C. Appl. Opt. 1972, 1 7 , 244-248. (17) Wellegehausen, B.; Laepple, L.; Welling, H. Appl. Phys. 1975, 6 , 335-340. (18) Teschke, 0.; Whlnnery, J. R.; Dlenes, A. I€€€ J. Quantum Electron. 1978, QE-12, 513-515. (19) Twarowski, A. J.; Kliger, D. S. Chem. Phys. 1977, 20, 253-258. (20) Rohde, R. S.; Buser, R. G. Appl. Opt. 1979, 18, 898-704. (21) Dovlci, N. J.; Harris, J. M. Anal. Chem. 1981, 53, 108-109. (22) Burke, R. W.; Deardorff, E. R.; Menls, 0. J. Res. Nat. Bur. Stand., Sect. A 1972, 76, 469-478. (23) Bevington, P. R. "Data Reductlon and Error Analysis for the Physlcal Sclences"; McGraw-Hill: New York, 1989; Program 11-5. (24) Dovichl, N. J.; Harris, J. M. Anal. Chem. 1980, 52, 2338-2342.
RECEIVED for review November 4, 1980. Accepted January 12,1981. This material is based upon work supported by the National Science Foundation under Grant CHE79-13177. Fellowship support (N.D.) by the Phillips Petroleum Co. and the American Chemical Society, Division of Analytical Chemistry (sponsored by General Motors), is acknowledged.
Enrichment of Nickel and Cobalt in Natural Hard Water by Donnan Dialysis Robert L. Wilson and James E. DINunzio" Department of Chemistry, Wright State University, Dayton, Ohio 45435
Donnan dlalysis enrichment was used for the determination of nlckel in natural rlver water by flame atomic absorptlon spectrophotometry. The optimum receiver electrolyte composition was found to be a compromlse between maxlmum enrlchrnent and the effects of the electrolyte on the signal to nolse ratio. The presence of phosphate In the sample was found to influence both the precision and accuracy of the method. However, adjustment of the sample pH prior to enrichment eiimlnated this problem. The use of cobalt as an internal standard Improved the preclslon and ellmlnated matrlx interferences.
Ion-exchange membranes have been used under conditions of Donnan dialysis ( I ) for a number of anlytical applications. In this process an ion-exchange membrane is used to separate solutions of differing ionic strength. As the system approaches Donnan equilibrium, trace level ions in the dilute sample soluton will migrate into the more concentrated receiver solution (2, 3). If the volume of the receiver solution is much
less than that of the sample, an enrichment of these ions can be accomplished. Since the rate of transfer of these ions is directly proportional to concentration, enrichment for a prescribed time can be used as a method of preconcentration prior to analysis. This technique has been used for the enrichment of both anions ( 4 , 5 )and cations (6, 7). Although Donnan dialysis enrichment has been used successfully as a preconcentration method prior to chemical analysis, few applications of the technique to natural samples have been reported. This may be due to the fact that a number of interferences are present in natural waters. The interferences include surfactants which can foul the membrane (8),chelating agents (9, IO),and an unknown and usually high ionic strength. These factors can interfere with the enrichment technique by altering the ion transfer rate across the membrane. The purpose of this paper is to report on the successful application of Donnan dialysis enrichment for the preconcentration of heavy metals from natural hard water. In this study Ni(I1) and Co(I1) were chosen as test species; however, since the enrichment process is nonspecific and operates on
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all ions of the appropriate charge sign (6, II), it is expected that the procedure can be applied to a large number of heavy metals. EXPERIMENTAL SECTION Procedure. Grab samples of natural water were collected from the Mad River. The sampling site was located approximately 3.4 miles downstream from a local water treat,ment plant. Immediately after collection 1 drop of 30% HzOz was added to each 2 L to preserve and oxygenate the sample;as required by the UV digestion procedure used. The sample was filtered through 0.45-pm regenerated cellulose filters, transferred to quartz flasks, and exposed to UV radiation (A- = 2537 A) in a photochemical reactor to oxidize organic complexing agents which might be present in the sample. The use of UV radiation for the oxidation of dissolved organics has been well characterized, being shown to be both a rapid and efficient process (12-14). Therefore, this technique was not extensively investigated in this study. In most cases the samples were collected,returned to the laboratory,prepared for digestion, and placed in the photochemical reactor overnight (about 16 h), since this was a convenient break point in the routine. However, there was no differencein enrichmentfor these samplesand those digested for as little as 2 h. After digestion the sample was adjusted to about pH 3.5 with concentrated HCl. This was done to ensure maximum enrichment and to redissolve the small amount of CaC03precipitate which formed during the digestion process. Deionized water was added to each quartz flask as required to replace the volume lost by evaporation during the digestion. Finally the resistance of the digested sample was measured. Metal enrichmentswere performed in a manner similar to that previously described (6). In all cases the diameter of the membrane in the dialysis cell was 3 cm and 200-mL aliquots of sample were enriched into 4.0 mL of receiver electrol.yk. The enrichment time was 1 h unless otherwise stated. Enrichment was initiated by placing the membrane face of the dialysis cell, containing the receiver electrolyte, in contact with the magnetically stirred sample solution. After the prescribed enrichment time the dialysis cell was removed from the sample and wiped and the receiver electrolyte transferred to a sample container. When necessary the receiver electrolyte was filtered thought 0.45-rmregenerated cellulose Titer using a syringe filter. Metals in the receiver electrolyte were then determined by atomic absorption spectrophotometry. Apparatus. A Rayonet PhotochemicalReactor (The Southern New England Ultraviolet Co., Middletown, CT) with mediumpressure mercury lamps (Ama = 2537 .&) was used to digest the natural water samples. Resistance measurements were made with a Yellow Springs Instrument Co. Model 31 conductivity bridge (Yellow Springs, OH) and Beckman CEC-BB1 (K = 1.00 cm-l) conductivity cell. Metals were determined by use of a Varian Techtron Model AA-6 atomic absorption spectrophotometerwith a Model BC-9 simultaneous background corrector. The cation-exchangemembranes used were Type P-1010 (RAI Research Corp., Hauppauge, Long Island, NK). The membranes were pretreated in a manner previously described (15)and were stored in the receiver electrolyte when not in use. All reagents used were analytical reagent grade, and solutions were prepared by using doubly distilled deionized water. RESULTS AND DISCUSSION On the basis of the reported enhancement in the transfer rate of trace metals through ion-exchange membranes in the presence of Al(III), a receiver electrolyte consisting of MgS04 and A1#~04)3 was chosen. The Al(II1) in the receiver electrolyte allows high enrichments to be achieved by interacting strongly with the fixed sites in the membrane. This reduces the attraction between the fixed sites and trace metals and allows them to diffuse rapidly across the membrane (6, 7). Initial experiments were aimed a t optimizing the receiver electrolyte to obtain a maximum enrichment of trace metals from high ionic strength samples and maximum sensitivity for their determination.
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Table I. Effect of Receiver Electrolyte Ionic Strength on the Enrichment of Nickel and Cobalt from High Ionic Strength Solutions enrichment [MgSO,], ionic factorb M strengtha Ni(I1) Co(I1) 0.20 0.60 0.80 1.00 1.25 1.50
0.81 2.41 3.21 4.01 5.01 6.01
6.34 8.78 9.46 9.62 9.75 10.54
4.76 8.28 9.32 10.02 9.94 10.70
Ni/Co 1.33 1.06 1.02 0.96 0.98 0.99
All solutions contained 5 x l o F 4M Al,(SO,),. Enrichment factor is defined as the ratio of the concentration of metal in the receiver electrolyte divided by the concentration of metal originally present in the sample. The ionic strength of the receiver electrolyte was found to control the enrichment rate of both Ni(I1) and Co(I1). Table Ishows the enrichment factors and relative enrichment for these ions from solutions containing 0.1 M NaCl as a function of the concentration of MgS04 and the ionic strength of the receiver electrolyte. The enrichment factor increases rapidly with increasing concentration of MgS04 up to 0.8 M; above this concentration the influence of the change in ionic strength of the receiver electrolyte on the enrichment factor is less. Similar experiments were performed to determine the influence of Alz(S04)3on enrichment. Receiver electrolytes containing 0.2 M MgSO, and Al2(so4)3ranging from 5 X lo4 to 5 X M were used. The upper concentration limit of AlZ(SO& was determined by the pH of the receiver electrolyte. Variation of Alz(S04)3over this range had no significant influence on the enrichment factor for either Ni or Co. Since the variation in A12(S04)3accounted for only a small change in the ionic strength of the receiver, this observation is not surprising. With high ionic strength solutions on each side, the permselectivity-the difference in the transfer of ions of opposite charge-of the membrane decreases (3,6). From the data in Table I it can be seen that the selectivity of the membrane for Ni and Co is also dependent on the ionic strengths of the separated solutions. For the lowest ionic strength receiver electrolyte, the relative enrichment of Ni to Co is 1.33. This indicates that Ni transfers at a faster rate than Co. As the concentration of MgS04 in the receiver electrolyte increases, the relative enrichment reaches a value of 1, indicating that the rates of diffusion of these ions through the membrane become equal when the concentration of MgS04 in the receiver electrolyte is 0.8 M or greater. Since the rate of diffusion of counterions through an ion-exchange membrane is dependent on the membrane selectivity (16), it follows that the difference in membrane selectivity for the two counterions decreases a t high receiver electrolyte ionic strength. The fact that the relative selectivity of the membrane for counterions decreases at high receiver electrolyte ionic strength is of importance in the selection of an internal standard. In this study Co(I1) was chosen as the internal standard counterion based on its exhibiting ion-exchange behavior similar to Ni(I1) (17). However, when receiver electrolytes of high ionic strength are used, the selection of the internal standard counterion becomes less critical. Since under these conditions differences in selectivity will be small, errors arising from these differences will be small. The role of the internal standard will then be to correct for experimental variables other than those due to the membrane transfer process. These factors include temperature, membrane area of the dialysis cell, and
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stirring rate of the sample solution. With nominal control of these factors the relative standard deviation of the method for the determination of Ni is 10%. By use of an internal standard, the relative standard deviation improved to 2%. Other factors must be considered when selecting the optimum receiver electrolyte. Of primary importance is the method of analysis to be used in the determination. On the basis of the data presented above, it would seem that the optimum receiver electrolyte would be one containing 1.50 M MgS04 and 5 X M Alz(S04)3.This receiver electrolyte is shown to produce the highest enrichment which should result in a lower detection limit for the method. In this study, however, the optimum receiver electrolyte was found to be 1.0 M MgS0, and 5 X M Alz(SOJ3. This receiver electrolyte was chosen because it was found to be compatible with the flame atomic absorption spectrophotometricmethod used to determine Ni and Co in the receiver electrolyte. Higher concentrationsof MgS0, in the receiver electrolyte were found to cause several problems such as increased flame noise, decreased aspiration rate, blockage of the aspirator, and the formation of salt deposits on the burner. These problems resulted in a higher detection limit for the enrichments using the higher concentrations of MgSO,. At MgS04 concentrations less than 1.0 M the detection limit was higher due to lower enrichment of Ni. With other methods of analysis which are not influenced by the electrolyte, the optimum receiver electrolyte would be that of the highest ionic strength. The ionic strength of the sample solution has been shown to influence the transfer of trace counterions in Donnan dialysis (5, 6). The ionic strength is directly related to “hardness” of natural waters, therefore hardness must be considered when using Donnan dialysis in natural water analysis. In a survey of water quality of Southwest Ohio (18) the average total hardness as CaC03 was found to be 320 mg/L. As is characteristic of hard water, calcium was found to be the major contributor to the water hardness. These facts are important because they represent two potential sources of interference with the technique. First, the high and variable ionic strenth of the sample can result in decreased transfer rates of trace counterions by decreasing the Donnan potential across the membrane. Second, the formation of insoluble CaS0, within the membrane and in the receiver electrolyte could foul the membrane resulting in re duced transfer rates or preventing enrichment entirely. Since the basic assumption of direct quantitative analysis using Donnan dialysis enrichmnt is that the rate of transfer is proportional to the concentration of the trace counterions in the sample, a reduction in transfer rate would result in a negative error for the analysis. Figure 1 shows the enrichment factor of Ni(I1) and the relative enrichment of Ni(I1) and Co(I1) as a function of the concentration of calcium in the sample. At concentrations less than about M (for CaClZ,ru = 0.003) the average enrichment factor is identical with that obtained from pure aqueous solutions of Ni and Co. This indicates that neither the ionic strength nor the presence of calcium affect the transfer process. At higher calcium concentrations the enrichment factor decreases. This is thought to be due primarily to the increased ionic strength of the sample (6,15). However, it is in this concentration range that precipitation of CaS04 in the receiver electrolyte begins to occur, and there is some evidence to indicate that this may be contributing to the decrease by partially blocking the membrane. When an identical experiment was performed by using NaCl in place of CaClZ,it was found that the enrichment factor was unchanged up to approximately M NaCl ( p = 0.01). If the decrease in enrichment factor was due solely to the ionic strength of the sample solution, the results of the two ex-
RESISTANCE
lo,
30pOO
4800
3 P [c
600
I.
2
70
, 1 I
Figure 1. Effect of calclum on the enrlchment of nickel and cobalt: receiver electrolyte 1.0 M MgS04, 5.0 X lop3M AI,(S04)B: 0,enrichment factor for nickel: A, Ni/Co enrichment factor ratlo.
periments should be identical. Although lower enrichments are obtained in the presence of high concentration of calcium, the formation of CaSO, precipitate in the receiver electrolyte does not affect the results obtained when an internal standard is used. As can be seen from Figure 1, the use of Co(I1) as the internal standard effectively corrects for errors due to the decreased enrichment factor even at very high concentrations of calcium in the sample. Also, analysis of the precipitate formed in the receiver electrolyte indicated no detectable loss of Ni(I1) or Co(I1) through coprecipitation with CaS04. Figure 1also shows the relationship between the enrichment factor and resistance of the solution. This gives a convenient means of determining the length of time required to obtain a desired enrichment factor. This is accomplished by measuring the resistance of the digested pH adjusted sample prior to enrichment. If the resistance is greater than 4800 Q an enrichment factor of about 20 can be expected to be obtained for a 1-h enrichment time. If the resistance is less than 4800 Q,longer enrichment time will be required to achieve a 20-fold enrichment. Initial experiments with natural water samples yielded substantially lower enrichment factors than what was expected based on the data in Figure 1. On the basis of the resistance of the river water samples (1100-1700 Q) it was expected to obtain enrichment factors ranging from 15 to 17 for a 1-h enrichment. Actual enrichment factors obtained from spiked river water samples (pH 7-8) were found to vary widely over the range 3-12. This led to an investigation of the influence of pH on the enrichment of Ni(I1) and Co(I1). Figure 2 shows the enrichment factor of Ni(I1) as a function of the sample pH. identical results were obtained for Co(I1). When the sample pH is in the range 3.5-5, the enrichment factor is identical with that obtained for pure standard solution containing only Ni and Co. Above pH 5 the enrichment factor appears to be influenced by both the pH and the nature of the anion present in the solution. Since both Ni and Co form insoluble hydroxides, the influence of pH as shown by the carbonate curve is not surprising. In fact above pH 10 no enrichment is obtained. In the presence of phosphate the enrichment factor decreases rapidly in the pH range 5-6.This behavior is distinctly different from that occurring in the presence of carbonate and indicates that some factor other than the effect of pH is operating in the transfer process. As was observed for natural water samples (pH 7-8), in the pH range 7-10, the enrichment
Anal. Chem. 1981, 53, 695-701
A
h Flgure 2. Influence of pH and sample composition on the enrlchment of nickel: receiver electrolyte 1.0 M MgS04, 5.0 X M A12(S04)3; 0, M phosphate; A, M carbonate: sample pH adjusted wRh HCI or NaOH; sample Ionic strength 3 X M adjusted with NaCI.
factors of both Ni(I1) and Co(I1) were found to vary widely. For example, enrichment factors obtained for Ni(I1) at pH 8 were 1.7,2.3,8.1, and 13.4. Above pH 10 no enrichment of either Ni(I1) or Co(I1) was obtained. This is identical with the data obtained for the carbonate-containing system and is due to the formation of insoluble metal hydroxides. The data in Figure 2 offers a means of eliminatingproblems due to both pH and phosphate in the sample by adjusting the pH of the sample to below 5. Adjusting the pH of the digested sample prior to measuring the resistance and enrichment resulted in improvements in both the enrichment factor and reproducibility. Enrichment factors obtained from pH-adjusted river water samples were in the range expected based on the data in Figure 1. Linear working curves were obtained from the spiked samples for the determination of Ni in river water after UV digestion, pH adjustment, and Donnan dialysis enrichment. A least-squares curve fit of the data gave a slope of 5.16, intercept of 0.007, and a correlation coefficient pf 0.999. This compares favorably with the data obtained for the enrichment
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of standard solutions containing only Ni and Co which gave a slope of 5.17 and a correlation coefficient of 0.999. Recoveries of Ni from spiked river water samples as determined from the standard solution working curve were 199 f 5 ppb for a 200-ppb spiked sample and 6.0 f 1.8 ppb for a 5-ppb spiked sample, each based on four replicates. The close agreement between the working curves obtained for standard samples and spiked river water samples is an indication of the successful application of Donnan dialysis enrichment to the determination of trace metals in natural hard waters. The ability to obtain accurate quantitative results using a working curve prepared from pure aqueous standard solutions greatly simplifies the procedure. Since it is not necessary to match the sample matrix one standard working curve can be used for a wide variety of samples of varying composition.
LITERATURE CITED Wallace, R. M. Ind. Eng. Chem. Process Des. Dev. 1967, 6 , 423. Blaedel, W.; Christensen E. Anal. Chem. 1967, 39, 1262. Blaedel, W.; Haupert, T Anal. Chem. 1966, 3 8 , 1305. Cox, J. A; Cheng, K. H. Anal. Chem. 1978, 50, 601. Lundqulst, G. L.; Washinger, G.; Cox, J. A. Anal. Chem. 1975, 47, 319. Cox, J. A.; DINunzlo, J. E. Anal. Chem. 1977, 49, 1272. Cox, J. A.; Twardowski, 2 . Anal. Chem. 1980, 52, 1503. Elsner, U.; Rottschafer, M.; Beriandi, F.; Mark, H. Anal. Chem. 1967, 39, 1466. Singer, P. “Trace Metals and Metal-Organic Interactions in Natural Waters”; Ann Arbor Science Publishers Inc.; Ann Arbor, MI, 1973. Shuman, M. S.; Cromer, J. L. Envlron. Sci. Techno/. 1979, 73,543. Helffereich, F. “Ion Exchange”; McGraw-Hill: New York, 1962: Chapter 8. Arrnstrong, F.; Williams, P.; Strickland, J. Nature (London) 1966, 21 f , 481. Klemeneij, A. M.; Kioosterboer, J. G. Anal. Chem. 1978, 48, 575. Goossen, J. T. H.; Kloosterboer, J. G. Anal. Chem. 1978, 50, 707. Blaedel, W. J.; Kissel, T. R. Anal. Chem. 1972, 44, 2109. Helfferich, F. “Ion Exchange”; McGraw-Hill: New York, 1962; pp 156, 304, 365. Zweig, G.; Sherma. J. “Handbook of Chromatography”; CRC Press: Cleveland, OH, 1972; Vol. 1. Miami Conservancy Dlstrlct, Dayton, OH, 1977, unpubllshed work.
RECEIVED for review October 22,1980. Accepted January 2, 1981.
Theory of Square Wave Voltammetry for Kinetic Systems John J. O’Dea, Janet Osteryoung, and Robert A. Osteryoung Department of Chemistty, State University of New York at Buffalo, Buffalo, New York 14214
The theoretical response for the appllcatlon of square wave voltammetry to systems complicated by electrode klnetlcs or by precedlng, followlng, or catalytic homogeneous chemical reactions is presented. Experimentally measurable parameters such as peak shifts, heights, and wldths are calculated and plotted as functlons of the approprlate rate constants. These curves are characteristic of the electrode process and provide a basts for the extractlon of klnetlc lnformatlon from the fast scan square wave experiment.
The theory and experimentalverification for the application of a generalized square wave waveform to a reversible system 0003-2700/81/0353-0695$01.25/0
have been presented (1, 2). This technique has also been shown to give well-defined peaks at concentrations levels of lo-’ M (3), thus ranking it among the most sensitive modern electroanalytical techniques. We believe that square wave voltammetry by virtue of its immunity to charging currents can be utilized in kinetic studies of chemical systems at concentrationsbelow those now typically employed with other techniques. Schaar and Smith have reported the estimation of kinetic parameters for a fast one-electron-transfer process at 8 X M concentration (4) using ac polarography. However, loss of sensitivity for irreversible reactions is a much more serious problem in ac voltammetry than in square wave voltammetry, as will be illustrated below. It should be possible in square wave voltammetry to obtain sufficiently precise data 0 1981 American Chemical Society
