Flow-through calcium-selective electrode: application in flow-injection analysis and ion chromatography

Analytica Chimica Acta, 242 (1991) 65-72 Elsevier Science Publishers B.V., Amsterdam

65

Flow-through calcium-selective electrode: application in flow-injection analysis and ion chromatography N. Kolycheva Department

of Analytical

*

Chemistry, D.I. Mendeleev Moscow Institute of Chemical Technology, I25 190 Moscow (U.S.S.R.)

H. Miiller Section Chemie, Technische Hochschule “Carl Schorlemmer” (Received

Leuna-Merseburg,

4200 Merseburg (Germany)

11 th June 1990)

Abstract The use of solid contact flow-through calcium-selective electrodes as potentiometric detectors in flow-injection analysis and non-suppressed ion chromatography is discussed. Owing to the high selectivity of the membrane electrode based on tetratolyl-m-xylylenediphosphine dioxide, it can be used to monitor trace amounts of calcium ions in the presence of a lOO-fold excess of alkali metal, ammonium and magnesium ions. The detection limit is about 1 x 10m6 M. The composition and thickness of the calcium selective membrane influence both the detection limit and selectivity of the electrode. The sensitivity of this potentiometric detector in ion chromatography relative to alkaline earth and heavy metals is significantly higher than that of a commercial conductivity detector. Keywords: Chromatography;

Flow system;

Ion exchange;

Potentiometry;

Ion-selective electrodes (ISEs) are widely applied as sensors in liquid flow analysers [l]. For example, Frenzel [2] recently indicated that more than 150 papers devoted to flow-injection potentiometry have been published since the early 1970s. Solid-state ISEs are the most widely used flowthrough electrodes [l], but plasticized calcium ion-selective electrodes are also widely applied in flow-through systems [3-61, including flow-injection analysis (FIA) for monitoring calcium ions in different environmental samples, industry and medicine. Application of the electrodes to detect cations in ion chromatography (IC) is limited especially because of their high sensitivity. An interesting solution to this problem was proposed by Trojanowicz and Meyerhoff [7,8], who used a highly selective potassium electrode and a pH electrode to detect some cations and anions by means of replacement IC. 0003-2670/91/$03.50

0 1991 - Elsevier Science

Publishers

Ion-selective

electrodes;

Calcium

The development and application of highly selective Ca*+ ISEs started in the 1960s and their continuation has been discussed in detail [l-9]. When neutral carriers are used as electrode active components, the electrodes show higher sensitivity and selectivity [lo-171 than those containing ion exchangers. Ca*+ ISEs based on tetratolyl-m-xylylenediphosphine dioxide (TXDPO) are characterized by high selectivity with respect to calcium ions over alkali metal and magnesium ions [16,17]. The metha isomer is preferred to the ortho isomer for calcium electrodes [17]. This paper discusses the dynamic characteristics (in an FIA system) of flow-through solid contact Ca*+ ISEs with a poly(viny1 chloride) (PVC) matrix membrane based on m-TXDPO. To effect reversible internal contact, an electrode of a second kind with solid membrane (AgCl-Ag,S mixture) was used. Application of the electrode as B.V.

66

N. KOLYCHEVA

a differential potentiometric detector pressed IC has been also studied.

for non-sup-

ml

AND

H. MiiLLER

min-’

EXPERIMENTAL

Equipment To measure the e.m.f. under batch conditions, a galvanic chain of the type shown in Scheme 1, and a pH/mV-meter (Model MV-88, Pracitronik, Dresden, Germany) were used. A schematic diagram of the flow-injection system is presented in Fig. 1. The system includes a multi-channel peristaltic pump (Model DP2-2, MLW, Ilmenau, Germany), a laboratory made rotary injection valve with a 30-~1 sample loop and the flow-through potentiometric detector. Polyethylene tubing (0.8 mm i.d.) was used to connect the injector and detector, the length being not longer than 30 cm. Potentiometric responses in both FIA and IC were monitored with a laboratory-made differential amplifier with two high-imAg. AgCl

KCL

saturated

solution

1

5 &

Fig. 1. Schematic diagram of FIA system with potentiometric detector. 1= Peristaltic pump; 2 = loop injector; 3 = flowthrough cell with two identical Ca2+ ISEs; 4 = differential amplifier with two high impedances; 5 = recorder; C = carrier electrolyte; S = sample; W = waste.

pedance inputs and recorded with a strip-chart recorder (Endim type, Messapparatewerke, Schlotheim, Germany). An ion chromatograph (Knauer, Berlin, Germany) supplied with a moderate-capacity separation column packed with cation-exchanger (0.19 meq/g) and a conductivity detector was used (Fig. 2). Eluate from the conductivity detector was

O.lM

Analyte

Plasticized

AgCL - Ag,S

CaCI,

solution

membrane

solid

Ag

mem-

brane Scheme 1.

Knauer

Eluent= reference solution

Ion Chromfltogreph

waste

c

Fig. 2. Schematic diagram of ion chromatographic cation determination using two detectors. LCP = liquid chromatographic pump; IV = injection valve; SC = separation column; CD = conductivity detector; PC = flow-through cell of potentiometric detector with two identical Ca’+ ISEs; DA = differential amplifier; R = recorder.

APPLICATIONS

OF FLOW-THROUGH

CALCIUM-SELECTIVE

ELECTRODE

61

pumped through the measuring channel of a differential potentiometric detector. Eluent used as a reference solution was passed through the second channel of the’detector.

Potentiometric detector Two similar Ca2+ ISEs were used as reference and indicator electrodes in a flow-through potentiometric cell of the differential type (Fig. 3). A solution contacted the ion-sensitive membrane by the windows (1.5 X 3.5 mm2) in the channels (125 mm x 1.5 mm id.). Short metal needles were inserted in the cell inlets and grounded to eliminate noise caused by the peristaltic pump. The detector cell volume was 25 ~1. The Ca2+ ISE with a solid contact consisted of a calcium-selective membrane plasticized in a PVC matrix and was stuck on the outer surface of the solid-state electrode (Keramische Werke, Hermsdorf, Germany). Grooves (0.3 mm deep) were marked on the solid membrane for better contact between polymeric and solid membranes. Calcium-selective membranes (thickness 0.2 and 0.6 mm) were prepared by evaporating a mixture consisting of (by mass) m-TXDPO (1.6%) onitrophenyl-n-octyl ether (o-NPOE) (65.2’%), PVC (32.6%) a salt (60 f 5 mol-% with respect to mTXDPO) containing a lipophilic anion, namely cesium 3,3-come-bis(undecahydro-1,2-dicarba-3cobalta-closododecabor)ate (CsDCC) or sodium tetraphenylborate (NaTPB) and 2 ml of tetrahydrofuran (THF). Smaller disks were cut from the larger membrane and stuck on the outer surface of solid membrane by THF. The electrode was kept

TABLE Electrode Electrode No.

Fig. 3. Diagram of flow-through potentiometric differential cell. 1= two identical Ca2+ ISEs with solid contact; 2 = cell body; 3 = measuring channel and reference channel (1.5 mm i.d.); 4 = calcium-selective membrane in PVC matrix; 5 = solid membrane (Ag,S and AgCl mixture); 6 = PVC body of Ca*+ ISE; 7 = internal contact; 8 = ground; 9 = window (1.5 x 3.5 mm) for electrode contact with a solution: 10 = metal needle (cell inlet).

before use in calcium chloride solution (0.1 M) for 1 day. Such Ca2+ ISEs function effectively for several months provided there is good contact between polymeric and solid membranes. If the polymeric membrane exfoliates it can be stuck again. Reagents Analytical-reagent grade chemicals and doubly distilled (from a quartz still), deionized water were used to prepare solutions of cations, carrier electrolyte solutions for FIA and the eluent [ethylenediammonium chloride (EDAH,Cl,), 5 x low4 M, pH 51 for IC. THF was distilled under metallic sodium in an inert gas atmosphere.

1 characteristics Anion in membrane phase, membrane thickness

of Ca *+ ISE with different Range of linearity

Pure CaCl I solution

under

batch

conditions

C&z+)]

Ca*+ solution

with background

Detection limit ’

electrolytes

(Ml 0.1 M LiCl

1

TPB-,

0.2 mm

1 x 1O-6-1

x

TPB-, DCC,

0.6 mm 0.2 mm

3 x 10-s-1 6 x 1O-5-1

x 10-2 a [38] x 10m2 a [43]

concentrations

membranes

(M) [slope (mV/log

2 3 a Higher

10

were not studied.

lOV* a 134 f 41

b According

3

x

lOK-1

c 5 x 1o-5-1 to IUPAC

0.1 M NaCl x lo-* x 10-i

recommendation.

a [28]

6

x

1O-6-1

x

10-s-l

[30]

5 =

’ Not examined.

x 1O-3 a [27] x lo-* a 1271

3 x lo-’ 3 x 1o-6 1 x10-s

N. KOLYCHEVA

68 RESULTS

AND

Batch conditions The electrode characteristics of Ca*+ JSEs with both thin (0.2 mm) and thick (0.6 mm) membranes containing CsDCC and NaTPB are listed in Table 1. The measurements were carried out in pure calcium chloride solutions and in the presence of background electrolyte (0.1 M LiCl or NaCl). Electrode selectivity was studied by the mixed solution procedure [l]. The potentiometric selectivity coefficients (K$$) are given in Table 2 (B being the interfering ion). Electrodes with a thin membrane containing NaTPB (electrode 1, Table 1) possessed the best characteristics. The selectivity of an electrode similar to electrode 1 but contaning o-NPOE with impurities (e.g., p-NPOE) was also studied (electrode 4). Both electrodes (1 and 4) had similar properties, but impurities in the plasticizer greatly influenced the selectivity of the electrode with respect to K+ and H+ ions (Table l), improving it in the former instance but decreasing it in the latter. As can be seen from the data in Fig. 4, the potential of electrode 1 is almost constant (& 1 mV) in lop3 and 10e4 M CaCl, in the pH ranges 6.7-2.5 and 6.7-5.5, respectively, whereas the corresponding TABLE

C,

electrodes

(M)

KL% 1

3

4

0.1 0.1 1x10-2 1x10-s 0.1 1 x 10-2 1x10-2 5~10~~ 1x10-s 1x10-2 1x10-3

5 x10-6 2 x1o-5 4 x10-s 7 x~O-~ 2 x10-4 1 x10-4 3 x~O-~ 6.3~10-~ 0.3

- a 1.5x10-4 5 x10-4 2.5x1o-2 1 x~O-~ 7 x10-4 1 x10-s 6.6~10-~ 0.1 0.1

7 2 8 3 3 4 _

(B) NH; Na+

Li+ K+ Mg2+ EDAH;+ Ba2+ Sr2+

a Dashes

H. MijLLER

10mv

00

---0+--G

0

7

0

o-+v d \

\ o-o-

cl

C

P

// 0 0-o

7

p/

/

do

o-o~z /

6

5

I

I

I

I

4

3

2

1

PH

Fig. 4. Influence of pH on potential of calcium electrodes (a and c) and (b and d) in 1 mM (a and b) and 0.1 mM (c and d) CaCl, solutions. Batch conditions.

2

Potentiometric selectivity coefficients for calcium 3 and 4 (see Table 1) under batch conditions (Method of mixed solutions, C, = constant) Interfering ion

I

DISCUSSION

AND

indicate

0.35

that the values were not determined.

x10-6 x10-s

x10-s x10-s x10-4 x10-3

1,

pH ranges for electrode 4 at the same CaCl, concentration are much smaller, 5.0-4.0 and 5.65.0, respectively. Hence electrode 1 with the thin membrane was characterized by a bet:er selectivity and detection limit than the other electrodes studied and known calcium electrodes with membranes of similar composition and liquid or solid (graphite) contacts [17]. Electrodes 1 and 4 were further studied as cation sensors in FIA and IC. Flow-injection analysis The choice of carrier electrolytes for study in FIA with the Cazf ISEs was determined first by the electrode selectivity (Table 2) and the requirements for an eluent to be used in IC. The follow-

APPLICATIONS

TABLE

OF FLOW-THROUGH

CALCIUM-SELECTIVE

69

3

Electrode Carrier electrolyte

characteristics

of Ca ‘+ ISEs in FIA mode (flow-rate

Concentration (M)

Electrode

functions

E =f(log

C)

Linear NaCl

1 x10-2 1 x10-s 1 x 10-4 =

NH&l NH,Cl NH&l EDAH,Cl,

0.1 1 x 10-a 1x10-s 5 x 1o-4 5 x 10-4

f

1 x 10-‘-l 8 x 10-‘-l 1 x 1o-4-1 3 x 1o-4-1 1 x 1O-3-1 1 x 1O-4-1 3 x 1O-4-1 5 x 10-5-3 1 x 1O-4-1 1 x 10-s-1 2 x 1o-4-1 5 x 10-5-2 1 x 1O-4-1 1 x 10-s-1

1.0-2.0

ml min-‘)

E =f(C)

range

Slope (mV/log

(M)

MgCl,

ELECTRODE

x 1O-2 x x x x x x x x x x x x x

1O-3 10-Z 10-2 1O-2 1O-3 1O-3 d 1o-4 1O-3 d 10-4 10-s 10-4 1O-3 d 10-4

Detection limit ’ (M)

C)

26

8 x 1O-6

16 35 115 29 18 25 14 25 14 33 19 16 8

Linear range (M) 3 x 10-6-1

Slope (mV/AC) x 10-4

6 x lo-’

3 x 10-4 6 x 10-s 3 x 1o-5

8 x10-s 4x10-s

3x10-6-1x10-4 2 1 3 5 4 2 1 3

x 10-4-5 ~10-~-2x x 10-s-1 x 10-6-3 x 10-s-1 x 1O-6-1 x 1o-5-1 x 1O-6-1

7 x 10-s 10K5 x 10-4 x 10-s x 1o-4 x 1O-5 x 10-4 x 1O-5

’ According to IUPAC recommendation. b AC = 1 X 10m4 M. ’ Detection limit was determined 0.90, n = 5. d Higher concentrations were not studied. e Poor signal reproducibility. ’ Very broad

-5

-4 log

-3

cca2+

Fig. 5. Calibration plots in FIA with Ca 2+ ISE and different EDAH,Cl,. Flow-rate, 1.0 ml mini.

b

7

20

calcium

carrier

40

60

concentration

electrolytes:

a0

0.1

11 16 10 21 7 16 7 16

statistically peaks.

0.04

0.04 0.08 0.1 with probability

100

[JJM]

1 = 10 mM NH,Cl;

2 = 1 mM NH,Cl;

3 = 0.5

(P)

10

N. KOLYCHEVA

ing carrier electrolytes were studied: 0.1 M LiCl, lo-i--10-’ M NaCl, 1O-‘-1O-4 M MgCl,, 1 x 10-l-5 X 1O-4 M NH,Cl and 5 x lop4 M EDAH,Cl, solution (pH 5). Electrode characteristics in FIA with some carrier electrolytes are given as examples in Table 3. The results obtained with MgCl, solution as a carrier were poorly reproducible and significant baseline drift was observed. NH,Cl solutions were the most promising owing to low reagent cost and satisfactory selectivity, signal reproducibility and sensitivity. Of all the carriers studied only 5 x lop4 M EDAH,Cl, can be used as an eluent for alkaline earth metal ions and other bi- and polycharged cations in non-suppressed IC. The electrode functions of Ca*+ ISEs in FIA systems with NH,Cl and EDAH,Cl, solutions as carriers are shown in Fig. 5. As can be seen from the data in Table 3, the detection limit for Ca*+ decreases with decrease in the carrier electrolyte (NH,Cl) concentration, down to 1 X 10m3 M. However, at lower carrier electrolyte concentrations signal broadening and an increase in electrode response time were observed. The influence of carrier flow-rate on the electrode dynamic characteristics was studied by injecting CaCl, solution (3 x lop3 M) into a carrier flow (lo-* M NaCl) at various flow-rates. The times corresponding to the maximum signal (tn) in FIA systems at flow-rates of 0.25, 0.9, 1.2, 1.6 and 4.0 ml min- ’ were 80, 60, 37, 25 and 10 s, respectively. However, if the carrier flow-rate was higher than 1.2 ml mm’, the electrode life time decreased drastically because of membrane exfoliation. Potentiometric selectivity coefficients under dynamic conditions were calculated as the ratio of

AND

H. MiiJLLER

the activities of calcium and interfering ion determined in solution where the e.m.f. of the electrode was the same [l]. The selectivity coefficients obtained in FIA systems with lo-* M NaCl and lo-3 M NH,Cl as carrier electrolytes are given in Table 4. Based on these KCa,a values, the selectivity can be considered to be independent of the nature of the carrier (NH,Cl or NaCl), slightly dependent on interfering ion concentration and dependent on carrier electrolyte concentration, being less at lower carrier electrolyte concentrations. The selectivity of the Ca*+ ISEs in FIA decreased in the order K+> NH: > Na+> Li+a Mg*+> H+> EDAH,Cl, - Ba*+> Sr*+, i.e., the sequence under both dynamic and batch conditions is the same for almost all ions (except NH: and K+). However, the absolute values for the selectivity coefficients measured in the FIA mode were almost an order of magnitude worse than KzztB under batch conditions. Ion chromatography As mentioned above, only EDAH,Cl, solution (5 X lop4 M) can be used as an eluent to separate alkaline earth metals by means of single-column IC. The optimum eluent flow rate was 1.4 f 0.4 ml mini. The IC system (Fig. 2) consisted of two different detectors, a conductivity detector (Knauer) and a differential potentiometric detector, connected in series. The signal delay from the potentiometric detector relative to the conductivity detector did not exceed 30 s. The sensitivity of the Ca*+ ISE-based detector with respect to alkaline earth metals was 100 times higher than that of the conductivity detector. For example, the detection limit for Ca*+ (sample

TABLE 4 Potentiometric selectivity coefficients of electrode 4 in FIA systems a Interfering ion, B

K+

NH:

Ca tmM) K &+,*X 1om3

100 0.2

100 0.7

10 0.3 b

a Carrier electrolytes 10 mM NH,CI and/or NaCI) = 1 mM.

Na+

Li+

100 1

100 4

Mg*+ 10 2

1 3

10 6

H+ 0.5 50 b

10 10

NaCI if not stated otherwise. b Concentration

1 40

Ba*+

sr*+

EDAH;+

1 50

1 80

0.5 50

of carrier electrolyte (NH,CI and/or

APPLICATIONS

I

OF FLOW-THROUGH

CALCIUM-SELECTIVE

ELECTRODE

OlmV Ba2+

d+

1.C

--

24

22

20

18

12

Time

10

6

6

4

2

0

[min]

Fig. 6. Chromatogram of cation mixture: NH:, K+, Na+ (300), Mg2+ (40), Zn*+ (25), Ca*+ (0.6). Sr2+ (2.5). Ba*+ (68 mg I-‘). Potentiometric detection with Ca*+ ISE. Eluent, 0.5 mM EDAH,CI,; flow-rate 1.2 ml min-‘.

volume 20~1) was 0.02 mg 1-r for the former but 4 of the mg 1-r for the latter. Another advantage highly selective calcium electrode is its low sensitivity to alkali metal, ammonium, magnesium and zinc ions, permitting the detection of alkaline earth metal ions, large organic cations and, based on the selectivity of the electrode [17], probably ions of heavy metals in the presence of a 100- or lOOO-fold excess of ammonium, alkali metal or magnesium ions. For example, if the concentrations of Mg*+, Naf (K+, Lit, NH:) and Zn” ions in the solution were not higher than 40-200, 300-500 and 25 mg ll’, respectively, Ca’+, Ba*+ and Sr*+ were easily detected at concentrations of 0.08-0.6, 2.5 and 6 mg l- ‘, respectively. A chromatogram of the mixture obtained with the Ca*+ ISE is presented in Fig. 6. However, the determination of calcium and strontium in a mixture was difficult (especially when the concentration of one ion was much higher than that of the other) owing to insufficient selectivity of both the separation column and the potentiometric detector. As a result of a study with model mixtures containing different amounts

71

of the ions, the lowest concentrations of calcium and strontium ions were shown to be detectable in the following mixtures: 1.2 and 10; 0.6 and 2.5; 2 and 5; and 2 and 2 mg l-‘, i.e., when the Ca*+ : Sr2+ mass ratio varied from 2 : 1 to 1: 10. Calibration graphs used to determine calcium or other ions with the potentiometric detector in complex mixtures can be conveniently plotted as peak height (mV) measured with respect to the baseline activity (concentration). As an example of the possibilities of potentiometric detection, two methods for the determination of calcium and magnesium in drinking water, namely, IC and FIA, were developed. As a rule the relative standard deviation (n = 5) did not exceed 0.02. The results obtained by both methods were in good agreement and agreed well with the results of EDTA titrations.

Conclusions The highly selective calcium electrode with a solid contact is applicable in a detector of both FIA and non-suppressed IC. The application of the detector in FIA permits the determination of trace calcium in the presence of a lOO-fold excess of alkali metal ions, Mg2+ and NH:. The sensitivity of the potentiometric detector with the Ca2+ ISE relative to alkaline earth metal ions was approximately two orders of magnitude higher than that of a commercial ion chromatograph conductivity detector (Knauer). This potentiometric detector seems to be useful in IC to solve special problems, such as the determination of calcium, other alkaline earths and heavy metal traces in the presence of large amounts of alkali metal, magnesium and zinc ions.

The gifts of m-TKDPO from Dr. G.V. Bodrin and Dr. Yu.M. Polikarpov (A.N. Nesmeyanov Institute of Organometallic Compounds, U.S.S.R. Academy of Sciences), of o-NPOE from Dr. A. Farmanovskii (V.I. Vernadskii Institute of Geochemistry and Analytical Chemistry, U.S.S.R. Academy of Sciences) and of CsDCC from Dr. K. Base (Institute of Inorganic Chemistry, CzechoSlovak Academy of Sciences) are gratefully acknowledged. The assistance of Bernd Hund-

72

N. KOLYCHEVA

hammer and Stefan Wilke is greatly appreciated. The authors also thank Alexander Zhukov for valuable consultations concerning Ca2+ ISEs.

REFERENCES J. Koryta and K. St&k, IontovC-Selectivni Elektrody, Academia, Prague, 1984. W. Frenzel, Analyst, 113 (1988) 1039. E.H. Hansen, J. Ruzicka and A.K. Ghose, Anal. Chim. Acta, 100 (1978) 151. P. Petak and K. &dik, Anal. Chim. Acta, 185 (1986) 171. J.D.R. Thomas and B.J. Birch, Analyst, 108 (1983) 1357. T.J. Cardwell, R.W. CatraIl, P.C. Hauser and J.C. Hamilton, Anal. Chim. Acta, 214 (1988) 359. M. Trojanowicz and M.E. Meyerhoff, Anal. Chem., 61 (1989) 787. M. Trojanowicz and M.E. Meyerhoff, Anal. Chim. Acta, 222 (1989) 95. W.E. Morf, The Principles of Ion-Selective Electrodes and Membrane Transport, AkadCmiai Kiado, Budapest, 1981, p. 270.

AND

H. MijLLER

10 K. Camman, Das Arbeiten mit Ionenselektiven Elektroden, Springer, Berlin, Heidelberg, New York, 1973. 11 A.M. Kapustin, G.M. Sorokina, M.V. Ryazanova, I.A. Zandeman, IS. Markovich and V.M. Dziomko, Electrokbimiya, 18 (1982) 1435. 12 S. Urs, D. Amman, E. Pretsch, U. Oesch and W. Simon, Anal. Chem., 58 (1986) 2282. 13 K. Suzuki, T. Tohda, A. Hiroscbi, M. Matsuzoe, H. Inone and T. Shirai, Anal. Chem., 60 (1986) 1714. 14 G.J. Moody, B.B. Saad and J.D.K. Thomas, Analyst, 113 (1988) 1295. 15 A.F. Zhukov, O.M. Petruhin, G.V. Bodrin, Yu.M. Polikarpov and M.J. Kabachnik, in E. Pungor (Ed.), Ion Selective Electrodes 5, Proceedings of 5th Symposium on Ion Selective Electrodes held at Mltraftired, Hungary, 9-13 October, 1988, Pergamon, Oxford, and Akadtmiai Kiado, Budapest, 1988, p. 529. 16 S.M. Bessis, A.F. Zhukov, Yu. I. Urusov, O.M. Petruhin, G.V. Bodrin, N.P. Nesterova, Yu.M. PoIikarpov and M.I. Kabachnik, Zh. Anal. Khim., 43 (1988) 1769. 17 E.N. Avdeeva, A.F. Zhukov, O.M. Petruhin, G.V. Bodrin, Yu.M. Polikarpov, and MI. Kabachnik, Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol., 32 (1989) N. 9.