- Email: [email protected]
Novel potentiometric monitoring combined with coulometric reagent generation
ANALmcA CHIMICA ACTA
ELSEVIER
Analytica
Chimica
Acta 319 (1996) 49-58
Novel potentiometric monitoring combined with coulometric reagent generation Gkza Nagy, Kl6ra T&h
*, Zs6fia Fehkr, Jen6 Kunovits
Research Group for Technical Analytical Chemistry of the Hungarian Academy of Sciences, Institute for General and Analytical Chemistry, Technical University of Budapest, GeUrt te’r 4., H-1111 Budapest, Hungary
Received23 May 1995; revised 12 September1995;accepted16 September1995
Abstract A potentiometric monitor incorporating a flow-through analysis channel has been developed based on the standard addition or subtraction principle. Programmed addition of standard or reagent was carried out at a given point of the flow-through system by current-programmed coulometry. A computer was used for data acquisition, for electric noise rejection, for signal recognition and for calculating the analytical results. The characteristics of the monitor system have been investigated under conditions optimised for the chloride determination, and the performance of it was checked by monitoring the chloride concentration in tap water. Keywords:
Potentiometry;Coulometry; Flow system
1. Introduction Continuous flow techniques constitute one of the important application fields of ion-selective electrode potentiometry [l-3]. Flow-through methods employing ion-selective electrode based detection are suited to laboratory discrete sample analysis as well as to monitoring and process control [4-71. In most potentiometric flow-through monitors incorporating ionselective electrodes the direct potentiometric measuring principle is employed. The reliability of the results obtained with direct potentiometry depends
* Corresponding
author.
Elsevier Science B.V. SSDI 0003-2670(95)00470-X
on the validity of the calibration data, and on the extent of potential drift. It is an advantage of standard addition and subtraction techniques in ion-selective electrode potentiometry, that under suitably selected experimental conditions it enables to determine a single component in complex natural samples, such as waters or biological samples even if the component in question is partly bound in a complex. On the other hand, using double or multiple standard addition or subtraction methods, the characteristic data of the detector cell can also be determined. That means, the cell can be calibrated in the same matrix. Actual concentration data can be obtained by a graphical method, using a Gran linearisation procedure [8], or by a
50
G. Nagy et al. /Analytica
computerised method based on Gran transformation or on non-linear regression analysis [9,10]. The potentiometric standard addition and sample subtraction methods were reviewed by Mascini [ll]. Horvai and Pungor [12] compared the calibration technique and the multiple standard addition with respect to reliability. Midgley [13] published an extensive review on the random errors in potentiometric known addition technique. The first steps towards the realisation of standard addition or subtraction techniques in flow-through systems have been made by Fleet and Ho [14]. Later Zipper et al. [15] with a Technicon AutoAnalyzer based computer-controlled system were able to determine the slope of the potentiometric measuring cell, in addition to sample concentration using volumetric standard addition method. In our laboratory a potentiometric flow-through monitor was developed. It uses standard addition or the sample subtraction principle. The addition of different amounts of a standard (primary ion) or a reagent is carried out by current-programmed coulometry. The construction of the monitor, its testing under optimised conditions as well as its application to the monitoring of chloride concentration in tap water is outlined in this paper.
Chimica Acta 319 (1996) 49-58
. the chemical reaction (if there is any involved) is fast (subtraction); * the effect of streaming potential and liquid junction potential is negligible. If only the sample solution, incorporating, e.g., an anionic species to be detected is streaming in the analysis channel, the cell voltage can be expressed as E,=Eb-Slogc,
(1)
where: Eb is the constant term of the potentiometric cell, which includes the standard potentials of the indicator and reference electrodes, as well as the liquid junction and streaming potentials, and S is the slope of the calibration curve of the indicator electrode determined in standard solutions. If a standard solution containing the component to be measured (in this case an anion) in a concentration of c,~ is introduced into the analysis channel before the detector cell at a flow rate of vst, then, after a transient period of time, the cell voltage changes to a value of E,=E;-Slog
ccvs+ cst%I vs + %t
(2)
Then, from the potential difference AE = E, -E, the unknown concentration c, can be expressed as:
2. Operation principle of the monitor
(3) The monitor incorporates a flow-through analysis channel with a selective potentiometric detector cell in which a sample solution of concentration c, flows at a constant rate of u,. At a certain point before the detector cell a reagent solution is introduced. The operation principle can be readily understood by taking the following simplifying assumptions into consideration: no distinction is made between ionic concentration and activity; it is assumed that components which interfere with the operation of the ion-selective detector electrode are absent, and the Nernst equation is valid; temperature changes due to chemical reaction and dilution are negligible; mixing is complete after the confluence point of the solution streams;
If a standard subtraction is carried out, that is, a reagent solution of c, concentration is added to the streaming sample solution at a flow rate of v*:,and the chemical reaction taking place is aS + bR = dP, + gP2
(4)
where S denotes the sample, R is the reagent, and P, and Pz are the products, respectively; the cell voltage after the subtraction can be expressed as
E,=E;-Slog
a CCV,- -cry b v, + VT
(5)
where a and b are stoichiometric constants. From the difference in the cell voltages, AE = E, -E,,
G. Nagy et al. /Analytica
the concentration of the sample solution can be expressed as a c, =
-cr. b
unknown concentration, and the actual values of S and Eb can be determined by successive approximation, based on the following equations:
v,
E, =Eb-Slog-
V,-(V,+VJ40+
i. 10-Y nFv,(l - 10-F)
ccv, vs + va
As shown by Eqs. 3 and 6, there is a well-defined correlation between the concentration of the sample solution and the cell voltage difference in both cases. This means that if all the experimental parameters are kept constant, the concentration of a streaming sample solution can be determined from the measured potential difference, AE, by using the experimental techniques outlined. Let us suppose that the standard or the reagent is prepared in an external, flow-through electrolysis cell by constant current coulometry. That is, the standard or reagent addition is carried out by electrolysis. The reagent generating electrolyte solution, after passing through the electrolysis cell, is continuously merged to the streaming sample solution and the combined stream passes through the detector cell. The programmed reagent or standard addition is ensured by employing a suitable generating currenttime program. In this case the equations given above are modified as follows: for standard addition: c, =
51
Chimica Acta 319 (1996) 49-58
(7)
(10)
(11) where E, is the cell voltage measured before standard or reagent addition; E, and E, are the cell voltages measured after the first and second standard or reagent addition, respectively; v, is the volumetric flow rate of the generating solution. A stationary solution version of the multiple standard addition method with volumetric addition was described by Brand and Rechnitz [17]. Similarly to their approach an other version for the evaluation of the double addition/subtraction data can be employed in cases when Eb and S are not known. Accordingly, let E, -El and E, -E, from Eqs. 9-11 be denoted by AE, and AE,, respectively, and then, after suitable transformations, the following equations can be obtained:
and for known subtraction:
c, =
a - .i b
(12)
nFvs(1 - 10-Y)
where i is the electrolysis current, F is the Faraday constant and n number of electrons involved in the electrolysis. As shown by Eqs. 7. and 8, c, is independent of the flow rate of the electrolyte used for reagent generation. If the slope of the electrode calibration curve is known, the unknown concentration c, can be calculated from AE, i and us. If the slope of the electrode calibration curve is not known from previous measurements, the concentration of a streaming sample solution can still be determined by standard addition or subtraction method. In such cases double or multiple standard addition or subtraction need to be performed. The
(13) Dividing Eq. 12. by Eq. 13 we get: a - .i
b
-4 AE2
=
l
c, vsnF a - .i b 2 c, vsnF
(14)
52
G. Nagy et al./Analytica
Using standard solutions of different concentrations and appropriate generating current values (ir and i2) we can determine the AE,/AE, vs. c, dependence for a given concentration range. When analysing a sample of unknown concentration, the relevant AE, and AE, values have to be measured using the same i, and i, generating current employed at calibration. Knowing the ratio of the AE values, the sample concentration can be read from the calibration curve. To employ this principle we do not need to know the slope of the electrode calibration curve, however, we suppose that its value is constant. As a matter of fact, different chemometric approaches were employed to obtain analytical results from the electrode potential changes measured with double standard addition/known subtraction methods when the slope of electrode calibration is not known. Li [18] worked out an iterative computer program for this. A slightly different, fast computer program was described by Wang [19]. High experimental accuracy was achieved when testing the applicability of the program.
Fig. 1. Schematic
Chimica Acta 319 (1996) 49-58
3. Experimental 3.1. The description of the monitor The scheme of the monitor developed is shown in Fig. 1. A two-compartment cell arrangement (1) with silver reagent generating electrode (2) and a platinum wire counter electrode (3) served for the reagent generation. The flow of the sample (4) and the reagent generating electrolyte (5) were propelled with peristaltic pumps (6) (LKEI Varioperpex 12000; type OL-602, LaborMIM, Hungary). The reagent and the sample streams merge dropwise in a drip vessel (7) and the combined stream after a dispersion coil (8) passes through a detector cell incorporating a microcapillary chloride ion-selective electrode (9) (type OP-Cl-7443, Radelkis, Hungary) as an indicator electrode, and a special flow-through double-junction calomel electrode (lo), as a reference one. The cell voltage was followed with a pH-meter (11) (type OP-211, Radelkis). The recorder output of it was connected to a personal computer (12) through an interface card (13) (type PCL-812). Home made
diagram of the monitor (for explanation
see text).
53
G. Nagy et al. /Analytica Chimica Acta 319 (1996) 49-58
time-programmed current generator (14) was connected to the generating electrodes. The main task to be performed by the computer was to determine the potential difference A E between the steady state values of the signal before and after the subtraction (or addition). The detector cell voltage was also recorded (type OP-814/l recorder, Radelkis) (15). All reagents used in this work were of analytical grade and doubly distilled water was employed for solution preparations.
4. Results and discussion 4.1. Evaluation of the monitor characteristics 4.1.1. Investigation of the electrode potential-time dependence of the monitor When a sample solution containing chloride ions at a constant concentration level is passed through the detector cell at a constant flow rate, a constant cell potential sets up, which depends on the chloride concentration (baseline). If the current generating the Ag+ reagent is turned on, the cell voltage changes corresponding to the decrease of chloride ion activity. After a transient time, a new constant potential level sets up. The potential of the indicator electrode
175 T-----
corresponds to the new, decreased chloride concentration level (subtraction line). Switching off the current, after a pre-set time the potential approaches its original, background value. An actual signal vs. time recording is shown in Fig. 2, as an example. The result of the measurement is calculated from the difference between steady-state sections of the base line and the subtraction line. The rate of analysis depends on the time needed to achieve steady-state conditions. The transient portion can be characterised by the time elapsed from the beginning of the current controlled generation to the point corresponding to 90% of the entire signal change (t,). The time necessary for the potential to restore its original value after switching off the generating current is shorter than that necessary for the subtraction line to be reached after the current has been switched on. Accordingly, at the flow rates applied (1.43 X 10-5-5.11 X lop5 dm3/s) t, was found to change between 50 and 120 s. The corresponding hold up volume was found to vary from 0.7 to 6.5 cm3. 4.1.2. Study of the effect of generating current intensity on the analytical result The most important experimental parameter of the monitor is the intensity of silver ion generating current. The effect of the generating current on AE has been studied experimentally. In the course of the
___~---_-_-~_-
170
165
Fig. 2. Trace of signal vs. time recording,
c = 1 X 1O-3 M; i = 1.8 mA; v, = 3.18 X 10-j
dm3/s.
54
0
G. Nagy et al. /Analytica
Chimica Acta 319 (1996) 49-58
0.1
0.4
0.2
0.3
0.5
0.6
0.7
0.8
0.9
i b4 Fig. 3. AE vs. i plot for potassium chloride standards
measurements a potassium chloride standard solution of constant concentration was passed through the analysis channel of the monitor at a constant flow rate (1.43 X 10m5 dm3/s). An aqueous solution of 10-l M for potassium nitrate, and 5 X 10m3 M for nitric acid was used as reagent generating electrolyte. The potential difference (AE) between the two steady state sections of the signal was measured at different generator current intensities. The AE values were
0
0.2
in the range of 10-4-10-3
plotted against the generating current. The AE vs. i plots obtained for different potassium chloride concentrations are shown in Fig. 3. For the comparison of the experimental curves with the theoretical ones it is reasonable to use the Gran-linearisation technique. The sample concentration can be calculated from Eq. 8. In our case the ratio of the stoichiometric constants a/b = 1 and n= 1.
0.4
0.6
i IrN Fig. 4. Linearised
M concentrations.
AE vs. i plot derived from Fig. 3.
0.8
1
G. Nagy et al./Analytica
0
I
I
I
0.1
0.2
0.3
55
Chimica Acta 319 (1996) 49-58
1 0.4 0.5 c. [mmol/dm3]
0.6
0.7
I
I
0.8
0.9
1
Fig. 5. AE vs. log c, function for different current intensities.
Employing 8 we get:
the Gran plot transformation
from Eq.
10-$=1-i
factors are constantiLThe theoretical and experimentally obtained lo- T -i relationships are shown in Fig. 4. The full lines denote the theoretical curves whereas the points are experimental values. The current stoichiometrically equivalent to the mass flow of the sample is given by the ordinate values at 0. Thus, if the flow rate is known, the chloride concentration in the sample solution can be calculated from
(15)
F% c,
Thus, theoretically, a linear relationship exists between lo- y and the generating current, if other
50 pi--i_,_--.--
.---I----
---I
F i%
4
I 10
_
__._._.___l_._..-_
__~._._~._.l.....
___+
.
I
__.“_...._.._._i.. _.._.. ___.____._..__...___.~.__.-__ __..-__.___.__
.._......_.
1 I
I !
0-t 0.03
I
0.035
0.04
0.045
/
0.05
v [ctn3/s] Fig. 6. Dependence
of AE on the volumetric
flow rate, c = 1 X lop3 M; i = 2.8 rr~4.
-I
0.055
56
G. Nagy et al./Analytica Chimica Acta 319 (1996) 49-58
the extrapolated current value. As shown in Fig. 4 the theoretical values agree well with the experimental results. This means that under the conditions used in the monitor it behaves as predicted by the theory. The theoretical AE values for a given generating current intensity at different sample concentrations can easily be calculated knowing the flow rate and S value of the detector. To compare the theoretical and the experimental A E - c, dependences at different generating current intensities, from the results shown in Fig. 3 A E - c, plots were made. These can be seen in Fig. 5. The series of curves obtained corresponds to the curves expected on the basis of Eq. 8.
Sr.(AE) 14 [%I
12
10
8
6
4.1.3. Investigation of the dependence of AE on the flow rate of the sample solutions In order to investigate this practically important dependence, a standard solution with chloride concentration of lop3 M and reagent generating electrolyte was passed through the system. The electrolysis current was 2.8 mA, while the length of the measuring cycle was 400 s. Different flow rates were employed and the AE values were measured. Fig. 6 shows the obtained AE values plotted against the flow rate of the standard solution. As can be seen from the figure, AE changes considerably with the flow rate of the chloride containing solution. This shows that it is essential to keep the flow rate of the sample stream constant during the measurements to be able to get reliable results with the apparatus. Based on the results shown, the approximate value of AE can be predicted for a given analyte concentration, knowing the flow rate of the sample solution and the intensity of the reagent generating current. In selecting the optimal conditions for the measurements, the effect of the experimental parameters on the precision of the results has to be taken into consideration. For this purpose the correlation between the AE values and the standard deviation of the results (experimental AE values or the corresponding calculated concentrations) has been studied experimentally. In this work potassium chloride solution of a concentration of lop3 M was pumped through the analysis channel at a flow rate of 5.51 X 10m5 l/s. The relative standard deviation of the AE values in percentage (& . 1001, (n = 5) were determined at different current intensities. In Fig. 7 the relative standard deviations of A E values are plotted
0
I
2
3
4. ’ WI
5
Fig. 7. Relative standard deviation (s,) of AE as well as c in the function of the electrolysis current at a preselected, constant flow rate. n=5; c= 1~10~~ M; vS=5..5x1O-5 dm3/s, AS,(AE) values (a 1; S,(c) values (0).
against the reagent generating current. It can be seen that the AE and i are in a well defined relationship. It can also be seen that the plotted relative standard deviations have a minimum. This means that from the point of view of precision an optimal reagent generating current can be selected to analyse a solution of given concentration. The best precision was obtained at current intensity of 3.5 mA, which produces a AE of about 30 mV. Accordingly, when operating the apparatus a current intensity should be chosen by which a AE of about 30 mV can be achieved. It is to be noted that this is in agreement with the general opinion about potentiometric standard addition and subtraction technique [16]. Obviously, for a monitor working with a single constant current, the above conditions exist only within a narrow concentration range. In order to investigate the performance of the
G. Nagy et al. /Analytica Chimica Acta 319 (1996) 49-58
19.1'
19.6
Fig. 8. Histogram
19.iss 20.6 AE WI
2O.j
ZO:6
of the obtained chloride concentration
apparatus, a current (3 mA) corresponding to a AE potential difference of about 20 mV was chosen. A lop3 M standard potassium chloride solution was passed through the sample line at a rate of 5.51 X 10m5 dm3/s. The measuring duty cycle was 400 s. In nearly 8 h continuous operation of the monitor 71 AE data as well as calculated concentration data were obtained. No significant drift of the AE and concentration values was observed during this time period. The distribution of the results (Fig. 8) can be considered as normal. The relative standard deviation
51
20.b
21.i
values.
calculated from 71 AE data was 1.95%, for the c, data 1.28%. 4.2. Monitoring water
of the chloride concentration
To investigate the practical applicability of the apparatus, the chloride concentration of tap water was continuously measured for 8 h. Tap water was passed through a vessel with an overflow pipe. The sample input tube took samples from the vessel
-l
I
2'
3
of tap
4
i
t IN Fig. 9. Results of tap water chloride concentration
monitoring.
8
58
G. Nagy et al. /Analytica
continuously and made measurements with a measuring cycle of 400 s, at a current of i = 2.0 mA and flow rate of vS= 5.51 X 10m5 dm3/s. The results of a one working day monitoring can be seen in Fig. 9. The consumption of the silver from the generating electrode was very small. However, while in continuous operation the silver electrode was checked and adjusted once a week.
References [l] L.I. Skeggs, Am. J. Clin. Path., 28 (1957) 311. [2] J. Ruzicka and E.H. Hansen, Flow Injection Analysis, Wiley, New York, 1981. [3] J. Koryta (Ed.), Medical and Biological Applications of Electrochemical Devices, Wiley, New York, 1980. [4] G. Nagy, 2s. Fehtr, K. T&h and E. Pungor, Hungarian Scientific Instruments, 41 (1977) 27.
Chimica Acta 319 (1996) 49-58 [5] K. Tbth, G. Nagy, Zs. Feher, Gy. Horvai and E. Pungor, Anal. Chim. Acta, 114 (1980) 45. [6] E. Pungor, Zs. Fehtr, G. Nagy and K. T&h! Crit. Rev. Anal. Chem., 14 (1983) 175. [7] K. T6th, G. Nagy, Zs. FehCr and E. Pungor, 2. Anal. Chem., 282 (1976) 379. [8] G. Gran, Analyst, 77 (1952) 661. [9] G. Horvai, L. Domokos and E. Pungor, Z. Anal. Chem., 292 (1978) 132. [lo] C.C. Westcott, Anal. Chim. Acta, 86 (1976) 269. [ll] M. Mascini, Ion-Selective Electrode Rev., 2 (1980) 17. [12] G. Horvai and E. Pungor, Anal. Chim. Acta, 151(1983) 383. [13] D. Midgley, Analyst, 112 (1987) 557. [14] B. Fleet and A.Y.W. Ho, in E.Pungor (Ed.), Ion-Selective Electrodes, Akadtmiai Kiad6, Budapest, p. 17. [15] 1.1. Zipper, B. Fleet and S.P. Perone, Anal. Chem., 46 (1974) 2111. [16] K. Cammann, Working with Ion-Selective Electrodes, Springer Verlag, Berlin, 1979, p. 140. [17] M.J.D. Brand and G.A. Rechnitz, Anal. Chem., 42 (1970) 1172. [18] H. Li, Analyst, 112 (1987) 1607. [19] J. Wang, Analyst, 115 (1990) 53.
ELSEVIER
Analytica
Chimica
Acta 319 (1996) 49-58
Novel potentiometric monitoring combined with coulometric reagent generation Gkza Nagy, Kl6ra T&h
*, Zs6fia Fehkr, Jen6 Kunovits
Research Group for Technical Analytical Chemistry of the Hungarian Academy of Sciences, Institute for General and Analytical Chemistry, Technical University of Budapest, GeUrt te’r 4., H-1111 Budapest, Hungary
Received23 May 1995; revised 12 September1995;accepted16 September1995
Abstract A potentiometric monitor incorporating a flow-through analysis channel has been developed based on the standard addition or subtraction principle. Programmed addition of standard or reagent was carried out at a given point of the flow-through system by current-programmed coulometry. A computer was used for data acquisition, for electric noise rejection, for signal recognition and for calculating the analytical results. The characteristics of the monitor system have been investigated under conditions optimised for the chloride determination, and the performance of it was checked by monitoring the chloride concentration in tap water. Keywords:
Potentiometry;Coulometry; Flow system
1. Introduction Continuous flow techniques constitute one of the important application fields of ion-selective electrode potentiometry [l-3]. Flow-through methods employing ion-selective electrode based detection are suited to laboratory discrete sample analysis as well as to monitoring and process control [4-71. In most potentiometric flow-through monitors incorporating ionselective electrodes the direct potentiometric measuring principle is employed. The reliability of the results obtained with direct potentiometry depends
* Corresponding
author.
Elsevier Science B.V. SSDI 0003-2670(95)00470-X
on the validity of the calibration data, and on the extent of potential drift. It is an advantage of standard addition and subtraction techniques in ion-selective electrode potentiometry, that under suitably selected experimental conditions it enables to determine a single component in complex natural samples, such as waters or biological samples even if the component in question is partly bound in a complex. On the other hand, using double or multiple standard addition or subtraction methods, the characteristic data of the detector cell can also be determined. That means, the cell can be calibrated in the same matrix. Actual concentration data can be obtained by a graphical method, using a Gran linearisation procedure [8], or by a
50
G. Nagy et al. /Analytica
computerised method based on Gran transformation or on non-linear regression analysis [9,10]. The potentiometric standard addition and sample subtraction methods were reviewed by Mascini [ll]. Horvai and Pungor [12] compared the calibration technique and the multiple standard addition with respect to reliability. Midgley [13] published an extensive review on the random errors in potentiometric known addition technique. The first steps towards the realisation of standard addition or subtraction techniques in flow-through systems have been made by Fleet and Ho [14]. Later Zipper et al. [15] with a Technicon AutoAnalyzer based computer-controlled system were able to determine the slope of the potentiometric measuring cell, in addition to sample concentration using volumetric standard addition method. In our laboratory a potentiometric flow-through monitor was developed. It uses standard addition or the sample subtraction principle. The addition of different amounts of a standard (primary ion) or a reagent is carried out by current-programmed coulometry. The construction of the monitor, its testing under optimised conditions as well as its application to the monitoring of chloride concentration in tap water is outlined in this paper.
Chimica Acta 319 (1996) 49-58
. the chemical reaction (if there is any involved) is fast (subtraction); * the effect of streaming potential and liquid junction potential is negligible. If only the sample solution, incorporating, e.g., an anionic species to be detected is streaming in the analysis channel, the cell voltage can be expressed as E,=Eb-Slogc,
(1)
where: Eb is the constant term of the potentiometric cell, which includes the standard potentials of the indicator and reference electrodes, as well as the liquid junction and streaming potentials, and S is the slope of the calibration curve of the indicator electrode determined in standard solutions. If a standard solution containing the component to be measured (in this case an anion) in a concentration of c,~ is introduced into the analysis channel before the detector cell at a flow rate of vst, then, after a transient period of time, the cell voltage changes to a value of E,=E;-Slog
ccvs+ cst%I vs + %t
(2)
Then, from the potential difference AE = E, -E, the unknown concentration c, can be expressed as:
2. Operation principle of the monitor
(3) The monitor incorporates a flow-through analysis channel with a selective potentiometric detector cell in which a sample solution of concentration c, flows at a constant rate of u,. At a certain point before the detector cell a reagent solution is introduced. The operation principle can be readily understood by taking the following simplifying assumptions into consideration: no distinction is made between ionic concentration and activity; it is assumed that components which interfere with the operation of the ion-selective detector electrode are absent, and the Nernst equation is valid; temperature changes due to chemical reaction and dilution are negligible; mixing is complete after the confluence point of the solution streams;
If a standard subtraction is carried out, that is, a reagent solution of c, concentration is added to the streaming sample solution at a flow rate of v*:,and the chemical reaction taking place is aS + bR = dP, + gP2
(4)
where S denotes the sample, R is the reagent, and P, and Pz are the products, respectively; the cell voltage after the subtraction can be expressed as
E,=E;-Slog
a CCV,- -cry b v, + VT
(5)
where a and b are stoichiometric constants. From the difference in the cell voltages, AE = E, -E,,
G. Nagy et al. /Analytica
the concentration of the sample solution can be expressed as a c, =
-cr. b
unknown concentration, and the actual values of S and Eb can be determined by successive approximation, based on the following equations:
v,
E, =Eb-Slog-
V,-(V,+VJ40+
i. 10-Y nFv,(l - 10-F)
ccv, vs + va
As shown by Eqs. 3 and 6, there is a well-defined correlation between the concentration of the sample solution and the cell voltage difference in both cases. This means that if all the experimental parameters are kept constant, the concentration of a streaming sample solution can be determined from the measured potential difference, AE, by using the experimental techniques outlined. Let us suppose that the standard or the reagent is prepared in an external, flow-through electrolysis cell by constant current coulometry. That is, the standard or reagent addition is carried out by electrolysis. The reagent generating electrolyte solution, after passing through the electrolysis cell, is continuously merged to the streaming sample solution and the combined stream passes through the detector cell. The programmed reagent or standard addition is ensured by employing a suitable generating currenttime program. In this case the equations given above are modified as follows: for standard addition: c, =
51
Chimica Acta 319 (1996) 49-58
(7)
(10)
(11) where E, is the cell voltage measured before standard or reagent addition; E, and E, are the cell voltages measured after the first and second standard or reagent addition, respectively; v, is the volumetric flow rate of the generating solution. A stationary solution version of the multiple standard addition method with volumetric addition was described by Brand and Rechnitz [17]. Similarly to their approach an other version for the evaluation of the double addition/subtraction data can be employed in cases when Eb and S are not known. Accordingly, let E, -El and E, -E, from Eqs. 9-11 be denoted by AE, and AE,, respectively, and then, after suitable transformations, the following equations can be obtained:
and for known subtraction:
c, =
a - .i b
(12)
nFvs(1 - 10-Y)
where i is the electrolysis current, F is the Faraday constant and n number of electrons involved in the electrolysis. As shown by Eqs. 7. and 8, c, is independent of the flow rate of the electrolyte used for reagent generation. If the slope of the electrode calibration curve is known, the unknown concentration c, can be calculated from AE, i and us. If the slope of the electrode calibration curve is not known from previous measurements, the concentration of a streaming sample solution can still be determined by standard addition or subtraction method. In such cases double or multiple standard addition or subtraction need to be performed. The
(13) Dividing Eq. 12. by Eq. 13 we get: a - .i
b
-4 AE2
=
l
c, vsnF a - .i b 2 c, vsnF
(14)
52
G. Nagy et al./Analytica
Using standard solutions of different concentrations and appropriate generating current values (ir and i2) we can determine the AE,/AE, vs. c, dependence for a given concentration range. When analysing a sample of unknown concentration, the relevant AE, and AE, values have to be measured using the same i, and i, generating current employed at calibration. Knowing the ratio of the AE values, the sample concentration can be read from the calibration curve. To employ this principle we do not need to know the slope of the electrode calibration curve, however, we suppose that its value is constant. As a matter of fact, different chemometric approaches were employed to obtain analytical results from the electrode potential changes measured with double standard addition/known subtraction methods when the slope of electrode calibration is not known. Li [18] worked out an iterative computer program for this. A slightly different, fast computer program was described by Wang [19]. High experimental accuracy was achieved when testing the applicability of the program.
Fig. 1. Schematic
Chimica Acta 319 (1996) 49-58
3. Experimental 3.1. The description of the monitor The scheme of the monitor developed is shown in Fig. 1. A two-compartment cell arrangement (1) with silver reagent generating electrode (2) and a platinum wire counter electrode (3) served for the reagent generation. The flow of the sample (4) and the reagent generating electrolyte (5) were propelled with peristaltic pumps (6) (LKEI Varioperpex 12000; type OL-602, LaborMIM, Hungary). The reagent and the sample streams merge dropwise in a drip vessel (7) and the combined stream after a dispersion coil (8) passes through a detector cell incorporating a microcapillary chloride ion-selective electrode (9) (type OP-Cl-7443, Radelkis, Hungary) as an indicator electrode, and a special flow-through double-junction calomel electrode (lo), as a reference one. The cell voltage was followed with a pH-meter (11) (type OP-211, Radelkis). The recorder output of it was connected to a personal computer (12) through an interface card (13) (type PCL-812). Home made
diagram of the monitor (for explanation
see text).
53
G. Nagy et al. /Analytica Chimica Acta 319 (1996) 49-58
time-programmed current generator (14) was connected to the generating electrodes. The main task to be performed by the computer was to determine the potential difference A E between the steady state values of the signal before and after the subtraction (or addition). The detector cell voltage was also recorded (type OP-814/l recorder, Radelkis) (15). All reagents used in this work were of analytical grade and doubly distilled water was employed for solution preparations.
4. Results and discussion 4.1. Evaluation of the monitor characteristics 4.1.1. Investigation of the electrode potential-time dependence of the monitor When a sample solution containing chloride ions at a constant concentration level is passed through the detector cell at a constant flow rate, a constant cell potential sets up, which depends on the chloride concentration (baseline). If the current generating the Ag+ reagent is turned on, the cell voltage changes corresponding to the decrease of chloride ion activity. After a transient time, a new constant potential level sets up. The potential of the indicator electrode
175 T-----
corresponds to the new, decreased chloride concentration level (subtraction line). Switching off the current, after a pre-set time the potential approaches its original, background value. An actual signal vs. time recording is shown in Fig. 2, as an example. The result of the measurement is calculated from the difference between steady-state sections of the base line and the subtraction line. The rate of analysis depends on the time needed to achieve steady-state conditions. The transient portion can be characterised by the time elapsed from the beginning of the current controlled generation to the point corresponding to 90% of the entire signal change (t,). The time necessary for the potential to restore its original value after switching off the generating current is shorter than that necessary for the subtraction line to be reached after the current has been switched on. Accordingly, at the flow rates applied (1.43 X 10-5-5.11 X lop5 dm3/s) t, was found to change between 50 and 120 s. The corresponding hold up volume was found to vary from 0.7 to 6.5 cm3. 4.1.2. Study of the effect of generating current intensity on the analytical result The most important experimental parameter of the monitor is the intensity of silver ion generating current. The effect of the generating current on AE has been studied experimentally. In the course of the
___~---_-_-~_-
170
165
Fig. 2. Trace of signal vs. time recording,
c = 1 X 1O-3 M; i = 1.8 mA; v, = 3.18 X 10-j
dm3/s.
54
0
G. Nagy et al. /Analytica
Chimica Acta 319 (1996) 49-58
0.1
0.4
0.2
0.3
0.5
0.6
0.7
0.8
0.9
i b4 Fig. 3. AE vs. i plot for potassium chloride standards
measurements a potassium chloride standard solution of constant concentration was passed through the analysis channel of the monitor at a constant flow rate (1.43 X 10m5 dm3/s). An aqueous solution of 10-l M for potassium nitrate, and 5 X 10m3 M for nitric acid was used as reagent generating electrolyte. The potential difference (AE) between the two steady state sections of the signal was measured at different generator current intensities. The AE values were
0
0.2
in the range of 10-4-10-3
plotted against the generating current. The AE vs. i plots obtained for different potassium chloride concentrations are shown in Fig. 3. For the comparison of the experimental curves with the theoretical ones it is reasonable to use the Gran-linearisation technique. The sample concentration can be calculated from Eq. 8. In our case the ratio of the stoichiometric constants a/b = 1 and n= 1.
0.4
0.6
i IrN Fig. 4. Linearised
M concentrations.
AE vs. i plot derived from Fig. 3.
0.8
1
G. Nagy et al./Analytica
0
I
I
I
0.1
0.2
0.3
55
Chimica Acta 319 (1996) 49-58
1 0.4 0.5 c. [mmol/dm3]
0.6
0.7
I
I
0.8
0.9
1
Fig. 5. AE vs. log c, function for different current intensities.
Employing 8 we get:
the Gran plot transformation
from Eq.
10-$=1-i
factors are constantiLThe theoretical and experimentally obtained lo- T -i relationships are shown in Fig. 4. The full lines denote the theoretical curves whereas the points are experimental values. The current stoichiometrically equivalent to the mass flow of the sample is given by the ordinate values at 0. Thus, if the flow rate is known, the chloride concentration in the sample solution can be calculated from
(15)
F% c,
Thus, theoretically, a linear relationship exists between lo- y and the generating current, if other
50 pi--i_,_--.--
.---I----
---I
F i%
4
I 10
_
__._._.___l_._..-_
__~._._~._.l.....
___+
.
I
__.“_...._.._._i.. _.._.. ___.____._..__...___.~.__.-__ __..-__.___.__
.._......_.
1 I
I !
0-t 0.03
I
0.035
0.04
0.045
/
0.05
v [ctn3/s] Fig. 6. Dependence
of AE on the volumetric
flow rate, c = 1 X lop3 M; i = 2.8 rr~4.
-I
0.055
56
G. Nagy et al./Analytica Chimica Acta 319 (1996) 49-58
the extrapolated current value. As shown in Fig. 4 the theoretical values agree well with the experimental results. This means that under the conditions used in the monitor it behaves as predicted by the theory. The theoretical AE values for a given generating current intensity at different sample concentrations can easily be calculated knowing the flow rate and S value of the detector. To compare the theoretical and the experimental A E - c, dependences at different generating current intensities, from the results shown in Fig. 3 A E - c, plots were made. These can be seen in Fig. 5. The series of curves obtained corresponds to the curves expected on the basis of Eq. 8.
Sr.(AE) 14 [%I
12
10
8
6
4.1.3. Investigation of the dependence of AE on the flow rate of the sample solutions In order to investigate this practically important dependence, a standard solution with chloride concentration of lop3 M and reagent generating electrolyte was passed through the system. The electrolysis current was 2.8 mA, while the length of the measuring cycle was 400 s. Different flow rates were employed and the AE values were measured. Fig. 6 shows the obtained AE values plotted against the flow rate of the standard solution. As can be seen from the figure, AE changes considerably with the flow rate of the chloride containing solution. This shows that it is essential to keep the flow rate of the sample stream constant during the measurements to be able to get reliable results with the apparatus. Based on the results shown, the approximate value of AE can be predicted for a given analyte concentration, knowing the flow rate of the sample solution and the intensity of the reagent generating current. In selecting the optimal conditions for the measurements, the effect of the experimental parameters on the precision of the results has to be taken into consideration. For this purpose the correlation between the AE values and the standard deviation of the results (experimental AE values or the corresponding calculated concentrations) has been studied experimentally. In this work potassium chloride solution of a concentration of lop3 M was pumped through the analysis channel at a flow rate of 5.51 X 10m5 l/s. The relative standard deviation of the AE values in percentage (& . 1001, (n = 5) were determined at different current intensities. In Fig. 7 the relative standard deviations of A E values are plotted
0
I
2
3
4. ’ WI
5
Fig. 7. Relative standard deviation (s,) of AE as well as c in the function of the electrolysis current at a preselected, constant flow rate. n=5; c= 1~10~~ M; vS=5..5x1O-5 dm3/s, AS,(AE) values (a 1; S,(c) values (0).
against the reagent generating current. It can be seen that the AE and i are in a well defined relationship. It can also be seen that the plotted relative standard deviations have a minimum. This means that from the point of view of precision an optimal reagent generating current can be selected to analyse a solution of given concentration. The best precision was obtained at current intensity of 3.5 mA, which produces a AE of about 30 mV. Accordingly, when operating the apparatus a current intensity should be chosen by which a AE of about 30 mV can be achieved. It is to be noted that this is in agreement with the general opinion about potentiometric standard addition and subtraction technique [16]. Obviously, for a monitor working with a single constant current, the above conditions exist only within a narrow concentration range. In order to investigate the performance of the
G. Nagy et al. /Analytica Chimica Acta 319 (1996) 49-58
19.1'
19.6
Fig. 8. Histogram
19.iss 20.6 AE WI
2O.j
ZO:6
of the obtained chloride concentration
apparatus, a current (3 mA) corresponding to a AE potential difference of about 20 mV was chosen. A lop3 M standard potassium chloride solution was passed through the sample line at a rate of 5.51 X 10m5 dm3/s. The measuring duty cycle was 400 s. In nearly 8 h continuous operation of the monitor 71 AE data as well as calculated concentration data were obtained. No significant drift of the AE and concentration values was observed during this time period. The distribution of the results (Fig. 8) can be considered as normal. The relative standard deviation
51
20.b
21.i
values.
calculated from 71 AE data was 1.95%, for the c, data 1.28%. 4.2. Monitoring water
of the chloride concentration
To investigate the practical applicability of the apparatus, the chloride concentration of tap water was continuously measured for 8 h. Tap water was passed through a vessel with an overflow pipe. The sample input tube took samples from the vessel
-l
I
2'
3
of tap
4
i
t IN Fig. 9. Results of tap water chloride concentration
monitoring.
8
58
G. Nagy et al. /Analytica
continuously and made measurements with a measuring cycle of 400 s, at a current of i = 2.0 mA and flow rate of vS= 5.51 X 10m5 dm3/s. The results of a one working day monitoring can be seen in Fig. 9. The consumption of the silver from the generating electrode was very small. However, while in continuous operation the silver electrode was checked and adjusted once a week.
References [l] L.I. Skeggs, Am. J. Clin. Path., 28 (1957) 311. [2] J. Ruzicka and E.H. Hansen, Flow Injection Analysis, Wiley, New York, 1981. [3] J. Koryta (Ed.), Medical and Biological Applications of Electrochemical Devices, Wiley, New York, 1980. [4] G. Nagy, 2s. Fehtr, K. T&h and E. Pungor, Hungarian Scientific Instruments, 41 (1977) 27.
Chimica Acta 319 (1996) 49-58 [5] K. Tbth, G. Nagy, Zs. Feher, Gy. Horvai and E. Pungor, Anal. Chim. Acta, 114 (1980) 45. [6] E. Pungor, Zs. Fehtr, G. Nagy and K. T&h! Crit. Rev. Anal. Chem., 14 (1983) 175. [7] K. T6th, G. Nagy, Zs. FehCr and E. Pungor, 2. Anal. Chem., 282 (1976) 379. [8] G. Gran, Analyst, 77 (1952) 661. [9] G. Horvai, L. Domokos and E. Pungor, Z. Anal. Chem., 292 (1978) 132. [lo] C.C. Westcott, Anal. Chim. Acta, 86 (1976) 269. [ll] M. Mascini, Ion-Selective Electrode Rev., 2 (1980) 17. [12] G. Horvai and E. Pungor, Anal. Chim. Acta, 151(1983) 383. [13] D. Midgley, Analyst, 112 (1987) 557. [14] B. Fleet and A.Y.W. Ho, in E.Pungor (Ed.), Ion-Selective Electrodes, Akadtmiai Kiad6, Budapest, p. 17. [15] 1.1. Zipper, B. Fleet and S.P. Perone, Anal. Chem., 46 (1974) 2111. [16] K. Cammann, Working with Ion-Selective Electrodes, Springer Verlag, Berlin, 1979, p. 140. [17] M.J.D. Brand and G.A. Rechnitz, Anal. Chem., 42 (1970) 1172. [18] H. Li, Analyst, 112 (1987) 1607. [19] J. Wang, Analyst, 115 (1990) 53.