**5. Potential response of solid state potentiometric chemical sensors, theoretical approach and analytical teaching experiment**

In this chapter a solid state potentiometric chemical sensors (PCS) used as detectors in the presented kinetic methods, performed in batch or flow-injection mode, are discussed. PCS make the use of the development of an electrical potential at the surface of a solid material when it is placed in a solution containing species which can be exchange (or reversibly react) with the surface. The species recognition process is achieved with a PCS through a chemical reaction at the sensor surface. Thus the sensor surface must contain a component which will react chemically and reversibly with the analyte in a contacting solution. The response of a solid state PCS to sensed ions in solution is governed by ion exchange or redox processes occurring between the electrode membrane and the solution. Since the transfer of the ions or electrons occurs across this membrane-solution interface, it is readily apparent that any changes in the nature and composition of the membrane surface will affect these processes and hence the response of the sensor. The potential of PCS in kinetic experiments is formed due to heterogeneous reaction at the surface of membrane and homogeneous reaction in contacting solution. The potential response of solid state PCS with Ag2S + AgI membrane has been extensively investigated in our laboratory. For better understanding the behavior of this sensor in kinetic experiments the following questions are discussed. i) Which chemical compound on the surface of the membrane is important for the response of the sensor? ii)Which heterogeneous chemical reaction (or reactions), occurring between the electrode membrane and the sensed ions in solution, forms the interfacial potential? iii) Which homogeneous chemical reaction (reactions) in solution is (are) important for the potential response of the sensor? Potentiometric measurements with PCS containing membrane prepared by pressing sparingly soluble inorganic salts can be used for teaching homogeneous and heterogeneous equilibrium. Learning objective is to distinguish between homogeneous and heterogeneous equilibrium, and between single-component and multicomponent systems [38, 39].

84 Analytical Chemistry

**Figure 5.** Flow-injection manifold configuration. Sample or standard solution (NAC); carrier stream (ultra pure water); reagent stream (1.0 103 mol L–1 Fe(III) and 1.0 103 mol L–1 TPTZ in acetate buffer solution, pH 3.6); peristaltic pump (flow rate 2.0 mL min1); injector valve (loop = 500 L); confluence point (Y-type); reactor in coiled form (length: 300 cm, i.d. 0.8 mm); spectrophotometric detector (

In order to evaluate the potential of the proposed method for the analysis of real samples, flow-injection spectrometric procedure was applied to different pharmaceutical formulations (granules, syrup and dispersible tablets) for the determination of NAC.

A FIA spectrophotometric procedure for determination of *N*-(2-mercaptopropionyl)-glycine (MPG), tiopronin, has been proposed [36]. Determination was also based on the coupled redox-complexation reaction between MPG, Fe(III) and TPTZ. This coupled reaction was usefully used in development of the FIA method for determination of ascorbic acid in pharmaceutical preparations [37]. The proposed method is simple, inexpensive, does not

Recorded peaks refer to samples A, B and C are showed in the Figure 6.

involve any pre-treatment procedure and has a high sample analysis frequency.

**theoretical approach and analytical teaching experiment** 

**5. Potential response of solid state potentiometric chemical sensors,** 

In this chapter a solid state potentiometric chemical sensors (PCS) used as detectors in the presented kinetic methods, performed in batch or flow-injection mode, are discussed. PCS make the use of the development of an electrical potential at the surface of a solid material when it is placed in a solution containing species which can be exchange (or reversibly react) with the surface. The species recognition process is achieved with a PCS through a chemical reaction at the sensor surface. Thus the sensor surface must contain a component which will react chemically and reversibly with the analyte in a contacting solution. The response of a solid state PCS to sensed ions in solution is governed by ion exchange or redox processes occurring between the electrode membrane and the solution. Since the transfer of the ions or electrons occurs across this membrane-solution interface, it is readily apparent that any

nm) equipped with flow cell (internal volume 160 L).

= 593

**Figure 6.** Fiagram chart and calibration curve (inlet) for spectrophotometric determination of NAC over the concentration range from 6.0 106 to 2.0 104 mol L1. Fiagram includes recorded peaks for three samples: (A) Fluimukan granules; (B) Fluimukan Akut Junior syrup and (C) Fluimukan Akut dispersible tablets

As it has been discussed, the determination of penicillamine was based on a batch and FIA experiments using PCS with AgI membrane. The membrane was prepared by pressing silver salts (AgI, Ag2S) and powdered Teflon (PTFE). This AgI-based membrane detector, sensitive to sulfhydryl group, can be applied to flow-injection determination of different compounds containing sulfur. In order to understand the effect of stirring or flowing to potential response of sensor, for both kind of kinetic experiment (batch and FIA), it is necessary to develop a picture of liquid flow patterns near the surface of sensor in a stirred or flowing solution.

Kinetic Methods of Analysis with Potentiometric

(27)

and Spectrophotometric Detectors – Our Laboratory Experiences 87

*<sup>K</sup>* (28)

+ + Ag RSH RSAg(s) H (26)

with appropriate equilibrium constant.

can be calculated.

equilibrium.

eq

eq

eq 20

**Figure 8.** New phase formation in Ndl and its adsorption on the surface of membrane.

compound, which cause precipitation of RSAg in acid media.

Now we can calculate the minimal concentration of penicillamine, or any other RSH

*K*

1.40 10

*K*

*K*

H Ag RSH

 

*<sup>K</sup>*

By using the experimentally established constant of solubility product [32] *K*sp, RSAg , the dissociation constant of penicillamine, [41] *K*a , and Equation (28) the equilibrium constant

11

 

The calculated value of equilibrium constant suggests completeness of the new phase formation reaction at the surface of membrane. In addition, it can be supposed that, by adsorption process, both parts of membrane, AgI and Ag2S, are covered with a thin layer of RSAg precipitate (Fig. 8). Under these conditions, the equilibrium activity of Ag+ ions and the corresponding response of PCS are governed by new heterogeneous

3.16 10 2.26 10

9

a

sp,RSAg

According to Skoog [40] three types of flow can be identified. *Turbulent flow* occurs in the bulk of the solution away from the electrode and can be considered only in stirred solution during batch kinetic experiment. Near the surface of electrode *laminar flow* take place. In FIA experiment only laminar flow exists in the tube. For both kind of kinetic experiments (batch and FIA) at 0.01 - 0.50 mm from the surface of electrode, the rate of laminar flow approaches zero and gives a very thin layer of stagnant solution, which is called the *Nernst diffusion layer* (Ndl). According Equation (13), the potential of sensor is determined by activity of Ag+ ion in Ndl.

When the membrane of the sensor, containing both Ag2S and AgI, is immersed in a solution with Ag+ or I ions heterogeneous equilibrium at the phase boundary is established. The potential difference between the solution phase and the solid phase of the sensor is built up by a charge separation mechanism in which silver ions distribute across the membrane/solution interface as shown in Figure 7.

**Figure 7.** Heterogeneous equilibrium at the phase boundary between AgI-based membrane and solution.

The stable potential of PCS with AgI + Ag2S membrane in contact with penicillamine (RSH) solution can be explained by the following consideration. According to the picture of liquid flow near the surface of sensor in a stirred or a flowing solution (Fig. 7) the potential of the sensor is determined by activity of Ag+ ion in Ndl. In FIA experiment PCS with AgI + Ag2S membrane (before injection of penicillamine) was in contact with flowing solution of Ag+ ion, and the concentration of Ag+ ions in solution including Ndl was 6.3010-6 mol L–1. The formation a new solid state phase in Ndl or/and at the surface of the sensing part of the tubular flow-through electrode unit may be expressed by the next reaction:

Kinetic Methods of Analysis with Potentiometric and Spectrophotometric Detectors – Our Laboratory Experiences 87

$$\text{Ag}^+ + \text{RSH} \rightleftharpoons \text{RSAg(s)} + \text{H}^+ \tag{26}$$

with appropriate equilibrium constant.

86 Analytical Chemistry

or flowing solution.

in Ndl.

solution.

with Ag+ or I-

membrane/solution interface as shown in Figure 7.

As it has been discussed, the determination of penicillamine was based on a batch and FIA experiments using PCS with AgI membrane. The membrane was prepared by pressing silver salts (AgI, Ag2S) and powdered Teflon (PTFE). This AgI-based membrane detector, sensitive to sulfhydryl group, can be applied to flow-injection determination of different compounds containing sulfur. In order to understand the effect of stirring or flowing to potential response of sensor, for both kind of kinetic experiment (batch and FIA), it is necessary to develop a picture of liquid flow patterns near the surface of sensor in a stirred

According to Skoog [40] three types of flow can be identified. *Turbulent flow* occurs in the bulk of the solution away from the electrode and can be considered only in stirred solution during batch kinetic experiment. Near the surface of electrode *laminar flow* take place. In FIA experiment only laminar flow exists in the tube. For both kind of kinetic experiments (batch and FIA) at 0.01 - 0.50 mm from the surface of electrode, the rate of laminar flow approaches zero and gives a very thin layer of stagnant solution, which is called the *Nernst diffusion layer* (Ndl). According Equation (13), the potential of sensor is determined by activity of Ag+ ion

When the membrane of the sensor, containing both Ag2S and AgI, is immersed in a solution

potential difference between the solution phase and the solid phase of the sensor is built up by a charge separation mechanism in which silver ions distribute across the

**Figure 7.** Heterogeneous equilibrium at the phase boundary between AgI-based membrane and

tubular flow-through electrode unit may be expressed by the next reaction:

The stable potential of PCS with AgI + Ag2S membrane in contact with penicillamine (RSH) solution can be explained by the following consideration. According to the picture of liquid flow near the surface of sensor in a stirred or a flowing solution (Fig. 7) the potential of the sensor is determined by activity of Ag+ ion in Ndl. In FIA experiment PCS with AgI + Ag2S membrane (before injection of penicillamine) was in contact with flowing solution of Ag+ ion, and the concentration of Ag+ ions in solution including Ndl was 6.3010-6 mol L–1. The formation a new solid state phase in Ndl or/and at the surface of the sensing part of the

ions heterogeneous equilibrium at the phase boundary is established. The

$$K\_{\text{eq}} = \frac{\left[\text{H}^+\right]}{\left[\text{Ag}^+\right] \cdot \left[\text{RSH}\right]} \tag{27}$$

$$K\_{\rm eq} = \frac{K\_{\rm a}}{K\_{\rm sp,RSAg}} \tag{28}$$

By using the experimentally established constant of solubility product [32] *K*sp, RSAg , the dissociation constant of penicillamine, [41] *K*a , and Equation (28) the equilibrium constant can be calculated.

$$K\_{\rm eq} = \frac{3.16 \times 10^{-11}}{1.40 \times 10^{-20}} = 2.26 \times 10^9$$

The calculated value of equilibrium constant suggests completeness of the new phase formation reaction at the surface of membrane. In addition, it can be supposed that, by adsorption process, both parts of membrane, AgI and Ag2S, are covered with a thin layer of RSAg precipitate (Fig. 8). Under these conditions, the equilibrium activity of Ag+ ions and the corresponding response of PCS are governed by new heterogeneous equilibrium.

**Figure 8.** New phase formation in Ndl and its adsorption on the surface of membrane.

Now we can calculate the minimal concentration of penicillamine, or any other RSH compound, which cause precipitation of RSAg in acid media.

$$K\_{\text{eq}} = \frac{\left[\text{H}^+\right]}{\left[\text{Ag}^+\right] \cdot c(\text{RSH}) \cdot a(\text{RSH})} = 2.26 \times 10^9 \tag{29}$$

$$\text{c(RSH)} \ge \frac{\left[\text{H}^+\right]}{\left[\text{Ag}^+\right] \cdot K\_{\text{eq}} \cdot a(\text{RSH})} \tag{30}$$

Kinetic Methods of Analysis with Potentiometric

and Spectrophotometric Detectors – Our Laboratory Experiences 89

glu L-glutatione

LOV lab-on-valve

NAC *N*-acetyl-L-cysteine Ndl Nernst diffusion layer

pen D-penicillamine phen 1,10-phenantroline

RSH thiol compound

**Author details** 

**6. References** 

1988.

1968.

*Croatia* 

H2A reduced form of ascorbic acid

MPG *N*-(2-mercaptopropionyl)-glycine

PCS potentiometric chemical sensor

PTFE polytetrafluoroethylene, Teflon

SIA sequential injection analysis TPTZ 2,4,6-trypyridyl-*s*-triazine

Njegomir Radić and Lea Kukoc-Modun

Chemistry. 1970;42(2):304-305.

*Department of Analytical Chemistry, Faculty of Chemistry and Technology, University of Split,* 

[1] Mottola HA. Kinetic aspects of analytical chemistry. New York: John Wiley & Sons

[2] Mark HB, Rechnitz GA. Kinetics in analytical chemistry. New York: John Wiley & Sons

[4] Brand MJD, Rechnitz GA. Surface films on glass membrane electrodes [1]. Analytical

[5] Srinivasan K, Rechnitz GA. Reaction rate measurements with fluoride ion-selective membrane electrode: Formation kinetics of ferrous fluoride and aluminum fluoride

[6] Radić N, Komljenović J. Kinetic potentiometric determination of Fe(III) using s fluoride

[7] Radić N. Determination of nanomole amounts of aluminium by use of a fluoride ion-

[8] Trojanowicz M, Hulanicki A. Microdetermination of aluminium with fluoride-selective

[9] Radić N, Bralić M. Kinetic - potentiometric determination of aluminium in acidic solution using a fluoride ion-selective electrode. Analyst. 1990;115(6):737-739. [10] Plankey BJ, Patterson HH, Cronan CS. Kinetics of aluminum fluoride complexation in

acidic waters. Environmental Science and Technology. 1986;20(2):160-165.

[3] Cattrall RW. Chemical Sensors. Oxford: Oxford University Press 1997.

complexes. Analytical Chemistry. 1968;40(12):1818-1825.

selective electrode. The Analyst. 1976;101:657-660.

electrode. Mikrochimica Acta. 1981;76(1-2):17-28.

ion-selective electrode. Croatica Chemica Acta. 1991;64:679-687.

$$\log\text{(RSH)} = \frac{\left[\text{RSH}\right]}{c\left(\text{RSH}\right)} = \frac{\left[\text{RSH}\right]}{\left[\text{R}\text{S}^{\cdot}\right] + \left[\text{RSH}\right]}\tag{31}$$

If we express [RS- ] with dissociation constant of RSH,

$$
\left[\text{RS}^-\right] = \frac{K\_{\text{a, RSH}} \cdot \left[\text{RSH}\right]}{\left[\text{H}^+\right]}
$$

we obtain

$$a\left(\text{RSH}\right) = \frac{\left[\text{H}^+\right]}{K\_{\text{a, RSH}} + \left[\text{H}^+\right]}\tag{32}$$

In 0.100 mol L–1 perchloric acid, where experiment was performed, (RSH) 1.

$$c(\text{RSH}) \ge \frac{0.100}{6.3 \times 10^{-6} \cdot 2.29 \times 10^{9} \cdot 1}$$

6 1 *c*(RSH) 7.0 10 mol L

This concentration of penicillamine may be estimated as the detection limit for describing experimental conditions. The solution of Ag+ was pumped as a reagent in a two-line flow manifold typically at a concentration of 10-5 mol L–1 with 0.1 mol L–1 perchloric acid as a pH and ionic-strength adjuster.

#### **List of abbreviations**


