## 2 Flow-based electrochemical detection
### 2.2. Potentiometric detection  
Potentiometric methods require a working electrode, a reference electrode, and a device for measuring the potentials. Potentiometry is the most widely used analytical technique because of its simplicity, versatility, and low cost. It is used to detect analytes in titration, biological fluids, etc. The flow injection analysis of hydrogen peroxide using a redox electrode and an Fe(II)-Fe(III) potential buffer solution has been developed [37]. The analysis of high concentrations of hydrogen peroxide up to \(\sim\)10 M was conducted successfully with a relative standard deviation of 0.7%. A differential potentiometric method for the determination of lysine based on a flow injection (FI) system has been reported. The lysine biosensor was prepared by chemically immobilized lysine oxidase on a nylon membrane and attached to an ammonium electrode. A circular ammonium electrode was used as a reference. By using differential potentiometry, the possible interferences of  
Figure 2: The FI manifold with amperometric detection using BDD as the working (WE) electrode. Optimal condition: potential, 1.0 V; carrier, 60 mM phosphate buffer at pH 5.  
endogenous ammonium can be partly corrected. The sensitivity can be improved from 20 to 40 mV per decade by using stopped-flow technique [38]. A highly sensitive method for the determination of manganese(II) using FIA with a flow-through type redox electrode detector has been developed [39]. This system utilized a redox reaction between manganese (II) and hexacyanoferrate(III) in near neutral media, and the signal can be obtained potentiometrically. Linearity was found over the concentration range from 10\({}^{-4}\) to 10\({}^{-7}\) M with a detection limit of 1 x 10\({}^{-7}\) M at a throughput of 20 samples per hour. This method was successfully applied to the determination of manganese (II) in soil samples. Polymer membrane type ion-selective electrodes for a potentiometric determination of phenyl-propanolamine hydrochloride have been constructed. A membrane of the first electrode was modified with the phenylpropanolamine-tetraphenylborate (PPA-TPB) ion-pair. Another electrode was modified with the phenylpropanolamine-phosphotungstate (PPA-PT) ion-associate. When applied in the FIA system, both sensors showed fast, stable, and Nernstian slopes over the concentration ranges of 10\({}^{-5}\) to 10\({}^{-2}\) M and 10\({}^{-5}\) to 10\({}^{-2}\) M in the case of PPA-TPB and PPA-PT, respectively. The advantages of the modified electrodes are its rapid response, simple operation, and low cost [40]. SIA with a potentiometric detection system has been widely used as an automation system for determination of both inorganic cations and anions, such as chloride, iodide, and nickel [41, 42]. Another two SIA system, Lab-at-Valve (SIA-LAV) [43] and Lab-on-Valve (SIA-LOV) [44], for the determination of inorganic ions using ion-selective electrodes have been reported. The LOV concept was introduced in 2000 by [PERSON] [45]. Sample-processing channels with a multipurpose flow cell were integrated on top of a selection valve. This concept can be used to perform wet chemistry at microlitre levels. A simpler designed, LAV, was introduced later [46]. In this new approach, the integrated device (sample-processing channel and detection unit) was simply attached to one port of the selection valve. LAV has more simplicity than LOV in terms of construction. The main advantages in terms of analytical performance should be the same. Apart from ion-selective electrodes, other electrodes such as carbon paste electrodes can also be used for potentiometric detection. This electrode material can be easily modified with many chemicals to obtain the desired properties or improve the selectivity. The modified carbon paste electrode for the potentiometric determination of the silver content of silver sulphradiazine (burning cream) in both static mode and flow injection analysis has been demonstrated by [PERSON] [47].