Complete micro-reference electrode

In case of potentiometric measurements with microelectrode arrays, complete micro-reference electrodes have been suggested. The term complete microreference electrode is used here for such reference electrodes that consist of the same parts as conventional macroelectrodes. Their operating principle is essentially identical to that of the latter. Major difference is the size. Needle-type reference electrodes are frequently used for miniaturisation. The Ag/AgCl system (or a similar one) is hosted in a capillary with dimensions in the millimetre range. The capillary is thinned at one end, and the orifice houses the liquid junction. Commercially available complete reference microelectrodes have capillaries with an outer diameter of 1-2 mm at the end. The liquid junction is realised by a ceramic frit or a fibre. A real micro-reference electrode has been described by Kitade and coworkers [26]. They used a commercially available Femtotip capillary tube as the electrode body. Such tips have a capillary with an outer diameter of 1 pm and are usually used for microinjection into adherent and suspension cells. The tip is filled with a KCl agar gel to realise the liquid junction. The bare part of the silver wire has to be covered with a polystyrene membrane to avoid any additional potential between the inner electrolyte solution and the bare silver wire. 

In many cases, where the reference electrode is not required to exhibit a specified and thermodynamically determined potential, the conventional reference electrode is replaced by a pseudo-reference electrode (see Chap. 14). These can be (1) in its most simple form, metal wires (or layers) of Au, Ag or Pt (cf. Fig. 11.1), or (2) in a more sophisticated form, Ag-salt covered Ag electrodes however, lacking the usual salt reservoir (e.g. KCl) and salt bridge afforded for a complete and thermodynamically controlled reversible reference electrode (see Preface and Chaps. 1-10). The metal layers can be deposited either by photolithographic processes or by screen printing methods (cf. above) [7]. Such micro-pseudo-reference electrodes are parts of voltammetric and amperometric analysing systems and commercially available. 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

---