The screen-printed (SP), reference electrode (RE) has been shown to be a crucial element of potentiometric sensors but it is also the stumbling block for reliable and accurate SP sensors. The easiest, most common and most environmentally friendly, type of RE is the Silver/Silver Chloride (Ag/AgCl) RE. Unfortunately, until now the only reliable RE of this kind is the conventional, liquid or gel-filled type. However, for most environmental and soil applications the use of the conventional RE is not an option, which raises the demand for a robust, rugged and low-cost version to replace the conventional RE. This paper presents a review of the various attempts to produce reliable, SP, Ag/AgCl REs, and explain why almost all of them never reach the commercialisation stage. The paper provides an overview of the main challenges that need to be overcome, details of the electrode's construction, an analytical comparison of their performance in terms of chloride susceptibility, cross-sensitivity and lifetime, and their suitability in different applications depending on their performance characteristics. (Sophocleous M.) Conventional Ag/AgCl reference electrode Construction of the conventional Ag/AgCl reference electrodeThe most common type of silver-silver halide electrode is the Ag/AgCl due to the availability of silver chloride and the use of potassium chloride as the electrolyte. The electrode is constructed using a silver wire, electroplated with silver chloride and then immersed into a 3.5 M or saturated potassium chloride (KCl) solution. The electrolyte is generally contained in a glass tube having a porous glass frit at the tip to allow ion exchange at the liquid-liquid junction. The conventional silver/silver chloride reference electrode has a relatively simple construction. A typical construction of such an electrode is shown in Figure 1.

The fabrication of prototype thick film silver-silver chloride electrochemical reference electrodes is described. Combinations of commercially available and proprietary thick film pastes have been used in their construction in a multi-layer planar configuration modelled upon the structure of the classic single junction silver-silver chloride reference electrode cell. Several variations in the basic electrode design were fabricated, involving combinations of one of three different paste formulations for the silver-silver chloride layer coupled with one of two combinations of paste formulation for the salt containment matrix. The relative performances of these different versions of reference electrode were evaluated in terms of their chloride ion sensitivity, hydration times required to achieve a stable potential and usable lifetime. It is shown that, depending on the processing methodology employed at certain stages in the fabrication of these devices, a large degree of variation in characteristics can be achieved and therefore exploited in the design of reference electrodes suitable for a range of specific applications.

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Lubricating oils are employed in machinery to minimise friction, remove heat and wear particles, protect against corrosion etc. Mineral base oils are derived from natural crude oil through a distillation process and are divided into five main groups depending on volume of sulphur, saturate and viscosity index, according to the American Petroleum Institute (API). As lubricating base oil oxidises, acidic by-products are formed and oil viscosity increases due to progressive polycondensation of higher molecular weight products. In this work, thick film (TF) potentiometric sensors based on ion-selective electrodes (ISE) have been developed for oil acidity measurement to enable online oil condition monitoring. A TF ruthenium oxide (RuO 2 ) working electrode and three types of TF reference electrodes were fabricated and tested in oxidised base oil samples prepared at the Shell Houston laboratories. The TF sensors were evaluated by comparison with acid number (AN) measurement and also compared with oil conductivity measurements using impedance spectroscopy. The results show that the conductivity, viscosity and AN of the oil samples increase with the level of oil oxidisation. The output of the TF sensors detected the acidity increase of the oxidised oil samples at both 50 and 80 °C and displayed a linear relationship with oil AN.

Lubricating oil of moving parts is a critical factor for the performance and longevity of cars and industrial engines. The lubricant degradation occurring during application can be detrimental to the proper operation of lubricated machines if the oil is not changed in a timely manner. However, the oil degradation process is often influenced by a number of parameters, such as the operating temperature and contamination. This offers a challenge to oil monitoring technologies. This study investigates the feasibility of detecting changes in oil acidity using Ion Selective Electrodes (ISE) fabricated using thick-film (TF) screen printing technology. The working electrode is a TF Ruthenium Oxide (RuO 2 ) ISE; the reference electrodes are three different types of TF reference electrodes and commercial glass pH reference electrode. Artificially degraded oil samples from oxidation tests and model oils produced by adding acid have been used for the evaluation of the sensors. Acid Number (AN) of all oil samples was obtained using conventional titration method. The results show that the TF electrodes responded linearly to the change in oil acidity under different temperatures. Their sensitivity in detecting oil acidity in the model oils is found to be much higher than that of the commercial electrode. Initial analysis also shows that the sensor responses decrease as the temperature increases.