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78 PCB007 MAGAZINE I APRIL 2018 NaCl and sulfuric acid in water. The other graphs are the pH of water as a function of dis- solved base or acid in moles per liter. Certain applications require that the activi- ty of an ion in solution be measured. This can be accomplished with a specific ion electrode which is designed to be sensitive to the ion whose concentration is being measured. These electrodes are similar in appearance to those employed to measure pH, but are construct- ed of glass-membrane electrodes, solid-mem- brane electrodes, liquid ion-exchange mem- brane electrodes, or silicone rubber-impreg- nated electrodes. The reference electrode is the same as that used for pH. The electrode output is read on a high-impedance voltmeter very similar to that used with pH electrodes. Many applications are possible using ion- selective techniques. In printed circuits, mea- surements are made of copper and chloride in acid sulfate plating baths, lead and fluorobo- rate in tin-lead fluoroborate plating solutions, cyanide (ductility promoter) in electroless cop- per solutions, permanganate in desmear/etch- back solutions, chromic acid in epoxy smear removal, and the sulfate in nickel sulfate plat- ing solutions for tab plating. The specific ion electrodes, are made for spe- cific ions, as listed in Table 2 with their range of sensitivity in ppm and preferred pH. Many times, specific ion electrodes use the same me- ter as pH electrodes. Oxidation-reduction potential (ORP) or re- dox measurements determine the oxidizing or reducing properties of a chemical reaction. A reduction is the opposite of oxidation. There can be no oxidation without an attending re- duction. For example, a ferrous ion may lose an electron and become a ferric ion (gaining increased positive charge) if a reduction, say, of cupric to cuprous ions (which is the reverse of this operation) occurs at the same time. As a sensor in ACSC, ORP or redox is used in control of the oxidizer in ferric chloride, cupric chloride and hydrogen peroxide/sulfuric acid etching, in measuring the Au (III) to Au (I) in gold tab plating, and to measure the copper ac- tivity in electroless copper baths. Specific Gravity A second universal sensor is specific gravi- ty. Although not a chemical analytic approach, it nonetheless can measure the concentration of dissolved solids in solutions. Six different methods are used in indus - try: hydrometers, displacers, hydrostatic head, radiation, weight in fixed volume, and vibrating U-tube (Table 3). The least expensive way to measure specific gravity (SG) is with a hydrometer. These can be purchased inexpen- sively. They come in many ranges, but all require the use manually, as shown in the beaker. The hydrometer is a dis- placer type of SG sensor; an- other is the total immersion displacer. The SG sensor seen in Figure 4 is made from a GEM level switch. A CPVC plastic rod is drilled out to a specific depth (Dd) and two CPVC plastic nuts are tuned Table 2: Many specific ion electrodes, their type, operating range and pH.