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APRIL 2018 I PCB007 MAGAZINE 77 Manual Procedures Figure 2 shows one of the oldest analytical techniques, which is the actual plating of pol- ished brass plates in the Hull Cell. Even with sophisticated analytical methods, the Hull Cell is still a final check on plating quality and con- tamination. This supports and is in addition to manual titrations. Electrochemical Sensors Electrochemical sensors like pH are some of the most commonly used for analysis, but equally important are ORP, ion-selective probes, capaci- tance and conductivity sensors, illus- trated in Table 1. Conductivity sensors will measure the amount of total dissolved solids in an electrolyte. It is common to use them where the concentration of a known salt, base or acid must be de- termined. The concentration of these solutions will vary the resistance of the solution, or the inverse of resis- tance, conductance. The conductivi- ty sensor typical measurement is in mhos per centimeter (reciprocal of ohm-centimeters). Lower conductiv- ity ranges of 0.01 to 100,000 microm- hos per centimeter are used for wa- ter purity such as boilers and chill- ers, or deionized water. Higher con- centration of electrolytes (50 to 1000 millimhos per centimeter) use electroless probes to avoid polarization effects of electrol- ysis. Conductivity (dissolved ionic concentra- tion) and pH (hydrogen-ion concentration) are very common sensors used in industry. Figure 3 shows graphs of the conductivity (in millim- hos /cm) versus the concentration of dissolved Table 1: The four major types of electrochemical sensors used in industry and their characteristics as well as relative costs. Figure 3: Conductivity and pH graphs: a) Conductivity of NaCl in water as a function of concentration and temperature; b) Conductivity of sulfuric acid as a function of concentration; c) pH of distilled water and the addition of an acid reagent; d) pH of distilled water and the addition of a base reagent.