pH Electrodes Designed and Manufactured by Hanna pH Measuring System pH Electrode The sensor half-cell of an electrochemical cell is typically composed of a special glass membrane that responds to a hydrogen ion concentration. Reference Electrode The half-cell of an electrochemical cell that supplies a stable voltage that is known, constant, and completely insensitive to the measurement solution. Changes in voltages generated from the pH sensor are measured versus this electrode’s voltage. High Input Impedance Meter The measurement device that processes the voltage from the electrochemical cell and converts it into a meaningful measurement unit (pH). The measurement is done with virtually zero current flow to prevent polarization of the electrodes. Modern pH meters also may provide sensor diagnostics, automatic buffer recognition, calibration reminders and user prompts. Chemical pH Buffers Buffers are stable, well-characterized standards used for calibration. Two or more pH buffers that bracket the sample pH range are suggested for the most accurate results. Thermometer or Temperature Probe A temperature measurement is desired during calibration and measurement to make adjustments to the Nernst slope factor. An auxiliary or built-in temperature probe ensures both calibration and measurement are automatically temperature compensated, thus eliminating error. Magnetic Stirrer Used in a laboratory setting, a magnetic stirrer together with magnetic stir bars continually agitate the buffer and/or samples to keep them homogenous, eliminating temperature or sample gradients. Electrode Design Ag/AgCl Internal Wire Buffered KCl Solution pH Sensitive Glass Internal Reference Wire High Concentration Electrolyte Solution (i.e. 3.5M KCl and AgCl Electrolyte) Salt Bridge Junction Connecting the Reference to the External Solution pH Half-cell Reference Half-cell Ag/AgCl Reference Wire Ceramic Junction Combination pH Electrode Inner Tube Housing the Membrane Sensing Wire Electrolyte Containing Silver Half-cells vs. Combination pH electrodes Until the 1970s, it was a common practice to offer two half cells separately, a glass pH sensor and a reference electrode. Today it is more common to use a single combined electrode that has both sensing and reference components. Reference electrodes still enjoy use in other electrochemical techniques and their use is often preferred with ion selective electrodes (ISE) half-cells. 2 pH 2.125 www.hannainst.com | electrodes
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