pH Meter

Overview

A pH meter measures how acidic or basic a solution is by reading the voltage of a glass electrode dipped into the sample. The benchtop version is built for the lab bench: a molded Housing with a flexible stand arm holds the pH Electrode upright over a beaker, the rear Input Panel takes the electrode through a BNC jack, and a touchscreen Display Module shows pH, millivolts, and temperature at once.

The electrode produces only a tiny voltage at an enormous source impedance, so almost the whole instrument exists to read that voltage faithfully and turn it into a number. A Temperature Probe sits next to the electrode for automatic temperature compensation, and a Buffer Storage Kit of standard solutions keeps the electrode calibrated. Power comes from a Power Module module that runs on a wall adapter or an internal rechargeable cell.

How it works

The sensing element is the Glass Bulb at the electrode tip. Its thin pH-sensitive glass develops an electrical potential across its wall that tracks the hydrogen-ion activity of the sample: about 59 mV per pH unit at 25 °C, the Nernstian slope. That potential only means something against a fixed reference, so the combination electrode also carries an internal Ag/AgCl Reference Wire half-cell bathed in Internal Buffer, and a second reference chamber of Reference Electrolyte that contacts the sample through a porous Reference Junction. The difference between the two half-cells is the measured cell voltage.

That voltage cannot be read by an ordinary meter, because the glass membrane has a source resistance of hundreds of megohms to gigohms; any current drawn would collapse the reading. The Main Board solves this with a High-Impedance Amplifier, an electrometer amplifier whose input impedance exceeds 10^12 ohms, so it buffers the electrode voltage while drawing essentially no current. The buffered signal then goes to a high-resolution ADC, and the microcontroller applies the Nernst equation, scaled by the live temperature from the probe, to convert millivolts into pH.

Because the glass slope and offset drift with age and temperature, the meter is calibrated against known Buffer Bottle before use, fitting a line through two or three standard points. The result, with mV and temperature, appears on the touchscreen, and between readings the electrode is capped to stay hydrated.