DIY pH Meter & Probe

Construct a circuit with an AC (alternating current) adapter and a TL082 dual JFET-input IC operational amplifier which has a high impedance input and prevents the need for the use of pH tables. Position the amplifier between the high impedance pH probe and the digital multimeter. Use ordinary wiring to connect the individual components.

Solder the amplifier directly to a BNC ("bouncer" or relay) panel controller using a soldering iron and a piece of flux. Use a 7-volt offset as a reference ground rather than an earth ground. Connect the multimeter to ground and pins 6 and 7 of the amplifier. Connect the pH probe to a non-inverting input. Power the circuit with two 9-volt batteries.

Insert the pH probe into the solution you wish to measure. Use the multimeter to measure the output voltage, which is directly proportional to the pH in this setup and produces an increase or decrease of 1 volt when there is a change in one unit of pH.

Interpret the voltage. Acids give negative voltages. Basic solutions give positive voltages. A pH probe produces about 59 mV per pH unit so, at a pH value of 7 (neutral pH) the probe produce 0 volts, as described by 66 Pacific.

Draw a graph of your results on graph paper (paper printed with squares). Put the full 0-14 pH scale along the X axis (the base line) and a range of steadily increasing voltage values up the Y axis (the vertical side line) from -300-mV to +300-mV. Draw a diagonal line with a straight rule from pH 14 to +300-mV.

Determine the pH value of the solution you are measuring. Locate your voltage on the Y axis. Follow along to the right until you reach the diagonal line. Now follow down in a direct line to the X axis. Read the corresponding pH at that point.