Measurement of acidity or alkalinity is very important in many industries and processes. Acidity or alkalinity is a measure of the relative ratio of Hydrogen (H+) ions to Hydroxyl (OH+) ions. If they exist in equal amounts, the solution is neutral, 7.0 pH. An increase of H+ will increase acidity, a lower pH, and an increase of OH+ will increase alkalinity, a higher pH.
Common electrodes used to measure pH will, based upon the Nernst equation at 25 °C, output 0 mV at 7.0 pH, +174 mV at 4.0 pH, and +/-174 mV at 10.0 pH. Although many measuring instruments today use complex microprocessor based circuits to measure pH, a low cost basic circuit can take advantage of the wide DPV-3 offset and span capability to simplify the external interface circuit.
Common electrodes used to measure pH will, based upon the Nernst equation at 25 °C, output 0 mV at 7.0 pH, +174 mV at 4.0 pH, and +/-174 mV at 10.0 pH. Although many measuring instruments today use complex microprocessor based circuits to measure pH, a low cost basic circuit can take advantage of the wide DPV-3 offset and span capability to simplify the external interface circuit.
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Probes for measuring pH usually require a very high input impedance buffer amplifier, (100 megohms or greater). This can be followed by an inverting high gain amplifier to output a useable voltage signal. For example with a total gain of -2000, pH 4.0 would output approximately +/-3.48 volts, pH 7.0 approximately 0.00 volts, and 10.0 pH approximately +3.48 volts.
These can be input directly into a DPV-3 set for the 5 Volt range, which can be adjusted to read the desired pH directly.
These can be input directly into a DPV-3 set for the 5 Volt range, which can be adjusted to read the desired pH directly.
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The DPV-3 can be easily calibrated to the output from a specific probe by inserting the probe into readily available standard pH 4.0, pH 7.0, and pH 10.0 solutions when making the adjustments.
Usually most measurements are made in either acidic or alkaline solutions, so best accuracy would be achieved by calibrating over the region of interest.
For example, if acidic measurements are of interest, insert the probe in a pH 4.0 solution and adjust the DPV-3 ZERO to display 4.00. Next insert the probe in a 7.0 pH solution and adjust the SPAN to display 7.00. It may require repeating this procedure several times until the readings stay at 4.00 and 7.00.
If alkaline measurements are of interest, the process above would be repeated using pH 7.0 and pH 10.0 respectively.
If both acidic and alkaline measurements are of interest, best overall accuracy would be achieved if the DPV-3 is calibrated with the probe in pH 4.0 and pH 10.0 solutions respectively.
Usually most measurements are made in either acidic or alkaline solutions, so best accuracy would be achieved by calibrating over the region of interest.
For example, if acidic measurements are of interest, insert the probe in a pH 4.0 solution and adjust the DPV-3 ZERO to display 4.00. Next insert the probe in a 7.0 pH solution and adjust the SPAN to display 7.00. It may require repeating this procedure several times until the readings stay at 4.00 and 7.00.
If alkaline measurements are of interest, the process above would be repeated using pH 7.0 and pH 10.0 respectively.
If both acidic and alkaline measurements are of interest, best overall accuracy would be achieved if the DPV-3 is calibrated with the probe in pH 4.0 and pH 10.0 solutions respectively.
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