User manual

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USB DrDAQ User's Guide 17

Using USB DrDAQ with pH Electrodes

The pH input on USB DrDAQ is a very high impedance input that is suitable for use

with any standard pH electrode. For most applications no calibration is required: just

plug in an electrode and measure. If, however, you require very accurate pH

measurements, then you should calibrate the probe before use (see below).

If you are using a pH probe and not getting the results you expect then the most

probable cause is a defective pH electrode. If cared for properly, pH electrodes will

last for a number of years (see the instructions supplied with the electrode). If they

are not stored properly, then they will be destroyed in a few weeks. Trying to

calibrate out errors from a defective probe will not work and may increase errors

further.

pH measurement and temperature calibration

The output from a pH electrode is proportional to absolute temperature (kelvin). A

temperature difference of 10 C will cause the probe output to change by

approximately 4%. This is a major source of error with most pH meters and data

loggers. To minimise these errors, a temperature sensor can be connected to Ext 1

and the DrDAQ will compensate for temperature changes.

Testing pH Electrodes

The best way to test a pH electrode is to use pH buffers (pH 4, pH 7 and pH 10 buffers

are widely available). If you do not have any pH buffers, then distilled de-ionised

water will have a pH near 7 (some still mineral water bottles have the typical pH

printed on the label). Most fizzy (carbonated) cola drinks have a pH in the 2.5 to 3

range.

With the pH electrode connected to USB DrDAQ, display the pH channel using PicoLog.

Measure the pH of each buffer in turn. Allow 30 seconds for the reading to stabilise

and be sure to wash the electrode in clean water before swapping solutions. If the pH

measured is within 1 pH of the expected value then you can be fairly sure the

electrode is working correctly. If the readings are wrong then the electrode is probably

defective. Replacement electrodes are available from Pico Technology.

If you suspect that the pH input on USB DrDAQ may be defective, then short out the

BNC connector using a terminator plug. The reading on the screen should be pH 7; if

not, then the most likely reason is that someone has calibrated the probe incorrectly.

Select File/New Settings in PicoLog to delete the user calibration and return to the

default calibration. If the reading is still wrong then contact technical support.

Calibrating for accurate pH measurements

As described above, calibration is only required when accurate (better than 0.5 pH)

measurements are required. Calibration should be performed just before the

measurements are made. User calibration information is stored separately in

PicoScope and PicoLog, so if you wish to measure pH accurately with both programs, 2

calibrations will be required.

To calibrate a pH electrode you will require at least 2 pH buffers (pH 4, pH 7 and pH

10 buffers are widely available). Calibration should be performed with the ambient

temperature close to 25 C. A container of clean water is also required to wash the

electrode before moving it from one buffer to another.

1. Connect the pH electrode to USB DrDAQ and display pH using either PicoScope or

PicoLog (depending on which program you are calibrating for).

2. Place the electrode in the first solution and wait for at least 30 seconds for the

readings to stabilise (gently stirring helps).