Short guide to the pH meter in combination with the ISFET pH electrode.

The pH ISFET electrode is not a traditional pH electrode dependent on an extremely vulnerable glass bulb made of a pH sensitive glass membrane! The metal H+ sensitive sensor at the end of the point of the pH ISFET electrode is encased in resin. The ISFET sensor is a metal semiconductor which has a special coating that makes it selectively sensitive to H+ . The electrode also contains electrolyte; a salt current conductive liquid. The electrolyte leaks outside via the diaphragm (the long white Teflon rod at the bottom/low down in the point). This allows contact to be established with the material of which the pH is to be measured. This means that there are two points of contact with the material being measured: the sensor and the diaphragm. And because these materials are tough the electrode is now much less susceptible to breakage and scratching.

Before you start, some important points to note:

ˆ This electrode imitates (“emulates”) a conventional pH electrode. For this reason the top of the thicker part of the electrode contains a small battery (this lasts some 700 hours and can be replaced) and some electronics. The advantage of this is that the electrode can be connected to any “old-fashioned” pH meter that has the correct connection. ˆ The electrode needs to be switched on before use. But: After use the electrode also needs to be switched off again. ˆ This intelligent electrode is capable of generating error messages; for example, when the battery is nearly empty or the electrode is dry. Consultation of the full instruction manual will ensure that you are not taken by surprise by these error messages. ˆ When stored, the point of the electrode should be kept wet. The small protective tube must contain fresh water, at least. The use of a strong solution of 3 Mol/l KCl of salt is preferred however. This can be purchased (art. no. 18.36.12). Replace the liquid frequently and keep the protective tube perfectly clean inside (cotton buds). ˆ The ISFET electrode can be applied directly into soft materials like dough, cheese, mud, peaty , soft loose sand and other such soft materials. In the case of drier and/or harder materials a hole should preferably be pre-drilled and the material moistened deeper down. This is normally achieved with distilled water (horticulture) but the use of water

with 1 Mol KCl (traditional agriculture) or water with 0.01 Mol CaCl2 (modern agriculture) to facilitate extraction is also possible. The electrode is then inserted a small distance into the material with light pressure or twisting. It is important to wait for a chemical equilibrium. This may take some minutes and in some cases even hours (in the case of with a high lime content). Measurements should be taken in several places to obtain an average. Please note: This field method differs from the standard method! The measurement results will therefore be (slightly) different! ˆ Never switch the lids of the jars of different buffer liquid. Rinse the electrode well with water, using a toothbrush, if necessary, during the adjustments and especially after it has been used. ˆ Keep all the items clean and dry during and after use. A pH meter is an extremely sensitive current meter. Makes sure, therefore that in wet weather the BNC plug is prevented from becoming wet by placing a sturdy plastic bag over the meter and the plug. Once a carry case has become wet it must be allowed to dry completely before being closed. Keep the liquids stored away from the meter as much as possible to minimise the effect of leakage from the buffer and maintenance liquids as the salt is aggressive, though the liquids are otherwise harmless.

Taking measurements:

Measurements are a waste of time, if components are not properly calibrated to each other. This electrode requires a different type of adjustment (“calibration to the standard”) than that used for traditional pH electrodes with glass bulbs. The procedure is as follows:

ˆ Check whether there are batteries in the meter by switching it on. ˆ Connect the electrode to the meter (pH/mV input). ˆ If you want to check the status of the batteries of the ISFET electrode use the MODE button to set the meter in the mV measuring mode. The figure that is displayed must be divided by 10. The result is the battery load percentage of the electrode. Refer to the full instruction manual for additional error messages. ˆ Remove the protective cover. The O-ring and collar should be pushed up out of the way. ˆ Rinse the shaft of the electrode with clean water or demineralised water. ˆ Connect a Temperature probe to the meter (banana plugs next to the BNC input). If a Temperature probe is not available the temperature of the adjustment liquid should first be introduced (refer to the general manual). ˆ Place both sensors in pH = 7.00 liquid (always use buffered liquid, never distilled water). ˆ Switch the meter on and press the MODE button until the pH measuring function has been activated. ˆ Strange values will now start to appear and possibly even the error message <<<<<<<. This is because the electrode also needs to be activated! ˆ The pH electrode is activated by pressing on the red dot at the top. ˆ The meter will show a value based on the last calibration. ˆ If the reading on the display is stable the emulation of the ISFET electrode then needs to be adjusted. ˆ To do this, press the red dot switch at the top of the electrode for 5 seconds. ˆ The electrode is now adjusted to your meter. ˆ You can now proceed with the regular adjustment on the pH 7.00 and (frequently also) 4.00 buffers. This adjustment depends on the make of meter to which the ISFET electrode is being connected. Below follows the procedure that applies to Multimeters 18.21, 18.26 and 18.28. For further information about specifications and operating options also refer to the instructions provided with the meter and the full instruction manual for the pH ISFET/ meter combination.

Calibration of Multimeters 18.21, 18.26 and 18.28

Note: If an error is made during operation, press the MODE key to go back through the steps in the menu.

Adjustment

Press CAL A calibration menu appears on the screen which shows: The adjustment buffer settings for 7.00 and 4.00 (as an example) CALIBRATE ..... GLP

If buffers with other values are displayed see below for the calibration of different adjustment buffers. Press CAL if the displayed adjustment standard values are correct and the adjustment can commence. The screen will display: Clean the electrode, place in pH … liquid and press CAL.

As the electrode has already been cleaned and placed in a buffer of 7.00, CAL can now be pressed. During the adjustment the screen will display: Buffer mV and °C and the observed values

If the adjustment was successful, a second adjustment will be prepared. The screen will display: Clean the electrode, place in pH … liquid and press CAL.

Next remove the electrode from the first adjustment solution Rinse the electrode with demineralised water Place the electrode in the second adjustment solution Press CAL to start the second adjustment The screen will display: Buffer mV and °C and the observed values of the first and second calibrations.

Press CAL to terminate the adjustment process and to start measuring.

If measuring is not commenced straight away, the electrode and meter should be switched off.

Modifying the adjustment solution settings:

Press CAL in the regular measuring mode. A calibration menu will appear on the screen: The adjustment buffer settings for 7.00 and 4.00 (as an example) CALIBRATE ..... GLP

Press the scroll arrow up or down until BUFFER 1 is reached. Press CAL to activate BUFFER 1. Alter the value using the <— or —> keys until it corresponds to the buffer you wish to use. Press CAL to record the selection Repeat for BUFFER 2.

Refer to the full instruction manual for information about measuring and specifications for the electrode and meter.

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