Note that despite being properly calibrated it has been my frequent experience that different pH meters will produce significantly different pH readings on the same solutions. This is especially true of lower quality electrodes because they can respond to other 'species' which are electrochemically similar to hydrogen ions, such as sodium. For this reason a hydroponic grower can get pH readings differing by more than 0.5 units when testing a nutrient sample with two different pH meters that have been correctly calibrated. Note that pH indicators cannot give false readings with colorless samples because they will only respond to hydrogen ions.

Storage of pH Electrodes

For a pH meter with a single combination electrode to work properly there must be an electrical connection between the electrode filling solution (usually potassium chloride – which is electrically conductive) and the sample solution. This is achieved via the use of a porous frit or wick (Fig 1.8a) in the glass wall separating the inner filling solution chamber from the sample. Both these devices allow the free (very slow) flow of the filling solution into the sample solution.

The most common cause of pH meter failure is due to blockage of this porous frit or wick. This is usually caused by dry storage of the electrode or leaving the electrode immersed for too long in concentrated solutions (e.g. concentrated nutrient). Dry storage causes dehydration of both the glass electrode and precipitation of salts within the interstices of the frit itself (Fig 1.8b). The consequences are reading drift, slow response times and decreased sensitivity (loss of accuracy and precision). To avoid these problems the electrode tip must be permanently stored (Fig 1.7) in a specially formulated storage solution such as Flairform's pH Electrode Storage solution.

NOTE: pH buffers 7.0 or 4.0 and distilled water are not suitable for storing pH electrodes as they can upset electrode calibration.
 

Cleaning pH Electrodes

The ‘frit’ and glass tip of pH meter electrodes invariably become contaminated with impurities (Fig 1.8b) causing symptoms such as reading drift, slow response times and decreased sensitivity (loss of accuracy and precision). Contamination may be so severe that re-calibration is not possible without prior cleaning.

Minimise this problem by filtering potentially problem (i.e. greasy or highly turbid) water samples prior to performing a pH measurement. Avoid placing electrodes in oily solutions as they can “coat” the electrode. Further, avoid measuring (or leaving electrodes for too long) in harsh chemicals. For example, highly caustic (e.g. pH Up, silica additives) or concentrated solutions (e.g. raw nutrient). These can damage electrodes and/or increase the need for electrode maintenance.

Regular cleaning will maximize electrode performance and ensure a longer working life. Some suitable cleaners are:

-  Organic contamination:  Use mild liquid detergent, acetone*, methylated spirit or toluol*. If toluol is necessary, rinse subsequently with acetone or methylated spirits and finally water.

-  Inorganic contamination:  Use dilute nitric or hydrochloric acid.

In hydroponics, because both organic and inorganic contaminations tend to co-occur, it is best to use an all-purpose cleaner such as Flairform’s Electrode Cleaner.

* Note that some cleaners may attack electrode hardware - particularly plastic. Therefore ensure to check with the electrode’s manufacturer before proceeding.