The pH Probe
Author: Paula Brooksby
First published in Aquarium World August 2013
The pH probe is a useful piece of equipment used to measure the acidity or alkalinity of water in the aquarium. The probe itself contains a glass bulb that comes in contact with the water. It is the thickness of this glass bulb that affects the measurement and hence the accuracy; therefore, anything that damages the glass should be avoided (scratches, exposure to chemicals, algae, etc.).
The glass bulb is special in that it can sense acid/hydrogen ions. When the bulb is exposed to water, the alkali ions in the glass bulb exchange with hydrogen ions in the water and this process generates a potential (voltage) difference. It is this potential difference that is measured and converted to a pH value that we understand when measuring our water acidity or alkalinity.
Inside the body of the glass bulb in most typical combination pH probe units there are two electrodes separated by a junction. These two electrodes include a measuring electrode which gets immersed in aquarium water and a reference electrode sealed in its reference solution (whose potential is always constant). The potential generated at the junction of these two electrodes is a consequence of the hydrogen ion concentration in the aquarium water and the difference between them gives the measurement of the aquarium water.
The junction between these two electrodes can become blocked by particulates and occasionally some metal ions can precipitate in the junction. If this occurs the electrode needs to be soaked in warm tap water to dissolve the material and unblock the junction.
In order to operate correctly, the pH electrode needs to be kept moist at all times as this keeps the glass hydrated. Hydration is required for the ion exchange process. Ideally the electrode should be kept in a buffered solution at either pH 4 or 7. It should never be stored in alkaline pH (as this can dissolve the glass) or distilled or deionized water (as this causes migration from the reference solution). If the pH electrode dries out it is best practice to recondition the glass by placing it in some tap water for 30 minutes prior to usage. All pH electrodes can run down with time and use (just like a battery) and as the electrode ages there are changes to the glass resistance. This in turn changes the electrode potential and affects accuracy of the results which is why they need regular calibration to correct for this gradual and continual change.
It is important to read the instructions specific for each meter but generally the pH electrode should be calibrated before each use and standard buffer solutions should always be used. Buffer solutions or tablets can be purchased for this purpose. Standard buffers come in three pH values: 4, 7 and 10. buffers should be stored away from heat and in tightly sealed containers. Always use freshly poured solutions for each calibration.
There are two types of calibration: one-point calibration (done at a single pH point provided that point is close to the expected pH of the tank water) or dual calibration (between two range points) which gives greater accuracy.
One-point calibration: Calibrate the meter using only one buffer. The value of the buffer should be close to the anticipated sample pH value.
Two-point calibration: Calibrate the meter using two buffers. One buffer is pH 7 and the other is pH 4 or 10. Choose the second buffer based on whether you are measuring an acidic solution (chose the pH 4 buffer) or an alkaline solution (choose the pH 10 buffer).
Dual calibration method:
If there is a temperature setting, the meter must correspond to the temperature of the buffers.
Place electrode into fresh pH 7 buffer at room temperature.
· Adjust meter to read 7.00 using the zero offset dial (for the one-point calibration method, at this point the electrode would be placed into a solution with a known pH value e.g. pH 7).
Withdraw the electrode from the solution and rinse with water.
· Wick the electrode dry by dabbing it briefly against a clean paper towel (do not rub dry as this can cause a static voltage to setup between the two electrodes).
· Place the electrode into the second buffer solution (pH 4 or 10) and adjust the meter with the slope or gain controls to make it read the same as the known buffer (refer to manufacturer instructions if needed).
· The pH electrode is now calibrated and ready to use.
A pH electrode is designed to operate at room temperature, which is nominally given as 25 °C. If probes are to be used at other temperatures, then the measurement observed needs to be corrected for the temperature variation. Tables are available if precise measurements are necessary, otherwise, when the temperature is anything other than 25 °C and the pH is anything other than 7, the correction is: 0.03 pH error/pH unit/10 °C. For example, in going from 25 to 35 °C and pH 7 to pH 4, the error is +0.09 - generally not a concern for most practical aquarium applications.
After each use, rinse the electrode with water and store it in soft (low salt) acidic solution. Commercial soaking solutions are available or you can make your own by mixing 1 M KCl (~0.75 g / 10 mL) adjusted to pH 4 with a few drops of concentrated acid. The level of filling (if this is an option for your electrode) should be kept at least 2/3 full and be open during use.
The life-span of a typical pH probe depends on the conditions in which they are used and the care with which they have been stored. For a probe that is only used occasionally and which is stored properly, it can continue to operate for a number of years but a probe in continuous use may last as little as 6 months. Even electrodes that are never used still age and deteriorate with time. When an electrode is failing it can typically be characterized by sluggish response, erratic readings or a reading that does not change. When this occurs the electrode can no longer be calibrated.
Reference: http://www.omega.com/Green/pdf/pHbasics_REF.pdf
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