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Electrodes are The Secret to Reliable pH Meter Readings
One of the handiest tools in a gardener’s tool belt is the pocket pH tester or meter. Within seconds, the meter’s display reveals the acidity or alkalinity of water and soil via the pH reading, indicating if nutrients are feeding into plants. For gardeners investigating why their plants are not thriving, the pH reading is, quite literally, a litmus test.
Priced around $100 or less, the handheld pH meter is also a worthy investment. Except, that is, when it begins to malfunction. Sluggish readings that take 15 to 30 seconds to register, erratic measurements that fail to stabilize, blatantly incorrect values, or no pH display at all, are complaints common to users of pocket pH meters. Further, gardeners using rainwater or treated wastewater to irrigate plants may find that the performance of their pH meter declines rather quickly.
Regardless of the type of pH meter, the problem can typically be traced back to the electrode itself. Single-junction electrodes are much more prone to problems and shorter lives than their double-junction counterparts.
pH electrodes fail because of reference electrode deterioration. This deterioration is a result of depletion, contamination, and clogging. The reference electrode’s ions (suspended in liquid, gel, or a polymer) are very slowly depleted with use. Electrolyte depletion occurs with all pH electrodes whether they are heavy-duty industrial electrodes, sophisticated laboratory electrodes, or pocket pH tester electrodes. Yet, if this were the only factor corrupting a pH reference electrode, electrode life would be long enough.
In addition to depletion from usage, contaminating ions from the water or soil sample can rapidly cause chemical reactions with the silver/silver chloride reference electrolyte solution commonly used in pH reference electrodes. This contamination produces sluggish, erratic, wrong, or even no pH electrode responses as the reference electrode wire spoils or the reference junction clogs. The result is swift electrode failure.
This deterioration proceeds most rapidly when the pH reference electrode is a single-junction electrode and the solution measured has high concentrations of ions that contaminate and then attack critical components of the reference electrode. Some of the most common applications that experience rapid failure of pH electrodes include testing plant nutrient solutions, measuring solutions with sulfides and/or metal ions, determining pH in food and wine, as well as wastewater analysis.
A pH electrode’s junction is the porous communication point between the electrolyte solution in the reference electrode and the sample itself. Basically the electrode solution must “leak” out into the sample in order for the electrode to achieve accurate results. In a single-junction electrode, the junction is in direct contact with the sample, allowing contaminants to pass directly into the electrolyte solution. The contaminants can then attack the reference electrode wire, sometimes quickly destroying the pH electrode itself.
In a double-junction electrode, ions that attack the pH reference electrode signal wire or react with the internal pH reference electrolyte (Ag/AgCl) take much longer to come into contact with the pH reference signal wire and internal pH reference wire. These contaminants must migrate through the first (outer) reference junction, build up a concentration in the cell filled with KCl reference electrolyte, and finally migrate through the second (internal) reference junction before coming into contact with the pH reference electrode signal wire and internal pH reference electrolyte.
This longer migration of contaminants to internal reference cells delays the pH reference electrode damage that ruins the pH electrode. This makes the double-junction pH reference electrode, and the entire pH electrode system, last much longer than single-junction pH electrode systems.
So it’s really no secret after all. When choosing a pH meter or tester, look for models using double-junction electrodes. These electrodes will deliver a longer life and higher level of performance. Of course, at some point the electrodes will need to be replaced and double-junction electrodes are a bit costlier to make, thus more expensive to purchase. For testing plant nutrient solutions, using rainfall or wastewater, or handling other potentially harsh fluids, the investment is well worth it. And your pH meter will show the difference.