A typical pH/ORP electrode compares the voltage between two wires, one of which is enclosed in a glass bulb while the other is submersed in a reference solution that contacts the sample. Contacting the sample is key to a responsive system, but it also introduces two weaknesses.
- The salt solution surrounding Wire 2 gets diluted by the sample over time.
- Wire 2 may be coated or react with chemicals in the sample, affecting response time and accuracy. This is known as electrode poisoning.
Differential electrodes eliminate these problems by encasing both wires in glass bulbs. The sample also cannot contact either wire so the electrode cannot be poisoned.
One of the glass bulbs is exposed to the sample, while the other is surrounded by a reference solution. For differential electrodes, a strong buffer may be used instead of a salt solution. The differential reference solution does still experience dilution but, by the nature of a buffer, it stays more stable than the salt solution used in typical pH electrodes. When dilution eventually affects the differential electrode solution, it is easily replaced within a minute using the reference capsules included with each sensor. Additional reference capsules are available for purchase.
Oh, and differential electrodes may be stored dry without adverse effects on performance.