by Marc Mason, business development manager, liquid analysis, Emerson Automation Solutions
Hi and welcome. pH sensors are a small thing that can have a huge impact on the smooth operation of your plant. When they’re operating correctly, you barely notice them. If they aren’t working well, they can shut you down. Various factors can cause damage to pH sensor glass electrodes – the process, temperature, sodium error, caustic, and hydrofluoric acid (HF).
- The Process: A glass pH electrode consists of an inert glass tube with a pH sensitive glass tip – either hemispherical (bulb) or flat in shape. The tip contains a fill solution with a known pH, and it is the influence of this solution on the inside of the glass tip versus the influence of the process solution on the outside that gives rise to its millivolt potential. Ideally, the pH electrode will have a slope (response) of -59.16 mV/pH, at 25C, but in practice, a new electrode may only have a slope of -57 to -58 mV/pH. As the electrode ages, its slope decreases.
- Temperature: In addition to changing millivolt output of the pH electrode, elevated temperatures accelerate the aging of the electrode. Extremely high or low temperatures can alternatively boil the fill solution or freeze it, causing the electrode tip to break or crack. Elevated temperatures can also affect the interior and exterior of the pH electrode differently, giving rise to asymmetry potential, which shifts the zero point of the pH electrode and changes its temperature behavior, thus leading to temperature compensation errors.
- Sodium Error: More correctly called alkali ion error, sodium ion error occurs at high pH, where hydrogen ion concentration is very low in comparison to sodium ion concentration. The sodium ion concentration can be so high relative to hydrogen ion concentration that the electrode begins to respond to the sodium ion. This results in a reading that is lower than the actual pH. Depending on the pH glass formulation, this can occur as low as 10 pH. Where accurate high pH readings are required, the upper pH limit of the pH electrode should be checked and a specially formulated, high pH electrode used if necessary. Compared to sodium, lithium ions will produce a larger error, while the effect of potassium ions is negligible.
- Caustic Components Attacking pH Electrodes: As noted earlier, high concentrations of hydroxyl ions shorten the life of pH electrodes. Solutions that approach 14 pH (equivalent to 4% caustic soda) can destroy a pH electrode in a matter of hours. There is nothing that can be done to prevent this, short of simply avoiding pH measurements in these solutions and using conductivity instead.
- Hydrofluoric Acid (HF): HF also dissolves pH glass, but there are pH glass formulations designed to resist destruction by HF which, when used within their limits, can give satisfactory electrode life. It is important to note that, while only HF attacks glass and not the fluoride ion (F-), hydrofluoric acid is a weak acid. Therefore, a solution can contain a relatively high concentration of fluoride ion at a high pH and do no damage to the electrode. But if the pH of the solution decreases, the fluoride ion will combine with hydrogen ion to form HF, which will damage the electrode. So, it is important to look at both pH and HF concentration to determine the impact on pH glass.
What kinds of problems have you encountered with your pH sensor glass electrodes?