Why is Calibration Required?
It is a simple question and a simple answer. Data is worthless if it is not correct, and calibration is required so we–the users–can verify that the electrode or sensor is giving us the correct data. It is one of the most important aspects of quality control and, in extreme cases, can cost tens of thousands of dollars and hours of time if even one electrode is off by 10%.
But you know all that. Why am I bothering to write this guide?
With the typical ego of a college graduate, I thought that the many laboratory classes taken to get a B.S. in Biology had thoroughly prepared me for the real world–or at least for basics like maintaining and calibrating an instrument. In general, I knew how to select and prepare buffers and perform a calibration, but there are many other factors not taught in school that affect the calibration process.
- What qualities of an electrode cause it to drift or become less accurate?
- How frequently is calibration required to maximize efficiency and accuracy?
- How is the health of an electrode determined quantitatively?
- When does an electrode actually require replacement to maximize value?
This guide goes over the specific qualities of pH, dissolved oxygen, ORP (redox), and conductivity electrodes that cause inaccuracies. It also covers questions 2-4 for any electrode.
Limiting Factors of Specific Electrodes
pH and ORP Electrodes
The reference solution is the limiting factor for most common types of electrodes. Rates of usage and contamination of the electrolyte depend entirely on how similar the chemical properties of the sample are to the reference solution. For example, an electrode in a low ionic solution will drift more rapidly compared to one in a high ionic solution because the reference electrolyte in most pH and ORP electrodes is 3.0M KCl–a high ionic solution–and the equilibrium force (and therefore dilution rate) is directly related to the difference between the reference and sample solutions.
Dissolved Oxygen Electrodes
Electrochemical dissolved oxygen electrodes are typically filled with a salt solution such as propionic acid. The electrodes typically have low diffusion rates so contamination is not a problem, but the salt solution does get used up eventually. Oxidation or reduction of the anode and cathode also become a problem over time.
For contacting conductivity electrodes, such as the one in the pHionics STs Series Conductivity, the limiting factor is cleanliness of the electrode cells. Algae or dirt will build up and increase resistance between the cells, causing inaccuracies, especially for 2-cell electrodes (Learn about the different conductivity electrodes here!). pHionics conductivity electrodes are exceptionally durable so the build-up can be removed with soap and a soft brush or fine sandpaper without fear of damage.