Since pH is one of the most important variables to control in hydroponic culture almost all hydroponic growers keep very close track of the pH in their reservoir. The easiest and most accurate way of measuring pH is with a pH probe or meter. These can be bought in any hydroponic store like Oasis Garden Hydroponics.
There are, however, several things that can go wrong with these probes when the glass membrane is not cared for properly. Here are some of the most crucial care guidelines.
1. Do not let them dry. How the sensor bulb works is by reading the potential difference between the pH of the inside reference solution and pH on the outside of the glass membrane. Hydronium (H3O+) ions interact strongly with the glass surface, which allows for this reading to occur. For measurements to be accurate the glass surface needs to be in equilibrium with the media that is being measured. So since there is KCl liquid on the inside, to be kept at equilibrium, there should be KCl liquid on the outside. If you let the electrode dry then the hydration of the surface will be lost and the equilibrium state will be much harder to achieve (a dry probe should be placed in a KCl solution for at least 4 hours before being used). Dry pH probes are therefore a not as accurate.
2. Do not keep them in water. This might be a better option than letting them dry out but this has a similar effect in that it alters the composition of the glass with time. Since the solution around the probe is much more diluted, with time ions in the glass will have no problem migrating away from the probe, creating defects within the glass that will mess with your sensor’s calibration. Ideally, you will want to store your pH probes in a concentrated KCl solution (usually around 150-300g/L) which will prevent any of these migration effects and will ensure that your probe remains stable in the longer term.
3. Do not measure very basic solutions. PH probes are made of glass, and the silicates that make up the glass will react to basic pH solutions. When the pH goes above 10 a pH probe will start to dissolve in solution, completely altering the surface of the probe and make the sensor lose calibration very quickly. In general, avoid measuring the pH of any solution above 10 so that this effect can be kept to a minimum.
4. Do not measure solutions with chemicals that react with glass. Besides basic solutions – where hydroxide ions dissolve glass – there are a variety of substances that can affect the performance of pH probes by reacting with the glass. This includes solutions containing silicate species and solutions containing fluoride ions.
5. Always clean the probe. When measuring solutions like hydroponic nutrient solutions the pH probe is usually subjected to an environment filled with a variety of microorganism contaminants. If the probe is not properly cleaned then microbes can form a biofilm over the glass that will seriously affect the accuracy of pH readings. A probe can be cleaned with a bleach or hydrogen peroxide solution to remove these contaminants but the probe will then need to be recalibrated as the film will have effectively changed the glass surface to some extent.