Effect of ionic strength on PH tests

In highly buffered (high KH) water, or a pH calibration solution different test methods will generally agree within a small tolerance. This is fine as most PH test systems are designed for lab use where solutions are highly buffered. However buffer strength can affect the results from different pH measurement systems when used in lower KH solutions. This is because in low ionic strength solutions neither the electrical (in pH probes) or chemical (liquid tests) response needed for the measurement are strong enough. 

It is important to remember that PH is a ratio that shows the balance of +/- ions in solution not the amount of ions, if you want to learn more on PH then please follow this link.

PH probes work by the concentration of hydrogen ions creating a milivolt output that the probes calculate a pH from. In a low ionic strength liquid, this reaction can be small and the pH probe may not have enough of a sensitivity to calculate an accurate pH. Therefore if you have a tropical discus tank with low KH and a probe system that is calibrated correctly in highly buffered solutions chances are it will not read correctly in the aquarium. An old or dirty probe in this situation it will probably report the same or similar PH value regardless of what is actually happening. In lab use in low buffer solutions different style probes and algorithms are used to the normal.

All test systems will report a value even if not correct and it’s not uncommon to see a difference of 1 pH point between different test methods and devices in low KH water.

Liquid based chemical test are also affected by the kH of the water. All liquids have a PH including the reagents in your test kit, in very low buffered solution they will often over whelm and adjust the PH of the water you wish to test.  

The chemistry on the seneye slide has been specifically calibrated to work in a kH that is more like that normal aquarium water.  We use two different algorithms based on marine and fresh water. If you believe you may have a problem with your slide then please read this.

Having some KH in the aquarium or pond is essential for fish and bacteria health; click here to learn more.

RO water should have a theoretical pH neutral of 7, but cannot be measured with traditional test methods discussed above as there is not enough ionic strength to create the reaction.