Q: I am looking to buy a pH electrode, many of them say “Na error over…” – what does this mean?
A: Sodium error is also known as alkaline error, and is a phenomenon that occurs at very high pH levels – usually pH 12 or over.
At very basic or alkaline pH levels, where Hydrogen ion concentration is relatively low compared to the Sodium ion concentration in the sample, you can see a Sodium error.
This is when the Sodium ion level is relatively so high that some of the H+ ions in the gel layer around the sensitive electrode membrane are replaced by Na ions. The electrode may eventually respond to Sodium ions instead of H+ ions, giving a falsely lower pH value than the real result.
The phenomenon can also be seen in solutions with a high Lithium content, and high temperatures can increase the effect.
To avoid alkaline error or sodium error;
- Check the limit of your pH electrode – most will state the pH range they are able to cover, and may state what value Sodium errors begin to occur. Usually this is at pH 12 or over.
- Measure highly alkaline solutions at room temperature where possible – high temperatures increase the alkaline error or sodium error effect
- Where you have to measure very alkaline solutions, ask your supplier for a specialist probe able to maintain accuracy in these conditions
For more information about pH electrodes and pH measurement, follow the links above or contact us with any questions;
One thought on “What is sodium error or Na error in a pH electrode?”
I have a process that requires 10% NaOH in an aqueous solution at ambient temp. In addition to the NaOH the solution will include 0.5 molar Na2S2O3 and reaction products as ions NO2- Cl- and Na2S4O6 in concentrations less than molar.
I realize pH will be adversely affected by the high pH. I plan to monitor the Na2S2O3 with ORP and hope there is a sodium electrode or pH electrode that will work in this environment.
Robert Richardson PhD
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