Changes to slope at higher pHs
Alkaline Error or Sodium Error occurs when pH is very high (e.g. pH 12) because Na+ concentration is high (from NaOH used to raise pH) and H+ is very low.
Electrodes respond slightly to Na+ and give a false low reading. This is related to the concept of selectivity coefficients where the electrode responds to many ions but is most selective for H+. This problem occurs because Na+ is 10 orders of magnitude higher than H+ in the solution.
High pH electrodes use a 0-14 pH glass. This electrode will read pH 14 (1 M NaOH) to be around pH 13.7 with a 0.3 pH sodium error.
A standard pH electrode uses a 0-12 pH glass. The electrode will read pH 14 (1 M NaOH) to be around pH 12.4 with a 1.6 pH sodium error.
Alkaline error
The alkaline effect is the phenomenon where H+ ions in the gel layer of the pH-sensitive membrane are partly or completely replaced by alkali ions. This leads to a pH measurement which is too low in comparison with the number of H+ ions in the sample. Under extreme conditions where the H+ ion activity can be neglected the glass membrane only responds to sodium ions. Even though the effect is called the alkaline error, it is only sodium or lithium ions which cause considerable disturbances. The effect increases with increasing temperature and pH value (pH > 9), and can be minimized by using a special pH membrane glass.
Sodium Ion Error
Although the pH glass measuring electrode responds very selectively to H+ ions, there is a small interference caused by similar ions such as lithium, sodium, and potassium. The amount of this interference decreases with increasing ion size. Since lithium ions are normally not in solutions, and potassium ions cause very little interference, Na+ ions present the most significant interference.
Sodium ion error, also referred to as alkaline error, is the result of alkali ions, particularly Na+ ions, penetrating the glass electrode silicon‐oxygen molecular structure and creating a potential difference between the outer and inner surfaces of the electrode. H+ ions are replaced with Na+ ions, decreasing the H+ ion activity, thereby artificially suppressing the true pH value. This is the reason pH is sometimes referred to as a measure of the H+ ion activity and not H+ ion concentration.
Na+ ion interference occurs when the H+ ion concentration is very low and the Na+ ion concentration is very high. Temperature also directly affects this error. As the temperature of the process increases, so does the Na+ ion error.
Depending on the exact glass formulation, Na+ ion interference may take effect at a higher or lower pH. There is no glass formulation currently available that has zero Na+ ion error. Since some error will always exist, it is important that the error be consistent and repeatable. With many glass formulations, this is not possible since the electrode becomes sensitized to the environment it was exposed to prior to experiencing high pH levels. For example, the exact point at which the Na+ ion error of an electrode occurs may be 11.50 pH, after immersion in tap water, but 12.50 pH after immersion in an alkaline solution.
Controlled molecular etching of special glass formulations can keep Na+ error consistent and repeatable.
This is accomplished by stripping away one molecular layer at a time. This special characteristic provides a consistent amount of lithium ions available for exchange with the hydrogen ions to produce a similar millivolt potential for a similar condition.
