Measurement Error
Electrochemical distortions occurring at high alkalinity levels cause ph electrodes to report values that are lower than the true hydrogen ion activity. In textile mercerization or reactive dyeing, sodium ion interference happens because the glass membrane begins to respond to sodium ions instead of hydrogen ions. This effect is also known as the alkaline error and becomes detectable when the ph exceeds 10 or 12.
The sensor effectively sees the abundant sodium ions as protons, leading to a false indication of higher acidity.
Chemical Mechanism
Glass membranes are designed to be selective for hydrogen, but the selectivity is not absolute in the presence of high concentrations of other monovalent cations. As the concentration of hydrogen ions drops in a caustic bath, the relative influence of sodium ion interference increases. Specialized low error glass can be used to manufacture electrodes that are less sensitive to this problem.
These sensors use different silicate compositions to maintain their selectivity at the top end of the ph scale. If a standard electrode is used, a correction table must be applied to the results to find the real ph of the liquor. Failure to account for this error leads to the under dosing of alkali and the incomplete fixation of dyes.
Detection Threshold
Severity of the distortion depends on the temperature and the specific concentration of sodium salts in the solution. Higher temperatures typically worsen the effect of sodium ion interference and increase the required correction.
Mitigation Strategy
Choosing the correct sensor for the specific range of the textile process is the most effective way to manage the problem. Maintenance teams should ensure that probes used in bleach ranges or mercerizing units are specifically rated for high sodium environments.
