In many industries, from food to steel, from microprocessors to chemicals including car making, the concentration of alkali during the various stages of the production process is of vital importance
The alkalis used in all these industries may very well differ in make up and concentration, eg, a few percent of alkali in a potato peeling plant or concentrated ammonia in a wafer plant.
They have one thing in common, that in all cases the concentration of the free hydroxyl ion and/or the total of free and bonded of the OH- play a decisive role in the process to produce a product with an acceptable quality.
It is very appealing to think that from measuring the pH value one can derive the alkali concentration.
However for several reasons however this is not the case.
The mV value generated by free hydroxyl ions at the glass electrode (later in the measurement device converted to pH) is often suppressed by other ions.
Or the alkali is not fully dissociated so the pH value only is expressing part of the alkali.
Even in those rare cases when all conditions are right, still the accuracy achieved will be poor.
This is explained by the fact that the glass electrode works according to Nernst's law, meaning the logarithm of the concentration creates a linear (mV) signal.
Reversibly, obtaining concentration from the pH value, the signal has to be de-logged, generating a big error .
The only method available to get the accuracy right is titration.
Because here the mV or pH value at the electrode is only a means to detect the equivalence potential, and from that the ml value of the consumed titrant (acid) coming from a burette.
This ml value is linear to the concentration of alkali.
Due to the steep shift in mV (pH) around the equivalence point the accuracy of the value is less significant .
With the availability of refined titration methods, controlled by the microprocessor, titrant volumes can be determined with an inaccuracy smaller than 0.05%.
Recommended analysers for on -line alkali analysis: for single steam and single chemistry applications: Applikon ADI2016 Titrolyzer.
For multiple streams and multiple chemistries: Applikon ADI2040 process analyser.
