Cation Column

The figure shows the arrangement of a Cation exchanger in the sampling system called Duplex type. The reason being that it uses 2 Cation columns, 1 duty and 1 standby.

The principle of Cation Exchanger is same as in any ion exchange process. This process is used in any D.M.Water plant. The D.M. Plant uses Anion Exchanger and Cation Exchanger to remove anions and cations from the water to finally make the water free from salts / dissolved impurities.

Only the Cation exchange principle is used for a different purpose in the sampling system. This is used to eliminate masking effects of known chemicals such as Ammonia or Dissolved Amines. We’d like to explain it in detail as below :

The conductivity measurement, as such, is a blind method of cross checking whether dissolved impurities are present in the sample or whether it is Ultrapure water. Ultrapure water is very poorly ionised and hence posses extremely low conductivity.

The moment one adds even a small amount of salt (say NaCl), the conductivity shoots up drastically. This can happen even with addition of desired chemicals as mentioned above. The typical reaction on the Cation column will be :

1)Na(+) + Cl(-) => H(+) ions => HCl + Na(+) ions
2)NH4(+) + OH (-) =>. H(+) ions => H2O + NH4(+) ions

In the Cation columns, resins are present charged with H(+) ions. These ions replace the +ve ions of any salt / dissolved impurity as it dissociates in water.

In case (1), the cations replace Na ions and the outcome is HCl, i.e. the corresponding Acid.
In case (2), the cations replace NH4 ions and the outcome is Water(H2O), i.e. Pure Water.

The conductivity in Case (1) is at 3 times as that imparted by the salt ( here NaCl ), while in case (2), the Conductivity imparted by the chemical ( here NH4OH ) gets eliminated as the outcome is pure H2O.

Thus Cation conductivity measurement eliminates the masking effects of known/desired chemicals.