Control of the process of acid ion exchange of water using the electrical conductivity of water
Acid ion exchange of water is performed to replace all cations found in water with hydrogen cations (H+). As a result, the following reactions occur on the cation exchange resin in the H-form:
Ca(HCO3)2 +2HR <-> CaR2 + 2H2CO3
Mg(HCO3)2 + 2HR <-> MgR2 + 2H2CO3
NaHCO3 + HR <-> NaR + H2CO3
CaCl2+ 2HR <-> CaR2 + 2HCl
MgCl2+ 2HR <-> MgR2 + 2HCl
NaCl + HR <-> NaR + HCl
CaSO4 + 2HR <-> CaR2 + H2SO4
MgSO4 + 2HR <-> MgR2 + H2SO4
Na2SO4 + 2HR <-> 2NaR + H2SO4
As it can be seen, only strong acids are present in water as a result of substitution of cations for hydrogen (in this case, sulfuric and hydrochloric acid). The resulting carbonic acid will turn into carbon dioxide gas, since strong acids are present in the water.
It is obvious that the amount of ions in the water after acid exchange process will be reduced by the amount of the original bicarbonate, since the bicarbonate will be removed as carbon dioxide. But how will the electrical conductivity of water behave?
We can immediately say that the electrical conductivity of the treated water will increase, since the hydrogen ion has much greater mobility than all other cations. But how will the electrical conductivity of the treated water decrease when the sodium cation slips into it? It is the presence of sodium in the treated water that indicates the need for cationite regeneration.
Let’s conduct a real experiment.
We will pass water through the H-cationic filter.
The source water has the following composition:
- Hardness – 1,5 mmol/l (1,5 mg-eq/l)
- Sodium – 0,98 mmol/l (0,98 mg-eq/l)
- Bicarbonate – 1,9 mmol/l (1,9 mg-eq/l)
- Sulfate – 0,72 mmol/l (1,44 mg-eq/l)
- Chloride – 0,64 mmol/l (0,64 mg-eq/l)
- Conductivity – 409 µS/cm
- рН – 7,0
As a result of acid exchange process, the electrical conductivity of water was determined only by sulfuric H2SO4 and hydrochloric HCl acids. In this case, the concentration of sulfuric acid is equivalent to the concentration of sulfate in the source water and accordingly the concentration of hydrochloric acid is equivalent to the concentration of chloride.
We get that the treated water contains:
Н2SO4 – 0,72 mmol/l
HCl – 0,64 mmol/l.
The measured electrical conductivity of the treated water was equal to 839 µS/cm.
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