The control of the process of acid ion exchange of water using the value of the specific electrical conductivity of water
Ivan Tikhonov
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.
The program I developed for calculating the electrical conductivity for water containing several ions showed the calculated value of electrical conductivity equaling 837 µS/cm. The calculated value of electrical conductivity for the source water is 410 µS/cm, while the measured value is 409 µS/cm.
The principles of calculating the electrical conductivity of natural water are described in the articles:
https://tiwater.info/en/the-influence-of-ion-composition-of-water-on-its-electrical-conductivity/
https://tiwater.info/en/the-method-of-controlling-the-process-of-ion-exchange-water-softening/
During the experiment, the electrical conductivity of the treated water was kept in the range of 841-837 µS/cm for a long time. Then the electrical conductivity of the water began to drop. The drop in the electrical conductivity of water is due to the fact that sodium began to slip into the treated water, which did not have time to exchange for hydrogen due to the depletion of resin by hydrogen.
Since sodium has a much lower mobility than hydrogen, it carries an electric charge much more slowly and, accordingly, the electrical conductivity of water decreases. The more sodium in the treated water, the less its electrical conductivity is.
Due to the difficulty in determining a small amount of sodium in the treated water, I used the developed calculation program to simulate the dependence of the electrical conductivity of the treated water on the concentration of sodium in the treated water. This dependence is shown in figure 1. The same figure shows the dependence of the electrical conductivity of the same treated water on the measured pH value. pH values were measured for each specific electrical conductivity value.
Figure 1
To confirm the correct operation of the calculation program, water was analyzed for alkalinity and hardness at a water conductivity value of 374 µS/cm and a pH of 6.54. The calculation program predicted a hardness salt concentration of 0.2 mmol/l and a bicarbonate concentration of 1.3 mmol/l in the treated water. As a result of the analysis, the concentration of hardness salts was 0.4 mmol/l and the bicarbonate concentration was 1.33 mmol/l.
The analysis results almost completely confirmed the calculated data.
The calculation program also predicted the lowest electrical conductivity of water equaling 262 µS/cm. Real measurements showed the lowest electrical conductivity equaling 268 µS/cm. These minor differences can be attributed to the error in determining the equivalent electrical conductivity of individual salts at different concentrations, and not to the methodology used in the calculation program. In fact, a minor error occurs because the influence of ion pairs on electrical conductivity is not accounted for by the program. The study of the formation of ion pairs is a separate, very interesting topic that explains the abnormal behavior of 2:2 salts when they are dissolved in water.
The lowest electrical conductivity of the treated water is achieved when strong acids completely disappear in the water and the formation of bicarbonates begins. When bicarbonates begin to form the pH of water is completely determined only by carbon dioxide.
As a result, when the bicarbonate concentration became equal to the initial concentration, the pH value became equal to the initial value of 7.0, but sodium prevailed as cations. In that case, the electrical conductivity of water was 421 µS/cm. The electrical conductivity of the source water was 409 µS/cm. The completely depleted acid exchanger started working as a softener.
Figure 2 shows the calculated dependence of the electrical conductivity of treated water on the concentration of sodium in the treated water. This is the same dependence as in figure 1 only taken at a larger scale.
Figure 2
As it can be seen, when the sodium concentration in the filtrate is 0.01 mmol/l (0.23 mg/l), the electrical conductivity value drops from 837 to 834 µS/cm. This slight change can be perceived as acceptable fluctuations, provided that the electrical conductivity of the source water can slightly change. But at the sodium concentration of 0.05 mmol/l, the electrical conductivity is 823 µS/cm. The difference is 14 µS/cm, which can be reliably recorded.
Thus, the control of the acid exchanger used as the first stage of preparation is quite possible if the ionic composition of the source water is sufficiently stable.
It should be noted that it is better to measure the electrical conductivity of the source and treated water using the same conductivity sensor.
