Changes in the electrical conductivity of natural water when adding hydrochloric and sulfuric acid to it

Ivan Tikhonov

Cooling tower, reverse osmosis, water acidification

This article is devoted to the problem of removing bicarbonate from natural water. Bicarbonate can be removed from water by converting it to carbon dioxide. In this case, part of the carbon dioxide dissociates in the water and reduces the pH value of the water. When the pH of the water becomes 4.5, it is assumed that the entire bicarbonate ion has turned into carbonic acid. You can convert bicarbonate ion to carbonic acid by adding hydrochloric or sulfuric acid to water. Thus, the amount of strong acid anions formed in water is equivalent to the amount of bicarbonate ion passed to carbon dioxide. If you add hydrochloric acid, then chlorides are formed, if sulfuric is added, then sulfates.

Scale formation processes can be effectively controlled by converting bicarbonate ion to carbonic acid. In other words, it is possible to avoid the deposition of salts of the so called temporary hardness. Temporary hardness is the salts of calcium carbonate CaCO₃, which are formed from the dissolved form of Ca(HCO₃)₂. As can be seen when converting bicarbonate (HCO₃) to carbon dioxide, the process of precipitation of temporary hardness salts is impossible.

The process of precipitation of temporary hardness salts significantly worsens the operational characteristics of the equipment. This is particularly true for the circulation loops of the fan cooling towers, installation of reverse osmosis water desalination, etc.

In these processes, there is no significant heating of water or water is not heated at all, but there is an increase in the salinity of water due to the peculiarities of the technological process. As a result, the carbon dioxide balance is disturbed and the precipitation of calcium carbonate salts begins.

Effective pretreatment of water for these processes is the dosing of hydrochloric or sulfuric acid into the source water. As a result, bicarbonate turns into carbon dioxide, but chlorides and sulfates also appear in the water, respectively.

The main disadvantage of the acid-to-water dosing process is the complexity of controlling the dosing process.

If you add more acid to the water than there is bicarbonate in the water, then the water will have strong acids and a very low pH of the water will be observed. Such water will have very strong corrosive properties. If the acid dosage is less than that of bicarbonate, it is possible to precipitate temporary hardness salts in a water-using equipment.

It is customary to monitor the process of acid dosing using the pH value. That is, if after dosing the acid into the water, the pH value of the resulting water is 4.5, then this indicates that all bicarbonate has turned into carbon dioxide. For any specific ionic composition of water, you can determine (calculate) the pH value that will correspond to a specific amount of bicarbonate in the water. But the problem is that the accuracy of measuring the pH value in real conditions leaves much to be desired. The pH sensors require constant maintenance and have a very short service life.

It is much easier to control water quality by measuring the specific electrical conductivity of water. The sensors for measuring the conductivity of water are very reliable and accurate even in the minimal price range. Electrical conductivity sensors can work successfully in relatively dirty water, which is completely excluded for pH sensors.

This raises the question of how to change the electrical conductivity of natural water after adding hydrochloric or sulfuric acid to it.

It turns out that it is very significant. The experiment shows that by maintaining a certain value of the difference in the electrical conductivity of water before and after dosing acid into it, it is possible to  control successfully the residual value of bicarbonate in the treated water.

Let’s go straight to the example.

There is the source water of the following quality:

• Conductivity – 385 µS/cm
• Alkalinity-2.0 mg-EQ/l
• Calcium-2.1 mg-EQ/l
• рН – 7,1.

This water is used to feed the fan cooling tower.

If the source water is not pre-treated, a fairly intense precipitation of calcium carbonate salts occurs in the fan cooling tower. There are two reasons for this. First – intensive aeration of water takes place in the cooling tower and, accordingly, carbon dioxide is carried away from the water. The carbon dioxide balance is disturbed and calcium carbonate begins to precipitate. The pH value of the circulating water in this case is usually about 8.0-8.3. And the second – the circulation loop due to the evaporation of most of the water in the cooling tower increases the salinity of water and, accordingly, the concentration of bicarbonates and calcium.

In this case, removing bicarbonates from the make-up water will avoid salt loss in the circulation loop. The main difficulty is that if you completely remove the bicarbonate ion from the water, this water will be extremely corrosive. If you leave a little bicarbonate in the make-up water, the pH of the circulating water will increase due to the removal of carbon dioxide in the cooling tower and the multiplicity of circulation, which will avoid hydrogen corrosion of the circulation loop.

Let’s determine the required bicarbonate concentration in the cooling tower’s make-up water.

To do this, it is necessary to determine the concentration of carbon dioxide in the circulating water. Since intensive aeration and evaporation of water takes place in the cooling tower, carbon dioxide is distilled from the circulating water with air. Ideally, the concentration of carbon dioxide in the water will be equal to the concentration of carbon dioxide in the air. The concentration of carbon dioxide in the air is about 0.4 mg/l. If we assume that the aerated water reaches a CO₂ concentration of about 0.5 mg/l, then the circulating water will contain 0.5/44=0.011 mg-EQ/l. For further calculations, we assume that the circulating water contains 0.01 mg-EQ/l of CO₂.

We will take the multiplicity of circulation, which is equal to 4. In this case, the concentration of calcium in the circulating water will be 2.1*4= 8.4 mg-EQ/l. We determine the required bicarbonate concentration in the make-up water, provided that the Langelier index of the circulating water is about 0.3. This value of the Langelier index will allow to avoid the deposition of calcium carbonate and hydrogen corrosion.

The bicarbonate concentration will be 0.7 mg-EQ/l for the Langelier index =0.3, the calcium concentration =8.4 mg-EQ/l, the water temperature =35 ⁰C and the CO₂ concentration = 0.5 mg/l.

This calculation is quite complex. A separate article will be devoted to this in the future.

Thus, we come to the conclusion that the permissible concentration of bicarbonate in circulating water should not exceed 0.7 mg-EQ/l. Accordingly, the make-up water concentration should be 0.7/4=0.175 mg-EQ/l. Let’s assume that the bicarbonate concentration in the make-up water should be-0.2 mg-EQ/l.

To achieve this concentration of bicarbonate, we dose hydrochloric acid into the make-up water. As a result, the following chemical reaction occurs.

Ca(HCO₃)₂+2HCl -> CaCl₂+ 2H₂CO₃ ->CaCl₂+2CO₂+2H₂O

As you can see, hydrochloric acid destroys bicarbonate, converting it to carbonic acid. The resulting calcium chloride does not form scale.

Thus, the electrical conductivity of the source water in particular was determined by calcium and bicarbonate ions. After dosing hydrochloric acid, the electrical conductivity of water is determined by calcium and chloride. Chloride can transfer much more electrical charge than bicarbonate because of its mobility (Almost twice as much). Accordingly, the electrical conductivity of water after adding hydrochloric acid should increase. At the same time, the more hydrochloric acid is added and the more bicarbonate is destroyed with its replacement with chloride, the more the electrical conductivity of the treated water will increase.

Figure 1 shows the dependence of changes in the electrical conductivity of water on the residual concentration of bicarbonate. Thus if the conductivity of the source water is 385 µS/cm with an initial bicarbonate concentration of 2.0 mg-EQ/l, after adding hydrochloric acid in an amount of 0.5 mg-EQ/l, the conductivity of water is equal to 390 µS/cm. In this case, the bicarbonate concentration will be equal to 1.5 mg-EQ/l. If 1.5 mg-EQ/l of hydrochloric acid is added to the water, the residual bicarbonate concentration will be 2.0-1.5=0.5 mg-EQ/l and, accordingly, the electrical conductivity will be 433 µS/cm.

Figure 1

As you can see, changes in the electrical conductivity values relative to the source water are significant and can be easily determined using a fairly cheap, but reliable water conductivity sensor.

Thus, in order for the bicarbonate concentration in make-up water to be equal to 0.2 mg-EQ/l, the electrical conductivity of such water should be 470 µS/cm. Accordingly, the difference in the value of electrical conductivity should be equal to 441-385 = 56 µS/cm.

If the difference is greater, the bicarbonate concentration in the water will be less than 0.2 mg-EQ/l, or there will be no bicarbonate at all. In this case, the circulation loop will start to corrode. If the difference is less, then at a given multiplicity of circulation, temporary hardness salts may precipitate.

It is necessary to measure both the source and treated water to account for possible changes in the electrical conductivity of the source water.

The main disadvantage of this method of monitoring the residual concentration of bicarbonate in make-up water is that it is applicable in the case of using the same water source to feed the cooling tower, since different water sources are likely to have a fundamentally different ionic composition of water.

Figure 2 shows the dependence of the residual bicarbonate in the treated water on the pH value of the treated water. As you can see, the concentration of bicarbonate equalling to 0.2 mg-EQ/l will be in the water with a pH value of 5.0-5.5. If you control the process of acid dosing by the pH value.

Figure 2

Figure 3 shows the dependence of the electrical conductivity of treated water on the pH value.

Figure 3

I have developed a program for calculating the value of electrical conductivity from the ionic composition of water. On the basis of this program the calculation of the electrical conductivity of water for different ionic compounds was done. The calculated and experimental data presented in figures 1, 2, 3 had very good convergence.

The calculation program was used to calculate the electrical conductivity of water in the case of dosing it with sulfuric acid. The calculation results are shown in figures 4,5,6.

Figure 4

Figure 5

Figure 6

As can be seen, when dosing sulfuric acid, the increase in the value of electrical conductivity is not as significant as in comparison with hydrochloric acid. This is due to the fact that divalent sulfate carries less electric charge than monovalent chloride in the presence of divalent cations (in this case, calcium and magnesium).

However the difference in the electrical conductivity values is sufficient to control the sulfuric acid dosing process using the electrical conductivity value.

For example, if the source water has an electrical conductivity of 325 µS/cm and a bicarbonate concentration of 1.6 mg-EQ/l, the electrical conductivity of water at a bicarbonate concentration of 0.2 mg-EQ/l will be 352 µS/cm. The difference is 352-325=27 µS/cm.

It is possible to calculate or experimentally determine the value of the electrical conductivity of water depending on the residual bicarbonate and to control effectively the dosage of acid in the source water.

I hope this information will be useful!