Determination of the carbon dioxide balance of water

The article shows how all forms of carbon dioxide in water can be determined by performing only one chemical analysis.

Carbon dioxide dissolved in water largely determines its corrosive properties. As a result of the dissolution of carbon dioxide (CO₂), carbonic acid (H₂CO₃) is formedinwater.Therefore, wateracquirescorrosiveproperties.Ifsuchwatercomes into contactwithmetals,thencorrosionwithhydrogendepolarization is possible.Allnaturalwatercontains carbon dioxide.Inthis case, threeforms of carbon dioxide are distinguished:free,semi-bound andbound.

The free form of carbon dioxide is carbon dioxide gas, which, when dissolved in water, provides its acidic properties. That is, when carbon dioxide gas is dissolved, an excess of hydrogen ions is formed in the water. Semi–bound carbon dioxide is the bicarbonate ion (H₂CO₃). The bound carbon dioxide is the carbonate ion (CO₃).

If only bound and semi-bound forms of carbon dioxide are present in the water, then such water does not contain “free” hydrogen ions, and corrosion with hydrogen depolarization does not occur in such water. This circumstance is important to understand in order to avoid corrosion in water supply systems, heating networks, supply paths of steam and hot water boilers and boilers themselves, as well as various water-using equipment. To determine the carbon dioxide balance of water, it is enough to perform only one analysis. Formally, these are two analyses, but it is quite easy to combine it into one analysis.

Let’s look at an example. There is a heating network with a maximum water temperature of 95 °C. This network should be supplied with softened, deoxygenated and decarbonized water. In this case, we are interested in the extent to which the water of the heating network and the supply water of the heating network are decarbonized, or in another way, whether the free form of carbon dioxide has been completely removed from it.

Carbon dioxide is dissolved in water in two stages (a two-stage process of carbon dioxide hydrolysis)

Stage 1:

СО₂ + Н₂О = Н₂СО₃ = Н⁺ + НСО₃^–

Stage 2:

2НСО₃^– = СО₃^– + Н₂СО₃ = СО₃^– + СО₂ + Н₂О

In the first stage, carbon dioxide in water dissociates into the hydrogen cation (H) and bicarbonate anion (HCO₃). In the second stage, carbonate anions (CO₃) and carbonic acid are formed in water. The second stage of the process is possible with the removal or neutralization of the resulting carbon dioxide.

Our goal is to determine whether there is free carbon dioxide in the water, which ensures the course of the hydrolysis process in the 1st stage. If free carbon dioxide is present, then such water is corrosive for the given conditions of the heating network, and this water will require complete removal of free carbon dioxide before it enters the heating network. That is, the carbon dioxide balance of such water is shifted towards dissolution (corrosion).

To determine the carbon dioxide balance of water or to determine the values of all three forms of carbon dioxide in a given water, it is necessary to conduct one analysis consisting of two existing actively used chemical analyses. This is an analysis of water for alkalinity by phenolphthalein and methylorange and an analysis for free carbon dioxide. These analyses are clearly interconnected. In this case, in contrast to the standard alkalinity analysis, another reagent (titrating solution) will be required. It is a solution of caustic soda (NaOH) with a concentration of 0.1 mol/l (0.1 n).

The sequence of the analysis. We take a sample of water – 100 ml. Add the phenolphthalein indicator to the water. If the water sample is stained, it means that there is no free carbon dioxide in the water. The pH value of such water is more than 8.3 pH units. Then we perform a standard water analysis for alkalinity. Titrate the sample with a solution of hydrochloric acid (0.1 n) until complete discoloration. The amount of hydrochloric acid used for titration is equal to the concentration of carbonates (plus hydrates under certain conditions) in mg-eq/l in the sample. Then we add the methyl orange indicator to the sample and continue titrating with hydrochloric acid solution until the color changes. The amount of hydrochloric acid solution used for titration in ml will be equal to the concentration of bicarbonates in the sample in mg-eq/l. Thus, there is no free carbon dioxide in this sample; semi–bound bicarbonate and bound carbonate are present. This water does not cause corrosion with hydrogen depolarization. The carbon dioxide balance of such water is shifted towards precipitation.

If the sample is not stained after the addition of phenolphthalein, then we begin to titrate it with a solution of caustic soda until a stable pink color appears. The amount of caustic soda used for titration in ml will be equal to the amount of free carbon dioxide in the sample in mg-eq/l. For example, titration took 0.3 ml of 0.1 n. NaOH. Then the concentration of free carbon dioxide in the sample will be equal to CO₂ = 0.3 mg-eq/l, or 0.3 *44 = 13.2 mg/l. Next, the methyl orange indicator is added to the sample and a standard alkalinity test is performed.

Such water already contains free carbon dioxide and corrosion with hydrogen depolarization is observed in it under the conditions of the heating network. This water will require either the distillation of carbon dioxide, or the addition of alkaline reagents to bind free carbon dioxide into semi–bound bicarbonate.

The following chemical reactions take place during this analysis.

Option 1. If there is free carbon dioxide in the water.

Titration with caustic soda solution in the presence of phenolphthalein.

СО₂ + NaOH = NaHCO₃

After all the free carbon dioxide is converted to semi-bound, the sample will turn red. The pH value of such a sample is 8.3 pH units.

Titration with hydrochloric acid solution in the presence of methyl orange.

NaHCO₃ + НСl = NaCl + CO₂ + H₂O

After all the bicarbonate decomposes with the release of CO₂, the color of the sample changes. The pH value of such a sample is 4.5 pH units.

Option 2. If there is no free carbon dioxide in the water.

In this case, a routine phenolphthalein and methylorange alkalinity test is performed.

It is necessary to take into account the fact that after adding a solution of caustic soda, the amount of bicarbonate in the sample will increase by the amount of free carbon dioxide. Therefore, when calculating bicarbonate by the amount of hydrochloric acid solution used for titration in the presence of methyl orange, it is necessary to subtract the value of the caustic soda solution (which spent on titration) from the obtained value of the spent hydrochloric acid solution in ml.

For example, for our heating network. After adding phenolphthalein to the sample, its color did not change. After adding 0.3 ml of 0.1 n sodium hydroxide solution to the sample, the sample acquired a stable pink color. This means that the sample contains 0.3 mg-eq/l (13.2 mg/l) of free carbon dioxide. Then the sample was titrated with hydrochloric acid solution. The color change occurred after adding 2.3 ml of acid solution to the sample. This means that the bicarbonate concentration in the sample is 2.3-0.3 = 2.0 mg/l. This is exactly the difference from a conventional alkalinity test.

Let’s look at another example in which the inhibition of corrosion processes takes a slightly different path.

As an example, there is a drinking water supply system using steel pipelines. A deep river is used as a source of water supply. The initial river water has the following chemical composition in terms of carbon dioxide forms:

рН = 7.8

НСО₃ = 2.5 mg-eq/l

СО₂ = 0.1 mg-eq/l, or 0.1*44 = 4.4 mg/l.

This water is in carbon dioxide equilibrium. The conditions in the river have developed in such a way that the CO₂ in the water is in equilibrium with the surrounding conditions (CO₂ in the air, CO₂ from various organic oxidation processes, etc.). At the same time, neither the dissolution of solid calcium carbonate nor its formation is observed. That is, under river water conditions, corrosion processes will be insignificant. But river water undergoes water treatment by coagulation with aluminum sulfate. As a result of aluminum hydrolysis, additional free carbon dioxide appears in the water, which leads to a decrease in the pH value. After water treatment, the water has a pH value of 7.0 and a bicarbonate value of 2.1 mg/l. Some of the bicarbonates decompose to form a hydrate to carry out the aluminum hydrolysis process.

It turns out that 2.5-2.1 = 0.4 mg-eq/l of bicarbonate decomposed to form hydrate and free carbon dioxide. The hydrate went to aluminum, and free carbon dioxide dissolved in water and caused a decrease in pH. Moreover, the amount of free carbon dioxide formed during the hydrolysis of aluminum is 0.4 mg-eq/l or 0.4 * 44 = 17.6 mg/l. In total, taking into account the initial free carbon dioxide, the water after water treatment contains 4.4+17.6 = 22 mg/l of CO₂.

Now the carbon dioxide balance of the water is shifted towards corrosion. In this case, corrosion of steel pipelines with hydrogen depolarization will occur. As a rule, these corrosion processes occur rather slowly due to the low water temperature. At the same time, the outlet water itself after water treatment meets all regulatory requirements.

The production of an analysis to determine the carbon dioxide balance of water using the method presented in the article makes it quite easy to track the shift in carbon dioxide equilibrium and we can understand whether corrosion processes or precipitation processes will be observed.

This technique is quite simple, but it allows you to evaluate the carbon dioxide equilibrium of water for any purpose.