The sodium salt of EDTA and calcium carbonate
I have long wanted to find out the question, how does trilon B interact with metal cations?
I’ll make a reservation right away that the conversation will be about low-pressure steam boilers.
Trilon B is the sodium salt of EDTA (Ethylenediaminetetraacetic acid) – Na2 – EDTA (Na2 – H2B).
Trilon B forms stable complex compounds with metal cations. The alkaline environment is the most favorable for the formation of a complex compound with a metal cation. This is due to the fact that the metal cation is replaced by two hydrogen cations. As a result, the pH of the environment decreases due to the appearance of free hydrogen cations, and the process of formation of a complex compound is inhibited. In an alkaline environment, hydrogen cations bind and the process continues.
Let’s look at the diagram of EDTA forms depending on the pH value.
Figure 1 shows this diagram.
Figure 1
This diagram is constructed using the following dissociation constants (their negative decimal logarithm pK):
рК1 = 1,99; рК2 = 2,67; рК3 = 6,16; рК4 = 10,26.
According to the diagram in Fig. 1, at a pH value of an aqueous solution of 4 or less, the following reaction occurs in the solution:
Н4В = Н+ + Н3В–1 (рК = 1,99)
That is, there is a free hydrogen cation and an anionic residue in the solution. At a pH of about 4, the solution contains only Н2В–2 and a free hydrogen ion.
If you start adding alkali to the solution to bind the hydrogen cation, then further dissociation of the anionic residue will begin.
Н3В–1 = Н+ + Н2В–2 (рК=2,67)
Н2В–2 = Н+ + НВ–3 (рК = 6,16) (1)
НВ–3 = Н+ + В–4 (рК = 10,26)
It turns out that the mosteffectivecomplexcompoundwithmetalcations will beformedat a pH of more than 9.
Now the question arises. With which metal cations will trilon B form complex compounds, with cations in the dissolved state or in the form of a solid precipitate?
The fact is that from time to time information appears about the possibility of washing boilers during their operation from all kinds of deposits, including calcium carbonate. To do this, it is recommended to add trilon B to the feed water of the steam boiler. Obviously, trilon B will bind the possible residual hardness of the feed water, and a stable complex compound will arise in the alkaline environment of boiler water. I.e., this residual hardness will not participate in the possible formation of scale or sludge in the boiler. But will an excess of trilon B in the feed water form complex compounds with metal cations of carbonate deposits in the boiler?
The issue is quite controversial. It can be assumed that before a complex compound is formed, the metal cation must be transferred to a dissolved state from the precipitate. In order to transfer calcium from the solid state of calcium carbonate to dissolved calcium bicarbonate, it is necessary that free hydrogen ions appear in the water, i.e. the environment becomes at least slightly acidic.
It turns out that the steam boiler must operate in a slightly acidic environment. This is a rather ambiguous decision. In any case, this is not the design mode of operation of the boiler.
In order to make sure that trilon B in the alkaline state is not able to dissolve and capture metal cations (at least calcium carbonate), the following simple experiment was performed.
Solidcalciumcarbonatewasadded to themetalcontainer.Then0.1n. (g-eql/l)trilonB solutionwasadded to thecontainer. The pHvalue of thissolution is 4.2-4.4.Thencausticsodawasadded to the solution.With the help of causticsoda, the pHvaluewasincreasedto9.1-9.2.Thenthesolutionwasboiledwithsolidcalciumcarbonate. The initialcalciumcarbonatecan be viewedinFig.2. The result of boiling is showninFig.3.
Figure 2
Figure 3
Conclusion: Calcium carbonate has not dissolved. The fact is that Trilon B was enough to dissolve several such volumes of calcium carbonate. But visually, the dissolution process was not observed. Interestingly, there was no formation of carbon dioxide bubbles that characteristic of the typical process of dissolution of calcium carbonate by acid.
Then 0.1n. trilon B solution was added to the same amount of calcium carbonate without the addition of alkali. The pH value of the solution turned out to be 4.2-4.4. During the heating process, active release of carbon dioxide bubbles was observed. The solution was not even brought to a boil. The temperature was maintained at about 80-90 0C. As a result, almost all of the calcium carbonate was dissolved (Fig. 4). A small amount remained, but most likely, the initial content of calcium carbonate was not 100 %.
If you continue to actively boil the solution and bicarbonates are contained in it, then, its pH value will increase and trilon B forms a stable compound with calcium.
It turns out that first, it is necessary to dissolve a solid precipitate of calcium carbonate by a standard reaction involving hydrogen (acid), and only then, a complex compound will be formed. Therefore, the current guidance documents indicate that trilon B must be mixed with other acids, for example, citric acid, in order to bring the pH of the solution to a value of 3.0.
Figure 4
Is it possible to wash the steam boiler from scale with trilon B during its operation? I think it’s technically possible. But the boiler will operate in a acidic mode, which is not its design mode of operation, with all possible consequences. Theoretically, the pH value of boiler water should be at least slightly less than 8.2. Only below this pH value free hydrogen cations will be present in boiler water, obtained by equation (1). In this case, corrosion of the boiler with hydrogen depolarization is possible. We can only hope for the inhibitory properties of trilon B.
