Hi, everyone.Recently, I want to change the acid buffer capacity of soil or soil colloid by using FeCl3 and Hcl. I found FeCl3 showed better performance in changing the acid buffer capacity of soil.Although I know the order of cation interchange, I can not fully understand why Fe3+ is more efficient than H+. Generally, hydrochloric acid is more acidic than ferric chloride.
But ionic radius of hydrogen ions is much smaller than ferric ions, thereby , former is more active , Birhanu...
Hi XIngxing,
Please see my answer to your other question. Humic acid is a polyelectrolyte or polyacid. It's pKa strongly depends on ionic strength of the surrounding medium. Why is Fe3+ more efficient? Because it is a trivalent ion and ionic strength (I) depends on the valency (V) of the ion; I is a function of CionV .....so Fe3+ is nine times more effective in increasing the ionic strength than a proton.
Kind regards, Leo
Higher charged central ions are better H+ donors and therefore more acidic. Fe3+ is therefore more acidic than H+. Similarly, FeCl3 is more acidic than FeCl2. Al3+ is a Lewis acid that creates H+ ions by attacking water molecules. Many metallic ions, like Fe3+, also act as Lewis acids.
Following link may be seen for an excellent presentation relevant to the topic:
http://www.css.cornell.edu/courses/260/Soil%20Acidity_Soil-Science-2600.pdf
Dear Nilay
Thank you for your kindly reply. However, I am wondering if it is right or suit to say that Fe3+ is more acidic than H+? I do not know how to judge the acidity of FeCl3 and HCl at the same concentration. Is lewis acid could be more acidic than Inorganic acid in some situations？
A nice presentation you provided! Thank you so much.
Sincerely
Xinxing Wu
But ionic radius of hydrogen ions is much smaller than ferric ions, thereby , former is more active , Birhanu...
So,
Strength of adsorption increases as:
-Cation valence increases
- Hydrated size decreases
- Strength of negative charge on colloid (soil) increases
Fe3+ is not more acid than H+. But H+ (or HCl) does not contribute to acid-base buffer capacity. How could it do that? Fe3+, like Al3+, does contribute to acid-base buffer capacity because of its character of Lewis acid (and also Brönsted acid). It reacts with water in the following way:
Fe3+ + H2O = FeOH2+ + H+
FeOH2+ + H2O = Fe(OH)2+ + H+
Fe(OH)2+ + H2O = Fe(OH)3 + H+
As long as there exist Fe3+ or hydroxy-iron species, when there is an input of H+ or OH- the above reactions go forward or backward, releasing or consuming H+. That is why soils rich in aluminium or iron are well buffered. (I am sorry that I cannot write subindices or superindices in the web page).
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