I mixed 1M NaCl with kaolinite, and the pH decreased significantly. I do not know why. If you have any papers or documents related to this, please help me. Any help is highly appreciated!
Please see the below links
https://www.gov.mb.ca/agriculture/crops/soil-fertility/soil-fertility-guide/soil-ph-and-salinity.html
http://www.tandfonline.com/doi/abs/10.1080/00103620802625377?journalCode=lcss20
Dear Professor Ewald Schnug,
You mean the dissociation of H+ from clay minerals? but Do you think that high cation concentration of Na+ affects the pH electrode?
For the saline soils, how to measure exactly the pH value?
Many thanks in advance!
I think there may be a number of different scenarios in which increased NaCl would lead to a lower pH in kaolinite suspensions. I would guess surface charge effects are most likely involved. Remember that kaolinite possesses both permanent and variable charge. The exact mechanism responsible could depend on the initial state of the kaolinite, i.e. what pH it had and the initial composition of the diffuse layer. Examples: if the pH is low, increased Na+ could displace Al3+ from the diffuse layer, which would decrease pH due to Al hydrolysis. If the pH is high, an increased ionic strength would lead to more >AlOH2+½ sites being converted to >AlOH-½ sites, which again causes acidification. Concerning kaolinite surface charge, see e.g. Schroth & Sposito (Clays Clay Miner. 45:85-91, 1997) and Tombasz & Szekeres (Appl. Clay Sci. 34:104-124, 2006)
Many thank Prf.Jon Petter Gustafsson and Prf. Ewald Schnug. Your information is very helpful!
The theory is described in the applied literature around the concept of the 'point of zero charge'.
Dear Duong Nguyen:
I think that the Kaolinite have high potential acidity (Great amount of H+ adsorbed). Perhaps, due to the way it was extracted. When the NaCl is applied, the Na + ions cause a displacement of the H + ions towards the soil solution. This causes a decrease in the pH value due to the higher amount of H+ ions in solution (active acidity).
Best regards
I think , responses from Ewald and Pablo are worth considering . Presence of kaolinites in any soil dominates under acidic soil condition having high potential acidity , which are formed under highly leached soil conditions. addition of NaCl will lead to desorption of hydrogen ions from kaolinites adsorption sites and cause a decrease in pH. The nice arguments have been given by Jon Petter in this regard ...Under what condition , diffuse layer will undergo changes leading to decrease or increase in soil pH. My understanding is , presence of kaolinites depicts the highly acidic soil conditions under the influence of high precipitation , facilitating leaching of bases...
Dear Prf. Jon Petter Gustafsson; Prf. Anoop Kumar Srivastava,
You please help me to explain more why the increase ionic strength would lead to more >AlOH2+½ sites being converted to >AlOH-½ sites, which again causes acidification.
Many thanks in advance!
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