Te recomiendo revisar la química de Buffer, existen variadas formulaciones donde el citrato puede actuar como una solución amortiguadora hasta pH 6.

Hola Nicolas,

La solución de ácido cítrico no tenia  base conjugada para que fuera un buffer, el pH era bien bajo. En la mayoría de los suelos causó una reducción del pH proporcional a la dosis, pero en solo en uno de los suelos el pH subió. En este caso el pH del suelo paso de 6.1 a 6.45 lo que esta cerca del pKa3 (6.4) del ácido. Que reacciones químicas podrían haber pasado causando este cambio? La hipótesis de que esta variación fuera causada por la microbiota me hace sentido, pero no veo una explicación química directa. Si tienes una te agradezco.

Saludos,

Daniel

I made the same observation with oxalic acid, but the rise in pH was not immediate, suggesting a biotic process most probably linked to metabolization of organic acids: if citric acid is absorbed then it disappears from the soil to be changed into carbon dioxide, further lost by diffusion in the atmosphere, then protons are consumed and pH rises up to an equilibrium level when all organic acids have been transformed by the microbial community. The soil is transformed in an external gut. The same rise in pH is observed in the gut of soil animals when they ingest acid soil. The phenomenon does not occur at high concentration most probably because of saturation of the microbial community, and it may vary from a soil to another depending on the microbial strains present and the availability of nutrients, in particular phosphorus and nitrogen, which are needed for the process to occur.

See:

https://www.researchgate.net/publication/41101590

Article Characterization of Proton Production and Consumption Associ...

May be due to the presence of reactive metals present in the soil, it may form complexes. That is why the excess dosage of citric acid may reduce the pH of the soil.

Sure, complexing with metals will produce protons, which are released from the hydrated organic molecule

Thank you Jean, Harish and Christina

 I had the same observation that soil pH increased with increasing concentration of citric acid. The increase might be associated with lower Al due complexation reaction with the acid forming chelate. Unfortunately, my publication was in Bahasa Indonesia as you can see in attachment

Thank you Zainal

Dear Daniel, Citric acids are weak acids meaning they slowly release protons. The less hydrogen ions held by the exchange complex of a soil considering the basic ions held, the greater the alkalinity of the soil.

The suggestion of microbial process dominating the response for non-toxic concentrations is almost certainly correct.  Aside from pasteurisation/sterilisation you could investigate the effect of microbial inhibitors.  Chloroform and toluene are frequently used and are moderately effective.

Dear Daniel, 

we had an experiment on water, if you add citric acid to lower EC (below 1500) probably you can see increasing pH but with high EC especially caused bu Sodium chloride you can see decreasing pH dramatically.  I think Cation of citric acid make HCl.   So it decreasing od increasing depends on your Soil's anion.

regards,

Javanshah

 What type of soil? The soils that had an increase in pH with low doses of citric acid, how would you characterize them? 

Perhaps the soil had prior bases in the soil, and the addition of citric acid (a weak acid), will result is partial dissociation, thus making a buffer. Once the reactants are neutralised, it will leave citric acid's conjugate base (citrate), which acts like a weak base, that may cause pH to increase.

I'm not 100% if this makes sense though.

I understand that before comment and suggesting the mechanism of increase in pH, it is essential to know the physical and chemical properties, the pH was measured in saturated paste or some soil to water suspension, type of clay minerals and OM contents. Without knowing these facts, it will be a matter of throwing stones into air.

Hi all, the data was published here in 2016 (Article Organic Acids Regulation of Chemical–Microbial Phosphorus Tr...

We found that pH decreased after CA application.

https://www.tandfonline.com/doi/abs/10.1080/03650340.2016.1249477?journalCode=gags20

https://link.springer.com/article/10.1007/s42729-021-00662-2

What type of soil is acidic or alkaline? It is possible that the added acids began to dissolve the bases, which led to an increase in the pH

I think its because it Release calcium citrate.

Calcium oxalate

May be bacterial flora are utilising citrate for growt. That is what happens in cancers as well. Citrate lyase Utilises NADH to reduce oxaloacetate to malate, and Malic enzyme prodduces NADPH to promote redox homeostasis in oxPPP.