I have designed 15-Runs with different substrate concentration, pH, and temperature parameters in order to produce hydrogen from cow dung + algae through dark fermentation technique. As per the literature, volatile fatty acids would generate which I have seen in a few runs as their pH has decreased. But in some Runs, it shows the opposite behavior and increased (4.97 to 5.77, 5.03 to 5.56, and so on). Please tell me why is it happening.
The increase in the pH here must be due to the presence of air, lead to decomposition along with production of volatile fatty acids this will definitely increase the pH value
The increase in pH increase may be due to the consequences of anaerobic fermentation and the production of VFA
pH is measurement of the ionisation of molecules. see the Herdenson Hasselbalch equation:
pH = pKa + Log Base/Acid.
As the equilibrium shift to the acid a lower pH, more H3O+ molecules are formed, the opposite if the base increases.
However, pH does not measure concentrations, just the equilibrium between both bases and acids.
For instance these two solutions have exactly the same pH. Although the concentrations is 10 greater..
pH = pKa + log 10/10 and pH = pKa log 100/100.
For VFA equilibrium the HH is
pH = pKa + log diss SCFA/undiss SCFA
In reality, there is a relationship between all molecules concentrations in a solution which depends on the temperature and the ionic strength of the fluid according to the Debye-Hückle limiting law for diluted solutions. But, that is another story.
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