What is a pH buffer?

Contributed by: NuclearNuggets
Thanks to: Shipperke
Submitted: 07-17-2003

Chemical equilibrium
To more deeply understand pH, we must first explore the concept of chemical equilibrium. The pH of pure water is considered neutral, because this is the point at which the autoprotolysis of water is just as favorable of a process as the reverse reaction. The chemical equation for the autoprotolysis of water is as follows:

H2O <- H+ + OH-

The equilibrium constant for this reaction is called Kw, which is equal to the product of the concentrations of hydrogen ion and hydroxide ion in solution in moles per liter (See Appendix 1 below to calculate the #moles/L)

The dissociation constant
Kw = [H+][OH-]

At this point, it is valuable to understand the p function itself. The p function of some value is equal to the negative logarithm of that value. So,

pH = -log[H+]
pKa = -log(Ka)

The value for Kw is 0.00000000000001 at 24?C, which is easier to write as pKw = 14. So, for pure water, we know that all H+ and OH- came from water molecules, and thus they are equal in number throughout the solution. Since they have the same value, we can use the above equilibrium expression as follows:

Kw = [H+][OH-] = [H+]2 = 10-14

Thus, [H+] = 10-7, and pH = 7.

Acids and Bases
Any chemical that increases the concentration of hydrogen ion in solution, or lowers the pH is an acid. Likewise, any chemical that increases the hydroxide concentration in solution is a base.

When an acid and its conjugate base are present in solution together, that solution is said to be a buffer, since it may react with acid or base without significant changes in pH. A hydroponic nutrient solution contains several conjugate acid-base pairs, since there are so many species present.

For a solution containing an acid HA and its conjugate base A-, the following equilibrium exists:
HA + H20 <- H3O+ + A-

For this protolysis equilibrium, the acid dissociation constant is given by:
Ka = [H3O+]*[A-]/[HA]

The pH is given by the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])

The term pKa refers to the p-function of the dissociation constant for that acid in water, similar to pKw for water. Notice from the equation above that as long as the acid and conjugate base are within one order of magnitude in concentration, additions of acid or base will not greatly affect the pH.

Buffering
The buffering capacity, or ability to resist change in pH, is greatest within one pH unit of the pKa for the acid. A complex equilibrium exists between the concentrations of all of the species present in the nutrient solution and the concentration of available hydrogen ions, making the nutrient solution a buffer over a very large range. This is why adding acid to pure water decreases the pH much faster than adding acid to the mixed nutrient solution.

Any species added to solution that can be either a proton donor (acid), or a proton acceptor (base), sets up a buffer.

ex)
You may have found that pure water you leave out in the air becomes slightly acidic over time. This is due to the absorption of CO2 from the atmosphere. The chemical process is as follows:

CO2 + H2O – H2CO3 <- H+ + HCO3-

Carbon dioxide reacts with water to form carbonic acid, which dissociates in water to hydronium and bicarbonate anion. This increases the concentration of H+ in solution, reducing the pH. The pKa of carbonic acid is 6.4, which is about the pH of pure water that has been exposed to the air.

ex) Potassium bicarbonate
Potassium is K+, bicarb is HCO3-. usually with diprotic acids like carbonic and sulfuric, the first H comes off pretty easily but since the ion has a -2 charge it holds onto the second proton fairly strongly.

Potassium bicarbonate is KHCO3, which dissociates to K+ and HCO3-. The bicarbonate anion can act as either an acid or a base. This makes it amphoteric.

Chelation
In chemical fertilization, EDTA salts are used as ?chelators?. The purpose is to form a more stable species in solution by using bidentate bonds. This means that the metal ion (such as Mg2+) will have two bonds for each EDTA molecule attached. This entropy of formation is higher for the EDTA complex, preventing the metal ions that you want to stay in solution from reacting to form insoluble compounds. Chelation makes the nutrient species more soluble, and thus more readily available for uptake.

What effect does pH have on elements in solution?
The element of interest to the plant must be present in an ionic form that can be transported by the roots. Changes in pH mean changes in concentration of H+ and OH-, which drive changes in equilibrium between various salt forms. For example… if the pH is too high, any available OH- will react with manganese or magnesium, or any of the various components of the nutrient solution.

Mg2+ + 2 OH- – Mg(OH)2

Magnesium hydroxide is not available for passive transport into the root system, but Mg2+ is. On a similar note, contamination by chlorine is bad for your solution, because MgCl2 is insoluble as well, and has a high rate constant of formation.

Appendix

1. Calculating Molar concentration
The molar concentration of a substance in solution is calculated by converting the mass of the substance into moles, and dividing that number by the liters of solution.
To make the conversion, you add up the atomic masses (from the periodic table of elements) for each atom in a single molecule of that substance. This is the molar mass. Divide the mass of the substance added to solution by the molar mass. This result is the number of moles. Divide this by the volume to get the molar concentration.

Let’s do an example:
We add 2.5g Epsom salts to 2 liters of water. The chemical formula for Epsom salt is MgSO4?7H2O.

The atomic masses are as follows:
Mg = 24.3 g/mol
S = 32.1 g/mol
O = 16.0
H = 1.01

Now remember to multiply each mass by the number of that species present in the molecule.
Total mass = 24.3 + 32.1 + (11*16.0) + (14*1.01) = 246.5 g/mol.

Now we convert grams to moles: 2.5g / 246.5 g/mol = 0.0101 mol.

Since we used two liters, we divide number of moles by 2, and the result, [MgSO4?7H2O] = [Mg+] = [SO4-] = 0.00507 mol/L = 0.00507 M.

Note: since Epsom salt is an ionic species, it dissociates in solution.