Buffers and Zwitterions

The September ‘08 issue of BYO has an interesting article on buffers that describes in pretty good detail how they work. The chemistry is slightly over my head, but I am able to make enough sense of it to understand the main points.

To start, it helps to know that PH is equivalent to the negative log of Hydronium ions (ph= –log |H30+|), which essentially means the concentration of hydronium ions. In water, a difference from the neutral ph of 7 occurs when the levels of H+ and OH- ions become unbalanced. Concentrations with more H+ have lower PH and are hence considered acids. Concentrations with more OH- have a higher PH and are hence considered bases. It’s best not to stray too far from the Hydronium description, because this is the heart of how buffers work.

Essentially, a buffer is a combination of a weak acid and its conjugate base, that when placed in solution balance each out in a way that holds the PH of the solution at a particular range. It’s not until additional acid is added at significant levels to overwhelm the balance that the PH can then drop at a standard rate. How does this work, its fairly intriguing actually. Take carbonic acid (H2CO3) and bicarbonate (HCO3) for example. Note that the difference between the acid and the base is at least one hydrogen ion. In solution the acid would disassociate one hydrogen ion and combine with water to create a new Hydronium ion (H30+). The extra hydronium ions would then combine with the extra CO3- ions to form a new HCO3+ or H2CO3+ ion. This process will continue in equilibrium holding the PH steady until additional acid is added is sufficient quantity to overwhelm the extra CO3- ions. This explains how Buffer 5.2 is able to hold the PH in the 5.2 range and why sometimes when adding acid you hit plateaus where the PH doesn’t change much.

The other aspect of buffers is the buffering capacity of the grain and produced wort. One of the primary bi-products of wort production are proteins to break down the starchs into sugars. This proteins are primarily composed of amino acids which also effect the PH of the wort. Amino acids are composed of a central carbon atom surrounded by 4 arms of ions depending on the type of amino acid. At a certain PH, called the isoelectric point, an amino acid will have both negative and positively charged arms. The resulting molecule has a neutral net charge and will buffer a solution at the isoelectric point, which is different for each amino acid. This type of molecule is called a zwitterion (german for hermaphrodite) because of the opposing charged arms. “Just as a weak acid and base will buffer a solution, a zwitterion will buffer a solution at its isoeletric point. If a acid is added to a solution, the minus end will absorb some of the hydrogen ions. If a base is added, the plus end will absorb some of the negation ion.”

The important thing to take away from the amino acids is that they are in greater concentration in the wort than the buffers in your brewing water. This means that the PH of your brewing water really isn’t that important. The article suggests that because of this the PH of your mash will be in the 5-6 range without any additional acid or bases, but I have never tried tested this. I have always used buffer 5.2 in the mash and adjusted the PH down to 5.2 using a little lactic acid. I will test the PH next time before the buffer to see what its stabilizes at. Unless my brewing water has some serious minerals, I imagine the article may be correct.