Having buffers such as Tris present can help protect against this; while these solutes also undergo transient concentration shifts during freezing and thawing, the ratio of acidic to basic forms of the buffer pair
should remain nearly constant and help prevent pH swings in the resulting microenvironments.
Each buffer system is made up of two compounds, together referred to as a buffer pair. One of the members of the pair is an acid because it is capable of donating a proton, thus lowering pH.
Table 1 Physiological Buffer Systems Buffer Pair [H.sup.+] Acceptor [H.sup.+] Donor Bicarbonate HC[O.sub.3.sup.-] [H.sub.2]C[O.sub.3] Phosphate HP[O.sub.4.sup.2-] [H.sub.2]P[O.sub.4.sup.-] Ammonia N[H.sub.3] N[H.sub.4.sup.+] Protein Protein Protein Buffer Pair Reaction pKa Bicarbonate HC[O.sub.3.sup.-] 6.1 + [H.sup.+] [left and right arrow] [H.sub.2]C[O.sub.