Acids and Bases: Buffers

As you can see when calculating the pH of the solution, only a small amount of strong acid is needed to change the pH dramatically. However, in some experiments it is desirable to maintain a fairly constant pH while the acid or base is added to the solution by reaction or experiment. Buffer zones are designed to fulfill this role. Chemists routinely use buffers to adjust the pH of the reaction. Biological discovery buffers are used in a variety of applications, from regulating blood pH to preventing urine from reaching pain acid levels.

The buffer is only weak acid and its conjugate base, or a mixture of weak base and its conjugate acid. Buffering agents control pH by reacting with any added acid or base. For example, consider the role of a buffer consisting of weakly alkaline ammonia, NH 3, and its conjugate acid NH 4 +. When HCl is added to the buffer, the proton of the NH 3 "absorbed" acid becomes NH 4 +. Since protons are fixed to ammonium ions, protons do not significantly raise the pH of the solution. When NaOH is added to the same buffer, the ammonium ion supplies protons to the base and turns into ammonia and water. Buffers are also used to neutralize alkalis

As shown in the above example, the buffer acts by using a weak acid instead of a strong acid or strong base. The proton of strong acid is replaced with ammonium ion (weak acid). The strong base OH - is replaced by weak base ammonia. These strong acid and base substitutes give an extraordinary capacity to adjust the pH to buffer

Buffering agents must be selected to control their appropriate pH range. The pH range of the buffer is given by Henderson-Hasselbalch's equation. For the purpose of induction, we assume a buffer consisting of acid, HA and its conjugate base A -. The acid dissociation constant pKa of the acid is,

Take -log of this expression, rearrange the terms so that each becomes positive, and give Henderson-Hasselbalch's expression.

When hydrogen ions (protons) or hydroxide ions are added or removed, the acid-base buffer imparts resistance to changes in the pH of the solution. Acid-base buffer usually consists of weak acid and its base (salt). Buffers are effective because the concentration of weak acids and their salts is large compared to the amount of added or removed protons or hydroxide ions. When protons are added to the solution from an external source, some of the alkaline components of the buffer are converted to weak acid components (thus, most of the protons are used up); hydroxide ions are added to the solution (Or equivalently) remove protons from the solution; the protons dissociate from several weakly acidic molecules in the buffer and convert them to bases in the buffer (thus eliminating the replenishment of most protons) ).

The buffer is only weak acid and its conjugate base, or a mixture of weak base and its conjugate acid. Buffering agents control pH by reacting with any added acid or base. For example, consider the role of a buffer consisting of weakly alkaline ammonia, NH 3, and its conjugate acid NH 4 +. When HCl is added to the buffer, the proton of the NH 3 "absorbed" acid becomes NH 4 +. Since protons are fixed to ammonium ions, protons do not significantly raise the pH of the solution. When NaOH is added to the same buffer, the ammonium ion supplies protons to the base and turns into ammonia and water. Buffers are also used to neutralize alkalis

The concept of acid and base in Bronsted is suitable for biochemists. Bronsted acids such as acetic acid can provide protons. Bronsted bases such as acetate can accept protons. The buffer consists of a Bronsted acid and its complementary Bronsted base. Edsall and Wyman (Biophysical Chemistry, Academic Pr, 1958) show that acetic acid can be protonated in concentrated sulfuric acid (as base). (See Super Acid) Equation pH = pKa + log /, which is easily obtained by taking the negative logarithm Ka = / of simple equilibrium relationship. Usually written as pH = pKa + log /. This is a relic of the early days when most buffers were acidic. There are some common basic buffers like Tris. The equation is more general when pH = pKa = log /.