Buffer Calculator

Introduction

A buffer is an aqueous solution able to preserve an approximately constant pH even when exposed to small amounts of acids or bases. To calculate the specific pH of a buffer, you need to use the Henderson-Hasselbalch equation. If the buffer consists of a solution of a weak acid of concentration [HA] and its conjugate base of concentration [A-] (e.g. acetic acid and sodium acetate), the equation has the form:

pH = pKa + log
[A-]
[HA]

Where pKa is the negative base-10 logarithm of the acid dissociation constant. If the buffer consists of a solution of a weak base of concentration [B] and its conjugate acid of concentration [BH+] (e.g. Tris and its protonated form), Henderson-Hasselbalch equation takes the form:

pH = pKa + log
[B]
[BH+]

Usage

Buffer Calculator allows you to calculate any of the four values (pH, pKa, [HA], [A-]) if you provide the other three values. So in addition to pH value calculations from pKa and concentrations, the app can also help you to calculate the buffer composition required to obtain a target pH value. As the pH value is determined by the ratio of dissociated and non-dissociated form of the weak acid or base, these concentrations can be easily derived from the required total concentration of the buffer. For example, if we are to prepare a 100 mM sodium acetate buffer solution of pH 4.0, we will first obtain the required ratio of acetate and acetic acid by entering 4.0 for pH, 4.76 for pKa and e.g. 10 for [A-]. The calculator will return 57.544 as the [HA] concentration, so the required proportion of acetate in the mixture with acetic acid is 10/(10+57.544) = 0.148. For the total 100 mM concentration we will prepare a solution containing 14.8 mM sodium acetate and 85.2 mM acetic acid.


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