Saturday, April 20, 2013

Solution pH of single conjugate acid- base pair

To calculate pH of a solution containing a single conjugate acid- base pair such as acetic acid and sodium acetate a simplified equation can be used. In order to use the equation, we need to know the concentration in Molarity for the acid (acetic acid), Ca, The concentration in Molarity for the base (sodium acetate), Cb, and Ka (acid ionization constant). The simplified equation is :

For example, if we have a solution with 0.1 M acetic acid (Ka = 1.76E-5) and 0.1 M sodium acetate then when we use the above equations we get pH = 4.76.

Another example, if we have a solution with 0.1 M ephedrine (weak base Kb = 2.3E-5 ) and 0.01 M ephedrine hydrochloride (conjugate acid Ka = 1E-14 / 2.3E-5 = 4.35E-10) then by applying the above equations with Ca = 0.01 M, Ka = 4.35E-10, and Cb = 0.1 M then we get pH= 10.36.

Keep in mind that having conjugate acid - base pair in solution makes a buffer. The above equation is actually another way to write the buffer equation (Henderson-Hasselbalch equation):

Feel free to write comments or questions.