Question

Tris is a weak base. The basic form of tris can be written tris0 and the protonated (conjugate acid) form can be written tris. The pKa of tris is 8.1. a. What is the ratio of the basic and the acidic forms of tris at pH 6.1? b. What is the useful buffering range of tris?

Answer 1

a) [Tris0] : [Tris] = 1 : 100

b) Range = 7.1 to 9.1

Step-by-step explanation:

a) Calculation of ratio of the basic and the acidic forms of tris

pH of a buffer is calculate using Henderson-Hasselbalch equation

`pH = pKa+log(Salt)/(Acid)`

Conjugate acid of Tris dissociated as

`Tris \leftrightharpoons Tris0 + H^+`

For tris,

Salt or Basic form = tris0

Acid or Acidic form = Tris

pKa = 8.1

pH = 6.1

`pH = pKa+log(Tris0)/(Tris)`

`6.1 = 8.1+log(Tris0)/(Tris)`

`log(Tris0)/(Tris) = -2`

`(Tris0)/(Tris) = antilog (-2)`

`(Tris0)/(Tris) = 10^(-2)`

[Tris0] : [Tris] = 1 : 100

b) Range of Tris

Range of any buffer is:

From (pKa -1) to (pKa+1)

So, range of Tris is:

From (8.1 - 1) to (8.1 +1)

or from 7.1 to 9.1