Prepare 1.5 liters of 0.25 M Tris buffer, pH 7.5.
Useful information:
pKa Tris = 8.1
Formula weight of Tris (hydroxymethyl aminomethane) base = 121.1
Formula weight of Tris hydrochloride (acid form) = 158.0
To prepare 1.5 liters of a 0.25 M Tris buffer with pH 7.5, you will need to know the Henderson-Hasselbalch equation, which relates the pH, pKa, and the ratio of the acidic and basic forms of Tris. The equation is as follows:
pH = pKa + log ([A-]/[HA])
Where:
pH = desired pH of the buffer (7.5 in this case),
pKa = dissociation constant of the acid (8.1 for Tris),
[A-] = concentration of the basic form (Tris base),
[HA] = concentration of the acidic form (Tris hydrochloride).
First, calculate the concentration of Tris hydrochloride (acid form) needed using the Molarity formula:
Molarity = moles of solute / volume of solution in liters
We want 1.5 liters of a 0.25 M Tris buffer, so the moles of Tris hydrochloride (acid form) can be calculated as:
Moles of Tris-HCl = Molarity × Volume
= 0.25 M × 1.5 L
Now, calculate the mass of Tris hydrochloride using the formula weight:
Mass = Moles × Formula weight
= Moles of Tris-HCl × Formula weight of Tris hydrochloride
Next, calculate the concentration of Tris base required using the Henderson-Hasselbalch equation. Rearrange the equation to solve for [A-]:
[A-] = 10^(pH - pKa) × [HA]
In this case, [HA] is the concentration of Tris hydrochloride you just calculated. Plug in the values:
[A-] = 10^(7.5 - 8.1) × [HA]
Calculate the moles of Tris base using the Molarity formula:
Moles of Tris base = Molarity × Volume
= 0.25 M × 1.5 L
Next, calculate the mass of Tris base using the formula weight:
Mass = Moles × Formula weight
= Moles of Tris base × Formula weight of Tris base
Now, add the calculated masses of Tris hydrochloride (acid form) and Tris base to 1.5 liters of distilled water and mix well to prepare 1.5 liters of a 0.25 M Tris buffer, pH 7.5.