How to Prepare a 50 mM pH 6 Citrate buffer starting from citric acid as the sole source of conjugate base.
Citric acid has three ionization constants; i.e., k1, k2, and k3.
pK1 = 3.13
pK2 = 4.76
pK3 = 6.39
Your tables may not agree exactly; use the numbers in your text/notes.
The pH you want in the solution should be as close as possible to one of the pKa values; therefore, I would use pK3.
Let's say you want to prepare 1 L of 0.05 M buffer. Weigh 0.05 mols citric acid solid, place in a conveniently sized flask, and add a NaOH solution, dropwise, until the pH reads 6.39, transfer the contents to a 1 L volumetric flask, add DI water to the mark on the flask, stopper and mix thoroughly. When finished you should have a solution that is 0.0355M in citric acid and 0.0145 M in NaH2C (sodium dihydrogen citrate) although the M concentration is not something that must be done before you have prepared the material. I did that just for the fun of it. If you will substitute those concentrations into the Henderson-Hasselbalch equation, you will see the pH is 6.00.
I am confused on the fact that if I am going to use the concentration that I got from the HH equation which was 0.0355M, then did Dr.Bob222 said that you weight 0.05 moles to add?
To prepare a 50 mM pH 6 citrate buffer starting from citric acid as the sole source of conjugate base, you will need to follow a series of steps.
First, let's consider the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the pKa of the acid/base pair:
pH = pKa + log([A-]/[HA]),
Where pH is the desired pH of the buffer solution, pKa is the acid dissociation constant of the acid/base pair, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid.
In this case, citrate is a weak acid, and we want to prepare a buffer at pH 6. The pKa of citric acid is around 3.13, which means the pH of the buffer should be higher than the pKa to favor the deprotonated form (conjugate base).
Here are the steps to prepare a 50 mM pH 6 citrate buffer:
Step 1: Calculate the concentration of citrate required.
To have a 50 mM buffer solution, you need to know the volume of the buffer you want to prepare. Let's assume you want to prepare 1 liter of buffer. In this case, the concentration of the citrate will be 50 mM (0.05 M).
Step 2: Calculate the ratio of [A-] to [HA] needed.
To calculate the ratio of [A-] to [HA] required for achieving pH 6, you will use the Henderson-Hasselbalch equation. Rearranging the equation, we get:
[A-]/[HA] = 10^(pH - pKa)
Substituting the values, we have:
[A-]/[HA] = 10^(6 - 3.13)
[A-]/[HA] = 10^2.87
[A-]/[HA] = 773.94
This means that the concentration of the conjugate base (citrate) should be around 774 times higher than the concentration of the acid (citric acid) to achieve a pH of 6.
Step 3: Prepare the buffer solution.
To prepare the buffer solution, you will need to dissolve the calculated amounts of citrate and citric acid in the desired volume of solvent, such as distilled water. Since the ratio of [A-] to [HA] is around 774, for every 1 mole of citric acid (HA), you will need approximately 774 moles of citrate (A-).
For example, if you want to prepare 1 liter of buffer at 50 mM concentration, you would dissolve 0.05 moles of citric acid and 38.69 moles of citrate in 1 liter of distilled water. These amounts are calculated using the molar masses of citric acid and citrate.
Once you have dissolved the citric acid and citrate in the solvent, you can adjust the pH of the solution to 6 using a pH meter and adding a small amount of a strong acid (like hydrochloric acid) or a strong base (like sodium hydroxide), if necessary.
By following these steps, you will be able to prepare a 50 mM pH 6 citrate buffer using citric acid as the sole source of the conjugate base (citrate).