I am following a procedure that requires a 100 mM NaHCO3 buffer, pH 8.5. I understand that one way to make the buffer would be to dissolve 84 mg NaHCO3 in 10 ml of deionized water. Will this always result in pH of 8.5? If not, what do I need to do to adjust the buffer to that pH?
I'm not sure if I should be adding HCl/NaOH or whether I should only be adding additional NaHCO3 to change the pH (if necessary).
Thanks.
The carbonate/bicarbonate anion system has two pKa values, one at 6.4 and one at 10.3. Therefore, at pH 8.5, you will have both carbonate and bicarbonate present. One simple way to make your buffer is to make two 100mM solutions, one of NaHCO3 and one of Na2CO3. You can then mix various proportions of the two until you get the proper pH. Another way is to calculate the proportion using the Henderson-Hasselbach equation. In our lab, we rely on the measured value, not the calculated value.
The method of dissolving 100 mM NaHCO3 in water followed by adjusting the pH by adding HCl/NaOH will always work.
Calculating the mixture of NaHCO3 and Na2CO3 that gives the desired pH is possible, but you will in many cases end up doing a bit of adjustment with HCl/NaOH to get the pH correct.
If you add additional NaHCO3 to change the pH the buffer will not be 100 mM.
Per definition, a buffer is a solution that resists pH changes. Therefore using HCl or NaOH to adjust its pH is not the best idea, unless only very slight changes are needed; in addition this will diminish the capacity of your buffer (i.e. its ability to absorb H+ or OH- ions without too much change in pH).
The best way to adjust the pH is to make use of Henserson-Hasselbalch equation (pH = pKa + log[CO3 2-]/[HCO3 -] by prepraring 0.1 M solutions of sodium carbonate and hydrogenocarbonate (bicarbonate) and then mixing corresponding quantities.
Note that the bicarbonate-carbonate buffer works best in the pH range 9.2-10.8. For reaching 8.5, you will need a large amount of bicarbonate and very small amount of carbonate.
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