Could someone please explain why many stock buffers ask you to adjust the solution's pH and subsequently increase the volume to a set amount?
For example:
Add 500g of powder X to 800mL of water.
Adjust the pH to 7.4.
Bring up the volume to 1L with additional water.
Doesn't this alter the pH of the stock solution? Or is there a generally-understood convention/standard procedure when preparing buffers to do this sort of thing so that it's universally known that a buffer with pH 7.4 doesn't actually have an exact pH of 7.4?
This question might be really simple, but it's something I've never understood and can't seem to find an answer to online. Any information would be greatly appreciated.
One reason for this procedure is the fact that if you add salts to the water, the end volume is unknown. Concentrations are defined as grams per L, which means that the end volume (not the added volume!) needs to be known exactly. You are right that the dilution of your adjusted solution may change the pH. In addition, the pH of some buffers is quite sensitive to temperature changes. BTW: Good buffer protocols (usually consisting only of two salts and water) do not require the adjustment of pH, only a final check.
The effect of dilution on buffered solution is insignificant.
The reason for adjusting the pH before bringing the solution up to the final volume with water is that if you calculate that 500 grams of powder X will give you C concentration of solution in 1 L of water, ...
and you add 500 grams of X to 1 L of water, it can take over 20 or 50 mL of acid or base to reach the proper pH. If the concentration of acid or base that you are using is very dilute then it will take even more acid or base to reach the proper pH. At that point the concentration of your solution is not "C". It is more dilute.
The point is follow the instructions and use a pH meter because you really don't know how much acid or base it will take to adjust the solutions pH. Also buffers are actually only valid buffers at pH = pKa +/- 1. Some buffers have more than one ionizable proton, so a broader range of pH is allowable for certain buffers. Furthermore, some chemicals only dissolve under acidic or basic conditions. For example, EDTA will not dissolve until pH >= 8 regardless of concentration.
I still do like the instructions and then add a few drops of acid or base near to the final volume to make sure the pH is correct.
One reason for this procedure is the fact that if you add salts to the water, the end volume is unknown. Concentrations are defined as grams per L, which means that the end volume (not the added volume!) needs to be known exactly. You are right that the dilution of your adjusted solution may change the pH. In addition, the pH of some buffers is quite sensitive to temperature changes. BTW: Good buffer protocols (usually consisting only of two salts and water) do not require the adjustment of pH, only a final check.
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