Lab Life Lesson #6 (continued):
Do you need an acidic buffer? If so, what pH should you use?
In our February newsletter, we discussed why you should use a simple acidic solution using phosphoric acid rather than using a phosphate salt. In this issue we want to continue the discussion, and explain further why many people (incorrectly) use the wrong pH for their buffer.
Remember the acid-base dissociation equilibrium:
When the pH is equal to the pKa of the acid, the two forms of the acid (HA and A-) are present in equal amounts. When the pH is one unit away from the pKa, then the ratio of the two forms is 10:1. That is, at one pH unit
below the pKa, the ratio of [HA]/[A-] is 10. At two pH units below the pKa, the ratio is 100, meaning that about 99% of the acid is in the HA form. Essentially, the solution contains only the HA form. At more than two pH units away from the pKa, the ratio is even larger, so smaller and smaller fractions are in the A- form.
These calculations are the origin of the "two pH units away from the pKa rule - if you are two pH units away from the pKa, then almost all of the compound is present in only one form." Unfortunately, many people have incorrectly interpreted this rule to mean that you should be
exactly two pH units away, when you actually just need to be
at least two units away!
Most weak organic acids have pKa values at about 4.2 and larger. So, any pH at 2.2 and smaller will keep almost all weak acids in their HA form (neutral, extractable into organic solvents, more retention in LC). At a pH of 6.2 and larger, almost all acids will be in their A- form (ionic, more water soluble, less retention in LC). The pH does not need to be exactly two units away.
To summarize, if you have weak organic acids present in your solution, and you want the acids in their neutral, HA form, then keep the pH near 2. You can use any acid that is compatible with your system. You usually will not need an actual buffer. If you want the acids in their ionic A- form, keep the pH at or larger than about 6.5. In this case you will need to use a buffer such as ammonium formate to maintain this pH.
Are you having problems choosing, preparing, or using buffers in your systems? Call us for a review and assistance. We have a long record of saving labs time and money on their procedures . . . but you have to contact us to see the benefit. |
The article "Investing to Save" by Dr. Merlin Bicking has been published in the December issue of Lab Manager Magazine.
This article discusses how laboratories can properly calculate the benefits of an investment in process improvements, technology changes, and even training. By learning to calculate Return on Investment (ROI), laboratory staff have more tools to sell their proposals to management.
The article is available on-line. Click here to view.
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ACCTA, Inc. |
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