I have seen in literature people using buffers such as PBS with pH 7.4, and dilute it with acid or base to create lower pHs like 2, 3,4 or higher pHs , will it still be called a buffer once buffering capacity is lost?
The buffering capacity (ability to resist changes in pH due to the addition of acid or base) diminishes as the pH of the solution gets farther from the pKa of the buffering group, and as the concentration of the buffering group is reduced. Sometimes, people use the term buffer incorrectly to mean the solution in use, whether or not it is actually acting as a pH buffer.
I agree Adam B Shapiro , but I have to use different pH ranges in my experiment , for that do I need to use different buffer system with different buffering capacity? Does it alter my experimental results because different buffers have different chemical composition and they will interact with my material accordingly. I am a bit confused here
Different buffers at the same pH can result in different effects on the function of your experiment. This could be due to the chemical composition and/or the ionic strength and/or the counterion. A good practice would be to test different buffer substances at the same pH to see if such an effect is occurring.
It isn't really possible to use a single buffer component for a very broad range of pH. There are some buffer systems that can cover a wide pH range by combining multiple components with different pKas.
Bis-tris-propane is an organic buffer that has a broad useful pH range of 6 to 9.5 due to having 2 pKas. Phosphate has multiple pKas (2.1, 7.2, and 12.7), allowing it to be used over 3 different pH ranges, but there are gaps in coverage.
The suitable pH range for a buffer is within 1 pH unit of the pKa, preferably 0.5 unit.
Some useful information about individual buffers and some two-component buffers can be found here:
https://www.sigmaaldrich.com/US/en/technical-documents/protocol/protein-biology/protein-concentration-and-buffer-exchange/buffer-reference-center
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