How to prepare carbonate and bicarbonate buffer

Written by john brennan
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How to prepare carbonate and bicarbonate buffer
Bicarbonate buffer solutions are useful for certain biochemical experiments. (mot solution image by Frédéric Massard from Fotolia.com)

Bicarbonate buffers can sometimes be useful for experiments in which non-phosphate buffers are preferred. One of the problems with preparing a bicarbonate/carbonate buffer, however, is that adding HCl to lower pH to the target value will cause carbon dioxide to be lost from solution. There is, however, a procedure that sidesteps this problem and will give you a buffered solution with a pH of about 10.3.

Skill level:
Moderate

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Things you need

  • Sodium bicarbonate
  • Deionised water
  • Calibrated pH meter
  • Sodium carbonate
  • Spatula
  • 2 beakers
  • Pipette
  • Scale
  • Stir bar

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Instructions

  1. 1

    Using the pipette, add 100ml of deionised water to the beaker.

  2. 2

    Measure 6g of sodium bicarbonate, add it to the deionised water and stir. This will prepare a supersaturated solution.

  3. 3

    Insert the calibrated pH meter and measure the pH of your solution; it should be about 8.4.

  4. 4

    Slowly add sodium carbonate to the solution while watching the pH until it reaches 10.3.

  5. 5

    Decant the clear liquid portion of your solution into the second beaker. Discard the remaining sediment.

  6. 6

    Measure the pH of your solution and dilute it to the desired volume.

  7. 7

    Seal tightly when storing.

Tips and warnings

  • A bicarbonate-carbonate buffer is best for target pH values within 1 of 10.3, which is the pKa2 value for carbonic acid.

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