Sodium citrate, or trisodium citrate, can buffer solutions in the pH range of about 3 to 6.2. Sodium citrate buffer, or citric acid buffer, is used for RNA isolation because it minimizes base hydrolysis of the RNA strands. This makes it an important step in the mRNA purification, during genomic research or for studying transcription. Citrate-based buffers can also aid the detection of antigens in fixed tissue preparations, because they break the cross-links formed between the antigens and the fixation media. Instructions below are for making a buffer of pH 6.
Time Required: 10 minutes
- There are two ways to go about making a sodium citrate buffer. First, determine whether you have both citric acid and the conjugate base, sodium citrate, to work with. You can still make the buffer if you only have citric acid.
- If you have both the acid and base, create a stock solution of each. Mix 21 g citric acid in 1 L distilled water, and 29.4 g sodium citrate in 1 L distilled water. If you only have citric acid, mix 2.1 g in just under 1 L distilled water and go to step 5.
- Mix 82 mL of the citric acid solution with 18 mL of the sodium citrate solution.
- Make the total volume of the mixture up to just under 1 L with distilled water. Leave enough volume to allow for addition of NaOH in the next step.
- Use 1M sodium hydroxide (NaOH) to adjust the pH of the mixture to 6.0, while gently stirring the solution using a magnetic stirrer.
- Make the final volume of the solution up to 1 L with distilled water using a volumetric flask.
What You Need
- citric acid
- 1M NaOH
- distilled water
- calibrated pH probe
- sodium citrate (optional)
- 1 L graduated cylinder
- 1 L volumetric flask
- Media bottles (3x 1L)
- magnetic stir bar
- magnetic stirrer