Citric acid buffer

Submitted by kdorfman on Tue, 11/16/2021 - 20:03

Citric Acid – Na2HPO4 Buffer Preparation, pH 2.6–7.61

From Sigma Buffer Reference Center

Citric acid monohydrate, C6H8O7 • H2O, MW 210.14; 0.1 M contains 21.01 g/L.

Na2HPO4, MW 141.98; 0.2 M contains 28.40 g/L, or Na2HPO4 • 2H2O, MW 178.05; 0.2 M contains 35.61 g/L.

pH x mL 0.1 M-citric acid y mL 0.2-Na2HPO4
2.6 89.10 10.90
2.8 84.15 15.85
3.0 79.45 20.55
3.2 75.30 24.70
3.4 71.50 28.50
3.6 67.80 32.20
3.8 64.50 35.50
4.0 61.45 38.55
4.2 58.60 41.40
4.4 55.90 44.10
4.6 53.25 46.75
4.8 50.70 49.30
5.0 48.50 51.50
5.2 46.40 53.60
5.4 44.25 55.75
5.6 42.00 58.00
5.8 39.55 60.45
6.0 36.85 63.15
6.2 33.90 66.10
6.4 30.75 69.25
6.6 27.25 72.75
6.8 22.75 77.25
7.0 17.65 82.35
7.2 13.05 86.95
7.4 9.15 90.85
7.6 6.35 93.65

For 2021 QBoC

Need pH 3,4,5,6,7

Start with

  • Citric acid (100 mM) (borrowed from P-Chem)
    • 2.65 g
    • 125 mL H2O
  • Sodium phosphate dibasic (200 mM)
    • MW = 268.07
    • 200 mM = 8.042 g
    • 150 mL H2O

Then make

pH x mL 0.1 M-citric acid y mL 0.2-Na2HPO4
3.0 39.725 10.275
4.0 30.75 19.275
5.0 24.25 25.75
6.0 18.425 31.575
7.0 8.825 41.175

Test pH of 1:10, 1:2

  • pH remains constant when diluted to 20% with LB.
  • So make 50 mL of each diluted to 40% with LB. Students will dilute it 1:1 with LB
    • 20 mL buffer
    • 30 mL water
  • Filter steriize, aliquot for students