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Drybench Calc.
Tris pH and pKa at your working temperature
A buffer's pKa at 4, 25 or 37 °C from Goldberg et al. 2002, and where a pH set at 25 °C ends up on ice.
Buffer pKa at temperature
A buffer's pKa at your working temperature, from the thermodynamic quantities of Goldberg, Kishore and Lennen 2002.
Reaction: HL+ = H+ + L · Formula weight 121.14 g/mol
goldberg-2002Full buffer recipes with ionic strength: phbuffers.org
Answer
- pKa at 4 °C
- 8.71
- pK 8.072 at 25 °C and zero ionic strength. Linear approximation -0.0279 per °C near 25 °C.goldberg-2002
- Estimated pH at 4 °C
- 8.64
- A buffer set to this pH at 25 °C reads about this at 4 °C.goldberg-2002
Open in the bench sheetThe bench sheet carries these inputs and offers the other calculators.
Worked example
- Tris: pK 8.072 at 25 °C, ΔrH° 47.45 kJ/mol, ΔrCp° -59 J/(K mol) (Goldberg et al. 2002).
- At 4 °C, by the paper's equation 12, the pK is 8.71; the linear slope near 25 °C is about −0.028 per degree.
- A Tris buffer set to pH 8.00 at 25 °C reads about 8.64 on ice: the ratio of base to acid does not change, the pK does.
Method
- pK at the working temperature. Goldberg RN, Kishore N, Lennen RM. Thermodynamic quantities for the ionization reactions of buffers. J Phys Chem Ref Data 2002;31:231-370 (read 6 October 2026).Checked against Goldberg's equation 12 in NumPy 2.4.6 within 0.01 on 15 buffers at 4, 25 and 37 °C (6 October 2026).
Constants
- goldberg-2002: Goldberg, Kishore and Lennen 2002, 298.15 K, zero ionic strength
Limits
- For research and education only, and not medical advice: the stock, dilution and buffer calculations here are not for dosing or clinical preparation.
- pKa values are thermodynamic values at zero ionic strength from Goldberg et al. 2002; ionic strength and buffer concentration shift the working pKa and are not applied here, so a buffer's final pH is for a meter to confirm.