Applications
Features
Calculation Examples pHcalc

In the following table, the calibrated measured and guaranteed pH values of SRM-buffers from the NIST internet site are compared with pHcalc Buffer Composer calculations. Please note, that these Buffers are ideally diluted and that the computations may yield a larger difference from measured pH values with higher concentrations and other substances.

Standard buffer Molality
[mol/kg]
guaranteed pH value
(NIST-SRM)
pHcalc Ver 2.0
pH calculation
KH2PO3
Na2HPO3
0.025
0.025
6.863 6.879
KH-phthalate 0.050 4.006 3.988
NaHCO3
Na2CO3
0.025
0.025
10.001 10.005
HEPES free acid
Na - HEPESate
0.025
0.025
7.364 7.370
MOPSO free acid
Na - MOPSOate
0.025
0.025
6.699 6.708
MOPSO free acid
Na - MOPSOate
0.04
0.04
6.676 6.679

In the following table, some standard buffers from literature [1] are compared with pHcalc Buffer Composer calculations

Standard buffer Molality
[mol/kg]
measured
pH [1]
pHcalc Ver 2.0
pH calculation
KH2-Citrate 0.05 3.776 3.739
TRIS*HCl
TRIS [2]
0.050
0.01667
7.788 7.780
AcOH
NaOAc
0.025
0.025
4.644 4.622

[1] Humeres, Quijano & Sierra, J.Braz.Chem.Soc. Vol 1 No3, 1990

[2] The temperature coefficiant of the pK value of Tris(hydroximethyl)aminomethane is in the room temperature range (about 15-25°C) approximately -0.0287 pK/°C: pKa(15°)=8.326, pKa(25°)=8.075,