Predict accurate acid/base dissociation constants from structure—the industry standard

The acid dissociation constant, Ka, is a measure of the tendency of a molecule or ion to keep a proton (H+) at its ionization center(s). It is related to the ionization ability of a chemical species and is a core property that defines chemical and biological behaviour. The pKa prediction model from ACD/Labs offers the following functionality:

  • Calculation of accurate acid and base pKa constants (pKa = -logKa) under standard conditions (25C and zero ionic strength) in aqueous solutions for every ionizable group within organic structures
  • Confidence intervals for all predicted pKa values, indicating their accuracy

Available Algorithms

The pKa prediction model incorporates two different predictive approaches—Classic and GALAS


The ACD/pKa Classic calculation algorithm may be trained by adding experimental data and is widely accepted as the industry standard for pKa prediction.

The algorithm uses Hammet-type equations and electronic substituent constants (σ) to predict pKa values for ionizable groups. Effects considered by the software include tautomeric equilibria, covalent hydration, and resonance effects in α, β-unsaturated systems.

Hammet-Type Equations—every ionizable group is characterized by several Hammet-type equations that have been parameterized to cover the most popular ionizable functional groups.

Sigma Constants—the internal training set contains >3000 derived experimental electronic constants. When the required substituent constant is not available from the experimental database, one of four algorithms are used to describe electronic effect transmissions through the molecular system.

Other technical characteristics of this algorithm include: Click to expand


Estimation of ionization constants using the GALAS algorithm is a multi-step procedure involving estimation of pKa microconstants for all possible ionization centers in the hypothetical state of an uncharged molecule ("fundamental microconstants"), with numerous corrections to these initial pKa values according to the chemical environment of the reaction center, and calculation of charge influences of ionized groups to neighbouring ionization centers. The Calculation routine utilizes a database of 4600 ionization centers, a set of ca. 500 various interaction constants and four interaction calculation methods for different types of interactions, producing a full range of microconstants from which pKa macroconstants are obtained. This allows for the simulation of a complete distribution plot of all protonation states of the molecule under different pH conditions.

Other technical characteristics of this algorithm include: Click to expand

ACD/Labs Product Suites

The Percepta prediction modules are available as bundles to offer cost savings for multiple modules, and provide related modules as a package.

Other Products

ACD/pKa Model predictions are available as a browser-based thin client application on ACD/Percepta Portal; a thick client product (ACD/Percepta for the Desktop); and a batch calculator (ACD/Percepta Batch).