How this calculation works
A polyprotic acid can occupy several protonation states at the same pH. One pKa comparison describes only a pair of adjacent states, so assigning the entire population from one comparison can miss important intermediate species. This page calculates all normalized fractions for one to six successive dissociations at an externally specified pH. It does not solve the pH of a prepared acid mixture from its total concentration.
Weight for species j = 10^(j pH − sum of first j pKa); fraction = weight/sum of weightsA worked example
For a monoprotic neutral acid with pKa 4.76 at pH 4.76, the two species have equal weights. Each fraction is one half and the mean formal charge is minus one half. With two pKa values, an intermediate species may dominate between them. Moving pH upward redistributes the total among states rather than creating additional material or changing the fraction sum.
Good to know
The charge convention is unsuitable without adjustment for an initially charged acid family, ampholytes with additional sites or coupled binding equilibria. This calculation concerns one sequence of ideal acid dissociations. Bulk electroneutrality and formal concentration are not solved. Use a separate equilibrium calculation when pH must be determined from added material rather than supplied as a known environmental condition.
Reference: OpenStax — polyprotic acids (opens in a new tab) ↗
What this calculation answers
A polyprotic acid can occupy several protonation states at the same pH. One pKa comparison describes only a pair of adjacent states, so assigning the entire population from one comparison can miss important intermediate species.
This page calculates all normalized fractions for one to six successive dissociations at an externally specified pH. It does not solve the pH of a prepared acid mixture from its total concentration.
The Weak acid or base equilibrium with water autoionization calculator solves the pH of one monoprotic weak species. An externally imposed pH here instead distributes a single family among several protonation states.
Use the right input conventions
Enter successive pKa values in nondecreasing order for one acid family. The fully protonated starting species is defined as neutral, so each removed proton decreases its formal charge by one.
The imposed pH must use conventions consistent with the supplied pKa values. The model treats the successive ratios ideally and does not automatically correct constants for solvent, temperature, ionic strength or counterions.
Interpret the result
The first result is the population-weighted formal charge. Individual rows show each species fraction and percentage; their fractions sum to one even when several states coexist substantially.
Weights are normalized in logarithmic form to avoid overflow when pH is far from a pKa. A numerically tiny displayed fraction therefore means the model population is small, not that a chemical species has been proven absent.
Work through the example
For a monoprotic neutral acid with pKa 4.76 at pH 4.76, the two species have equal weights. Each fraction is one half and the mean formal charge is minus one half.
With two pKa values, an intermediate species may dominate between them. Moving pH upward redistributes the total among states rather than creating additional material or changing the fraction sum.
Where the model stops
The charge convention is unsuitable without adjustment for an initially charged acid family, ampholytes with additional sites or coupled binding equilibria. This calculation concerns one sequence of ideal acid dissociations.
Bulk electroneutrality and formal concentration are not solved. Use a separate equilibrium calculation when pH must be determined from added material rather than supplied as a known environmental condition.
A few questions, answered
Does mean charge determine the solution pH?
No. Here pH is an input; the charge describes the acid-family population under that imposed condition.
What changes in the comparison?
Raising pH by two units shifts a monoprotic family toward its deprotonated state while preserving total fraction one.