Solubility and pH

General Chemistry • Solubility and Complex Ion Equilibria

Written by STEM Calculators Team Published April 18, 2026 Updated April 18, 2026

Solubility and pH — Hydroxide Salts

Analyze sparingly soluble hydroxides of the form M(OH)_n(s) ⇌ Mⁿ⁺(aq) + n OH⁻(aq). This calculator can check precipitation at a chosen pH or pOH, or compute the threshold pH at which precipitation just begins. Pure water at 25 °C is assumed, so K_w = 1.0 × 10⁻¹⁴.

Quick data paste or CSV

Recognized keys: mode, n, M, Ksp, M0, pKind, pVal, precision.

Import CSV

Hydroxide salt

Cation symbol

Hydroxide stoichiometry n

M(OH) M(OH)₂ M(OH)₃

Core inputs

K_sp

Initial [Mⁿ⁺]₀ (mol·L⁻¹)

Displayed significant figures

Mode 1 — Check precipitation at a given pH or pOH

pH specification

Give pH Give pOH

Value of pH or pOH

Ready

Rate this calculator

0.0 /5 (0 ratings)

Be the first to rate.

Your rating

Name (optional) Review (optional)

You can update your rating any time.

Solubility and pH for metal hydroxides

For sparingly soluble hydroxides, solubility depends strongly on pH. The tool models:

\[ \mathrm{M(OH)_n}(s) \rightleftharpoons \mathrm{M^{n+}}(aq) + n\,\mathrm{OH^-}(aq) \]

with

\[ K_{sp} = [\mathrm{M^{n+}}]\,[\mathrm{OH^-}]^{\,n} \]

At \(25^\circ\text{C}\): \(K_w = [\mathrm{H_3O^+}][\mathrm{OH^-}] = 1.0\times10^{-14}\), so

\[ \mathrm{pH} = -\log_{10}[\mathrm{H_3O^+}],\quad \mathrm{pOH} = -\log_{10}[\mathrm{OH^-}],\quad \mathrm{pH} + \mathrm{pOH} = 14.00. \]

Ion product \(Q_{sp}\) and precipitation

For a solution with known \([\mathrm{M^{n+}}]_0\) and pH (or pOH), the ion product is

\[ Q_{sp} = [\mathrm{M^{n+}}]_0\,[\mathrm{OH^-}]^{\,n}. \]

\(Q_{sp} < K_{sp}\): unsaturated, no precipitation.
\(Q_{sp} \approx K_{sp}\): saturated, at equilibrium.
\(Q_{sp} > K_{sp}\): supersaturated, precipitation occurs.

In “Check precipitation at given pH” mode the calculator:

Converts pH or pOH to \([\mathrm{OH^-}]\) using \(K_w\).
Computes \(Q_{sp} = [\mathrm{M^{n+}}]_0[\mathrm{OH^-}]^{n}\).
Compares \(Q_{sp}\) with \(K_{sp}\) to decide if precipitation occurs.
If \(Q_{sp} > K_{sp}\), uses \[ [\mathrm{M^{n+}}]_{\text{eq}} = \frac{K_{sp}}{[\mathrm{OH^-}]^{n}} \] to estimate the remaining dissolved metal and the fraction precipitated.

Threshold pH (onset of precipitation)

At the pH where precipitation just begins, only a tiny amount has precipitated, so \([\mathrm{M^{n+}}]_{\text{crit}} \approx [\mathrm{M^{n+}}]_0\). Then

\[ K_{sp} \approx [\mathrm{M^{n+}}]_0\,[\mathrm{OH^-}]_{\text{crit}}^{\,n} \quad\Rightarrow\quad [\mathrm{OH^-}]_{\text{crit}} = \left(\frac{K_{sp}}{[\mathrm{M^{n+}}]_0}\right)^{1/n}. \]

From this the tool finds \(\mathrm{pOH}_{\text{crit}}\) and \(\mathrm{pH}_{\text{crit}} = 14 - \mathrm{pOH}_{\text{crit}}\). For pH values higher than \(\mathrm{pH}_{\text{crit}}\), the hydroxide becomes less soluble and precipitation is favored.

pH is treated as known (e.g. from a buffer or separate acid–base calculation). All relations assume \(25^\circ\text{C}\) so that \(K_w = 1.0\times10^{-14}\).

Frequently Asked Questions

How does pH affect the solubility of metal hydroxides?

Metal hydroxides dissolve to produce OH-, so changing [OH-] changes the ion product and the position of equilibrium. Higher pH (higher [OH-]) generally favors precipitation and lowers solubility.

What is Qsp and how is it used to predict precipitation?

Qsp is the ion product computed from current ion concentrations, using the same form as Ksp. If Qsp is greater than Ksp, the solution is supersaturated and precipitation is favored; if Qsp is less than Ksp, no precipitation is expected.

What is the threshold pH for precipitation?

The threshold pH is the pH at which precipitation just begins, meaning Qsp is approximately equal to Ksp. The calculator finds the critical [OH-] from Ksp and [M^n+]0, then converts it to pOH and pH at 25 C.

Why does the calculator assume pH + pOH = 14.00?

At 25 C, Kw is taken as 1.0 x 10^-14, so pKw = 14.00 and pH + pOH = 14.00 follows directly. If temperature changes, Kw can change and the relationship may differ.

Does this tool account for buffers or complex-ion formation?

No, pH is treated as an input value and the model focuses only on the hydroxide solubility equilibrium using concentrations. Effects such as buffering capacity, complex ions, or activity coefficients are not included.

Solubility and pH

Solubility and pH — Hydroxide Salts

Hydroxide salt

Core inputs

Mode 1 — Check precipitation at a given pH or pOH

Mode 2 — Threshold pH for onset of precipitation

Calculation steps

Rate this calculator

Frequently Asked Questions

Solubility and pH — Hydroxide Salts

Hydroxide salt

Core inputs

Mode 1 — Check precipitation at a given pH or pOH

Mode 2 — Threshold pH for onset of precipitation

Calculation steps

Rate this calculator

Frequently Asked Questions

Related calculators

Questions related to this topic