Metal precipitation by pH

How much of each metal stays dissolved as the pH rises, precipitated as its hydroxide or as its sulfide. Pick the metals in your water and a discharge limit: the green band is the pH where every one of them is below it.

Discharge limit mg/L

No single pH gets every metal below 0.5 mg/L: aluminium climbs back above it at pH 7.1, before manganese(II) falls below it at pH 10.2. Two stages would: one near pH 6.3, then one near 12.1.

At pHopens where the most of your metals, 4 of 6, meet the limit; tap the chart or type a pH to move it

Iron(III)✓ under 0.5

Aluminium✗ over 0.5

Copper✓ under 0.5

Zinc✓ under 0.5

Nickel✓ under 0.5

Manganese(II)✗ over 0.5

Metal	Lowest at pH	Dissolved there	Below 0.5 mg/L between
Iron(III)	8.5	3.8 × 10^-5 mg/L	pH 3.7 to 13.3
Aluminium	6.3	0.17 mg/L	pH 5.6 to 7.1
Copper	9.7	0.0023 mg/L	pH 6.9 to 12.9
Zinc	9.6	0.44 mg/L	pH 9.3 to 10.0
Nickel	10.2	0.047 mg/L	pH 9.0 to 12.1
Manganese(II)	12.1	0.0036 mg/L	pH 10.2 to 14.0

Learning zone

Reading the curves, and why a plant reads higher

Each curve falls as the pH rises, because more hydroxide pushes the metal out as a solid, then reaches a minimum, the best precipitation pH for that metal. Past it, zinc, lead, aluminium and chromium climb again: their hydroxides dissolve back as hydroxo complexes. That rebound is why one pH cannot always clear a mixed water, and why a plant treating zinc and manganese together often runs two stages.

These are equilibrium curves at 25 °C for clean water. A real effluent sits higher. Ammonia, EDTA, citrate and other complexers hold metal in solution; sulphate and carbonate change the solid that forms; fine particles that pass the clarifier count as dissolved on a filtered sample; and a young sludge is more soluble than an aged one. Iron works the other way: ferric hydroxide floc co-precipitates other metals, and arsenic and selenium, below what their own curves would allow.

Sulfides are far less soluble than hydroxides, often by many orders of magnitude, and work at a lower pH, which is why they reach limits a hydroxide cannot. The curves depend on how much dissolved sulfide is left, so set that box to the residual you run. In practice that residual is a hazard and a cost: hydrogen sulfide off-gasses below about pH 8, excess sulfide must be removed before discharge, and the sludge is finer and harder to settle.

Where these constants come from

Every constant is from one database, so the set agrees with itself, and each one links to the lines it sits on.

Parkhurst, D.L. and Appelo, C.A.J. (2013). Description of input and examples for PHREEQC version 3. U.S. Geological Survey Techniques and Methods, book 6, chapter A43. The program the database ships with.
minteq.v4.dat, the MINTEQA2 version 4 database distributed with PHREEQC 3 (file revision 11091, 21 April 2016), at commit 9d08ff5.
Hydrogen sulfide, H+ + HS− = H2S: lines 2670 to 2671.
Iron(III): Ferrihydrite, Fe(OH)3 (lines 10993 to 10995); FeOH2+ to Fe(OH)4− (lines 1006 to 1031).
Iron(II): Fe(OH)2 (lines 10989 to 10991); FeOH+ to Fe(OH)3− (lines 982 to 999); FeS, fresh precipitate (lines 10545 to 10547).
Aluminium: Al(OH)3, amorphous (lines 10865 to 10867); AlOH2+ to Al(OH)4− (lines 686 to 711).
Chromium(III): Cr(OH)3 (lines 11033 to 11035); Cr3+ (lines 526 to 527); CrOH2+ (lines 1094 to 1095); Cr(OH)3, dissolved (lines 1102 to 1103); Cr(OH)4− (lines 1110 to 1111). The file writes chromium(III) on Cr(OH)2+; these values are its reactions rewritten on Cr3+ using lines 526 to 527, and the conversion is ours.
Copper: Cu(OH)2 (lines 10953 to 10955); CuOH+ to Cu(OH)4 2− (lines 838 to 863); Covellite, CuS (lines 10513 to 10515).
Zinc: Zn(OH)2, amorphous (lines 10897 to 10899); ZnOH+ to Zn(OH)4 2− (lines 750 to 775); ZnS, amorphous (lines 10465 to 10467).
Nickel: Ni(OH)2 (lines 10965 to 10967); NiOH+ to Ni(OH)3− (lines 894 to 911); NiS alpha, the fresh precipitate (lines 10525 to 10527).
Cadmium: Cd(OH)2, amorphous (lines 10925 to 10927); CdOH+ to Cd(OH)4 2− (lines 782 to 807); Greenockite, CdS (lines 10477 to 10479).
Lead: Litharge, PbO (lines 10845 to 10847); PbOH+ to Pb(OH)4 2− (lines 606 to 647); Galena, PbS (lines 10457 to 10459).
Manganese(II): Pyrochroite, Mn(OH)2 (lines 11017 to 11019); MnOH+, Mn(OH)3−, Mn(OH)4 2− (lines 1054 to 1071); MnS, green (lines 10561 to 10563).

Revised 16 October 2026

Cite this page

formula.expert. (2026, October 16). Metal precipitation by pH: hydroxide and sulfide. https://formula.expert/reference/metal-precipitation