Calculate pH, pOH, and hydrogen ion concentration. Visualize the pH scale with color indicators and buffer capacity curves.
Enter any one of pH, pOH, or hydrogen ion concentration and the tool derives the other two, then places the result on a 0-14 scale banded in indicator colors, with a buffer capacity curve for buffered solutions. The arithmetic rests on two defining relations, pH = -log10[H+] and pH + pOH = 14, the second a consequence of water's ion product at 25 degrees C. The step most people trip on: the whole scale assumes 25 degrees C and dilute solutions, so a 10^-8 M HCl solution is not pH 8 - water's own hydrogen ions make the real answer about 6.98, still acidic.
A concrete input and expected output from the current implementation.
Input
Hydrogen ion concentration [H+] = 2.5 x 10^-4 mol/L
Expected output
pH = 3.60, pOH = 10.40, [OH-] = 3.98 x 10^-11 mol/L. The marker is placed at 3.60 on the 0-14 scale, in the orange-red (acidic) band.
By definition pH = -log10(2.5 x 10^-4) = 3.60. At 25 degrees C, Kw = [H+][OH-] = 1.0 x 10^-14, so pOH = 14 - 3.60 = 10.40 and [OH-] = 10^-10.40 = 3.98 x 10^-11 mol/L; the two concentrations multiply back to about 1.0 x 10^-14, confirming consistency.
The three quantities come from two relations. pH = -log10[H+] by definition, inverted to [H+] = 10^-pH; and pH + pOH = 14, which follows from the water ion product Kw = [H+][OH-] = 1.0 x 10^-14 at 25 degrees C, inverted to [OH-] = 10^-pOH. Entering one value fixes the others, and moving 1.0 unit on the pH scale multiplies or divides the concentration by ten.
The result is plotted on a 0-14 band colored like common indicator dyes: red-orange at the acidic end, green near 7, blue to violet at the basic end. The bands track real transition ranges, such as methyl orange (3.1-4.4), bromothymol blue (6.0-7.6), and phenolphthalein (8.2-10.0). For buffers, the capacity curve shows pH change per added acid or base; capacity peaks at pH = pKa, the Henderson-Hasselbalch midpoint.
No - neutral means [H+] = [OH-], which lands at pH 7 only at 25 degrees C because Kw = 1.0 x 10^-14 there. As temperature changes, so does Kw: at 37 degrees C neutral is about 6.8, at 0 degrees C about 7.5. This tool computes 25 degree C values, which suits room-temperature lab work but not biological or industrial conditions.
Match the indicator's transition range to the pH at the equivalence point, not the starting pH of the solution. A strong acid-strong base titration ends near pH 7, so bromothymol blue (6.0-7.6) fits; a weak acid titrated with strong base ends above 7, where phenolphthalein (8.2-10.0) is the usual pick. The scale's color bands show these ranges directly.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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