| Sample case | Mode | Key input | Expected pH (25°C) | Note |
|---|---|---|---|---|
| 0.010 M strong acid (n=1) | Strong acid | C=0.010 | 2.00 | Monoprotic, complete dissociation |
| 0.100 M strong base (n=1) | Strong base | C=0.100 | 13.00 | Monohydroxide, complete dissociation |
| 0.100 M weak acid (Ka=1.8e−5) | Weak acid | C=0.100 | 2.88 | Quadratic solution |
| Buffer with equal acid/base amounts | Buffer | pKa=4.76 | 4.76 | Henderson–Hasselbalch |
- pH = −log10([H+])
- pOH = −log10([OH−])
- pH + pOH = pKw (at a chosen temperature)
- Kw = 10^(−pKw) and [H+][OH−] = Kw
- Strong acid: [H+] ≈ C·n
- Strong base: [OH−] ≈ C·n
- Weak acid: solve x^2 + Ka·x − Ka·C = 0, x=[H+]
- Weak base: solve x^2 + Kb·x − Kb·C = 0, x=[OH−]
- Buffer: pH = pKa + log10(n_base/n_acid)
Note: very concentrated solutions may require activity coefficients for best accuracy.
- Pick a calculation mode that matches your chemistry.
- Enter your concentration, constants, or buffer amounts.
- Set pKw if temperature differs from 25°C.
- Press Calculate to view results above the form.
- Use Download CSV or Download PDF for reporting.
Why pH Matters in Aqueous Chemistry
pH summarizes hydrogen ion activity on a logarithmic scale, letting you compare acidity across samples quickly. A one‑unit pH change reflects a tenfold change in effective [H+]. In routine aqueous work, values near neutral indicate balanced [H+] and [OH−], while lower values signal corrosive or reactive acidity. Higher values suggest alkalinity that can affect solubility, precipitation, and reaction rates. In environmental monitoring, pH shifts alter metal solubility, microbial activity, and treatment chemical demand significantly.
Choosing the Right Input Mode
This calculator supports several laboratory scenarios. Use direct [H+] or [OH−] when you have ion concentrations from calculations or titration points. Select strong acid or strong base when dissociation is effectively complete and equivalents n represent released H+ or OH− per formula unit. Choose weak acid or weak base when Ka or Kb governs equilibrium. For buffer solutions, enter pKa and the base‑to‑acid amount ratio.
Temperature and the pKw Setting
Water autoionization changes with temperature, so the pKw setting lets you adapt results beyond standard conditions. When pKw shifts, the neutral point becomes pKw/2 rather than a fixed value. Keeping pKw consistent across a dataset improves comparability, especially when samples are warmed, cooled, or measured in process streams. If you are unsure, use your instrument’s temperature‑compensated reference or a validated handbook value.
Weak Electrolytes and Exact Solutions
For weak acids and bases, the calculator solves the quadratic equilibrium expression instead of relying on the x≪C shortcut. This matters when solutions are dilute, the dissociation constant is relatively large, or accuracy is critical for specifications. The output also reports percent ionization, which helps confirm whether the weak‑electrolyte assumption is reasonable. If percent ionization is high, simplified approximations can noticeably shift pH.
Interpreting Outputs and Reporting
Results include pH, pOH, [H+], and [OH−], plus a neutral reference based on your chosen pKw. Use classification to label solutions as acidic, basic, or effectively neutral for quick screening. Ion concentrations support downstream calculations such as corrosion risk, cleaning validation, and buffer capacity checks. Exporting CSV aids traceable records, while the PDF report is useful for audits, lab notebooks, and client deliverables.
FAQs
1) What is the difference between pH and pOH?
pH tracks hydrogen ion level, while pOH tracks hydroxide ion level. In water they are linked by pH + pOH = pKw, so knowing one lets you compute the other for the same temperature.
2) When should I use the buffer mode?
Use buffer mode for a weak acid/conjugate base pair where both components are present. Enter pKa and the base-to-acid amount ratio to apply the Henderson–Hasselbalch relationship.
3) Why can results fall outside 0–14?
The 0–14 range is a common approximation for dilute aqueous solutions near room temperature. Concentrated acids or bases, high ionic strength, or different pKw values can shift apparent pH beyond that range.
4) How do I pick a pKw value?
Use 14.00 for standard 25°C work. For other temperatures, use a trusted reference table, your meter’s temperature-compensated settings, or validated lab SOP values to keep results consistent.
5) How are weak acid and weak base cases solved?
The calculator solves the equilibrium quadratic for x instead of using the x≪C shortcut. This improves accuracy for dilute solutions or larger Ka/Kb values, and it reports percent ionization for context.
6) Can I apply this to non‑aqueous solvents?
Not reliably. pH definitions, dissociation behavior, and autoprotolysis constants differ in other solvents. Use solvent-specific models and constants, or report acidity using methods appropriate to that medium.