pH Calculator

Q: What is the Henderson-Hasselbalch equation and when do I use it?

The equation pH = pKa + log([A-]/[HA]) calculates the pH of a buffer solution. Use it when you have a mixture of a weak acid and its conjugate base. Maximum buffer capacity occurs when [A-] = [HA], meaning pH = pKa. The effective buffer range is pKa +/- 1 pH unit.

Q: How do I prepare a PBS buffer at pH 7.4?

For 1 liter of 10x PBS stock: dissolve 80 g NaCl, 2.0 g KCl, 14.4 g Na2HPO4, and 2.4 g KH2PO4 in distilled water, verify pH 7.4, and fill to 1 L. Dilute 1:10 for working concentration (137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, 1.8 mM KH2PO4).

Q: Why does the Tris buffer entry warn about temperature dependence?

Tris has a pKa shift of -0.028 per degree Celsius, which is unusually large. A buffer prepared at pH 8.07 at 25 C will have pH approximately 7.73 at 37 C and 8.66 at 4 C. This means you must adjust pH at the temperature you will actually use the buffer, not at room temperature.

Q: Which indicator should I use for a weak acid-strong base titration?

Phenolphthalein (pH 8.2-10.0) is ideal because the equivalence point pH is above 7 due to conjugate base hydrolysis. For example, titrating acetic acid with NaOH gives an equivalence point at pH 8.72. Methyl orange would change color too early and give an inaccurate endpoint.

Q: What Ka and Kb values are included in the reference?

The reference table lists Ka values for HF (6.8 x 10^-4), HNO2 (4.5 x 10^-4), CH3COOH (1.8 x 10^-5), H2CO3 (Ka1 4.3 x 10^-7), H2S (Ka1 1.0 x 10^-7), H2PO4- (Ka2 6.2 x 10^-8), HCN (6.2 x 10^-10), NH4+ (5.6 x 10^-10), and phenol (1.0 x 10^-10), along with Kb values for NH3, CH3NH2, and pyridine.

Q: Does neutral pH change with temperature?

Yes. The water ion product Kw increases with temperature, so neutral pH (where [H+] = [OH-]) decreases. At 0 C neutral pH is 7.47, at 25 C it is 7.00, at 37 C (body temperature) it is 6.81, and at 100 C it is 6.14. The reference lists Kw values at five temperatures.

Free reference guide: pH Calculator

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About pH Calculator

The pH & Equilibrium Reference is a searchable quick-reference covering the full scope of acid-base chemistry. It is organized into five categories: pH Fundamentals (pH/pOH definitions, water autoionization Kw at various temperatures, H+ concentration conversion tables, common substance pH values), Strong Acid & Base (pH formulas for monoprotic and diprotic strong acids like HCl, H2SO4, NaOH, Ca(OH)2), Weak Acid & Base (Ka-based pH approximations, acetic acid, carbonic acid, phosphoric acid, and a table of key Ka/Kb values), Buffer Solutions (Henderson-Hasselbalch equation, buffer capacity, PBS/acetate/Tris/HEPES buffer preparation recipes), and Titration & Indicators (strong/weak acid-base titration curves with volume-by-volume pH data, and a comprehensive indicator color-change range chart).

This reference is designed for chemistry students, laboratory technicians, biochemistry researchers, environmental scientists, and anyone who needs to quickly recall pH formulas, look up dissociation constants, or prepare buffer solutions. Each entry provides worked numerical examples, not just formulas. For instance, the weak acid entry shows step-by-step calculation of 0.1 M acetic acid pH with verification of the 5% approximation rule, and the PBS entry gives exact gram quantities of NaCl, KCl, Na2HPO4, and KH2PO4 for 10x stock preparation.

All content is rendered client-side in the browser with no server requests. The reference supports dark mode for comfortable lab use and is fully responsive across desktops, tablets, and mobile devices. No account registration, no software download, and no data collection.

Key Features

pH and pOH definitions with conversion formulas between [H+], [OH-], pH, and pOH including worked examples
Water ion product Kw values at five temperatures (0, 25, 37, 50, 100 C) showing how neutral pH shifts with temperature
Strong acid/base pH calculations for HCl, NaOH, H2SO4 (including second dissociation Ka2 = 0.012), and Ca(OH)2
Weak acid/base pH approximation formulas with Ka/Kb table covering HF, HNO2, CH3COOH, H2CO3, H3PO4, HCN, NH3, and more
Henderson-Hasselbalch equation with worked buffer calculations and buffer capacity analysis at pH = pKa
Complete buffer recipes for PBS (10x and 1x), acetate buffer (pH 4.0-5.5), Tris-HCl (with temperature correction), and HEPES
Titration curve data for strong acid-strong base and weak acid-strong base with pH at each volume increment
Indicator color-change range chart from methyl violet (pH 0) to alizarin yellow (pH 12) including phenolphthalein and BTB

Frequently Asked Questions

How do I convert between pH and hydrogen ion concentration?

Use [H+] = 10^(-pH) to convert pH to concentration, and pH = -log10[H+] for the reverse. For example, pH 3.00 corresponds to [H+] = 1.0 x 10^-3 M. The reference includes a full conversion table from pH 0 to pH 14 with both [H+] and [OH-] values.

How do I calculate the pH of a weak acid solution?

For a weak acid HA with known Ka and concentration C, use the approximation [H+] = sqrt(Ka x C) when C >> Ka. The pH is then -log[H+]. For example, 0.1 M acetic acid (Ka = 1.8 x 10^-5) gives [H+] = 1.34 x 10^-3 M and pH = 2.87. The entry also shows how to verify the approximation holds (degree of dissociation < 5%).

What is the Henderson-Hasselbalch equation and when do I use it?

The equation pH = pKa + log([A-]/[HA]) calculates the pH of a buffer solution. Use it when you have a mixture of a weak acid and its conjugate base. Maximum buffer capacity occurs when [A-] = [HA], meaning pH = pKa. The effective buffer range is pKa +/- 1 pH unit.

How do I prepare a PBS buffer at pH 7.4?

For 1 liter of 10x PBS stock: dissolve 80 g NaCl, 2.0 g KCl, 14.4 g Na2HPO4, and 2.4 g KH2PO4 in distilled water, verify pH 7.4, and fill to 1 L. Dilute 1:10 for working concentration (137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, 1.8 mM KH2PO4).

Why does the Tris buffer entry warn about temperature dependence?

Tris has a pKa shift of -0.028 per degree Celsius, which is unusually large. A buffer prepared at pH 8.07 at 25 C will have pH approximately 7.73 at 37 C and 8.66 at 4 C. This means you must adjust pH at the temperature you will actually use the buffer, not at room temperature.

Which indicator should I use for a weak acid-strong base titration?

Phenolphthalein (pH 8.2-10.0) is ideal because the equivalence point pH is above 7 due to conjugate base hydrolysis. For example, titrating acetic acid with NaOH gives an equivalence point at pH 8.72. Methyl orange would change color too early and give an inaccurate endpoint.

What Ka and Kb values are included in the reference?

The reference table lists Ka values for HF (6.8 x 10^-4), HNO2 (4.5 x 10^-4), CH3COOH (1.8 x 10^-5), H2CO3 (Ka1 4.3 x 10^-7), H2S (Ka1 1.0 x 10^-7), H2PO4- (Ka2 6.2 x 10^-8), HCN (6.2 x 10^-10), NH4+ (5.6 x 10^-10), and phenol (1.0 x 10^-10), along with Kb values for NH3, CH3NH2, and pyridine.

Does neutral pH change with temperature?

Yes. The water ion product Kw increases with temperature, so neutral pH (where [H+] = [OH-]) decreases. At 0 C neutral pH is 7.47, at 25 C it is 7.00, at 37 C (body temperature) it is 6.81, and at 100 C it is 6.14. The reference lists Kw values at five temperatures.