pH Calculator
pH calculator for [H+], [OH-], pOH, strong and weak acids and bases, pKa and Ka, Henderson-Hasselbalch buffers and mixing two solutions.
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How the pH Calculator Formula Works
pH measures how acidic a water solution is. It is the negative base 10 logarithm of the hydrogen ion concentration in moles per litre, so each step of 1 on the scale is a tenfold change. A solution with [H+] of 0.001 mol per litre has pH 3, and pure water at 25 C, with 1 × 10−7, has pH 7. The same idea applies to pOH and the hydroxide concentration, and at 25 C pH plus pOH is 14.
The calculator has five modes. Convert switches between pH, pOH, [H+] and [OH-]. Strong acid or base treats the acid as fully dissociated and adds water's own ionisation, which matters below about 1 × 10−6 mol per litre. Weak acid or base solves the exact quadratic from Ka or pKa and also shows the √(Ka C) shortcut and the percent ionised, so you can see when the approximation is fair.
Buffer uses the Henderson-Hasselbalch equation, pH = pKa + log10 ([A-] / [HA]). Mixing combines two strong solutions by averaging the net acidity, [H+] minus [OH-], by volume, which is the correct replacement for averaging the pH numbers. Everything is for 25 C and dilute solutions.
pH = -log10 [H+]; pOH = -log10 [OH-]; pH + pOH = 14; weak acid: x = (-Ka + √(Ka2 + 4 Ka C)) / 2; buffer: pH = pKa + log10 ([A-] / [HA])- [H+]mol/LHydrogen ion concentration.
- [OH-]mol/LHydroxide ion concentration.
- Ka, Kbmol/LAcid or base dissociation constant; pK = -log10 K.
- Cmol/LStarting concentration of the acid or base.
- [A-] / [HA]ratioBase form over acid form in a buffer.
- + 1 more input below
- [H+]
- mol/L
- Hydrogen ion concentration.
- [OH-]
- mol/L
- Hydroxide ion concentration.
- Ka, Kb
- mol/L
- Acid or base dissociation constant; pK = -log10 K.
- C
- mol/L
- Starting concentration of the acid or base.
- [A-] / [HA]
- ratio
- Base form over acid form in a buffer.
- Kw
- mol2/L2
- Ionic product of water, 1 x 10^-14 at 25 C.
Using the pH Calculator
Enter what do you want to calculate?
Choose from Convert pH, pOH, [H+] and [OH-], pH of a strong acid or base, pH of a weak acid or base (Ka or pKa), Buffer pH (Henderson-Hasselbalch), Mix two strong solutions.
Enter you know the
Choose from Hydrogen ion concentration [H+], Hydroxide concentration [OH-], pH, pOH.
Enter value (a concentration, a ph or a poh)
Type the figure you already have, then press Calculate.
Enter concentration unit (ignored for ph and poh)
Choose from mol per litre (M), millimolar (mM), micromolar (uM), nanomolar (nM).
Enter strong
Choose from Acid (such as HCl), Base (such as NaOH).
Enter concentration
Type the figure you already have, then press Calculate.
Enter concentration unit
Choose from mol per litre (M), millimolar (mM), micromolar (uM), nanomolar (nM).
Enter h+ or oh- released per formula unit
Choose from 1 (HCl, NaOH), 2 (Ba(OH)2, H2SO4 first approximation), 3.
Enter weak
Choose from Acid (uses Ka or pKa), Base (uses Kb or pKb).
Enter constant is given as
Choose from pKa or pKb, Ka or Kb.
Enter constant (pka, pkb, ka or kb)
Type the figure you already have, then press Calculate.
Enter concentration
Type the figure you already have, then press Calculate.
Enter concentration unit
Choose from mol per litre (M), millimolar (mM), micromolar (uM), nanomolar (nM).
Enter pka of the buffer acid
Type the figure you already have, then press Calculate.
Enter base form [a-] (any unit, same as the acid)
Type the figure you already have, then press Calculate.
Enter acid form [ha] (same unit)
Type the figure you already have, then press Calculate.
Enter ph of solution 1
Type the figure you already have, then press Calculate.
Enter volume of solution 1 (any unit)
Type the figure you already have, then press Calculate.
Enter ph of solution 2
Type the figure you already have, then press Calculate.
Enter volume of solution 2 (same unit)
Type the figure you already have, then press Calculate.
Read the result
Press Calculate to update the result. Until you do, the last result is dimmed and marked as out of date, so you always know which numbers belong to the values in the boxes.
A Real Worked Example
A solution has a hydrogen ion concentration of 1 mM, which is 0.001 mol per litre. In the Convert mode, pH = -log10 (0.001) = 3.
The pOH is 14 − 3 = 11, and [OH-] is 1 × 10−11 mol per litre. A solution at pH 3 is a thousand times higher in [H+] than a neutral solution at pH 6, not three times.
Strong base: 5 mM barium hydroxide
Choose strong, base, 5 mM, with 2 hydroxides per formula unit. The effective [OH-] is 2 × 0.005 = 0.01 mol per litre, so pOH = 2 and pH = 14 − 2 = 12.
Water's own ionisation adds almost nothing at this strength, so the exact and simple answers agree to three places.
Weak acid: 0.1 M acetic acid, pKa 4.76
Choose weak acid, pKa 4.76 and 0.1 M. Ka = 1.74 × 10−5. The exact quadratic gives [H+] = 1.31 × 10−3, so pH = 2.883.
The approximation √(Ka C) gives 2.880, a difference of -0.003. Only 1.310 percent of the acid is ionised, so the shortcut is fair here.
Acetate buffer with twice as much base
A buffer has pKa 4.76 with base form 0.2 and acid form 0.1. The ratio is 2, so pH = 4.76 + log10 (2) = 4.76 + 0.301 = 5.061.
The base form is 66.67 percent of the total. Equal amounts would give pH 4.76, and a ratio of 10 would give 5.76.
Mix 100 mL of pH 2 with 300 mL of pH 4
The net acidity is 0.01 mol per litre for the first solution and 0.0001 for the second. The mixture has (0.01 × 100 + 0.0001 × 300) / 400 = 0.002575 mol per litre, so pH = 2.589.
Averaging the pH numbers by volume would give 3.5, which is wrong. The calculator shows both so you can see the gap.
[H+] from a pH of 3.5
Choose pH in the Convert mode and enter 3.5. [H+] = 10−3.5 = 3.1623 × 10−4 mol per litre, which is 0.00031622776602.
The pOH is 10.5 and [OH-] is 3.1623 × 10−11 mol per litre. Half a pH unit is a factor of about 3.16 in concentration.
What Else to Know
The pH Formula and How to Calculate pH
pH is the negative base 10 logarithm of the hydrogen ion concentration, pH = -log10 [H+], with [H+] in moles per litre. Because it is a logarithm, one pH unit is a tenfold change in concentration. A solution at pH 3 has ten times the [H+] of one at pH 4.
To calculate pH on a calculator, enter the concentration, press the log key and change the sign. For 0.001 mol per litre, log is -3 and pH is 3. To go back, raise 10 to the power minus pH. At pH 3.5 that gives 3.16 × 10−4 mol per litre.
This pH calculator has five modes so one tool covers the usual jobs: convert, strong acid or base, weak acid or base, buffer and mixing. The formula table lists each formula with an example. Everything assumes 25 C and dilute solutions.
pH, pOH, [H+] and [OH-]: Converting Between Them
The hydroxide side has its own scale, pOH = -log10 [OH-]. In water at 25 C the product [H+] x [OH-] is Kw = 1 × 10−14, which means pH + pOH = 14. So pH 3 goes with pOH 11, and [OH-] of 0.01 mol per litre goes with pOH 2 and pH 12. The scale table lists all of it from 0 to 14.
The Convert mode takes any one of [H+], [OH-], pH or pOH and returns the other three, with concentrations in M, mM, uM or nM. It also accepts values outside 0 to 14. Pure water at 25 C has [H+] = [OH-] = 1 × 10−7 and pH 7, which is neutral.
For scale, Wikipedia's pH article gives vinegar as 2 to 3, milk as 6.5 to 6.8, sea water as 7.5 to 8.4 and bleach as 12.5. The common solutions table adds the [H+] each implies.
pH of Strong Acids and Bases
A strong acid or base dissociates completely, so [H+] or [OH-] equals the concentration times the number of ions released per formula unit. 0.01 M hydrochloric acid has pH 2. 5 mM barium hydroxide releases two hydroxides each, so [OH-] is 0.01 and pH is 12. The calculator has a box for 1, 2 or 3 ions per formula unit.
At very low concentrations, water itself supplies about 1 × 10−7 mol per litre of each ion, so the simple formula fails. The exact result is [H+] = (C + √(C2 + 4 Kw)) / 2. For 1 × 10−8 M HCl that gives pH 6.978, not 8. The strong acid table compares the simple and exact answers from 1 M down to 1 × 10−8.
Very concentrated acids and bases are also not ideal, because ions interact and the concentration no longer matches the activity. Treat answers beyond about 1 M as rough.
Weak Acids and Bases: Ka, pKa and the Exact Quadratic
A weak acid only partly ionises, so you need its acid dissociation constant, Ka, or pKa = -log10 Ka. For HA with starting concentration C, Ka = x2 / (C - x), where x is the [H+]. The exact solution is x = (-Ka + √(Ka2 + 4 Ka C)) / 2, and pH = -log10 x. Acetic acid has a pKa of 4.76, so 0.1 M acetic acid is at pH 2.883.
The usual shortcut assumes C - x is about C, which gives x = √(Ka C) and pH = 2.88. It is fair when less than about 5 percent of the acid ionises. The weak acid table shows the exact and approximate pH for acetic acid from 1 M to 0.00001 M, with the percent ionised, and the Ka table shows how the gap grows as the acid gets stronger. When it does, use the exact answer. The calculator shows both.
Weak bases work the same way with Kb and [OH-], then pH = 14 - pOH. Water's ionisation is ignored in the weak modes, so they suit solutions where x is much larger than 1 × 10−7. A soda ash solution is a weak base too: enter its Kb, found as Kw divided by the Ka of its conjugate acid.
Buffers and the Henderson-Hasselbalch Equation
A buffer holds the pH steady by pairing a weak acid with its conjugate base. The Henderson-Hasselbalch equation estimates its pH: pH = pKa + log10 ([A-] / [HA]). With equal amounts of base and acid, the pH equals the pKa. Ten times more base raises it by one unit, and ten times less lowers it by one.
The ratio table shows the base and acid share for each pH relative to pKa. A buffer works best within about one pH unit of its pKa, which is why the buffering agent table gives a range of pKa minus 1 to pKa plus 1. Values there come from Wikipedia's buffer solution article: citric acid 3.13, 4.76 and 6.40, MES 6.15, MOPS 7.20, HEPES 7.48, Tris 8.07 and CHES 9.3.
For a histidine buffer, take the pKa of the histidine group nearest your target pH from the supplier's datasheet, then enter the base and acid amounts. The amounts can be in any unit as long as both use the same one.
Mixing Two Solutions and Why You Cannot Average pH
It is tempting to average the pH of two solutions. That is wrong, because pH is logarithmic. The right quantity to average is concentration. For strong acids and bases, the net acidity [H+] minus [OH-] is conserved, so average it by volume and convert back.
Mixing 100 mL of pH 2 with 300 mL of pH 4 gives 2.589, not the 3.5 that averaging the numbers would suggest. Mixing pH 2 with pH 12 in equal volumes gives 7, because the acid and base cancel. The mixing table lists six pairs with the true and the averaged values.
The mixing mode assumes strong acids and bases, with no buffers or weak acids. If a solution contains a buffer, its pH moves much less than this calculation predicts, and you need its full equilibrium or a calibrated meter.
Limits, Safety and Related Calculators
The results are estimates for education and planning. They assume 25 C, dilute solutions, ideal behaviour and concentrations in place of activities. Acids and bases can be hazardous, so follow the product label and the safety data sheet, wear the protective equipment they call for, and never mix chemicals unless the label says it is safe. Measure real solutions with a calibrated pH meter.
For percentages and ratios, try the percentage calculator. To work out pool salt, see the pool salt calculator. For values between two table points, the linear interpolation calculator can help.
Reference Tables
| Substance | pH | [H+] at the low pH (mol/L) | [H+] at the high pH (mol/L) |
|---|---|---|---|
| Gastric acid | 1.0 to 1.5 | 1.00 × 10−1 | 3.16 × 10−2 |
| Vinegar | 2 to 3 | 1.00 × 10−2 | 1.00 × 10−3 |
| Orange juice | 3.3 to 4.2 | 5.01 × 10−4 | 6.31 × 10−5 |
| Milk | 6.5 to 6.8 | 3.16 × 10−7 | 1.58 × 10−7 |
| Pure water at 25 C | 7 | 1.00 × 10−7 | 1.00 × 10−7 |
| Sea water | 7.5 to 8.4 | 3.16 × 10−8 | 3.98 × 10−9 |
| Ammonia solution | 11.0 to 11.5 | 1.00 × 10−11 | 3.16 × 10−12 |
| Bleach | 12.5 | 3.16 × 10−13 | 3.16 × 10−13 |
| pH | pOH | [H+] (mol/L) | [OH-] (mol/L) |
|---|---|---|---|
| 0 | 14 | 1 | 1 × 10−14 |
| 1 | 13 | 1 × 10−1 | 1 × 10−13 |
| 2 | 12 | 1 × 10−2 | 1 × 10−12 |
| 3 | 11 | 1 × 10−3 | 1 × 10−11 |
| 4 | 10 | 1 × 10−4 | 1 × 10−10 |
| 5 | 9 | 1 × 10−5 | 1 × 10−9 |
| 6 | 8 | 1 × 10−6 | 1 × 10−8 |
| 7 | 7 | 1 × 10−7 | 1 × 10−7 |
| 8 | 6 | 1 × 10−8 | 1 × 10−6 |
| 9 | 5 | 1 × 10−9 | 1 × 10−5 |
| 10 | 4 | 1 × 10−10 | 1 × 10−4 |
| 11 | 3 | 1 × 10−11 | 1 × 10−3 |
| 12 | 2 | 1 × 10−12 | 1 × 10−2 |
| 13 | 1 | 1 × 10−13 | 1 × 10−1 |
| 14 | 0 | 1 × 10−14 | 1 |
| Concentration (mol/L) | Simple pH | Exact pH | Difference |
|---|---|---|---|
| 1 | 0.000 | 0.000 | 0.000 |
| 1.0 × 10−1 | 1.000 | 1.000 | 0.000 |
| 1.0 × 10−2 | 2.000 | 2.000 | 0.000 |
| 1.0 × 10−3 | 3.000 | 3.000 | 0.000 |
| 1.0 × 10−4 | 4.000 | 4.000 | 0.000 |
| 1.0 × 10−6 | 6.000 | 5.996 | -0.004 |
| 1.0 × 10−7 | 7.000 | 6.791 | -0.209 |
| 1.0 × 10−8 | 8.000 | 6.978 | -1.022 |
| Concentration (mol/L) | Exact pH | Approximate pH | Percent ionised |
|---|---|---|---|
| 1 | 2.381 | 2.380 | 0.42 |
| 1.0 × 10−1 | 2.883 | 2.880 | 1.31 |
| 1.0 × 10−2 | 3.389 | 3.380 | 4.08 |
| 1.0 × 10−3 | 3.909 | 3.880 | 12.34 |
| 1.0 × 10−4 | 4.470 | 4.380 | 33.89 |
| 1.0 × 10−5 | 5.149 | 4.880 | 71.00 |
| Ka | pKa | Exact pH | Approximate pH | Percent ionised |
|---|---|---|---|---|
| 1 × 10−1 | 1 | 1.209 | 1.000 | 61.80 |
| 1 × 10−2 | 2 | 1.568 | 1.500 | 27.02 |
| 1 × 10−3 | 3 | 2.022 | 2.000 | 9.51 |
| 1 × 10−4 | 4 | 2.507 | 2.500 | 3.11 |
| 1 × 10−5 | 5 | 3.002 | 3.000 | 1.00 |
| 1 × 10−7 | 7 | 4.000 | 4.000 | 0.10 |
| 1 × 10−9 | 9 | 5.000 | 5.000 | 0.01 |
| pH - pKa | Ratio [A-] / [HA] | Base form (percent) | Acid form (percent) |
|---|---|---|---|
| -2 | 0.010 | 1.0 | 99.0 |
| -1 | 0.100 | 9.1 | 90.9 |
| -0.5 | 0.316 | 24.0 | 76.0 |
| +0 | 1.000 | 50.0 | 50.0 |
| +0.5 | 3.162 | 76.0 | 24.0 |
| +1 | 10.000 | 90.9 | 9.1 |
| +2 | 100.000 | 99.0 | 1.0 |
| Buffering agent | pKa | Useful range (pKa - 1 to pKa + 1) |
|---|---|---|
| Citric acid, pKa1 | 3.13 | 2.13 to 4.13 |
| Citric acid, pKa2 | 4.76 | 3.76 to 5.76 |
| Citric acid, pKa3 | 6.40 | 5.40 to 7.40 |
| Acetic acid | 4.76 | 3.76 to 5.76 |
| MES | 6.15 | 5.15 to 7.15 |
| MOPS | 7.20 | 6.20 to 8.20 |
| Phosphate (H2PO4-) | 7.20 | 6.20 to 8.20 |
| HEPES | 7.48 | 6.48 to 8.48 |
| Tris (conjugate acid) | 8.07 | 7.07 to 9.07 |
| CHES | 9.3 | 8.3 to 10.3 |
| Solution 1 (pH, mL) | Solution 2 (pH, mL) | Mixture pH | Averaged pH numbers |
|---|---|---|---|
| 2, 100 | 12, 100 | 7.000 | 7.000 |
| 2, 100 | 4, 300 | 2.589 | 3.500 |
| 3, 50 | 5, 50 | 3.297 | 4.000 |
| 1, 100 | 3, 100 | 1.297 | 2.000 |
| 4, 100 | 10, 100 | 7.000 | 7.000 |
| 11, 100 | 13, 300 | 12.876 | 12.500 |
| Mode | Formula | Example |
|---|---|---|
| Convert | pH = -log10 [H+]; pOH = -log10 [OH-]; pH + pOH = 14 | [H+] = 0.001 gives pH 3 |
| Strong acid or base | [H+] = (C + √(C2 + 4Kw)) / 2 | 0.01 M HCl gives pH 2 |
| Weak acid | x = (-Ka + √(Ka2 + 4 Ka C)) / 2 | 0.1 M, pKa 4.76 gives 2.883 |
| Weak base | x = [OH-] from Kb, then pH = 14 - pOH | 0.1 M, pKb 4.75 gives 11.122 |
| Buffer | pH = pKa + log10 ([A-] / [HA]) | 0.2 / 0.1 at pKa 4.76 gives 5.061 |
| Mixing strong solutions | net [H+] - [OH-] averaged by volume | pH 2 and pH 12 equal volumes give 7 |
| Mistake | Effect | Fix |
|---|---|---|
| Averaging pH values | Off by a lot when the pH values differ | Average concentrations, not pH numbers |
| Using -log C for a very dilute acid | pH above 7 for an acid | Include water's ionisation, as the strong mode does |
| Using the approximation for a strong or dilute weak acid | Error of a tenth of a pH unit or more | Use the exact quadratic when ionisation exceeds 5 percent |
| Mixing up pKa and Ka | Off by many orders of magnitude | pKa = -log10 Ka; choose pK or K in the calculator |
| Assuming pH 7 is neutral at every temperature | Neutral pH moves with temperature | The calculator is for 25 C only |
Frequently Asked Questions About the pH Calculator
How to calculate pH in a calculator?
Take the negative base 10 logarithm of the hydrogen ion concentration in mol per litre: pH = -log10 [H+]. On a scientific calculator, enter the concentration, press log, then change the sign. For 0.001 mol/L, log is -3, so pH = 3. In this tool, choose Convert, pick [H+], and enter the value.
What is a pH formula?
The basic formula is pH = -log10 [H+], where [H+] is the hydrogen ion concentration in mol per litre. For bases there is the matching pOH = -log10 [OH-], and at 25 C pH + pOH = 14. For weak acids and buffers other formulas apply, including pH = pKa + log10 ([A-]/[HA]).
Is there a formula to calculate pH?
Yes, and the right one depends on the solution. A strong acid fully dissociates, so [H+] equals its concentration. A weak acid needs its Ka and a quadratic or the √(Ka C) shortcut. A buffer uses Henderson-Hasselbalch. The calculator has a mode for each, and the formula table lists them.
How do I convert Ka to pH?
For a weak acid of concentration C, solve x2 / (C - x) = Ka for x = [H+], then pH = -log10 x. The shortcut is x = √(Ka C). For 0.1 M acetic acid with Ka = 1.74 × 10−5, x = 1.32 × 10−3 and pH is about 2.88, or 2.883 from the exact quadratic.
How do I calculate pH from pKa?
Convert pKa to Ka with Ka = 10−pKa, then use the weak acid method with the concentration. In a buffer, the pH is simpler: pH = pKa + log10 ([A-]/[HA]). With equal amounts of acid and base, the pH equals the pKa. Choose the pK option in the weak acid mode or use the buffer mode.
How do I convert OH to pH?
Find pOH = -log10 [OH-], then pH = 14 - pOH at 25 C. For 0.01 mol/L hydroxide, pOH = 2, so pH = 12. Choose [OH-] in the Convert mode and enter the concentration. A hydroxide concentration of 1 × 10−4 gives pOH 4 and pH 10.
What is pOH and how do I find it?
pOH is the negative base 10 logarithm of the hydroxide ion concentration, pOH = -log10 [OH-]. You can also get it from pH: pOH = 14 - pH at 25 C. A solution with pH 11 has pOH 3 and [OH-] of 0.001 mol per litre. The scale table lists all values from 0 to 14.
How do I find [H+] from pH?
Raise 10 to the power of minus pH: [H+] = 10−pH. At pH 3.5 that is 3.16 × 10−4 mol per litre, and at pH 7 it is 1 × 10−7. Enter the pH in the Convert mode and the calculator shows [H+], [OH-] and pOH.
What is the pH of pure water?
At 25 C pure water has [H+] = [OH-] = 1 × 10−7 mol per litre, so its pH is 7, which is neutral. The ionic product of water, Kw, is 1 × 10−14 at that temperature. The value of Kw changes with temperature, so the neutral pH is not exactly 7 at other temperatures.
How do I calculate the pH of a strong acid?
A strong acid fully dissociates, so [H+] equals its concentration times the number of acidic protons. For 0.01 M HCl, pH = 2. For very dilute acid, water adds to [H+]: 1 × 10−8 M HCl is pH 6.98, not 8. The calculator includes this correction.
How do I calculate the pH of a weak acid?
Use Ka and the concentration. The exact quadratic is x = (-Ka + √(Ka2 + 4 Ka C)) / 2, then pH = -log10 x. For 0.1 M acetic acid with pKa 4.76, pH = 2.883. The percent ionised is 1.31, so the √(Ka C) shortcut is good here.
When can I use the weak acid approximation?
When the acid is less than about 5 percent ionised, so that C minus x is close to C. The ionised share is x / C. For 0.1 M acetic acid it is 1.31 percent, so the shortcut works. At 0.0001 M it is 33.9 percent, so use the exact quadratic.
What is the Henderson-Hasselbalch equation?
pH = pKa + log10 ([A-] / [HA]), where A- is the base form and HA the acid form. It estimates the pH of a buffer. Equal amounts give pH = pKa, ten times more base gives pKa + 1, and ten times less gives pKa - 1. For pKa 4.76 and a 2 to 1 ratio, pH = 5.061.
How do I work out a histidine buffer pH?
Use the buffer mode with the pKa of the histidine group nearest your target pH, taken from your supplier's datasheet or a reference, then enter the amounts of the base and acid forms. Equal amounts give a pH equal to that pKa. A histidine buffer works best within about one pH unit of the pKa.
How do I calculate the pH of two mixed solutions?
For strong acids and bases, average the net acidity, [H+] minus [OH-], by volume, and convert back. Mixing 100 mL of pH 2 with 300 mL of pH 4 gives pH 2.589, not 3. Buffers and weak acids need their full equilibria, so treat results as estimates.
Can I average pH values?
Not directly. pH is a logarithm, so averaging the numbers is misleading, as the mixing table shows. A pH 2 and a pH 12 solution in equal volumes average to 7 by number, and also neutralise to about 7 in reality, but pH 2 and pH 4 in equal volumes gives 2.30, not 3. Average concentrations first.
What is the pH of vinegar?
Wikipedia lists vinegar at about 2 to 3. For a 5 percent acetic acid solution, about 0.833 mol per litre at 60.052 g per mole, the weak acid mode with pKa 4.76 gives pH 2.42. Real vinegar varies with the brand and other ingredients, so measure it if it matters.
How do I find the pH of a soda ash solution?
Sodium carbonate is a weak base, because the carbonate ion takes a proton from water. Use the weak base mode with its Kb, which is Kw divided by the Ka of hydrogen carbonate, so pKb = 14 - pKa2, and enter the concentration. Look up pKa2 in a reference. The result is an estimate for education, not for dosing a pool.
Can pH be negative or above 14?
Yes. The pH scale is not limited to 0 to 14. A concentrated strong acid with [H+] above 1 mol per litre has a negative pH, and a concentrated base can exceed 14. At those strengths, activity differs a lot from concentration, so a simple calculation is only a rough guide.
Why does temperature matter for pH?
The ionic product of water, Kw, changes with temperature, so pH + pOH is 14 only near 25 C, and pure water is neutral at pH 7 only there. Ka values also depend on temperature. This calculator is fixed at 25 C. For other temperatures, treat the answer as approximate.
How accurate is this pH calculator?
It is exact for the formulas it uses, which assume dilute solutions at 25 C and use concentrations instead of activities. Real solutions, mixtures and buffers differ, and a calibrated pH meter is the way to measure. Estimates are for education and planning. Follow product labels and safety data sheets when handling acids and bases.
References
- [1] Wikipedia contributors. “pH.” Wikipedia. Accessed 2026-10-07.
- [2] Wikipedia contributors. “Henderson-Hasselbalch equation.” Wikipedia. Accessed 2026-10-07.
- [3] Wikipedia contributors. “Buffer solution.” Wikipedia. Accessed 2026-10-07.
- [4] Wikipedia contributors. “Acid dissociation constant.” Wikipedia. Accessed 2026-10-07.
- [5] Wikipedia contributors. “Self-ionization of water.” Wikipedia. Accessed 2026-10-07.
- [6] Wikipedia contributors. “Acetic acid.” Wikipedia. Accessed 2026-10-07.
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