pH Calculator: Strong, Weak Acid-Base + Buffer & Titration

pH Calculator

strong · weak · buffer · titration — with validity checks & visual scale

I know…

Molarity (M)

H⁺/OH⁻ per formula

Full dissociation assumed (e.g. HCl, NaOH). Use 2 for H₂SO₄ or Ca(OH)₂.

Reference list (optional) Picks only pre-fill the constant — the Ka/Kb field always stays fully editable

Type

Ka

Concentration (M)

Direction

pKa

[A⁻] (M)

[HA] (M)

Target pH

Single-point estimate for a STRONG acid titrated with a STRONG base (not a full curve).

Acid Conc. (M)

Acid Vol. (mL)

Base Conc. (M)

Base Vol. Added (mL)

Result

Choose a mode and calculate to see pH, pOH, ion concentrations, approximation validity and the result placed on a color-coded 0–14 scale

All calculations assume Kw = 1×10⁻¹⁴, which is specific to 25 °C — neutral pH shifts slightly at other temperatures (about 6.14 at 100 °C, informational only, not calculated here). The weak acid/base square-root approximation is applied only after checking C/Ka; when C/Ka < 500 the exact quadratic solution is shown and preferred. Very dilute strong solutions (below ~10⁻⁶ M) need water autoionisation corrections that this tool flags but does not perform. Activity effects are ignored, so results are ideal-solution values; the titration mode covers strong acid–strong base chemistry only.

pH Calculator: Strong Acids, Weak Acids, Buffers and a Titration Point Estimator

The standard weak acid shortcut, pH from the square root of Ka times concentration, quietly breaks down once your concentration gets too close to your dissociation constant and most calculators apply it blindly without ever checking whether it was actually valid for your numbers. This pH calculator checks that validity every time, using the ratio of concentration to Ka, and shows you the exact quadratic solution alongside the approximation whenever the shortcut starts to drift. Beyond weak acids and bases, it also handles strong acids and bases with full dissociation, buffer pH in both directions using its built-in Henderson-Hasselbalch calculator, either pH from a ratio, or the exact ratio you need for a target pH and a single-point strong acid-strong base titration estimator, all placed on a color-coded 0 to 14 pH scale so you can see exactly where your result lands.

How to Use

This tool has four tabs: Strong, Weak, Buffer and Titration.

Step 1: Using Strong mode

Select what you already know under “I know…”: Strong acid molarity, Strong base molarity, [H⁺] concentration, [OH⁻] concentration, or pH itself if you want to work backward to find the ion concentrations.
Enter the corresponding Value.
Enter H⁺/OH⁻ per formula, how many protons or hydroxide ions that compound releases per formula unit on full dissociation, 1 for HCl or NaOH, but 2 for sulphuric acid or calcium hydroxide.
Tap Calculate pH. This mode assumes full dissociation throughout, appropriate for strong acids and bases only.

Step 2: Using Weak mode

Optionally pick a compound from the Reference list, acetic acid and several other common weak acids and bases are included, to pre-fill its Ka or Kb. This is a convenience only, the constant field always stays fully editable for any compound.
Select Type: Weak acid (Ka) or Weak base (Kb).
Enter the Ka or Kb value directly, scientific notation like 1.8e-5 is accepted and the Concentration.
Tap Calculate pH. The result shows both the standard square-root approximation and the exact quadratic solution side by side, along with your C/Ka ratio and a clear note on whether the approximation was actually valid for these numbers, using 500 as the threshold, and which result the tool is actually using as the headline answer.

Step 3: Using Buffer mode

Select your Direction: pH from pKa plus the [A⁻]/[HA] ratio, or the required ratio for a target pH, if you already know what pH you’re aiming for and need to know how much conjugate base and acid to combine.
Enter the pKa of your weak acid.
If calculating pH, enter [A⁻] and [HA] concentrations. If calculating the required ratio instead, enter your Target pH.
Tap Calculate Buffer.

Step 4: Using Titration mode

This mode gives a single-point estimate for a strong acid titrated with a strong base, not a full titration curve.
Enter the Acid Concentration and Acid Volume.
Enter the Base Concentration and Base Volume Added so far.
Tap Estimate pH at this Point. The result tells you whether you’re before, at or past the equivalence point and calculates the pH from whichever reagent is left in excess.

Step 5: Exporting your result

Use Print / PDF for a clean printable copy or Copy to paste the figures into your lab report.

Key Features

Strong acid/base mode handling full dissociation, with a proton or hydroxide count for polyprotic species like H₂SO₄ or Ca(OH)₂, converting freely between pH, pOH, [H⁺] and [OH⁻]
Weak acid/base mode with the standard square-root approximation, an explicit C/Ka validity check against the 500 threshold, and the exact quadratic solution shown alongside whenever the approximation starts to drift
Buffer mode via Henderson-Hasselbalch, working in both directions, pH from a known ratio, or the required ratio for a target pH
Single-point strong acid-strong base titration estimator, showing whether you’re before, at, or past the equivalence point
Color-coded 0 to 14 pH scale bar, showing exactly where your result sits at a glance
Never-locked Ka/Kb field, the reference list is a convenience only, any compound’s constant can always be typed in directly .

Formula / Logic Used

Strong Acid/Base

 $pH = -\log_{10}[H^+], \qquad pOH = -\log_{10}[OH^-], \qquad pH + pOH = 14 \ \text{(at 25°C)}$

Weak Acid, Approximation and Validity Check

$[H^+] \approx \sqrt{K_a \times C}, \qquad \text{valid when } \frac{C}{K_a} > 500$

Weak Acid, Exact Quadratic (used whenever the approximation is questionable)

$[H^+] = \frac{-K_a + \sqrt{K_a^2 + 4K_a C}}{2}$

Henderson-Hasselbalch (Buffer)

$pH = pK_a + \log_{10}\left(\frac{[A^-]}{[HA]}\right)$

Rearranged for the required ratio at a target pH:

$\frac{[A^-]}{[HA]} = 10^{(pH – pK_a)}$

Titration Point (Strong Acid-Strong Base, Single Point)

$[H^+]\ \text{or}\ [OH^-]_{excess} = \frac{|C_a V_a – C_b V_b|}{V_a + V_b}$

Whichever reagent has more total moles remaining, acid or base, determines whether the result is calculated as an excess [H⁺] or an excess [OH⁻] and if the two are exactly balanced, the point sits at the equivalence point, pH 7 for a strong acid-strong base pair.

Who Should Use This Tool

Diploma and B.Tech Chemical Engineering, pharmacy, and biology students learning acid-base equilibrium, buffer preparation, and titration concepts. also useful for lab technicians checking a buffer’s expected pH before preparation or verifying a weak acid calculation where the standard shortcut might not actually be reliable.

Frequently Asked Questions (FAQs)

1. When is the square-root approximation for weak acid pH not accurate?

The approximation, [H⁺] roughly equal to the square root of Ka times concentration, assumes the acid barely dissociates at all, which stops being a safe assumption once your concentration gets too close to Ka itself, typically when C divided by Ka drops below about 500. This tool checks that exact ratio every time and automatically shows the more accurate exact quadratic solution whenever the approximation is questionable.

2. How do I calculate the pH of a buffer solution?

Use the Henderson-Hasselbalch equation, pH equals pKa plus the log of the ratio of conjugate base to weak acid concentration. This tool calculates this directly and can also work backward from a target pH to tell you the exact ratio of conjugate base to acid you’d need to combine.

3. Why does H₂SO₄ need a different setting than HCl in Strong mode?

Sulphuric acid releases two H⁺ ions per formula unit on full dissociation compared to hydrochloric acid’s one, so at the same molarity, sulphuric acid actually produces double the hydrogen ion concentration and therefore a lower pH. This tool’s “H⁺/OH⁻ per formula” field accounts for this directly, so you get the correct answer for polyprotic acids and bases like H₂SO₄ or Ca(OH)₂ as well as simple monoprotic ones.

4. What does the titration mode’s “before, at, or past equivalence point” result actually mean?

Before the equivalence point, there’s still more acid than base has been added, so excess H⁺ determines the pH; past it, excess base determines the pH instead, and exactly at it, the two exactly balance and the pH sits at 7 for a strong acid-strong base pair. This tool calculates which of these three regions your specific volumes fall into and applies the correct calculation for that region.

5. Can this tool calculate a full titration curve, not just one point?

No, the Titration mode gives a single-point pH estimate for a specific combination of acid and base volumes that you enter, useful for checking one particular point along a titration, rather than plotting the entire curve from start to equivalence and beyond. It’s also limited to strong acid-strong base chemistry specifically, weak acid or weak base titrations follow a different, more complex curve shape that this simplified single-point method doesn’t cover.

Related Tools

Molarity / Normality Calculator – work out the concentration of your acid or base before checking its pH
Chemical Dilution Calculator – dilute a stock acid or base to the concentration you need
Distillation Column Basic Sizing – for the separation side of the same chemical process