Titration pH at equivalence calculator
A 0.05 M salt of a weak acid with a Kb of 5.6 × 10⁻¹¹ gives a pH slightly above 8 at the equivalence point. Enter the salt concentration and Kb to get hydroxide, pOH, pH and the fraction hydrolyzed, with every substitution shown. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Salt concentration
- 0.05
- Kb
- 0.0000000001
- Concentration
- 0.05
- C0
- 0.05
- Unit concentration
- mol/L
- Kb
- 0.0000000001
A titration pH at equivalence calculator answers the classic weak-acid question: once every mole of HA has been converted to the conjugate-base salt A-minus by a strong base, what is the solution pH? Enter the analytical salt concentration in moles per litre and the conjugate-base Kb (often Kw over Ka of the parent acid); the tool treats the equivalence-point mixture as a monoprotic weak base, solves Kb equals x squared over C minus x for hydroxide, then returns pOH, pH equals fourteen minus pOH at twenty-five degrees Celsius, fraction hydrolyzed, undissociated salt left, an acidity label, and step-by-step substitution you can paste into a lab notebook. Exam prep, titration-curve checkpoints, quality-control worksheets, and automated graders all need that same dilute conjugate-base path without a full aqueous speciation package.
What equivalence-point pH means for a weak-acid titration
In a monoprotic weak-acid titration with a strong base, the equivalence point is the stoichiometry where moles of titrant equal moles of analyte HA. At that composition the dominant solute is the salt of the conjugate base A-minus, not free HA and not excess hydroxide. The salt hydrolyzes in water according to A-minus plus water in equilibrium with HA plus OH-minus.
With activities approximated by molar concentrations, the base dissociation constant is Kb equals the product of HA and hydroxide divided by free A-minus. If the analytical salt concentration after mixing is C moles per litre and x is the equilibrium hydroxide concentration, mass and charge balance for a pure salt solution give Kb equals x squared over C minus x. That quadratic has the physical root x equals negative Kb plus the square root of Kb squared plus four Kb C, all over two.
From hydroxide you compute pOH as minus the base-ten logarithm of x, then pH as fourteen minus pOH when Kw is one times ten to the minus fourteen near twenty-five degrees Celsius. Because the conjugate of a weak acid is a weak base, the equivalence-point pH is typically greater than seven—never assumed to be seven as in a strong-acid strong-base titration.
Teachers and automated graders care about rejecting zero salt concentration as much as the happy path: a silent clamp would hide a data-entry error when volumes cancel or a stock was never diluted. The check path recomputes the same quadratic and logs with the fixed rounding used for published fields, so a successful response always ships with steps you can assert in tests or lab notebooks.
Practical tips, limits, and related searches
Keep salt concentration and Kb inside the published absolute range so the discriminant and logarithms cannot overflow into non-finite results the solver must reject. Use moles per litre consistently for the mixed solution at equivalence; if you know millimoles of salt and total volume in millilitres, form C equals millimoles over millilitres before calling rather than mixing unit systems inside one request.
When Ka of the parent weak acid is tabulated, set Kb equals one times ten to the minus fourteen divided by Ka at the same temperature assumption used here. The exact quadratic is used rather than the common square-root approximation, so cases where the degree of hydrolysis is not tiny still match textbook quadratic solutions.
Prefer this capability when you already know analytical C of the salt and Kb at equivalence; if your source is only free weak base concentration outside a titration context, use a dedicated weak-base pH path instead. Round-trip intuition is easy: recomputing Kb from published oh and undissociated should recover the input Kb within the same twelve-decimal rounding for well-behaved dilute cases, and confirming acidity stays basic for typical weak-acid salts with pH greater than seven.
What you can do with it
Homework checks for equivalence-point pH
Students enter textbook salt C and Kb (or Kw/Ka) for weak-acid titrations and compare hand-written [OH−], pOH, and pH against a deterministic quadratic result before submitting.
Lab titration-curve checkpoints
Instructors and technicians estimate the expected basic pH at the color-change region from diluted salt concentration after mixing, without a full speciation package.
FAQ
How is pH calculated at the equivalence point of a weak-acid titration?
At equivalence the solution is a salt of the conjugate base.
What happens if salt concentration is zero?
Analytical salt concentration at equivalence must be strictly positive; zero concentration is not allowed.
Why is equivalence-point pH usually greater than 7?
The conjugate base of a weak acid hydrolyzes to produce OH−, so the salt solution is basic. Strong-acid strong-base equivalence (pH ≈ 7) is a different model.
Is the titration pH at equivalence calculator free?
The browser tool is free and runs locally.
How do I get Kb if I only know the Ka of the acid?
Divide the water constant by the Ka of the parent acid. Near 25 °C the water constant is 1 × 10⁻¹⁴, so a weak acid with a small Ka gives a conjugate base with an even smaller Kb.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/chem/titration-ph-at-equivalence \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"salt_concentration":0.05,"kb":5.6e-11}'const res = await fetch("https://api.kit.forhosting.com/chem/titration-ph-at-equivalence", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"salt_concentration": 0.05,
"kb": 5.6e-11
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/titration-ph-at-equivalence",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"salt_concentration": 0.05,
"kb": 5.6e-11
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/titration-ph-at-equivalence", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"salt_concentration":0.05,"kb":5.6e-11}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"salt_concentration":0.05,"kb":5.6e-11}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/titration-ph-at-equivalence", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"salt_concentration": 0.05,
"kb": 5.6e-11
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.titration_ph_at_equivalence",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_abs | 1000000000000 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Zero salt concentration is rejected with a clear invalid_input error so a blank volume cell or a canceled aliquot never silently yields a fake neutral pH.
How to call the API and what comes back
Both salt concentration and Kb must be strictly positive; values at or below one times ten to the minus fifteen are treated as zero and rejected along with any explicit negative. Send a JSON body with two required finite numbers: salt_concentration (aliases concentration, c_salt, salt_c, c0, c, molarity, salt_molarity) in moles per litre for the conjugate-base salt at equivalence, and kb (aliases Kb, base_kb, dissociation_constant, k_b, conjugate_kb). Absolute values must stay within the published limit of one trillion so intermediate products remain finite in IEEE double precision on the Worker and in the browser. On success the response echoes rounded salt_concentration, concentration, and c0, kb and Kb, unit_concentration mol/L, oh_minus and oh as equilibrium hydroxide, h_plus from Kw over hydroxide, poh and pOH, ph and pH, alpha and fraction_dissociated as x over C, undissociated as C minus x, formula strings for the equilibrium and pH relations, reaction A-minus plus water, point set to equivalence, kw and pkw constants, acidity as acidic, neutral, or basic, message as a short English summary, and steps as an ordered array of substitution lines. Failed validation—missing fields, non-finite numbers, values outside absolute limits, zero or non-positive salt concentration or Kb, or a non-finite computed pH—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price. Prefer sending numeric JSON rather than pre-formatted strings when you control the client; string numbers are accepted only when they match a strict finite numeric pattern including optional scientific notation for values such as five point six e minus eleven.
Practical tips, limits, and related searches
When teaching, pair classic examples: zero point zero five molar acetate salt with Kb about five point six times ten to the minus eleven yields a mildly basic equivalence pH near eight point seven; a more dilute salt with the same Kb yields a lower pH closer to seven; and salt concentration equal to zero demonstrates the hard invalid_input path. This landing targets people searching for a titration pH at equivalence calculator, equivalence-point pH of a weak acid, or salt concentration and Kb online; it is not a full titration-curve simulator, not a strong-acid equivalence tool that assumes pH seven, and not a pre-equivalence buffer calculator using Henderson–Hasselbalch. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately send zero.
Integration use cases
Unit tests for chemistry helpers
Generate known-good basic equivalence pH cases plus deliberate zero salt concentration rejections for coding kata and automated grading pipelines.
Technical questions
How do I get Kb if I only know Ka of the weak acid?
At 25 °C with Kw = 1×10⁻¹⁴, use Kb = Kw / Ka, then pass that Kb with the salt concentration.