pKa + pKb calculator
Acetic acid's pKa of 4.76 means its conjugate base, acetate, has a pKb of 9.24: enter any pKa and get the conjugate pKb from the rule that both add up to 14 in water at 25 °C, plus a pair label. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Pka
- 4.76
- PKa
- 4.76
- Pkb
- 9.24
- PKb
- 9.24
- Sum
- 14
This pKa plus pKb calculator turns a known acid dissociation constant on the logarithmic scale into the conjugate base pKb using the standard aqueous relation that the two values sum to fourteen at twenty-five degrees Celsius.
Why pKa and pKb sum to fourteen
For any conjugate acid–base pair in water at twenty-five degrees Celsius, the product of the acid dissociation constant Ka and the base dissociation constant Kb equals the ion-product of water Kw, which is one times ten to the minus fourteen under standard classroom conditions. Taking the negative base-ten logarithm of both sides of Ka times Kb equals Kw yields pKa plus pKb equals pKw, and pKw equals fourteen when Kw is ten to the minus fourteen.
Therefore the conjugate base strength is fully determined once you know the acid pKa: pKb equals fourteen minus pKa. Acetic acid near pKa four point seven six implies a conjugate acetate pKb near nine point two four; a strong mineral acid with negative pKa implies a very large pKb for its conjugate base; a very weak acid with pKa above seven implies a conjugate base that is relatively stronger on the pKb scale.
That identity is the entire search intent of this landing: people looking for a pKa plus pKb calculator, pKa pKb sum of fourteen, or conjugate pKb from pKa want a single subtraction with explicit steps—not a buffer solver, not a Ka lookup table, and not a temperature-dependent Kw model. Teachers, lab notebooks, and automated graders care about rejection as much as the happy path: a silent fallback would hide a data-entry error and produce non-finite results.
Practical tips, limits, and related searches
Keep pKa inside the published absolute range so fourteen minus pKa cannot overflow into a non-finite pKb the solver must reject. Use a consistent definition of pKa from your source table or experiment; if your literature quotes Ka only, convert with pKa equals minus log ten of Ka before calling this tool rather than mixing raw Ka and pKa units inside one request.
Prefer this capability when you already know pKa and only need the conjugate pKb under the standard sum-to-fourteen model; if your source is only measured Ka or Kb, convert to the logarithmic scale first with a dedicated sibling tool. Round-trip intuition is easy: fourteen minus the published pKb recovers the original pKa within the same twelve-decimal rounding for typical aqueous values, and confirming pair labels flip correctly when you move pKa across zero and seven.
What you can do with it
Lab notebook conjugate-pair tables
Technicians fill conjugate pKb columns from published acid pKa for reports, buffer design notes, and method sheets.
FAQ
Does this adjust for temperature or non-aqueous solvents?
Temperature-dependent Kw and other solvents are out of scope.
Is the pKa + pKb calculator free?
The browser tool is free and runs locally.
How do I calculate pKb from pKa?
Subtract the pKa from 14 and you have the pKb of the conjugate base. If your table lists only Ka, first turn it into pKa by taking minus its base-ten logarithm, then subtract.
Why do pKa and pKb add up to 14?
For a conjugate pair in water, Ka times Kb equals the ion product of water, 1 × 10⁻¹⁴ at 25 °C. Taking minus the logarithm of both sides turns that product into pKa plus pKb equal to 14.
What does a negative pKa say about the conjugate base?
A strong mineral acid with a negative pKa has a conjugate base with a very large pKb, so the base is extremely weak. At the other end, a very weak acid with a pKa above 7 has a relatively stronger conjugate base.
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/pka-pkb-sum \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"pka":4.76}'const res = await fetch("https://api.kit.forhosting.com/chem/pka-pkb-sum", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"pka": 4.76
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/pka-pkb-sum",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"pka": 4.76
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/pka-pkb-sum", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"pka":4.76}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"pka":4.76}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/pka-pkb-sum", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"pka": 4.76
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.pka_pkb_sum",
"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.
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
Non-numeric pKa—missing fields, blank cells, text, or non-finite values—is rejected with a clear invalid_input error so a typo never silently produces NaN. Homework checks, conjugate-pair tables, exam review, automated graders, and API pipelines all need that same closed formula without a spreadsheet or a full aqueous speciation package.
Why pKa and pKb sum to fourteen
Related quantities such as Ka from pKa, Kb from pKb, percent ionization, and buffer pH from the Henderson–Hasselbalch equation are intentionally out of scope so this page stays focused on the fourteen-sum identity for homework, lab prep, and API automation without a full aqueous speciation package or spreadsheet macros.
How to call the API and what comes back
Send a JSON body with one required finite number: pka (aliases pKa, p_ka, acid_pka, pka_value). Absolute value must stay within the published limit of one trillion so intermediate results remain finite in IEEE double precision on the Worker and in the browser. pKa must be numeric and finite; missing fields, blank strings, pure text, Infinity, and NaN are rejected with invalid_input. On success the response echoes rounded pka and pKa, sets pkb and pKb to fourteen minus that value, sum to the integer fourteen, formula to the standard rearrangement string, pair to a classroom band such as strong_acid_weak_conjugate, moderate, or weak_acid_strong_conjugate, message to a short English summary, and steps to an ordered array of substitution lines. Failed validation—missing pKa, non-numeric input, values outside absolute limits, or a non-finite computed pKb—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.
Practical tips, limits, and related searches
When teaching, pair classic examples: acetic acid pKa about four point seven six yields pKb about nine point two four; pKa seven yields pKb seven for the water-centered midpoint; pKa zero yields pKb fourteen; and a non-numeric string such as not-a-number demonstrates the hard invalid_input path that protects callers from silent NaN. This landing targets people searching for a pKa plus pKb calculator, pKa pKb equal fourteen, or conjugate base pKb from acid pKa online; it is not a Ka-from-pKa converter, not a buffer calculator, and not a temperature-dependent Kw solver. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately send text.
Integration use cases
Homework checks for pKa + pKb = 14
Students enter textbook acid pKa values and compare hand-written conjugate pKb against a deterministic result before submitting.
Unit tests for chemistry helpers
Generate known-good pKa→pKb cases plus deliberate non-numeric rejections for coding kata and automated grading pipelines.
Technical questions
How do you calculate pKb from pKa?
At standard aqueous conditions, pKa + pKb = 14, so pKb = 14 − pKa. This tool returns the rounded pKb, the fixed sum, and substitution steps.
What happens if pKa is non-numeric?
Missing, blank, text, Infinity, and NaN values are rejected with invalid_input so the API never returns a silent non-finite pKb.
What pKb does acetic acid have if pKa ≈ 4.76?
pKb ≈ 9.24. Enter pKa = 4.76 to see the rounded result and steps under the sum-to-fourteen model.