Buffer pOH calculator
Get the pOH of a weak-base buffer from pKb and two concentrations: with pKb 4.75 and 0.1 mol/L each of base and conjugate acid, the pOH is 4.75 and the pH at 25 °C is 9.25. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Pkb
- 4.75
- Conjugate acid
- 0.1
- Base
- 0.1
- Ratio
- 1
- Log ratio
- 0
- Poh
- 4.75
- Ph 25c
- 9.25
A weak-base buffer resists pOH (and pH) change when small amounts of acid or base are added because both the free base B and its conjugate acid BH+ are present in appreciable amounts. Classroom problems, lab notebooks, and formulation pipelines therefore need a reliable way to turn three numbers—pKb, [BH+], and [B]—into the buffer pOH without re-deriving the log ratio every time.
How a weak-base buffer holds its pOH
A buffer made of a weak base B and its conjugate acid BH+ resists changes in pOH, and therefore in pH, when small amounts of acid or base are added, because both partners are present in appreciable amounts. Its pOH follows the base form of the Henderson–Hasselbalch equation: pKb plus the base-10 logarithm of the conjugate acid concentration divided by the base concentration. When the two are equal, the logarithm is zero and pOH equals pKb exactly, 4.75 in the example with 0.1 mol/L of each. The conjugate acid is often added as a soluble salt, such as ammonium chloride for ammonia.
Reading the result and avoiding slips
More conjugate acid than base pushes the pOH above pKb, and more base pulls it below. Both concentrations go in mol per litre. A base concentration of zero is refused, because the ratio would have no meaning, and negative concentrations are refused too. The pH shown next to the pOH assumes 25 °C, where the two add up to 14, so a pOH of 4.75 gives a pH of 9.25. If your problem is framed in pH, read that value, but keep in mind that pOH is the main result for a base buffer.
Henderson–Hasselbalch for bases: inputs and the pOH formula
Start from a weak base B with known base dissociation constant expressed as pKb, the negative base-10 logarithm of Kb. In a buffer that also contains the conjugate acid BH+ (often as a soluble salt such as ammonium chloride for ammonia), the simplified equilibrium relation rearranges to pOH equals pKb plus log10 of the ratio of conjugate acid concentration to base concentration. When the two concentrations are equal, the log term is zero and pOH equals pKb exactly—the familiar half-equivalence or equimolar buffer point.
When [BH+] is larger than [B], the ratio exceeds one, the log is positive, and pOH rises above pKb; when the free base dominates, pOH falls below pKb.
What you can do with it
Homework and exam verification
Students compute buffer pOH from pKb and two concentrations, compare equimolar cases where pOH equals pKb, and get a clear error when base concentration is submitted as zero by mistake.
Lab notebook and buffer prep checks
Technicians verify the expected pOH of ammonia–ammonium or similar weak-base buffers before titration or storage, using the same deterministic formula as the course key.
FAQ
What happens if base concentration is zero?
The Henderson–Hasselbalch ratio [BH+]/[B] is undefined when [B] is zero, so the engine rejects the input and does not charge.
When does pOH equal pKb?
When conjugate acid and base concentrations are equal, log10 of the ratio is zero, so pOH equals pKb exactly (the equimolar buffer point).
Is the buffer pOH calculator free?
The browser tool runs free on your device.
Does the response include pH?
The primary result remains poh from the base-buffer equation.
How do I get the pH of an ammonia buffer?
Read the pH shown next to the pOH. It assumes 25 °C, where pH and pOH add up to 14, so in the example with pKb 4.75 and equal concentrations, a pOH of 4.75 means a pH of 9.25.
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/buffer-poh \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"pkb":4.75,"conjugate_acid":0.1,"base":0.1}'const res = await fetch("https://api.kit.forhosting.com/chem/buffer-poh", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"pkb": 4.75,
"conjugate_acid": 0.1,
"base": 0.1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/buffer-poh",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"pkb": 4.75,
"conjugate_acid": 0.1,
"base": 0.1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/buffer-poh", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"pkb":4.75,"conjugate_acid":0.1,"base":0.1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"pkb":4.75,"conjugate_acid":0.1,"base":0.1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/buffer-poh", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"pkb": 4.75,
"conjugate_acid": 0.1,
"base": 0.1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.buffer_poh",
"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
This capability applies the Henderson–Hasselbalch equation for bases, pOH equals pKb plus the base-10 logarithm of the conjugate-acid-to-base concentration ratio, with deterministic floating-point arithmetic and clear invalid_input errors when the base concentration is zero or when concentrations are non-positive.
Henderson–Hasselbalch for bases: inputs and the pOH formula
Send three required fields as JSON numbers or numeric strings: pkb for the pKb value (aliases pKb, p_kb, base_pkb), conjugate_acid for [BH+] in mol per liter (aliases acid, acid_concentration, salt, bh_plus, conjugate_acid_concentration), and base for [B] in mol per liter (aliases base_concentration, weak_base, b_conc). Identical inputs always produce identical JSON because the path is pure arithmetic with no network, no random seed, and no wall clock.
Reading the response: ratio, pOH, and the 25 °C pH helper
The field ph_25c is a convenience conversion that assumes the common 25 °C water ion product with pH plus pOH equal to fourteen; use it for quick display when your problem is framed in pH, but treat pOH as the primary contract for base-buffer work. The formula string is always the Henderson–Hasselbalch base form, and method is always henderson_hasselbalch_base so downstream code can branch without sniffing shapes. Missing any of the three required fields, non-numeric values, negative concentrations, zero conjugate acid, and zero base all fail with invalid_input and are not charged. The product rule that base concentration cannot be zero is intentional: division by zero would make the log ratio undefined, and the API never returns Infinity or a silent NaN in place of an honest error. Pair this endpoint with a pKb-from-Kb converter when you only know Kb, or with a pH–pOH converter when you already have pOH and need [OH−] or pH alone.
API usage, pricing, lab workflows, and practical limits
Call the free in-browser buffer pOH calculator for homework checks, lecture demos, and quick lab notebook verification without installing a spreadsheet or a chemistry CAS. Concentrations should stay positive and practical: extremely large or tiny values may still be finite mathematically but are rarely meaningful for aqueous buffers taught in general chemistry. Send a JSON body with pkb, conjugate_acid, and base set to finite numbers in the units of your problem statement—almost always mol per liter for both concentrations. Use the prepaid API when a tutoring platform, formulation tool, quality-control script, or curriculum generator needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Authors shipping SDKs can map poh and ph_25c directly into UI readouts while showing ratio and log_ratio in a worked-solution panel so students see the same intermediate steps they write on paper when applying Henderson–Hasselbalch by hand.
Integration use cases
Tutoring apps and chemistry pipelines
Lesson builders and auto-graders call a stable prepaid endpoint for Henderson–Hasselbalch base-buffer pOH instead of reimplementing log10 ratios in every language.
Technical questions
How do I calculate buffer pOH with this tool?
Send JSON with pkb, conjugate_acid ([BH+] in mol/L), and base ([B] in mol/L). The response includes poh = pkb + log10([BH+]/[B]), plus ratio and log_ratio.