pOH calculator
0.1 mol/L of hydroxide ions is pOH 1, while the pure-water level of 1 × 10⁻⁷ mol/L is the neutral pOH 7: type a known [OH−] and get the pOH, an alkalinity label and steps to copy into your notebook. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Concentration
- 0.0000001
- Oh minus
- 0.0000001
- Unit concentration
- mol/L
- Poh
- 7
- POH
- 7
A pOH calculator turns hydroxide ion concentration into the alkaline side of the acidity scale every chemistry lab and textbook uses. Enter [OH−] in moles per litre and the tool returns pOH equals minus log base ten of that concentration, a short alkalinity label—acidic, neutral, or basic—and step-by-step substitution you can copy into a notebook. Homework checks, titration prep, exam review, and automated graders all need that same closed formula without a spreadsheet or a full aqueous speciation package.
What pOH and [OH−] mean together
pOH is the common logarithmic measure of hydroxide ion activity expressed as a simple scale for aqueous solutions. By definition at standard classroom precision, pOH equals the negative base-ten logarithm of the hydroxide ion concentration in moles per litre: pOH equals minus log ten of [OH−]. When [OH−] is one times ten to the minus seven moles per litre, the pure-water reference at about twenty-five degrees Celsius, pOH is seven and the solution is called neutral on that scale.
Larger [OH−] means a smaller pOH and a more basic or alkaline solution; smaller [OH−] means a larger pOH and a more acidic solution. Concentrated strong bases can yield pOH below zero when [OH−] exceeds one mole per litre, and strongly acidic solutions can push pOH above fourteen when [OH−] is extremely small. Those extremes are still valid under the same formula; what is never valid is a zero or negative concentration, because the logarithm is only defined for positive real numbers.
Teachers and automated graders care about that rejection as much as the happy path: a silent fallback would hide a data-entry error. The check path recomputes minus log ten of the published concentration with the same fixed rounding used for the poh field, so a successful response always ships with steps you can assert in tests or lab notebooks.
Related classroom quantities such as pH and the water ion product Kw are intentionally out of scope here so this landing stays focused on the single search intent of converting a known [OH−] into pOH.
Practical tips, limits, and related searches
Keep [OH−] inside the published absolute range so minus log ten of concentration cannot overflow into a non-finite pOH the solver must reject. Use moles per litre consistently; if your source reports millimoles per litre or moles per cubic centimetre, convert before calling rather than mixing unit systems inside one request. Prefer this capability when you already know [OH−] and only need pOH; if your source is only a weak-base Kb and concentration of undissociated base, solve for [OH−] first with the appropriate equilibrium model.
Round-trip intuition is easy: ten to the power of minus the published pOH recovers the original concentration within the same twelve-decimal rounding for typical aqueous values, and confirming alkalinity flips correctly when you move [OH−] across the pure-water reference of one times ten to the minus seven moles per litre.
What you can do with it
Homework checks for pOH = −log10([OH−])
Students enter textbook hydroxide ion concentrations and compare hand-written pOH and alkalinity labels against a deterministic result before submitting.
Lab notebook conversion of meter-free [OH−]
Technicians convert a known or calculated [OH−] in mol/L into pOH for reports when only concentration data is available.
FAQ
How do you calculate pOH from hydroxide ion concentration?
Use pOH = −log₁₀([OH⁻]) with [OH⁻] in mol/L. This tool returns the rounded pOH, alkalinity class, and substitution steps.
Does this calculate pH or buffer pH?
No. It only converts a known [OH⁻] into pOH. Use a dedicated pH or buffer tool if you need those models.
Is the pOH calculator free?
The browser tool is free and runs locally.
Can pOH drop below 0 or rise above fourteen?
Yes. A concentrated strong base with more than 1 mol/L of hydroxide gives a pOH below zero, and a strongly acidic solution with very little hydroxide can push it above fourteen. Only zero or negative concentrations are refused.
What if I only know the Kb and concentration of a weak base?
Work out the hydroxide concentration first with a weak-base equilibrium model, then enter that [OH−] here. This calculator starts from a known hydroxide level and does not solve the equilibrium for you.
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/poh-from-hydroxide \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"concentration":1e-7}'const res = await fetch("https://api.kit.forhosting.com/chem/poh-from-hydroxide", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"concentration": 1e-7
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/poh-from-hydroxide",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"concentration": 1e-7
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/poh-from-hydroxide", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"concentration":1e-7}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"concentration":1e-7}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/poh-from-hydroxide", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"concentration": 1e-7
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.poh_from_hydroxide",
"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 or non-positive concentration is rejected with a clear invalid_input error so a blank cell or a negative typo never silently produces a non-finite log.
How to call the API and what comes back
The concentration 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 one required finite number: concentration (aliases oh_minus, oh_concentration, hydroxide, hydroxide_ion, hydroxide_ion_concentration, c_oh, molarity_oh) in moles per litre. 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. On success the response echoes rounded concentration and oh_minus, sets unit_concentration to mol/L, poh and pOH to the same rounded value, formula to the string pOH equals minus log ten of [OH−], alkalinity to acidic, neutral, or basic, message to a short English summary, and steps to an ordered array of substitution lines. Failed validation—missing concentration, non-finite numbers, values outside absolute limits, zero or non-positive concentration, or a non-finite computed pOH—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 one e minus seven.
Practical tips, limits, and related searches
When teaching, pair classic examples: [OH−] equals one times ten to the minus seven yields pOH seven and neutral; [OH−] equals zero point one for a strong monoprotic base approximation yields pOH one and basic; [OH−] equals one times ten to the minus fourteen yields pOH fourteen and acidic; and concentration equals zero demonstrates the hard invalid_input path. This landing targets people searching for a pOH calculator, pOH from hydroxide ion concentration, or minus log ten of [OH−] online; it is not a buffer calculator, not a titration curve tool, and not a pH or Kw solver. 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 acidic, neutral, and basic pOH cases plus deliberate zero-concentration rejections for coding kata and grading pipelines.
Technical questions
What happens if concentration is zero or negative?
pOH is undefined for non-positive [OH⁻]. The API returns invalid_input instead of taking a log of zero or a negative number.
What concentration gives pOH 7?
At the pure-water reference, [OH⁻] = 1×10⁻⁷ mol/L gives pOH = 7 and alkalinity neutral.