pH to pOH calculator
pH 7 at 25 °C means pOH 7 and 1 × 10⁻⁷ mol/L of both hydrogen and hydroxide ions: enter a pH from 0 to 14 and get the pOH and both ion concentrations in moles per litre. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Ph
- 7
- Poh
- 7
- H plus
- 0.0000001
- Oh minus
- 0.0000001
- Hydrogen ion mol l
- 0.0000001
pH and pOH are complementary scales that describe how acidic or basic an aqueous solution is at a given temperature. At twenty-five degrees Celsius the ion product of water is one times ten to the minus fourteen, so pH plus pOH always equals fourteen, and the hydrogen and hydroxide ion concentrations follow directly from base-ten powers of the negative scale values. This capability takes a single pH number in the closed classroom range from zero through fourteen and returns pOH together with both ion concentrations in moles per litre, using a pure deterministic formula with no network calls and no random behaviour.
What pH, pOH, and ion concentrations mean at 25 °C
pH is defined as the negative base-ten logarithm of the hydrogen ion activity; in dilute ideal solutions it is treated as the negative log of the molar hydrogen ion concentration. pOH is the same construction for the hydroxide ion. At twenty-five degrees Celsius pure water has equal concentrations of both ions at one times ten to the minus seven moles per litre, which places the neutral point at pH seven and pOH seven. Acidic solutions have higher hydrogen ion concentration and lower pH; basic solutions reverse that picture.
The water autoionization equilibrium enforces Kw equals one times ten to the minus fourteen, so the product of the two ion concentrations is fixed and the two logarithmic scales sum to fourteen. Use this calculator when you already know pH and need the complementary pOH and both molar ion concentrations for homework checks, lab worksheets, or automated grading without installing a full chemistry suite.
What you can do with it
Lab notebook and LIMS helpers
Bench apps convert recorded pH into complementary pOH and molar [H+]/[OH−] fields for sample records without re-deriving Kw formulas in every language.
FAQ
Is the pH to pOH calculator free?
The browser tool runs free on your device.
Can I send pOH or ion concentrations as input?
Converting from pOH or from [H+]/[OH−] is a different product intent and is not accepted here.
What do I get for a strong acid at pH 0?
A pOH of 14, a hydrogen ion concentration close to 1 mol/L and a hydroxide ion concentration close to 1 × 10⁻¹⁴ mol/L. The two concentrations always multiply to that same small number at 25 °C.
Why do pH and pOH always add up to 14?
At 25 °C the ion product of water is fixed at 1 × 10⁻¹⁴, so the hydrogen and hydroxide concentrations multiply to that value. Taking minus the logarithm of both sides turns that product into a sum of 14.
What happens if the pH is outside 0 to 14?
The calculator rejects it and asks for a value in that range, both ends included. The conversion follows the classroom scale at 25 °C, so readings outside it are not converted.
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/ph-poh-convert \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"ph":7}'const res = await fetch("https://api.kit.forhosting.com/chem/ph-poh-convert", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"ph": 7
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/ph-poh-convert",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"ph": 7
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/ph-poh-convert", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"ph":7}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"ph":7}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/ph-poh-convert", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"ph": 7
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.ph_poh_convert",
"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
What pH, pOH, and ion concentrations mean at 25 °C
Send the required field ph as a JSON number or numeric string; aliases pH and p_h are accepted. Values must lie in the closed interval from zero to fourteen inclusive—outside that band the request fails with invalid_input and is not charged. Identical inputs always produce identical JSON because the path is pure arithmetic with no wall clock, no seed, and no remote chemistry database.
Reading the response fields and the conversion formulas
The formula string restates pOH equals fourteen minus pH, hydrogen ion concentration equals ten to the power of minus pH, and hydroxide ion concentration equals ten to the power of minus pOH. For a neutral example with ph seven you get poh seven and both ion concentrations at one times ten to the minus seven. For a strong acid example with ph zero you get poh fourteen, hydrogen ion concentration near one, and hydroxide ion concentration near one times ten to the minus fourteen. Prefer the structured numbers for pipelines; use formula when a human-readable caption is enough for a report or slide. On success the response echoes the input ph, reports poh equal to fourteen minus ph, and returns h_plus and oh_minus as the hydrogen and hydroxide ion concentrations in moles per litre. The same concentrations are also exposed under the longer keys hydrogen_ion_mol_l and hydroxide_ion_mol_l so SDK clients can pick either naming style. temperature_c is always twenty-five and kw is always one times ten to the minus fourteen, making the temperature assumption explicit in every payload. The method label is always ph_poh_kw_25c so downstream code can branch without sniffing shapes. Missing ph, non-finite ph, and any value below zero or above fourteen fail with invalid_input.
API usage, pricing, classroom workflows, and practical limits
Call the free in-browser pH to pOH calculator for homework checks, lecture demos, and quick lab-bench verification without installing spreadsheet macros or a local chemistry package. The model does not adjust Kw for other temperatures, does not solve buffer equilibria, and does not accept pOH or ion concentrations as primary inputs—those are different search intents and different products. Send a JSON body with the required finite number ph inside zero through fourteen. Use the prepaid API when a tutoring platform, LMS auto-grader, curriculum generator, or lab notebook service needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Pair this endpoint with molar mass or dilution tools when the next step is solution prep, or keep it standalone when the only question is the pH–pOH–ion triangle at room temperature. Authors shipping SDKs can map h_plus and oh_minus directly into UI tables that show pH, pOH, and both concentrations side by side—the same layout students write by hand when they convert scales on paper.
Integration use cases
Homework and exam verification
Students enter a measured or given pH and check pOH plus both ion concentrations against answer keys; out-of-range pH returns a clear invalid_input error instead of a silent wrong number.
Tutoring and curriculum generators
Lesson builders produce worked pH–pOH examples with structured JSON for flashcards, step sheets, and auto-graded drills at the fixed 25 °C classroom convention.
Technical questions
How do I convert pH to pOH with this calculator?
Send JSON with ph set to a number between 0 and 14 (for example ph: 7). The response includes poh (14 − pH), h_plus ([H+] in mol/L), and oh_minus ([OH−] in mol/L) at 25 °C.
What happens if pH is outside 0 to 14?
The request fails with invalid_input. This capability accepts only the closed classroom range 0..14 inclusive; values outside that band are rejected and not charged.
At what temperature are the formulas valid?
All results assume 25 °C with Kw = 1.0×10⁻¹⁴ so that pH + pOH = 14. The response always includes temperature_c: 25 and kw: 1e-14.