Buffer pH calculator
Get the pH of a weak-acid buffer from pKa and two concentrations: with pKa 4.76 and 0.1 mol/L each of acid and conjugate base, the pH is exactly 4.76. A pOH for 25 °C is shown as well. 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
- Conjugate base
- 0.1
- Acid
- 0.1
- Ratio
- 1
- Log ratio
- 0
- Ph
- 4.76
- Poh 25c
- 9.24
A weak-acid buffer resists pH change when small amounts of strong acid or base are added because both the undissociated acid HA and its conjugate base A− are present in useful amounts. Classroom problems, lab notebooks, and formulation pipelines therefore need a reliable way to turn three numbers—pKa, [A−], and [HA]—into the buffer pH without re-deriving the log ratio every time.
Henderson–Hasselbalch for acids: inputs and the pH formula
Start from a weak acid HA with known acid dissociation constant expressed as pKa, the negative base-10 logarithm of Ka. In a buffer that also contains the conjugate base A− (often as a soluble salt such as sodium acetate for acetic acid), the simplified equilibrium relation rearranges to pH equals pKa plus log10 of the ratio of conjugate base concentration to acid concentration. When the two concentrations are equal, the log term is zero and pH equals pKa exactly—the familiar half-equivalence or equimolar buffer point.
When [A−] is larger than [HA], the ratio exceeds one, the log is positive, and pH rises above pKa; when the free acid dominates, pH falls below pKa. Use the same concentration unit for both species so the ratio is dimensionless and meaningful; almost every general-chemistry problem uses mol per liter for both fields.
What you can do with it
Homework and exam verification
Students compute buffer pH from pKa and two concentrations, compare equimolar cases where pH equals pKa, and get a clear error when acid concentration is submitted as zero by mistake.
Lab notebook and buffer prep checks
Technicians verify the expected pH of acetate–acetic acid or similar weak-acid buffers before titration or storage, using the same deterministic formula as the course key.
FAQ
What happens if acid concentration is zero?
The Henderson–Hasselbalch ratio [A−]/[HA] is undefined when [HA] is zero, so the engine rejects the input and does not charge.
When does pH equal pKa?
When conjugate base and acid concentrations are equal, log10 of the ratio is zero, so pH equals pKa exactly (the equimolar buffer point).
Is the buffer pH calculator free?
The browser tool runs free on your device.
Does the response include pOH?
The primary result remains ph from the acid-buffer equation.
What happens to the pH when there is more conjugate base than acid?
It rises above pKa, because the ratio of base to acid is greater than one and its logarithm is positive. When the acid dominates, the logarithm turns negative and the pH falls below pKa.
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-ph \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"pka":4.76,"conjugate_base":0.1,"acid":0.1}'const res = await fetch("https://api.kit.forhosting.com/chem/buffer-ph", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"pka": 4.76,
"conjugate_base": 0.1,
"acid": 0.1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/buffer-ph",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"pka": 4.76,
"conjugate_base": 0.1,
"acid": 0.1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/buffer-ph", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"pka":4.76,"conjugate_base":0.1,"acid":0.1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"pka":4.76,"conjugate_base":0.1,"acid":0.1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/buffer-ph", 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,
"conjugate_base": 0.1,
"acid": 0.1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.buffer_ph",
"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 acids, pH equals pKa plus the base-10 logarithm of the conjugate-base-to-acid concentration ratio, with deterministic floating-point arithmetic and clear invalid_input errors when the acid concentration is zero or when concentrations are non-positive.
Henderson–Hasselbalch for acids: inputs and the pH formula
Send three required fields as JSON numbers or numeric strings: pka for the pKa value (aliases pKa, p_ka, acid_pka), conjugate_base for [A−] in mol per liter (aliases base, base_concentration, salt, a_minus, A_minus), and acid for [HA] in mol per liter (aliases acid_concentration, weak_acid, ha, HA). 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, pH, and the 25 °C pOH helper
For weak-base buffers that use pKb and [BH+] with [B], use the sibling buffer pOH calculator instead of forcing acid-side labels onto a base problem. The field poh_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 pOH, but treat pH as the primary contract for acid-buffer work. The formula string is always the Henderson–Hasselbalch acid form, and method is always henderson_hasselbalch_acid so downstream code can branch without sniffing shapes. Missing any of the three required fields, non-numeric values, negative concentrations, zero conjugate base, and zero acid all fail with invalid_input and are not charged. The product rule that acid 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 pKa-from-Ka converter when you only know Ka, or with a pH–pOH converter when you already have pH and need [H+] or pOH alone.
API usage, pricing, lab workflows, and practical limits
Call the free in-browser buffer pH 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. Equimolar acetate–acetic acid near pKa 4.76 is a classic sanity check: equal concentrations must return pH equal to pKa before you trust uneven ratios or automated pipelines. Send a JSON body with pka, conjugate_base, and acid 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 ph and poh_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 acid-buffer pH instead of reimplementing log10 ratios in every language.
Technical questions
How do I calculate buffer pH with this tool?
Send JSON with pka, conjugate_base ([A−] in mol/L), and acid ([HA] in mol/L). The response includes ph = pka + log10([A−]/[HA]), plus ratio and log_ratio.