Buffer pH after acid addition
See how far a buffer's pH falls when strong acid goes in: adding 0.02 mol to 0.1 mol each of weak acid and conjugate base with pKa 4.76 leaves 0.12 mol and 0.08 mol, and the pH drops to 4.583908740944. 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
- Moles acid
- 0.1
- Moles base
- 0.1
- Moles added
- 0.02
- Moles acid final
- 0.12
- Moles base final
- 0.08
- Ratio
- 0.6666666667
- Log ratio
- -0.1760912591
When a strong monoprotic acid is poured into a weak-acid buffer, the conjugate base A− is protonated to form more HA, the mole ratio of the pair shifts, and the buffer pH falls according to the Henderson–Hasselbalch equation written on the final moles. Students, lab technicians, and formulation tools need that new pH without rebuilding the stoichiometry by hand every time. This capability takes pKa, the initial moles of weak acid and conjugate base, and the moles of strong acid added, applies A− plus H+ yields HA, and returns the post-addition pH with intermediate mole amounts, ratio, and log ratio.
Stoichiometry of strong acid addition to a weak-acid buffer
Start from a closed weak-acid buffer with known pKa and initial mole amounts of the undissociated acid HA and its conjugate base A−. When you add n moles of strong monoprotic acid, each mole of H+ converts one mole of A− into one mole of HA. The engine therefore computes final base moles as initial base moles minus moles added, and final acid moles as initial acid moles plus moles added.
Those final moles share the same solution volume, so their ratio equals the concentration ratio that enters Henderson–Hasselbalch: pH equals pKa plus the base-10 logarithm of final A− moles over final HA moles. Concentrations alone are not required because textbook problems usually state moles of acid added directly; if you only have molarities and a volume, multiply both species by the same volume before calling.
Zero addition leaves the ratio unchanged and recovers the ordinary static buffer pH, which is a useful sanity check against equimolar acetate systems near pKa 4.76.
What you can do with it
Homework: pH after adding strong acid to a buffer
Students enter pKa and moles of HA, A−, and HCl, check that pH falls below the equimolar value, and see a clear error when the acid load exhausts the conjugate base.
Lab notebook buffer adjustment
Technicians estimate the new pH after spiking a known acetate or phosphate buffer with a measured amount of strong acid before titration or storage.
FAQ
How do I calculate buffer pH after adding strong acid?
The engine applies A− + H+ → HA and returns ph = pKa + log10(n_A_final / n_HA_final).
Do I enter concentrations or moles?
Enter moles for HA, A−, and the strong acid added. If you only know molarities and a shared volume, multiply both species by that volume first so the ratio stays correct.
Is the buffer pH after acid addition calculator free?
The browser tool runs free on your device.
What if I add more strong acid than the buffer has base?
The tool refuses it. Once the added acid equals or exceeds the conjugate base, the base is used up and the mixture is no longer a simple buffer, so any pH it reported would be invented.
What does each mole of strong acid do to the buffer?
It turns one mole of conjugate base into one mole of weak acid. The base goes down and the acid goes up by the amount added, and the new ratio of the two sets the pH.
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-after-addition \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"pka":4.76,"moles_acid":0.1,"moles_base":0.1,"moles_added":0.02}'const res = await fetch("https://api.kit.forhosting.com/chem/buffer-after-addition", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"pka": 4.76,
"moles_acid": 0.1,
"moles_base": 0.1,
"moles_added": 0.02
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/buffer-after-addition",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"pka": 4.76,
"moles_acid": 0.1,
"moles_base": 0.1,
"moles_added": 0.02
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/buffer-after-addition", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"pka":4.76,"moles_acid":0.1,"moles_base":0.1,"moles_added":0.02}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"pka":4.76,"moles_acid":0.1,"moles_base":0.1,"moles_added":0.02}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/buffer-after-addition", 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,
"moles_acid": 0.1,
"moles_base": 0.1,
"moles_added": 0.02
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.buffer_after_addition",
"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
If the added acid equals or exceeds the conjugate base present, the engine returns invalid_input instead of inventing a buffer pH outside the buffer region.
Stoichiometry of strong acid addition to a weak-acid buffer
Send four required fields as JSON numbers or numeric strings: pka for the acid strength (aliases pKa, p_ka, acid_pka), moles_acid for initial HA moles (aliases moles_ha, n_ha, ha_moles, acid_moles), moles_base for initial A− moles (aliases moles_a, n_a, base_moles, conjugate_base_moles, salt_moles), and moles_added for strong-acid moles (aliases strong_acid_moles, acid_added, n_added, moles_strong_acid).
Reading the response and the exhaustion error
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 a problem is framed in pOH, but treat ph as the primary contract. The formula string documents the post-addition Henderson–Hasselbalch path, and method is always buffer_after_strong_acid so clients can branch without sniffing shapes. Missing required fields, non-numeric values, negative mole amounts, non-positive initial base, and any case where moles_added is greater than or equal to moles_base all fail with invalid_input and are not charged. That exhaustion rule is intentional: once the conjugate base is fully consumed, the mixture is no longer a simple buffer of both members of the pair, and inventing a pH from a zero or negative base mole amount would be dishonest. Pair this endpoint with the static buffer pH calculator when you only need pH from concentrations without addition, or with pKa-from-Ka converters when you know Ka rather than pKa.
API usage, pricing, lab workflows, and practical limits
Call the free in-browser buffer pH after acid addition calculator for homework checks, lecture demos, and quick lab notebook verification without installing a spreadsheet. Mole amounts should stay non-negative and practical: extremely large or tiny values may still be finite mathematically but are rarely meaningful for aqueous buffers taught in general chemistry. A classic acetate check uses pKa 4.76 with equal 0.10 mol of HA and A− and a small strong-acid spike such as 0.02 mol: final moles become 0.12 and 0.08, the ratio falls below one, and pH drops slightly below pKa as expected. Send a JSON body with pka, moles_acid, moles_base, and moles_added set to finite numbers in the units of your problem statement—almost always moles for the three amount fields and dimensionless pKa for the acid strength. 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 the final mole fields 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 buffer stoichiometry by hand.
Integration use cases
Tutoring apps and auto-graders
Lesson builders call a stable prepaid endpoint for post-addition Henderson–Hasselbalch instead of reimplementing mole bookkeeping in every language.
Technical questions
What if the added acid exceeds the conjugate base?
The request fails with invalid_input. When moles_added is greater than or equal to moles_base, the buffer base is exhausted and a simple buffer pH is not returned.
Does zero addition still work?
Yes. When moles_added is zero, final moles equal initial moles and pH matches the ordinary Henderson–Hasselbalch result for that pair.