---
title: "Third Dissociation in General Chemistry II"
description: "Third dissociation is the third stepwise proton loss in a polyprotic acid, with its own equilibrium constant and pH effects in General Chemistry II."
canonical: "https://fiveable.me/general-chemistry-ii/key-terms/third-dissociation"
type: "key-term"
subject: "General Chemistry II"
unit: "Unit 3"
---

# Third Dissociation in General Chemistry II

## Definition

Third dissociation is the third ionization step of a polyprotic acid, where the species loses its third proton and forms a more highly deprotonated ion. In General Chemistry II, you treat it as a separate equilibrium with its own K value.

## What It Is

Third dissociation is the third step in a polyprotic acid’s sequence of proton losses. In General Chemistry II, that means you are looking at the point where a species that still has one more acidic proton gives up its third H+, producing the triply deprotonated form or, if the molecule started with more protons, the next more negatively charged ion in the series.

The big idea is that dissociation does not happen all at once. Each proton leaves in its own equilibrium, so the first, second, and third dissociations each get their own constant. By the time you reach the third dissociation, the molecule is usually carrying more negative charge, so it is harder to remove another proton. That is why the third dissociation is usually much weaker than the first one.

You can see this stepwise pattern in polyprotic acids such as phosphoric acid. The first proton leaves most easily, the second is harder, and the third is harder still. Chemically, that happens because the leftover ion holds onto its remaining proton more tightly and because removing another H+ makes an already negative species even more negative.

This is why you cannot use one single Ka value for the whole acid if the molecule can lose more than one proton. Each dissociation step changes the dominant species in solution, and each one affects pH a little differently. For many problems, the third dissociation is small enough to ignore in a rough estimate, but in a full equilibrium setup, you still need to know it exists and when it matters.

A common way this shows up is in phosphoric acid, H3PO4. After the first and second dissociations, the third step forms PO4^3-. That third step is much less favorable than the first two, but it still matters when you are tracking species distribution, buffer behavior, or the pH of a solution containing phosphate.

## Why It Matters

Third dissociation shows you how polyprotic acids behave one proton at a time instead of as one big acid reaction. That matters any time you are writing equilibrium expressions, comparing Ka values, or deciding which species is actually present in solution.

It also keeps you from making bad pH assumptions. If you treat a triprotic acid like it only has one dissociation, you can miss the later equilibria that control the solution at higher pH. That shows up in acid-base problems where you have to identify the dominant form, estimate pH, or interpret a titration curve with multiple equivalence points.

In Gen Chem II, third dissociation also connects directly to buffer chemistry. The phosphate system is a classic example, and the third dissociation helps explain why different phosphate species dominate at different pH values. If you know which dissociation is active, you can predict which ion is acting as an acid, which one is acting as a base, and how the mixture will respond to added acid or base.

## Connections

### Polyprotic Acid

Third dissociation only exists for an acid that can donate more than one proton. The concept makes more sense when you track the whole ionization series, because each step changes the charge and the strength of the remaining acidic proton. If the acid is diprotic, there is no third dissociation to analyze.

### Dissociation Constant

The third dissociation has its own dissociation constant, often written as Ka3. That value tells you how favorable the third proton loss is compared with the earlier steps. In practice, Ka3 is usually much smaller than Ka1 and often smaller than Ka2, so the third step contributes less to the overall acidity.

### [Second Dissociation](/general-chemistry-ii/key-terms/second-dissociation)

The second dissociation comes right before the third one in a polyprotic series, and comparing the two shows the stepwise drop in acidity. By the time the third proton is removed, the species is usually already more negatively charged, so the next proton is harder to remove. That pattern is one reason later Ka values get smaller.

### [phosphate buffer](/general-chemistry-ii/key-terms/phosphate-buffer)

The phosphate buffer system depends on phosphate species that come from successive dissociations of phosphoric acid. The third dissociation helps create the fully deprotonated phosphate ion, which matters when you are thinking about pH ranges where different phosphate forms dominate. It is a good example of how multiple equilibria shape buffer behavior.

## On the AP Exam

A problem set question might give you a polyprotic acid and ask which equilibrium constant belongs to the third proton loss, or which ion is formed after the third dissociation. You may also need to compare Ka1, Ka2, and Ka3, rank acid strength across steps, or decide whether the third step can be ignored in a pH approximation.

In a titration problem, the third dissociation can show up as the final ionization step after the second equivalence region, especially for triprotic acids like phosphoric acid. On quizzes and exams, the main move is to track species carefully, write the equilibrium for the correct step, and match the proton count to the correct ion name or formula.

## third dissociation vs second dissociation

These are easy to mix up because both are later steps in a polyprotic acid. The second dissociation is the loss of the second proton, while the third dissociation is the loss of the third proton. The third step is usually weaker because the molecule is already more negatively charged, so the remaining proton is held more tightly.

## Key Takeaways

- Third dissociation is the third stepwise proton loss from a polyprotic acid or the corresponding ionization step in a polyprotic base system.
- Each dissociation step has its own equilibrium constant, so the third dissociation is not described by the same K value as the first or second step.
- The third dissociation is usually weaker than the earlier ones because the species is already more negatively charged.
- In General Chemistry II, you use third dissociation when comparing Ka values, solving equilibrium problems, and reading titration or buffer behavior.
- Phosphoric acid is a classic example, because its third proton loss leads toward the fully deprotonated phosphate ion.

## FAQs

### What is third dissociation in General Chemistry II?

It is the third step in which a polyprotic acid loses a proton, forming a more highly deprotonated ion. In Gen Chem II, you treat it as a separate equilibrium with its own Ka value instead of folding it into one overall reaction.

### How is third dissociation different from second dissociation?

Second dissociation is the loss of the second proton, while third dissociation is the loss of the third proton. The third step is usually less favorable because the molecule already has more negative charge, so the next proton is harder to remove.

### Does every polyprotic acid have a third dissociation?

No. Only acids with at least three ionizable protons can have a third dissociation. A diprotic acid stops after the second step, so there is no third ionization to write or calculate.

### Why can the third dissociation sometimes be ignored?

In many weak polyprotic acids, Ka3 is so small that the third step changes the pH very little compared with the first and second steps. You still need to know it exists, but for a quick approximation it may be negligible unless the problem specifically asks for full speciation.

## Related Study Guides

- [3.4 Polyprotic acids and bases](/general-chemistry-ii/unit-3/polyprotic-acids-bases/study-guide/bvhJudnXCbiUesvu)

## About This Document

Canonical Fiveable pages are available as Markdown at the same path plus `.md`.

- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
- [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing
- [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`)
- [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`)

## Structured Data

```json
{"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/general-chemistry-ii/key-terms/third-dissociation#resource","name":"Third Dissociation in General Chemistry II","url":"https://fiveable.me/general-chemistry-ii/key-terms/third-dissociation","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/general-chemistry-ii/key-terms/third-dissociation#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:35.298Z","isPartOf":{"@type":"Collection","name":"General Chemistry II Key Terms","url":"https://fiveable.me/general-chemistry-ii/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/general-chemistry-ii/key-terms/third-dissociation#term","name":"third dissociation","description":"Third dissociation is the third ionization step of a polyprotic acid, where the species loses its third proton and forms a more highly deprotonated ion. In General Chemistry II, you treat it as a separate equilibrium with its own K value.","url":"https://fiveable.me/general-chemistry-ii/key-terms/third-dissociation","inDefinedTermSet":{"@type":"DefinedTermSet","name":"General Chemistry II Key Terms","url":"https://fiveable.me/general-chemistry-ii/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is third dissociation in General Chemistry II?","acceptedAnswer":{"@type":"Answer","text":"It is the third step in which a polyprotic acid loses a proton, forming a more highly deprotonated ion. In Gen Chem II, you treat it as a separate equilibrium with its own Ka value instead of folding it into one overall reaction."}},{"@type":"Question","name":"How is third dissociation different from second dissociation?","acceptedAnswer":{"@type":"Answer","text":"Second dissociation is the loss of the second proton, while third dissociation is the loss of the third proton. The third step is usually less favorable because the molecule already has more negative charge, so the next proton is harder to remove."}},{"@type":"Question","name":"Does every polyprotic acid have a third dissociation?","acceptedAnswer":{"@type":"Answer","text":"No. Only acids with at least three ionizable protons can have a third dissociation. A diprotic acid stops after the second step, so there is no third ionization to write or calculate."}},{"@type":"Question","name":"Why can the third dissociation sometimes be ignored?","acceptedAnswer":{"@type":"Answer","text":"In many weak polyprotic acids, Ka3 is so small that the third step changes the pH very little compared with the first and second steps. You still need to know it exists, but for a quick approximation it may be negligible unless the problem specifically asks for full speciation."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"General Chemistry II","item":"https://fiveable.me/general-chemistry-ii"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/general-chemistry-ii/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 3","item":"https://fiveable.me/general-chemistry-ii/unit-3"},{"@type":"ListItem","position":4,"name":"third dissociation"}]}]}
```