Synthetic-intermediate context
Attributed HSDB records identify phthalic acid in synthesis of compounds including phenolphthalein and phthalimide. [3]
01
THE ESSENTIALS
Phthalic Acid in 30 Seconds
Two neighboring ring carbons carry COOH groups. Their adjacent arrangement is positional isomerism, not a cis/trans alkene designation. The neutral acid is achiral.
Phthalic acid is the ortho, or 1,2, benzene diacid. It is distinct from phthalic anhydride and from the meta and para positional isomers; heating data need to retain chemical-change qualifiers. [1] [3]
Safety at a glance — Sigma 402915 classifies serious eye damage; TCI lists skin and eye irritation. Retain the more severe exact-product warning rather than reducing the summary to mild irritation. Control solid exposure and account for dehydration/chemical change on heating; use the applicable product SDS. [4] [5] [6]
A condensed structural formula keeps the functional groups visible. [1]
02
GO A LITTLE DEEPER
Understanding Phthalic Acid
THE SIMPLE EXPLANATION
The functional groups
Two neighboring ring carbons carry COOH groups. Their adjacent arrangement is positional isomerism, not a cis/trans alkene designation. The neutral acid is achiral. [1]
A LITTLE DEEPER
What the structure changes
Heating can remove water to form phthalic anhydride. That reaction changes formula and identity; it is not ordinary boiling of the unchanged acid. Stepwise acid dissociation and dehydration are different processes. [1] [12]
Technical detail Identity, dissociation & specification
The molecular formula specifies the neutral free acid. A commercial solution, salt, ester or anhydride needs its own composition and specification. Quantitative references retain temperature, pressure, product identity and unresolved method qualifiers. [1] [2] [4] [5] [6]
Acid–base reference values describe equilibria under specified conditions. They are not a substitute for a validated formulation or an assay on the purchased lot.
03
MAKE THE CONNECTION
How Phthalic Acid Compares
Similar names and functional groups do not mean interchangeable chemicals.
SIDE-BY-SIDE COMPARISON
Phthalic Acid vs Isophthalic Acid
Ortho aromatic dicarboxylic acid
versusMeta aromatic dicarboxylic acid
Fixed comparison for this prototype. Acid selectors are disabled; additional comparisons are coming later.
| Property | Phthalic Acid | Isophthalic Acid |
|---|---|---|
| Molecular formula | C₈H₆O₄ [1] | C₈H₆O₄ [7] |
| Molecular weight | 166.13 g/mol [1] | 166.13 g/mol [7] |
| Carbon atoms | 8 [1] | 8 [7] |
| Carboxyl groups | 2 [1] | 2 [7] |
| Structure | 1,2-C₆H₄(COOH)₂ [1] | 1,3-C₆H₄(COOH)₂ [7] |
| Chemical family | Ortho aromatic dicarboxylic acid [1] | Meta aromatic dicarboxylic acid [7] |
| Identity focus | Free acid [1] | Free acid [7] |
Chemical identities are compared here; supplier grades, concentrations, and stereochemical specifications are separate purchasing attributes. pKa values retain their stated conditions; no quantitative solubility ranking is implied.
When Might You Choose One Instead of the Other?
Choose phthalic acid for a method specifying the ortho free acid; choose isophthalic acid for a meta aromatic-diacid target. If a synthesis calls for phthalic anhydride, obtain or prepare that defined compound under the method rather than substituting free acid without accounting for water and reaction chemistry. [1] [3] [7]
Specified starting material
Phthalic Acid
Match 1,2-C₆H₄(COOH)₂ and the method’s required chemical form.
Isophthalic Acid
Match 1,3-C₆H₄(COOH)₂; verify the identity before substituting a related acid.
Before choosing: match the reaction or formulation, then verify the exact form, grade, assay basis, and impurities. These contexts do not establish suitability for an industrial or regulated application.
04
FROM STRUCTURE TO APPLICATION
What Is Phthalic Acid Used For?
Synthetic-intermediate context
Attributed HSDB records identify phthalic acid in synthesis of compounds including phenolphthalein and phthalimide. The finished dye or imide properties belong to those derivatives. Industrial phthalic-anhydride routes are not automatically free-acid uses. [3]
Compendial analytical-test context
Wako separately sells a Japanese Pharmacopoeia general-tests phthalic-acid product for testing hypromellose phthalate. This is a direct analytical use, not qualification of the priced reagent products as pharmaceutical ingredients. [14]
06
SOURCE WITH CONTEXT
Phthalic Acid Suppliers
Compare specific products and package options. Listings are not ranked recommendations.
3 suppliers · 3 products · 7 package listingsLatest supplier check: October 3, 2026
Package sizes stay grouped under the supplier product. Assay methods, product forms, and regional markets remain visible. TCI uses GC and titration, Wako uses capillary GC and Sigma’s ACS reagent assay method is unstated. Positional identity, water and anhydride impurity need their own controls. ACS or JP test-reagent terminology does not establish pharmaceutical-ingredient suitability.
Scroll horizontally if needed. Supplier and package identity stay visible.
| Supplier | Product / Grade | Purity / Assay | Package Size | Public Price | Normalized Price | Documentation | Last Checked | Action |
|---|---|---|---|---|---|---|---|---|
| TCI AmericaPhthalic Acid | Named grade not statedP0287 · 25 g | GC: minimum 99.0% Neutralization titration: minimum 99.0% | 25 g | $21.00USD · US public price | Not comparedGrades, assay bases, or forms differ | Document notes: TCI America
| View Product: TCI America, P0287 · 25 g (opens new tab) | |
| Named grade not statedP0287 · 500 g | GC: minimum 99.0% Neutralization titration: minimum 99.0% | 500 g | $58.00USD · US public price | Not comparedGrades, assay bases, or forms differ | View Product: TCI America, P0287 · 500 g (opens new tab) | |||
| Sigma-Aldrich / MerckPhthalic Acid | ACS reagent402915-25G | Minimum 99.5%; method not stated | 25 g | $43.50USD · US public price | Not comparedGrades, assay bases, or forms differ | Document notes: Sigma-Aldrich / Merck
| View Product: Sigma-Aldrich / Merck, 402915-25G (opens new tab) | |
| ACS reagent402915-250G | Minimum 99.5%; method not stated | 250 g | $87.20USD · US public price | Not comparedGrades, assay bases, or forms differ | View Product: Sigma-Aldrich / Merck, 402915-250G (opens new tab) | |||
| ACS reagent402915-1KG | Minimum 99.5%; method not stated | 1 kg | $121.00USD · US public price | Not comparedGrades, assay bases, or forms differ | View Product: Sigma-Aldrich / Merck, 402915-1KG (opens new tab) | |||
| FUJIFILM WakoPhthalic Acid | Wako Special Grade163-02462Research / further manufacturing. Japan stock does not confirm US availability. Distinct from the separately qualified Japanese Pharmacopoeia general-tests product. | 99.0+% (Capillary GC) | 25 g | $23.00USD · US catalog list price | Not comparedGrades, assay bases, or forms differ | Document notes: FUJIFILM Wako
| View Product: FUJIFILM Wako, 163-02462 (opens new tab) | |
| Wako Special Grade167-02465Research / further manufacturing. Japan stock does not confirm US availability. Distinct from the separately qualified Japanese Pharmacopoeia general-tests product. | 99.0+% (Capillary GC) | 500 g | $71.00USD · US catalog list price | Not comparedGrades, assay bases, or forms differ | View Product: FUJIFILM Wako, 167-02465 (opens new tab) |
TCI America
Phthalic Acid
Neutralization titration: minimum 99.0%
Named grade not stated
25 g
P0287 · 25 g- Public price
- $21.00USD · US public price
- Normalized price
- Not comparedGrades, assay bases, or forms differ
500 g
P0287 · 500 g- Public price
- $58.00USD · US public price
- Normalized price
- Not comparedGrades, assay bases, or forms differ
Documentation
Document notes: TCI America
- A product-specific certificate retrieval option was visible; no lot certificate was reviewed. Open the product page and use its certificate section.
- A current regional SDS was not verified in this pass.
- Origin-document availability was not verified. Country of origin is unverified and is not inferred.
Last checked:
Sigma-Aldrich / Merck
Phthalic Acid
ACS reagent
25 g
402915-25G- Public price
- $43.50USD · US public price
- Normalized price
- Not comparedGrades, assay bases, or forms differ
250 g
402915-250G- Public price
- $87.20USD · US public price
- Normalized price
- Not comparedGrades, assay bases, or forms differ
1 kg
402915-1KG- Public price
- $121.00USD · US public price
- Normalized price
- Not comparedGrades, assay bases, or forms differ
Documentation
Document notes: Sigma-Aldrich / Merck
- A product-specific certificate retrieval option was visible; no lot certificate was reviewed. Open the product page and use its certificate section.
- A current regional SDS was not verified in this pass.
- Origin-document availability was not verified. Country of origin is unverified and is not inferred.
Last checked:
FUJIFILM Wako
Phthalic Acid
Wako Special Grade
Research / further manufacturing. Japan stock does not confirm US availability. Distinct from the separately qualified Japanese Pharmacopoeia general-tests product.
25 g
163-02462- Public price
- $23.00USD · US catalog list price
- Normalized price
- Not comparedGrades, assay bases, or forms differ
500 g
167-02465- Public price
- $71.00USD · US catalog list price
- Normalized price
- Not comparedGrades, assay bases, or forms differ
Documentation
Document notes: FUJIFILM Wako
- A product-specific certificate retrieval option was visible; no lot certificate was reviewed. Open the product page and use its certificate section.
- A current regional SDS was not verified in this pass.
- Origin-document availability was not verified. Country of origin is unverified and is not inferred.
Last checked:
Documentation evidence
COA = Certificate of Analysis · SDS = Safety Data Sheet · COO = Certificate of Origin.
Lookup means a retrieval option was visible, not that a lot certificate was reviewed. Check supplier means availability or the applicable document was not verified.
Prices may vary by quantity, account, contract, geography, and availability. These products have different grades, assay bases, or forms; no normalized comparison is shown. Foreign-market prices are reference listings only. No currency conversion, mass-to-volume conversion, or ranking is applied. Confirm equivalent specifications and a current quote before comparison. [4] [5] [6]
Verification limits & placement policy
Thermal transition: CONFLICT UNRESOLVED. Attributed records include about 210 °C water loss, 230 °C decomposition and a sealed-tube reference near 191 °C; Wako gives 196–202 °C. Measurement context explains some differences, but not every discrepancy. No single ordinary melting constant. Water solubility: CONFLICT UNRESOLVED. TCI gives 5.7 g/L at 20 °C, while an attributed NTP record gives less than 1 g/L at 20 °C; cause not established. A 25 °C reference cannot resolve the same-temperature conflict. Numeric main field omitted. pKa: OMITTED. Compiled stepwise values at 25 °C lack fully verified measurement/ionic-strength context; no numeric positional-isomer ranking. Boiling point: OMITTED. Decomposition/dehydration and a predicted boiling estimate are not a measured normal boiling point.
Only the displayed identity, specification, packages, public prices and stated documentation evidence were reviewed. Current stock, lead times, shipping eligibility, contract pricing, lot certificates, and regulatory suitability are unverified.
There are no paid placements. Future paid listings will show “Sponsored” in both views; payment will not determine chemistry interpretation or imply a ranking.
07
PLAN A LARGER ORDER
Need Phthalic Acid in Bulk?
Turn your requirements into a clear supplier brief.
Bulk RFQ feature — coming soon. Prototype form: submissions are not currently sent to suppliers. Use sample details to prepare an on-screen brief.
Prepare a bulk request Prototype form
08
A BUYER’S CHECKLIST
Before You Buy Phthalic Acid
Selections remain only while this page is open.
09
FOR YOUR REFERENCE
Phthalic Acid Technical Reference
Identity, product reference properties, and handling context.
Identification Names, formula & identifiers
- Principal CAS
- 88-99-3
- Identity focus
- Free acid
- Formula
- C₈H₆O₄
- Molecular weight
- 166.13 g/mol
- IUPAC name
- phthalic acid
- Connectivity SMILES
- C1=CC=C(C(=C1)C(=O)O)C(=O)O
- Identity SMILES
- C1=CC=C(C(=C1)C(=O)O)C(=O)O
- InChI
- InChI=1S/C8H6O4/c9-7(10)5-3-1-2-4-6(5)8(11)12/h1-4H,(H,9,10)(H,11,12)
- PubChem CID
- 1017
Connectivity SMILES describes connectivity; it is not an independent stereochemical assignment. CAS and product-form distinctions are supported by the supplier listings. [1] [4] [5] [6]
Physical Properties Product references & conditions
Solid at 20 °C. Thermal references include decomposition or loss of water to anhydride; no universal clean melting or normal boiling point is claimed. [13] [4] [5]
- Thermal behavior
- Sigma 402915: 210–211 °C (decomposition), literature reference. Other methods and references differ; see uncertainty notes. [5]
- Solid density reference
- Attributed Ullmann reference: 1.593 g/cm³ at 15 °C; not bulk powder density [13]
Hydrate, concentration, stereoisomer, and temperature can change the relevant specification. Product reference data are not a reviewed lot certificate.
Acid/Base Properties Verified values & missing data
A numeric pKa was not verified for the selected identity in this review; no value is inferred. [2]
Thermal transition: CONFLICT UNRESOLVED. Attributed records include about 210 °C water loss, 230 °C decomposition and a sealed-tube reference near 191 °C; Wako gives 196–202 °C. Measurement context explains some differences, but not every discrepancy. No single ordinary melting constant. Water solubility: CONFLICT UNRESOLVED. TCI gives 5.7 g/L at 20 °C, while an attributed NTP record gives less than 1 g/L at 20 °C; cause not established. A 25 °C reference cannot resolve the same-temperature conflict. Numeric main field omitted. pKa: OMITTED. Compiled stepwise values at 25 °C lack fully verified measurement/ionic-strength context; no numeric positional-isomer ranking. Boiling point: OMITTED. Decomposition/dehydration and a predicted boiling estimate are not a measured normal boiling point.
Temperature, ionic strength, and the use of activities versus concentrations matter in quantitative work. These reference values are not buffer recipes.
Safety at a Glance Product-specific hazards
Sigma 402915 classifies serious eye damage; TCI lists skin and eye irritation. Retain the more severe exact-product warning rather than reducing the summary to mild irritation. Control solid exposure and account for dehydration/chemical change on heating; use the applicable product SDS. [4] [5] [6]
Hazard classifications can vary by concentration, product, supplier and jurisdiction. Review the SDS for the exact product before handling. This summary does not replace the SDS.
Supplier safety information and SDS retrieval (opens new tab)
A product-page safety classification is not verification of every regional SDS. Confirm product code, chemical form and current local document.
Review supplier documentation10
COMMON QUESTIONS
Phthalic Acid FAQ
What distinguishes Phthalic Acid?
Two neighboring ring carbons carry COOH groups. Their adjacent arrangement is positional isomerism, not a cis/trans alkene designation. The neutral acid is achiral. [1]
How does it differ from Isophthalic Acid?
Isophthalic acid places the carboxyls at 1,3 instead of 1,2. The formula and molecular weight are identical, but adjacency enables the characteristic phthalic-anhydride relationship. Neither assay nor formula alone identifies the positional isomer. [7]
What must a buyer specify?
TCI uses GC and titration, Wako uses capillary GC and Sigma’s ACS reagent assay method is unstated. Positional identity, water and anhydride impurity need their own controls. ACS or JP test-reagent terminology does not establish pharmaceutical-ingredient suitability. [4]
Are the listed products directly price-comparable?
No. The table retains differing grades, assay bases, forms, and markets. No normalized ranking or currency conversion is applied. [5]
What reference data still need checking?
Thermal transition: CONFLICT UNRESOLVED. Attributed records include about 210 °C water loss, 230 °C decomposition and a sealed-tube reference near 191 °C; Wako gives 196–202 °C. Measurement context explains some differences, but not every discrepancy. No single ordinary melting constant. Water solubility: CONFLICT UNRESOLVED. TCI gives 5.7 g/L at 20 °C, while an attributed NTP record gives less than 1 g/L at 20 °C; cause not established. A 25 °C reference cannot resolve the same-temperature conflict. Numeric main field omitted. pKa: OMITTED. Compiled stepwise values at 25 °C lack fully verified measurement/ionic-strength context; no numeric positional-isomer ranking. Boiling point: OMITTED. Decomposition/dehydration and a predicted boiling estimate are not a measured normal boiling point.
Can I request a bulk quote here?
The RFQ is a prototype that prepares an on-screen brief. Nothing is sent to suppliers. Confirm actual bulk terms directly with the supplier.
11
TRACE THE INFORMATION
Sources & Editorial Notes
Identity and compiled reference values are separated from supplier-specific specifications. Structural comparisons and buyer guidance are editorial interpretation. Sources were checked October 3, 2026; supplier information can change.
- PubChem · chemical identity (opens new tab)
Neutral molecular identity verified independently through PUG REST; CAS also checked on all selected supplier products.
- PubChem / HSDB · dissociation references (opens new tab)
No numeric field promoted: insufficient conditions or unresolved conflicting records.
- PubChem / HSDB · application references (opens new tab)
Attributed application records reviewed claim by claim. Free acid, intermediate, derivative and biological context are distinguished.
- TCI America · commercial product (opens new tab)
Direct product checked October 3, 2026. Identity, listed assay methods, packages, market, prices and documentation interfaces inspected. Lot certificates and stock are not verified.
- Sigma-Aldrich / Merck · commercial product (opens new tab)
Direct product checked October 3, 2026. Identity, listed assay methods, packages, market, prices and documentation interfaces inspected. Lot certificates and stock are not verified.
- FUJIFILM Wako · commercial product (opens new tab)
Direct product checked October 3, 2026. Identity, listed assay methods, packages, market, prices and documentation interfaces inspected. Lot certificates and stock are not verified.
- Isophthalic Acid · identity (opens new tab)
Primary structural comparison. Practical interpretation does not establish suitability.
- Benzoic Acid · identity (opens new tab)
Related identity and structural relationship.
- Maleic Acid · identity (opens new tab)
Related identity and structural relationship.
- Fumaric Acid · identity (opens new tab)
Related identity and structural relationship.
- Isophthalic Acid · dissociation references (opens new tab)
Only independently reviewed values are promoted into the comparison; no condition-free acidity ranking.
- OpenStax · Carboxylic-acid structure and properties (opens new tab)
General carboxylate resonance, acidity and structural interpretation; not authority for this acid’s quantitative constants.
- PubChem / HSDB · attributed physical references (opens new tab)
Attributed references reviewed; product, phase, pH and method gaps stated rather than inferred.
- Wako · JP general-tests product (opens new tab)
Separate 100 mg product, JP test context and distinct specification; not a priced row here.
Review limits
Thermal transition: CONFLICT UNRESOLVED. Attributed records include about 210 °C water loss, 230 °C decomposition and a sealed-tube reference near 191 °C; Wako gives 196–202 °C. Measurement context explains some differences, but not every discrepancy. No single ordinary melting constant. Water solubility: CONFLICT UNRESOLVED. TCI gives 5.7 g/L at 20 °C, while an attributed NTP record gives less than 1 g/L at 20 °C; cause not established. A 25 °C reference cannot resolve the same-temperature conflict. Numeric main field omitted. pKa: OMITTED. Compiled stepwise values at 25 °C lack fully verified measurement/ionic-strength context; no numeric positional-isomer ranking. Boiling point: OMITTED. Decomposition/dehydration and a predicted boiling estimate are not a measured normal boiling point.
This informational prototype is not a supplier, quotation service, or endorsement of a product’s fitness for an application. Buyers should verify the applicable specification, certification, and regulatory requirements for their intended use.