Hydrogen phthalate ion Polarity

Is C₈H₆O₄ (Hydrogen phthalate ion) Polar or Nonpolar? Molecular Polarity & Structure Analysis

Figure 1: 2D Lewis Structure (C ₈H ₆O ₄; bent oxygen centre)

Polarity Parameters of C₈H₆O₄

CAS Registry Number	88-99-3
Molecular Formula	C8H6O4
Molecular Weight	166.13 g/mol
Valence Electrons	62
Lewis Structure Guide	View C8H6O4 Lewis Structure Guide
Dipole Moment (μ)	Not applicable — the species carries a net charge, so it is described by its charge, not by a molecular dipole (which for a charged species also depends on the choice of origin) — the molecule is flexible: across the low-energy conformers the calculated value spans ≈1.00–3.66 D, and the measured gas-phase dipole is a population average inside that range
Dielectric Constant (εr)	Not quoted — the Debye estimate would inherit a dipole that this method does not deliver reliably for this class of compound
Snyder Polarity Index (P′)	Not conventionally assigned
XlogP3-AA	1.08 (Crippen calculated, RDKit) (computed descriptor)

Note: Snyder P′ is a chromatographic solvent scale developed for conventional liquid solvents; no dipole magnitude is quoted: the species carries a net charge, so its dipole depends on the choice of origin, so the polarity verdict rests on symmetry and structure alone; no relative permittivity is quoted, because the Debye estimate would inherit an unreliable dipole; XlogP is a calculated partition descriptor, not a direct measure of solvent polarity.

1. Molecular Polarity Analysis of Hydrogen phthalate ion: Geometry & Dipole Vector

To determine whether Hydrogen phthalate ion (C₈H₆O₄) is polar or nonpolar, the decisive question is whether the bond dipoles of the molecule cancel once its three-dimensional shape is taken into account. Hydrogen phthalate ion is a carboxylic acid, built from 2 × carboxylic acid (–COOH). As set out in the C₈H₆O₄ Lewis Structure Guide, C₈H₆O₄ has 12 heavy atoms, 1 ring system and 2 rotatable bonds. ....

The bond polarities follow from electronegativity. The largest differences in the structure are C–O (ΔEN ≈ 0.89), so electron density is pulled toward oxygen. In three dimensions, the oxygen centre is bent; the 2 non-bonding pairs on oxygen compress the X–O–X angle below the tetrahedral value. As a result, the species is a covalently bound ion, so it is described by its net charge and by how that charge is delocalised over the structure, not by a molecular dipole. The dipole is conformation-dependent: across the low-energy conformers the calculation spans ≈1.00–3.66 D, and the value quoted above is the one for the lowest-energy conformer. For a flexible molecule the measured gas-phase dipole is the Boltzmann average over that population, so it lies between the two, not at either end.

For formulation and process work, the useful consequences are these: the calculated topological polar surface area is 74.6 Ų, 2 hydrogen-bond donors and 2 hydrogen-bond acceptors, the Crippen calculated partition coefficient is logP ≈ 1.08 and the calculated molar refractivity is 40.36 cm³ mol⁻¹. A logP in this range means the molecule is amphiphilic enough to cross between phases, which is often the useful behaviour in extraction and formulation. PubChem’s curated experimental record adds: Hydrogen phthalate ion is less than 1 mg/mL at 68 °F (NTP, 1992).

2. Hydrogen phthalate ion application:

PubChem’s use and manufacturing record describes Hydrogen phthalate ion as follows: ... These applications are what make the polarity question practical — solvent choice, extraction recovery, cleaning and formulation all turn on how strongly the molecule interacts with polar phases.

The measured physical behaviour recorded for this substance is consistent with that picture. Solubility: less than 1 mg/mL at 68 °F (NTP, 1992). Physical Description: Phthalic acid appears as white crystals or fine white powder.. Density: 1.593 at 68 °F (NTP, 1992) - Denser than water;. Melting Point: 410 to 412 °F (decomposes) (NTP, 1992). These are database values and should be read with the conditions (temperature, purity, pressure) that the source states.

No dilute-gas permittivity is reported here: the Debye estimate would have to inherit a dipole moment that this method does not deliver reliably for this class of compound (the species carries a net charge, so its dipole depends on the choice of origin), and for an ionic substance the equation does not apply at all.

Structurally the reactivity sits in 2 × carboxylic acid (–COOH). That functional inventory, together with the unresolved dipole, is what predicts the solubility and adsorption behaviour: polar groups give the molecule a handle on polar solvents and on polar surfaces, while the hydrocarbon portion resists them.

PubChem’s GHS record for this substance lists H360D ***: May damage the unborn child [Danger Reproductive toxicity] and H315 (47.5%): Causes skin irritation [Warning Skin corrosion/irritation]. Those classifications, not the polarity of the molecule, normally determine packaging, labelling, ventilation and transport requirements.

Recommended Analytical Quality-Control Strategy

• Identity confirmation: FTIR or Raman against a reference spectrum, supported by GC–MS or LC–MS where the compound is amenable; this is the fastest way to catch a mis-labelled lot.
• Assay / purity: GC-FID or GC-MS for volatile material, HPLC with a validated detector for non-volatile or thermally labile material; state the basis (area %, assay on dried substance, or titrimetric) on the certificate of analysis.
• Water content: Karl Fischer titration is the default for a substance whose polarity and hydrogen-bonding capacity make it hygroscopic; moisture is the single most common cause of out-of-specification lots.
• Related substances: a gradient HPLC or GC method capable of resolving the synthesis by-products and the isomeric impurities typical of this structure.
• Acid–base / ionisable content: a validated acid–base titration or potentiometric determination, since the ionisable group controls both the assay and the behaviour in use.

In short, specify Hydrogen phthalate ion by assay, water content, impurity profile, packaging and transport class, and state the intended application on the enquiry. The polarity analysis above explains behaviour in use; it does not replace a specification.

3. Polarity Comparison: C₈H₆O₄ and Related Industrial Fluids

The comparison below places C₈H₆O₄ next to structurally related substances from the same catalogue. Dipole moments come from one consistent protocol (GFN2-xTB//MMFF94, gas phase, single conformer) and are quoted only where that protocol has been validated; where it has not, the cell says so instead of giving a number. Permittivities are dilute-gas estimates from the Debye equation, logP values are Crippen calculations, and the solubility column is a qualitative inference from the calculated descriptors rather than a measurement.

The spread across this table is the point: composition alone does not set the polarity verdict. C₈H₆O₄ sits where it does because of the balance between its polar functional groups and the asymmetry of its shape, and any of the neighbouring entries can be the better choice once the temperature, the phase and the required polarity window are fixed.

4. Expert R&D Commentary

5. Regulatory, Safety & Sustainability Trends

Regulatory status is specific to the substance and to the jurisdiction, and it changes. For CAS 88-99-3 the authoritative public sources are the ECHA registered-substance database for the EU, and the OSHA and NIOSH tables for occupational exposure in the United States.

PubChem’s GHS record for this substance lists H360D ***: May damage the unborn child [Danger Reproductive toxicity] and H315 (47.5%): Causes skin irritation [Warning Skin corrosion/irritation]. These statements are the ones that must appear on the label and in section 2 of the safety data sheet; they, rather than the polarity of the molecule, drive classification, packaging group and transport documentation.

No occupational exposure limit for this substance is quoted in this article, because none was present in the database record used here. Publishing a limit that is not on the record would be worse than publishing none: obtain the applicable value from the OSHA Z-table, the NIOSH pocket guide or the equivalent national instrument before designing the ventilation or monitoring programme.

From a compliance and sustainability standpoint, the polarity of C₈H₆O₄ is a means rather than an end. It determines which solvent can replace another, how completely the substance can be recovered from a stream, and how much energy the separation will cost — so it belongs in the process justification, while the hazard classification and the exposure limits belong in the safety case.

6. Guidechem's Procurement Tips: What to Specify for Hydrogen phthalate ion

A purchase enquiry for Hydrogen phthalate ion (CAS 88-99-3) should state the grade, assay basis, permitted impurity limits, water content, packaging, and the transport and regulatory status required for the destination market. Where the substance is used as a reagent or intermediate, the impurity profile usually matters more than the nominal percentage.

Because polarity governs phase behaviour, it also governs recovery: a polar molecule is easier to strip from a non-polar stream and harder to dry than a non-polar one, and vice versa. Confirm the Guidechem supply listing for current availability and compare lots on the certificate of analysis rather than on the label alone.

Frequently Asked Questions (FAQs)

References & Academic Databases

1. PubChem Compound Summary: Hydrogen phthalate ion, CAS RN 88-99-3, CID 1017. National Center for Biotechnology Information. pubchem.ncbi.nlm.nih.gov
2. ECHA Substance Information: Hydrogen phthalate ion, CAS 88-99-3 — REACH registration status, harmonised classification and regulatory context. European Chemicals Agency.
3. NIST Chemistry WebBook / CCCBDB: experimental geometry, vibrational data and dipole-moment compilations for Hydrogen phthalate ion. National Institute of Standards and Technology.
4. CRC Handbook of Chemistry and Physics, 92nd ed. (Haynes, W. M., ed.), CRC Press, 2011 — electronegativity, dielectric constant and dipole-moment tables.
5. Nelson, R. D.; Lide, D. R.; Maryott, A. A.: Selected Values of Electric Dipole Moments for Molecules in the Gas Phase, NSRDS-NBS 10, National Bureau of Standards, 1967.
6. Snyder, L. R.: solvent-polarity classification and the P′ scale for liquid chromatographic solvents. The P′ scale is defined only for conventional liquid solvents and is not assigned to substances outside that scope.
7. Bannwarth, C.; Ehlert, S.; Grimme, S.: GFN2-xTB — an accurate and broadly parametrised self-consistent tight-binding quantum chemical method with multipole electrostatics and density-dependent dispersion contributions. J. Chem. Theory Comput. 2019, 15 (3), 1652–1671.
8. RDKit / Halgren, T. A.: MMFF94 force field for geometry generation (J. Comput. Chem. 1996, 17, 490–519) and the RDKit cheminformatics toolkit used for the Crippen logP, TPSA and hydrogen-bond descriptors quoted here.