The molecular structure of ATMP•Na₅ is specific and reflects a precise, industrially relevant state of neutralization. It is not simply the fully neutralized salt of the ATMP acid.
Here is a detailed breakdown:
1. Core Molecular Structure
The “ATMP” core is Aminotris(methylenephosphonic acid), with the molecular formula N(CH₂PO₃H₂)₃ in its fully protonated, acid form (H₆L).
ATMP is a hexaprotic acid, meaning it can donate up to six protons (three from the three phosphonic acid groups, each of which is diprotic, plus one from the ammonium group –NH⁺– in its protonated state).
2. The Meaning of “Na₅”
The notation •Na₅ indicates that five of the six acidic protons have been replaced by sodium ions (Na⁺). This results in a molecule with a net charge of -5.
The structure in its dominant form at neutral to alkaline pH can be represented as:
Na₅[NH⁺(CH₂PO₃⁻Na⁺)(CH₂PO₃⁻Na⁺)(CH₂PO₃⁻Na⁺)] or more succinctly,
Na⁺₅ · [N(CH₂PO₃²⁻)₃]⁵⁻
3. Detailed Structural Representation & Key Features
A more informative graphical description is:
Na⁺ Na⁺ Na⁺
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O=P-O⁻ O=P-O⁻ O=P-O⁻
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H₂C-N⁺-CH₂ CH₂
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O=P-O⁻ O=P-O⁻ O=P-O⁻
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Na⁺ Na⁺ Na⁺
Key Points of the Structure:
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Central Nitrogen: Remains positively charged (ammonium form, -N⁺H-). In ATMP•Na₅, the proton on the nitrogen is NOT replaced by Na⁺. This is crucial. The pKa of this ammonium proton is relatively high (~10-12), so under normal storage and application pH (often 1-2 for the concentrated product, 6-9 in diluted application), it remains protonated.
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Three Phosphonate Groups: Each phosphonate group (–CH₂–PO₃²⁻) is in the fully deprotonated, dianionic state. This is the key to the “Na₅” count:
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Each –PO₃²⁻ group balances two Na⁺ ions.
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Three groups × 2 Na⁺ = 6 Na⁺.
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However, the +1 charge on the central ammonium nitrogen offsets one of these sodium ions.
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Therefore, the net is 6 – 1 = 5 Na⁺ ions associated with the molecule to achieve electroneutrality.
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Counter-Ions: The five Na⁺ ions are dissociated in aqueous solution, surrounding the large, negatively charged organic anion. In the solid state (if crystallized), they would be closely associated with the oxygen atoms of the phosphonate groups.
4. Why Not Na₆? The Fully Neutralized Salt
The fully neutralized hexasodium salt (ATMP•Na₆) would require the deprotonation of the central ammonium group to a neutral amine (–N–), forming N(CH₂PO₃³⁻)₃ with a charge of -6, balanced by 6 Na⁺. This only occurs at very high pH (>> 12), which is not practical for water treatment applications. ATMP•Na₅ represents the salt form that is stable and effective in the typical application pH range of 6-10.
5. Commercial Product Reality
What is sold as “ATMP•Na₅” is typically a ~40-45% aqueous solution (expressed as ATMP acid equivalent). It is prepared by partially neutralizing crude ATMP acid with sodium hydroxide (NaOH) to a target pH (often between 1.5 and 2.5 for the concentrated product).
Important Nuance: The commercial product is a complex mixture in equilibrium, not a pure single species. It contains:
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The dominant ATMP pentaanion species described above.
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Some ATMP tetraanion (H₂L⁴⁻) and hexaanion (L⁶⁻) species.
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Excess free sodium ions from the neutralization process.
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Impurities from synthesis (sodium orthophosphite, sodium chloride, etc.).
Summary: The “Exact” Structure
The exact, dominant molecular structure of ATMP•Na₅ in its commercially relevant form is:
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Empirical Formula: C₃H₁₂NO₉P₃Na₅
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Core Ion: [NH(CH₂PO₃)₃]⁵⁻ (a pentavalent anion)
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Charge-Balancing Cations: 5 Na⁺
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Key Feature: Central protonated ammonium nitrogen (–NH⁺–) and three fully deprotonated phosphonate groups (–PO₃²⁻).
This structure makes ATMP•Na₅ highly soluble, less corrosive to handle than the free acid, and immediately active in the pH range of most cooling water systems. It is the optimized “delivery form” of the ATMP molecule for industrial use.