PBTC•Na₄ refers to the tetra-sodium salt of 2-Phosphonobutane-1,2,4-tricarboxylic acid. Its composition is not simply PBTC mixed with sodium hydroxide; it’s a specific, partially neutralized form with defined properties for commercial use.
Here’s a detailed breakdown of its main components and their significance.
1. Primary Active Component: PBTC Tetra-Anion
The core functional molecule is the PBTC ligand in its tetra-valent anionic form. This is created by neutralizing three of PBTC’s carboxylic acid groups and one of its phosphonic acid protons.
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Chemical Structure:
[HO₃P–C(COOH)(CH₂COOH)–CH₂–CH₂–COO]⁴⁻ • 4Na⁺ -
Neutralization State:
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Phosphonate Group (–PO₃H₂): One proton is removed, forming
–PO₃H⁻. -
Three Carboxylate Groups (–COOH): All three are fully deprotonated to
–COO⁻.
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Why This State? This specific neutralization (usually to pH ~4-5 in concentrated solution) offers an optimal balance:
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High Water Solubility: The sodium salt is highly soluble, allowing for easy handling and dosing of concentrated solutions (typically 40-50% active).
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Stability: Minimizes the risk of precipitation during storage.
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Performance Ready: It readily dissociates in application water to provide the active PBTC anion for scale inhibition and corrosion control.
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Key Property: This tetra-sodium salt form is the standard commercial product sold as “PBTC” or “PBTC-Na”. When you buy liquid PBTC, you are almost always buying PBTC•Na₄ in an aqueous solution.
2. Solvent: Water
PBTC•Na₄ is supplied as an aqueous solution, typically with an active content of 40% to 50% by weight. Water is the carrier that enables easy pumping, metering, and dilution.
3. Impurities & By-products (Minor but Important Components)
During the industrial synthesis of PBTC (which involves the Arbusov or Michaelis–Becker reaction and hydrolysis), several impurities may be present in trace amounts in the final product:
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Orthophosphate (PO₄³⁻): A hydrolysis or oxidation by-product of the phosphonate group. Its presence should be minimized (<0.5%) as it can contribute to calcium phosphate scaling.
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Chloride (Cl⁻): If synthesis involves phosphorus trichloride (PCl₃) or other chlorinated reagents, residual chloride ions may remain. This is corrosive and levels are controlled.
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Unreacted Starting Materials: Trace amounts of phosphorous acid (H₃PO₃), maleic acid, or itaconic acid (common raw materials) may be present.
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Color Bodies & Organic By-products: Minor colored impurities may form during high-temperature synthesis, giving the commercial product its characteristic pale yellow to amber color. High-quality producers control this.
4. Stabilizers and Additives (Product-Specific)
Depending on the manufacturer and intended shelf life, the commercial formulation may include:
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pH Stabilizers: To maintain the solution at its target pH (usually around 4-5) and prevent degradation or crystallization.
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Corrosion Inhibitors for Storage: Low levels of sodium nitrite or other inhibitors may be added to protect carbon steel shipping containers from internal corrosion.
5. Comparison with the Acid Form and Other Salts
| Form | Chemical Formula | Typical Form | pH (of conc. soln.) | Solubility | Commercial Use |
|---|---|---|---|---|---|
| PBTC Acid | C₇H₁₁O₉P | White crystalline solid | Very acidic (~1-2) | Low | Rare, mainly for chemical synthesis |
| PBTC•Na₄ (Tetra-sodium salt) | C₇H₇O₉PNa₄ | Clear amber liquid (40-50%) | ~4-5 | Very High | Standard commercial product |
| Fully Neutralized Salt | C₇H₄O₉PNa₆ | Solid or paste | Alkaline (>10) | High | Less common, used in specific high-pH formulations |
Why Na₄ and not Na₆? Full neutralization (removing all protons, including the second phosphonate proton and all acidic protons) creates a very alkaline, hygroscopic product that is more difficult to handle and offers no significant performance advantage in most water treatment applications.
Summary of Main Components:
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Active Ingredient (≥95%): The tetra-sodium salt of PBTC in aqueous solution.
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Solvent (50-60%): Water.
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Controlled Impurities (<1-2% total): Orthophosphate, chloride, residual acids, and organic color bodies.
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Possible Additives (Trace): pH buffers or storage corrosion inhibitors.
When specifying or using PBTC, it’s crucial to understand that you are working with PBTC•Na₄. Its performance, handling (corrosivity is much lower than the acid), compatibility, and dosing calculations are all based on this specific chemical form. Always refer to the manufacturer’s technical data sheet for the exact active content, pH, density, and impurity profile.