Orthophosphoric acid (H₃PO₄) usages
Why Orthophosphoric acid (H₃PO₄) is commonly used for pH adjustment in High-Performance Liquid Chromatography (HPLC) analysis, in the pharmaceutical industry.
Orthophosphoric acid (H₃PO₄) usages in Chromatography
Orthophosphoric acid (H₃PO₄) is commonly used for pH adjustment in Chromatography (HPLC) analysis, as it provides a stable and easily controllable pH, is relatively non-reactive with most analytes, and allows for precise adjustments within the desired pH range in addition to that it has unique chemical properties and practical advantages.
Precise pH control is critical in HPLC, particularly in reversed-phase chromatography, because it influences analyte retention, peak shape, and reproducibility.
Below are the Detailed reasons why orthophosphoric acid is preferred in HPLC Analysis.
1. pH Control and Buffering Capacity :
Wide buffering range: It has three dissociation constants (pKa₁ = 2.15, pKa₂ = 7.20, pKa₃ = 12.35), enabling it to form buffers across a wide pH range (especially at pH 2–7, which is typical for pharmaceutical separations).
Stable pH adjustment: Its ability to provide stable and reproducible pH levels minimizes fluctuations that could impact analyte retention times and peak symmetry.
Compatibility with common buffers: It is often paired with salts like potassium dihydrogen phosphate (KH₂PO₄) to create robust phosphate buffers.
2. Low UV Absorbance (UV Transparency) :
In UV detection, commonly used in pharmaceutical HPLC, mobile phase components must not interfere with analyte detection.
It exhibits minimal UV absorbance at low wavelengths (e.g., 210-220 nm), making it ideal for detecting compounds with weak chromophores without adding background noise.
3. Compatibility with HPLC Systems :
Non-volatile nature: While volatile acids like trifluoroacetic acid (TFA) are used in LC-MS, orthophosphoric acid’s non-volatile nature makes it ideal for UV and refractive index detectors, ensuring stable baselines.
No precipitation risks: Phosphate buffers formed with orthophosphoric acid are highly soluble in aqueous media, reducing the risk of salt precipitation that can clog HPLC columns.
4. Chemical and Thermal Stability :
Orthophosphoric acid is chemically stable and can withstand the high pressures and temperatures in HPLC without degradation.
It resists hydrolysis and does not introduce reactive impurities that could interact with pharmaceutical compounds.
5. Improves Peak Shape and Resolution :
Suppression of silanol interactions: In silica-based reversed-phase columns, residual silanol groups can cause tailing, especially with basic drugs. Adjusting pH with orthophosphoric acid suppresses these interactions, leading to sharper, more symmetrical peaks.
Ionization control: Precise pH control influences the ionization state of analytes, optimizing retention times and improving resolution between structurally similar compounds.
6. Regulatory and Safety Considerations :
Pharmacopoeial acceptance: Major pharmacopeias (USP, EP, BP) endorse orthophosphoric acid for pH adjustment in pharmaceutical analysis, Safety and handling.
7. Cost and Availability :
Economical and readily available: It is cost-effective and Easily available
What is the Difference Between Phosphoric acid and orthophosphoric acid ?
“Phosphoric acid” and “orthophosphoric acid” are essentially the same thing, meaning there is no practical difference between the two, “orthophosphoric acid” simply uses the prefix “ortho” to specifically denote the standard form of phosphoric acid, distinguishing it from other related phosphoric acids like polyphosphoric acids – and is the term often used in the pharmaceutical industry when discussing this compound.
In short Phosphoric acid and orthophosphoric acid are the same thing. The term “orthophosphoric acid” is used to distinguish this acid from other phosphoric acids, such as pyrophosphoric acid.
