The pH of formalin typically ranges from 6.7 to 7.1. For specifically buffered formaldehyde solutions, such as a 4% buffered formaldehyde, the pH is precisely maintained at 6.9.
Understanding Formalin and its pH
Formalin is an aqueous solution of formaldehyde, usually containing 37-40% formaldehyde by mass, often with a small amount of methanol (typically 10-15%) to prevent polymerization. Its pH is a critical physicochemical property that influences its stability, effectiveness as a fixative or preservative, and safety.
The near-neutral pH range (6.7-7.1) for general formaldehyde solutions is essential for many applications. When specifically buffered, the pH can be precisely controlled, as seen with the 4% buffered solution at pH 6.9.
Key Physicochemical Information
To better understand formalin, here are some of its core physicochemical properties:
| Property | Value |
|---|---|
| Boiling point | 100 °C (at 1013 hPa) |
| Density | 1.017 g/cm³ (at 20 °C) |
| pH value | 6.7 - 7.1 (in H₂O) |
| Buffered pH | 6.9 (for 4% buffered formaldehyde) |
Factors Influencing Formalin pH
Several factors can influence the pH of a formalin solution:
- Buffering: The presence of buffering agents (e.g., phosphates) is the primary way to stabilize and maintain a specific pH, preventing significant fluctuations. This is why a buffered solution has a precise pH of 6.9.
- Formaldehyde Concentration: While the pH range is generally consistent for typical formalin concentrations, extreme dilutions or highly concentrated solutions might exhibit slight variations.
- Age and Storage Conditions: Over time, formaldehyde can slowly oxidize to formic acid, especially when exposed to air and light. This process decreases the pH, making the solution more acidic. Methanol is often added to commercial formalin to inhibit this polymerization and oxidation.
- Temperature: Changes in temperature can slightly affect the dissociation of water and any acids or bases present, leading to minor pH shifts.
Practical Implications of Formalin pH
Maintaining the correct pH of formalin is crucial for various applications, particularly in histology, pathology, and certain industrial processes:
- Tissue Fixation: In histology, neutral-buffered formalin (NBF) with a pH around 6.8-7.2 is preferred for tissue fixation. An acidic pH can lead to the formation of "formalin pigment" (acid formaldehyde hematin) in tissues, which complicates microscopic examination. Conversely, a highly alkaline pH can degrade tissue components.
- Preservation: For long-term preservation of biological specimens, a stable pH prevents degradation and maintains the integrity of the sample.
- Chemical Stability: An appropriate pH ensures the stability of the formaldehyde solution itself, minimizing its degradation into formic acid or its polymerization into paraformaldehyde, which would reduce its effectiveness.
- Material Compatibility: The pH can also influence the compatibility of formalin with various materials it comes into contact with, such as storage containers or equipment.
Understanding and controlling the pH of formalin is therefore vital for its effective and reliable use in diverse scientific and industrial settings. For more comprehensive information on formaldehyde, its properties, and safety data, you can consult resources like PubChem.
