Basicity is the most critical quality parameter of Poly Aluminium Chloride (PAC) that determines its coagulation efficiency, pH sensitivity, and suitability for different applications. Understanding basicity enables water treatment professionals to select the optimal PAC grade for their specific needs and achieve superior treatment results.
What Is PAC Basicity?
Basicity (B value) represents the degree of hydrolysis of the aluminum species in PAC. It is mathematically defined as the molar ratio of hydroxyl groups to aluminum ions:
Basicity (%) = [OH⁻] / (3 × [Al³⁺]) × 100
In simpler terms, basicity indicates how much of the aluminum’s charge has already been neutralized by hydroxyl groups during manufacturing. This pre-hydrolysis is what gives PAC its superior performance compared to non-hydrolyzed coagulants like aluminum sulfate.
| Basicity Range | Classification | OH/Al Ratio | Typical Appearance |
|---|---|---|---|
| 0 – 20% | Very Low | 0 – 0.6 | Clear to light yellow liquid |
| 20 – 40% | Low | 0.6 – 1.2 | Light yellow liquid |
| 40 – 60% | Medium | 1.2 – 1.8 | Yellow to amber liquid / White to yellow powder |
| 60 – 75% | High | 1.8 – 2.25 | Dark amber liquid / Yellow powder |
| 75 – 85% | Very High | 2.25 – 2.55 | Dark brown liquid / Deep yellow powder |
How Basicity Affects Coagulation Performance
1. Charge Neutralization Efficiency
Higher basicity PAC contains more polynuclear aluminum species (such as Al₁₃ and Al₃₀ polymers) with high cationic charge density (+7 to +10 per molecule). These species provide superior charge neutralization of negatively charged colloidal particles, resulting in faster and more complete coagulation. Lower basicity PAC contains more monomeric aluminum species (Al³⁺) that are less efficient at charge neutralization.
2. pH Stability and Alkalinity Consumption
Higher basicity PAC consumes less alkalinity from the water because fewer hydrolysis reactions need to occur after dosing. This means:
- Less pH depression after PAC addition
- Reduced or eliminated need for pH adjustment chemicals (lime, soda ash, caustic soda)
- More stable coagulation across varying raw water pH
- Better performance in low-alkalinity waters
3. Floc Formation and Settling
PAC with higher basicity (60-85%) produces larger, denser flocs that settle faster. This is because the pre-formed aluminum polymers act as flocculation nuclei, promoting rapid aggregation. Lower basicity PAC (10-40%) produces smaller, lighter flocs that require longer settling times or additional flocculant aids.
4. Residual Aluminum
Higher basicity PAC leaves lower residual aluminum in treated water. This is critical for drinking water treatment, where regulations typically require residual aluminum < 0.2 mg/L. The pre-hydrolyzed species in high-basicity PAC are more completely utilized in the coagulation process, leaving fewer soluble aluminum species behind.
Selecting the Right Basicity for Your Application
| Application | Recommended Basicity | Reason |
|---|---|---|
| Drinking Water (surface, low-turbidity) | 60 – 80% | Low residual Al requirement; wide pH tolerance |
| Drinking Water (groundwater) | 45 – 65% | Moderate turbidity, stable water quality |
| Municipal Wastewater | 50 – 70% | Good balance of performance and cost |
| Industrial Wastewater (general) | 60 – 80% | Maximum COD and color removal |
| Textile Dyeing Wastewater | 70 – 85% | Superior color removal; wide pH tolerance |
| Paper Mill White Water | 40 – 60% | Specific charge requirements for fiber retention |
| Metal Finishing Wastewater | 60 – 80% | Heavy metal co-precipitation |
| Swimming Pool Water | 30 – 50% | Gentle coagulation without pH disturbance |
How to Test PAC Basicity
Basicity is determined according to national and international standards (GB/T 22627, EN 883, AWWA B408):
Standard Method (Simplified)
- Dissolve a known mass of PAC sample in distilled water.
- Add a known excess of standardized hydrochloric acid (HCl) to fully decompose all hydroxyl groups.
- Add potassium fluoride (KF) to complex aluminum ions, releasing hydroxide from the aluminum-fluoride reaction.
- Titrate the released hydroxide with standardized sodium hydroxide (NaOH) solution using phenolphthalein indicator.
- Calculate basicity from the titration results.
HydroChemix provides certified basicity test reports with every shipment. Each batch is tested according to GB/T 22627-2014 standards with complete quality documentation including Al₂O₃ content, basicity, pH, insoluble matter, and heavy metal limits.
PAC Basicity and Product Forms
PAC is available in both powder and liquid forms, with basicity availability varying by form:
- PAC Powder (Spray-Dried): Available in basicity 40-85%. Spray-dried powder has the highest Al₂O₃ content (28-30%) and best solubility. Shelf life: 24 months.
- PAC Powder (Drum-Dried): Available in basicity 40-75%. Slightly lower Al₂O₃ (26-28%) but more economical. Shelf life: 12 months.
- PAC Liquid: Available in basicity 40-70%. Typically 10-18% Al₂O₃ content. Convenient for direct dosing but higher transportation cost per ton of active ingredient.
Key Takeaways
- Higher basicity (60-85%) = Faster coagulation, wider pH range, lower residual Al, less sludge. Best for drinking water and demanding industrial applications.
- Medium basicity (40-60%) = Good all-around performance, economical. Suitable for general wastewater treatment.
- Lower basicity (10-40%) = Specialized applications where slower, gentler coagulation is desired.
- Always test different basicity grades with jar tests on your actual water before making a final product selection.
Contact HydroChemix to request PAC samples with different basicity grades for jar testing. Our technical team provides free consultation to help you select the optimal PAC specification for your treatment process.