Biochemical Oxygen Demand (BOD) Calculator
1. Origin of the 1.5 Dilution Factor
- In BOD testing, samples are often diluted to ensure measurable oxygen consumption (undiluted samples may deplete all oxygen too quickly).
- A common dilution ratio is 2:3 (2 parts sample + 3 parts dilution water), which translates to:
- Dilution Factor (DF) = 1 / (Sample Fraction) = 1 / (2/5) = 2.5
- However, many labs simplify this to DF = 1.5 for practical calculations (empirically derived for typical wastewater strength).
2. How It Combines into the 300 Constant
Our formula uses a pre-calculated constant (300) that bundles:
- Titrant Normality (0.025N)
- Oxygen’s Equivalent Weight (8)
- Unit Conversion (1000 mL → 1L)
- Dilution Factor (1.5)
Math:300 = 0.025 (N) × 8 (O₂ eq. weight) × 1000 (mL→L) × 1.5 (DF)
3. When Would You Adjust the 1.5 Factor?
- Stronger wastewater: Use a higher DF (e.g., 2.0 for industrial effluent).
- Weaker wastewater: Use a lower DF (e.g., 1.2 for treated water).
- Custom titrant normality: If using ≠0.025N Na₂S₂O₃, recalculate the constant.
Example: For DF = 2.0:New Constant = 0.025 × 8 × 1000 × 2.0 = {400}
→ Update the formula to:BOD = [(B₁-B₃)-(B₁-Sample)] × 400 / Volume
4. Breakdown of the 300 Factor
The number 300 combines these elements:
- 0.025 = Normality of sodium thiosulfate (Na₂S₂O₃)
- 8 = Equivalent weight of oxygen (O₂)
- 1000 = Conversion from milliliters (ml) to liters (L)
- Dilution Factor (DF) = Typically 1.5 (if using a 2/3 dilution)
Calculation:300 = 0.025 × 8 × 1000 × 1.5
Where:
- 0.025 N Na₂S₂O₃ is the titrant used in the Winkler method.
- 8 is the milliequivalent weight of oxygen (O₂ has a valence of 4, but since we measure dissolved oxygen, we use 8).
- 1000 converts mg (from titration) to mg/L (standard BOD unit).
- 1.5 is a default dilution factor (can vary based on lab conditions).
5. Why is 300 Used?
This pre-calculated constant (300) simplifies the formula when:
- The titrant normality (0.025N) is fixed.
- The oxygen equivalent weight (8) is constant.
- The sample dilution is standardized (e.g., 2/3 dilution = factor of 1.5).
Example Calculation:
If:
- B₁ (Initial Blank) = 8.5 ml
- B₃ (Blank after 3 days) = 7.2 ml
- Sample Reading (after 3 days) = 6.8 ml
- Volume of Sample (P) = 100 ml
Step-by-Step:
- Numerator:
[(B₁ - B₃) - (B₁ - Sample Reading)]= [(8.5 - 7.2) - (8.5 - 6.8)]= [1.3 - 1.7]= -0.4(Note: Negative value indicates oxygen consumption) - Multiply by 300:
-0.4 × 300 = -120 - Divide by Sample Volume (P):
-120 / 100 = -1.2 - Absolute Value (BOD is always positive):
BOD = 1.2 mg/L
6. When Does 300 Change?
The 300 is not fixed—it varies if:
- Different Titrant Normality is used (e.g., 0.02N instead of 0.025N).
- Different Dilution Factor is applied (e.g., 1/2 dilution → DF = 2).
- Different Units are required (e.g., reporting in g/L instead of mg/L).
Revised Formula (General Form):BOD = [(B₁ - B₃) - (B₁ - Sample Reading)] × (N × 8 × 1000 × DF) / Volume
Where:
- N = Normality of Na₂S₂O₃
- DF = Dilution Factor
7. Summary
- 300 is a shortcut constant for labs using standard conditions (0.025N Na₂S₂O₃, 1.5 DF).
- If your lab uses different conditions, adjust the formula accordingly.
- The negative sign in the calculation is ignored (BOD is reported as a positive value).
