1. How do I prepare BOD dilution water?
- When preparing BOD dilution water use deionized or distilled water, tap water is not suitable.
- When using commercial nutrients for your dilution water make sure:
- The reagents have not expired
- The reagents have been stored properly
- You follow the manufacturer’s instructions
- If you make your own nutrients (i.e. stock solution) make sure:
- The stock solutions were stored at 4-7° C in the dark
- The correct reagents were used (Phosphate buffer, Magnesium sulfate heptahydrate, calcium chloride, ferric chloride hex hydrate)
- The nutrients were prepared in accordance with Standard Methods
- Add 1ml of each stock solution to 1 liter of deionized or distilled water (if you use commercial nutrients, follow the manufacturer’s instructions)
- The pH of the dilution water should be between 6.8 and 7.2
- It is best to bring your dilution water to 20°±3°C before use. We suggest preparing the BOD dilution water the night before. Store in the incubator overnight, not exceeding 24 hours.
- Saturate your BOD dilution water with DO by shaking vigorously or aerating with organic-free filtered air.
2. What do I check if my blanks (i.e. water check) are greater than 0.2 mg/L after the 5-day incubation?
The dilution water should have a DO uptake rate no greater than 0.2 mg/L over the 5-day test period. If the DO uptake rate exceeds 0.2 mg/L, you should check the following:
- Reagents have not been contaminated or expired
- Your glassware has been properly cleaned
- You have calibrated the DO meter properly
- Analyst-Check for consistency/accuracy
- Incubation temperature-consistently holds a temp of 20°C
- Tubing / air stones for mold or mildew (mold and mildew will release more oxygen after the 5-day incubation period)
3. Can DI water be over filtered?
Yes. Standard Methods gives us different grades of DI water to use depending on the purpose. Over filtration can result in bacterial inhibition. Thus the dilution water blanks will meet criteria, but the seeded controls and the GGA controls will most likely fail with low result. Make sure you make your BOD dilution water in accordance with Standard Methods.