Cleaning of Colorimetric Tubes for BOD Determination

Abstract: Accurate biochemical oxygen demand (BOD) measurement requires exceptionally clean glassware. This note describes a reliable procedure for cleaning colorimetric tubes (or BOD bottles) to remove organic residues, ensure no interfering substances remain, and avoid toxicity to microorganisms.

1. Introduction

BOD determination relies on microbial activity. Any organic or inorganic contaminant on tube walls can falsely increase oxygen demand or inhibit bacteria. Colorimetric tubes used in BOD tests must be rigorously cleaned, rinsed, and verified free of residues. The following protocol is simple yet effective.

2. Cleaning Procedure

Step 1 – Immediate rinsing after use

Empty the tube contents. Rinse immediately with tap water to remove loosely attached sample material. Do not let residues dry inside.

Step 2 – Detergent washing

Soak tubes in a non?phosphate, low?foaming laboratory detergent solution (e.g., 2% Liquinox or equivalent) for 15–30 minutes. Scrub inner walls with a tube brush. Avoid abrasive powders that scratch glass.

Step 3 – Acid soak (for stubborn deposits)

If organic films remain, soak tubes in 10% HCl or 10% H?SO? for 1–2 hours. Note: Never use chromic acid – it leaves toxic chromium residues.

Step 4 – Thorough rinsing

Rinse each tube at least five times with tap water, then three times with deionized (DI) water. Ensure no detergent foam remains.

Step 5 – Final DI water rinse and drying

Give a final rinse with fresh DI water. Invert tubes on a clean rack and air?dry. Alternatively, dry in an oven at 60?°C (do not exceed 80?°C). Avoid using cloth towels that may leave lint.

3. Quality control check

Periodically perform a blank test: fill a cleaned tube with DI water, add nutrient buffer and seed, measure BOD after 5 days. The blank BOD should be ≤?0.2?mg/L. Higher values indicate residual contamination.

4. Important precautions

No phosphate?based detergents – phosphate interferes with BOD nutrient buffer.

No organic solvents (acetone, ethanol) – they leave traces that consume oxygen.

Separate glassware – designate tubes exclusively for BOD to avoid cross?contamination from other analyses (e.g., COD, metals).

Never use bleach (chlorine) – chlorine residues inhibit microbes.