Hach-Lange BODTRAK II: Respirometric Biochemical Oxygen Demand (BOD) - Manual (Page 34)

BODTrak™ II procedures
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5.7.5 Chlorine
Any chlorine in the sample must be removed prior to testing. Keep the sample at
room temperature for 1 to 2 hours before a test to dissipate low chlorine
concentrations. If any chlorine remains after sitting for 2 hours or if the chlorine
concentration is high, add sodium thiosulfate to remove the chlorine:
1. In a 250 mL erlenmeyer flask add 100-mL of sample.
2. Add 10 mL of 100 g/L potassium iodide solution and
10 mL of 0.02 N sulfuric acid standard solution to the erlenmeyer flask.
3. Add 3 droppers of starch indicator solution and swirl to mix.
4. Titrate from dark blue to colorless with 0.025 N Sodium Thiosulfate standard
solution.
5. Calculate the quantity of sodium thiosulfate standard solution necessary to
dechlorinate the remaining sample:
6. Add the necessary quantity of 0.025 N sodium thiosulfate standard solution to
the sample and mix fully. After 10 to 20 minutes, do the BOD test.
5.7.6 pH effect
Low BOD test results occur when sample pH is outside the range of 6 to 8. Keep
this pH to simulate source sample conditions or adjust the pH to neutrality (buffered
at pH 7). Use 1.0 N (or weaker) sulfuric acid to neutralize caustic samples. Use 1.0
N (or weaker) sodium hydroxide to neutralize acidic samples. When samples are
pH adjusted, they should also be seeded.
5.7.7 Supersaturation
Equilibrate supersaturated cold samples (containing more than 9 mg/L dissolved
oxygen at 20 °C) to saturation:
1. Heat or cool the sample temperature to approximately 20 °C.
2. Half fill a sample bottle with sample.
3. Shake for 2 minutes or aerate with filtered compressed air for 2 hours.
mL of Sodium Thiosulfate
mL used ( ) mL sample to be dechlorinated ( )
100
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