Hach DR2400 - Page 768

Toxicity
Tox ici ty
Page 4 of 6
Toxicity_None_Other_Eng_Ody.fm
13. Repeat step 12 for all
samples and dilutions. Be
sure to record each
absorbance.
14. Allow the solutions in
the tubes to react until the
absorbance of the control
decreases 0.60 ±0.10 abs.
This takes 45–75 minutes.
Invert occasionally.
Reaction time varies
according to temperature,
age of the culture, bacteria
concentrations, etc.
15. After the absorbance
of the control has
decreased to 0.60 ±0.10
abs., place the blank into
the cell adapter.
16. Touch Zero.
The display will show:
0.000
17. Place the control into
the cell holder.
Touch
Read. The
absorbance value of the
Control will appear.
Record this value.
18. Repeat step 17 for
each sample or dilution.
Record each absorbance
value.
19. Calculate the
% Inhibition:
Where:
∆A = Initial absorbance
value – Final absorbance
value
Note: Some toxins increase
respiration and will give a
negative % Inhibition on all
respiration-based toxicity tests.
After repeated testing, samples
that give a % Inhibition that is
more negative than –10%
should be considered toxic.
Repeat
Step 12
HRS MIN SECHRS MIN SEC
B
Zero
C
Repeat
Step 17
%I 1
A∆
sample
A
control
∆
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⎛⎞
– 100×=