Hach DR/850, Procedures Manual, Page: 432 / 460

432
17.
After the absorbance
of the “control” vial has
decreased 0.60 ±0.10
absorbance units, place
the blank in the adapter.
Tightly cover the vial
with the instrument cap.
18.
Press:
ZERO
The cursor will move to
the right, then the
display will show:
0.000 ABS
19.
Place the “control”
vial in the cell holder.
Tightly cover the vial
with the instrument cap.
Record the absorbance
value of the control.
20.
Place each sample
or dilution vial in the
cell holder. Tightly
cover the vial with the
instrument cap. Record
each absorbance value.
21.
Calculate the %
Inhibition as follows:
Δ
A is the initial
absorbance value minus
the final absorbance
value.
See the example
following this step.
Note: Some toxins increase
respiration and will give a
negative % inhibition on all
respiration-based toxicity
tests. After repeated testing,
samples which always give a
% inhibition in Step 21 that
is more negative than –10%
should be considered toxic.
%I 1
Δ
Abs sample
Δ
Abs control
--------------------------------
⎝ ⎠
⎛ ⎞
– 100
×
=
TOXTRAK TOXICITY TEST,
continued
Example
The control tube (C) has an initial absorbance of 1.6 and decreases
to 1.0 Abs. The sample tube has an initial absorbance of 1.7 and
decreases to 1.3 Abs.
Δ
Abs. Sample 1.7 1.3 – 0.4 = =
Δ
Abs. Control 1.6 1.0 – 0.6 = =
%I 1
0.4
0.6
-------
⎝ ⎠
⎛ ⎞
–
⎝ ⎠
⎜ ⎟
⎛ ⎞
x
100 =
%I 33.3 =