12 / 59
Ref. No. [UMCES] CBL 2016-013
ACT VS16-04
12
Storage at temperatures above the sample temperature cause the loss of I
2
due to the thermal
expansion of the solution of 0.025 mL ·°C
–1
for a 125 ml sample (Carignan
et.al. 1998).
Sample Titration Procedures
Whole bottles were titrated using a Metrohm automated model 916 Ti-Touch titrator equipped with
a 10-mL burette and a Metrohm Pt ITrode. The Pt ring of the electrode was polished weekly. The
titrator was used in the dynamic equivalence point titration (DET) mode, with a measuring point
density of 4, a 1.0-µL minimum increment, and a 2 mV·min
-1
signal drift condition. In this method,
the solution’s potential (controlled by the
I
2
/I
–
and
"
#
$
%#&
"
'
$
(#&
– redox couples) was monitored
after successive additions of titrant, where optimal increment volumes are calculated by the
titrator’s software. During titration, the size and rotation speed of the magnetic stirring bar was
controlled in such a way that complete mixing of the I
2
generated during standardization occurred
within 3 - 4 s, without vortex formation. To reduce the titration time (3 - 4 min) and I
2
volatilization, an initial volume of titrant equivalent to 85–90% of the expected O
2
concentration
was added at the beginning of the titration. Because the molar volume of water and the normality
of the titrant vary appreciably with temperature, care was taken to standardize the titrant and
conduct all titrations of a given batch of samples at constant temperature (± 1°C).
(1) The stopper of the BOD bottle was removed and, using a wash bottle fitted with a 200-µL
pipette tip, the I
2
present on the side and conical part of the stopper was rinsed into the BOD bottle
with 1 - 2 mL of distilled water.
(2) BOD bottles (Corning No. 5400-125) had been selected to accommodate the displacement of
the electrode without having to remove any volume of the fixed sample.
(3) The stirring bar was inserted into the bottle using plastic or stainless steel forceps.
(4) The delivery tip and the electrode were immersed, the stirrer turned on and the titration begun.
The electrode was not allowed to touch the neck of the bottle.
(5) Once the titration was complete, the equivalence point volume (
V
T) was noted
Thiosulfate Standardization
The Thiosulfate was standardized at room temperature as the first and last step in daily analysis.
Either triplicate assays of a fixed volume of iodate standard was run, or a range of volumes
(≥ 3) bracketing the normal sample titration range (e.g. 0.500, 1.000, 1.500, 2.000 mL for well
oxygenated waters.) A clean BOD bottle and clean glassware were dedicated to this purpose.
(1)
Insert a stirring bar into a 200 mL beaker.
(2) With mixing add 1.0 mL of the H
2
SO
4
reagent followed by 1.0 mL of the alkaline iodide and
then 1.0 mL Mn
2+
reagent.
(3) Using a gravimetrically calibrated pipet add a suitable volume of the KIO
3
standard to the
stirring solution
(4) Insert the electrode and delivery tube and immediately begin titration
(5) The normality of the thiosulfate is calculated from the equivalence point volume as Vol
KIO3
/
Vol
Thio
)* N KIO
3
using replicates of single KIO
3
volume additions or from the slope of a range of
KIO
3
addition volumes.
Blank determination
Reagent blanks were determined as follows:
(1) A volume of 1-2 L of site water was brought to a boil in a clean glass reagent bottle.
(2) Boiled, degassed water was cooled and poured into 125 ml sample flasks and sparged with N
2