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Ref. No. [UMCES] CBL 2016-010
ACT VS16-01
12
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 (eg. 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
for
no less than 30 minutes.
(3) The sample was then rapidly fixed as a normal sample, and on the auto titrator.
(4) A global reagent blank taken as the mean of samples fixed at each test site (0.078 ± 0.020, n=5) and
used to correct all reference values.