Hach DR/700 Procedures Manual page 664

OXYGEN, DISSOLVED, SHR,
continued
SAMPLING AND STORAGE
The foremost consideration in sampling with the High Range Dissolved
Oxygen AccuVac Ampul is to prevent the sample from becoming
contaminated with atmospheric oxygen. This is accomplished by
capping the ampul with an ampul cap in the interval between breaking
open the ampul and reading the absorbance. If the ampul is securely
capped, the ampul should be safe from contamination for several hours.
The absorbance will decrease by approximately 3% during the first hour
and will not change significantly afterwards.
Sampling and sample handling are important considerations in obtaining
meaningful results. The dissolved oxygen content of the water being
tested can be expected to change with depth, turbulence, temperature,
sludge deposits, light, microbial action, mixing, travel time and other
factors. A single dissolved oxygen test rarely reflects the accurate over-
all condition of a body of water. Several samples taken at different
times, locations and depths are recommended for most reliable results.
Samples must be tested immediately upon collection although only a
small error results if the absorbance reading is taken several hours later.
ACCURACY CHECK
The results of this procedure may be compared with the results of a
titrimetric procedure or dissolved oxygen meter.
STATISTICAL EVALUATION
A single operator repetitively tested samples of the same solutions, using
one DR/700 and two representative lots of AccuVac Ampuls. Testing
samples, the standard deviation was ±1.08 mg/L O
24.0 mg/L O .
2 2
Testing zero concentration samples, the limit of detection was 1.60 mg/L
O . The limit of detection was calculated as three times the standard
2
deviation when testing zero concentration samples (Adapted from
Analytical Chemistry, 1980, 52, 2242-2249).
INTERFERENCES
The following do not interfere at a level of 10 mg/L which is in excess of
3+ 2+ 2+ 2+ 2+ -
naturally occurring levels of Cr , Mn , Fe , Ni , Cu and NO .
2
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