TM 5-6630-215-12
of the titration.
If the oxygen content of the sample is very high,
it may be necessary to refill the micro burette before the endpoint is
reached.
In this case, a record should be kept of the total amount of
sodium thiosulfate solution used in the titration.
NOTE
Each major division on the mini burette is equiva-
lent to 0.2 mg/l dissolved oxygen and each minor
division to 0.04 mg/l.
Calculation - Dissolved Oxygen:
If it required 18
major and 2 minor divisions to reach the titration
endpoint, the reading would be (18 x 0.2) plus (2
x 0.04) = 3.68 mg/l.
Sample Handlings are important considerations in
obtaining meaningful results.
The dissolved
oxygen content of the water being tested can be
expected to change with depth, turbulence, temper-
ature,
sludge deposits, lights, microbial action,
mixing, travel, time, and other factors.
A single
dissolved oxygen test rarely reflects the accurate
overall condition of a body of water.
Several
samples taken at different times, locations and
depths are recommended.
The solvability of oxygen
in pure water at various temperatures and pressures
is given in para 2-12 and table 2-5.
e.
Zinc Test Kit (fig. 2-10).
(1) Zinc Concentrations in most water supplies average about 1
mg/l but may range as high as 50 mg/l in some areas.
Though zinc is
commonly found in many natural waters, the deterioration of galvanized
iron and the leaching of brass can add substantial amounts. Industrial
effluents may contribute large amounts of zinc, and high concentra-
tions suggest the presence of lead and cadmium, common impurities
from the galvanizing process.
(a) Zinc is essential to human metabolism and has been found
to be necessary for proper body growth.
(b) High concentrations of zinc in water act as stomach irri-
tants but the effects are temporary.
Concentrations above 5 mg/l show
no harmful physiological effects but can cause a bitter taste and/or
an opalescence in alkaline drinking water.
(c) For the above reasons, the maximum recommended zinc con-
centrations in public water supplies has been set at 5 mg/l.
2-29
