It is difficult to obtain a truly typical
sample when collecting surface water samples.
More meaningful results
can be obtained by running a series of tests with samples taken from
several locations and depths and at different times.
The results can
then be used to establish patterns applicable to that particular body
As little time as possible should lapse between
collecting the sample and making the analysis.
Depending on the
nature of the test, special precautions in handling the sample may
also be necessary to prevent natural interferences such as organic
growth or loss or gain of dissolved gaes.
c. Best Test Temperature.
All of the calorimetric and titri-
metric tests described in this manual should be performed with sample
temperature between 20°C (68°F) and 25°C (77°F).
tests require closer temperature control, it will be indicated in the
Volume Measurement Accuracy.
The sample cells are marked to
indicate approximately 25 ml or 50 ml.
In most tests it is a suffi-
ciently accurate to fill to the mark without premeasuring in a gradu-
In tests where volume measurements are critical, the
procedure specifies the appropriate method.
Never pipet chemical solutions or unknown water
samples by mouth.
(1) The moist, warm interior surfaces of the mouth form an
incubator which will harbour and breed germs, bacteria, viruses and
harmful toxic ingredients.
Always prevent lacerated areas in the gums
and interior mouth surfaces from coming in contact with the sample
water, this will provide and entry into the body by direct absorption.
When pipetting by mouth a certain amount of absorption by inhalation
can be experienced.
Over-suction of suspect water can cause an
accidental swallowing or at the least a flooding of the mouth area.
Placing the pipet in the mouth after rinsing (para 2-4) in the suspect
water will be direct introduction.
A safety pipet filler is provided
in the preventive medicine set to permit transfer of chemicals and
unknown water samples without using the mouth.
(2) If a sample must be diluted,
it is strongly recommended that
the pipet be used for volume measurement.
Accuracy is important since
a slight mistake in the measurement of a small sample will cause a
substantial error in the result; for instance a 0.1 ml mistake in a
measurement of a 1.0 ml sample produces a 10 percent error in the test