LaMotte DC1600 Colorimeter - Manual (Page 6)

CHEMICAL TESTING
WATER SAMPLING FOR CHEMICAL ANALYSIS
TAKING REPRESENTATIVE SAMPLES
The underlying factor to be considered for any type of water sampling is whether or not the sample is truly representative of
the source. Some of the ways to properly collect a representative sample are as follows:
Sample as frequently as possible.
Collect a large sample or at least enough to conduct whatever tests are necessary.
Make a composite sample for the same sampling area.
Handle the sample in such a way as to prevent deterioration or contamination before the analysis is performed.
Perform analysis for dissolved gases such as dissolved oxygen, carbon dioxide, and hydrogen sulfide immediately at the site
of sampling. These factors, as well as samples for pH, cannot be stored for later examination.
Make a list of conditions or observations which may affect the sample. Other considerations for taking representative
samples are dependent upon the source of the sample. Taking samples from surface waters involves different considerations
than taking samples from impounded and sub-surface waters.
SAMPLING OF OPEN WATER SYSTEMS
Surface waters, such as those found in streams and rivers, are usually well mixed. The sample should be taken downstream
from any tributary, industrial or sewage pollution source. For comparison purposes samples may be taken upstream and at
the source of the pollution before mixing.
In ponds, lakes, and reservoirs with restricted flow, it is necessary to collect a number of samples in a cross section of the
body of water, and where possible composite samples should be made to ensure representative samples.
To collect samples from surface waters, select a suitable plastic container with a tight fitting screw cap. Rinse the container
several times with the sample to be tested, then immerse the container below the surface until it is filled to overflowing and
replace the cap. If the sample is not to be tested immediately, pour a small part of the sample out and reseal. This will allow
for any expansion. Any condition which might affect the sample should be listed.
Sub-surface sampling is required to obtain a vertical profile of streams, lakes, ponds, and reservoirs at specific depths. This
type of sampling requires more sophisticated sampling equipment.
For dissolved oxygen studies, or for tests requiring small sample sizes, a Water Sample Bottle (LaMotte code 1060) will serve
as a subsurface or in-depth sampler. This weighted device is lowered to the sampling depth and allowed to rest at this depth
for a few minutes. The water percolates into the sample chamber displacing the air which bubbles to the surface. When the
bubbles cease to rise, the device has flushed itself approximately five times and it may be raised to the surface for
examination. The inner chamber of the sampling device is lifted out and portions of the water sample are carefully
dispensed for subsequent chemical analysis.
A Snap-Plunger Water Sampler (LaMotte code 1077) is another “in-depth” sampling device which is designed to collect
large samples which can be used for a multitude of tests. Basically, this collection apparatus is a hollow cylinder with a
spring loaded plunger attached to each end. The device is cocked above the surface of the water and lowered to the desired
depth. A weighted messenger is send down the calibrated line to trip the closing mechanism and the plungers seal the
sample from mixing with intermediate layers as it is brought to the surface. A special drain outlet is provided to draw off
samples for chemical analysis.
SAMPLING OF CLOSED SYSTEM
To obtain representative samples from confined water systems, such as pipe lines, tanks, vats, filters, water softeners,
evaporators and condensers, even different considerations are required because of chemical changes which occur between
the inlet and outlet water. One must have a basic understanding of the type of chemical changes which occur for the type
of equipment used. Also, consideration should be given to the rate of passage and retaining time for the process water.
Temperature changes play an important part in deciding exactly what test should be performed. Process water should be
allowed to come to room temperature, 20–25°C, before conducting any tests.
For drawing off samples from an outlet pipe such as a tap, allow sample to run for several minutes, rinsing the container
several times before taking the final sample. Avoid splashing and introduction of any contaminating material.
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