Hach DR2400 - Section 3 Chemical Analysis; 3.1 Sample Collection, Preservation, and Storage

Chemical Analysis
5940022ChemAnal.fm Page 27
Section 3 Chemical Analysis
3.1 Sample Collection, Preservation, and Storage
Correct sampling and storage are critical for accurate testing. Sampling devices
and containers must be thoroughly clean to prevent carryover from previous
samples. Preserve the sample properly; each procedure has information about
sample preservation.
3.1.1 Collecting Water Samples
Use a clean container. Rinse the container several times with the water to be
sampled before taking the sample. Document the location and procedure used
for each sample taken. For example:
3.1.1.1 From a tap
Take samples as close as possible to the source of the supply. This lessens the
influence of the distribution system on the sample. Let the water run long
enough to flush the system. Fill sample containers slowly with a gentle stream to
avoid turbulence and air bubbles.
When testing well water, let the pump run long enough to draw fresh
groundwater into the system. Collect a sample from a tap near the well.
3.1.1.2 From open waters
Sample as near the middle of the body of water as is practical; at least several
feet from the shore or edge of the tank.
Take the sample under the surface of the water. If you are using a capped
container, submerge it before removing the cap.
3.1.1.3 Types of Containers
Table 8 lists recommended containers for specific parameters.
• Polypropylene and Polyethylene — These are the least expensive
containers.
• Quartz or TFE (tetrafluoroethylene, Teflon
®
)—These are the best, and the
most expensive.
• Glass—Glass provides a good general-purpose container. Avoid using
soft-glass containers to collect samples for metals testing in the microgram-
per-liter range.
When determining silver, store samples in dark containers such as amber or
brown glass.
Acid washing will thoroughly clean sample containers before use.
3.1.1.4 Acid Washing
If a procedure suggests acid washing, follow these steps:
a. Clean the glassware or plasticware with laboratory detergent. Phosphate-
free detergent is best. (When determining phosphates, phosphate-free
detergent must be used.)
b. Rinse well with tap water.
c. Rinse with a 1:1 hydrochloric acid solution or a 1:1 nitric acid solution.
(Nitric acid is best when testing for lead or other metals.)