Sample Collection, Preservation And Storage - Hach DR/850 Procedures Manual

CHEMICAL ANALYSIS INFORMATION

Sample Collection, Preservation and Storage

Correct sampling and storage are critical for accurate testing. For greatest
accuracy, thoroughly clean sampling devices and containers to prevent
carryover from previous samples. Preserve the sample properly; each
procedure has information about sample preservation.
•
The least expensive containers are polypropylene or polyethylene.
•
The best and most expensive containers are quartz or PTFE
(polytetrafluoroethylene, Teflon).
•
Avoid soft glass containers for metals in the microgram-per-liter range.
•
Store samples for silver determination in light-absorbing containers,
such as amber bottles.
Avoid contaminating the sample with metals from containers, deionized
water or membrane filters. Thoroughly clean sample containers as
described under Acid Washing Bottles.
Preservation slows the chemical and biological changes that continue after
collection. These processes may change the amount of a chemical species
available for analysis. Normally, analyze the samples as soon as possible
after collection, especially when the analyte concentration is expected to
be low. This also reduces the chance for error and minimizes labor.
Preservation methods include pH control, chemical addition, refrigeration
and freezing. Table 4 gives the recommended preservation for various
substances. It also includes suggested types of containers and the
maximum recommended holding times for properly preserved samples.
Preserve aluminum, cadmium, chromium, cobalt, copper, iron, lead,
nickel, potassium, silver and zinc samples for at least 24 hours by adding
one Nitric Acid Solution Pillow 1:1 (Cat. No. 2540-98) per liter of
sample. Check the pH with pH indicator paper or a pH meter to assure the
pH is 2 or less. Add additional pillows if necessary. Adjust the sample pH
prior to analysis by adding an equal number of Sodium Carbonate
Anhydrous Powder Pillows (Cat. No. 179-98). Or raise the pH to 4.5 with
Sodium Hydroxide Standard Solution, 1 N or 5 N. Correct for the added
volume of the preservatives; see Correcting For Volume Additions.
, continued
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