SECTION 2,
continued
27
Letting the sample stand for a period of time to remove air bubbles is not
recommended. Particulates that cause turbidity may settle, and the sample
temperature may change. Both conditions may alter the turbidity of the sample
resulting in a measurement that is not representative of the original
sample turbidity.
2.6.4.1
Application of Vacuum
Apply vacuum with any convenient, clean, oil-free vacuum source. The vacuum
lowers the atmospheric pressure above the sample allowing trapped gas bubbles to
escape. Vacuum works well with non-viscous samples, such as water, that do not
contain volatile components. Application of vacuum to viscous, volatile samples
(such as paint resins) may cause volatile components to come out of solution, and
intensify the bubble problem.
To apply vacuum, use a sample degassing kit equivalent to Cat. No. 43975-00
(Sample Degassing Kit) as shown in Figure 4 or Cat. No.43975-10 (Sample
Degassing and Filtration Kit). These kits contain a syringe and stopper for vacuum
degassing. An electric or hand-operated pump equivalent to Cat. No. 14697-00 or
14283-00, respectively, also may be used.
Figure 4
Sample Degassing
2.6.4.2
Addition of Surfactant
Limit the use of surfactants (surface-action agents) to severe problems when other
degassing techniques prove ineffective. Surfactants change the surface tension of
the water causing the release of entrained gases. Hach Company recommends a
surfactant such as Triton X-100 (a Rohm and Haas Product, Hach Cat. No. 14096-
32) or equivalent. Add one drop of Triton X-100 in the sample cell prior to
sample addition.
Note: Any turbidity contributed by
the addition of surfactant
is negligible.
This technique is particularly effective when water is supersaturated with air.
Changing the surface tension may accelerate settling of turbidity-causing
particles. Mix the sample well, and measure as soon as possible. Overmixing may
cause the surfactant to foam. Rinse sample cells thoroughly between
measurements to prevent accumulation of residual surfactant in the cells.
Tài liệu sưu tầm của Trung Sơn - www.tschem.com.vn - 028.3811.9991 - [email protected]
Tài liệu sưu tầm của Trung Sơn - www.tschem.com.vn - 028.3811.9991 - [email protected]
