Environmental Engineering Reference
In-Depth Information
automatically adjust chlorine concentrations in the system. In contrast, the
use of portable sensors or colorimetric test kits requires technicians to take
a sample and read the results. The technician then initiates required actions
based on the results of the test.
Several methods are currently available to measure chlorine in water sam-
ples, including
•
N
,
N
-diethyl-
p
-phenylenediamine (DPD) colorimetric method
• Iodometric method
• Amperometric electrodes
• Polarographic membrane sensors
It should be noted that these methods differ with regard to the specific type
of analyte, their range, and their accuracy. In addition, these methods have
different operations and maintenance requirements; for example, DPD sys-
tems require periodic replenishment of buffers, whereas polarographic sys-
tems do not. Users may want to consider these requirements when choosing
appropriate sensors for their systems.
Portable Cyanide Analyzers
Portable cyanide detection systems are designed to be used in the field to
evaluate for potential cyanide contamination of a water asset. These detec-
tion systems use one of two distinct analytical methods—either a colorimet-
ric method or an ion-selective method—to provide a quick, accurate cyanide
measurement that does not require laboratory evaluation. Aqueous cyanide
chemistry can be complex. Various factors, including the pH and redox poten-
tial of the water, can affect the toxicity of cyanide in that asset. Although per-
sonnel using these cyanide detection devices do not need to have advanced
knowledge of cyanide chemistry to successfully screen a water asset for cya-
nide, understanding aqueous cyanide chemistry can help users to interpret
whether the cyanide concentration represents a potential threat. For this rea-
son, a short summary of aqueous cyanide chemistry, including a discussion
of cyanide toxicity, is provided below. For more information, the reader is
referred to Greenburg et al. (1999).
Cyanide (CN
-
) is a toxic carbon-nitrogen organic compound that is the
functional portion of the lethal gas hydrogen cyanide (HCN). The toxicity of
aqueous cyanide varies depending on its form. At near-neutral pH,
free cya-
nide
(commonly designated as CN
-
, although it is actually defined as the total
of HCN and CN
-
) is the predominant cyanide form in water. Free cyanide is
potentially toxic in its aqueous form, although the primary concern regard-
ing aqueous cyanide is that it could volatilize. Free cyanide is not highly
volatile (it is less volatile than most VOCs, but its volatility increases as the
pH decrease below 8). When free cyanide does volatilize, it volatilizes in its
