Once all free cyanide is consumed, the excess silver ions react with an
indicator (rhodamine, dithiozone, and murexide, for example) that
changes color once in contact with free silver ions. However, because
silver ions can also remove cyanide from other complexed species,
the color might disappear after reaching the actual endpoint, thereby
leading to false results. Hence, this well-established method requires
experienced users and special analytical equipment, and the reagents
needed can have limited bench lifetimes.
Corrin-Based Indicators
The most recent technology to detect free cyanide relies on its
binding to corrin-based indicators. In this case, the indicator not only
senses, but also removes cyanide from the solution. A color change of
the immobilized indicator, from orange to violet, indicates cyanide's
presence in solution. This method allows for naked-eye detection of
free cyanide without specialized equipment, organic solvents, and/or
toxic substances. The market availability of this technology is limited,
but CyanoGuard AG is currently commercializing test kits based on
this method.
Why Is It Important To Evaluate Your Method Of Choice
For Free Cyanide Detection?
In many cases, the complexity of your samples might hinder
your detection method of choice, leading to false results,
unnecessary treatments, and even involuntary disposal of
cyanide into surface waters. While most methods perform well
in laboratory settings, where the sample composition is known
and appropriate equipment is available, their performance
might vary in industrial settings. Unfortunately, this is a
common situation when testing complex matrices containing
known and unknown interfering chemical substances.
When using chlorinating agents, ions such as nitrites
and sulfides might interfere with the indicator due to their
reactivity towards chloramine T. This is a common situation in
INSTRUMENTATION
25
wateronline.com n Water Innovations
SELECTIVITY SPEED SIMPLICITY SAFETY EQUIPMENT
CHLORINATING
REAGENTS
Low tolerance towards several
metal ions, nitrites, sulfides,
and thiocyanate
2-15 minutes Experienced users Possible exposure to toxic or
harmful substances
None and/or dedicated
equipment
SILVER NITRATE
TITRATION
Low tolerance towards several
metal ions, thiosulfate, and
thiocyanate
< 15 minutes Experienced users Harmful for the environment;
can cause irritation if inhaled
Analytical laboratory
equipment
CORRIN-BASED
INDICATORS
High tolerance towards most
interferences except sulfide
< 60 seconds Nonexperienced
users
Harmless reagents None