environmental health
Is There Cyanide in My Drinking Water?
by Michael F. Delaney, PhD, director, Laboratory Services, Massachusetts Water Resources Authority and
Charles Blodget, laboratory supervisor III, Laboratory Services, Massachusetts Water Resources Authority
From a laboratory perspective, cyanide is
the “Baddest” Bad Actor. Of all the tests
routinely performed in environmental
laboratories, it can be argued that
quantifying cyanide in drinking water
is the worst in terms of method
performance. 1 The analytical chemist’s
goal is to collect, preserve, transport, store,
pre-treat and test a drinking water sample
while maintaining an unchanging target
analyte concentration.
But this is where cyanide’s diverse
chemical reactions give it a bad
reputation: sample preservation
(dechlorination) with sodium hydroxide
treatment in the field and lab treatments
prior to analysis to prevent potential
chemical interferences can actually
result in free cyanide formation within
a sample. Regulatory drinking water
testing is prescriptive, so laboratories are
required to follow sample collection and
preservation regulations and analytical
methods even though this can lead to
false detections. 2
The US Environmental Protection Agency
(US EPA) acknowledges 3 that these false
positives are possible in some treated
drinking water types (e.g., chloraminated
water) using the prescribed preservation
and testing procedures. However, all
cyanide detects must nonetheless be
reported in a public water supply’s (PWS’s)
annual Consumer Confidence Report,
and this presents a risk communication
chall enge. Every year hundreds of PWSs
report cyanide detects, 4 of which most
are likely to be false positives compared
to the cyanide amount originally present
in the drinking water sample. These
false detections are exacerbated by
inconsistent state-required minimum
reporting levels (MRLs) for cyanide ranging
from the “detection limits” in 40 CFR
141.23 to the regulatory MCL. 5
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LAB MATTERS Spring 2018
Figure 1. Free cyanide formation when dechlorination is with ascorbic acid or thiosulfate. (Derived from reference 2.)
Cyanide Analytical Methods
Free cyanide is the US Safe Drinking Water
Act-regulated analyte, but samples are
commonly “screened” using total cyanide
(i.e., free cyanide plus all metal-cyanide
complexes). If these samples have total
cyanide above the MCL, they need to
be retested for free cyanide. US EPA has
indicated that the total cyanide screening
methods are “easier, faster and cheaper”
than the free cyanide methods, 6 but this is
no longer true. The total cyanide method
digestion/distillation step to prevent
interferences is labor intensive and uses
strongly acidic and high temperature
conditions that can cause free cyanide
to form.
We have found EPA method OIA-1677,
DW (2004) to be a preferable alternative
because it is less labor intensive, uses
gentle chemical conditions and is able to
selectively detect free cyanide directly.
This automated method combines flow
injection analysis with a semipermeable
membrane to isolate hydrogen cyanide
(HCN), which is converted to cyanide
ion (CN-) for selective electrochemical
detection, avoiding the harsh conditions
of a total cyanide digestion/distillation.
While this method was developed for
available cyanide (i.e., free cyanide plus
weak complexes), it can be used to
measure free cyanide by eliminating the
ligand exchange reagent, which releases
CN- from the weak complexes.
Challenges in Cyanide Analysis
Field sample preservation and lab
interference treatments can present
challenges in many cyanide drinking
water methods. Massachusetts Water
Resources Authority (MWRA) studies
have shown 2 that free cyanide can form
even in deionized water using these
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