The VGA-76 Operation Manual recommends the use of
Digestion Procedures sodium borohydride as the reducing
agent in the determination of mercury, the solution containing
0.3% NaBH4 in 0.5% NaOH. It has been found in the present
study that while sodium borohydride provides a slightly
more sensitive response than stannous chloride (25% SnCl2
w/v in 20% v/v HCI) for nitric acid standard solutions, a
severe interference exists in the analysis of fish sample
digests.
Figure 2 shows the response to mercury standards, prepared
in 40% v/v nitric acid, obtained with sodium borohydride and
stannous chloride as alternative reductants. We conclude
that stannous chloride should be used routinely because of
the interference observed with NaBH4.
Many