15
preferably performed with a pipette
to minimize mixing with oxygen, and
a maximum of 10% of the vial volume
should be left as headspace . Close the
container with a gas tight lid and shake
vigorously .
3 . Obtain zero reading
The signal at zero H
2
S can be obtained by immersing the sensor tip
into one of the vials with calibration buffer . Note the signal, which
is the calibration value for zero H
2
S partial pressure (S0) . This signal
should be 0-30 pA (otherwise see “Troubleshooting”) .
4 . Obtain sulfide standard readings
Calibration points within the expected range of measurement are
prepared by injecting suitable amounts of the S
2-
stock solution
anaerobically into the calibration buffer with a micro-syringe
(the stock solution should be diluted at least 10 times) . Mix the
solutions . If you have not added reductant, oxygen that dissolve
in the calibration solution will oxidize the sulfide, so the stock
solution should be added immediately after the oxygen removal
(e .g . N
2
bubbling) and the calibration should be done immediately
after adding the stock solution .
For each calibration solution, measure the calibration values
by removing the rubber stopper and immerse the microsensor
tip into the solution . Read the signal and plot it against the
concentration .
The most precise calibration curve is obtained by fixing
the calibration solutions with Zn-acetate and subsequently
determining the total sulfide concentration using the Cline
method (Cline 1969) . This may, due to the nature of the method,
be difficult if a reductant has been added previously .
c
alibration
in
the
field
For an easy and portable field calibration, prepare the following
reagents in the lab . Bring the reagents and a 1 ml s needle
to the field .
