Hanna Instruments HI 4115 - Manual (Page 4)

6 7
tions. Standards with concentrations < 100 ppm should
be made daily.
Solutions for Sulfide Measurements
SAOB, Hanna HI 4015-00 : Add content of SAOB reagent
B bottle to the bottle with SAOB reagent A. Cap bottle
and mix well until completely dissolved. Reagent that is
brown has been oxidized and should be discarded.
Sulfide Standards:
Sulfide standards are susceptible to air oxidation and
therefore are not commercially available.
Sulfide stock solution: Weigh approximately 100 gram of
reagent grade Sodium Sulfide nonahydrate
(Na
2
S ·9H
2
O) into a 1 liter volumetric flask that contains
a volume of distilled or deionized /deareated water.
Swirl to dissolve, bring to volume, then transfer to a
tightly sealed 1 L container with minimal head space
and allow solution to equilibrate overnight.
133.5 ppm standard (approximate concentration):
Transfer 5.00 mL stock solution and 250 mL Sulfide
Antioxidant buffer SAOB (Hanna HI 4015-00) to a to
500 mL volumetric flask. Mix well then bring to volume
with distilled or deionized /deareated water. Hanna HI
4015-00 SAOB, should be clear to pale straw in color
after mixing. Note: higher concentrations of standard can
also be prepared and titrated if samples are greater in
concentration. The sensor is useable from 32,100 to
0.003 ppm.
Determine exact concentration of the prepared standard
by doing a titration with Lead Perchlorate standard (user
prepared) on a weekly basis. The exact concentration of
the standard is found by titrating 10 mL of the standard
with 0.01 M Lead Perchlorate standard; (Pb(ClO
4
)
2
).
Pb 2+ + S 2- > PbS
Use the electrode pair as an indicator for S 2- to determine
the endpoint. See Section XII.
VVVVV
s. s. s. s. s.
= Volume of the standard sulfide solution used
(10 mL)
VVVVV
t t t t t
= Volume of the Lead Perchlorate titrant used to reach
endpoint (mL))
Weekly standard in ppm
C (ppm)= (0.01) *32.07 V
t
/ V
s.
Using volumetric pipettes and glassware make dilutions
daily from the titrated standard whose value has been
determined. Choose standards to bracket the concentra-
tion of the samples.
50 mL of HI 4000-00 ISA should be added to each 100
mL flask of standard before being brought to volume.
VII VII VII VII VII. General Guidelines General Guidelines General Guidelines General Guidelines General Guidelines
•
Calibration standards and sample solutions should
have the same ionic strength. ISA or SAOB should be
added to both samples and standards in the same
ratio.
•
Calibration standards and sample solutions should
be at same temperature.
•
Thermally insulate beaker with standard or sample
from magnetic stirrer.
•
Calibration standards and sample solutions should
be stirred at the same rate using identically sized TFE
coated stir bars.
•
Rinse electrode pair with distilled or deionized water
between samples and gently dab off using soft
disposable absorbent toweling. Do not rub sensor sur-
face.
•
Presoaking silver sensor in a dilute standard
(
<10 -3 M) will help to optimize response.
•
A scratched, pitted, or tarnished pellet surface can
cause drift, poor repeatability or a loss of low level
response. Restore optimum response can be by
removing the damaged surface with the microabrasive
strip HI 4000-70.
Calculate as follows:
Weekly standard in Molar concentration
C (moles/liter)= (0.01) * V
t
/ V
s.