Chlorinity titration
Equipment:
Titrator or autotitrator capable of measuring single µL amounts
Graduated flask 100 mL
magnetic stirrer
beakers
Reagents:
sea water standard
0.1 N AgNO3 solution in brown flask
pre-weighted K-chromate (4.2 g), K-dichromate (0.7 g)
Prepare indicator solution:
Dissolve 4.2 g K-chromate and 0.7 g K-dichromate in 100 mL pure water.
This solution is stable over months. Caution this solution is toxic and should be handled stored and disposed with care.
Analysis:
Add 5 mL pure water and 100 µL indicator solution100 µL sample to get a clear yellow solution. Put beaker on a white paper on the magnetic stirrer.
Stir well while adding known quantities of silver nitrate solution to get a full change of color from clear yellow to a dirty orange (“ bad orange juice”).
Calibration:
Perform the titration with standard seawater. If standard seawater is not available filtered real seawater is sufficient in most cases. The chlorinity of an IAPSO standard with a salinity of 34,998 is 19,2787 g/kg, since Cl [g/kg] = salinity / 1.81537.
This is equal to a Chloride concentration of 556 mM and regarding some effect of Bromide (0.86 mM) in the setup above 0.557 mL of silver nitrate solution should be used for titration to the color change.
Determine a conversion factor f that accounts for the real amount obtained yb the setup and operator f = Vexpected / Vreal
the concentration of Cl+Br may then be calculated from
Cl+Br[M] = f VAgNO3 [L] c AgNO3 [M] V-1sample [L]
With the setup above 0.1 ml sample titrated with 0.1 M AgNO3 the conversion to molar concentrations of Cl+Br simplifies to
cCl+Br[mM] = f VAgNO3 [µL]
High concentrations of sulfide interfere with the titration. Sulfide may be stripped by treating samples with 0.05M suprapure nitric acid (mix equal amounts of both sample and acid). Yet the time needed to evaporate the sulfide may also cause evaporation of sample.
Reference