APPARATUS:
Eppendorfpipettes (100-1000 µl und 1-5 cm³)
Volumetric flasks (50 cm³, 1000 cm³)
10 beakers (20 ml) + magnetic stir bars
Titration unit with dark glass bottle
Magnetic stirrer
Various bottles to store the reagents

REAGENTS:
EDTA-Stock standard: MERCK-Titriplex III-Solution™  EDTA (0.1 mol/l). Dilute with pure water to a concentration of 0.03 mol/l (=work standard)
Buffer: 67.5 g NH₄Cl are added to 570 cm³ of NH₄OH (32%), the final volume being made up to 1000 cm³ with pure water.
Indicator: 50 mg Eriochrome Black-T are dissolved in 50 ml Ethanol (80%). A fresh batch should be made bevor each site or every other day.
IAPSO (certificated standard seawater)

TITRATION:
To 500 µl sample aliquot add 5 cm³ of pure water. Add 1 cm³ of ammonia buffer and 100 µl indicator solution. Start stirring. Color change is reddish to blue.
With this method you measure Ca, Mg and Sr. The content of Sr is very low, so it can be neglected. Ca has to be measured separately with another method and the result has to be subtracted from the result of the magnesium-titration.

CALIBRATION:
The calibration is carried out with IAPSO seawater standard which is titrated like the samples.
The f-factor is determined as ratio of the theoretical EDTA consumption (1.0773 cm³ for IAPSO with Ca + Mg + Sr = 64.64 mM) and the EDTA consumption during IAPSO titration (VEDTA).
 

CALCULATION:

INTERFERENCES:
High concentrations of H₂S (> 1mM) in the sample have no effect on the measurement whereas  CaCO₃-precipitations might occur. Therefore, measurements have to be carried out right after sampling.