Method / Description Calcium

Eppendorfpipettes (100-1000 µl und 1-5 cm3)
Volumetric flasks (100 cm3, 250 cm3)
approx. 10 beakers (10 cm3) + magnetic stir bars
Titration unit with dark glass bottle
Magnetic stirrer
Various bottles to store the reagents

EGTA stock solution: 3.8 g EGTA are dissolved in 30 cm3 of 1M NaOH (4 g/100 cm3) and diluted to 100 cm3. This yields a 0.1 M EGTA solution. From this a 10 mM EGTA solution can be made by appropriate dilution.
Borate buffer: 5 g of borax (Na2B4O7*10 H2O) and 15 g of sodium hydroxide are dissolved in 250 cm nanopure water.
Indicator: 40 mg of GHA are dissolved in 100 cm3 ethanol. Should be made fresh each day that titrations are carried out.
IAPSO (certificated standard seawater)

Transfer 0.5 cm3 sample into a 10 cm3 beaker and add 3 cm3 pure water. While stirring, add 0.5 cm3 of GHA solution and 0.5 cm3 of buffer solution. Stir for about 3 min., unless you have quite high calcium values, in which case you must start titrating immediately after adding the buffer (the color tends to fade after 5 or 6 minutes). Add 2 cm3 butanol when the reddish color starts to diminish. When the color becomes even less reddish, stop stirring and wait for the butanol to separate. Examine the color and start stirring again while adding a small amount of the 10 mM EGTA titrant. When red color fails to reappear, the titration is finished.

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 EGTA consumption (0.527 cm3 for IAPSO with 10.55 mM Ca) and the the volume of the EGTA solution consumed during titration of IAPSO (VEGTA).      


or easier for the amounts of sample and concentrations used here:

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