Analytical Method: Selenium in Water

Instruments and Reagents

Product No. 109748 Spectroquant^® Photometer NOVA 30 or
Product No. 109751 Spectroquant^® Photometer NOVA 60
Product No. 114944 Rectangular cells 50 mm
Product No. 100264 Formic acid 98-100% for analysis
3,3‘-Diaminobenzidine tetrahydrochloride (CAS 868272-85-9), e. g. Sigma-Aldrich Art. 32750
Product No. 109992 TitriplexIII^® solution 0.1 mol/l
Product No. 105432 Ammonia solution 25% for analysis
Product No. 108389 Toluene for gas chromatography
Product No. 109584 MColorpHastTM pH-indicator strips, pH 2.0 - 9.0
Product No. 106649 Sodium sulfate anhydrous for analysis
Product No. 116754 Water for analysis
Membrane filter with 0.45 μm pore diameter

Measuring Range

0.05 – 2.00 mg/L

Reagent 1

Fill 9.4 mL formic acid 100% in a calibrated or conformity-checked 100-mL volumetric flask. Make up to volume with water for analysis and mix.

Reagent 2

Dissolve 0.5 g 3,3’-diaminobenzidine tetrahydrochloride in 100 mL chilled water for analysis. Prepare freshly each day and store in an amber glass bottle.

Reagent 3

Fill 48 mL ammonia solution 25% for analysis in a calibrated or conformity-checked 100-mL volumetric flask. Make up to volume with water for analysis and mix.

Sample Preparation

The sample must be clear; if not, filter through a membrane filter with 0.45 μm pore diameter.

Analysis

Mix 30 mL nearly neutral sample solution with 2 mL reagent 1, 2 mL Titriplex III® solution 0.1 mol/l and 2 mL reagent 2. Adjust the pH value of the mixture with reagent 3 to 2.5 and set aside for 45 min at a dark place. Adjust the pH value of the mixture again with reagent 3 to 7.0, transfer into a 100-mL separation funnel, add 15 mL water for analysis and 7 mL toluene for analysis. Shake the mixture in the closed funnel for 1 min. Allow to separate and discharge the lower water phase. Fill the toluene extract in a test tube, add some anhydrous sodium sulfate and shake. Fill the clear, dried toluene extract in a 50 mm rectangular cell and measure at 445 nm against pure toluene.

Calculation when measuring the absorbance value using NOVA photometers without programming the method in the memory of the instrument:

Concentration [mg/l] = Absorbance x 0.654 – 0.037

Calculation:

Result = measured value in mg/l Se⁴⁺

Statistical Data:

Standard deviation: 0.019 mg/l

Coefficient of variation: 2.9 %

Reference

1. Fries J, Getrost H. 1975. Organische Reagenzien für die Spurenanalyse. Darmstadt: E. Merck.