Analytical Method for Selenium in Water

This method for measuring selenium in water uses the Spectroquant^® Photometer and a clear, filtered sample to evaluate the results at 445 nm. 

Instruments and Reagents Required

Spectroquant^® Photometer NOVA 30 (Product No. 109748) or
Spectroquant^® Photometer NOVA 60 (Product No. 109751)
Rectangular cells 50 mm (Product No. 114944)
Formic acid 98-100% for analysis (Product No. 100264 )
3,3‘-Diaminobenzidine tetrahydrochloride (CAS 868272-85-9) (Product No. 32750)
TitriplexIII^® solution 0.1 mol/l (Product No. 109992)
Ammonia solution 25% for analysis (Product No. 105432)
Toluene for gas chromatography (Product No. 108389)
pH-indicator strips, pH 2.0 - 9.0 (Product No. 1.09584)
Sodium sulfate anhydrous for analysis (Product No. 106649)
Water for analysis (Product No. 116754)
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. Adjust 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 fresh 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. Adjust volume with water for analysis and mix.

Sample Preparation

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

Analysis

1. Mix 30 mL of nearly neutral sample solution with 2 mL reagent 1, 2 mL Titriplex III^® solution 0.1 mol/L and 2 mL reagent 2. 
2. Adjust the pH value of the mixture with reagent 3 to 2.5 and set aside for 45 minutes in the dark. 
3. 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 minute. 
4. Allow to separate and discharge the lower water phase. 
5. Fill the toluene extract in a test tube, add some anhydrous sodium sulfate and shake. 
6. 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 %