Sulfide
Dissolved sulfide is determined with the DR 3900 photometer from HachLange by methylene blue method (after Cline, 1969).
Analytical principle
Dimethyl-p-phenylene-diamine forms an intermediate with hydrogen sulfide that transforms to leucomethylene blue. Leucomethylene blue is oxidized to methylene blue by ferric iron.
Measurement principle
The extinction of this color complex is determent with a VIS spectrophotometer (wavelength 670 nm) for both samples and calibration standards and sample concentrations are calculated from the resulting standards calibration curve.
Sample preparation
Please not, sulfide is volatile! Samples and standards have to be preserved immediately with zinc acetate to precipitate sulfide as ZnS.
1.5 mL sample is mixed with 0.6 mL ZnAc solution (this is enough to precipitate the sulfide from 1.5 mL of sample with up to 300 ppm or approx.10 mM/L sulfide).
Solutions
Color reagent: 0.4 g N,N-Dimethyl-1,4-phenylenediamin-dihydrochloride (C8H12N2 x 2HCl) to be dissolved in 100 mL 30% HCl (stable for several months if stored in a dark bottle).
Catalyst: 1.6 g FeCl3 x 6 H2O to be dissolved in 100 mL 30% HCl (approx. 60 mM/L ferric iron) (stable for months).
Zinc acetate solution: 1.04 g ZnAc ( C4H6O4Zn x 2 H2O) in 100 mL pure water (approx. 50 mM/L Zn).
Dilution water and Blank: 14 ml ZnAc in 50 ml (MilliQ or artificial sea water, dependent on sample type).
Calibration solutions:
Pre-dilution solution 50 ppm: 0.5 mL sulfide standard 1000 ppm (in 1% NaOH) diluted with 9.5 mL O2-free water (dilution factor DF20 ). To be freshly produced before analysis.
From this stock solution, the following calibration standards are produced:
|
Conc [ppm] |
DF |
Aliquot std [µl] |
ZnAC [ml] |
Aliquot MilliQ or ASW [ml] |
|
0.0 |
0 |
0 |
2,86 |
7,14 |
|
0.2 |
250 |
40 |
2,86 |
7,1 |
|
0.5 |
100 |
100 |
2,86 |
7,04 |
|
0.75 |
66,66 |
150 |
2,86 |
6,99 |
|
1.0 |
50 |
200 |
2,86 |
6,94 |
|
1.25 |
40 |
250 |
2,86 |
6,89 |
|
1.5 |
33,33 |
300 |
2,86 |
6,84 |
Measurement
1 ml of prepared standard or sample solution is filled in measurement cuvettes and 30 µl color reagent and 60 µl catalyst is added (shake the precipitated sample before taking an aliquot for measurements). Shake the prepared sample aliquots and measure the absorbance/extinction at a wavelength of 670 nm after a 30 min reaction time.
Device
- Start the VIS spectrophotometer and wait until the system has booted
- Press SINGLE WAVELENGTH on the touch screen
- Enter the cuvettes one by one into the device and measure absorbance
- 1.) Measure a blank (dilution water)
- 2.) Measure the bundle of calibration solutions
- 3.) Measure a bundle of samples (at least 10 in a row).
Calculation:
Enter the measured values of the calibration standards into an Excel data sheet and calculate the sample concentrations from the resulting standards calibration curve.
Documentation:
Make sure to document all data in the respective lab book, the Excel worksheet and the resulting sample data in the offshore database DIS.
