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Marine Geochemistry - Laboratory Methods

Sulfide detection with Merck Photometer
note! sulfide is volatile ! Both samples and standards have to be preserved immediately. Sulfide is preserved by precipitation as ZnS by addition of sufficient amounts of zinc acetate. For preservation mix 1.5 mL sample with 0.5 mL ZnAc solution (see below) (=dilution factor of 1.33). 2mL or 4 mL glass vial with screw cap. This is enough to precipitate the sulfide from 1.5 mL samples with up to 450 ppm sulfide (approx. 14 mM/L). Before taking an aliquot for measurement shake well (use ultrasonic bath if the precipitate does not form an evenly distributed cloud).
We now usually do a second more diluted aliquot already upon sampling: 100 µL sample + 900 µL of ZnAc. In case the concentration is high, it is easier and more precise to work directly from a more diluted sample than trying to dilute the sample with the ZnS precipitate.
method 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. Reference:  Cline (1969) Limnology and Oceanography https://doi.org/10.4319/lo.1969.14.3.0454
setup
solutions  
color reagent

Dissolve 0.4 g N,N-Dimethyl-1,4-phenylenediammonium dichloride (C8H12N2 *· 2HCl Merck # 103067, MW 209.12) in 100 mL 10 molar hydrochloric acid (HCl, 30%).
This solution is stable for several months if stored refrigerated in a dark bottle .
catalyser solution Dissolve 1.6 g FeCl3* 6 H2O (Merck #103943, MW 270.33) in 100 mL 10 molar hydrochloric acid (HCl 30%).
(approx. 60 mM/L ferric iron). This solution is long term stable
zinc acetate solution Dissolve 0.52 g ZnAc ( C4H6O4Zn * 2 H2O, Merck # 108802 MW 219.49) in 50 mL  oxygen-free pure water
(approx. 56 mM/L Zn)
zinc acetate solution
for dilution and blank
 Dissolve 0.625 g ZnAc ( C4H6O4Zn * 2 H2O, Merck # 108802 MW 219.49) in 250 mL oxygen-free pure water
(approx. 14 mM/L Zn) this is equivalent to the Zn concentration in the mix sample +ZnAc, so for dilution you keep the same Zn level
standard stock

always prepare fresh!! sodium sulfide crystals have to be stored dry (in an exsiccator)
150 ppm sulfide solution:
Dissolve 112 mg Na2S * 9 H2O ( Merck # 106636, MW 240) in 100 mL oxygen-free pure water (approx. 4.7 mM/L sulfide)
work solution

15 ppm sulfide: dilute standard stock by a factor of 10 (1+9).(approx.0.47mM/L sulfide) with the zinc acetate solution for dilution (14 mM/L)
always prepare fresh
dilution water and blank see above
work standards always prepare fresh.
concentration work sulfide solution dilution water
ppm mL mL
0.0 0.0 10
0.15 0.1 9.9
0.3 0.2 9.8
0.6 0.4 9.6
0.75 0.5 9.5
0.9 0.6 9.4
1.2 0.8 9.2
1.5 1.0 9.0
 Check sample 1.5 200 µL 150 ppm std stock
 19.8

analysis conditions

Depending from the expected sulfide concentration, dilute the sample to a concentration below 1.5 ppm. Total volume needed for the small cuvette is 1 mL.
1. Always shake well prior to taking aliquots since the precipitate contains the analyte. We now put samples in a overhead shaker for 2h.
2. If there is a strong sulfide odor, try a high dilution first
3. add 30 µL color reagent
4. add 60 µL catalyser solution
6. shake well - allow 30 min reaction time.
7. determine extinctions for both standards and samples at 670 nm (on the Merck photometer shown above method 239 @ 660nm) and calculate concentrations from the calibration curve in an Excel worksheet

If the concentrations are above the calibrated range (0-1.5 ppm) start over with the samples from Step 2 and increase the dilution factor.
tips + tricks

Pure water for preparing standard solutions should be stripped with Argon to avoid oxidation while diluting (bubble for 20min with Argon)
last edit

 Nov 2018

page created by M.Kölling & T.Feseker