Annex B-8 Appendix 4: Technical note on the determination of hydrogen sulfide in seawater

1. Introduction

Hydrogen sulphide is a poisonous gas that readily dissolves in water. The sulphide is formed in stagnant waters, where the oxygen has been consumed by bacteria oxidizing organic matter to carbon dioxide, water, and inorganic ions. Sulphate-reducing bacteria then use the oxygen bound in sulphate ions as an electron acceptor while reducing the sulphate ions to sulphide. No higher life forms can exist in water containing hydrogen sulphide, and these areas are thus turned into oceanic deserts. Hydrogen sulphide in a water sample is easily detected by its characteristic smell, even at concentrations lower than those measurable with the method below.

2. Methods

The reference method for sampling and determination of hydrogen sulphide in the Baltic area is the spectrophotometric method described in Fonselius et al. (1999). This book should be consulted for exact reagent compositions and procedures. For concentrations up to approximately 250 µM, the method by Fonselius et al. (1999) is recommended. Samples with higher concentrations can be treated in two different ways. Samples containing higher concentrations may be diluted after precipitation with a zinc acetate solution containing 2 g l^-1 of gelatin (Grasshoff and Chan, 1971). This solution can be homogenized and diluted. However, higher levels of sulphide are better quantified using the method by Cline (1969).

3. Sampling

Samples are taken from ordinary hydrocast bottles immediately after the oxygen samples, using the same sampling technique (cf. ‘Technical Notes on the Determination of Dissolved Oxygen in Sea Water’). If no oxygen is present, the sulphide samples should be taken first. Sulphide reacts with many metals, and the samplers should thus preferably be all-plastic. 50–100 ml oxygen bottles are recommended.

The two reagents are added simultaneously using piston pipettes or dispensers. The tips of the pipetting devices should be close to the bottom of the bottle. No air bubbles should be trapped in the bottle. Note that the amount of reagents added have to be adjusted according to the size of the bottles used. As concentrations rather than amounts are measured, no exact knowledge of the bottle volume is required.

Samples that cannot be analysed within 48 hours may be preserved with zinc acetate, which precipitates the sulphide as zinc sulphide. The preserved samples can be stored for a few months, if light and temperature changes are avoided. Prior to analysis, the reagents are added in the same way as for unpreserved samples. When the bottle is turned, the precipitate dissolves easily, and the colour develops normally.

4. Analytical procedures

Absorbances are measured in a spectrophotometer or a filter photometer at 670 nm. Measurements should be performed no sooner than 1 hour and no later than 48 hours after the reagent addition.

5. Analytical quality assurance

The following QA elements must be satisfied:

The performance of the photometer with regard to absorbance and wavelength correctness must be checked and documented using a certified set of filters, or by an equivalent method.

The reagents must be calibrated using the procedure described in Fonselius et al. (1999). For measuring volumes in this procedure, only calibrated or class A glassware should be used. It is essential that the working solutions are freshly prepared, and that the sulphide content of the stock solution is measured, not calculated from the weighing of Na₂S (as Na₂S of sufficient purity is not available).

New reagents should be prepared at one-year intervals. The old reagents always must be checked against the newly prepared reagents in order to prove their stability.

No stable solutions are available for control charts. The difference between double samples in a control chart with zero as the reference line provides information on both precision and the validity of the subsampling. Ideally, the result (Sample 1 – Sample 2) should be evenly distributed around zero. Any deviations from this suggest subsampling problems.

6. Reporting of results

The concentration of hydrogen sulphide is usually expressed as µmol l^-1 (µM), or in some cases as ml l^-1 H₂S or as negative oxygen.

X µmol l^-1 S^2– = X ´ 22.41 ´ 10^–3 ml l^-1 H₂S

Y ml l^-1 H₂S = Y ´ 10³ / 22.41 µmol l^-1 S^2–

Z µmol l^-1 S^2– = -0.044 ´ Z negative oxygen units (ml l^-1)

7. Precision

Using the method recommended in Fonselius et al. (1999), the analytical precision will be approximately ±1 µmol l-1.

8. References

Cline, J.D. 1969. Spectrophotometric determination of hydrogen sulfide in natural waters. Limnology and Oceanography, 14: 454–458.

Fonselius, S., Dyrssen, D., and Yhlen, B. 1999. Determination of hydrogen sulphide. In Methods of seawater analysis, 3rd edition. Ed. by K. Grasshoff et al. Wiley-VCH, Germany.

Grasshoff, K., and Chan, K.M. 1971. An automatic method for the determination of hydrogen sulphide in natural waters. Analytica Chimica Acta, 53: 442–445.

Last updated: 29.10.2012 (Annex number changed from Annex B 9 to Annex B 8)