Acid Volatile Sulfides (AVS) in Sediment An Environmental Chemistry Experiment Henry Brouwer Redeemer College, Ancaster, ON, Canada L9K 1J4
With the increased interest in environmental chemistry, more student experiments to measure chemical parameters of particular ecosystems are desirable. This experiment describes a simple method to measure acid volatile sulfides (AVS), a form of reduced sulfur found in sediment. I t may be used a s a n undergraduate laboratory for a n environmental chemistry course, as a n experiment to illustrate the use of a n ion-selective electrode in chemical analysis, or a s part of a project involving the study of sediment. Sulfur is widespread in the environment and exists in many different forms: sulfides 6 3 , polysulfides (S,%), elemental sulfur (Ss), thiosulfate (S2032-), sulfite (SO3%), sulfate (SO&), and various organic sulfur compounds. Under oxic conditions, sulfides may he oxidized to sulfates, a common problem in mines where the oxidation of sulfide minerals (caused by exposure to air, water, and bacteria) forms sulfuric acid, producing acidic mine wastes. In waters or sediments containing a high concentration oforganic matter ns in the cair of~wamps,scwagc, or polluted waters^ thc chemical oxygen demand mar he so h ~ g h that the sediment becomes an&ic, resulting in the reduction of oxidized forms of sulfur to sulfides. One such reduced sulfur compound is hydrogen sulfide, a toxic gas (TLV 10 ppm) often associated with swamps, sewers, sour natural gas, and rotten eggs. Anoxic sediments with a high sulfide content tend to be very black in color due to insoluble metal sulfides, primarily FeS. In aqueous solution, hydrogen sulfide is in equilibrium with the bisulfide and sulfide ions:
with pK, = 7.05 and pK2 = 18.51 a t 20 "C ( I ) . Sulfide concentrations usually are reported a s the sum of all three species. Sulfides are known to be toxic to aquatic organisms; levels as low a s 15 ppb caused chronic toxicity to Hexaeenia limbata (2)while 0.5 nnm resulted in acute toxicity i o rainbow trout and salmon (3).The sulfide level in sediment, though, is not necessarily a n indicator of sediment toxicity, because the sulfides may be biologically unavailable, due to the low solubility of many metal sulfides. Knowing the concentration of sulfides in sediment provides a measure of the reduced (anoxic) state of the sediment and may help to explain why only certain types of aquatic organisms are found in a particular area. Analysis of sulfides in sediment is complicated by the wide range of sulfur compounds commonly found in sediment. Because H2S exists in equilibrium with other sulfides, it is not possible to analyze only for H2S. The most common form of reduced sulfur reported in the literature is called acid volatile sulfides (AVS). Acidifymg sediment results in rapid dissolution of iron monosulfide minerals (such a s amorphous FeS) and other reactive sulfide species with the liberation of the sulfide a s gaseous H2S. Pyrite (FeS2), a more stable reduced sulfur compound, requires hot hydrochloric acid and stannous chloride digestion for dissolution (4). 182
Journal of Chemical Education
The orocedure described below for AVS determination in s e d i m k t utilizes a simple diffusion chamber in which the H2S released by acidification of the sediment is absorbed by a special trap solution. The concentration of the sulfide ions in this solution is then measured ~otentiometricallv using a sulfide ion-selective electrode. Experimental Electrodes and Meter
The sulfide iou-selective electrode, sensitive t o w or S", is available from most scientific supply houses (e.g., Canadawide Scientific Cat. No. 66540-02, $239.12). A calomel reference electrode (a pH combination electrode may also be used: conned the outer terminal to the meter) and a hieh impedance digital meter with a resolution of a t least 1& com~letethe a ~ ~ a r a t rewired us to measure emf. The solution: (about 10h) shouldbe stirred a t a constant rate for the emf measurements. Trap Solution The solution for t r a ~ o i n ethe hvdroeen sulfide. called a sulfide antioxidant b i f t e r ? ~ sol;tion ~ ~ ~ j (51, c&sists of 2 M NaOH (to convert H2Sto S2-), 0.1 M EDTA(to complex metals that may catalyze the oxidation of S"), and 0.1 M ascorbic acid (to prevent oxidation of S"). Calibration A stock sodium sulfide solution (0.1 M) is prepared from freshly washed crystals (to remove oxidized materials) of Na9S.9H90 and stored a t 4 "C. This solution is standardized ;a a n iodometric titration of 1.00 mL with 0.025 M sodium thiosulfate (see any standard analvtical text for procedure). The stock solution (1.0 mL) is added to SAOB solution (3.0 mL) and diluted to 10 mL in a volumetric flask. The emf of 10-fold dilutions over the range to M Na2S is measured to calibrate the electrode system. Acid Volatile Sulfides (A VS) in Sediment Wet sediment (about 1-2 g for samples low in AVS and 0.2 g for high levels of AVS) along with 2 mL of water is added to a 20-mL scintillation vial (A) closed with a cap having a polyethylene cone insert or with a rubber stopper. A 15 x 45 mm vial (B) containing 3 mL of SAOB solution is placed inside the scintillation vial (A) after which 2 mL of 2 M HCI is quickly added to the sediment and the vial (A) immediately closed to prevent gases from escaping. The vial (A) is swirled gently to mix the acid and sediment and allowed to stand for an hour (with occasional swirling) during which time the H2S diffuses into the SAOB solution. While waiting for the next step the standards may be prepared and measured. After removing the inner vial (B) with a nair of tweezers. the exterior is wTped off and the SAOBsolLtion transferred to a 10-mL volumetric flask or a test tube marked a t 10.0 mL; the vial (B) is rinsed several times, the solution brought to volume, mixed well, and its sulfide concentra~~~~~~
tion measured using the electrodes. Although the sulfide solutions in SAOB are stable for a t least 6 h, measnrements of the standards and samples should done a s close together a s possible a s there may be some drifting in the emf measurements. Calculations
The [S2-I may be determined directly from a calibration curve prepared by plotting -emf versus log[S2-I, or the calculations may be done using a spread sheet program. Alinear regression analysis of the calibration data using +mf and ln[S2-I as the x- and y-values, respectively, provides the constant and slope for the data. The sulfide ion concentration in the solution is calculated using the equation [s? = exp K-emf of sample) - eonstant)/slope (1) The quantity of AVS (pmolelg wet sediment) is given by quantity of AVS = X mom x 10 mL x (1 L/l000 mL) x (lo6 pmal/mol)/(massof sediment) = X x lo4 pmoVg sediment (2) where X = concentration of sulfide. For comparison purposes, the quantity of AVS/(g wet sediment) will he adequate. To convert to dry weight basis (the more usual means of reporting AVS), a known mass of wet sediment is dried in a n aluminum dish a t 70 "C overnight and the mass of AVS converted to mole AVS/(g dry sediment). Discussion AVS levels a s high a s 100 umol (in industriallv. ~olluted . sediments) to a s low a s 0.01 pmoVg dry sediment (in uncontaminated. oxic sediments) have been measured with this procedur~.From a n analytical point of view, the precision of this method is rather low (relative standard deviation is about 15%),due primarily to two reasons. First of all, sediment is a heterogeneous mixture and with the small samples used in this experiment, representative
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sampling is difficult. Secondly, a n error of 1 mV in emf readings leads to a significant error (about 8%)in concentration. Because the sulfide ion is a divalent species, a potential change of about -29 mV per 10-fold dilution is ohtained, a s compared to the theoretical value of -59.16 mV for a monovalent ion. The convenience and speed of analysis by ISE is a definite advantage over colorimetric or chemical analysis, even though the measurements may he less precise. Three (or more) samples may he analyzed readily in triplicate for AVS in a three-hour lab. Once the solutions are ready, less than a minute is required per emf measurement. Two ISE's are adequate for a class of 20 students working in pairs. The sediment sample may he obtained in several ways, depending on the information desired. Asimple grab sample from several locations within the body of water under investigation may he used to map the anoxic state of the sediment. Seasonal changes in AVS may be measured by freezing samples collected monthly over several seasons. To obtain a sulfide profile of the sediment with depth, a sediment core obtained with the simple coring device described recently in this Journal (6)may be divided into 1or 2-cm sections and analyzed for AVS. Since sulfides are readily oxidized, handling of the sediment is important for good results. Fresh sediment should be used a s quickly as possible, with minimum exposure to air. If samples need to he stored, they should be placed in airtight containers and kept a t a low temperature, or even frozen. Literature Cited 1. Schoonen, M. A. A ; Barnes, H. L. Gemhim C&mochirn.Acto 1988.52.649-654, 1975.415-18. 3. o m , J. A,: ~ u e d aA,; , csmaw, J. A,: ~ i e t oF: . ~ e o y o M. . J.: ~arazons.J . V BUIL. Enuimn. Contorn. h r i m l . 1993,50,165170. 4. Mome.J.W:Millem.F. J.;Comwell.J.C.:Rickad,D.Ea~hSci.Rau 1987,24.1-42. 5. Amwolo. T A.; Cresaer, M . S. Analyst 1981.116. 595699. 6. Delumyea, R D.; McCleary, D. L. J. C h r n Edue 1998.70, 172-173. 2. Oseid, D.M.; Smith, L. L.EnuimnmnlolEnlorn011gy
Volume 72 Number 2 February 1995
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