## 4500-S 2

 

## SULFIDE*

4500-S 2

 

## A. Introduction

1. Occurrence and Significance

Sulfide is often present in groundwater and sediment. It isproduced by decomposition of organic matter and bacterial reduction of sulfate. It is sometimes found in industrial or municipal wastewater. Hydrogen sulfide escaping into the air from sulfide-containing wastewater causes odor nuisances. The threshhold odor concentration of H 2 S in clean water is between 0.025 and 0.25  g/L. Gaseous H 2 S is very toxic and has claimed the lives of numerous workers. At levels toxic to humans it interferes with the olfactory system, giving a false sense of the safe absence of H 2 S. It attacks metals directly, and indirectly hascaused serious corrosion of concrete sewers because it is oxidized biologically in the presence of oxygen to H 2 SO 4 on the pipe wall. Dissolved H 2 S is toxic to fish and other aquatic organisms.

Hydrogen sulfide combines with iron and other metals innatural sediments and sludges to form slightly-soluble precipitates. Acid-volatile sulfide (AVS) is an important class of metal sulfides in these anoxic environments. The determination of AVS concentrations has become more prevalent because AVS isconsidered to be a key binding phase for controlling bioavailability of toxic metals in anoxic sediments.

AVS typically is determined by a purge-and-trap method in which hydrochloric acid is used to volatilize AVS at room temperature. The metals associated with the sulfides can be determined from the supernatant of the purged sample solution by using methods such as those in Part 3000. The hydrogen sulfide produced is trapped in zinc-acetate absorbing solution. AVS concentrations are measured by iodometric titration of the ZnS precipitated in the trap (4500-S 2

 .F). Certain minerals, including iron pyrite, are partially digested by the AVS reagentsat elevated temperatures, which may result in a significant overestimation of AVS. Iron pyrite can be partially digested to the extent of less than 10% of the total pyrite present. The addition of stannous chloride (SnCl 2 ) prevents the oxidation of sulfides by any liberated ferric iron.

2. Categories of Sulfides

Four categories of sulfide in water, wastewater, and sediment can be operationally defined:

a. Total sulfide includes dissolved H 2 S and HS  and acidvolatile metallic sulfides present in particulate matter. The pKa2 of H 2 S is so high that the concentration of S 2

 is negligible at all pH values. Copper and silver sulfides are so insoluble that they do not respond in ordinary sulfide determinations; they can be ignored for practical purposes.

b. Dissolved sulfide is that remaining after suspended solids have been removed by flocculation and settling. Flocculation and settling are used to separate dissolved and particulate sulfide because sulfide may be oxidized during filtration. Centrifugation also may be used.

c. Acid-volatile sulfide includes amorphous iron monosulfides, including mackinawite (FeS), greigite (Fe 3 S 4 ), and pyrrhotite (FeS), and amorphous monosulfides of other metals. Pyrite, another iron sulfide mineral, is not included in the acid-volatile sulfides.

d. Un-ionized hydrogen sulfide can be calculated from theconcentration of dissolved sulfide, the sample pH, and the conditional ionization constant of H 2 S.

Figure 4500-S 2

 :1 shows analytical flow paths for sulfide determinations under various conditions and options. None of the operationally defined sulfide categories includes pyrite or marcasite (FeS 2 ).

3. Sampling and Storage

Collect water samples with minimum aeration. Either analyzesamples immediately after collection or preserve with zinc acetate solution for later analysis. To preserve a sample for a total sulfide determination, put zinc acetate and sodium hydroxide solutions into sample bottle before filling it with sample. Use 0.2 mL 2M zinc acetate solution per 100 mL sample. Increase volume of zinc acetate solution if the sulfide concentration is expected to be greater than 64 mg/L. The final pH should be at least 9. Add more NaOH if necessary. Fill bottle completely and stopper.

Sample sediments and sludges under nitrogen atmosphere if possible. Store samples at 4°C or frozen, and analyze within 2weeks (1 month for frozen samples) of collection. Do not freezedry because acid-volatile sulfide may decompose as a result of oxidation artifacts.

4. Qualitative Tests

A qualitative test for sulfide often is useful. It is advisable inthe examination of industrial wastes containing interfering substances that may give a false negative result in the methylene blue method (D).

a. Antimony test: To about 200 mL sample, add 0.5 mL saturated solution of antimony potassium tartrate and 0.5 mL 6N HCl in excess of phenolphthalein alkalinity.

Yellow antimony sulfide (Sb 2 S 3 ) is discernible at a sulfide concentration of 0.5 mg/L. Comparisons with samples of known sulfide concentration make the technique roughly quantitative. The only known interferences are metallic ions, such as lead, which hold the sulfide so firmly that it does not produce Sb 2 S 3 , * Approved by Standard Methods Committee, 2000. Editorial revisions, 2011. Joint Task Group: 21st Edition—(4500-S 2

 .A, H, I, J )—Thomas R. Holm (chair), Robert P. Fisher, Martin S. Frant, Christian Gagnon, Lorne R. Goodwin; 20th Edition—(4500-S 2

 .I)—Scott Stieg (chair), Bradford R. Fisher, Owen B. Mathre, Theresa M. Wright.

1and dithionite, which decomposes in acid solution to produce sulfide.

b. Silver-silver sulfide electrode test: Dilute sample 1:1 with alkaline antioxidant reagent (see 4500-S 2

 .G.3 a ). Measure elec

trode potential relative to a double-junction reference electrode and estimate the sulfide concentration from an old calibration curve or the example calibration curve in the electrode manual. This gives a reasonable estimate of sulfide concentration if the electrode is in good condition.

c. Lead acetate paper and silver foil tests: Confirm odors attributed to H 2 S with lead acetate paper. On exposure to thevapor of a slightly acidified sample, the paper becomes blackened by formation of PbS. A strip of silver foil is more sensitive than lead acetate paper. Clean the silver by dipping in NaCN solution and rinse. C AUTION : NaCN is toxic; handle with care. Silver is suitable particularly for long-time exposure in the vicinity of possible H 2 S sources because black Ag 2 S is perma

nent, whereas PbS slowly oxidizes.
5. Selection of Quantitative Methods

Iodine oxidizes sulfide in acid solution. A titration based on this reaction is an accurate method for determining sulfide at concentrations above 1 mg/L if interferences are absent† and if loss of H 2 S is avoided. The iodometric method (F) is useful for standardizing the methylene blue colorimetric methods (D, E, and I) and is suitable for analyzing samples freshly taken from wells or springs. The method can be used for wastewater andpartly oxidized water from sulfur springs if interfering substances are removed first. The automated methylene blue method with distillation (I) is useful for a variety of samples containing more than 1 mg S 2

 /L.

The methylene blue method (D) is based on the reaction of sulfide, ferric chloride, and dimethyl- p -phenylenediamine to produce methylene blue. Ammonium phosphate is added after color development to remove ferric chloride color. The procedure is applicable at sulfide concentrations between 0.1 and 20.0 mg/L. The automated methylene blue method (E) is similar to Method D. A gas dialysis technique separates the sulfide from the sample matrix. Gas dialysis eliminates most interferences, including turbidity and color. The addition of the antioxidantascorbic acid improves sulfide recoveries. The method is applicable at sulfide concentrations between 0.002 and 0.100 mg/L.

Potentiometric methods using a silver/sulfide ion-selective electrode (G) may be suitable. The sulfide concentration can beestimated from the potential of the electrode relative to a reference electrode, but careful attention to details of procedures and frequent standardizations are needed to secure good results. The electrode is useful particularly as an endpoint indicator for titration of dissolved sulfide with silver nitrate. The ion-selective electrode method is unaffected by sample color or turbidity and is applicable for concentrations greater than 0.03 mg/L. † Many substances can reduce iodine; all of these are potential interferences in procedures using this chemistry. Figure 4500-S 2

 :1. Analytical flow paths for sulfide determination. SULFIDE (4500-S 2

 )/Introduction

26. Preparation of Sulfide Standards Take care in preparing reliable stock solutions of sulfide for calibration and quality control. Prepare sulfide standards from sodium sulfide nonahydrate (Na2 S  9H 2 O) crystals. These crys

tals usually have excess water present on the surface, in additionto a layer of contamination from oxidation products (polysulfides, polythionates, and sulfate) of sulfide reacting with atmospheric oxygen. Further, solutions of sulfide are prone to ready oxidation by dissolved and atmospheric oxygen. Use reagent water to prepare sulfide standards and sample dilutions. Degas the water with either argon or nitrogen. Purchase the smallest amount of solid standards possible and keep no longer than 1 year. Preferably handle and store solid sulfide standards and stock solutions in an inert atmosphere glove bag or glove box to reduce contamination due to oxidation.

Preferably remove single crystals of Na2 S  9H 2 O from re

agent bottle with nonmetallic tweezers; quickly rinse in degassed reagent water to remove surface contamination. Blot crystal dry with a tissue, then rapidly transfer to a tared, stoppered weighing bottle containing 5 to 10 mL degassed reagent water. Repeat procedure until desired amount of sodium sulfide is in weighing bottle. Avoid excess agitation and mixing of the solution with atmospheric oxygen. Quantitatively transfer and dilute entire contents of weighing bottle to an appropriate size volumetric flask with degassed reagent water to prepare a known concentration sulfide stock solution (3.75 g Na2 S  9H 2 O diluted to a final volume of 500 mL will give a stock solution of which 1.00 mL 1.00 mg S 2

 ). Standardize stock solution using the iodometric method, 4500-S 2

 .F. Alternatively, purchase precer

tified stock solutions of sulfide. Verify concentration of stock solution daily using the iodometric method (F). Store stock solution with minimum headspace for no more than 1 week.
7. Bibliography

C RUSE , H. & R.D. P OMEROY . 1969. Hydrogen sulfide odor threshold. J. Amer. Water Works Assoc. 61:677. KARCHMER , J.H., ed. 1970. The Analytical Chemistry of Sulfur and Its Compounds. Wiley-Interscience, New York, N.Y. NICKLESS , G., ed. 1970. Inorganic Sulphur Chemistry. Elsevier Publ., Amsterdam, The Netherlands. U.S. ENVIRONMENTAL PROTECTION A GENCY . 1974. Process Design Manual for Sulfide Control in Sanitary Sewerage Systems. Publ. 625/1-74- 005. B AGARINAO , T. 1992. Sulfide as an environmental factor and toxicant: Tolerance and adaptations in aquatic organisms. Aquat. Toxicol. 24:21. B UTLER , I.B., M.A.A. S CHOONEN & D.T. RICKARD . 1994. Removal ofdissolved oxygen from water: a comparison of four common techniques. Talanta 41:211.



## 4500-S 2

 

## B. Separation of Soluble and Insoluble Sulfides
Unless the sample is entirely free from suspended solids (dissolved sulfide equals total sulfide), to measure dissolved sulfide first removeinsoluble matter. This can be done by producing an aluminum hydroxide floc that is settled, leaving a clear supernatant for analysis.
1. Apparatus

Glass bottles with stoppers: Use 100 mL if sulfide will be determined by the methylene blue method and 500 to 1000 mL if by the iodometric method.
2. Reagents

a. Sodium hydroxide solution, NaOH, 6 N

. b. Aluminum chloride solution: Because of the hygroscopic and caking tendencies of this chemical, purchase 100-g bottles of AlCl 3  6H 2 O. Dissolve contents of a previously unopened 100-g bottle in 144 mL distilled water. 3. Procedure

a . To a 100-mL glass bottle, add 0.2 mL (nominally 4 drops) 6N NaOH. Fill bottle with sample and immediately add 0.2 mL (4 drops) AlCl 3 solution. Stopper bottle with no air understopper. Rotate back and forth about a transverse axis vigorously for 1 min or longer to flocculate contents. Vary volumes of these added chemicals to get good clarification without using excessively large amounts and to produce a pH of 6 to 9. If a 500- or 1000-mL bottle is used, add proportionally larger amounts of reagents.

b. Let settle until reasonably clear supernatant can be drawn off. With proper flocculation, this may take 5 to 15 min. Do not wait longer than necessary.

c. Either analyze the supernatant immediately or preserve with 2N zinc acetate (see 4500-S 2

 .C).



## 4500-S 2

 

## C. Sample Pretreatment to Remove Interfering Substances or to Concentrate the Sulfide
The iodometric method suffers interference from reducing substances that react with iodine, including thiosulfate, sulfite, and various organic compounds, both solid and dissolved. Strong reducing agents also interfere in the methylene blue method (D) by preventing formation of the blue color. Thiosulfate at concentrations about 10 mg/L may retard color formation or SULFIDE (4500-S 2

 )/Sample Pretreatment

3completely prevent it. Ferrocyanide produces a blue color. Sulfide itself prevents the reaction if its concentration is very high, in therange of several hundred milligrams per liter. To avoid the possibility of false negative results, use the antimony method to obtain a qualitative result in industrial wastes likely to contain sulfide but showing no color by the methylene blue method. Iodide, which is likely to be present in oil-field wastewaters, may diminish color formation if its concentration exceeds 2 mg/L. Many metals (e.g., Hg, Cd, Cu) form insoluble sulfides and give low recoveries.

Eliminate interferences due to sulfite, thiosulfate, iodide, andmany other soluble substances, but not ferrocyanide, by first precipitating ZnS, removing the supernatant, and replacing it with distilled water. Use the same procedure, even when not needed for removal of interferences, to concentrate sulfide. The automated methylene blue method (E) is relatively free from interferences because gas dialysis separates the sulfide from the sample matrix.

1. Apparatus

Glass bottles with stoppers: See 4500-S 2

 .B.1.

2. Reagents

a. Zinc acetate solution: Dissolve 220 g Zn(C 2H 3 O 2 ) 2  2H 2 O in 870 mL water; this make s 1 L solution.

b. Sodium hydroxide solution, NaOH, 6N

. 3. Procedure

a. Put 0.20 mL (4 drops) zinc acetate solution and 0.10 mL (2 drops) 6N NaOH into a 100-mL glass bottle, fill with sample, and add 0.10 mL (2 drops) 6N NaOH solution. Stopper with no air bubbles under stopper and mix by rotating back and forth vigorously about a transverse axis. For the iodometric procedure, use a 500-mL bottle or other convenient size, with proportionally larger volumes of reagents. Vary volume of reagents addedaccording to sample so the resulting precipitate is not excessively bulky and settles readily. Add enough NaOH to raise the pH above 9. Let precipitate settle for 30 min. The treated sample is relatively stable and can be held for several hours. However, if much iron is present, oxidation may be fairly rapid.

b. If the iodometric method is to be used, collect precipitate on a glass fiber filter and continue at once with titration according to the procedure of Method F. If the methylene blue method (D) is used, let precipitate settle for 30 min and decant as much supernatant as possible without loss of precipitate. Refill bottle with distilled water, shake to resuspend precipitate, and quickly withdraw a sample. Ifinterfering substances are present in high concentration, settle, decant, and refill a second time. If sulfide concentration is known to be low, add only enough water to bring volume to one-half or one-fifth of original volume. Use this technique for analyzing samples ofvery low sulfide concentrations. After determining the sulfide concentration colorimetrically, multiply the result by the ratio of final to initial volume. No concentration or pretreatment steps to remove interferences are necessary for Method E.



## 4500-S 2

 

## D. Methylene Blue Method

1. Apparatus

a. Matched test tubes, approximately 125 mm long and 15 mm OD.

b. Droppers, delivering 20 drops/mL methylene blue solution. To obtain uniform drops hold dropper in a vertical position and let drops form slowly.

c. If photometric rather than visual color determination will be used, either:

1) Spectrophotometer, for use at a wavelength of 664 nm with cells providing light paths of 1 cm and 1 mm, or other path lengths, or

2) Filter photometer, with a filter providing maximum transmittance near 660 nm.

2. Reagents

a. Amine-sulfuric acid stock solution: Dissolve 27 g N

,N

-dimethyl- p -phenylenediamine oxalate* in an iced mixture of 50 mL conc H 2 SO 4 and 20 mL distilled water. Cool and dilute to 100 mL with distilled water. Use fresh oxalate because an old supply may be oxidized and discolored to a degree that results in interfering colors in the test. Store in a dark glass bottle. When this stock solution is diluted and used in the procedure with a sulfide-free sample, it first will be pink but then should become colorless within 3 min.

b. Amine-sulfuric acid reagent: Dilute 25 mL amine-sulfuric acid stock solution with 975 mL 1  1 H 2 SO 4 . Store in a dark glass bottle.

c. Ferric chloride solution: Dissolve 100 g FeCl 3  6H 2 O in 40 mL water.

d. Sulfuric acid solution, H 2 SO 4 , 1  1. e. Diammonium hydrogen phosphate solution: Dissolve 400 g (NH4 ) 2HPO 4 in 800 mL distilled water.

f. Methylene blue solution I: Use USP grade dye or one certified by the Biological Stain Commission. The dye content should be reported on the label and should be 84% or more. Dissolve 1.0 g in distilled water and make up to 1 L. This solution will be approximately the correct strength, but because of variation between different lots of dye, standardize against sulfide solutions of known strength and adjust its concentration so 0.05 mL (1 drop) 1.0 mg sulfide/L.

Standardization—Prepare five known-concentration sulfide standards ranging from 1 to 8 mg/L as described in 4500- S 2

 .A.6, or proceed as follows: Put several grams of clean, washed crystals of Na2 S  9H 2 O into a small beaker. Add some

- what less than enough water to cover crystals. Stir occasionally * Eastman catalog No. 5672 has been found satisfactory for this purpose. SULFIDE (4500-S 2

 )/Methylene Blue Method

4for a few minutes, then pour solution into another vessel. This solution reacts slowly with oxygen but the change is insignificant if analysis is performed within a few hours. Prepare solution daily. T o 1 L distilled water, add 1 drop of Na2 S solution andmix. Immediately determine sulfide concentration by the methylene blue procedure and by the iodometric procedure. Repeat, using more than 1 drop Na 2 S solution or smaller vol

umes of water, until at least five tests have been made, with arange of sulfide concentrations between 1 and 8 mg/L. Calculate average percent error of the methylene blue result as compared to the iodometric result. If the average error isnegative (i.e., methylene blue results are lower than iodometric results), dilute methylene blue solution by the same percentage, so a greater volume will be used in matching colors. If methylene blue results are high, increase solution strength by adding more dye.

g. Methylene blue solution II: Dilute 10.00 mL of adjusted methylene blue solution I to 100 mL with reagent water.

3. Procedure

a. Color development: Transfer 7.5 mL sample to each of two matched test tubes, using a special wide-tip pipet or filling to marks on test tubes. If sample has been preserved with zinc acetate, shake vigorously before taking subsample. Add to Tube A 0.5 mL amine-sulfuric acid reagent and 0.15 mL (3 drops) FeCl 3 solution. Mix immediately by inverting slowly, only once. (Excessive mixing causes low results by loss of H2 S as a gas before it has had time to react). To Tube B add 0.5 mL 1  1 H2 SO4 and 0.15 mL (3 drops) FeCl 3 solution and mix. The presence of S 2

 will be indicated by the appearance of blue color in Tube A. Color development usually is complete in about 1 min, but a longer time often is required for fading out of the initial pink color. Wait 3 to 5 min and add 1.6 mL (NH4 ) 2HPO 4 solution to each tube. Wait 3 to 15 min and make color comparisons. If zinc acetate was used, wait at least 10 min before making a visual color comparison.

b. Color determination: 1) Visual color estimation—Add methylene blue solution I or II, depending on sulfide concentration and desired accuracy, dropwise, to the second tube, until color matches that developed in first tube. If the concentration exceeds 20 mg/L, repeat test with a portion of sample diluted tenfold.

With methylene blue solution I, adjusted so 0.05 mL (1 drop)

1.0 mg S 2

 /L when 7.5 mL of sample are used:
mg S 2

 /L no. drops solution I  0.1 (no. drops solution II) 2) Photometric color measurement—A cell with a light path of 1 cm is suitable for measuring sulfide concentrations from 0.1 to 2.0 mg/L. Use shorter or longer light paths for higher or lowerconcentrations. This method is suitable for sample concentrations up to 20 mg/L. Zero instrument with a portion of treated sample from Tube B. Prepare calibration curves on basis of colorimetric tests made on Na2 S solutions simultaneously ana

lyzed by the iodometric method, plotting concentration vs. absorbance. A linear relationship between concentration and absorbance can be assumed from 0 to 1.0 mg/L.

Read sulfide concentration from calibration curve.
4. Precision and Bias

In a study by two chemists working in the same laboratory, the standard deviation estimated from 34 sets of duplicate sulfide measurements was 0.04 mg/L for concentrations between 0.2 and 1.5 mg/L. The average recoveries of known additions were 92% for 40 samples containing 0.5 to 1.5 mg/L and 89% for samples containing less than 0.1 mg/L.
5. Quality Control

The quality control practices considered to be an integral part of each method are summarized in Table 4020:I.
6. Bibliography
P OMEROY , R.D. 1936. The determination of sulfides in sewage. Sewage Works J. 8:572. NUSBAUM , I. 1965. Determining sulfides in water and waste water. Water Sewage Works 112:113.



## 4500-S 2

 

## E. Gas Dialysis, Automated Methylene Blue Method

1. Apparatus

Automated analytical equipment: An example of the continuous-flow analytical instrument consists of the interchangeable components shown in Figure 4500-S 2

 :2.

The sampler is equipped with a mixer to stir samples before analysis and the gas dialysis membrane, which is maintained at room temperature, separates H 2 S from the sample matrix.

2. Reagents

a. N,N- dimethyl- p -phenylenediamine stock solution: Dissolve 1 g N,N -dimethyl- p -phenylenediamine dihydrochloride in 500 mL 6N HCl. Prepare fresh monthly. Store in an amber bottle. b. N,N- dimethyl- p -phenylenediamine working solution: Dilute 190 mL N,N -dimethyl- p -phenylenediamine stock solution to 1 L. Store in an amber bottle. Prepare weekly.

c. Ferric chloride stock solution: Dissolve 13.5 g FeCl 3  6H 2 O in 500 mL 5N HCl. Store in an amber bottle. Prepare fresh monthly.

d. Working ferric chloride solution: Dilute 190 mL ferric chloride stock solution to 1 L. Store in an amber bottle. Prepare fresh weekly.

e. Hydrochloric acid, HCl, 6N: f. Sodium hydroxide stock solution, NaOH, 1 N

. g. Sodium hydroxide, NaOH, 0.01 N: Dilute 10 mL NaOH stock solution to 1 L. SULFIDE (4500-S 2

 )/Gas Dialysis, Automated Methylene Blue Method

5h. Sulfide stock solution, 1.00 mg S 2

 /1.00 mL: See 4500- S 2

 .A.6.

i. Sulfide intermediate standard solution: Dilute 10 mL sulfide stock solution to 1 L with water. Prepare fresh daily. Standardize by iodometric titration method, 4500-S 2

 .F. 1 mL  0.01 mg S 2



.

j. Sulfide tertiary standard solution: Dilute 50 mL sulfide intermediate solution to 500 mL with 0.01 N NaOH. Prepare fresh daily. Use standardization value from ¶ i above to determine exact concentration. 1.00 mL  0.001 mg S 2



.

k. Working sulfide standard solutions: Prepare a suitable series of standards by diluting appropriate volumes of sulfide tertiary standing solutions with 0.01 N NaOH. Prepare fresh daily.

l. Zinc acetate preservative solution: Dissolve 220 g Zn(C 2H 3 O 2 ) 2  2H 2 O in 870 mL water (this make s 1 L solution).

3. Procedure

For unpreserved, freshly collected samples and sulfide working standards, add, in order, 4 drops 2N zinc acetate, 0.5 mL 6NNaOH, and 400 mg ascorbic acid/100 mL. For preserved samples, add 0.5 mL 6N NaOH and 400 mg ascorbic acid/100 mL. Shake well. Let precipitate settle for at least 30 min. Pour a portion of well-mixed sample or working standard into a sample cup. Set up manifold as shown in Figure 4500-S 2

 :2 and follow the general procedure described by the manufacturer. Determine absorbance at 660 nm.

4. Calculation

Prepare standard curves by plotting peak heights of standards processed through the manifold against S 2

 concentration in the standards. Compute S 2

 sample concentration by comparing sample response with standard curve.

5. Precision and Bias

In a single laboratory, samples with S 2

 concentrations of 0.012, 0.015, 0.034, and 0.085 mg/L had standard deviations of0.001, 0.001, 0.001, and 0.001 mg/L, respectively, with coefficients of variation of 8.3%, 6.3%, 2.9%, and 1.2%, respectively. In two environmental samples with added S 2

 , recoveries were 104.2 and 97.6%. Figure 4500-S 2

 :2. Sulfide manifold. SULFIDE (4500-S 2

 )/Gas Dialysis, Automated Methylene Blue Method

66. Quality Control The quality control practices considered to be an integral part of each method are summarized in Table 4020:I. 7. Bibliography FRANCOM , D., L.R. G OODWIN & F.P. D IEKEN . 1989. Determination of low level sulfides in environmental waters by automated gas dialysis/ methylene blue colorimetry. Anal. Lett. 22:2587.



## 4500-S 2

 

## F. Iodometric Method

1. Reagents

a. Hydrochloric acid, HCl, 6N

. b. Standard iodine solution, 0.0250N: Dissolve 20 to 25 g KI in a little water and add 3.2 g iodine. After iodine has dissolved, dilute to 1000 mL and standardize against 0.0250N Na2 S 2 O 3 , using starch solution as indicator.

c. Standard sodium thiosulfate solution, 0.0250N: See Section 4500-O.C.2 e
.

d. Starch solution: See Section 4500-O.C.2d

. 2. Procedure

a. Measure from a buret into a 500-mL flask an amount of iodine solution estimated to be an excess over the amount of sulfide present. Add distilled water, if necessary, to bring volume to about 20 mL. Add 2 mL 6N HCl. Pipet 200 mL sample into flask, discharging sample under solution surface. If iodine color disappears, add more iodine until color remains. Back-titrate with Na2 S 2 O 3 solution, adding a few drops of starch solution asendpoint is approached, and continuing until blue color disappears.

b. If sulfide was precipitated with zinc and ZnS filtered out, return filter with precipitate to original bottle and add about 100 mL water. Add iodine solution and HCl and titrate as in ¶ a above. 3. Calculation
One milliliter 0.0250N iodine solution reacts with 0.4 mg S 2

 :

mg S 2

 /L [( A B )

 ( C D )]  16 000 mL sample

where: A mL iodine solution, B normality of iodine solution, C mL Na2 S 2 O 3 solution, and D normality of Na2 S 2 O 3 solution.

4. Precision

The precision of the endpoint varies with the sample. In cleanwaters it should be determinable within 1 drop, which is equivalent to 0.1 mg/L in a 200-mL sample.

5. Quality Control

The quality control practices considered to be an integral part of each method are summarized in Table 4020:I.



## 4500-S 2

 

## G. Ion-Selective Electrode Method

1. General Discussion

a. Principle: The potential of a silver/sulfide ion-selective electrode (ISE) is related to the sulfide ion activity. An alkaline antioxidant reagent (AAR) is added to samples and standards to inhibit oxidation of sulfide by oxygen and to provide a constant ionic strength and pH. Use of the AAR allows calibration in terms of total dissolved sulfide concentration. All samples andstandards must be at the same temperature. Sulfide concentrations between 0.032 mg/L (1  10  6M ) and 100 mg/L can be measured without preconcentration. For lower concentrations, preconcentration is necessary.

b. Interferences: Humic substances may interfere with Ag/SISE measurements. For highly colored water (high concentration of humic substances), use the method of standard additions to check results. Sulfide is oxidized by dissolved oxygen. Sulfide oxidation may cause potential readings to drift in the direction of decreasing concentration, i.e., to more positive values. Flush surface of samples and standards with nitrogen to minimize contact with atmospheric oxygen for low-level measurements. Temperature changes may cause potentials to drift either upward or downward. Therefore, let standards and samples come to the same temperature. If samples cannot be analyzed immediately, preserve dissolved sulfide by precipitating with zinc acetate (4500-S 2

 .C).

c. Quality control (QC): The QC practices considered to be an integral part of each method are summarized in Table 4020:I.
2. Apparatus

a. Silver/sulfide electrode. * b. Double-junction reference electrode.

* Orion 941600, or equivalent. SULFIDE (4500-S 2

 )/Ion-Selective Electrode Method

7c. Electrode polishing strips. † d. pH meter with millivolt scale, capable of 0.1-mV resolution. Meters that can be calibrated in concentration and that perform standard-additions calculations are available.

e. Electrochemical cell: Make suitable cell from a 150-mL beaker and a sheet of rigid plastic (PVC or acrylic) with holes drilled to allow insertion of the electrodes and a tube for flushingthe headspace with nitrogen. Alternatively, purchase a polarographic cell with gas transfer tube.‡

f. Gas dispersion tube: Use to deaerate water for preparing reagents and standards.

g. Magnetic stirrer and stirring bar: Use a piece of styrofoam or cardboard to insulate the cell from the magnetic stirrer.
3. Reagents

a. Alkaline antioxidant reagent (AAR): To approximately 600 mL deaerated reagent water (DRW) in a 1-L volumetric flask, add 80 g NaOH, 35 g ascorbic acid, and 67 g Na2H 2EDTA. Swirl to dissolve and dilute to 1 L. The color of freshly prepared AAR will range from colorless to yellow. Store in a tightly capped brown glass bottle. Discard when solution becomes brown.

b. Lead perchlorate, 0.1 M: Dissolve 4.60 g Pb(ClO 4 ) 2  3H 2 O in 100 mL reagent water. Standardize by titrating with Na2H 2EDTA. Alternatively, use commercially available 0.1 M Pb(ClO 4 ) 2 solutions.

c. Sulfide stock solution, 130 mg/L: See 4500-S 2

 .A.6, and dilute 13.0 mL of 1.00 mg S 2

 /mL stock to 100.0 mL with AAR. Alternatively, add 500 mL AAR and 1 g Na2 S  9H 2 O to a 1-L volumetric flask; dissolve. Dilute to 1 L with DRW. Use deaerated artificial seawater (DASW), Table 8010:III, or 0.7M NaCl ifsulfide concentrations are to be determined in seawater. Standardize stock solution by titrating with 0.1 M Pb(ClO 4 ) 2 . Pipet 50 mL sulfide stock solution into the electrochemical cell. (Use 10 mL with a small-volume polarographic cell.) Insert Ag/S electrode and reference electrode and read initial potential. Titrate with 0.1 M Pb(ClO 4 ) 2 . Let electrode potential stabilize and record potential after each addition. Locate equivalence point as in Section 4500-Cl  .D.4a . Alternatively, linearize the titration curve. 1 Calculate the function F 1 for points before the equivalence point.

F 1 ( V o  V)10 E

m where:
V

o volume of stock solution, mL, V titrant volume, mL, E potential, mV, and
m slope of calibration curve, mV/log unit. Plot F 1 as a function of titrant volume. Extrapolate to find the intersection with the x-axis; that is, the equivalence point. Calculate sulfide concentration in the stock solution from:

C V eq [Pb ]

V

o where:
C sulfide concentration, mg/L, V

eq equivalence volume, mL, [Pb ] concentration of Pb in titrant, mg/L, and

V

o volume of stock solution, mL. Store stock solution in a tightly capped bottle for 1 week or less. The stock solution also can be standardized iodometrically (see 4500-S 2

 .E). C AUTION : Store in a fume hood.

d. Sulfide standards: Prepare sulfide standards daily by serial dilution of stock. Add AAR and Zn(C 2H 3 O 2 ) 2 solutions to 100-mL volumetric flasks. Add sulfide solutions and dilute to volume with DRW (or DASW). Refer to Table 4500-S 2

 :I for volumes. Prepare at least one standard with a concentration less than the lowest sample concentration.
4. Procedure

Check electrode performance and calibrate daily. Check electrode potential in a sulfide standard every 2 h. The procedure depends on the sulfide concentration and the time between samplecollection and sulfide determination. If the total sulfide concentration is greater than 0.03 mg/L (1  10  6 M

) and the time delay is only a few minutes, sulfide can be determined directly. Otherwise, precipitate ZnS and filter as described in 4500-S 2

 .C.

a. Check electrode performance: Pipet 50 mL AAR, 50 mL DWR, and 1 mL sulfide stock solution into the measurement cell. Place Ag/S and reference electrodes in the solution and read potential. Add 10 mL stock solution and read potential. The change in potential should be

 28  2 mV. If it is not, follow the troubleshooting procedure in the electrode manual.

b. Calibration: Place electrodes in the most dilute standard but use calibration standards that bracket the sulfide concentrations in the samples. Record potential when the rate of change is less than 0.3 mV/min. (This may take up to 30 min for very lowsulfide concentrations, i.e., less than 0.03 mg/L.) Rinse electrodes, blot dry with a tissue, and read potential of the next highest standard. For a meter that can be calibrated directly in concentration, follow manufacturer’s directions. For other meters, plot potential as a function of the logarithm (base 10) of the sulfide concentration. For potentials in the linear range, calculate the slope and intercept of the linear portion of the calibration plot.

c. Sulfide determination by comparison with calibration curve, no ZnS precipitation: Add 40 mL AAR, 0.15 mL (3 drops) zinc acetate, and 50 mL sample to a 100-mL volumetric flask. Dilute to 100 mL with AAR. Pour into the electrochemical cell and † Orion 948201, or equivalent. ‡ EG&G Princeton Applied Research K0066, K0060, G0028, or equivalent. TABLE 4500-S 2

 :I. D ILUTION OF S ULFIDE S TOCK S OLUTION FOR PREPARATION OF S TANDARDS (100 ML T OTAL V OLUME)

Dilution Alkaline Antioxidant

Reagent

mL Sulfide Solution Sulfide Solution

mL 1 M Zinc Acetate

mL 1:10 45 Stock 10 0.15 1:100 50 Stock 1 0.15 1:1 000 45 1:100 10 0.14 1:10 000 50 1:100 1 0.15 SULFIDE (4500-S 2

 )/Ion-Selective Electrode Method

8insert the electrodes. Record potential when the rate of change is less than 0.3 mV/min. Read sulfide concentration from the calibration curve. Alternatively, for potentials in the linear range, calculate the sulfide concentration from:

S Tot 10E

 b
m

where:
E electrode potential and b and m are the intercept and slope of the calibration curve. For a meter that can be calibrated directly in concentration, follow the manufacturer’s directions.

d. Sulfide determination by comparison with calibration curve, with ZnS precipitation: Place filter with ZnS precipitate in a 150-mL beaker containing a stir bar. Wash sample bottle with 50 mL AAR and 20 mL DRW and pour the washings into the beaker. Stir to dissolve precipitate. Remove filter with forceps while rinsing it into the beaker with a minimum amount of DRW. Quantitatively transfer to a 100-mL volumetric flask and dilute to mark with DRW. Pour into the electrochemical cell and place the electrodes in the solution. Measure potential as in ¶ c above. Calculate sulfide concentration (¶ c above).

e. Sulfide determination by standard addition with or without ZnS precipitation: Measure the Ag/S-ISE electrode potential as in ¶ c or d above. Add sulfide stock solution and measure potential again. Calculate sulfide concentration as follows:

C o fC s (1  f )10E s  E o m

 1

where: C

o and C s sulfide concentrations in sample and known addition, E

o and E s potentials measured for sample and known addition, m slope of calibration curve (approximately 28 mV/log S 2

 , and f ratio of known-addition volume to sample volume. f. Sulfide determination by titration: Use the same procedure as for standardizing the sulfide stock solution (4500-S 2

 .G.3 c ).The minumum sulfide concentration for determination by titration is 0.3 mg/L (10  5M ).

5. Precision

For sulfide determination by comparison with the calibrationcurve, the relative standard deviation varies with the sulfide concentration. RSD values of 23% for 0.0091 mg/L and 5% for 0.182 mg/L have been reported. 2 (0.0091  g/L was below the range for which the potential varied linearly with the logarithm of thesulfide concentration, i.e., the Nernstian range.) For sulfide determination by standard addition, the precision is greatest if the amount of sulfide added is as large as possible while staying within the linear range. 3

6. References

1. GRAN , G. 1952. Determination of the equivalence point in potentiometric titrations. Part II. Analyst 77:661. 2. B AUMANN , E. 1974. Determination of parts per billion sulfide in water with the sulfide-selective electrode. Anal. Chem. 46:1345. 3. RATZLAFF , K.L. 1979. Optimizing precision in standard addition measurement. Anal. Chem. 51:232.
7. Bibliography

O RION RESEARCH , INC . 1980. Instruction Manual for Silver-Sulfide Electrode. V IVIT , D.V., J.W. B ALL & E.A. JENNE . 1984. Specific-ion electrode determinations of sulfide preconcentrated from San Francisco Bay waters. Environ. Geol. Water Sci. 6:79.



## 4500-S 2

 

## H. Calculation of Un-ionized Hydrogen Sulfide
Hydrogen sulfide (H 2 S) and bisulfide ion (HS  ), which to

gether constitute dissolved sulfide, are in equilibrium with hydrogen ions:

H 2 S o H   HS 

The fraction of sulfide present as un-ionized H 2 S can be estimated with an error of less than 40% from Figure 4500- S 2

 :3. If more accuracy is needed, use the methods given below. For both fresh water and seawater, it is convenient to define “conditional” dissociation constants, which are valid for the temperature and ionic strength of the water of interest. In the following mass-action equation for fresh water, K FW is a mixedequilibrium constant that relates the hydrogen ion activity (calculated from the pH) and the concentrations of H 2 S and HS  : K  FW [H  ]  HS   H 2 S The square brackets indicate concentrations and the braces indicate activity. The value of pK FW for H 2 S is approximately 7.0  0.3 for the ionic strengths and temperatures likely to be encountered in water-quality monitoring. For seawater, it is convenient to use a stoichiometric equilibrium constant (KSW ), which relates the concentrations of H  , HS  , and H 2 S:

K  SW  H   HS   H 2 S The mass-action equations can be rearranged to give: SULFIDE (4500-S 2

 )/Calculation of Un-ionized Hydrogen Sulfide

9pH

 pK   log  HS   H 2 S In this equation, pK  can be either pK  FW or pK  SW. The fraction of un-ionized H 2 S can either be read from Figure 4500-S 2

 :3 or calculated with the following equation:

H 2S   H 2 S S T  1 10

pH

 pK   1

where:

S T total dissolved sulfide concentration.
1. Calculation for Fresh Water ( I  0.01 M ) Calculate ionic strength I as in Table 2330:I. Read value of pK FW from Table 4500-S 2

 :II. Sample calculation: Total sulfide concentration, 1.5 mg S 2

 /L; pH, 6.87; temperature, 10°C; ionic strength, 0.04. From Table 4500-S 2

 :II, pK FW 7.11.
pH

 pK  FW   0.24
10pH

 pK  FW  10  0.24  0.575

H 2S  1 1  0.575  0.63

0.63 1.5  0.95
The concentration of un-ionized H 2 S is 0.95 mg S 2

 /L.
2. Calculation for Seawater, Estuarine Water, and Brackish Water

This procedure is the same as that for fresh water. A potentialsource of error is the determination of the hydrogen ion concenFigure 4500-S 2

 :3. Proportion of H 2 S and HS  in dissolved sulfide. SULFIDE (4500-S 2

 )/Calculation of Un-ionized Hydrogen Sulfide

10tration. If the pH electrode is calibrated using NIST buffers as in Section 4500-H  , then a correction factor2 must be determined. Add acid (HNO 3 , HCl, or HClO 4 , not H 2 SO 4 ) to artificial sea

water diluted to the salinity of interest and at the temperature of interest to give an acid concentration of 0.001 N. (Prepare artificial seawater as in Table 8010:III, substituting NaCl for Na2 SO 4 on an equimolar basis and omitting NaF, SrCl 2  6H 2 O, H 3 BO 3 , KBr, Na2 SiO 3  9H 2 O, Na4EDTA, and NaHCO 3 .) Measure the pH. The difference between the negative logarithm of the known acid concentration and the measured pH is the correction factor. For example, if the acid concentration is 0.001 N and the measured pH is 3.15, the correction factor is 0.15. Subtract 0.15 from measured pH values to get p cH, the negative logarithm of the hydrogen ion concentration. (The pH in fresh water corresponds to the negative logarithm of the hydrogen ion activity. ) Alternatively, calibrate the pH electrode with Tris* buffer in artificial seawater diluted to the salinity of interest and at the temperature of interest. 3 Read pK SW from Table 4500-S 2

 :III. Calculate the fraction of un-ionized H 2 S as for fresh water.

Sample calculation: Total sulfide concentration, 1.5 mg S 2

 /L; temperature 10°C; pH, 7.15; salinity 25‰. From Table 4500- S 2

 : III, pK SW 6.87.
pH

 pK  SW  0.28
10pH

 pK  SW  10 0.28  1.91 H 2S  1 1  1.91  0.34

0.34 1.5  0.51

The concentration of un-ionized H 2 S is 0.51 mg S 2

 /L.

3. References
1. M ILLERO , F. J. 1986. The thermodynamics and kinetics of the hydrogen sulfide system in natural waters. Mar. Chem. 18:121. 2. S IGEL , H., A. D. Z UBERBUHLER & O. Y AMAUCHI . 1991. Commentson potentiometric pH titrations and the relationship between pHmeter reading and hydrogen ion concentration. Anal. Chim. Acta. 255:63. 3. M ILLERO , F. J. 1986. The pH of estuarine waters. Limnol. Oceanogr. 31:839.
4. Bibliography
D’ YACHKOVA , I.B. & I.L. KHODAKOVSKIY . 1968. Thermodynamic equilibria in the systems S-H 2 O, Se-H 2 O and Te-H 2 O in the 25–300°C temperature range and their geochemical interpretations. Geochem. Internat. 1108. ARCHER , D.G. & P. WANG . 1990. The dielectric constant of water and Debye-Huckel limiting law slopes. J. Phys. Chem. Ref. Data 12:817. S TUMM , W. & J.J. M ORGAN . 1991. Aquatic Chemistry, 3rd ed. John Wiley & Sons, New York, N.Y. * Trishydroxymethylaminomethane. TABLE 4500-S 2

 :III. C ONDITIONAL FIRST D ISSOCIATION C ONSTANT OF H YDROGEN S ULFIDE , S EAWATER * Temperature °C pK’ SW at Given Salinity 5‰ 10‰ 15‰ 20‰ 25‰ 30‰ 35‰ 0 7.17 7.12 7.09 7.07 7.06 7.06 7.06 5 7.08 7.02 6.99 6.97 6.96 6.96 6.96 10 6.99 6.93 6.90 6.88 6.87 6.86 6.86 15 6.91 6.85 6.82 6.80 6.78 6.78 6.77 20 6.83 6.77 6.74 6.72 6.70 6.69 6.69 25 6.76 6.70 6.66 6.64 6.63 6.62 6.61 30 6.70 6.63 6.60 6.57 6.56 6.55 6.54 * Values calculated according to Millero 1

. TABLE 4500-S 2

 :II. C ONDITIONAL FIRST D ISSOCIATION C ONSTANT OF H YDROGEN S ULFIDE , FRESH WATER *

Temperature °C pK FW at Given Ionic Strength 0.00 mol/L 0.005 mol/L 0.01 mol/L 0.02 mol/L 0.03 mol/L 0.05 mol/L 0.10 mol/L 0 7.36 7.33 7.32 7.30 7.29 7.27 7.24 5 7.28 7.25 7.23 7.22 7.21 7.19 7.16 10 7.20 7.16 7.15 7.13 7.12 7.10 7.07 15 7.12 7.09 7.08 7.06 7.05 7.03 7.00 20 7.05 7.02 7.00 6.99 6.97 6.96 6.92 25 6.98 6.95 6.94 6.92 6.91 6.89 6.86 30 6.92 6.89 6.87 6.86 6.84 6.83 6.79 * Values calculated according to Millero 1

. SULFIDE (4500-S 2

 )/Calculation of Un-ionized Hydrogen Sulfide

11

## 4500-S 2

 

## I. Distillation, Methylene Blue Flow Injection Analysis Method

1. General Discussion

a. Principle: Water and wastewater samples are distilled intoa sodium hydroxide trapping solution and the distillate is analyzed. Hydrogen sulfide (H 2 S) reacts in acid media and in the presence of ferric chloride with two molecules of N , N-dimethylp -phenylenediamine to form methylene blue. The resulting color is read at 660 nm.

b. Sample preservation: Because H 2 S oxidizes rapidly, ana

lyze samples and standards without delay. To preserve samples, add 4 drops 2M zinc acetate to 100 mL sample and adjust pH to

9 with 6M NaOH, then cool to 4°C. Samples are distilled into a trapping solution resulting in 0.25M NaOH matrix.

Also see 4500-S 2

 .A, B, and E, and Section 4130, Flow Injection Analysis (FIA).

c. Interferences: This method measures total sulfide, which is defined as the acid-soluble sulfide fraction of a sample. Total sulfide includes both acid-soluble sulfides, such as H 2 S, and acid-soluble metal sulfides present in suspended matter. This method does not measure acid-insoluble sulfides, such as CuS.

Most nonvolatile interferences are eliminated by distillation. Strong reducing agents inhibit color formation at concentrations of several hundred milligrams per liter. Iodide interferes at concentrations greater than 2 mg I/L.

Also see 4500-S 2

 .A and B. d. Quality control (QC): The QC practices considered to be an integral part of each method are summarized in Table 4020:I.

2. Apparatus

a. Distillation apparatus consisting of a glass or polypropylene micro-distillation device* capable of distilling 6 mL or more of sample into a 0.25M NaOH final concentration trapping solution.

b. Flow injection analysis equipment consisting of: 1) FIA injection valve with sample loop or equivalent. 2) Multichannel proportioning pump. 3) FIA manifold (Figure 4500-S 2

 :4) with cation exchange column and flow cell. Relative flow rates only are shown in Figure 4500-S 2

 :4. Tubing volumes are given as an example only; they may be scaled down proportionally. Use manifold tubing of an inert material, such as TFE.

4) Absorbance detector, 660 nm, 10-nm bandpass. 5) Injection valve control and data acquisition system.

3. Reagents

Use reagent water ( 10 megohm) to prepare carrier for all solutions. To prevent bubble formation, degas carrier and buffer with helium. Pass He at 140 kPa (20 psi) through a helium degassing tube. Bubble He through 1 L solution for 1 min.

a. Sodium hydroxide diluent, NaOH, 0.25M: In a 2-L volumetric flask, dissolve 20 g NaOH in approximately 1800 mL water. Dilute to mark and mix with a magnetic stirrer until dissolved. Store in a plastic container.

b. Hydrochloric acid, HCl, 3 M: To a tared 1-L container, add 752 g water and then slowly add 295 g conc HCl. Invert to mix.

c. Hydrochloric acid, HCl, 0.20M: To a tared 1-L container, add 983.5 g water. Then add 19.7 g conc HCl. Invert to mix.

d. N,N- dimethyl-p- phenylenediamine: In a 1-L volumetric flask dissolve 1.0 g N,N -dimethyl- p -phenylenediamine dihydrochloride, (CH 3 ) 2NC 6H4NH 2  2HCl, in about 800 mL 3 M HCl (¶ b above). Dilute to mark and invert to mix. If solution appears dark, it is likely that the N,N -dimethyl- p -phenylenediamine dihydrochloride is decomposed; discard, and use fresh reagent.

e. Ferric chloride: In a 500-mL volumetric flask dissolve 6.65 g ferric chloride hexahydrate, FeCl 3  6H 2 O, in about 450 mL 0.20M HCl (¶ c above). Dilute to mark with water and invert to mix.

f. Stock sulfide standard, 100 mg S 2

 /L: In a 1-L volumetric flask dissolve 0.750 g sodium sulfide nonahydrate, Na2 S  9H 2 O, in approximately 900 mL NaOH diluent (¶ a above). Dilute to mark and invert to mix. Standardize as in 4500-S 2

 .F; also see 4500-S 2

 .A.6.

g. Standard solutions: Prepare sulfide standards in desired concentration range, using stock standard (¶ f above), and diluting with NaOH diluent (¶ a above).

h. Sulfuric acid distillation releasing solution, H 2 SO 4 , 9M: To a tared 500-mL container, add 150.0 g water, then add slowly while swirling, in increments of 40 g, 276 g conc H 2 SO 4 . C AUTION : Solution will become very hot. Allow to cool before using.

4. Procedure

a. Distillation: This procedure is designed for the determination of sulfides in aqueous solutions, solid waste materials, oreffluents. To preserve and remove sulfide from interfering substances, distill samples immediately after collection.

Follow manufacturer’s instructions for use of distillation apparatus. Add sufficient 9M H 2 SO 4 (4500-S 2

 .I.3 h ) to sample to dissolve ZnS (s), digest total sulfides, and release the sulfide as hydrogen sulfide gas. Immediately place sample on-line with the receiving vessel or collector tube and distill hydrogen sulfide and water in the sample into a 0.25M trapping solution.

* Lachat Instruments MICRO DIST, or equivalent. Figure 4500-S 2

 :4. FIA sulfide manifold. SULFIDE (4500-S 2

 )/Distillation, Methylene Blue FIA Method

12b. Flow injection analysis: Set up a manifold equivalent to that in Figure 4500-S 2

 :4 and follow method supplied by the man

ufacturer or laboratory standard operating procedure. Follow quality control protocols outlined in 4020.

5. Calculations

Prepare standard curves by plotting absorbance of standards processed through the manifold versus sulfide concentration.

6. Precision and Bias (MDL)

a. Method detection level (MDL): A 200-  L sample loop was used in the method described above. Using a published method 1
, analysts ran 21 replicates of 0.02-mg S 2

 /L standard. These gave a mean of 0.024 mg S 2

 /L, a standard deviation of 0.0021 mg S 2

 /L, and MDL of 0.006 mg S 2

 /L.

b. Precision: Ten injections of a distilled 0.8 mg S 2

 /L stan

dard gave a mean of 0.82 mg S 2

 /L, a standard deviation of 0.0054 mg S 2

 /L, and percent relative standard deviation of 0.66.

7. Reference

1. U.S. ENVIRONMENTAL PROTECTION A GENCY . 1984. Definition and procedure for the determination of method detection limits. Appendix B to 40 CFR 136 Rev. 1.11 Amended June 30, 1986. 49 CFR 43430.



## 4500-S 2

 

## J. Acid-Volatile Sulfide

1. Apparatus

See Figure 4500-S 2

 :5. a. Reaction vessel , 250-mL, 3-neck flask, standard taper. b. Gas traps , 125-mL gas scrubbers. c. Dropping funnel , standard taper to fit reaction vessel. d. Purge-gas tube , standard taper to fit reaction vessel. e. Tubing , TFE or polypropylene, 0.635 cm (0.25-in.) OD, to connect reaction vessel to gas traps.

f. Syringe , plastic, 5 mL, lower end cut off to inject sediment into reaction vessel.

g. Compressed gas , cylinder of high-purity nitrogen, regulator, needle valve, rotameter or flow meter (optional).
2. Reagents

a. Hydrochloric acid solution, 9N: Add 186 mL conc HCl to about 50 mL distilled or deionized water. Dilute to 250 mL.

b. Stannous chloride solution, 0.53 M : Dissolve 50 g SnCl 2 in 250 mL 9N HCl solution.

c. Sodium hydroxide solution , NaOH, 2N. d. Zinc acetate solution, 1 M, 22%: Dissolve 220 g Zn(CH 3 COO) 2  2H 2 O in 1 L distilled or deionized water.

e. Alkaline zinc solution: Add 150 mL 22% zinc acetate solution to 850 mL 2N NaOH solution. f. Reagents for the iodometric titration method: See Section 4500-S 2

 .F.

3. Procedure

Add 15 mL stannous chloride/hydrochloric acid solution to digestion vessel and 100 mL alkaline zinc solution to each trap. Adjust nitrogen flow rate to approximately 40 to 60 mL/min and flush system for 10 min. Add approximately 6 g fresh wet sediment to digestion vessel using a 5-mL syringe with its distal ends removed. Close outlets with clamps and gently stir the suspension. Allow H 2 S generation to take place at room temper

ature (25  3°C) for 3 h while stirring and with the nitrogen flowing. Combine the solutions from the two traps and titrate as described in Section 4500-S 2

 .F.

4. Precision and Bias

For marine sediment samples (n 6), an average of 97% of the added ferrous monosulfide was recovered by the diffusion method. Reproducibility of this measurement performed on four samples with concentrations of around 40 to 150  mol/g dry weight (1.3 to 4.8 mg S/g) was better than 5%.

5. Quality Control

The quality control practices considered to be an integral part of each method are summarized in Table 4020:I.

6. Bibliography

C HANTON , J.P. & C.S. M ARTENS . 1985. The effects of heat and stannous chloride on the active distillation of acid volatile sulfide from pyrite-rich marine sediment samples. Biogeochemistry 1:375. M ORSE , J.W. & J.C. C ORNWELL . 1987. Analysis and distribution of iron sulfide minerals in recent anoxic marine sediments. Mar. Chem. 22:55. Figure 4500-S 2

 :5. Apparatus for acid-volatile sulfide analysis. SULFIDE (4500-S 2

 )/Acid-Volatile Sulfide

13H SIEH , Y.P. & C.H. YANG . 1989. Diffusion methods for the determination of reduced inorganic sulfur species in sediments. Limnol. Oceanogr. 34:1126. ALLEN , H.E., G.-M. FU & B.-L. D ENG . 1993. Analysis of acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) for the estimation of potential toxicity in aquatic sediments. Environ. Toxicol. Chem. 12:1441. LASORSA , B. & A. C ASAS . 1996. A comparison of sample handling and analytical methods for determination of acid volatile sulfides in sediment. Mar. Chem. 52:211. LEONARD , E.N., A.M. C OTTER & G.T. ANKLEY . 1996. Modified diffusion method for analysis of acid volatile sulfides and simultaneously extracted metals in freshwater sediment. Environ. Toxicol. Chem. 15:1479. SULFIDE (4500-S 2

 )/Acid-Volatile Sulfide

14