-4 nalyst, December, 1973, 1701. 98, $$. 851-856 851 Absorptiometric Determination of Trace Amounts of Sulphide Ion in Water BY S. A. RAHIM, A. Y. SALIM* AND MRS. S. SHEREEF (Depavtment of Chemistry, College of Science, M o d University, Iraq) A sensitive and rapid method is described for the determination of minimal concentrations of soluble sulphide in water. It is based on the reduction of iron(II1) to iron(I1) by sulphide a t pH 3 in the presence of 1,lO-phenanthroline reagent, forming theorange - red complex [(C,2H,N2)3Fe]2+. The method is subject to some interferences, which fortunately are not usually present in water supplies. Interferences of many acidic and basic radicals can be successfully overcome by recovery of hydrogen sulphide by steam distillation from the samples acidified with sulphuric acid.Down to 0-5 pg or a concentration of 33 pg 1-1 of sulphide, if 15 ml of sample are available, can be detected by the method. ANAEROBIC decomposition of sulphur-bearing organic materials produces hydrogen sulphide, the presence of which, or its salts, is an indication of poor water quality. The rotten-egg odour of hydrogen sulphide is very offensive. Its odour threshold is as low as 0.0011 mg 1-l.l The toxicity of hydrogen sulphide to aquatic life has been reported to have widely ranging values. It can be toxic to some fresh-water fish in concentrations well below 1 mg 1-1.2 Corrosion of metallic and concrete structures by sulphides is important and costly. Sulphides also corrode a wide variety of metals and alloys rapidly.Oxidation of hydrogen sulphide produces sulphuric acid that will attack concrete. Few methods are available for the determination of small amounts of sulphide in water. The best known are probably the titration method involving the use of iodine and thiosul- hate,^ and the colorimetric method based on the reaction that takes place under suitable conditions between 9-amino-NN-dimethylaniline and sulphide ions in the presence of iron( 111) , which results in the formation of methylene blue.4 The accuracy of the latter method is reported to be &lo per cent. if very careful use of the technique is made.4 A fluorimetric method has been described5 in which the sulphide ion is caused to react with the non- fluorescent palladium complex of 8-hydroxyquinoline-5-sulphonic acid at pH 9.2, thus liberating the strongly fluorescent free ligand.More recently6 an absorptiometric method has been described based on the green colour that is formed when sulphide ions are treated in ammoniacal solution with iron(II1) in the presence of excess of nitrilotriacetic acid, colloidal basic iron(II1) sulphide solution being used as a reference. The recommended procedure can be applied down to 8mg1-1 of sulphide, but the absorbance has to be measured 2 to 12 minutes after carrying out the test. In this paper, we describe a method for the determination of small amounts of sulphide, based on the reaction between sulphide ion and iron(II1) at pH 3 in the presence of 1,lO-phenanthroline. In the course of an investigation of the elimination of sulphides from natural water7 by chemical treatment, it was observed that with the gradual addition of iron(II1) chloride t o sulphide solutions the following series of reactions occurs : ( a ) A dark green coloured solution is first formed when up to one molar proportion of iron(II1) is added to the sulphide at high pH.(b) Addition of a slight excess of iron(II1) results in the immediate formation of a black precipitate. (c) Further addition of iron(II1) causes gradual dissolution of the black precipitate, until its complete dissolution when a total of two molar proportions of iron(II1) have been added with respect to the original sulphide, together with the precipitation of colloidal sulphur. Increasing the molar proportion of iron (111) to more than 2 has no effect.(d) * Present address : Faculty of Engineering, Alexandria University, Alexandria, Egypt. (Q SAC and the authors.852 RAHIM, SrZLIM AKD SHEREEF : ABSORPTIOMETKIC 1)ETERMINATIOS 01; [ A nalyst, \-d. 98 The reactions that take place are very probably as follows: (a) S2- + Fe3+ + OH- = Fe(0H)S (b) 3Fe(OH)S + 4Fe3+ = 6Fe2+ + Fe(OH), + 3s 3Fe(OH)S Fe(oH!, 2FeS + Fe(OH), + S 2Fe3+ + FeS = 3Fe2+ + S 2Fe3+ + S2- = 2Fe2+ + S (c) with the probable over-all equation : The determination of small concentrations of sulphide ion in solution by methods previously described in the literature was inconvenient because these methods lacked sensi- tivity, simplicity or stability. Determination of sulphide from the iron(I1) liberated by the reaction of sulphide with excess of iron( 111) was considered, but of the different methods available,8 the colorimetric method in which 1,lO-phenanthroline is used is probably the most satisfactory.Both iron(I1) and iron(II1) form coloured complexes with 1,lO-phenanthroline ; the reddish orange iron(I1) complex absorbs at 510nm, and both the iron(I1) and the yellow iron(II1) complexes have identical absorbances a t 396 nm, the amount being additive. Thus the reading a t 396 nm gives the total iron, while that at 510 nm gives iron(I1) only, which is equivalent to the original sulphide. EVOLUTION OF THE ANALYTICAL PROCEDURE- When a solution containing about 1 mg of sulphide was mixed with about 10 mg of iron(II1) in the presence of 1,lO-phenanthroline, a very deep orange - red colour (characteristic of ferroin) developed, accompanied by a slight turbidity due to colloidal sulphur.The colour was so intense that the mixture could be diluted to more than 10 litres and the solution was still distinctly coloured. It was therefore possible to decrease the concentration to a very low value, and it was surprising that, when dealing with 0.1 mg of sulphide or less in a total volume of 50 ml, no formation of colloidal sulphur was observed, a clear orange - red solution being obtained, which was stable for long periods. As the colour intensity is proportional to the concentration of iron(II), resulting from the reduction of iron(II1) by sulphide, the above observation suggests that it may be made use of for the determination of small amounts of sulphide. These experiments revealed that if appropriate solutions were mixed to give 1 to 100 pug of sulphide, 200 to 700 pg of iron(II1) and 3 to 10 mg of 1,lO-phenanthroline in a total volume of 25 ml, clear orange-red solutions were obtained that could be read.It was also found that it was best to maintain the pH between 3 and 4 (preferably at 3) because at higher pH values slight turbidity resulting from the presence of iron(II1) hydroxide occurred, and at lower pH values gradual fading of the colour took place. A buffer solution of pH 3 was used to control the pH. A study was then carried out in order to determine the conditions of maximum absorption, e.g., optimum time of colour development, sequence of addition of reactants, time between additions and proportions of reactants.Only a slight variation in absorption was observed when the standing time was extended from 30 minutes to 2 hours, as shown in Table I for the determination of 15 pg of sulphide in a final volume of 25 ml. TABLE I ABSORBANCE IN THE IRON(III) - SULPHIDE - 1 ,~O-PHENANTHROLINE SYSTEM 15 pg of S2- + 0-2 mg of Fe3+ + 3 mg of 1,lO-phenanthroline + 5 ml of acetate buffer in a final volume of 25 ml Standing time . . . . 0 15 30 45 1 2 24 Absorbance (1-cm cell) . . 0.36 0-368 0.370 0-370 0.370 0.372 0-45 Attempts were then made to find the optimal concentrations of reactants. minutes minutes minutes hour hours hours Also, only slight changes in absorbance were observed with different standing times (0 to 30 minutes) between additions of subsequent reagents or between the mixing of the reagents and dilution to the mark.A composite solution containing iron(II1) and acetate buffer wasDecember, 1973; TRACE AMOUKTS O F SULPHIDIS I 0 9 I?; WATER 853 made for simplicity. A composite iron(II1) - buffer - 1,1O-phenanthroline solution was un- suitable because it developed a red colour on standing for long periods. It was found that the best procedure was to mix 1,lO-phenanthroline solution with iron(II1) - buffer reagent, add sulphide, dilute to the mark and read the absorbance after 30 minutes to 2 hours against a reagent blank, Having thus established the best conditions for the determination, a test was made to check the adherence to the Beer-Lambert law. It was found that this law was obeyed (Table 11) in the range 0-5 to 25 pg of sulphide in a final volume of 25 ml.On extrapolation, the calibration graph passed almost through the origin when the readings were made against a reagent blank (with all materials except sulphide). Readings with amounts of sulphide above 25 pg showed slight negative deviation. The molar absorptivity of sulphide ion corresponds to 1-98 x 10' 1 mo1-1 cm-l. TABLE I1 ABSORBAXE OF THE IRON(III) - SULPHIDE - 1, 10-PHENANTHROLINE SYSTEM 7 ml of iron(II1) - buffer reagent containing 0.2 mg of iron(II1) + 3 ml of 0.1 per cent. 1,lO-phenanthroline + 0-5 to 25 pg of S2- diluted to final volume of 25 ml. Readings taken after 1 hour, with a l-cm cell, against reagent blank Sulphide solution/ml 0.1 0.2 0-3 0-6 0.8 1.0 2.0 3-0 5.0 Sulphidelpg . . . . 0.5 1.0 1.5 3.0 4.0 5.0 10 15 25 Absorbance .. . . 0.010 0.022 0.036 0.070 0.10 0-12 0.25 0.37 0.62 In order to facilitate interpretation, a calibration graph of iron(I1) was required. Iron(II1) chloride solutions containing from 10 to 70 pg of iron(II1) were reduced with hydroxyl- ammonium chloride and 1 , 10-phenanthroline was added.s Comparison of the calibration graph for sulphide with that for iron [the molar absorptivity of iron(I1) was founds to be 1.03 x 1041mol-lcm-l] showed that similar absorbances were obtained with 1 part of sulphide and 3.35 parts of iron, corresponding to a molar ratio of Fe3+ to S2- of 1.92: 1, which is slightly lower than that expected according to the equation ~ 2 - + 2~e3f = 2Fe2+ + S In order to test the accuracy and precision of the proposed method, recovery experiments were carried out on sulphide solutions containing 7, 10 and 15pg of sulphide.The results (Table 111) showed that both precision and accuracy were good. TABLE I11 RECOVERY OF SULPHIDE ADDED TO DISTILLED WATER Sulphide Sulphide Standard deviation Coefficient of added1 Number of Mean found r variation, per cent. PF: determinations absorbance (mean)/ pg Absorbance Irg 7 10 16 I5 0.172 6-94 0.0073 0.295 4.2 17 0.248 10.00 0.008 0.32 3.2 11 0.370 14.90 0.013 0.52 3.5 The interfering effect of cations was not examined in detail as most of them are not compatible in solution with the sulphide ion. Of the anions examined (Table IV), however, the following did not interfere in 100-fold excess on a mass basis relative to sulphide: chloride, sulphate, carbonate, hydrogen carbonate, bromide and nitrate.Many oxidising, reducing and complexing agents were found to interfere seriously, e.g. ., thiosulphate, sulphite, nitrite, citrate, tartrate and oxalate. Fortunately, it has been found that in the presence of most interfering agents, except those which react with sulphide in acidic medium, the determination can be carried out easily by acidifying the sample with sulpliuric acid in a micro-Kjeldahl distillation apparatus and steam distilling the liberated hydrogen sulphide, which is directly absorbed in a solution containing iron(II1) - buffer and 1,lO-phenanthroline. Losses of hydrogen sulphide are 15 per cent. or less and can be minimised by taking the usual precautions to prevent oxidation by atmospheric oxygen.Phosphate, cyanide and EDTA had little effect.854 RAHIM, S.4LIM AND SHEREEF: ABSORPTIOMETRIC DETERMINATION OF [,4ndySt, VOl. 98 TABLE IV INTERFERING EFFECT OF SOME ANIONS ON THE ABSORBANCE OF SULPHIDE SOLUTIONS Added anion Chloride Sulphite Phosphate Nitrite Sulphate Hydrogen Carbonate Oxalate Citrate Tartrate Thiosulphate Fluoride Iodide Cyanide EDTA Nitrate Bromide carbonate Absorbance of sulphide solution containing the anion tested in- control l0-fold excess 102-fold excess 103-fold excess 104-fold excess - - - - - Sample Blank Sample Blank Sample Blank Sample Blank Sample Blank - - 0.21 0.04 0-21 0-04 0.21 0.04 - 0.21 0.04 0.24 0.31 0.94 1-08 - - - - 0.21 0.04 0.21 0.04 0.22 0.07 0.23 0.12 0.25 0.18 0.21 0.04 - - 0.04 0.06 0.06 0.06 0-08 0.08 0.21 0.04 - - 0.21 0.04 0.22 0.04 0.22 0.05 f A \ - 0.20 0.04 0.20 0.04 0.20 0.04 0.20 0.04 - - 0.20 0.04 0.19 0.03 0.20 0.04 0.20 0.04 - - 0.20 0.04 0.20 0.18 0.39 0.31 0.45 0.43 - - 0.22 0.04 0.37 0.13 0.41 0.28 0.34 0.31 - - 0.22 0.04 0.21 0.08 0.32 0.19 0.51 0.43 - - 0.22 0.04 0.31 0.10 1.08 1-05 - - - - 0.22 0.04 0.21 0.04 0.21 0.06 0.98 0.04 - - 0.22 0.04 0.24 0.04 0.24 0.05 0.34 0.21 - - 0.22 0.04 0.23 0.04 0.24 0.05 0.30 0.11 - - 0.22 0.04 0.22 0.04 0.20 0.01 0.19 0.01 - - 0.19 0.04 - I 0.18 0.03 0.17 0.03 0.17 0.03 0.19 0.04 - I 0.19 0.04 0.19 0.04 0.19 0-04 The blank contained all the reagents except the substance to be determined (sulphide), and was measured against distilled water.Although it is recommended that the absorbance should be measured against a reagent blank, the absorptions of each sample and its blank were measured separately against distilled water in order to show the effect of each inter- fering ion examined and its corresponding blank (containing no sulphide).However, sub- traction of the absorption of each blank from its corresponding sample absorption (both measured against distilled water) gives the absorption of the sample against the reagent blank. This procedure is preferred to that of driving off the liberated hydrogen sulphide with a slow stream of nitr~gen,~ because (i) the apparatus is simple and compact, (ii) the use of extremely pure nitrogen is avoided, (iii) recovery of hydrogen sulphide takes place in a shorter time, e.g., 20 minutes, and (iv) the hydrogen sulphide recovered is directly absorbed in the test reagent. The presence of excess of zinc ions interferes in the determination as zinc forms a complex with l,lO-phenanthroline.a Consequently pre-treatment with zinc a ~ e t a t e , ~ ~ ~ in order to preserve and purify or concentrate, or both, the sulphide, should be followed by the above steam-distillation procedure. METHOD APPARATUS- Readings werelmade on a Unicam SP500 spectrophotometer. REAGENTS- All reagent solutions should be prepared with recently boiled and cooled distilled water, and protected from atmospheric oxygen as much as possible. Stock sodium sulphide solution-Add 10 per cent.sodium hydroxide solution dropwise to a saturated solution of hydrogen sulphide in water, until a pH of 11 is reached. Determine the exact molarity of the solution i~dimetrically~ (0.1 M).The stock solution can be kept for 2 to 3 days but its concentration should be checked immediately before each dilution. Dilute sodium sulphide solation-Dilute 10 ml of the above solution with water to 200 ml. This solution should be freshly prepared. Standard sodium sulphide solution, 5 mg Z-1(1.56 x M)-Ddute sufficient stock sodium sulphide solution (1 to 2 ml, measured to the nearest 0.005 ml; alternatively, 20 to 40 ml of dilute sodium sulphide solution) with water to 1 litre. This solution should be freshly prepared daily. 1 ml of solution = 5 pg of S2-.December, 19731 TRACE AMOUNTS OF SULPHIDE ION IK WATER 855 Stock iron(I1I) chloride solution, 0.5 M-Dissolve 10 g of anhydrous iron( 111) chloride in water, make the volume up to 100 ml, leave the solution overnight, then filter it.Determine the molarity of the iron solution gravimetrically. Standard iron(II1) chloride solution, 100 mg l-l (1.79 x M)-Dilute sufficient stock iron(II1) chloride solution (3 to 4 ml, measured to the nearest 0.01 ml) with water t o 1 litre. Acetate bufer solution, pH 3-Dissolve 35 g of crystalline sodium acetate in 200 ml of 1,lO-Phenanthroline solution, 0.1 per cent.-Dissolve 0-1 g of the monohydrate in hot Iron(III) - bufer composite reagent-Mix 2 volumes of standard iron(II1) chloride 1 ml of solution = 100 pg of Fe3+. water, then add glacial acetic acid until a pH of 3 is reached. water (70 "C). solution with 5 volumes of acetate buffer solution. PROCEDURE- Pyeparation of calibration graph-Transfer 7-ml portions of iron(II1) - buffer reagent to 25-ml standard flasks.To each flask add 3 ml of 0.1 per cent. 1,lO-phenanthroline solution, then add 0-1 to 10-ml aliyuots of the 5 mg 1-1 sulphide test solution (i.e., 0.5 to 50 pg of S2-) and dilute the contents of the flasks to 25ml. Measure the absorbance of each solution 4 to 2 hours from the time of preparation in l-cm cuvettes a t 510 nm against a reagent blank. Use this procedure for unknown solutions containing 0-03 to 100 mg 1-1 of sulphide by using sample volumes containing 0.5 to 25 pg of sulphide. After cooling the solution, make the volume up to 100 ml. APPLICATION TO SOME KATURAL WATERS The sulphide contents of two natural waters were determined iodimetrically arid by the The results given in Tables V and VI show that a satisfactory recovery of present method.sulphide was obtained in the presence of high concentrations of several interfering ions. TYPICAL TABLE V Results expressed in mg 1-l ANALYSES O F TWO NATURAL WATERS FROM THE VICINITY OF MOSUL Val Lie determined 13 arnmam Al- A1 il p l l . . . . . . . . 6.8 Total solids . . . . . . 570 Total hardness* . . . . 130 MgCO, . . . . . . 29 CaCO, . . . . . . 97 Na,CO, . . . . . . Na,SO, . . . . . . 35.5 __ NaCl . . . . . . . . 330 * Calculated as CaCO, (mg 1-l). .\in Kibrit 5.4 1650 265 160 88 83 1183 - CONCLUSIONS The proposed method for determining sulphide can be applied down to 0.03 mg 1-l of ulphide if 15 ml of sample are available, and up to 100 mg 1-1 in 25-ml test samples. The nsitivity is high and the accuracy is good.The method is specially recommended for the termination of sulphide in natural waters in which almost all interfering materials are not kely to be present. TABLE VI ANALYSES OF THE NATURAL WATERS FOR SULPHIDE CONTENTS Sample volume Sulphide present in sampIe by Sulphide found in sample by \\'ater analysed taken/ml iodimetric method/pg present method/ pg ammam A1-Alil 0.5 1 2 Iiibrit 0.1 0.5 I 2-25 4.5 9 1.6 8.01 16.03 2.15 4.4 9.08 1.2 7.20 16.6856 RAHIM, SALIM AND SHEREEF The exact composition of the red absorbing species is known and is referred to as (C,,H,N,) ,Fez+. Stoicheiometry of the reaction between sulphide and iron( 111) ions indicates that 1 mol of S2- produces 2 mol of Fe2+, probably according to the equation 2Fe3+ + S2- = S + 2Fe2+ Although when present in higher concentrations sulphur separates in colloidal form, at low concentrations no turbidity arising from separation of sulphur is observed. The precision of the method was tested by replicate analyses of a 1-58 x low4 M sulphide solution, which gave a mean coefficient of variation of 3.6 per cent. The sensitivity was 0-0016 pg ml-1. 1 . 2. 3 . 4 . 5. 6. 7 . 8. 9. REFERENCES Klein, L., “River Pollution 11: Causes and Effects,” Butterworths, London, 1962. Doudoroff, P., and Katz, M., Sewage Ind. Wastes, 1950, 22, 1432. Vogel, A., “A Text Book of Quantitative Inorganic Analysis,” Third Edition, Longmans, London, 1966, p. 370. A.P.H.A., A.W.W.A. and F.S.I.W.A., “Standard Methods for the Examination of Water and Waste Water,” Thirteenth Edition, American Public Health Association, New York, 197 1, pp. 555 and 336. Hanker, J. S., Gelberg, A., and Witten, B., Analyt. Ckem., 1958, 30, 93. Rahim, S. A., and West, T. S., Talanta, 1970, 17, 85. Salem, A. Y., and Rahim, S. A., Report to Mosul University, 1971. Sandell, E. B., “Colorimetric Determination of Trace Metals,” Third Edition, lnterscience Pub- Pomeroy, R., Analyt. Chern., 1954, 26, 571. lishers, New York, 1959, p. 520. Received Octobev 30th, 1972 Amended April 27th. 1973 Accepted July 27th, 1973