AutaZyst, August, 1968, Vol. 93, &5. 535-539 535 A Titrimetric Method for Determining Total Sulphur in Mineral Soils BY D. S. JENKINSON (Pedology Department, Rothamsted Experimental Station, Harpenden, Hevts.) A rapid method for the determination of total sulphur in mineral soils is described. The various sulphur compounds in soil are first oxidised to sulphate by potassium dichromate in phosphoric acid, and the sulphate thus formed is then reduced to sulphur dioxide by heating with activated charcoal. The evolved sulphur dioxide is trapped in hydrogen peroxide, barium per- chlorate added to the resulting sulphuric acid to precipitate barium sulphate, and the excess of barium determined by titration against standard potassium sulphate, with sulphonazo I11 as indicator. The proposed method gave acceptable results when tested on pure compounds and on soils of known sulphur content.No interferences are to be expected from normal soil con- stituents. WHEN soils containing charcoal are strongly heated with phosphoric acid, sulphur dioxide is evolved from any sulphates that may be present. A new method, based on this observation, was developed for determining total sulphur in soil that compares favourably, both in speed and simplicity, with other current chemical techniques for this determinati0n.l 12 s3 Inorganic sulphur is often present in soil as sulphate, held by anion exchange, but can be found, particularly in soils from arid regions, as calcium and magnesium sulphates ; it occurs as sulphides in certain soils developed under waterlogged conditions.Some of the organic sulphur in soil is combined in sulphur-containing a-amino-acids, and some as ester sulphate, but the chemical nature of much of the soil organic sulphur is obscure; for a review see Freney.* In all chemical methods for determining total sulphur in soil, the different sulphur compounds are first quantitatively converted into a single form; the sulphur in this is then isolated from the soil residue for determination. Analytical methods for determining total sulphur in soil have been briefly discussed by Bloomfield2 and by Bardsley and Lancaster.1 The proposed method is in three steps : organic and inorganic soil sulphur are oxidised to sulphate by refluxing with potassium dichromate and dehydrated phosphoric acid (oxidation step); the sulphate thus formed is reduced to sulphur dioxide by heating with activated charcoal in phosphoric acid (reduction step); the evolved sulphur dioxide is oxidised to sul- phuric acid by hydrogen peroxide, barium perchlorate added to precipitate barium sulphate, and the excess of barium determined by titration against standard potassium sulphate solution , with sulphonazo I11 as indicator.5 Back-titration is used rather than direct titration because of the erratic colour changes that sometimes occurred at the beginning of direct titrations, caused, most probably, by the absence of barium sulphate precipitation nuclei. Preliminary work showed that it was essential to pre-treat the phosphoric acid by heating to 370" C with activated charcoal if low blank values were to be obtained.During this pre- treatment the orthophosphoric acid is dehydrated, giving a mixture of ortho, pyro and meta- phosphoric acids. The charcoal and the dehydrated acid are then both used as reagents in the determination. Use of the dehydrated acid has an incidental advantage in that excess of dichromate remaining after oxidation of the organic matter during the first step is decomposed at the temperature of reflux (about 305" C). If, for example, 88 per cent. orthophosphoric acid is used in the oxidation step, the reflux temperature is lower (about 170" C), excess of dichromate remains in the reaction mixture and then reacts violently when heated with charcoal during the reduction step. 0 SAC and the author.536 JENKINSON A TITRIMETRIC METHOD FOR [AaaZyst, Vol.93 METHOD APPARATUS- This is shown in Fig. 1. The gas stream carrying sulphur dioxide from the 150-ml flask, A, is sucked through traps, B and C , each containing a sintered-glass thimble (porosity No. 2, diameter 14mm) to stop acid spray, before going through a water condenser and into the absorption trap, D. The absorption trap also contains a sintered-glass thimble of the same size and porosity as those in traps B and C. The sintered-glass thimble in the absorption trap, D, must fit snugly into the dimple in the absorption flask, almost touching the bottom. Construction of the absorption flask is simplified if, as in Fig. 1, a screw-thread adaptor with 1 Sulphur dioxide Scale 3 0 I5 cm I " ' I I I I 7-- Jf - B19 C 4 BI vacuum line needle valve A = 150-ml reduction flask 6 and C = Traps containing a sintered- glass thimble (porosity No. 2, diameter 14 mm) D = Absorption trap S = Stopper Fig.1. Details of the apparatus a PTFE washer (Quickfit and Quartz Ltd.) is used to connect the tube carrying the sinter to the B19 cone. Connections are as short as possible and made with poly(viny1 chloride) tubing. The incoming air is passed through a U-tube containing self-indicating soda limeAugust, 19681 DETERMINING TOTAL SULPHUR I N MINERAL SOILS 537 to remove oxides of sulphur. To facilitate control of the gas flow, a length of capillary tubing (1 cm long, 0-25-mm i.d.) is inserted between the U-tube containing soda lime and flask A. Except for the 50-ml absorption flask (D, Fig. l), all the glassware is available commercially. REAGENTS- Phosphoric acid-Heat 750 ml of 88 per cent.analytical-reagent grade orthophosphoric acid with 15g of activated charcoal (Merck's Pro Analysi grade) in a distillation flask connected to a water condenser. Collect 200ml of the distillate, then allow the suspension to cool to about 150" C, and filter under suction through a No. 3 glass sinter, discarding the first few millilitres of filtrate. Traces of suspended charcoal in the filtered phosphoric acid can be disregarded. Any crystals that appear on standing can be re-dissolved by keeping the acid overnight in an oven at 80" C. A burette, 3 cm in diameter with a tap and "jet" of 6-mm id., is convenient for dispensing the viscous acid. Active charcoad-Wash the charcoal remaining on the sinter with three 250-ml portions of boiling distilled water, making sure that the charcoal is well dispersed in each wash, and dry overnight at 105°C.Barium perclzlorate-A 0.03 N solution of barium perchlorate trihydrate (general-purpose reagent grade) in water. Standard sulphate solution-Dissolve 0.271 7 g of analytical-reagent grade potassium sulphate, dried overnight at 105" C, in water and make up to 1 litre; 1 ml of this solution contains 0.05 mg of sulphur. Iutdicator-A 0.1 per cent. solution of sulphonazo I11 in water. Potassium dichromate-Analytical-reagent grade. Hydrogen peroxide, 100 voZ-Microanalytical-reagent grade. A cetone-Analytical-reagent grade. PROCEDURE- Weigh 1 g (less if the sulphur content exceeds 500 p.p.m.) of air-dried soil (ground to pass a 30-mesh sieve or finer) into the dry reaction flask.Moisten with 1 ml of water, add 2 g of potassium dichromate and add 25 ml of phosphoric acid. Connect the flask to a water- cooled reflux condenser, bring the solution to the boil, swirl the flask, and heat it under steady reflux for 10 minutes. Allow to cool, add 0.5 g of activated charcoal and 4 ml of water and connect the flask to the apparatus as shown in Fig. 1. Place 0.2 ml of hydrogen peroxide in the absorption trap, D. Adjust the stream of air passing through the apparatus to give a steady rapid flow of bubbles in the absorption trap. Heat the reduction flask on a bunsen burner: if foaming occurs when the mixture begins to boil, momentarily remove the flame. After the mixture has come to the boil, heat for 5 minutes.The rate of heating should be such that steam reaches the top of the water condenser between 2 i and 34 minutes after coming to the boil. Two minutes after the burner is turned off, remove stopper, S, then disconnect the absorption trap and wash the sinter inside and out with 15ml of acetone, adding the washings to the absorption flask. Add 3 drops of indicator and run in 1 ml of barium perchlorate by using a Van Slyke pipette. If the solution remains purple add a further 1 ml of perchlorate. Titrate against the standard potassium sulphate solution, with a 10-ml burette. Carry out a blank determination, omitting the soil, with the same volume of barium perchlorate solution. Sulphur, mg = (ml of potassium sulphate solution required by blank - ml of potassium Between determinations, empty the water collected in traps B and C.If the reduction mixture is allowed to cool in the flask it solidifies: to avoid this pour it out while still liquid and then wash the flask with hot water. RESULTS AND DISCUSSION The colour change is from blue to purple. sulphate solution required by sample) x 0.05. Table I gives the sulphur contents of some organic and inorganic compounds as found by the proposed method, together with the calculated values. For analysis, 2 g of potassium dichromate and 25 ml of phosphoric acid were added to 1 ml of an aqueous solution of the compound containing about 0.4mg of sulphur. The rest of the determination was as described above. In Table I and subsequent tables results are means of triplicate determinations unless otherwise indicated.Results were quantitative, within +2 per cent.538 JENKINSON : A TITRIMETRIC METHOD FOR [Arcalyst, VOl. 93 TABLE I SULPHUR CONTENTS OF SOME ORGANIC AND INORGANIC COMPOUNDS Compound* Sulphamic acid? . . .. .. S-Benzylthiuronium chloride? . . Sulphanilic acid'; . . .. .. Methionine . . .. .. .. Cystine . . .. .. .. Copper(I1) sulphate pentahydratel Potassium sulphate: . . .. .. Sulphur calculated, per cent. 33-0 16.8 18-5 21.6 26.7 12-8 18-4 Sulphur by proposed method, per cent. 32.9 16.7 18.7 21-3 26.7 13-0 18.1 * All dried over anhydrous magnesium perchlorate. t Microanalytical-reagent grade. 1 Analytical-reagent grade. TABLE I1 SULPHUR CONTENTS OF SOME ANALYSED Sulphur nominal, Material* per cent.Basicslag? . . .. . . .. 0.16 Anthracite? . . .. . . . . 2-33 Iron pyrites . . .. .. .. 44.6 Shale . . .. .. . . .. ' 5.07 Portland cement? . . .. .. 1.21 MATERIALS Sulphur by proposed method, per cent. 0.197 2.25 354 (44.0): 1.23 5.24 * All results (except pyrites) on oven-dried (24 hours at 80" C) basis. t Obtained from the Bureau of Analysed Samples Ltd. : Result in parenthesis is after hydrogen peroxide treatment. Table I1 gives the sulphur contents of five analysed materials. The nominal sulphur contents of the basic slag, anthracite and Portland cement were those given by the Bureau of Analysed Samples Ltd. ; those given for pyrites and shale were obtained by the gravimetric procedure described by Vogel.g Acceptable agreement was obtained, except with pyrites. Unchanged pyrites crystals were found in the reaction mixture at the end of the dichromate oxidation stage.The more coarsely ground the pyrites, the less was oxidised. Hydrogen peroxide in the cold will oxidise pyrites.' More sulphur was recovered (see Table 11) from pyrites if, instead of moistening with 1 ml of water before oxidation, 1 ml of 100-vol hydrogen peroxide was added, the mixture left to stand overnight, and the analysis then continued as above. This peroxide treatment is probably only necessary with soils containing much coarsely crystalline pyrites. TABLE I11 SULPHUR CONTENTS OF SOME ROTHAMSTED SOILS soil No.+ 1 2 3 4 6 6 sampling depth. inches 0 to 6 0 to 6 0 to 9 9 to 18 18 to 27 0 to 9 organic carbon, pH percent. '7.8 2.43 8.1 0.97 7.8 2.89 7.9 0.8 1 7.7 0.66 4-6 1.98 Carbonate carbon, per cent.0.14 0.16 0.19 0.04 0 0 Nitrogen, per cent. 0.246 0-107 0.261 0.098 0.076 0.168 clay' per cent. 18 18 24 50 62 20 * All results on oven-dried (24 hours at 80' C) basis. Sulphur by proposed method, p.p.m. 611 200 419 167 108 326 Sulphur by Bloomfield's method, p.p.m. 492 187 41 1 149 116 330 Table I11 gives the sulphur contents of a range of Rothamsted soils as determined by Bloomfield's method2 and by the proposed method. Agreement was close. Table IV gives the sulphur contents of some Australian soils as found by the proposed method and that of Steinbergs, Iismaa, Freney and Barrow ; with these, discrepancies between the two methods were slightly greater than with the Rothamsted soils, but neither method gave values consistently higher than those by the other.August, 19681 DETERMINING TOTAL SULPHUR I N MINERAL SOILS 539 INTERFERENCES- The addition of 1000 p.p.m.of Fe 3+ , Pb2+, AP+, CUB+, Co2+, Mn2+, Sn2+, Sr 2+, Zn2+,Mg2+, Ba2+, NH,+, K+, Na+, Se0,2-, F-, C1-, Br-, B,0,2-, NO,- or NO2- tosoil 2 (Table 111) did not significantly alter the value found by the proposed method for the sulphur content of this soil (200 p.p.m.). Of the cations and anions tested, iodide alone interfered; in the presence of 1000 p.p.m. of iodide (as potassium iodide) the sulphur content of soil 2 was found to be 171 p.p.m. PRECISION- minations. method is precise enough for most soil work. Iodine concentrations as large as this are most unlikely in soils. Table IV gives the sulphur contents of six soils; each value is the mean of four deter- The standard errors of these means range from +2 to +_9 p.p.m., so that the TABLE IV SULPHUR CONTENTS OF SOME AUSTRALIAN SOILS soil No.* 1 2 3 4 6 6 Sulphur by Sampling Organic proposed depth, carbon, Nitrogen, method, Type inches pH per cent.per cent. p.p.m. Red - brown earth 0 to 4 6.9 0.66 0.043 68 f 3.2t Red-brown earth 0 to 4 6.8 2.23 0.136 168 f- 3.1 Red - brown earth 0 to 4 6.9 2-70 0.162 209 f 2.1 Podsol 0 to 2 5.6 3-89 0-306 326 f 1.9 Alpine humus Oto 4 4.9 7.60 0.409 442 f 3.9 Kraznozem 0 to 2 6.1 7.71 0.676 964 f 9.4 * All results on oven-dried (24 hours at 80” C) basis. 1 Standard error of mean (four replicates). CONCLUSIONS Sulphur by S teinbergs Iismaa, Freney and Barrow’s method,* p.p.m.67 167 199 343 473 928 The proposed method is rapid and may be of value when the number of determinations to be carried out does not warrant more complex apparatus. It is convenient to perform the oxidation step in batches of six and to duplicate the apparatus in Fig. 1, so that two reductions can be done at the same time. Six determinations then take 19 hours. The sensitivity is adequate for the range of sulphur contents common in mineral soils. If needed, the sensitivity could probably be increased by abandoning the titrimetric finish, collecting the sulphur dioxide in sodium tetrachloromercurate instead of hydrogen peroxide, and deter- mining the sulphur dioxide colorimetrically by the rosaniline method.* A disadvantage is that the hot phosphoric acid attacks the glass reaction flask, A : flask life is limited to twenty to fifty determinations.BudESinskjr and Krumlov6 have recently shown that dimethyl sulphonazo I11 is a slightly better indicator than sulphonazo I11 for the titration of sulphate with barium perchlorate.$ This reagent could probably be used as an alternative to sulphonazo I11 in the proposed method. I thank Dr. C. H. Williams for supplying the Australian soils, together with the results in Table IV characterising them, and also British Drug Houses Ltd. and Dr. B. BudZ:Sinsk$ for samples of sulphonazo 111. REFERENCES 1. Bardsley, C. E., and Lancaster, J. D., in Black, C . A., Evans, D. D., White, J. L., Ensminger, L. E., and Clark, F. E., Editors, “Methods of Soil Analysis,” American Society of Agronomy Inc., Madison, 1965, Part 2, p. 1102. 2. Bloomfield, C., Analyst, 1962, 87, 686. 3. Steinbergs, A., Iismaa, O., Freney, J. R., and Barrow, N. J., Analytica Chim. Acta, 1962,27, 168. 4. Freney, J. R., in McLaren, A. D., and Peterson, G. H., Editors, “Soil Biochemistry,” Marcel 5. BudEhskq, B., Analyt. Chem., 1966, 37, 1159. 6. Vogel, A. I., “A Textbook of Quantitative Inorganic Analysis, Theory and Practice,’’ Second 7. Gad, M. H., and Le Riche, H. H., Geochim. Cosmochim. Acta, 1966, 30, 841. 8. West, P. W., and Gaeke, G. C., Analyt. Chem., 1966, 28, 1816. 9. BudE!Sinskf, B., and Krumlovb, L., Analytica Chim. Acta, 1967, 39, 376. Dekker Inc., New York. 1967, p. 229. Edition, Longmans, Green & Co., London, New York and Toronto, 1951, p. 406. Received January lst, 1968