Analyst, February, 1968, Vol. 93, pp. 97-100 97 Determination of Sulphate by Automatic Colorimetric Analysis BY MORRIS E. GALES, JUN., WILLIAM H. KAYLOR AND JAMES E. LONGBOTTOM (US. Department of the Interior, Federal Water Pollution Control Administration, Division of Pollution Surveillance, 1014 Broadway, Cincinnati, Ohio 45202, U.S.A .) An automatic colorimetric method for determining sulphate in natural water in the range of 5 to 400 mg per litre is presented. The method is based on the precipitation of barium sulphate and the release of the coloured acid chloranilate ion. A manifold has been developed for the determination of sulphate on the Technicon AutoAnalyzer. The proposed method will run 15 samples per hour. SULPHATE, which is widely distributed in nature, is present in natural water in concentrations ranging from near zero to several thousand milligrams per litre, and may also be discharged in numerous industrial wastes, such as those from tanneries, pulp mills, textile mills and other plants in which sulphuric acid is used.In the past, analytical procedures used for determining sulphate in water and waste discharges were cumbersome and tedious, e.g., the time-consuming standard gravimetric method,l and the titrimetric method, which is not very accurate or precise. This investigation was undertaken to develop a method for determining sulphate in natural water in the range of 5 to 400 mg per litre on the Technicon AutoAnalyzer. Several methods were tried : Technicon's turbidimetric method,2 the methylthymol blue method? and the barium chloranilate method, as described by Bertolocini and Barney4 and Agterdenbos and Martini~s.~ The turbidimetric procedure was abandoned because the barium sulphate coated the inside of the mixing coils and the flow cell, and the thymol blue method proved too sensitive for the range desired.The chloranilate method gave the best results. In the chloranilate procedure, when solid barium chloranilate is added to a solution containing sulphate, barium sulphate is precipitated, releasing the highly coloured acid chloranilate ion. The colour intensity in the resulting chloranilic acid is proportional to the amount of sulphate present. EXPERIMENTAL REAGENTS- and dilute to 1 litre with distilled water. 6.4 ml of acetic acid and dilute to 1 litre with distilled water. hydrated disodium salt of EDTA in distilled water and dilute to 1 litre.(Na,SO,) in distilled water and dilute to 1 litre. APPARATUS- Barium chloranilate-Add 3 g of barium chloranilate (BaC,C1,0,) to 333 ml of ethanol Acetate buffer, p H 4.63-Dissolve 13.6g of sodium acetate in distilled water. Add Sodium hydroxide - EDTA solzction-Dissolve 65 g of sodium hydroxide and 6 g of Ion-exchange resin-Dowex 50W-X8, ionic form, H+, 20 to 50 mesh. Stock sulphate solution, 1000 mg per litre-Dissolve 1.479 g of dried sodium sulphate Standard Technicon AutoAnalyzer equipment and accessories. Heating bath with double delay coil, at 45" C. Double delay coil. Single delay coil. Continuous jilter. 0 SAC and the authors.98 Lm GALES, KAYLOR AND LONGBOTTOM DETERMINATION OF [AIzabSf, VOl.93 ion exchange - I &Barium chlorani late ,065 ,045 - - 4.63 Buffer Continuous filter ,025 1. I00 - EDTA-NaOH Sample Recorder Colorimeter 520mj.1 Sm = Small mixing coil D40 = 40-foot Delay coil Lm = Large mixing coil 080 = 80-foot Delay coil Fig. 1. Manifold for determination of sulphate on the AutoAnalyzer: sampling time, 2-0 minutes; 1 wash tube MANIFOLD PROCEDURE- Alternating samples and wash tubes were pumped at the same rate of 5.0 ml per minute for 2 minutes per tube and passed through an ion-exchange resin to remove cations, because calcium, aluminium and iron will precipitate the chloranilate. The ion-exchange column consisted of 12 inches of $-inch 0.d. sleeving, packed with Dowex 50W-X8. 80 409 Fig. 2. Recording of the sulphate standard, mg per litre )portioning I per litre Fig.3. Precision of the barium chloranilate procedure (raw river water)February, 19681 SULPIIATE BY AUTOMATIC COLORIMETRIC ANALYSIS 99 The barium chloranilate is added to water and ethanol to form a slurry, and the mixture placed on a magnetic stirrer. Ethanol is added to decrease the solubility of barium sulphate. Air is pumped into the reagent line to keep the reagent moving. After emerging from the ion-exchange column, the sample is mixed with the buffer and the solid barium chloranilate and passed into a 45" C heating bath to increase the reaction rate. After a delay time of 15 minutes, by passing the sample through a series of delay and mixing coils, the barium sulphate and the unused barium chioranilate are removed by iiltra- tion. The sample, before passing into the flow cell, is mixed with sodium hydroxide - EDTA reagent to remove any barium sulphate that may come through the filter. The colour is measured at 520mp in a 15-mm flow cell.The entire flow diagram for this procedure is shown in Fig. 1 and the calibrztion curve obtained with it in Fig. 2. TABLE I RECOVERY OF SULPHRTE ADDED TO ARiciiNSAS RIVER WATER Total, Recovered, Recovered, Amount added mg per litre mg per litre per cent. - - i ! 31 10 mg per litre . . { I] 90 60 mg per litre . . { ;; q 94 100 mg per litre . . { E} 101 None . . .. .. [ 31 40 47 7 8 47 J 133 102 133 10.2 METHOD EVALUATON- To check the recovery of known amounts of sulphate, 10, 50 and 100mg per litre of sulphate were added to a sample of Arkansas river water.Ninety per cent. of the 10-mg per litre, 94 per cent. of the 50-mg per litre and 101 per cent. of the 100-mg per litre sample were recovered. These results are shown in Table I. Two samples containing different concentrations of sulphate were run through the system to determine the precision of the automatic procedure. The results, depicted in Fig. 3, show this method to be reproducible. Additional results confirming the reproducibility are given in Table 11. TABLE I1 PRECISION OF THE AUTOMATIC SULPHATE PROCEDURE Number of Coefficient of river water Range, Mean, St an dard variation, samples mg per litre rng per litre deviation per cent. 6 50 to 52 50.8 f 0-3 0.6 9 92 to 97 94.0 & 1.0 1.1 10 168 to 174 170.0 f 3.0 1.8 The reliability of this procedure was checked by comparing the results obtained by the AutoAnalyzer with the standard gravimetric methodl and a turbidimetric methodG on a group of representative river water samples.These comparisons, shown in Tables I11 and IV, indicate that this method is of comparable accuracy.100 GALES, IUE’LOR AND LONGBOTTOM TABLE 111 COMPARISON OF THE GRAVIMETRIC METHOD WITH THE BARIUM CHLORANILATE METHOD ON RIVER-WATER SAMPLES Gravimetric method , mg per litre 324 303 187 283 252 Barium cliloranilate method, mg per litre 340 318 192 276 246 Gravimetric I AutoAnalyzer, per cent. 95 95 97 102 10s TABLE IV COMPARISON OF THE TURBIDIMETRIC METHOD WITH THE BARIUM CHLORANILATE METHOD ON RIVER-WATER SAMPLES Concentration, 0 to 50 mg per litre Concentration, 60 to 400 mg per litre Turbidimetric Barium chloranilate Turbidimetric Barium chloranilate method method method method 8 6 4 13 16 15 13 13 10 45 35 23 Average deviation .. 10 6 6 16 15 15 20 17 10 42 30 22 2.2 mg per litre 91 215 235 239 240 80 85 105 225 115 320 - 82 196 220 219 236 72 80 103 231 110 320 - 8.9 mg per litre CONCLUSION The method can be used to determine sulphate in the range of 5 to 400 mg per litre without any sample dilution. The accuracy and precision are equal to, or better than, those methods used for comparative purposes in this study, and those reported for the standard turbidimetric meth0d.l REFERENCES 1. Orland, H. P., Editor, “Standard Methods for the Examination of Water and Wastewater,” Twelfth Edition, American Public Health Association Inc., New Yorlr, 1965. 2. “Technicon AutoAnalyzer Methodology-Sulfate VIb,” Technicon Controls Inc., Chauncey, New York. 3. Lazrus, A. L., Hill, K. C., and Lodge, J. P., “A New Colorimetric Microdetermination of Sulfate Ion,” Technicon Symposia 1965, Automation in Analytical Chemistry, Mediad Inc., New York, 1966, p. 291. Bertolocini, R. J., and Barney, J. E., Analyt. Chern., 1957, 29, 283. Agterdenbos and Martinius, Talanla, 1964, 11, 875. Water & Sewage Analysis Procedures, Catalogue No. 9, Hach Chem. Co., Ames, Iowa, 1966, p. 42. Received May 2nd, 1967 4. 5. 6.