Analyst, February, 1967, Vol. 92, p p . 115-117 115 Determination of Sulphate in the Presence of Soluble Silicate BY M. AZEEM (Pakistan Atomic Energy Commission, P.O. Box No. 31 12, Karachi, Pakistan) A method is described ior determining sulphate in the presence of soluble silicate. The sulphate was precipitated as barium sulphate in an acidic medium, after adding dimethylformamide. The formation of a stable complex between Si (OH), and dimethylformamide is considered. DURING an investigation of the SiF, - SF, system, an ethereal solution of dimethylformamide (DMF) was used for separating the sulphur tetrafluoride from the mixture; silicon tetra- fluoride was absorbed in the solution thereby producing solid SiF4.2DMF.l ,2 However, sulphur tetrafluoride is slightly soluble3 in ether, and can be determined in solution by precipitation of barium sulphate. The ethereal solution and the complex SiF4.2DMF were first hydrolysed with ice-cold water, and then made alkaline with sodium hydroxide solution.The solution, containing the sulphite, silicate, fluoride and sodium ions and dimethylform- amide, was treated with hydrogen peroxide and then analysed for sulphate. For the gravi- metric determination of sulphate in the presence of soluble silicate the logical procedure would be to separate silica in the first stage, and then precipitate barium sulphate from the filtrate and the washings. When the recommended procedures4~5 were used for precipitating silica from the alkaline sulphate - silicate solution containing dimethylformamide, the yield of silica was found to be abnormally low.Therefore, an excess of dimethylformamide was added to a standard solution of sulphate - silicate, followed by the addition of an excess of hydrochloric acid. The solution was boiled, evaporated to one quarter of its original volume and then diluted. The solution was crystal clear. The acidified solution was then placed on a steam-bath for 48 hours. At the end of this period the solution was clear and colourless. This showed that dimethylformamide acts as a hold-back carrier for the silicate, and barium sulphate can be precipitated directly from the solution. METHOD REAGENTS- All reagents should be of analytical-reagent quality. Potassium sulphate. Sodium sul9hate. Sodium silicate, Na2Si03.9H20, su$$lied by Fislzev Scienti$c Company.Dimethyl f ovmamide. PROCEDURE- A standard solution of potassium sulphate containing an equivalent amount of sodium silicate was prepared. To a 25-ml portion of the solution, 1 to 2 ml of dimethylformamide were added. The solution was then diluted to 250 ml and acidified with hydrochloric acid. The solution was heated to boiling, and an excess of 5 per cent. barium chloride solution was added slowly with constant stirring. The solution was heated on a steam-bath for a few hours to granulate the precipitate. The supernatant liquid was decanted through a Whatman No. 42 filter-paper. The precipitate was washed, transferred to the filter-paper and then ignited in a weighed silica crucible to a constant weight, as described in the recom- mended p r o c e d ~ r e .~ , ~ The results of the analysis are given in Table I.116 AZEEM: DETERMINATION OF SULPHATE [Analyst, Vol. 92 TABLE I RESULTS OF THE GRAVIMETRIC DETERMINATION OF BARIUM SULPHATE I N THE PRESENCE OF SOLUBLE SILICATE Weight of K,SO, in 25 ml of the solution containing Na,SiO,, g 0.2438 0.2438 0.2438 0.2438 0.2438 0-2204 0.2204 0.2204 0.2204 0.2204 Weight of BaSO, calculated in 25 ml of the solution, g 0,3265 0-3265 0.3265 0.3265 0.3265 0.2952 0.2952 0.2962 0.2952 0.2952 Weight of BaSO, found in 25 ml of the solution, g 0-3258 0.3252 0.3271 0-3280 0.3269 0.2950 0.2945 0.2961 0.2965 0.2958 In another series of experiments a 25-ml aliquot of a standard solution of sodium sulphite containing an equivalent amount of sodium silicate was treated with 2 ml of 30 per cent.hydrogen peroxide to oxidise the sulphite ions. The solution was heated to decompose excess of the peroxide and then analysed for sulphate according to the above procedure. The results of the analysis are given in Table 11. TABLE I1 DETERMINATION OF SULPHITE BY PRECIPITATION OF BARIUM SULPHATE I N THE PRESENCE OF SOLUBLE SILICATE Weight of Na,SO, in Calculated weight of Weight of BaSO, Amount of Na,SO, 25 ml of the solution containing Na,SiO,, the solution, the solution, BaSO, found, BaSO, from 25 ml of found in 25 ml of calculated from g g g g 0.1637 0.3032 0-3025 0.1633 0.1637 0.3032 0.3047 0.1645 0.1637 0.3032 0.3042 0.1642 0-1637 0.3032 0-3035 0.1639 0.1637 0.3032 0.3020 0.1635 0.1450 0.2685 0.2672 0.1443 0.1450 0.2685 0.2700 0.1457 0.1450 0-2685 0.2692 0.1453 0.1450 0.2685 0.2888 0.1451 0.1812 0.3357 0.3352 0.1810 0.1812 0.3357 0.3365 0.1817 0.1812 0.3357 0.3370 0.1s19 The purity of the solid sodium sulphite was checked by the iodimetric m e t h ~ d .~ , ~ It was observed that short exposure of the compound to the atmosphere, as experienced in weighing, does not appreciably affect its purity (Table 111). TABLE 111 DETERMINATION OF THE PURITY OF SODIUM SULPHITE Weight of iodine coiisumed by b" g g 0.1552 0- 1548 0.3128 0.1465 0.2954 0.1466 0,2692 0.1336 0.1335 0.1282 0.2579 0.1280 Weight of Na,SO, calculated from Weight oE Na,SO, taken, Na,SO,, the amount of iodine consumed, RESULTS AND DISCUSSION The gravimetric values for barium sulphate are within k0-5 per cent. of the theoretical value.The concentration of either sulphate or silicate ions does not appear to affect the results. The values in Table I1 show that this method can be used satisfactorily for determining sulphite in the presence of silicate. From the above observations and results it is concludedFebruary, 19671 I N THE PRESENCE OF SOLUBLE SILICATE 117 that the soluble silicate forms a very stable complex with dimethylformamide, which is not affected by strong acids. It is suggested that hydrochloric acid reacts initially with the aqueous silicate forming Si(OH), which in turn interacts with dimethylformamide, a strong electron donor. Presumably the complex is of the type Si(OH),(DMF). (where x = 1 or 2), and is analogous to SiF,(DMF),.1,2 This work was carried out at the McMaster University, Hamilton, Ontario, Canada. I thank Dr. R. J. Gillespie for providing the laboratory facilities and for his encouragement. REFERENCES 1. 2. Muetterties, E. L., Ibid., 1960, 82, 1082. 3. 4. 5. Piper, T. S., and Rochow, E. G., J . Amer. Chem. SOL, 1954, 76, 4318. Azeem, M., “The Structures of SF,.BF, and Some Related Compounds,” Ph.D. Thesis, MchIaster Vogel, A. I., “A Text Book of Quantitative Inorganic Analysis,” Third Edition, John Wley Scott, W. W., Editor, “Standard Methods of Chemical Analysis,” Fifth Edition, D. Van Nostrand Received January 31st, 1966 University, Canada, 1965. & Sons, New York, 1961; pp. 370, 464 and 583. Co. Inc., New York, 1939; pp. 127, 803 and 926.