Sulphate gravimetric method

Occasionally it is desired to determine sulphurous acid by a gravimetric method it is then usual to expel the sulphur dioxide from the solution under examination by distilling in an atmosphere of carbon dioxide and oxidise the gas to sulphuric acid by absorption in bromine water or iodine solution, subsequently adding barium chloride and weighing the precipitated barium sulphate. This method also gives accurate results volumetrieally if steps are taken to prevent loss of iodine by volatilisation in the current of carbon dioxide. The excess iodine is titrated with a solution of sodium thiosulphate.8... 

Gravimetric Methods.—The main difficulty is to obtain the cobalt in the form of one of its compounds entirely free from nickel or other metal. This may be accomplished by the cyanide method described under Wet Tests above. The solution, after separation of the nickel, is evaporated with dilute sulphuric acid until white fumes are evolved. On addition of water a solution of cobalt sulphate results, and the cobalt may be precipitated in a variety of ways, for example as oxide with hypobromites or persulphates,2 as sulphide, as cobalti-nitroso p-naphthol, or as basic carbonate.3 In each case the precipitate is ignited and reduced to metallic cobalt in a current of pure hydrogen. 

Gravimetric Methods.—When the chromium is in solution as a chromic salt, it can be precipitated completely from the boiling solution by the addition of ammonium hydroxide in the presence of ammonium salts. The precipitate may be dried at 100° C. and converted by gentle ignition to the dark green sesquioxide and weighed as such. Ammonium sulphide or hydrazine sulphate may also be used as the precipitant. 

For the determination of higher quantities of sulphates a gravimetric method is used (weighable form of BaSO ) [13,14, 32]. In the case of lower concentrations polarographic [54] or nephelometric methods are used [13]. 

In the classic gravimetric method the sulphate is precipitated and weighed as barium sulphate. Although it has been applied to seawater for more than 150 years, the method is still preferred because of its high precision and the minimum amount of equipment required to perform the determination. The procedure by Bather and Riley (1954) is described here in detail. 

Discussion. Some of the details of this method have already been given in Section 11.11(C), This procedure separates aluminium from beryllium, the alkaline earths, magnesium, and phosphate. For the gravimetric determination a 2 per cent or 5 per cent solution of oxine in 2M acetic add may be used 1 mL of the latter solution is suffident to predpitate 3 mg of aluminium. For practice in this determination, use about 0.40 g, accurately weighed, of aluminium ammonium sulphate. Dissolve it in 100 mL of water, heat to 70-80 °C, add the appropriate volume of the oxine reagent, and (if a precipitate has not already formed) slowly introduce 2M ammonium acetate solution until a precipitate just appears, heat to boiling, and then add 25 mL of 2M ammonium acetate solution dropwise and with constant stirring (to ensure complete predpitation). 

If there is no available salt of constant composition, such as copper(II) sulphate or iron(III), aluminium or chromium alums, it is advisable to prepare first a stock solution of an approximate concentration, slightly higher than required and to determine the concentration by a gravimetric or volumetric method. After suitable calculations the solution is diluted with pure solvent to obtain a solution containing exactly, e.g., 1 mg/ml of the given element. In some cases, standard solutions are obtained by dissolving a precisely weighed amount of the element in its pure form. 

Precipitation was very frequently used in the past as a method of determining certain substances in water by direct gravimetric analysis. Even today, for example for sulphate determination, precipitation from an acidified... 

Sulphate concentrations may also be determined accurately by potentiometric back-titration of excess Ba + with a mercury electrode following the precipitation of BaS04 Mucci, 1991). The seawater sample is freed from most seasalt cations with an ion-exchange-column. Then the eluate is reacted with an excess of barium, and after filtration of precipitated BaS04, the solution is titrated potentiometrically with an EGTA solution (see also Section 11.2) to the endpoint. The method applies over a wide range of salinities and sulphate concentrations in 1 mL or less of seawater and marine pore water samples, however, it is somewhat less precise (c.v. of about 0.6 %) than the simple gravimetric procedure described. 

The classical method for calcium is by gravimetric determination as oxalate. A hot solution of calcium salt in hydrochloric acid is treated with ammonium oxalate or oxalic acid and neutralised with ammonia the precipitate is washed with dilute ammonium oxalate solution and is then ignited to calcium carbonate or oxide, or is converted to calcium sulphate by treatment with sulphuric acid. An alternative volumetric finish is to dissolve the precipitated oxalate in sulphuric acid and titrate with potassium permanganate. 

Similar to hydrogen sulphide and sulphides, sulphur dioxide and sulphites can be determined by titration with iodine or by oxidation to sulphate followed by gravimetric determination as barium sulphate. Additionally a spectrophotometric method was developed based on the colour of p-nitroaniline which is discharged on acidification but is restored by adding formaldehyde and sulphur dioxide or sulphite. 

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