Sulphate gravimetric determination

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). 

Discussion. This gravimetric determination depends upon the separation and weighing as elementary selenium or tellurium (or as tellurium dioxide). Alkali selenites and selenious acid are reduced in hydrochloric acid solution with sulphur dioxide, hydroxylammonium chloride, hydrazinium sulphate or hydrazine hydrate. Alkali selenates and selenic acid are not reduced by sulphur dioxide alone, but are readily reduced by a saturated solution of sulphur dioxide in concentrated hydrochloric acid. In working with selenium it must be remembered that appreciable amounts of the element may be lost on warming strong hydrochloric acid solutions of its compounds if dilute acid solutions (concentration <6M) are heated at temperatures below 100 °C the loss is negligible. 

And the qualities we are looking for are always realized as a matter of degree, these being best reflected by a proper statement of measurement uncertainty. This can also be illustrated by a top-down example from gravimetry suppose that NIST fails to incorporate in their measurement procedure one correction after another the quality of results for the gravimetric determination of sulphate as barium sulphate (for a recent account of... 

JJ Gravimetric Determination of Sulphate in Comprehensive Analytical Chemistry. C. Willson, and D. Willson, Eds., Vol. 1C. Elsevier Publishing Company. Amsterdam, Netherlands. 1962. p. 282. 

Gravimetric determination as barium sulphate after distillation General... 

The volume of the water sample to be employed for gravimetric determination of sulphate must be measured so that it contains between 10 and 500 mg sulphate ions. Where necessary, a larger volume of water must be reduced to 400 ml, and/or in the case of lower concentrations of sulphate ions to 100 ml. 

An ion-selective electrode (ISE) determination of sulphide from sulphate-reducing bacteria was compared with a gravimetric determination. The results obtained were expressed in milligrams of sulphide. 

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. 

The gravimetric determination of elemental sulphur adopted by the U.S.P.y depending on oxidation of sulphur to sulphate with hydrogen peroxide, is essentially the following ... 

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. 

No satisfactory direct gravimetric procedure is available but nitrite can be oxidised to nitrate by permanganate or cerium(IV) and then determined in that form. The determination of total nitrate + nitrite is an important analysis, e.g. for soil samples. Nitrite may be destroyed using urea, sulphamic acid or hydrazine sulphate the reaction with the former is ... 

The detection and determination ot the perchlorates.—The perchlorates give no precipitates with silver nitrate or barium chloride soln. cone. soln. give a white crystalline precipitate with potassium chloride. Unlike all the other oxy-acids of chlorine, a soln. of indigo is not decolorized by perchloric acid, even after the addition of hydrochloric acid and they do not give the explosive chlorine dioxide when warmed with sulphuric acid unlike the chlorates, the perchlorates are not reduced by the copper-zinc couple, or sulphur dioxide. Perchloric acid can be titrated with —iV-alkali, using phenolphthalein as indicator. The perchlorates can be converted into chlorides by heat and the chlorides determined volumetrically or gravimetrically they can be reduced to chloride by titanous sulphate 28 and titration of the excess of titanous sulphate with standard permanganate they can be fused with zinc chloride and the amount of chlorine liberated can be measured in terms of the iodine set free from a soln. of potassium iodide and they can be... 

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... 

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. 

There are a few occasions when these rules do not apply, e.g. when the solid is to be decomposed by heat and the filter paper destroyed, a process known as ashing. Some gravimetric analyses, such as the determination of sulphate as barium sulphate, require retention of the precipitate from gravity filtration since BaS04 is too fine to be collected on a vacuum filtration system. BaS04 is thermally stable up to 600 C, so the filter paper can be burned away. 

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]. 

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... 

This can be done gravimetrically or volumetrically. In either case it is necessary to do a parallel determination on a sample that has not been hydrolysed, unless the hydrolysed sample had been rendered free of sulphate before hydrolysis, e.g. by extraction with dry ethanol or propan-2-ol. 

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. 

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 chloride obtained by reduction with the acid ferrous sulphate solution may be determined gravimetrically as silver chloride, after reboiling with nitric acid (Harvey). 

Similar to sulphites, thiosulphates can be titrated against standardised iodine solution. Following oxidation to sulphate by hydrogen peroxide/ammonia solution, thiosulphates can be determined gravimetrically as BaS04 (See above section for details). 

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