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Detection of Barrium

Dry Tests for Barrium

Volatile barium salts give a yellowish-green coloration to the non-luminous flame. Like the other alkaline earths, barium has a characteristic spectrum. The most prominent line to be observed by the eye has a wave-length 5535.5 Å. By de Gramont's photographic method the lines 4934 and 4554 A are the most distinctive, but the line 3891.8 is also a sensitive ray.

When heated with sodium carbonate on charcoal the oxide is not formed, so that a brightly luminous mass is not obtained, and this affords a distinction from the other alkaline earths.

Wet Tests for Barrium

By the spectroscopic method of Riesenfeld and Pfutzer already mentioned under calcium and strontium, barium in a solution of concentration 0.0006 mgm. per c.c. may be detected by the lines 6497, 4934, and 4554 Å. The lines 6142 and 5536 A also show faintly.

The chief method of separating barium from admixture with the other alkaline earths, namely by precipitation of the chromate in acetic acid solution, has already been incidentally mentioned under calcium and strontium. Other confirmatory tests may be applied - for example, the precipitation of the sulphate by strontium sulphate, or of the chromate by strontium chromate, and the precipitation of the fluosilicate by hydrofluosilicic acid in aqueous or alcoholic solution, or by aniline fluosilicate. Precipitation by alcoholic thiosulphate has also been suggested.

Similar microchemical tests to those employed in the case of calcium or strontium may be applied.

Quantitative Estimation of Barium

The determination of barium by precipitation of the sulphate is the method in most general use, and, therefore, the one which has received the most careful study. Its great drawback, in addition to the effect of the medium on the solubility of the precipitate, which must be considered in all precipitation methods, is the pronounced tendency of barium sulphate to occlude foreign salts, whether by chemical combination or physical adsorption. There is a large literature on the subject, and some of the results may be briefly summarised.

Excess of barium chloride should be avoided. Alkali and ammonium nitrates and sulphates are occluded to a greater extent than chlorides. Salts of heavy metals and aluminium are carried down, and also calcium sulphate, but the latter at the same time apparently prevents the occlusion of potassium, sodium, magnesium, iron, or cobalt compounds. Adsorption may be diminished by rapid stirring.

Alkali and ammonium salts, ferric chloride, free chlorine and bromine, hydrochloric, nitric, and metaphosphoric acids increase the solubility. A slight loss may also take place on ignition under certain circumstances. Arbitrary corrections to be added or subtracted from the calculated result, according to the procedure employed, have been suggested, but it is probably preferable to calibrate the method used by pure sodium sulphate or potassium sulphate in strongly acid solution. It has also been stated that, owing to partial compensation of opposing errors, a very good uncorrected determination may be made by precipitating rapidly.

The general behaviour of the barium sulphate precipitate, for example, the dependence of the amount of foreign substance taken up on the concentration of the solution and on the fineness of the precipitate, seems to indicate that the phenomenon is one of adsorption at the surface.

The precipitation of barium chromate is also employed as a method of estimation. Ammonium dichromate solution in the presence of ammonium acetate, or of acetic acid and ammonium chromate, may be used. In the presence of strontium a solid solution of strontium chromate in barium chromate is apparently formed, and two precipitations are necessary to give a satisfactory result.

The precipitation of the chloride by hydrochloric acid and ether might be employed for the estimation of barium in the presence of strontium chloride, or precipitation by a mixture of acetone and acetyl chloride in the presence of calcium and magnesium. Precipitation of barium as the fluosilicate may also be used in the presence of strontium and calcium.

Precipitation by chromate is employed in the volumetric estimation of barium, the latter being precipitated by a known amount of chromate and the excess chromate being titrated by potassium iodide and thiosulphate. Barium may also be precipitated as oxalate and estimated volumetrically. Another volumetric method depends on the precipitation of the iodate, treatment of the latter with hydrochloric acid and potassium iodide, and titration of the liberated iodine. Strontium and calcium must not be present. For an approximate volumetric estimation, precipitation by alcoholic thiosulphate might be used.

Electrical Conductivity Methods of Barrium Detection

Electrical Conductivity Methods may be employed as in the case of calcium and strontium. The sulphate, chromate, fluosilicate, and carbonate are suitable for precipitation. For estimating calcium, strontium, and barium in a mixture by this method the following procedure may be adopted. The solution is divided into three parts. To the first is added two volumes of alcohol, and the solution is titrated with lithium sulphate. This gives the total titration for barium, strontium, and calcium. After addition of one volume of alcohol the second portion is titrated with lithium chromate, giving barium and strontium; and the third portion, also with the addition of one volume of alcohol, is titrated with cupric fluosilicate, giving barium. From these data the percentages of the three may be calculated.

Electrolytic Methods of separation and estimation have also been suggested.

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