A Manual of Chemical Analysis : qualitative and quantitative by George S. Newth

Author:George S. Newth [Newth, George S.]
Language: eng
Format: mobi
Tags: Chemistry, Analytic -- Handbooks, manuals, etc
Published: 2015-03-24T04:00:00+00:00

FIG. 47-

about fifteen minutes; it is then cooled in the desiccator and weighed.

In case any of the calcium carbonate has become converted into the oxide in spite of care in the heating, the residue is moistened with a few drops of a strong solution of ammonium carbonate, which is then evaporated to dryness by placing the crucible in the steam-oven. When dry, it is reheated for a few minutes over the heated gauze, and again weighed.

If this process results in any increase in the weight, it shows that in the first heating some of the carbonate had been decomposed ; the series of operations should, therefore, be repeated until the weight remains constant.

In this particular instance the form of combination in which the metal to be estimated is weighed, happens to be the same as that in which it exists in the original compound, namely, the carbonate; hence the weight of the product obtained will be the same as that of the substance taken for analysis.

The calcium carbonate is now converted entirely into calcium oxide by heating the crucible to a strong red heat by means of a Bunsen flame for about ten minutes, and finishing off with a blowpipe flame. The crucible is placed in the desiccator to cool, and is then weighed. It is afterwards reheated with the blowpipe for a few minutes and weighed again, the process being repeated until no further loss of weight results.

Magnesium.

Epitome of Process. —The magnesium is precipitated by sodium phosphate, in the presence of ammonium chloride and ammonia, as ammonium magnesium phosphate, NH 4 MgPO 4 ,6H 2 O. This is afterwards converted by heat into magnesium pyrophosphate, Mg 2 P 2 O 7 , in which form the metal is weighed. Factor—

(Mg 2 P 2 O 7 ) 222 : (Mg 2 ) 48 = i : 0-2162 Estimation of Magnesium in Magnesium Sulphate,

MgSO 4 ,7H 2 O.—Take about 75 gram. The recrystallised salt is weighed out into a beaker, and dissolved in a rather smaller bulk of water than usual (about 60 to 80 c.c.). A quantity of ammonium chloride solution is added, equal to about one-half the volume of the solution of magnesium sulphate, and then ammonia in moderate excess.* To this is added a solution of hydrogen disodium

* Should the addition of ammonia cause precipitation, more ammonium chloride must be introduced until this precipitate is redissolved. Excess of ammonia, over and above that which is required for the formation of the ammonium magnesium phosphate, is necessary to ensure complete precipitation, as the phosphate is slightly soluble in water, but insoluble in the presence of free ammonia.

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