An Introductory Course of Quantitative Chemical Analysis eBook

This eBook from the Gutenberg Project consists of approximately 188 pages of information about An Introductory Course of Quantitative Chemical Analysis.

From the corrected volume of the iodine solution required to oxidize the antimony, calculate the percentage of antimony (Sb) in the stibnite.

[Note 1:  Antimony chloride is volatile with steam from its concentrated solutions; hence these solutions must not be boiled until they have been diluted.]

[Note 2:  Antimony salts, such as the chloride, are readily hydrolyzed, and compounds such as SbOCl are formed which are often relatively insoluble; but in the presence of tartaric acid compounds with complex ions are formed, and these are soluble.  An excess of hydrochloric acid also prevents precipitation of the oxychloride because the H^{+} ions from the acid lessen the dissociation of the water and thus prevent any considerable hydrolysis.]

[Note 3:  The action of hydrochloric acid upon the sulphide sets free sulphureted hydrogen, a part of which is held in solution by the acid.  This is usually expelled by the heating upon the water bath; but if it is not wholly driven out, a point is reached during dilution at which the antimony sulphide, being no longer held in solution by the acid, separates.  If the dilution is immediately stopped and the solution warmed, this sulphide is again brought into solution and at the same time more of the sulphureted hydrogen is expelled.  This procedure must be continued until the sulphureted hydrogen is all removed, since it reacts with iodine.  If no precipitation of the sulphide occurs, it is an indication that the sulphureted hydrogen was all expelled on solution of the stibnite.]

[Note 4:  Ammonium hydroxide is added to neutralize most of the acid, thus lessening the amount of sodium bicarbonate to be added.  The ammonia should not neutralize all of the acid.]

[Note 5:  The reaction which takes place during titration may be expressed thus: 

Na_{3}SbO_{3} + 2NaHCO_{3} + I_{2} —­> Na_{3}SbO_{4} + 2NaI + H_{2}O + 2Co_{2}.]

[Note 6:  If the end-point is not permanent, that is, if the blue of the iodo-starch is discharged after standing a few moments, the cause may be an insufficient quantity of sodium bicarbonate, leaving the solution slightly acid, or a very slight precipitation of an antimony compound which is slowly acted upon by the iodine when the latter is momentarily present in excess.  In either case it is better to discard the analysis and to repeat the process, using greater care in the amounts of reagents employed.]

CHLORIMETRY

The processes included under the term !chlorimetry! comprise those employed to determine chlorine, hypochlorites, bromine, and hypobromites.  The reagent employed is sodium arsenite in the presence of sodium bicarbonate.  The reaction in the case of the hypochlorites is

NaClO + Na_{3}AsO_{3} —­> Na_{3}AsO_{4} + NaCl.

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An Introductory Course of Quantitative Chemical Analysis from Project Gutenberg. Public domain.
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