An Introductory Course of Quantitative Chemical Analysis eBook

This eBook from the Gutenberg Project consists of approximately 220 pages of information about An Introductory Course of Quantitative Chemical Analysis.

The caustic alkali may also be determined by first using phenolphthalein as an indicator, which will show by its change from pink to colorless the point at which the caustic alkali has been neutralized and the carbonate has been converted to bicarbonate, and then adding methyl orange and completing the titration.  The amount of acid necessary to change the methyl orange to pink is a measure of one half of the carbonate present.  The results of the double titration furnish the data necessary for the determination of the caustic alkali and of the carbonate in the sample.]


Procedure.—­Weigh out two portions of the acid of about 1 gram each.  Dissolve these in 50 cc. of warm water.  Add two drops of phenolphthalein solution, and run in alkali from the burette until the solution is pink; add acid from the other burette until the pink is just destroyed, and then add 0.3 cc. (not more) in excess.  Heat the solution to boiling for three minutes.  If the pink returns during the boiling, discharge it with acid and again add 0.3 cc. in excess and repeat the boiling (Note 1).  If the color does not then reappear, add alkali until it does, and a !drop or two! of acid in excess and boil again for one minute (Note 2).  If no color reappears during this time, complete the titration in the hot solution.  The end-point should be the faintest visible shade of color (or its disappearance), as the same difficulty would exist here as with methyl orange if an attempt were made to match shades of pink.

From the corrected volume of alkali required to react with the oxalic acid, calculate the percentage of the crystallized acid (H_{2}C_{2}O_{4}.2H_{2}O) in the sample (Note 3).

[Note 1:  All commercial caustic soda such as that from which the standard solution was made contains some sodium carbonate.  This reacts with the oxalic acid, setting free carbonic acid, which, in turn, forms sodium bicarbonate with the remaining carbonate: 

H_{2}Co_{3} + Na_{2}Co_{3} —­> 2HNaCO_{3}.

This compound does not hydrolyze sufficiently to furnish enough Oh^{-} ions to cause phenolphthalein to remain pink; hence, the color of the indicator is discharged in cold solutions at the point at which bicarbonate is formed.  If, however, the solution is heated to boiling, the bicarbonate loses carbon dioxide and water, and reverts to sodium carbonate, which causes the indicator to become again pink: 

2HNaCO_{3} —­> H_{2}O + Co_{2} + Na_{2}Co_{3}.

By adding successive portions of hydrochloric acid and boiling, the carbonate is ultimately all brought into reaction.

The student should make sure that the difference in behavior of the two indicators, methyl orange and phenolphthalein, is understood.]

[Note 2:  Hydrochloric acid is volatilized from aqueous solutions, except such as are very dilute.  If the directions in the procedure are strictly followed, no loss of acid need be feared, but the amount added in excess should not be greater than 0.3-0.4 cc.]

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