~, Thomas M ~ C I I I I O U ~C.S.C. St. Edward's University Austin, Texas 78704
A Combined Gravimetric-Volumetric Analysis for Organic Chemistry
A structured course devoted exclusively to quantitative analysis is becoming harder t o find these days; in its place individual quantitative experiments are now routinely included in and oreanic chemistrv laboratories. This - ~ eeneral paper describes a relatively simple organic chemistry exneriment which utilizes titration, equivalent weight determination, paper chromatography, and gravimetric weighing. Surprisingly, with some preliminary work by the instructor or laboratory assistant, this entire procedure can be carried nut in one 4-hr laboratory session. In essence this experiment deals with a mixture of two simple organic acids, one of which should be formic acid and the other acetic, propionic, or n-butyric. T h e student is given about one gram of the mixture and asked to find the apparent equivalent weight (neutralization equivalent) of the solution a s is, the identity of the second acid, and the amount of both.
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Experimental Upon receiving his unknown, the student carefully weighs out about 0.150 g in a 50-ml flask. All data in this experiment should contain three significant figures (a three decimal top loading balance should be used): sample weight, normality, titration volume, equivalent weights, and percentages. The weighed sample, diluted with 1 or 2 ml of distilled water, is titrated (50-ml buret) with a standard 0.250 N FaOH solution to the phenolphthalein end point. One lambda (0.001 ml) of this solution is used to spot a paper (19 X 14 em Whatman *1 paper) chromatogram, which can easily hold 10 unknowns and several knowns. To the titrated solution 10 ml of a buffered HgC12 solution is added, plus a small pinch of solid hydroxylamine hydrochloride. With a reflux condenser the mixture is heated on a steam bath for 1 hr. The mercuric chloride will oxidize the formic acid quantitatively and in turn will be reduced to calomel, Hg2C12, which will appear as a white, insoluble pre~ipitate.~ During the heating period the paper chromatography3 can be started and a fritted filtering crucible weighed. At the end of the hour, the Hg2CI2 is collected in the crucible, washed with water, acetone, and dried briefly in a 105°C oven. A final weighing of the dry and cooled crucible concludes the eravimetric data. Aker the solvent has risen at least 11 or 12 cm, the paper is removed from the Berzelius beaker (caution, corrosive solvent) and dried in a stream of warm air for about 40 mi". The dry paper is dipped or sprayed with 0.015% methyl red in alcohol which reveals yellow spots on a pink background. The spots should he marked in pencil as soon as they are discernible, for they often fade completely. Typical R values are: 0.10, 0.17, 0.29, and 0.43 for farmate, acetate, propionate, and n-hutyrate, respectively. T h e apparent neutralization equivalent of t h e original sample is obtained from the titration d a t a
N.E
=
sample weight X 1,OM) ml of NaOH x normality
The percentage of formic acid in the original sample is obtained from the weight of H g ~ C l zcollected 0.0375 X wt. of Hg,CI, X 100 % Formic acid = weight of sample Calculation of the percentage of the second acid requires t h e above two values and the identity of the second acid, obtained from the chromatography. In the following equation the only unknown value is the percentage (by weight) of the second acid 460 x (% of formic acid)
+ (Mol wt of 2nd acid) X (%) -- N.E,
100 Formic acid is generally not available as 100% pure material b u t usually contains 3-12% by weight of water. For better results, the above equation should be modified by replacing the 46.0 by the number obtained by dividing 46.0 by the actual percentage of formic acid in the original supply bottle. In making u p the unknowns, the instructor should keep the amount of formic acid in all samples below 25% of the total weight thus guaranteeing t h a t there will be an adequate amount of HgClz present to completely carry out the oxidation of the formic acid. A shorter variation of this experiment can be realized by using a mixture of two relatively pure acids, omitting the HgClz oxidation, but retaining t h e titration and paper chromatography. In such a case, the amount of each acid can be obtained from the equation
+
(Mol wt of tst acid) X y% (Mol wt of 2nd acidX100 - y)% = N.E. 100 where y is the percentage of the first acid and 100 - y the percentage of the second. 'Buffered HgC12 solution made from 80 ml of water, 10 ml of glacial acetic acid, 10 g of crystalline sodium acetate, and 4 g of HgC12. ZWelcher, F. J., "Standard Methods of Chemical Analysis," 6th Ed., D. Van Nostrand & Co., b!ew York, 1963, vol. 2, p. 590. ZA liter Berzelius beaker containing 6 ml of 88% phenol, 2 ml of methyl acetate, and 0.6 ml of water sealed with a plastic or glass covering. See McCullough, CSC, Brother Thomas, J Chromatogr., 44, 188 (1969).
Volume 51, Number 4, April 1974
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