Analysis of Organic Acids A Freshman Laboratory Experiment John R. Griswold and Richard A. Rauner Cedar Crest College, Allentown, PA 18104 The determination of the molecular weizht of a carboxvlic acid by titration with standard base is ;sually a part of aualitative organic analysis schemes' and has also found its way into singG laboratory experiments.2J I t is usually desirable to employ several laboratory techniques in a given student experiment, thereby increasing exposure to the methods and their mutually supporting roles. This is especially true in laboratory experiences for nonscience majors, who often follow a one-year course sequence involving general chemistry, organic chemistry, and biochemistry. Because nonscience majors' exposure to organic chemistry is limited, we have attempted to maximize the laboratory experience within the allowed time frame. While the instructor has options for modification or expansion, we have found that the experiment described herein fits a 3-h session with meaningful results. In the experiment students select unknown carboxylic acids, determine their melting points, and investigate their soluhility behavior in water and ethanol. Based on solubility, they then select the appropriate indicator and titrate several accurately weighed samples to determine the molecular weight. The calculated molecular weight and melting point are then used in combination with each other in identifying the carboxylic acid from a list of possibilities. Solubility is checked in pure water, 50% ethanol, and, if necessary, 95% ethanol, the latter calling for hromthymol blue as the titration indicator. The experimentation gives the student an appreciation for the svstematic use of ~roceduresto arrive a t comuound identification, stressing the principle of stoichiometri in an acid-base titration, the melting poinr as a criterion for identification, and the importance-of an accurate solubility ohservation in the choice of a titration indicator. Experimental Students select unknown carboxylic acids and determine their melting points using electric melting point devices.' Once the student has observed and recorded a tieht - meltinaranae. - - . the remainingprocedures are carried out. Soluhility is determined on a basis of relative quantities of solute and solvent used in the titration. This is approximately 0.2 g in 50 mL of solvent. Therefore, about 40mg (small amount on the end of s steel spatula) is tested with 10 mL of water in a 18- X 150-mm test tube, with agitation at room temperaturefor about 1min. If solubili-
' Shriner, R. L.; Fuson, R. C.; Curtin, D. Y.; Morrill. T. C.
The Systematic Identification of Organic Compounds, 6th ed.; Wiley: New York, 1980; p 268. Boikess, R. S.; Edelson, E. Chemical Rinclples in the Laboratory, 3rd ed.; Harper and Row: New York, 1985; pp 323-328. Kauffman,G. 6.; Houghten, R. A. J. Chem. ~duc.1867.44, 408. "esults were obtained using the Mel-Temp apparatus: Laboratory Devices, Cambridge. MA 02139. Indicator solutions were prepared as described in the CRC Ha& bmk of ChemistryandPhysics, 63rd ed.; CRC: Boca Raton. FL, 1982: ~D158. Analvtical balances. sensltlve to 0.1 ma. -. are recommended. - --~ ~ ~ e vwee found r ; that 0haus four-beam arm balances give salislactory results if carefuly zeroed and read. In any event, at least 1 mg accuracy is needed. ~
516
Journal of Chemical Education
~
~~~
ty is not completely achieved, the procedure is repeated with 50150 water+thanol. Either of these solvent ayatema allow for the use of phenolphthalein as the titration indicator. If the rample will not dissolw in the water-ethanol medium, then 959 ethanol is required
for the titration with hromthymol blue as the indicato~.~ using 125mL Erlenmever flasks. at least two samoles of about 0.2 e each are are then neeuratelv wiiehed m a n anslvtical -~~~~~~~~ ,~ bal&e.eThe aam~lei dias,lved'in 51)mLof thechosen medium, 2-4 drops iheappropriate indicator are added, and the solutions are titrated u, their endpoints using standard NaOH solution with a molarity of about 0.1. Our approach to the analysis of the titration is given in the discussion section. Once the titration results are translated into molecular weights and the results averaged, a table is consulted for unknown identification using the observed melting point and the calculated moleeular weieht. - ~ " ~ ~ ~ After the experimentation is completed, students submit their results, which include the identification, observed melting point. titration medium selected, indicator, and the molecular weight with calculationsand data. ~~e~~~~~
~
~
~
~~
~~
~
of
~~
~~
~
~
Dlscusslon This exoeriment is recommended for students who have had a t least some prior experience in titrations and endpoint observation. We chose the 16 acids in the table for several reasons. They are all solids melting above 100 O C and are therefore easily transferred and handled. Melting points are grouped, requiring that a melting point and molecular weight must be used toeether t o arrive at a valid identification. The melting pointdeterminations consume aportionof the laboratory period, since their accuracy is important in the identification process. However, this activity is speeded u p in two ways. Since all of the melting points on the list are above 100 "C, the devices need not he recooled to ambient temperature between determinations. Also, students can be
Selected Carboxyllc Aclds Acida
Melting Pointb
mtoluic 108-110 109-111 arelaic OL-mandeiic 120-122 benzoic 122-123 dkmaiic 131-133 tmn~cinnarnic 133-134 maieic 134-136 ~ehi~robBnz~i~ 138-140 acetylsalicylic 140-142 benzilic 150-153 adipic 152-154 citric (anhydrous) 152-154 salicylic 156-160 p-tertbutylbenroic 165-167 170-172 ~4+)-tartaric ptaiuic 160-182
Acidlc
Titrating
Gram
Moi. Wt.
Mediumc
1
136
2 1 1
188 152 122
2
134
H20
1
148
50150
2 1 1 1 2
116 156 180 228 146
H20
3 1
192 136 178 150 136
1
2 1
ethanol 50150 H20
50150
50150 50150
50150 50150 H20
50150 eman01 H20
ethanol
1\11 acids were obtained from Aldrich Chemical Co., Milwaukee. WI, and were us& without further purlticstion. bAo given in the Aldrich catalog. =Determined from oolubillfy tes* in our laboratay.
advised to do a rough rapid heating to identify an approximate melting range, followed by a more refined run in which the anticipated range is traversed slowly for accurate results. Information on the titratine solvent svstems used. eiven in the table, is not initially provided to the students'& order that thev ex~erimentallvdetermine their own media for the analysis. u&ng standard NaOH with a concentration of about 0.1 M, monocarboxylic acids require between 10 and 20 mL of hase to reach the endpoint, with dicarboxylic acids requiring twice this amount. We have found it best to avoid the term "normality" in the prelaboratory remarks on the experiment, since the concepts are best understood on the basis of moles and stoichiometric relationships in the molecular equations, using appropriate numerical examples. If the acid contains one carboxyl group, then eq 1shows the simple neutralization process: R-COOK
+ NaOH
-
R-COO-Na'
+ H,O
(1)
The endpoint is recognized by the clear to pink phenolohthalein color chanee in aoueous or water-ethanol media and the yellow to blue changk with hromthymol blue in 95"ko ethanol. If the acid is dicarboxvlic, then 1 mol rewires twice the molar quantity of hase to achieve the endpoint: HOOC-CH,-COOH
-
+ 2NaOH
Na' -00C-CH,C00-Na+
+ 2H20
(2)
When titrating an unknown acid, it is not initially known how many carboxyl groups are present. It is usually best to assume that one group is present, perform the titrations, and determine the average molecular weight. If the molecular weight turns out to he a very low value (the carboxyl group alone weighs 45 mass units) and if the calculated value bears no resemblance to those on the list provided, then it is
advisable to assume two carboxyl groups. In this case, the molar ratio in the numerical solution yields a molecular weight which often makes more sense and agrees with a carboxylic acid on the list. For example, assume that 15.95 mL of 0.1017 M NaOH were required to titrate a 0.2000-g sample of unknown acid. Then,
Inspection of the table reveals that benzoic acid has a molecular weight of 122, about 1% from the calculated value. A dicarboxylic acid will require a greater volume of hase to reach the endpoint, and the final term in eq 3 is altered to read 2 mol OH-: 1 mol acid. This trial-and-error method works well as the investigator assumes progressively more carboxyl groups until a realistic and identifiable molecular weight is obtained. The experiment usually yields molecular weights well within 1%of the true values, and our results with nonmajors often rival those of the students in the analytical and qualitative oreanic courses. Also, there is no reason whv this experiment cannot he used with a select group of high &hool students. perhaps with modification if sufficient melting point de&s are not available. The experiment is relativel; safe and consumes only small quantities of material, although some initial expenditure may be needed to acquire a suitable stock of carboxylic acids such as those in the table. Acknowledgment
We with to thank Cindy Lillie for her useful discussions and the Chemistry 102 class at Cedar Crest College, spring 1989, for their experimental results.
Volume 67
Number 6
June 1990
517
