V O L U M E 24, NO. 4, A P R I L 1 9 5 2
XI.
Table '\lean Cuncentration lienzyl .\lcohol, \Il./L. 1 02 1 b5 2 00 2.48
2.91
3.47 3 87 4 02 4 43
~
685
Precision and Sensitivity i
l
Of
Sample. MI. 'I,. 50 50
50 50 50 50 20 50 50
~
S~t a n di a ~~d ~ ~ Deviatio:), 7cStandard Del-., I I V Z(d1) ! I . * Ytd. Del-. 111.'I,. ( \lean Concn. ) x 100 1 2 9 -0 O i &0 07 ;to 04 r O I1 +0 13 * 0 13
23.2 1 3 5 1 3 2 1 3 8 1 3 7 1 3 4 12.2 12.9 1 3 0 13.9 zt3 6 13.8 +2.2
liieTi'iCaflask :tiid niakc. u p to 100 nil. wit11 distilled water. Trannfer 5 nil. of each solution to a correspoiidirrg colorimeter tube, add 5 nil. of concentrated sulfuric acid, and stir with a glass rod. Place the colorimeter tuijee :tt once in a beaker partly filled viith boiling wst,er and heat for exactly 10 minutes. Run cold wat,er into the beaker holding colorimeter tubes for 5 minutes. Stir with a glass rod t o dislodge air bubbles and t o niix in any water that may have recondensed near the top of the solution. Determine the per cent of light transmitBteda t approximately 410 n i p , using distilled wat,er as a st:m(Iard for 1 0 0 ~ otransmit,tance. Froni these resuhs prepire a graph plotting the per cent, of liglit transmitted against thp milliliters of 1)enzyl alcohol per liter of 3olution.
Analysis of Sample. l+c:iusc the scniitivity of the rcaction is liniitcd to only a certain range of benq-1 alcohol concentrations, it is ric~esaaryto dilute :I simple ileing analyzed so that its concc,nti,ation falls nithin tlir sensitive range. Onc~of nine different dilutions \vi11 bring :in>- s:rniplt~ into the sensitive range, so that any (*oncentration including pure hc111z~.l alcohol can be determ i n d . The sa~npleneed not he soluble in water in all proport,ious. Any dilution that fails to give complete solution should be omitted :md only tlilutioli:: that give homogeneous solutions ~1iouIdbe t,wted further. If the :tpproxini:ite concentration is
known, it is sufirient to test only t h e dilution th:tt applies to that concentratioti. Table I gives the proper dilution for each concentration range of benzyl alcohol and the factor by n.hich the reading from the graph should be multiplied in order to determine the concentration of benzyl alcohol in the original sample. After dilut,ing the sample t o obtain ni:iximuni sensitivity, transfer 5 nil. to a colorimeter t,ulie and continue as in t,he calibration of the photelometer. To obtain the Iienzyl alcohol concentration of t h e sample, multiply the wading from the graph t)y the factor lihted in Table I. If a suhstnnce is present whose aqueous solution darkens on heating with sulfuric acid, it may he po,wible to ovei'(*onie the interference hy selective extraction h d o r e aixtlyais. If B su1)st:tnce is present v-hose sulfate is inpoluhle, such :ts lead or Imiuni, t h r sample should he h a t e d n.ith :i reagent, poasibl~. sodium carhonate, to remove the interfering sul)st:tnce before sulfuric acid is added. PRECIS103 4 Y D S E N S I T I V I T Y
Tai)le I1 slion-s the prerisioii obtained n licn threcx mixes of benzyl alcohol were made up at enrh c~oiic.fiiti.:itioiiand mal! was run in quadruplicate, giving it total of t\vc.lvr tests at each concentration. The average per c*ctit stnticl:ird deviation \vas found to he :ibout &37, for wnwntrtttionP of h n z y l alcohol above 0.10 nil. per liter. If thcx cwiirt~iitratiotiis less than 0.10 ml. of h ~ ~ n alcohol z ~ l per liter of solution. t h c ~preckion dwreases rapidly \\-itti tlrop in concentrittioii. LITER.4TLRE CITE11
(1) Callaway, Joseph. Jr., a n d Reziiek. Solomon, .J. As.suc. Ofic.d g r . Chemists, 1 6 , 285-9 (1933). ( 2 ) Vohler, H., a n d Himmerle, IT.,,Uitf. Lehozsm. H,yg., 30,284-320 (1939 j .
( 3 ) JIohler. H.. and I-Iiirnmei,le, IT,,Z . ~ M Z . C h e m , , 122, 202-9 ( I 941 ). (4) Sal-es, Y. R., Hnlr. Chim. A c t n . 27, 942%; (1944). ( 5 ) Rees, H. I*.,a n d Anderson, D. H., -L~-.r., 21, YS!j-O1 (1949 I , (6) Shriner, 1%.L., a n d Rcrger. Arthur, J . O r g . C h e m . , 6, 3306-18 (1941 ). R w m \ - b : c f n i re\-iew Augilzt 2 , Ig51.
Acrei7ted .January 5 , 1952.
Titration of Very Weak Acids with Lithium Aluminum Amides T 4 K E K C EIIGL-CHI, ,JESLS4 COSCEE4,
iVD
ROY KC'K-I\.IO'I'O
Srhool of Pharmacy, I'nirersity of Wisronsin, Madison, E i a .
'R
ICCESTLY it vias shown that r.strenlely weak acids such as :rlcohols, amines, and certain hydrocarbons could be detrrniined quantitative12 with lithium aluniinuni hydride as the reagent, (2-6). Iliguchi and Zuck ( 3 ) were able to titrate a n u m t w of these compounds with visual indicators using the h!.tlride as the base. Prrvious direct titrations of weak acids have h e m limited mainly to relatively strong acids such as phenols, rarhosylir acids, et(-., the strongest hase generally used * being of the alkoxide type ( 1 ) . Litliiuni :rluminum h)-dritie is not Liltogether satisfactory as the basic. reagent, because it may reart, Lvith reducible functional g r o u p such as aldehydr, ketone, ester, and other nonacidic groul)ings that may tie contained within the molecules. The prewnt investigation WNL: initiated to test the feasibility of using lithium aluminum aniides as hases in the titratfon of extremely w t ~ t h :ncitls. It was hoped that such h ~ s e sn-ould he capahlr of
reacting with hydroxyl groups hut ~ o u l dnot affectt such functional group^ a' carbonyl and cstei,, s.hic*h ai'e i,etlured hy the hydride. THEORY
For quantitative determination the conjugate arid of the base used for the titration must be considerahly weaker acid than the acid being titrated. Thus it is possible to titlate niost mineral acids and c:irbosj-lir acids with hydi,osyl ions because water is a much ~ v e a k ~acid r than thew. I t is possihlr to titrate phenols with alkoxides because alcohols are much weaker acids. For titration of alcohols ant1 other acids of si~nilarstrength, a salt of an arid much weaker than alcohol is necessary. As most amines fall into this class, it is reasona1)le to expect that the nic~tallic. amidc. rould Iw used to titrate su1.h \veal< acids.
686
ANALYTICAL CHEMISTRY
Lithium aluminum hydride, which was prebiously de\eloped as a reagent for titration of t e r y weak acids, is not altogether satisfactory as the basic reagent because of its strong reducing action; aldehydes, Letones, esters, and man? other nonacidic compounds undergo reaction. The present investigation was initiated to test the feasibility of using lithium aluminum amides as bases in the titration of these extremely weak acids. 1,ithium aluminun~amides of di-n-but?laniine, n-but: lamine, piperidine, and
The "neutralization" of a weak acid, alcohol, I)? a metallic aniitl~can be represented by the equat,ion
R*S.\I
+ HOR e 110R + R i S f i
The equilibrium should be far to the right, as the proton affinity of nitrogen kernels is generally much greater than that of oxygen. The present invest'igation has been limited to the study of lithium aluniinuiii amides, principally because of the ease of preparation of these compounds and their ready solubility in organic solvents. Neither the structure nor the nature of the chemical bonds in these lithium aluminum amides has been clearly established. I t is probable, however, that the nitrogenaluminum bonds in these cornpounds partake n ~ u c hniore of a covalent character than those formed betvieen nitrogen and alkali metals. Exact evaluation of this factor on the comparative basic strength of alkali metal amides and lithium aluminum amide is admittedly beyond the wope of t.he present investigators. I t seems reasonable, however, to assume that t'he loss in ionic character of these bonds Tvould tend to reduce the protophilic strength of amides. The lithium aluminuni amides, in other words, are probably not as strong bnses as the corresponding alkali metal amitles. CHOICE O F A l l I h E S A h D IADICATOR5
A number of different lithium aluminum amides n ere prepared by reaction of the amines and lithium aluminum hydride in tetrahydrofuran. I n each case the mole ratio of the reactants wa5 adjusted so that onlv one hydrogen - was displaced from each amine molecule. Replacement of the second hydrogen usually resulted in precipitation of the reaction product, probably because of formation of polymeric structures. Of all the amides prrpared, only that formed from ammonia proved to be insoluble in tetrahydrofuran or other ethrrs. .4niides of n-butylanine, di-n-butylamine, aniline, pyrrolidine, and piperidine nere very soluble in these same solvents. The aniline salt was not suitable for titration Xvork because of its greatei tendency to darken on standing. The remaining amides -worked about equally 11ell. The range of indicator types available foi titration with lithium aluminum amides is conY 10. siderably greater than those for use n ith lithium 4 aluminum hydride, as the reductive tendencies m of the amides are much less than those of the 0 hydride, permitting use of indicators that con0 05' Y tain reducible groups. Anthraquinone, nitroaniline, Michler's ketone, and phenothiazine M e1 e among the many compounds tried. None, however, 0 gave as sharp a change as p-phenylaminoazobenzen~, the indicator mhich was developed for the hydiide titrations. Further search would probably uncover Figure 1. a number of better and more selective indicators.
pyrrolidine were found to be suitable basic reagents for titration of alcohols and phenols. With excess base, compounds like acetophenone and benzophenone behave like monobasic acids. Benzoic acid in great excess appeared to take up 3 equivalents of base per mole. Lithium aluminum amides can he substituted for lithium aluminum hydride for titration of alcohols and phenols. The present investigation is an extension of titrations in nonaqueous media into an extremel: alkaline region.
EXPERI\IERT 4L
Apparatus. The equipment employed was essentially the same as that described by Higuchi and Zuck (3). Reagents. The preparation of the reagents used, except that of the amide solution, has been described. The latter solution was prepared as follows: A solution of lithium aluminum hydride in tetrahydrofuran of about 1 A' strength was prepared according to the method of Higuchi and Zuck. This solution was analyzed for its hydride content with a standard alcohol solution, using p-phenylaminoazobenzene aa the indicator. A slight excess of a dry amine on a mole for equivalent of hydride present basis was added to the solution. This was then stored in a light-tight bottle under nitrogen. Procedure. The procedure developed by Higuchi and Zuck was followed closely, all titrations being conducted under nitrogen and only oven-dried glassware being used. A 4the ~ reaction time did not appear to be critical, the titrations were gerieially carried out nearly immediately after mixing the saniples with amide. RESULTS AUD DISCUSSION
The results of application of the titrimetric method to a numl v r of alcohols and phenols are phon-n in Table I. The end points \yere moderately sharp in every case. I t is evident from the data that the nlet,hod is applicable to titration of these hydroxyl eompounds. In Tahle I1 are shown results of t,it,rationof two ketones, vvhich seem to behave partly as monobasic acids. This can be rationalized for acetophenone on assumption of enolization, but the explanation is not applicable to hiBnzophenone. It is possible
IL
0 5
10 MlLLlEP
I5
OF
20 EXCESS
25
3 0
35
40
BASE
Effect of Excess Amide Concentration on Amount of Base Reacting with Benzoic Acid
687
V O L U M E 2 4 , N O . 4, A P R I L 1 9 5 2 Tahle I .
Titration of ilcohols and Phepols with Lithium i l iini i n ii m Pi peridide"
Sa1np.r Ethyl alcohol
Eqiiirxli~ntof Amide C o n w m e d / l l o l e of Sample 1 008 0.997 1 027 1.010 1.020 1 089 1 034 1 038 1 040
n - A n i > - i alcoliol
0.987 0 988 0.9Y:! 0.971 1.02P
I 025 1.01,5 1 028
u-Propyl alcohol
Table 1 I . Titration of Acetophenone and Benzophenone w - i t h Lithium .iluminum n-Dibutyl Amide" Sample Acetophenone
Equivalent of .Xniicle Consiimed/3Iole o i Sample 1 01 1.01
1.02 Brnzophrnone
ther studies. I t is possible that the amide solution ma!- corit:iiii a number of reactive species. Results of titration of t'wo esters and an anid anhytlriile :irtb given in Table 111. The monobasic acid nat,ure of ester linlingw is illustrated by the behavior of benzyl benzoate, the acidit?- proliably residing with the C 4 bond. I n the case of phenyl salic)-Iate, the ester linkage again took up one equivalence of Iiasc. the second equivalence being due to the acidir hydroxyl. On the basis of the above reasoning, the dihasicait>-of phthalic. nnhytiritle is not unespected, as it contains two C=O groups. ii few nitrogm-containing compounds, as shown in Tahlc 11-, n-rre also investigated. Carbazole, which contains a rather acidic hydrogen, and acetanilide, which has a C=O bond, behaved strictll- as monobasic acids. Phthalimide appeared to take u p varying aniouiits of base n-ith possible maximum of three eqnivalence, two for t h r t x o C=O groups and one for the acidic hydrogen on the nitrogen. The hcshavior of acetamide is, a t the moment, unaccountatile. Lithium aluniinurii amides, like the corresponding hyciritlc, hold considerable promise as anal>-tical reagents, because they permit routine titration of weakly acidic substances hitherto possible only on a research basis. Until the chemistry of t!ie reactions of the lithium aluminum amides with various functional types is further clarified, however, the method should be applied with caution t o compounds containing functional groups other than hydroxyl.
1.06,
7::;
I;ivcfoldescers of baaeb 1.06) 0.871' 0 .852 I,mz than twofold excev of haye 0.801j
1
'Table I\'. Titration of Some Nitrogen-Containing Compounds with Lithium Aluminum Pyrrolidide Sample .Icetanilide
Equivalent of Amide Consumed/lIoie of Sample 1.03 1.03 1.04
Acetamide
l a b l e 111. Titration of Benzyl Benzoate, Phenyl Salic>-late,and Phthalic Anhydride with Lithium .4luminuni Amides Sample
hinide Used
Benzyl benioare
n-Dihutyl amide
Phenyl salic! late
Pyrrolididr
Phthalic anh)-dride
Pyrrolidide
Equivalent of Aniide Conauined/ Mole of Sample 1.05 0.99 1.08 2.05 2.04 2.06 1.98 2.00 2.01
that the ketones may behave as Lewis acid. Further investiyations are needed to clarify the problem. Titrat.ion of benzoic acid indicated that the proportion of the amide taken up was a function of the excess of the base present. This is illustrated in Figure 1, where the equivalents of the base taken up per mole of benzoic acid are shown as a function of the excess haye. The exact' esplmation of this behavior requires fur-
Carbazole P h t halimide
1.23 1.24 1.29 0.992 0.984 1.008 2.19 2.35 2.48
LITERATURE CITED (1) Fritz, J. S.. and Lisicki, N. hl., ANAL.CHEY.,23, 589 ( l ! X l j . (2) Higuchi, T., Lintner, C. J., and Schleif, R. H., Science. 1 1 1 , 63 (1950). (3) Higuchi, T., and Zuck, D. A., J . Am. Chem. SOL,73, 2Gi6 (1951). (4) Hochestein, F. 9., Ibid., 71, 305 (1949). ( 5 ) Krynitsky, J. A., Johnson, J. E., and Carhart, H. W., Ihid., 70, 486 (1948). (6) Lintner, C. J.,Schleif, R. H., and Higuchi, Takeru, ASAL.CHEM., 22,534 (1950). RECEIVED for review .4irgirst 29, 1931. llccepted December 2 1 , 1951. Presented a t the XIIth International Congress of Pure and Applied C h e r n k t r s , New York, September 10 t o 13, 19.51. This project v a s slipported i n part b y the Research Corninittee of the Graduate School from funds supplied by the Wisconsin Alumni Rerearch Fovndation.
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