(6) Clayton, R. hI,, Xature 174, 1059 (1954). (7) R. R. A,. Fiset. M. L.. ~, Coombs. ‘ Brit. J . Exptl. Pathol. 35,472 (1954). (8) Coombs, R. R. A., Howard, A. N., Mynors, L. S., Ibid., 34,525 (1953). (9) Coombs, R. R. A., Howard, A. N., Wild, F., Ibid., 33, 390 (1952). (10) Coons, A. H., “General Cytochemical Methods,” Vol. 1, p. 399, ed. by J. F. Danielli, hcademic Press, New York, 1958. (11) Coons, -4.H., Kaplan, H., J . Exptl. iMed. 91, 1 (1950). (12) Coons, 8.H., Leduc, E. H., Connolly J. LI., Ibzd., 102, 49 (1958). (13) Coone, -4.H., Leduc, E. H., Kaplan, hI. H., Ibid., 93, 173 (1951). (14) Coons, -4.H., Snyder, J. C., Cheever, F. S., Murray, E. S., Ibid., 91,31 (1950). (15) Edwards, P. R., Bruner, D. >Tr., Univ. of Kentucky Agr. Expt. Station, Circ. 54 (1942). (16) Elek, S. D., Brit. J . Erptl. Pathol. 30, 484 119491. (17) Eiek, S . D., Levy, E., Ibid., 31, 358
(27) Kaplan, LISH., J . Immunol. 80, 254 (1958). (28) Kaplan, M. H., Coons, A. H., Deane, H. W., J . Exptl. Med. 91, 15 (1950). (29) Korngold, L., J. Immunol. 77, 119 (1956). (30) Korngold, L., Lipari, R., Cancer 9, 183 (1956). (31) Landsteiner, K., “Specificity of Serological Reactions,” rev. ed., Harvard Univ. Press, Cambridge, Mass., 1945. (32) Landy, AI., Am. J . Public Health 44, 1059 (1954). (33) Lapresle, C., Bull. SOC. chim. biol. 37, 969 (1955); Ann, inst. Pasteur 89, 654 f 1955). (34) Lapresie, C., Durieux, J., Ibid., 94,38 (1957). (35) McKenna, J. M., Proc. SOC.Exptl. Biol. X e d . 95, 519 (1957). (36) Marshall, J. D., Eveland, W. C., Smith, C. IT.,Ibid., 98, 898 (1958). (37) Marshall, J. M.,Exptl. Cell Research 6,240 (1954). (38) Marshall, J. AI., J . Exptl. X e d . 94,
(18) Fdou, G. J., Finch, S. C., Detre, K. D., J . Immunol. 80,324 (1958). 119) Gordon. M. A.. J . Invest. Dermatitis 31, 123 (1958). ‘ (20) Grabar, P., Atti V I congress0 intern. rnicrobiol., Roma 2, 169 (1953). (21) Grabar, P., Courcon, J., Ilberg, L. T., Loutit, J. F., Merrill, J. P., Compt. rend. 245,950 (1957). (22) Grabar, P., Sowinski, N. W., Generaux, B. D., Nature 178,430 (1956). (23). Grabar, P., Williams, C. A., Biochem. Bzovhvs. Acta 17,67 (1955). (24) Hiramoto, R.; Engel, K., Pressman, D., Proc. SOC.Exptl. Biol. Med. 97, 611 (1958). (25) Jennings, R. K., Malone, F., J . Immunol. 72. 411 (1954). (26) Kabat, E. A:, Mayer, 11. M., ’ ‘Experimental Immunochemistry,” Charles C Thomas, Springfield, Ill., 1948.
(39) Mood M. D., Ellis, E. C., Updyke, E. I,., J. 8acteriol. 75, 553 (1958). (40) Munoz, J., “Serological Approaches to Studies of Protein Structure and Metabolism,” p. 55, Rutgers University Press, New Brunswick, h’. J., 1954. (41) Munoz, J., Becker, E. L., J . Immunol. . 65,47 (1950): (42) Neff, J. C., Becker, E. L., Ibid, 78, 5 (1957). (43) Neter, E., Bacteriol. Rev. 20, 166 (1956). (44) Noyes, W. F., Mellors, R. C., J . Exvtl. Med. 106. 555 (1957). (45) Oakley, C. ‘L., Fulthorpe, A. J., J . Path. Bacteriol. 65, 49 (1953). (46) Osler, -4.S., Hawrisiak, &I. M., Ovary, Z., Siqueira, M., Bier, 0. G.; J . E r d . M e d . 106.811 f 1957). (47) O&hterlony, O:, Acta Paihol. Microbid. Scand 25, 187 (1948). (48) Ouchterlony, O., Arkiv Kemi,
(\ 1- -w,n) - -
i ~ n ~ i \
01
Yl
(1JC)l).
Xineral. Geol. 26B, No. 7, 43 (1949). (49) Ouchterlony, O., Progr Allergy 5, 1 f1958). (50) Oudin, J., Discussions Faraday SOC. 18,351 (1954). (51) Oudin, J., “Methods in Medical Research,” Vol. 5, p. 335, Year Book Publishers, Chicago, 1952. (52) Ovary, Z., Intern. Arch. Allergy and Applied Immunol. 3,293 (1952). (53) Pope, C. G., Stevens, M. F., Brit. J . Exptl. Pathol. 39, 150 (1958). (54) Poulik, AI. D., Can. J . Mea. Sci. 30,417 (1952). (55) Poulik. >I. D., Suture 177, 982
1
(1954). (59) Schuchardt, L. F., Munoz, J., Verwey, T I T . F., Ibzd., 80, 237 (1958). (60) Sheldon, IT. H , Proc. SOC. Erptl. Biol. Med 84, 165 (1953). (61) Silverstein, A. M., J . Histochem. 5, 94 (1957). (62) Stevens, K. RI., McKenna, J. M., J . Erptl. Med. 107,537 (1958). (63) Surgalla, RI. J., Bergdoll, M. S . Dack, G. M., J . Immunol. 69, 357 (1952). (64) Telfer, W. H., Killiams, c. M., J . Gen. Physiol. 36,389 (1953). (65) Uriel, J , Grabar, P., Ann. inst. Pasteur 90,427 (1956) (66) Uriel. J., Scheidegger, J. J., bull. SOC. chim. biol. 37, 165 (1955). (67) Williams, C. A., Grabar, P., J Immwnol. 74, 158 (1955). (68) Wilson, G. S., Miles, A. “Topley and Wilson’s Principles of Bacteriology and Immunity,” 4th ed., Williams and Wilkins, Baltimore, Md., 1955. (69) Wodehouse, R. P., Ann. Allergy 14, 121 (1956). for review January 12, 1959. RECEIVED hccepted April 6, 1959.
END OF SYMPOSIUM
Nonaqueous Titration of Organic Acids, Anhydrides, Acyl Halides, Strong Inorganic Acids, and Reactive Alkyl Halides in Various Mixtures ABRAHAM PATCHORNIK and SARAH EHRLICH ROGOZlNSKl Department of Biophysics, The Weizmann Institute o f Science, Rehovoth, Israel
( A method is presented for the determination of milligram quantities of the individual components of complex mixtures of organic and inorganic acids, acyl halides, anhydrides, and alkyl halides. The accuracy of individual determinations compares favorably with that obtained b y simple titrations in pure solutions. All titrations are carried out in nonaqueous media, using inexpensive readily available apparatus. Three standard solutions of bases are used with a single
indicator. The method has proved valuable for routine as well as research purposes.
T
increasing interest in acid-base titrations in nonaqueous media is evidenced by the large number of papers published in recent years. An excellent review of this literature has appeared recently ( 7 ) . Procedures developed during work on the synthesis of substituted N-carboxyHE
histidine anhydrides (6) permit the selective determination of hydrochloric acid, acid chlorides, acid anhydrides, organic acids, and reactive alkyl halides such as benzyl chloride (CYchlorotoluene) in mixtures. They have proved useful in determining the purity of acid chlorides and acid anhydrides in general. When pure alkyl halides were analyzed, results were comparable to those obtained by the classical Carius method. All titrations are carried out in anVOL. 31, NO. 6, JUNE 1959
@
985
Table I.
Titration of Hydrogen Chloride in Dioxane Found, Mg.
NaOCH3 ( B U ) ~ K 63.4 62.8 63.4 62.0 63.4 62.0 53.5 53.6 52.8 53.6 53.1 53.3
AgNOa 63.5 63.5 63.4 53.4 53.4 53.3
Triton B 64.5 64.4 64.5 52.8 53.9 53.2
Table II. Titration of Acid Chlorides with Sodium Methoxide after Reaction with Alcohol
Compound Benzoyl chloride Oxalvl chloride Fumaroyl chloride p-Nitrobenzoyl chloride
Taken,
Found, Recovery,
Mg.
RIg.
yo
97.24 50.88 50.88
97.0 50.8 50.9
99.8 99.8 100.0
23.10 23.10
23.3 23.2
100.9 100.4
58.44 58.44
56.7 56.7
97.0 97.0
101.64 88.23
99 2 87.2
97.6 98.8
Table 111.
hydrous media (usually methanol, ethyl alcohol, dioxane, or pyridine) using thymol blue as the indicator. This indicator is red in the presence of strong acids; yellow with weak organic acids, their chlorides and anhydrides, amines and their salts; and blue in the presence of strong bases such as sodium methoxide or Triton I3 (trimethylbenzylammonium hydroxide). DISCUSSION
The over-all procedure may be illustrated by considering a hypothetical mixture having the composition: HCI RCOCl (RC0)ZO RCOOH RCI. Molar ratios of the components of such a mixture change because of interaction of some of the components. Three standard solutions of basesnamely, tributylamine in dioxane, sodium methoxide in methanol-benzene, and Triton B in pyridine-are used as follows: One aliquot of the mixture is titrated with tributylamine to determine free strong acid. Then alcohol is added to form hydrochloric acid by reaction with the acid chlorides. The liberated acid is titrated with either sodium methoxide or tributylamine. The total acidity is determined on a
+
+
+
+
Titration of Benzoyl Chloride by Different Methods
Control Detn., Mg. (Volhard)d 11.9 11.7 11.7
Found, Mg. B~3iYb
Taken, Mg. 11.71
XaOCHp Triton Bc 11 so 11.8 11.8 11.80 11.9 11.7 11 80 11.7 11 8 11 73 11.8 11 9 11.8 a Procedure I1 for determination of acyl chlorides. * Same procedure as for titration with sodium methoxide. Titration performed after addition of 0.2 ml. of water and boiling sample a few minutes. RIilligrams calculated assuming 1 mole of acyl chloride requires 2 moles of Triton B. Volhard determination performed after boiling sample with mater. Table IV.
Compound Benzyl chloride (a-chlorotoluene)
Titration of Alkyl Halides (Procedure IV) Weight, Found,
Dibromoethylene +Butyl iodide (1-iodobutane) p-Nitrobenzyl chloride
RIg.
Rig.
36.27 66.43 61.01 66.43 108.47 83.70 20.72 115.90 47.68
35.8 658 60.5 66.2 107.2 83.6 20.5 115.7 47.7 32.3 52.2 40.5 32.0 43.1 56.8 56.8 101.3 101.3 300,5 168.8
31.85
Phenacyl bromide (2-bromoacetophenone) Bromo diethyl acetylurea Isoamyliodide (1-iodo-3-methylbutane) Cetyl bromide ( 1-bromohexadecane) Amyl bromide (1-bromopentane)
986
ANALYTICAL CHEMISTRY
52.58 40.46 32.14 42.42 57.27 57.27 108.52 108.52 304.32 171.49
Recovery, Reaction 7% Time 98.7 6 99.1 15 90.2 20 99.7 30 98.8 25 99.9 30 98.8 5 99.8 20 100.0 25 101.4 25 99.3 15 100.1 25 99.6 20 101.6 20 99.2 25 99.2 30 93.4 30 93.4 60 98.7 20 98.4 30
second aliquot by titration with sodium methoxide, in which case acid chlorides and anhydrides behave as monobasic acids. The neutralized solution is boiled with aniline, after which the titration is continued to determine the anilinium chloride formed from reactive alkyl chlorides. At this point analytical values have been established for HC1, RCOCl, and RX, and for the sum of (RC0)20 and RCOOH. A third aliquot is titrated with Triton B for total acidity. The fact that acyl halides and anhydrides behave as dibasic acids toward this reagent permits the calculation of the amount of anhydride and free carboxylic acid present. The characteristic behavior of each of the components of the mixture described above toward the indicator and the standard bases is described as follows: Strong acids, such as hydrochloric, give a faint red in dioxane in the presence of thymol blue. On the addition of alcohol the red deepens. The acid can be titrated with equal accuracy using tributylamine, sodium methoxide, or Triton B (Table I). When tributylamine in dioxane is added to a solution of pure hydrochloric acid in dioxane in the presence of thymol blue, the color of the solution remains faintly red until one equivalent has been added. At this point the color changes sharply to bright yellow. An excess of the titrant causes no further change in color of the indicator. None of the other four components of the mixture described above interferes in this titration. HC1 (Bu)3N 4 (Bu)JSHCl
+
When the solution of pure hydrochloric acid is titrated with sodium methoxide in benzene-methanol, the red color first deepens because of the methanol and finally changes to yellow at the end point. Addition of a very small excess of base causes a second change to deep blue. The amount of titrant consumed between the two end points is a measure of weakly acidic impurities present in the solvents. On titration of this acid solution with Triton B in pyridine, the color turns yellow immediately because of the presence of excess pyridine and changes to blue after the addition of one equivalent of the quaternary base. HC1 CsHsN CbHaNHCl CsHaHCl (CHa)3NC?H,OH+ CBHbN ( C H ~ B N C ~ H &Hz0 ~
+ +
+
+
+
Weak acids such as carboxylic acids and alkyl ammonium salts give a yellow color with thymol blue in dioxane. They consume one equivalent of sodium methoxide or Triton B to the blue end point. Acid chlorides, when pure, give a faint pink with thymol blue in dioxane solu-
tion. Addition of a trace of tributylamine changes the solution to bright yellow except with extremely reactive acid chlorides such as chloroacetyl chloride and oxalyl chloride, where the addition of tributylamine fails to change the red color of the indicator. Thus strong acids cannot be differentiated from such acyl halides by this procedure. Acid chlorides react with ethyl alcohol to yield one equivalent each of hydrogen chloride and the corresponding ethyl ester. RCOCl
+ EtOH
RCOOEt
4
+ HC1
Titration of the liberated acid with tributylamine or sodium methoxide gives the amount of acid chloride present (Table 11). When acid chloride solutions are contaminated with free strong acids the latter must be titrated with tributylamine prior to the addition of alcohol. Alkyl halides, weak acids, and acid anhydrides do not interfere. Acid chlorides behave as monobasic acids toward sodium methoxide in organic solvents. RCOCl
+ NaOCHa
+
RCOOCHs
+ NaCl
However, in pyridine solution, acid chlorides are titrated as dibasic acids with Triton B, if no alcohols are present (Table 111).
+
RCOCl 2( CHI)BNC~H~OH + RCOON(CH~)~C.IH?"+'(CH~)~NC~H~CI Hz0
+
Alkyl halides react with aniline to yield the corresponding phenyl alkyl amines and anilinium halide. The anilinium salt in the aniline solution can RC1
+ BCIH~NH,
+
+
CBH~NH~CI CsHsNHR
be titrated with sodium methoxide to the yellow-blue end point of thymol blue. Even where the reaction of the halide with aniline is more complex resulting in elimination of hydrogen halide from the molecule or leading to different substitution products, all the halogen is finally converted into anilinium halide and can be subsequently titrated with sodium methoxide. Because alkyl halides react extremely slowly with sodium methoxide a t room temperature, they may be determined in the presence of all other components of the mixture after neutralization of the latter with sodium methoxide (Table IV) Anhydrides of organic acids impart a yellow color to thymol blue and are titrated as monobasic acids with sodium methoxide to the blue end point of the indicator.
.
(RC0)rO
+ NaOCH3 -., RCOOCH, + RCOONa
On titration with Triton B, 2 moles of
V.
Titration of Anhydrides (Procedure 111) Sodium Methoxide Triton Ba AnWeight, Found, Recovery, Weight, Found, Recovery, hydride mg. mg. % mg. mg. % Acetic 13.27 13.42 101.2 13.27 13.6 102.7 13.27 13.42 101.2 13.27 13.5 101.9 Succinic 76.42 75.80 99.3 30.93 30.4 98.4 16.16 15.9 98.4 16.6 102.2 Maleic 22.17 21.90 98.8 16.27 50,30 101.6 Benzoic 51.00 101.4 32.86 33.4 25.9 100.5 25.76 25.7 99.7 25.76 Phthalic 49.74 50.20 101.0 16.25 16.3 100.3 16.25 16 3 100.3 16.25 16.1 99.2 Butyric 46.99 46.90 99.8 29,04 28.6 98.6 29.04 28.1 96.8 29.04 28.3 97.5 Propionic 23.34 23.40 100.2 22.93 22.9 100.0 63.11 62.60 99.2 22.93 22.6 88.7 75.59 75.20 99.5 20.61 20.8 101.0 ,. Amount of anhydride calculated assuming 1 mole of anhydride requires 2 moles of Triton B. Table
the titrant are absorbed, provided no alcohols are present, (RC0)sO
+ 2(CHa)aNC?H?OH
-C
2RCOO(CH3)aNCyH7
Table Acetic
VI. Acid
+ H20
The difference in behavior of anhydrides toward these two bases permits their determination in the presence of other acidic components, including acyl halides, as shown in the procedure below (Tables V and VI).
Acetic acid Acetic anhvdride Acetic acid Acetic anhydride Acetic acid Acetic anhydride
Titration of Mixtures of and Acetic Anhydride
Weight, Found, Recovery, Mg. hfg. % 15.50 15.5 100.0 13.27 15.50
13.4 15.3
100.9 98 7
13.27 15 50
13.3 15.4
100.3 09.3
13.27
13.5
101.7
EXPERIMENTAL
Apparatus. Automatic burets ( 5 to 10 ml.) equipped with 1-liter reservoirs and tubes to protect contents from atmospheric carbon dioxide and R-ater. Reagents. Dioxane, reagent grade, purified according to Vogel (8). Pyridine, reagent grade, distilled a t atmospheric pressure. Aniline, reagent grade, freshly distilled a t atmospheric pressure. All solvents must be tested for acidity. When 1 drop of 0.1N sodium methoxide fails to produce a distinct blue color with thymol blue in 5 ml. of the solvent, purification must be repeated. Thymol blue, 0.2% in dioxane. Tributylamine, 0.1N. Commercial grade tributylamine was dried for 24 hours over potassium hydroxide pellets, refluxed for 4 hours over metallic sodium, and distilled a t atmospheric pressure, and the fraction distilling between 213 and 214' C. was collected. A 0.1N solution was prepared immediately by dissolving 23.8 ml. in dioxane to make 1 liter. This was transferred to the automatic buret and standardized against 0.1N hydrochloric acid in dioxane. Sodium methoxide (2), 0.1N. Approximately 2.3 grams of clean metallic sodium was dissolved in 200
ml. of absolute methanol in the buret reservoir. Then 800 ml. of benzene was added and the base was standardized against pure benzoic acid in absolute ethyl alcohol to the blue end point of thymol blue. Trimethylbenzylammonium hvdroxide (Triton B), 0.1N. Fifty milliliters of commercial grade 30% Triton B in water was dissolved in 950 ml. of pyridine. If not clear, water was added in small aliquots until solution was achieved. (In no case was more than 4 to 5% of water needed.) The solution was standardized against pure benzoic acid in alcohol to the blue end point of thymol blue. PROCEDURES
All titrations were performed in 25ml. Erlenmeyer flasks with the tip of the buret dipping into the solutions, which were magnetically stirred with glasscoated iron wires. The contents of the flasks were protected from atmospheric carbon dioxide during titration by a vented rubber stopper permanently mounted on the buret tip. One to 3 drops of indicator solution was added for each titration. Burets of 2-, 5-, or 10-ml. capacity were used for 0.02 to 1 mmole. When amounts to be determined were less than 0.02 mmole, VOL. 31, NO. 6, JUNE 1959
987
Table VII.
Titration
of Mixtures of Strong Acid, Acid Chloride, and Weak Acid
(Procedure 11) Hydrochloric Acid Benzoyl Chloride Benzoic Acid Taken, Found, Recovery, Taken, Found, Recovery, Taken, Found, Recovery, mg. mg. % mg. mg. 76 mg. mg. % 105.3 100.3 35.39 35.5 100.3 40.60 39.9 98.3 105.0 105.3 100.3 35.5 100.3 39.9 98.3 105.3 36.5 103.1 101 5 105 0 104 3 99 3 41 2 36.3 102.6 99 8 105 0 105 0 100 0 40 5 36.4 102.9 105 0 104 7 99 7 41 6 102 4
titrations were performed with a syringetype ultramicroburet (Agla, Burroughs Wellcome 8: Co. London, 0.5-ml. capacity) in a test tube. I. Titration of Strong Acids. The solution of strong acid, in a neutral anhydrous solvent, is titrated with 0.1N tributylamine (yellow end point). This base must be used when the strong acid is in a mixture with acyl halides. However, when acyl halides are known to be absent, the strong acid should be titrated with 0.1.V sodium methoxide (yellow end point). If pure, strong acids may also be titrated with Triton B (blue end point). Table I compares results obtained with these three bases. 11. Titration of Mixtures of Strong Acids, Acyl Chlorides, and Weak Acids. A 2- to 5-ml. aliquot of a dioxane solution of the sample is placed in a 25-ml. Erlenmeyer flask, 2 drops of indicator is added, and the strong acid titrated with 0.1N tributylamine as in Procedure I (TJ. Ethyl alcohol ( 5 ml.) is added and the mixture heated in a water bath a t 80' C. for 5 t o 10 minutes. The solution is then titrated with 0.1N sodium methoxide to the yellow end point, heating and titration being repeated until the yellow does not change to red upon further heating (Tz).An equal aliquot is titrated with 0.l.V sodium methoxide to the blue end point (T3). Calculation: I Meq. strong acid = TI X normality of tributylamine (XI,) Meq. acyl halide = T S X normality of sodium methoxide ( N I ) Meq. total acidity = Ta X iV2 Meq. weak acid = Ts X N? - (TIN, T2Nd 111. Titration of Mixtures of Acid Anhydrides and Weak Acids. A 2ml. aliquot of a dioxane solution of the sample is placed in a n Erlenmeyer flask. Pyridine (1 ml.) and water (0.1 ml.) are added and the mixture is heated on a steam bath for 1 t o 10 minutes depending on the reactivity of the anhydride. Pyridine (10 ml.) is added and the solution titrated with 0.1N Triton B to the blue end point (TJ. (Normality of Triton B = N 3 ) . An equal aliquot is titrated with 0.1N sodium methoxide to the blue end point
+
(T3).
Calculation: Meq. acid anhydride
(TGV~- T3;Vz)
Meq. weak acid = 2Tah'z
=
- TJVI
IV. Titration of Reactive Alkyl Halides. The sample and 5 ml. of 988
ANALYTICAL CHEMISTRY
aniline are placed in a 25-ml. Erlenmeyer flask equipped with an air condenser. The mixture is refluxed for 3 to 30 minutes (depending on reactivity), cooled, and diluted with aniline (10 ml,). It is then titrated with 0.1N sodium methoside to the green-blue end point, using 3 drops of the indicator solution ( T J . Calculation: X NP
Meq. alkyl halide
=
7'5
RESULTS
KO procedure is given for mixtures containing all five of the components under consideration. While it is possible, as indicated above, t o carry out such a procedure, the results would have little practical meaning because of internal reactions which might seriously affect the accuracy of individual determinations. For example, the following equilibrium is known to exist. HC1
+ (RC0)zO 2 RCOOH + RCOCl
However, in the case of mixtures of hydrochloric acid, benzoyl chloride, and benzoic acid this reaction is slow and it was possible to obtain reasonably accurate results when such a mixture was analyzed (Table VII). Strong acids give a faint pink with the indicator in dioxane solutions. When titrations of mixtures are to be made as in Procedure 11, this color may deepen on the addition of polar solvents such as nitromethane, acetonitrile, or chloroform. I n Procedure 11,two aliquots are used rather than one to avoid errors from the esterification of the weak acid which occurs when it is heated in the presence of strong acid and alcohol. This makes it often impossible to obtain accurate values for the weak acid unless a second aliquot is used. When acyl halides are titrated with sodium methoxide in alcohol to the yellow end point, one equivalent of base is consumed. The addition of a very small excess of titrant immediately produces a blue color. When oxalyl chloride is so titrated, the yellow end point is obtained after the addition of two equivalents of base. However, further addition of titrant does not produce the blue end point until one additional equivalent has been added. Apparently, dimethyl oxalate reacts
with 1 mole of sodium methoxide to form the addition compound (1). CHaO,
,ONa C ' \OCHI
Aromatic halides such as chlorobenzene give no reaction with aniline when treated according to Procedure IV. Attempts to react it with benzylamine failed, even though it was heated in a bomb tube for 10 hours a t 260' C. More reactive aromatic halides such as p-chlorophenol react slowly but cannot be determined accurately by this procedure. When volatile alkyl halides are determined, the reaction with aniline is carried out in a sealed tube. Values obtained for alkyl halides in mixtures are sometimes low because of side reactions-for example, RCOONa
+ RX
-,RCOOR
+
NaS
In the titration of acyl halides or acid anhydrides with Triton B in pyridine solution a small amount of R-ater is added to accelerate complete hydrolysis, thus permitting greater speed and accuracy. Alcohol must be absent because its presence leads t o esterification and, hence, low values. The Triton B titration of acid anhydrides (Procedure 111) cannot be applied to X-carboxy-a-amino acid anhydrides (SCA), because these do not react stoichiometrically with Triton B. However S-carboxy-a-amino acid anhydrides may be determined in mixtures containing acid chlorides, weak and strong acids, and reactive alkyl halides. Such a mixture is commonly encountered in the conventional preparation of N-carboxy-a-amino acid anhydrides
($1. APPLICATIONS
These procedures were used extensively in the course of recent work on the synthesis of carbobenzoxy derivatives of histidine, imidazole, and benzimidazole (6). Typical applications of these techniques in connection with work on amino acids (4) include the following determinations: purity or concentration of acyl chlorides such as acetyl chloride, benzoyl chloride, carbobenzyloxy chloride; small amounts of phosgene in organic solvents; halogen content of chloro- or bromoacetyl-a-amino acid derivatives; bromine content of poly(vinyl bromide) and its copolymers with styrene; kinetics of the cyclization of carbobenzyloxy-a-amino acid chlorides to the corresponding N-carboxy-aamino acid anhydrides. The relative specificity and accuracy
in determining reactive alkyl halides have enabled alkoxy1 determinations on higher alkyl groups which are not amenable to the Zeisel method. For example, the benqloxy group of carbobenzyloxy compounds, benzyl ethers, and benzyl esters is quantitatively converted to the benzyl bromide which, after extraction with toluene, was determined by Procedure 11’. This continuing.
LITERATURE CITED
(1) Bender, M. L., J. Am. Chem. SOC. 75,5986 (1953). (2) Fritz, s. J.1 Lisicki, N. b1.1 A S A L . CHEM.23,589 (1951). (3) Katchalski, Ephraim, Advances i n Protein Chem. 6, 123 (1951). (4) Patchornik, Abraham, Ph.D. disserta-
tion, Hebrew University, Jerusalem, Israel, 1956. ( 5 ) Patchornik, Abraham, Berger, Arieh, Katchalski, Ephraim, J . Am. Chem. SOC.
79,5227 (1957). (6) Ibid.,. 79, 6416 (1967). (7) Riddick, J. A., ANAL.CHEX 30, 793 (1958). (8) Vogel, A. I., “Practical Organic Chemistry,” p. 175, Longmans, Green, London, 1948.
RECEIVED for review October 16, 1958. Accepted February 16, 1959. Work suported b research grant (PHS H-2279) From U. . National Institutes of Health, Public Health Service.
8
Conductometric Titration of Very Weak Acids FRANC0 GASLlNl and LUClO ZION NAHUM Research Division, Vita Mayer &
Co.,Milan, Italy
,The determination of weak and very weak acidic groups, particularly phenols, is carried out by dissolving the sample in an aqueous solution of a weak nitrogenous base (ammonia) present in excess and performing the titration with lithium hydroxide conductometrically. Mono- and polybasic acids (carboxylic and phenolic acids, enols, imides) have been titrated in this manner with accurate reproducible results. Intersection angles are obtained which are as satisfactory as those given by strong acids using the usual conductometric method. The method makes it possible to reveal and titrate very weak acidic groups which were not revealed by the usual conductometric method. A better differentiation i s obtained between different functions of polybasic acids.
T
conductometric titration of very weak acids with strong bases ( 1 ) is hindered by hydrolysis phenomena which, as the neutralization proceeds, cause a progressive nonlinear increase of the conductance of the solution, due to the increase of the hydroxyl ion concentration. Therefore, the weaker the acid to be titrated, the more inclined and shorter the straight line with which the titration curve begins. ilsa consequence, large angles are obtained between the neutralization line and the excess base line, which are a source of inaccuracy in the location of the end point; also the accurate extrapolation of the initial straight line becomes, as a result of its brevity, difficult or even impossible. hforeover, in the case of weak acids in very dilute solution it is often impossible to get quantitative results-e.g., nonquantitative ralues are obtained in the titration of phenols in water-alcohol solution with lithium hydroxide, by KoltHE
tion equilibrium of the acid, HA: HA B aABH+ If the weak base added is, for instance, ammonia, the conductometric titration with lithium hydroxide consists substantially in the neutralization not of H 3 0 + ions but of NH4+ ions. From this aspect the ammoniacal solution of an acid, HA, having as dissociation constant in n-ater K , = may be considered similar to the solution of a n acid having as a dissociation constant,
+
1 2 3 LCLLMC. O F T I T R A R T
4
5
UL.
Figure 1. Comparison of conductometric titrations of vanillin with 2.48 1 N lithium hydroxide in absence and presence of weak bases Total solution volume, 1 2 0 ml. 1 . 9 5 % ethyl alcohol = 13 mi., sample = 1.059 gram, N = 0 2. Triethylamine = 6 ml., sample = 1.021 gram, N = 30 3. 1 4 . 7 6 N ammonia = 6 ml., sample = 0.945 gram, N = 3 0 Ordinate values for each titration curve are shifted b y an amount N which i s indicated for each compound
hoff’s method, for concentrations lower than 0.04 equivalent per liter (3, 4). I n this work, reference is made to the results obtained in performing conductometric titrations with lithium hydroxide, of phenols and other weak and very weak acids previously dissolved in aqueous solutions of ammonia or other weak nitrogen bases present in excess. The preliminary addition of an excess of a weak base has a double function. First, the presence of the weak base, B, shifts towards the right the dissocia-
+
neglecting the variation of activity coefficients due to the presence of an excess of ammonia. The considerable increase in the dissociation of the acid, H-4, not only leads to a marked improvement in the angle a t the equivalence point, so that frequently n here it was obtuse in the absence of a weak base i t now becomes acute, but also makes it possible to titrate groups not previously titratable by standard conductometric methods, as, for instance, salicylic acid and thiolignin. Secondly, because aqueous solutions of weak bases are generally good solvents for acidic compounds, water-insoluble compounds can usually be titrated without having to use mater-organic solvent mixtures n-hose adverse influence on the angle a t the equivalence point is knovx. During the titration, before the equivalence point, the hydroxyl ions of the titrant combine n-ith the B H + cations originating from the n-eak base, B. The variation of the conductance depends, therefore, on the difference between the mobilities of the Li+ and B H + ions as \yell as on the variation of the concentration of the BH+ ion, which in turn deVOL. 31, NO. 6, JUNE 1 9 5 9
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