Conductometric Titration of Weak Acids in Dimethyl Sulfoxide Using Dimsylsodium Reagent L. Kenneth Hiller, Jr.’ Procter d Gamble Company, Miami Valley Laboratories, Cincinnati, Ohio 45239 The conductometric titration of low levels (0.05-0.50 milliequivalent) of a large number of weakly acidic organic compounds containing one or more acidic functional groups with “dimsylsodium” (the sodium salt of methylsulfinyl carbanion) as titrant in dimethyl sulfoxide solvent is reported. The advantages of the conductometric technique over the visual indicator, triphenylmethane, as the titration end-point method for alcohols and difunctional organic acids are discussed. The conductometric titration curves give information concerning the relative acidity of the acidic functional group(s) of the samples and water impurity in the solvent. For example, tert-butanol is titrated after all residual water has reacted, and two distinct breaks are observed, one for the residual water and one for tert-butanol.
THE PREPARATION and use of methylsulfinyl carbanion or “dimsyl” ion, the conjugate base of dimethyl sulfoxide (DMSO), as a n intermediate in a wide variety of organic synthetic reactions is well known (1-4). The carbanion is prepared by the reaction of a n alkali metal hydride or amide with DMSO as given in Reaction 1. The sodium salt has been named “dimsylsodium” (5). 0
NaH
0
t
t
+ CHsSCH3
-t
CH3SCH2-
+ Na+ + Hz t
1 Present address, The Procter & Gamble Company, Winton Hill Technical Center, Cincinnati, Ohio 45224
(1) E. J. Corey and M. Chaykovsky, J . Amer. Chem. Soc., 84,
866 (1962). (2) Ibid., 87, 1345 (1965). (3) I. D. Entwistle, R. A. W. Johnstone, and B. J. Millard, J. Chem. Soc., Sect. C, 302 (1967). (4) W. I. Lyness, D. E. O’Connor, and J. S . Berry, U. S. Patent 3,288,860,(1966). ( 5 ) G. G. Price and M. C. Whiting, Chem. Znd. (London), 775 (1963). (6) E. C. Steiner and J. M. Gilbert, J . Amer. Chem. Soc., 85, 3054 (1963). (7) C . D. Ritchie and R. E. Uschold, ibid., 89, 1721 (1967); 90,
30
e
Moles Reagent/Moles Sample Reagent: ; [DMSO-Na+]“ [DMSO-K+]* [D MSO-Cs+Ic 1 .oo 0.64 0.0 1.02 0.99 ...
Compound Water Phenol Methanol 1.05 ... ... Ethanol ... 0.31 ... 2-Propanol 1.02 ... ... Butanol 0.98 0.45 ... rert-Butanol 1.06 0.33 ... 2-Methyl-2butanol 1.01 ... ... 1-Octanol 0.96 ... ... 2-Octanol 0.94 ... ... Benzyl alcohol 0.99 ... ... Ethylene glycol 1.02 ... ... Diethylene glycol 2.04 ... ... Glycerol 1.56 1 .oo ... a Reference 5; triphenylmethane used as visual indicator. b Reference 6; triphenylmethane used as visual indicator. Reference 7; glass-calomel electrode potentiometric titration; only one end point observed for the titration of p-toluenesulfonic acid monohydrate.
(1)
The strongly basic carbanion reacts quantitatively with many weak acids including triphenylmethane to produce the deep red color of the triphenylmethide ion, a convenient titrimetric visual indicator (5). Of particular importance is the reactivity of the base toward hydroxy-containing compounds. Price and Whiting (5) titrated several alcohols including water quantitatively with 1M dimsylsodium using triphenylmethane as the indicator. Steiner and Gilbert, (6) however, found that the reaction of the dimsyl ion with alcohols depended upon the choice of cation. In their study, dimsylpotassium reacted with triphenylmethane after only about one half of the alcohol had reacted. Water exhibited a similar behavior. Ritchie and Uschold (7) found that dimsylcesium did not react with water at all when they potentiometrically titrated p-toluenesulfonic acid monohydrate with a modified glass electrode.
2821 (1968).
Table I. Reported Results of Titrations of Alcohols with Various Dimsyl Reagents
All of these studies were made in DMSO solvent, and the results are summarized in Table I. The differences in the reactivity of the various dimsyl reagents with alcohols are thought to be the result of the degree of cation association with the alkoxides formed. The apparent acidity of a n alcohol is increased when the activity (basicity) of the alkoxide is decreased (6, 8). Whereas, the reported pK. values of alcohols are 16-19, Steiner and coworkers (9) have demonstrated how the pK’s of methanol, ethanol and tert-butanol are affected by the cation present (Cs, K , Na, and Li). The pK, values (in DMSO) of 27, 27.4, and 29.2 for methanol, ethanol, and tert-butanol, respectively, have been assigned. The pK, of triphenylmethane is reported to be between 28 and 33 (6, 7). The results given in Table I indicate that the metal alkoxide dissociation constants are in the order Cs > K > Na. Recently, Steiner and coworkers (10) have determined values for the dissociation constants of several alkali metal alkoxides; for methanol the order is K > Na > Li. For analytical purposes dimsylsodium and dimsyllithium appear t o be the dimsyl reagents of choice for effecting a complete reaction with hydroxy-containing compounds. (8) D. J. Cram, “Fundamentals of Carbanion Chemistry,” Academic Press, New York, 1965, pp 40-42. (9) E. C. Steiner, J. D. Starkey, J. M. Tralmer, and R. 0. Trucks, 153rd Meeting American Chemical Society, Division of Perroleum Chemistry, Miami Beach, Fla., April 1967. (10) E. C. Steiner, R. 0. Trucks, J. D. Starkey, and J. H. Exner, 156th Meeting American Chemical Society, Division of Polymer Chemistry, Atlantic City, N. J., September 1968.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
The purpose of this study was to investigate the usefulness of the dimsylsodium reagent as a n analytical titrant of small amounts (0.05-0.50 meq) of hydroxy-containing compounds. The titration of many other weakly acidic compounds is also reported. The use of the visual indicator triphenylmethane which reacts with dimsylsodium instantaneously is a convenient and rapid end-point technique for samples that are not colored or whose reaction products are not colored. However, preliminary experiments indicated that many of the hydroxycontaining compounds yielded end points which were difficult t o determine visually because of a somewhat sluggish reaction with the reagent. The use of a glass-calomel electrode system for potentiometric titrations in DMSO has been reported (7, 11-13), however, at high p H (>lo) the response of the glass electrode is extremely slow. Since the hydroxy-containing compounds are extremely weak acids, the necessity of working in solutions of high p H could not be avoided. The unusually long titration times required with the potentiometric technique were therefore not desirable. Conductance measurements in DMSO have until recently been limited to studies of the conductance behavior of a large number of inorganic salts (24, 15), and to the determination of the dissociation constant of a weak base (di-n-butylamine) in DMSO (IZ),and of uncharged and monovalent cation acids in DMSO (16). Steiner et al. (10) have recently obtained conductivity data demonstrating that the dimsyl salts of the alkali metals are predominantly dissociated and have determined the dissociation constants. (Log KO for the Li salt is -2.43 and for the Na salt, -2.00.) I n obtaining this conductivity data, these authors corrected for water impurity in the solvent by conductometric titration with dimsylsodium or dimsyllithium. The evaluation of the conductometric technique seemed appropriate because of the ionic dissociation of the basic reagent, dimsylsodium. Comparisons were made between the end points observed conductometrically and visually for hydroxy-containing compounds. The applicability of the conductometric technique in determining successive end points for compounds containing more than one active hydrogen was tested to gain information about the relative acidities of the active hydrogens as well as the nature of the reaction products. EXPERIMENTAL.
Materials. Matheson, Coleman and Bell (MCB) Spectrograde methylsulfoxide (DMSO) was used throughout these experiments and was stored in 1-liter bottles over activated Linde type 4A Molecular Sieves. The conductivity of the solvent at ambient room temperature (about 25 "C) was 3.5 =t0.2 X lo-' ohm-' cm-I. [The conductivity of pure DMSO is stated to be between 2 X and 3 X ohm-' cm-l although values over a hundred times larger have also been reported (14).] The residual water content, or rather, total acidic impurities in the solvent, measured by titration with dimsylsodium was typically about 2.2 mM, or 40 ppm of water. Sodium hydride was obtained as the 50z mineral oil dispersion (Metal Hydrides, Inc.). All other chemicals were reagent grade and were used without further purification. 2-Naphthol (MCB reagent grade) used (11) K. K. Barnes and C. K. Mann, ANAL.CHEM., 36,2502 (1964). (12) I. M. Kolthoff and T. B. Reddy, Inzorg. Chem., 1, 189 (1962). 40, 1148 (1968). (13) R. Morales, ANAL.CHEM., (14) J. N. Butler, J . Electroa/?al.Chem., 14, 89 (1967). (15) P. G. Sears, G. R. Lester, and L. R. Dawson, J. Phys. Chem., 60, 1433 (1956). (16) I. M. Kolthoff, M. K. Chatooni, Jr., and S. Bhowmik, J . Amer. Chem. SOC.,90,23 (1968).
in standardizing the dimsylsodium reagent gave the following analysis. Calcd for ClaHsO: C, 83.3; H , 5.59; 0, 11.1. Found: C , 83.2; H , 5.6; 0, 11.1. National Cylinder Gas dry nitrogen was used in a n all copper and glass tube line with connection made with the shortest possible lengths of Tygon. Conductance Apparatus. Conductance measurements were carried out on a n Industrial Instruments type RC-16 Conductivity Bridge (*1% accuracy). The Beckman Number CEL-KO1 conductivity cell was used. The cell constant was calculated to be 0.103 cm-I from conductance measurements on aqueous KCl solutions. Preparation of Reagent. The dimsylsodium reagent was prepared according t o the method of Corey and Chaykovsky ( 2 ) by adding 200 ml of DMSO t o 0.6-1.0 gram of 50% NaH/mineral oil which had previously been washed with two 10-15 mi portions of petroleum ether to remove the mineral oil. The reaction was allowed to proceed under nitrogen at 50-60 O C with stirring and required 3-4 hours for completion. The reaction was judged to be complete with the cessation of hydrogen gas evolution. The reagent solution was clear and pale green. The reagent could be stored under nitrogen in the reaction flask until needed for use. As a n extra precaution enough mineral oil was added t o the flask to form a layer 1 cm thick over the reagent solution. The reagent can be stored at room temperature for at least a week with only a 1-3 loss in activity. Approximately fifty batches of reagent were prepared without difficulty. [Caution : Pressure explosions have been reported as a result of adding N a H (3.27 moles) t o DMSO (19.5 moles) and heating to 50" with mechanical stirring (17, la).] Titration Procedure. An adapter tube (reducing bushing) was fitted to one of the necks of the reaction flask to receive a male ground glass tube to which a serum cap was secured. The reagent was transferred from the reaction flask to the storage bulb of a 5-ml analytical buret with a 50-ml syringe fitted with a stopcock and a 6-inch 18-gauge hypodermic needle. The buret was modified by making a glass tubing T-connection between the storage bulb and the graduated buret above the solution level to allow the reagent to be stored under nitrogen. The titration cell was a 180-ml tall form beaker fitted with a No. 10 rubber stopper which had holes drilled to accommodate a gas inlet tube (for nitrogen), a catheter of Teflon (Du Pont) (which delivered the reagent from the buret), a conductance cell, and a hole t o allow for solvent and sample delivery. The nitrogen gas was passed through a solution of 80 ml of DMSO, 0.05M in dimsylsodium, to remove any acidic impurities. Atmospheric water, carbon dioxide, and oxygen will react with the reagent and, therefore, must be excluded. All titrations were performed at ambient temperatures (about 25 "C). For titrations using the visual indicator triphenylmethane, 15 ml of DMSO and five drops of the indicator solution (1 solution of triphenylmethane in DMSO) were added t o the titration beaker. The residual water in the solvent was titrated prior to the addition of a sample. Two or three samples were successively titrated in each 15-ml aliquot of solvent. Conductometric titrations were performed in one of two ways depending upon whether or not the conductometric behavior of the residual water in the solvent could be distinguished from that of the sample. The first way (Method I) is analogous t o that used in all visual end-point titrations; namely, the residual water was titrated first to a conductometric end point and then the sample added and titrated. This is illustrated in Figure 1. The second way (Method 11) involved simply titrating the sample and residual water together. This method was used when a definite portion of the
z
(17) Chem. Eng. News, 44,48 (April 11, 1966). (18) G. L. Olsen, ibid., 44, 7 (June 13, 1966).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
31
~~
~~
Table 11. Conductometricand Visual (Triphenylmethane Indicator) End-Point Titrations with Dimsylsodium in DMSO Solvent (Moles Reagent)/(Moles Sample)" Compound Cpd. No. Visual Conductometric Comment sb 1 Water 0.97 f 0.00 (2) A 2 p-Toluene sulfonic acid 1.95 f 0.01 (3) Monohydrate 3 Methanol 0.88 f 0.02 (3) 1.04 f 0.01 (2) A 4 fert-Butanol 0.84 f 0.04 (5) 1.00 f 0.02 (4) A, B, C 5 1-Decanol 0.98 f 0.01 (3) 1.04 A, D 6 6-Undecanol 0.81 1.02 f 0.02 (3) A, B, c 7 1-Dodecanol 1.02 f 0.01 (2) A, B, C 8 2-Dodecanol 0.89 f 0.04 (2) 0.93 f 0.04 (2) A, B, C 9 1-Tetradecanol 0.97 f 0.01 (3) 1.03 f 0.01 (2) A, D 10 2-Tetradecanol 0.95 f 0.04 (2) 1.O2fO0.00(2) A, D 11 2-Hexadecanol 0.84 f 0.02 (2) 0.91 f 0.01 (2) A, D 12 1-Octadecanol ... E 13 Cyclohexanol 0.74 1.12 f 0.07 (2) A, B, c 14 1,2-Propanediol 1.91 f 0.04 (3) 1.91 f 0.01 (2) F, G 15 1,3-Propanediol 1.97 f 0.03 (3) 2.01 f 0.03 (2) F 0.82 16 Benzyl alcohol 0.96 f 0.01 (3) 17 Benzhydrol 1.01 f 0.01 (3) 0.99 f 0.02 (3) 18 Triphenylmet hanol 1.01 f 0.02 (6) 1.07 19 Hydroquinone 2.03 f 0.06 (3) 1.01 2.00 20 Pyrogallol 3.03 =! 0.02 (2) 0.94 3.03 o,o '-Biphenol 21 1.97 f 0.00 (3)
::y;: 1.94
22 23 24 25 26 27 28 29 30 31 32 33
1-Naphthol 8-Amino-2-naphthol 8-Hydroxyquinoline Lauric acid Myristic acid Palmitic acid Stearic acid Benzoic acid Anthranilic acid m-Aminobenzoic acid p-Aminobenzoic acid Salicylic acid
34
Acetylsalicylic acid
2.68
35
Glycolic acid
1.84 f 0.03 (3)
36
Mandelic acid
1.98 f 0.01 (3)
37
Benzilic acid
1.94
38 39 40 41
Acrylic acid Methacrylic acid trans-Cinnamic acid Succinic acid
1.00 f 0.01 (3) 0.98 f 0.00 (3) 1.01 f 0.00 (3) 2.06 f 0.00 (3) . .
42 43 44 45
Azalaic acid Sebacic acid Fumaric acid Maleic acid
2.00 f 0.00(3) 2.00 f 0.01 i3j 2.00 f 0.00 (3) 2.00 f 0.01 (3)
46 47 48 49
Ethylenediaminetetraacetic acid Nitrilotriacetic acid Mellitic acid Tartaric acid
50
2-H ydroxyhexadecane
1.00 f 0.00 (2) 0.91 f 0.00(2) 0.99 f 0.01 (3) 1.OO 1.OO 1.OO
...
E
1.Go 1.01 1.00 1.03 2.00 f 0.01 (3) f 0.06 (4)
B, J, K
1.20 2.58
{2, : B. G
:::;) 1.04
2.01
51 52 53 54
sulfonic acid 3-Hydroxyhexadecane sulfonic acid 4-Hydroxy hexadecane sulfonic acid 2-Aminoethanol
0
1.98
... 3.59 f 0.09 (3)
1.02 3.70
1.03 1.00 0.98
2-tert-Butylaminoethanol
~
32
0.95 2.05 2.01
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
1.08 1.02
B, G F
Table 11. (continued) (Mole Reagent)/(Moles Samplep Visual Conductometric 1.04 0.94 1.10 3.02 2.91 f 0.01 (3) 1.09
Cpd. No. 55 56 57 58
Compound 2-Ethylaminoethanol 1-Amino-2-propanol 2,2’,2’’-Nitrilotriethanol 2-(2-Aminoethylamino) ethanol 59 2-Amino-2-(hydroxymethyl)2.11 f 0.01 (3) 1,3-propanediol Aniline 60 ... o-Nitroaniline 61 62 m-Nitroaniline p-Nitroaniline 63 Diphenylamine 64 1.00 f 0.01 (3) N,N’-Diphenylbenzidine 65 4-Aminopyridine 66 0.97 f 0.00 (3) Acetamide 1.01 f 0.01 (3) 67 Benzamide 68 1.00 f 0.01 (3) 69 Acetanilide 1.00 f 0.01 (2) 2-Hydroxyacetanilide 1.97 f 0.03 (3) 70 3-Hydroxyacetanilide 2.01 f 0.04 (3) 71 72 4-Hydroxyacetanilide 2.00 0.02 (3) 73 Saccharin 1 .OO i 0.01 (3) Acrylamide 74 1.01 f 0.01 (3) 75 Methacrylamide 0.99 f 0.01 (3) 76 Thiourea 1.01 f 0.00 (3) Triphenylmethane 77 Diphenylmethane 78 a Average value and average deviation are given. Number of trials is given in parenthesis if * Comments refer to conductometric titrations. Bracket indicates data from the same trial. A. Method I. B. Method 11. C. Less acidic than water by Method 11. D. Cannot distinguish from water by Method 11. E. Incompletely soluble. F. Total by Method I. G. First acidic proton more acidic than water. H. Total by Method 11. I. Second acidic proton less acidic than water by Method 11. J. More acidic than water. K. Highly colored solution. L. Second acidic proton more acidic than water by Method 11. M. No reaction observed.
1.01 2.13 .,
1.00 f 0.03 (3) 0.88 0.97 0.99 1.88 0.7
Commentsb A, J
A, J F A, J B, G F M A, B,J A, J A, J
B,J A A, B, J
*
conductometric titration curve could be assigned to the reaction of water; this is illustrated in Figures 3, 4, 6 , 7, 8, and 9. Fifty-milliliter aliquots of DMSO were used in all conductometric titrations; only one sample could be titrated per solvent aliquot to achieve meaningful results. Approximately 15 minutes were required for each titration. All conductance measurements were corrected for changes in the solution volume during the titrations. End points were determined by drawing the best straight lines through the conductivity data points and measured as the points of intersection. Above each titration curve figure is a solid line interrupted by arrows indicating the theoretical end point for each acid function being titrated. RESULTS AND DISCUSSION
Titration results for seventy-eight acidic compounds are given in Table 11. Unless otherwise noted, visual (triphenylmethane indicator) and conductometric titration results in Table I1 are separate experiments. The dimsylsodium reagent is no more difficult to use than other air and water sensitive reagents, and its stability is adequate for analytical purposes. Water. The conductometric titration behavior of the residual acidic impurities in the spectrograde DMSO solvent and of added water was identical. It is proposed that essentially all of the reactive acidic solvent impurities is water.
... ...
M M
more than one.
This behavior is shown in Figures 1 and 2. Initially, the conductivity increases as sodium and hydroxide ions are formed because of the reaction with water; the conductance .then becomes relatively constant as NaOH is either precipitated or forms tight ion pairs. [The solubility of NaOH in DMSO is reported to be only 1 X M (14).1 Just past the end point, the excess dimsylsodium causes a sharp increase in the conductivity giving rise to a n unambiguous titration end point. The presence of residual water in the solvent allows the determination of conductometric titration end points for those compounds that are weak acids (but stronger than water) and which form soluble dissociated salts upon reaction with dimsylsodium. Thus, during the titration of o-nitroaniline shown in Figure 9, the conductivity increases as the reagent reacts initially with the sample. A sharp end point is produced when excess dimsylsodium reacts with the residual water (constant conductance during its titration). I n the absence of water, excess dimsylsodium would give rise to a further increase in the conductivity making it almost impossible to determine the end point accurately. The quantitative reaction of dimsylsodium with water and the well-defined conductometric end point for the titration suggest that the reagent would be a suitable substitute for the Karl Fischer reagent in certain applications.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
33
f
Sample Added
I
511
41-
0
0
I
ml
I
I
2
3
I 4
1
5
DIMSYLSODIUM
Figure 1. Conductometric titration of 0.005 ml of added HzO in 50 ml of DMSO with 0.1101M dimsylsodium
Alcohols. Titration data for sixteen aliphatic alcohols (compounds 3-18) are given in Table 11. In all cases, the conductometric technique gave sharper end points (Method I) and generally more quantitative results than did the visual indicator method. The visual end points were usually difficult to judge because of precipitation of sodium alkoxide which made the solutions cloudy. For three samples (cpds. 6, 7, 13) conductometric titrations were performed in the presence of the indicator, triphenylmethane, and they revealed the visual end point to be occurring prior to the conductometric end point. Several of the secondary alcohols and tert-butanol (cpd. 4) could be titrated conductometrically in the presence of the residual water in the solvent to yield two breaks (Method 11). The water reacted first and then the alcohol. This behavior is shown for 6-undecanol (cpd. 6) in Figure 3 and tert-butanol in Figure 4. The mono-hydroxy alcohols showed the same general conductometric behavior as water; however, the two diols (cpds. 14, 15) tested behaved quite differently in that the first hydroxyl group was more acidic than water and formed a soluble dissociated sodium salt (increasing conductance). This is shown for 1,3-propanediol in Figure 5 . The first break
2
I
ml
3
4
34
2 ml, DIMSYLSODIUM
3
Figure 3. Conductometric titration of 0.0438 gram of 6-undecanol with 0.1057M dimsylsodium
occurs after only 80% of the first hydroxy proton has reacted. After the first hydroxy proton has reacted, the residual water reacts with the reagent. Before it has completely reacted, the second hydroxy proton of the diol begins to react giving rise to a rounded curve and a break of no quantitative significance. Quantitative results for conductometric titrations could be obtained for the diols only by Method I. Benzyl alcohol and triphenylmethanol (cpds. 16, 18) are conveniently titrated with dimsylsodium using either the visual indicator or conductometric end-point technique. These alcohols are both more acidic than water. The con-
0
5
DIMSYLSODIUM
Figure 2. Conductometric titration of 0.005 ml of added HzO in 50 ml of DMSO with 0.1082M dimsylsodium
I
0
2
I
0
I
I
0
I
0
2
I ml
3
4
DIMSYLSODIUM
Figure 4. Conductometric titration of 0.0190 gram of tertbutanol in 50 ml of DMSO with 0.1101M dimsylsodium
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
I 2
I
0
I
1
2
3
4
ml DIMSYLSODIUM
Figure 5. Conductometric titration of 0.0094 gram of 1,5propanediol in 50 ml of DMSO with 0.1021M dimsylsodium ductance increases during the titration in contrast to the other aliphatic alcohols tested. Therefore, the reaction of the residual water in the solvent serves to locate the titration end point. The conductometric titration curve of benzhydrol (cpd. 17) by Method 11, however, exhibits a conductance maximum after only 50 of the compound has reacted, and no precise end point is obtained for the alcoholic proton. Both hydroxy protons of o,o’-biphenol (cpd. 21) react quantitatively with dimsylsodium before water does giving three titration breaks (Figure 6). Carboxylic Acids. The titration of carboxylic acids with dimsylsodium poses no problems using either the visual indicator or the conductometric technique. These acids are essentially undissociated in DMSO and are more acidic than water. In general, dicarboxylic acids (cpds. 41-45) and fatty acids (cpds. 25-28) form precipitates upon reaction with dimsylsodium. No precipitate was observed for maleic acid (cpd. 45, Figure 7) even though the conductance decreases during the titration of the second acidic proton. The titration of the residual water in the solvent causes the conductance
0
I 0
I
I
I
2 3 ml DIMSYLSODIUM
1
1
4
6
Figure 6. Conductometric titration of 0.0255 gram of o,o’biphenol in 50 ml of DMSO with 0.1047M dimsylsodium
I I
0
ml
I 2 DIMSYLSODIUM
I
1 . 3
4
Figure 7, Conductometric titration of 0.0144 gram of maleic acid in 50 ml of DMSO with 0.1061M dimsylsodium to remain roughly constant, giving the titration curve a rounded portion after the titration of the acid protons. Hydroxy Carboxylic Acids. The conductometric titration behavior of salicylic acid (cpd. 33) is analogous to that of the dicarboxylic acids and o,o’-biphenol. The carboxylic acid function is titrated first with an accompanying increase in conductance. The reaction of the hydroxyl proton occurs next with decreasing conductance followed by the reaction of the residual water (constant conductance). For benzilic acid (cpd. 37, Figure 8) the hydroxyl proton is less acidic than water and, thus, the titration of the residual water interrupts the titration of the carboxylic acid and hydroxyl functions of benzilic acid. Note the similar titration behavior of 1,3-propanediol in Figure 5. The titration curve
0
0
1
2
3
4
5
6
7
8
9
ml. DIMSYLSODIUM
Figure 8. Conductometric titration of 0.0345 gram of benzilic acid in 50 ml of DMSO with 0.0638M dimsylsodium ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
35
22 2c 18 T 16
-5 I
E 14
r
0
In’
0 12 X
>.
5- I O I-
u
28 2
0
u
6 4
2 0 rnl
DIMSYLSODIUM
Figure 9. Conductometric titration of 0.03828 of o-nitroaniline in 50 ml of DMSO with 0.0966M dimsylsodium
for benzilic acid by Method I1 does give quantitative end points in contrast to 1,3-propanediol. The conductometric titration curve for glycolic acid (cpd. 35) is similar to that of salicylic acid rather than benzilic acid suggesting that the hydroxyl proton of glycolic acid is more acidic than water. This result is unexpected. Hydroxyhexadecanesulfonic Acids. Conductometric titrations of 2-, 3-, and 4-hydroxyhexadecanesulfonic acids (cpds. 70-72) were performed by Method 11. These acids are strong acids in DMSO and ostensibly completely dissociated. Although the conductometric titration curves were not identical in shape, three breaks were observed for the 2- and 3-hydroxy acids corresponding to the titration of the sulfonic acid proton, the residual water (in the solvent and sample), and the hydroxy proton, respectively. If the residual water
36
in the solvent is titrated before addition of the sample, the water in the sample may be calculated from the titration curve. The 4-hydroxyhexadecanesulfonic acid titration curve produces only two breaks, the second of which occurs when all the water and hydroxy protons have been titrated. Aliphatic Amines. The dimsylsodium reagent was unreactive toward primary and secondary aliphatic amines. (Addition of the amines to DMSO solution made slightly basic with dimsylsodium did not cause the red color of the triphenylmethide ion to disappear). The following amines were tested: propylamine, butylamine, sec-butylamine, tertbutylamine, allylamine, diethylamine, dipropylamine, dibutylamine, and ethylenediamine. Hydroxylamines. The reagent reacts 1 :1 with hydroxylamines indicating a stoichiometric reaction with the hydroxy group. Both the visual indicator and the conductometric (Method I) end-point technique were used. Table 11 includes results for several hydroxylamines (cpds. 53-59). Conductometric titrations by Method I1 did not give an analytically useful titration curve as the conductance is observed to increase initially, pass through a maximum and then decrease until excess reagent is added. Aromatic Amines. Although aniline is unreactive to dimsylsodium, the reagent reacts with nitroanilines (cpds. 61-67) to give highly colored solutions which mask the end-point color of triphenylmethane. It is therefore necessary to titrate these compounds using the conductometric technique. The aniline derivatives are all more acidic than water. The conductometric titration of o-nitroaniline (Figure 9) illustrates this point as well as the necessity of the presence of residual water in the solvent for determining the end point by Method 11. This condition also applies to diphenylamine (cpd. 64). Amides. Amides react quantitatively with dimsylsodium and are more acidic than water. The visual indicator may be conveniently used. Titration results for a variety of amides are given in Table I1 (cpds. 67-74). Triphenylmethane. Both triphenylmethane and diphenylmethane (cpds. 77-78) do not yield a conductometric end point as both react with dimsylsodium to form dissociated sodium salts. ACKNOWLEDGMENT
The author thanks D. F. Kuemmel for his helpful discussions and encouragement and Ted Williams who performed some of the titrations. The author expresses his gratitude to E. G . Steiner for his comments on the paper and for calling the author’s attention to Reference (10).
RECEIVED for review August 26, 1969. Accepted November 10, 1969.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
