Conductimetric Titrations of Acids and Bases in Benzene and Dioxane by Arthur A

U. S. Department of Commerce Research Paper RP1794 N alional Bureau of Standards Volume 38, May 1947

Part of the Journal of Research of the National Bureau of Standards

Conductimetric Titrations of Acids and Bases In Benzene and Dioxane By Arthur A. Maryott

Acid-base tit rations were made conductimetrically in pure benzene and dioxane with picric, t ri chloroacetic, and camphorsulfonic acids, together with primary, secondary, and tertiary amines. Although the conductances of the solutions were extremely low, lower by a'factor of 10-8 or more tllan in water, the t itrations gave sharp end points which generally ,;ere accurate to 1 percent or better. The unusual, though similar behavior of all titrations involving trichloroacetic or camphorsulfonic acid, where the conductance of t he salt was enhanced greatly by the presence of free acid, ,,·as interpreted in term of a reaction between salt and acid leading to the formation of a complex anion, The occasional variations in conductanc with time, in one instance suggesting slow attainment of some secondary ionic equilibrium, and the effect upon the t itration curves of the addition of a small amount o(methYl a lcohol to the solvent are disc ussed.

1. Introduction In solvcnts like benzene which serve primarily as inert diluents, dilu te solutions of acids or bases While the·'conductimetric method has been em have almost negligibly low conductances. There ployed as a gencral procedure for acid-base titra is practically no tendency for the solvent to react tions in water and has been extended to certain with the acid or base to form ions a would be the nonaqueous and mixed solvents, its applicntion to case in an amphoteric solvent like water. When purely nonpolar solvents such as benzene and the olution contains both acid and base, however, dioxane which have very low dielectric constants they combine with the transfer of a proton from remains practically an unexplored field . One case the acid to the base 2 to form a true salt. Since has been reported by La Mer and Downes [1 ] 1 the solvent has a low dielectric constant and has in which trichloroacetic acid was titrated with little tendency to stabilize individual ions through diethylamine in benzene. The titration curve solvation the extent of dissociation of the salt shown by these workers exhibited an interesting into free,' or conducting, ions is quite small. The and rather unexpected behavior. On titration of behavior of certain types of organic salts, partic the acid with the base, the conductance first rose ularly the tertiary and quarternary ammonium to a maximum well before the equivalence point halides and picrates, have been extensively inves was reached 'and then decreased to a minimum tigated by Walden [2] and by Kraus [3] and co somewhere in the vicinity of the equivalence point. workers. The very low conductances of these It was not apparent from their data, however, solutions indicate that only a minute fraction of whether there was· any definite break in the titra the salt, someth;ng of the order of a millionth of tion curve at the equivalence point which would 1 percent, is dissociated into free ions. The re be suitable for analytical purposes. The present mainder exists as nonconducting ion pairs, or work wns undertaken not only to explore the aggregates of ion pairs, held together largely by possibility of extending an analytical methou, but co ulomb torces. Although the equivalent C011- also in the hope that additional information con cerning the behavior of electrolytes in these,media 2 The definition of acid employed here is that oCBr onstedrat.ber than the more might be obtained. generalized electronic concept oC Lewis. Although compo.Ullds owing their acidity to an electron defici ency can be titrated with the aId or colored IUd l· I Figures in brackets indicate the li terature references at the end oC this cators, these acids are not suitable for tbe present purpose since an ionic prod paper. uct having a measurable conductance is required.

Conductirnetric Titrations 527 740279- 47--6

L_ ductance varies in a rather complex manner with III. Apparatus and Procedure concentration, and suggests a multiple series of A direct-current, high-resistance bridge, General equilibria involving th~ formation of triple ions Radio Co. type 544-B, was used to measure the and more highly associated ionic products, the resistances of the cell during the titrations. These specific conductance increases continually with resistances were of the order of 10 7 to 1010 ohms. concentration. Conductimetric titrations in these Sensitivity as well as convenience and rapidity of solvents might be expected to follow a simple pat measuring these high resistances, rather than aCCll tern. The extent of ionization of the salt, though racy, were determining factors in selecting the small, should be considerably greater than that of method. Throughout the required range, the either the acid or the base, so that the conductance resistances could be determined to 1 percent, would be determined almost entirely by the con which is as precisely as the logarithmic dial could centration of salt formed. Then the conductance be read. The use of direct current introduces the would be expected to increase more or less uni possibility that the mea ~ ured resistances may be formly until the equivalence point is reached, and influenced by the effects of electrode polarization. remain substantially constant upon the addition Should these effects cause the resistance of the cell of excess titrating solution. It is evident from to vary appreciably with time, they might interfere the experimental results that only a part of the with the titrations. In a majority of cases, either titrations conform to this simple pattern. no such drifts were observed or else they were The choice of acids and bases is limited by the sufficiently small so as not to interfere with detec solubility, particularly of the salt formed, and by tion of the equivalence points. the requirement that they be sufficiently strong A photograph of the cell and titrating assembly to insure complete neutralization. In these re is shown in figure 1. The cell consists of an inner spects picric and trichloroacetic acids together with various aliphatic primary, secondary, and tertiary amines were for the most part suitable. Titrations were generally made in both dioxane and benzene. To determine the effect of a small amount of polar impurity, in some cases approxi mittely 1 percent of methyl alcohol was also added to the solvent before titrating. Camphorsulfonic acid was used in several titrations in dioxane. Good breaks in the titration curves, corresponding tQ the equivalence points calculated from aqueous standardizations of the various solutions, were observed' in practically all cases.

II. Materials Reagent-grade trichloroacetic acid was stored in a vacuum desiccator until used. Picric acid and dl-camphor-sulfonic acid were recrystallized, re

spectively, from benzene and dioxane, and dried FIGURE I. - Assembly used in conductimetric titratious in in a vacuum oven. The various amines, obtained benzene and dioxane. from Eastman Kodak Co., and Sharples Chemi cals, Inc., were used without additional treatment. section of soft glass, on which are mounted two To remove moisture the solvents, benzene and concentric platinum electrodes and their leads, dioxane, wen? refluxed and distilled over sodium. and an outer U-shaped container of Pyrex glass.

528 Journal of Research lAs the cell was also used in measurements of di All titrations were made at 25° ± 0.01 ° C in an electric constants, the lead from the outer elec- oil bath. This close control of temperature main trode is tubular and serves as a shield for the tained constant condition for comparative pur I lead from the inner electrode. In addition to the poses, and the titrations could be made satis electrode compartmen t, the outer section con factorily at ordinary room temperature. The tains two enlarged bulbs to accommodate excess t ime required to obtain a complete titration curve liquid and to facilitate mL-.,::ing the solutions in was about 20 minutes. Because of uncertain Ithe manner to be described. The electrodes are puri ty, particularly of the ba es, and in order to

18.0 cm high, with the outer one having a diameter have a check upon the reliabili ty of the conducti of 2.4 cm. The spacing betwern electrodes is metric end points, all stock solutions were stand [approximately 0.2 cm. A sufficiently good esti ardized with aqueous olutions of sodium hydrox mate of the cell constant, k, is obtained from the ide and hydrochloric acid, using phenolphthalein int3relectrode capacitance, 0, by the formula, and methyl red as indicators. Solutions of amine l k = djA = 1/(47r 0). This capacitance in air, de in benzene were added to excess acid and back termined by calibration with pure benzene, is 27.4 titrated with alkali to eliminate loss of amine Icm so that k= 0.00291 em- I. through volatilization from the benzene layer. Stock solutions were made up in volum etric flasks from weighed quantities of acid 01' base so IV. Experimental Results that the concentrations were approximately 0.2 molar. To the cell containing 100 ml of solvent A number of preliminary titrations were made Iwa s added 5 ml of tock solution of acid (or base) using a variety of amines with picric acid or from a pipette. This solution was then titrated trichloroacetic acid in benzene. While the general with a stock solution of base (or acid) from a 10- shapes of the titration curves were found to be ml burette. The concentration of salt at the dependent upon the acid, and in some cases, upon I equivalence point was thus around 0.009 molar the class of amine, no essential qualitative differ in all cases. As the titrating solution was roughly ence was no ted within a particular class of amine. twenty times as concentrated as the solution to be Consequently, in the titrations r eported, only I titrated, the effect of dilution was minimized. triethylamine, di-n-butylamine, a,nd n-heptyl I Compressed air, dried by passage through a tube amine representing, respectively, the tertiary, containing calcium chloride, was used to achieve secondary, and primary classes of amines were mixing upon each addition of the titrating solu used. Frequently, both forward and reverse tion by first forcing the solution into one of the titrations were made. The forward titrations are I enlarged bulbs and then into the other . The defined as those in which the base is added to burette was attached to the cell opening through the acid, and the reverse titrations as those in a cork stopper, which also contained a tube pass which the acid is added to the base. ing to the compressed air line. With the cell While representative data for several of the i virtually airtight, any danger of contaminating the titrations are given in table 1, it seemed generally I solution with atmospheric moisture, which might preferable to present the results in graphical form. effect the conductance during the titration, was The various titration curves plotted on semi-loga eliminated. Very small additions of stock solu rithmic paper are shown in figures 2 to 6. Figures 2 to tion, not over three or four drops, were made near 5 show forward titrationson th~ l eft side and reverse the equivalence point. Whether such small addi titrations on the right. The specifi c conductance i tions were necessary depended upon the shape of plotted on the logarithmic scale. Plotted as ab .. the titration curve. In some cases the curves scissa is the function X, defined by X = (B - A )jD . were substantially linear over an appreciable A, represents the equivalents of acid, and B , the region on either side of the equivalence point, equivalents of base in the solution at that partic while in others the marked curvature of the plots ular point in the titration. D , is the equivalents I required that evaluation of the end point be of acid or base initially to be titrated and is thus based upon points within a rather limited region. identical with A for the forward and with B for

Conductim.etric Titrntions 529 the reverse titration. The values of A , B, and D are based upon the concentrations of the stock 2 solutions determined by the aqueous standardiza -iO 10 r----4-~~~---~---_r---+_--~ tions. Up to the equivalence point, X measures the fraction of acid or base not yet neutralized, and beyond this point the fractional degree of w <.) overtitration. The forward titrations start with z ...« negative values of X , and the reverse titrations <.) => 0 z with positive values. The equivalence point 0 <.) -II corresponds to X = O. <.) 10 ;;: U w a. Vl TARLE I. - Representative dala on conduclimetric titrations

1. Titrations with picric acid (0.2000 molar) and di-n-butylamine (0.2013 molar) in benzene

Forward titration Reverse titration 1-0 12 L-____~ ______~ ____~L- ____~ ______~ ____~

I -I -0.5 0 iO.5 0 ~O . 5 ~I Base Specific Acid Specific added X conductance added X conductance ------FIGU R E 2.- Forward (left) an; reverse (right) ml ml titratio ~ 1.50 - 0. 698 0.39XlOll 1.50 0. 702 0.59X I011 curves with picric acid and di-n-butylamine. 3.00 -.396 1.16 3. 00 . 403 1. 40 Curves: 1, in benzene; 2, in benzene containing 1 percent of methyl alcohol: 3.50 -. 296 1. 53 4.00 . 205 2.06 3, in dioxane. 4. 00 -.195 1.91 4. 25 . 155 2.26 4. 25 - . 145 2.12 4.50 .105 2. 46 4.50 -.094 2.36 4. i 5 . 056 2.64 4.60 - .074 2. 45 4. 90 . 026 2. 74 4.70 -. 054 2.53 4.98 . 010 2.85 4. SO -. 034 2. 63 5.09 -. 011 2.91 -10 4. 90 -.014 2.72 5.17 -. 028 2.91 10 5.00 . 006 2. 78 5.26 -. 045 2.91 5.10 . 025 2.78 5.50 - .092 2.91 -I 5.20 .045 2.80 6.00 -. 193 2.88 5.30 .066 2.81 w 5.50 .106 2.85 u z 6.00 .207 2.97 « ....u c=> z II. Titrations with trichloroacetic acid (0. 1994 molar) and triethyla- 0 u mine (0.1939 molar) in dioxane. - I I u 10 I ;;: U 1.00 - 0. S06 0. 044 1.00 0. 794 w 0.033 a. 200 -.611 . 092 2.00 . 588 .036 Vl 3.00 - . 416 . 114 3.00 .383 .. 049 4. 00 -. 222 . I SO 4.00 . 177 .074 4. 25 -. 172 . ISO 4.25 . 126 .081 4.50 -.124 . 174 4. 50 . 074 .090 4.75 -.076 . 159 4.75 .023 . 101 4.83 - . 060 . 150 4. 81 . 010 . 104 4.90 -.047 . 142 4.87 -.002 . 11 0 10-12 L-____~ ______~ ____~ ____~ ______~ _____J 4.98 -.031 .130 4.93 -. 015 . 123 5.07 -.014 . 116 5. 00 - .029 . 139 -I -0. 5 o iO.5 o .0.5 . 1 5. 15 . 002 . 109 5.06 - .040 . 161 )( 5.23 .017 . 108 5. 12 -. 054 . 178 . . I 5.50 . 070 . 107 5. 25 -. 080 . 197 FIGUR E 3.- Forward (left ) and reverse (right) t ~ tratwn 6.00 . 167 .107 5.50 -. 131 . 285 curves with picric acid and triethylamine. 6.50 -.337 . 415 Curves: I, in benzene; 2, in benzene containing 1 percent of methyl alcohol; 1 3, in dioxane.

530 Journal of Research I - I I -II 10 ~----~----~~~~1-----_1------1------1 10 ~----~------r-----~----~r--==-i------1

.., (,) '"z0 z I-.. ~ U (,) :> 0 15z z 0 0 (,) - 12 - 12 I ~ 10 (,) 10 ... ~ ..,u ..,u Q. A- I/) If)

-13 10 ~ ____-L ______~ ____~ ____~ ______~ ____~

- I -0.5 o ± 0 .5 o +0.5 +1 - I - 0 .5 o + 0 .5 x -I - 0 .5 o + 0 .5 X fIGFRE 4.--Forward (left) and reverse (right) titration F I G UR)!; 6.- Pm·ward titmtion CUI'ves using trichloroacetic curves with trichloroacetic acid and tTiethylamine. acid (left) and camphorsulfonic acid (right ). Curves: 1, in benzene; 2, in bcnzene containing 1 percent of methyl alcohol; Curvcs wi th trichloroacetic acid and n-heptylamine: 1, in bcnzene; [2, in 3, in dioxane; 4, in dioxanc containing 1 percent of methyl alco hol. benzene co ntaining 1 pcrccnt methyl alcohol. Curves nsing camphorsul I foni c acid in dioxane: 3, with triethylamine; 4, with di·n·butylamine. 1. Titrations With Picric Acid a n d Di-n-butylamine Curves represen ting the titrations with picric acid and di-n-butylamine are shown in figure 2.

w Those numbered 1, 2, and 3 are the titrations in <..) ~ - 12 benzene, in benzene containing 1 percent by <..) 10· 1----I--I------\+~-,._+------.lH_----_1----__I => a volume of methyl alcohol, and in dio:x1ane, respec z o <..) tively. Conductances of the initial solutions containing only free acid or base arc not shown on the plots. These conductance were les than one-hundredth the conducance of the al t at the equivalence point. All curves, both for the

· 13 forward and reverse titrations, have the same 10 general shape and show a pronounced break at the -I -0.5 o = 0 5 o .1 "J5 equivalence point. The conductance increases continually until the equivalence point is reached IFIGURE 5.- Forward (left) and reverse (right ) titration curves with trichloroacetic acid and di-n-butylamine. and remains subs tan tially constant thereafter. This is the typical behavior in cases wj:wre the I Curves: 1, in benzene; 2, in bcnzene containing 1 percent of methyl alcohol; conductance is largely determined by the concen- I'In dioxane.

Con du cti:metric Titration s 53 1 tration of salt formed during the neutralization, the latter case. To study this effect further andl and the titrations bear a formal analogy to con to ascertain which is the proper value for the con·· ductimetric titrations of weak acids and bases in ductance of the salt, a solution of purified triethyl aqueous media, where the extent of ionization of ammonium picrate, approximately 0.009 molar, the acid and base is relatively small compared to was introduced into the cell. Upon the addition that of the salt. A comparison of the forward of one drop of the solution of picric acid, the con titration curve with the corresponding curve for ductance began to drift and continued until itl the reverse titration shows that they are nearly reached a steady value in about 10 minutes, superimposable. Unlike many of the titrations which was approximately 30 percent higher thanl to follow, there is no indication of any specific the original. Subsequent small additions of acid interaction between salt and free acid or base of did not produce any further drifts in conductance.1 such a nature as to influence markedly the con On back titrating with the stock solution of tri-. ductance of these solutions. The conductances ethylamine to give a drop excess, the conductancel are not very different in the two solvents, benzene drifted back approximately to its original value. and dioxane, and the addition of the small amount This change was not dependent upon the continual of alcohol to the benzene has merely the expected passage of current through the solution. It is effect of shifting the titration curve to higher evident from the above experiment and from the' values of conductance. instability noticed during the initial part of the forward titration that this drift in conductance 2. Titra tions With Picric Acid and Triethylamine toward abnormally high values is associated with When similar titrations, shown in figure 3, are solutions containing both salt and free picric acid'l made with picric acid and triethylamine rather The process is reversible and possibly suggests a than di-n-butylamine, the results are quite dif slow attainment of some ionic equilibrium involv ferent and dependent upon the solvent. Curves ing a reaction between the acid and the salt. The 1, 2, and 3 are again the titrations in benzene, in possibility that relatively slow reactions may occur benzene containing 1 percent of methyl alcohol, in such solvents has been suggested by Dietz and and in dioxane, respectively. With the first Fuoss [4], who found that the conductances ofl additions of base during the forward titration in mixtures of tributylammonium chloride and lead benzene, the conductance is unstable and tends abietate in toluene drifted wi th time in a manneri to drift rapidly with time toward higher values. which indicated a slow ionic reaction between As the titration proceeds, these drifts become less these two salts. The failure of larger excesses of pronounced and practically vanish after somewhat picric acid to produce additional drifts indicates a more than half of the acid is neutralized. The saturation effect requiring only a very small con-l conductance then steadily increases in the ex centration of acid. pected manner until a second drift, this time Curves 2 in figure 3 represent titrations with toward decreasing conductance, occurs at or very the same solutions, the difference being that 1 per close to the equivalence point. The reverse cent by volume of methyl alcohol was added tol titration shows a corresponding drift toward the benzene before titrating. Unlike the titra-: increasing conductance in the immediate vicinity tions in pure benzene, no drifts in conductance of the equivalence point, but none during the were noted during any part of either the forward initial part of the titration. The regions of or reverse titrations. The shapes of both curves marked instability are indicated by the dotted are similar and now resemble those obtained with portions of the curves. picric acid and di-n-butylamine. From a comparison of the forward and reverse Still different behavior is shown by the titra titrations it would appear that there are two quite tions in dioxane. The conductance during the different values representing the conductance of forward titration, instead of leveling off beyond triethylammonium picrate. The conductance the equivalence point, increases at a faster rate near the equivalence point when approached from than before. In the reverse titration the conduc the forward titration is about 20 percent greater tance reaches a maximum value when only about than for the reverse titration, although the con 80 percent of the base has been neutralized and centration of salt is about 10 percent greater in then decreases rather rapidly up to the equiva-

I 532 Journal of Research lence point. Beyond this point the conductance In several instances, excess base eauses an remains practically constant. Conductances were appreciable increa e in conductance. For ex table throughout the titrations. In view of the ample, the forward titration curves for trichloro relatively low conductance of the base, the C011- acetic acid with di-n-butylamine and n-heptyla ductance of solutions containing both salt and mine in benzene, curve 1 in figure 5, and curve 1 free base are unexpectedly h igh. The conduct in figure 6, respectively, show a gradual ri e in ance at the mfiximum of the reverse titration is conductance beyond the equivalonce point. orne 3.7 X 10-11 compared to a value of 1.4 X 10- 11 at interaction between salt and base is indicated, but the corresponding point in the forward titration the effect is relatively less important than in the where the concentration of salt is the same. There case of acid and salt, and is insufficient to cause is evidently some rather strong interaction be a maximum in the curve for the reverse titration. tween the salt and base in dioxane of such a The maxima. in the forward titrations are more nature as to increase markedly the extent of iO'l1- pronounced in benzene than they are in dioxane ization over that produced by the salt alone. or in benzene containing the small amount of alcohol. A comparison of the titrations of tri 3. Titrations With Trichloroacetic Acid chloroacetic acid with n-heptylamine in benzene find in benzene containing 1 per'cent of methyl Figures 4, 5, and the left ide of figure 6, show alcohol, curves 1 and 2, respectively, in fi gure 6, the curves for titrations with trichloroacetic acid. shows that addition of alcohol has nearly elimi All curves for the forward titrations are quali ta nated the maximum and all but obsecured the tively similar. They rise to a maximum between end point. The effect of adding alcohol to half neutralization and the equivalence point. dioxane is shown for the reverse titrations with Immediately beyond the equivalence point, the trichloroacetic acid and triethylamine. The titra conductance tends to level off so that breaks in tion in pure dioxane is represented by right-hand the CUl"Yes at the equivalence points are very curve 3 in figw'e 4, and that after the alcohol had pronounced. Curves for the forw:ard and reverse been added by curve 4. In the latter case, the titrations have entiTely differellt shapes . The end poin t is much less pronounced, and it would conductance increases throughout the reverse probably not be evident at all upon the addition titration but at a faster rate beyond the eq uiva of somewhat larger amounts of alcohol. lence point than before. The breaks, though no t so striking as in the case of the forward titrations, 4. Titrations With Camphorsulfonic Acid are still quite definite. It is apparent from the e curves that solutions The very limited solubility of camphorsulfonic containing free acid as well as salt have abnor acid precluded its use in titrations in benzene at mally high conductances. The outstanding illus the desired concentration. Two titrations were tration is the titration of trichloroficetic acid with made in dioxane, using triethylamine and di-n triethylamine, curve 1 of figure 4. The maxi- butylamine, curves 3 and 4 in figure 6, respectively. The appearance of maxima in these curves, just J mum occurs at X = 0. 3, which corresponds to a solution containing 0.0065-molar salt and 0.0027- as in the titrations with trichloroacetic acid, shows molar acid. The specific conductance of this a similarity in behavior of the two acids. solution is about 1. 5 X 10- 11, whereas that at the 5. Accura cy of End Points equivalence point corresponding to a substaniaUy higher salt concentration of 0.0090 molar is only Except for the titrations with picric acid and 2.0 X 10- 12 • The drop in conductance just before t,riethylamine in pure benzene, the end points were the equivalence point is extremely sharp. On determined by the intersection of smooth curves, titrating the last 5 percent of acid there is a three usually linear, drawn tmough points immediately fold drop in conductfince. This behavior sug on each side of the break in the titration curve. gests n specific reaction between acid and salt Agreement with the equivalence points calculated leading to much greater ionic dissociation than from the independently determined concentrations can be attribu ted to the ordinary dissociation of of the stock solutions was within 3 percent in the salt itself. every case. The average deviation was some",hat

Conductimetric Titrations 533 less than 1 percent and within the experimental instance, would seem to be required. 3 Although uncertainties. In the special case of titrations independent data in dilute solutions are lacking, with picric acid and triethylamine in benzene, the amine salts of carboxylic acids do form complexes end point was indicated by the previou<;ly de with excess acid under certain conditions. Ralston scribed drift in conductance which was evidently [5] has obtained evidence from the freezing point confined to a region within 1 percent of the true diagram of the system, octadecylamine-acetic acid, equivalence point. Because of the nature of the of the presence not only of the 1: 1 compound end point, however, this particular titration is less representing the simple salt bu t also of a compound suitable than the others; and it is apparently not containing two molecules of acid to one of base. applicable to certain other tertiary amines. Prideaux [6] found that the conductance of the Preliminary titr ations with tribu tylamine or two component system , diethylamine-propionic triamylamine, although qualitatively similar to acid, when corrected for changes in viscosity had those with triethylamine, gave a slower and less a maximum value in the neighborhood of 70 mole pronounced drift in conductance, which did not percent of acid, and concluded that a complex give a satisfactory indication of the end point. salt having a higher conductance than the simple Although the effect of alcohol wa s not studied in salt was formed. these prelimina.ry titrations, its addition would, That carboAJ'lic acids associate into dimers in all probability, improve the titrations in the through the formation of hydrogen bonds in the manner observed with triethylamine. vapor phase and in solvents like benzene is 'well established [7 ]. The reaction b etweel). simple salt V. Discussion and acid, formulated by HB+ . . A - + HA= HB+ . . HA2-, might be attributed to the forma It is apparent from the various graphs th at the tion of a complex anion having a similar structure titrations, with the exception of those with picric except that only one hydrogen bond could be acid and triethylaminc in pure benzene, follow one formed. For the case of trichloroacetic acid, this of two gencral patterns. The simplest type in structure would be represented by cludes all the titrations involving picric acid and ( - ) di-n-butylamine, and those with picric acid and o o triethylamine ill benzene con taining the small ;/ amount of alcohol. Corresponding curves for the CI 3C - C C - C Cia. "'"#' forward and reverse titrations have the same O- H o general shape, and the conductance is controlled " almost entirely by the concen tratioll of salt Formation of such a complex salt would be ex formed during the neutralization. For the second pected to enhance the conductance of the soIL! tion. type, curves for thc forward and reverse titrations The larger size of the complex anion and the better have entirely different, though complementary, shielding of its charge should lead to a high er shapes. In all forward titrations employing tri degrce of dissociation of ion pairs into free ions, chloroacetic acid or camphorsulfonic acid, the than for the simple salt, by diminishing the attrac cond uctance passes through a maximum wcll tive forces between thc oppositely charged ions. before the equivalen ce point is reached. This It is interesting to note that the reaction be maximum occurs in the reverse titration in the tween acid and salt still takes place when dioxane particular case involving picric acid and tri rather than benzene is the slovcnt. Dioxane, like ethylamine in dioxane. Except in the titrations other solvents the molecules of which contain with picric acid and triethylamine, tIle character acceptor atoms, tends to destroy hydrogen bonds of the curves did not depend upon the solven t 3 'The high conductances of solutions containing salt and acid together used. were noted by La Mer and Downes in the titration of trichloroacetic acid with dicthylamine ill benzene, previously mentioned [IJ. These authors In order to accoUll t for the behavior of titrations concluded that there was a Debye salt efTect wbich increased greatly tbe ionic of this second type, some specific reaction between dissociation of the acid. However, tbe ionic strellgtbs of these solutions, as evidenced by thei r extremely low cond uctan ces, are so small that sucb an salt and acid, or between salt and base in one interpretation would hardly seem valid.

534 Journal of Research between other molecules because of its own ten data which fail to show any marked rise in con dency to form hydrogen bonds with them. Car ductance during the forw3,rd titration after the boxylic acids, for example, do not associate into equivalence point has been reached. This fact· dimers in dioxane. Consequently, a complex anion is not too surprising as the alts exist almost of the type postulated above would hardly be entirely in the form of ion pairs 0 that the he expected to be sufficiently stable in dioxane to give havior of the cation is influenced by the nature the observed maxima in the titration curves, of the attached anion. unless an exceptionally stable structure resulted. The anomalous behavior of titrations with pic Although the presence of a negative charge on the ric acid and triethylamine in benzene has been complex ion should tend to strengthen this con discussed previously. In a few of the other titra ventional type of hydrogen bonding, it would tions, small variations in conductance with time appear more likely that the ion has a somewhat took place which were not confined to any partic different structure. R esonance in the carboxylate ular portion of the titration curves. Since a ion gives both carbon-o),."ygen bonds a partial dou ble direct current bridge was employed, it is quite bond character and makes them equivalent. In possible that these variations were associated the complex anion the proton of the second carb with some polarization p))enomenon and could be oxyl group could assume a position where it was eliminated by an alternating CUl'rent method. equally shared by all four oxygen atoms as indi These drifts were sometimes toward lower and cated by the structure ometimes toward higher conductances. In the ( - ) titrations with picric acid and di-n-bu·tylami ne in o a benzene, for example, drifts toward lower conduc J '-- Lance , or higher resistaJ1Ce, were noted. The con CI 3 C - C H "c- c C1 3 • /' ductance immediately after addiLion of the titrat ' 0 "'" a in g solution and mixing was several percent higher Rathe)' than the planar configuration shown , the than the steady value reached in 10 or 15 seconds. four oxygens would, in all probability, be located R eversal of the polarity of the bridge caused an tetrahedrally around the proton. Because of the immediate increase in conductance t o or slightly compact structure and the gain in energy through above the initially observed value which was then complete resonance of both carboxyl groups, this followed by a decrease to the same steady value configuration should be more stable than thc as before. Subsequent mL,-:ing also tended to previous structure involving the more co nven tional restore the original yalue. A rather detailed type of hydrogen bond. study of an apparently analogous effect also with. The similarity in behavior of trichloroacetic direct current, has ' been reported by Fuoss and acid and eamphorsulfonic acid indicates a corre Elliott [8] in which the electrolyte was tri-n sponding interaction betwe en the latter acid and butylammollium picrate and the solvent, tricresyl its salt. A bonding of the sulfoni c acid group phospllate, a highly viscous medium with a rela with the sulfonate ion in a manner analogous to tively low dielectric constan t. The attainmell t of t hat formulated in the case of Lh o carboxylic acid a steady state required a much longer time and would appear likely. the change in conductance was much greater in The behavior of the titrations involving picric this case. These authors attributed this slow acid and triethylamine in dioxane, where solutions i.ncrease in effective resistance of the development containing both salt and base have unexpectedly of a space charge in the solution near the elec high conductances, suggests a strong reaction trodes which deceased the potential gradient between salt and base leading to the formation of across the main body of the solution. a hydrogen-bonded complex cation, as R 3N+ - H Drifts toward hi gher conductances, observed I R 3 in this particular case. The possibility for example in titrations with trichloroacetic of forming this arne complex cation exists in other acid and n-heptylamine in dioxane, oc curred more cases, for example, in the titrations of trichloro slowly and several minutes were required to reach acetic acid or camphorsulfonic acid with triethyl a steady state. As before, mixing the solution amine in dioxane. Its actual existence to any restored th e initial value. This drift cannot be significant extrnt is r\lled out by the experimcntal attributed to the development of a spa,ce charge .

Conductimetric Titrotions 535 r·~

On reversal of the polarity, however, an effect titrations, but additional experimen tal data will similar to that described in connection with the be required befor e a more complete interpretation t itrations involving picric acid and di-n-butyl in terms of reaction equilibria can be made. The amine was also observed, so that there would ap data also are of interes t in connection with the pear to be a superposition of at least two different general problem of acid-base equilibria in media of eff ec ts. Observation of these drifts was more or low dielectric constant. While in cer tain in less inciden tal since they did not interfere with the stances such reactions do seem to follow a simple titrations, provid ed a consistent procedure was mass law [9] , in others the behavior is more com followed in taking the readings. Where the drifts plicated [10]. In addition to factors such as as were relatively slow, the initial conductances, re sociation of the acids and salts, the formation of producible on repeated mixing, were used. The complex salts as suggested by the present studies final or steady values were used where the drift may introduce additional complications, particular was quite rapid. In a majority of cases the ly in cases where cal'bo:xyli c or sulfonic acids are systems werc quite stable and no yariation in employed. conductance was observed either on reversal of VI. References the polarity or on mixing. W'heJ'e drifts were [II V. K La Mer and H. C. Downes, J. Am. Chem. observed, addition of the small amount of alcohol Soc. 53, 888 (1931). t o the solvent eliminated them . To explain these [21 P. "'VaIden, Z. physik. Chern. H7, 1 (1930). drifts in cert ain systems and their absence in [31 C. A. Kraus, J. Franklin Inst. 225, 687 (1938). others will require a more thorough investigation [41 V. Dietz and R. M. Fuoss, J. Am. Chem. Soc. 60, employ ing both alternating and direct CLllTent 2394 (J 938) . [5 ] W. O. Pool, H . J . H arwood, and A. \Y. Ralston, methods. J. Am. Chern. Soc. 67,775 (1945). The results of the present investigation show [6] R. N. Coleman and E. B. R. Prideaux, J. Chem. that conductimetric titrations of acids and bases Soc. 1937, 1022. are feasible in solvents of the lowest dielectric [7] J. Karle and L. O. Brockway, J. Am . Chern. Soc. 66, constants even though the conductances are 574 (1944). [8] R. M. Fuoss and M. A. Elliott, J. Am. Chern. Soc. 67, lower by a factor of 10- 8 than they would be in 1939 (1945). water. The diversified behavior of these systems, [9] ::\Iarion Mac'lean Davis and P. J. Schuhmann, un not anticipated from studies in the usual solven ts, published data, National Bureau of Standards. makes generalizations difficult. Qualitatively, the [10] Y. 1<. La Mer aod H. C. Downes, Chem. Rev. 13, assumption of the formation of a complexion 47 (1933). accounts for the behavior of a n lIlll bel' of the WASHING'l'ON, F ebruary 12, 1947.

536 Journal of Research

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