method can be destroyed with peroxide (IO), which, in turn, must be eliminated before nitrate can be determined. Organic substances which are reasonably stable under the conditions of the method do not interfere considerably, even if they are nitrogenated. Preliminary experiments showed that satisfactory, although somewhat low, results can be obtained for nitrate in the presence of equal quantities of gelatin or urea. ACKNOWLEDGMENT
The authors wish to express their ap-
preciation to Kalter B. Mors for his interest in this work and t o Mary Nebel, Wayne State University, Detroit, hIich., for revision of the English manuscript. LITERATURE CITED
(1) Assoc. Offic. Agr. Chemists, “Official hlethods of Analysis,” 8th ed., p. 13, 1955. (2) Baumgarten, P., Bey. deut. chem. Ges. 71, 80 (1938). (3) Berglund, E., Bull. S O C . chim. [2], 29, 422 (1878). (4) Bowler, W. W., Arnold, E. A., ANAL.CHEN.19, 336 (1943).
(5) Brasted, R. C., Zbid., 23, 980 (1951). ( 6 ) Zbid., 24, 1111 (1952). (7) Cumming, W.M., Alexander, W. A., Analust 68, 273 (1943). (8) Gottlieb, 0. R., “Titrimetria Gasombtrica,” Bol. 42, p. 47, Tnstituto
de Quimica Agricola, Ministbrio da Agricultura, Rio de Janeiro (1955);
Anal. Chim. Acta 13, 531 (1955). (9) Hirozawa, S. T., Brasted, R. C., ANAL.CHEM.25. 221 (19531. (10) Johnson, C . M., blrich, A,; Zbid., 22, 1526 (1950).
RECEIVEDfor review May 9, 1957. Accepted December 5, 1957. Work supported by Conselho Nacional de Pesquisas, Brazil.
Basic Behavior of Molecules and Ions in Acetjc Anhydride C. A. STREULI Analytical Research laborafory, American Cyanamid ,Titration of neutral and anionic bases in acetic anhydride has shown that a linear relationship exists between pK,(H20) values and half neutralization potentials in acetic anhydride. The relationship is, however, different for the neutral bases than for ions. Anions are stronger bases in acetic anhydride than in water relative to the neutral compounds. It is also possible to titrate halide salts directly in the solvent. Chlorides and iodides may be resolved, but this is not possible for chlorides and bromides or bromides and iodides. Anions which form weak acids in water solution are leveled in basic strength in acetic anhydride.
T
utility of acetic anhydride as a titration medium, either alone or in conjunction with another solvent, has been noted by Gremillion (4), Fritz (S), and Usanovich ( I S ) . A number of compounds which do not exhibit basic properties in water, acetic acid, or acetonitrile may be quantitatively titrated in the anhydride. The principal drawback of this solvent is its reactivity with some solutes to produce new products, usually weaker bases. The work described was initiated to determine if weak anionic bases might be titrated in this solvent, to give a more detailed picture of acid-base behavior in the anhydride, and to establish, if possible, a relationship between pK,(H20) and half neutralization potential (HNP) of weak bases in acetic anhydride. The substances used as standards were chosen to give as large a range of pK, values as possible, but were limited to those substances which showed a minimum chance of being acetylated by the solvent. In the case of the amines, HE
Co., Stamford, Conn.
this restricted the choice to tertiary amines and heterocyclics. Two assumptions must be made in viewing qualitatively acid-base behavior in acetic anhydride: Hydrogen acids behave (and bases react with hydrogen acids) in much the same manner as they do in a protonated solvent, such as acetic acid; and liquid junction potentials, while unknown, are constant and do not distort the titration curves.
PROCEDURE
Weigh out 1 mmole of the sample to be tested, dissolve it in acetic anhydride, and dilute to 100.0 ml. in a volumetric flask. Pipet 25.0 nil. of this solution into a EO-ml. beaker, add 75 ml. of anhydride, and titrRte with
REAGENTS AND APPARATUS
Reagents. The acetic anhydride used as solvent was Baker and Adamson reagent made. which was not further purifiedu. ‘ PERCHLORIC ACIDSOLUTION, 0.05N. This was prepared by diluting 4.2 ml. of 72% acid with about 500 ml. of acetic acid and then diluting this to 1 liter with acetic anhydride. The solution became highly cofored on standing, but color formation did not affect the normality. The acid was standardized against potassium hydrogen phthalate (11). Xost of the organic compounds mere Eastman Kodak White Label. Quaternary ammonium salts were obtained from the southwestern Chemical Co. Other salts were reagent grade. Apparatus. All titrations were performed using a Precision-Dom Recordomatic titrator. Solutions were agitated with a magnetic stirrer during the titration. The electrodes mere a conventional glass electrode and a silver-silver chloride reference cell. The latter was made from the shell of a Leeds & Northrup calomel electrode, and contained a coiled silver wire, electrically coated with silver chloride, immersed in acetic anhydride which had been saturated with silver and lithium chlorides.
‘ 0
ll-ullJ P 40 80 80 100 120 % Neutralization
Figure 1 . Titration of neutral nitrogen bases in acetic anhydride a. Acetanilide b. Acetamide c. Caffeine d. Urea e. Methylurea f. N,N-Dimethylaniline g. Quinoline h. Pyridine 1. N,N-Diethylanlline k. N,N-Dimethylbenzylamine I. Tri-n-butylamine
VOL. 30, NO. 5,
MAY 1958
997
the standardized acid. Potentiometric curves were recorded automatically. Three aliquots were usually run, and blanks were run on each batch of solvent. Some of the salts which had limited solubilitv in the anhvdride were initiallv dissolved in 1 or 3 ml. of water and then diluted with the anhydride. Although the water was removed by conversion to acetic acid, the salts remained in solution. Samples run in solutions that contained water initially gave identical results with those that did not. RESULTS AND DISCUSSION
The titration curves obtained for some nonionic bases and some salts, corrected for blanks, were plotted as per cent neutralization against potential (Figures 1 and 2). No leveling for the neutral bases tested mas observed. However, all salts derived from weak acids which did not undergo reaction with the solvent were leveled. The two families of curves show that amines, amides, ureas, and salts all have similar basic behavior in the anhydride. Halide salts are readily titrated in this solvent without the addition of mercuric acetate and the consequent leveling r e action. This has not been possible in solvents previously reported (9, 12). The nonleveling properties of the solvent should make possible the differential titration of the salts of strong acids in general as well as weak neutral bases. The slope of the titration curve is essentially linear between the values of 20
and 80% neutralization. The slope for the stronger aliphatic amines and leveled anions is 1.3 f 0.1 mv. per % neutralization. For the other weaker bases, this slope is 1.0 f 0.1 mv. per % neutralization. This corresponds to the buffered region for weak bases in water, although in water this slope is about 0.9 mv. per % neutralization, as calculated from the Nernst equation. In Figure 3, pK,(H20) values for the compounds used as standards have been plotted against half neutralization potential in the anhydride. Two sets of relationships are apparent based on the charge of the original base. Keutral bases show an unleveled linear relationship, with heterocyclic compounds the only group of nitrogen bases showing large deviations. Average deviation from the median line, excluding heterocyclics, is A0.15 pK, unit; the u value is 10.35 unit. Individual values are given in Table I, which contains essential data for all compounds used as standards. The slope of the neutral base line is 51 mv. per pK,(H,O). Hall (5) obtained a slope of 59 mv. per pK,(H20) for this relationship in acetic acid and Frita (2) obtained 100 mv. per pK,(H20) for acetonitrile. Of the three solvents, acetonitrile is clearly the best differential medium for neutral bases. Titrations for samples which did not undergo acetylation may be considered quantitative. Gremillion (4) claims that acetylatable molecules do not react a t 0” C. and may be titrated in the an-
hydride. No attempt was made to duplicate these conditions. A few conipounds which did undergo acetylation also give quantitative results, because the products of the acetylation are still reasonably strong bases. These data are included in Table 11. Univalent anions show a pK,(H20)HNP relationship differing from that of the nitrogen bases. As shown in Figure 3, the median line is neither coincident nor parallel to that determined by the neutral bases. Leveling occurs for the salts of weak acids. The slope of the unleveled portion of the line is 34 mv. per pK,(H20). Average deviation is *0.05 pK, unit, and u is 10.1 unit. Briefly, the two relationships illustrate that anions are, relative to uncharged molecules, stronger bases in acetic anhydride than they are in water. The lower dielectric constant of the anhydride (E = 21) favors the formation or retention of an uncharged species and should, therefore, increase the base strength of anions in general. The absence of water from this system should also permit the titration of bases TT-eaker than water. Salts leveled by the solvent react to produce mixed anhydrides and acetate ions, the latter being the titratable species. Titrations of a variety of molecules with unknown or poorly defined pK, values are illustrated in Figure 4. Pertinent data for these as well as other neutral bases and salts are listed in Table 11. The calculated pK, values mere obtained by use of the appropriate
iooom
Figure 3. Base strength in water and in acetic anhydride
800
I
I
0
EO
40
60
BO
I00
120
Ye NeutroI~zofion
Figure 2. Titration of anionic bases in acetic anhydride A. Potassium iodide B . Potassium bromide C.
D. E. F.
lithium chiorlde Potassium nitrate Sodium acetate, sodium formate, and sodium lbarbital Potassium hydrogen phthalate and sodium fluoride
998
ANALYTICAL CHEMISTRY
0
HNP
(AcZO)
calibration line in Figure 3. Some of the compounds are of sufficient interest to be considered in more detail. Ai-Methyl-and N-n-propylacetanilide are both somewhat stronger bases than the parent compound. Similar behavior has been noted with , N-substituted anilines. The calculated pK. values are about one unit less than those calculated b y Hall from acetic acid data (6). Xtrilotrispropionitrile gives a vivid illustration of the effect of negative substitution on base strength. Although IO00
the nitrile groups are insulated from the nitrogen by two saturated carbon atoms, the pKa value of the molecule is nine units less than that of the parent tri-npropylamine. Compounds such as diphenylguanidine and anthranilic acid appear to be completely acetylated to a single product which can be titrated quantitatively. The acetylation product of diphenylamine gives low results. Neither propio- nor acetonitrile showed any indication of basicity when titration of these was attempted in this solvent.
1
-r
t
200'
0
I
20
I 40
I
60
I 80
I
100
1
120
% Neutroiizati
Figure 4. Titration of various neutral bases in acetic anhydride A. Acetanilide D. Dimethyl sulfoxide 8. Acrylamide E. Nitrilotrispropionitrile C. 1 -Methyl-2-pyrroRdinone
Table I. Titration of Standards in Acetic Anhydride
PK. Compound HNP(Acz0) Lit. Calcd. ApK. Tri-n-butylamine 148 9.85 9.95 0.10 N,N-Dimethylbenzylamine 196 9.02 9 . 0 1 -0.01 N,N-Diethylaniline 322 6.52 6.55 0.03 N-Ethyl-N-methylaniline 358 5.99 5.85 -0.14 Pyridine 336 5.30 6.28 0.98 X,N-Dimethylaniline 389 5.21 5.25 0.04 Isoquinoline 328 5.30 6.43 1.13 369 5.06 5.63 0.57 Quinoline Caffeine 63 3 0.61 0.49 -0.12 hlethylurea 604 0.90 1.05 0.15 625 0.50 0.64 0.14 Urea Phenylurea 660 -0.30 -0.04 0.26 712 -0.48 -1.05 Acetamide -0.5i Potassium iodide 581 -10.74 -10.7 0.0 Potassium bromide 485 -7.74 -7.9 -0.2 Lithium chloride 375 -4.74 -4.i 0.0 Potassium nitrate 258 -1.34 -1.3 0.0 197 2.9 Leveled Potassium hydrogen phthalate 195 3.2 Leveled Sodium fluoride 218 3.67 Leveled Sodium formate 220 4.72 Leveled Sodium acetate 220 7.8 Leveled Sodium barbital a Acetylates partially.
Compound
1
i
iI
Acetanilide N-h-Methylacetanilide N-n-Propylacetanilide
Nitrilotrispropionitrile 1-Methyl-2-pyrrolidinone
dcrylamide Diphenylguanidineb Anthranilic acidb Dimethyl sulfoxide Triphenylphosphine
Tetra-n-butylammonium iodide
HNP (AczO)
mi. of acid
Figure 5. Titration of a mixture of quaternary ammonium chlorides and iodides
98.8 98.9 98.7 98.8 98.6 98.7 99.1 98.3 98.5 a
a
a a
99.4 99.8
...
96.6 100.1 91.0 98.7 97.8 99.8
iodide Tetramethylammonium iodide Tetra-n-butylammonium bromide Tetramethylammonium chloride Aniline hydrochloride Guanidine nitrate* Ammonium acetateb Sodium p-toluenesulfonate Trisodium phosphateb Sodium dihydrogen phosphate* a Acetylates partially. b Acetylated product titrated.
PK. Calcd.
Purity Found
-2.9 -1.5 -1.5 1.4 -0.3 -2.0
94.5 99.3 100.6 99.0 97.7
Neutral Bases 809 737 736 584 676 760 479 735 605 620
...
a
...
99.6 99.0 100.0 97.0
560
-10.1
100.2
550
-9.8
99.8
568
-10.3
100.7
455
-7.0
109.9
364 809 4i9 714 549 581
-4.4
-9.7 (-10.7)
100.1 100.8 97.6 91.7 96.2 99.0
587
(-10.8)
...
...
1.0
Anion Bases
Tri-n-butylmethylammonium
0
Purity Found
Table II. Titration of Various Compounds in Acetic Anhydride
1000
800
Dimethyl sulfoxide gives a n excellent titration curve and quantitative results. Neither dipropyl sulfide or sulfone exhibited any basic properties. This solvent should offer a rapid and accurate method for determining sulfoxide in the presence of the other two types of compounds. Aromatic nitroso compounds may also show basicity. Ethers such as di-n-butyl ether and tetrahydrofuran exhibited no basic properties to hydrogen ion when dissolved in acetic anhydride. Triphenylphosphine, the phosphorus
... ... ...
VOL. 30, NO. 5, M A Y 1958
999
analog of triphenylamine, is titratable in the anhydride while the nitrogen compound is not. No base constant was calculated, as there was some evidence of acetylation of the phosphine. Quaternary ammonium halides all gave quantitative titrations. Calculated pK. values are in agreement with literature values. Chlorides and iodides could be resolved as shown in Figure 5. The sample titrated contained 20.4 mg. of tetramethylammonium chloride and 91.9 mg. of tetrabutylammonium iodide. The amounts found were 20.4 and 90.7 mg., respectively. Chlorides and bromides, or bromides and iodides, give only one break in the titration curve. Bisulfates could not be titrated. This may be due either to the high acidity of sulfuric acid or to the formation of a superacid by reaction of solute and solvent. Russell (IO) has noted a reaction between the anhydride and sulfuric acid to yield a mixed anhydride of high acidity: (CH&0)20
+ H2SO4
e
-+
0 0
I1
CHa OS-OH
II
+ CHsCOiH
Titration of monosodium phosphate resulted in a titration curve indicating the presence of a very weak base, pK. -10.8, which is as weak as iodide ion, In aqueous solution the dihydrogen phosphate ion has a PI(, of 1.96. Reaction with the solvent may have produced a superacid: HzPOI-
+ 2(CHsCO)ZO
(CHaC0)zP-0-
II
+ (CHpC0)20
CH~C-ONHI
-+I
0
I/
+
CHsC-NH1 2CHaC02H (4) The titration of aniline hydrochloride is quantitative. That for ammonium acetate is not, implying the reaction may proceed partially to imide formation.
+
LITERATURE CITED
0
//
0
+ 2CHSCOzH
(1) Edwards, J. O., J . Am. Chem. SOC.
(2)
When trisodium phosphate is titrated, two breaks appear in the titration curve. The first end point occurs after the addition of two equivalents of acid, and the second after addition of the third equivalent. The titration curve again illustrates formation of a superacid for the final part of the titration. More condensed molecules should be stronger acids than parent compounds, as illustrated by the strength of pyro acids over the less condensed parent. Other salts such as aniline hydrochloride and ammonium acetate react with the solvent to give amides, which are then the entity titrated.
(1)
0
The corresponding base would be, of course, exceedingly weak.
76,1540 (1954).
12) Fritz, J. S., ANAL. CHEM.25. 407
(1953). ‘ Fritz, J. S., Fulda, M. O., Ibid.,25, 1837 (1953). Gremillion, A. F., Ibid., 27, 133 (1955). Hall, N. F., J. Am. Chem. SOC.52, 5115 (1930). Kolthoff, J. M. Furman, Tu’. H., “Potentiornetr{c Titrations,” p. 329, Wiley, New York, 1926. Lange, N. A., “Handbook of Chem(7) istry,” 4th ed., pp. 1220-1, Handbook Pub., Sandusky, Ohio 1941. Lemarie, H., Lucas, H. J., f. Am. Chem. SOC.73, 5198 (1951). Pifer, C. W., WoIlish, E. G., Schmall, hl., J. Am. Pharm. ASSOC., Sci. Ed. 42, 509 (1953). Russell, J., J. Am. Chem. SOC.60, 1345 (1938). Seaman, W., Allen, E., ANAL.CHEM. 23, 592 (1951). (12) Streuli, C. A., Ibid., 27, 1827 (1955). (13) Usanovich, M., Yatsimirskii, K., J . Gen. Chem. U.S.S.R. (Eng. Transl.) 11, 954 (1941). RECEIVED for review August 22, 1957. Accepted December 14,1957.
Amperometric Titration of Fluoride with Thorium Using a Rotating Palladium Electrode W. E. HARRIS Department o f Chemisfry, University of Alberta, Edmonton, Alberta, Canada )The amperometric titration method for determining traces of fluoride is performed in a solution containing potassium bromide, potassium sulfate, and aerosol, and buffered with monochloroacetic acid and sodium monochloroacetate. Best results are obtained with about 100 to 200 y of fluoride per 100 ml. of solution, although as little as 20 y in 100 ml. can b e determined. Moderate amounts of chloride, nitrate, sulfate, perchlorate, borate, calcium, or magnesium cause no interference but large amounts slightly decrease the sensitivity. Aluminum and more than 1 mg. per liter of phosphate interfere. A simplified, inexpensive apparatus is described.
A
of determining traces of fluoride was desired which would
METHOD
0
ANALYTICAL CHEMISTRY
be fast, accurate, and subject to a minimum number of interferences. The amperometric titration method d e veloped is performed in a solution containing potassium bromide, potassium sulfate, aerosol, and buffered with monochloroacetic acid and sodium monochloroacetate. Best results are obtained with about 100 to 200 y of fluoride per 100 ml. of solution, although as little as 20 y can be determined. Moderate amounts of chloride, nitrate, sulfate, perchlorate, borate, calcium, or magnesium cause no interference, but large amounts slightly decrease the sensitivity. Aluminum and more than 1 mg. per liter of phosphate interfere. A simplified, inexpensive apparatus suitable for these fluoride analyses is described. The principal methods for the deter-
mination of fluoride involve either the measurement of the bleaching effect of fluoride upon a colored organic salt of a metal such as thorium or zirconium or the titration by standard thorium solutions using an indicator such as Alizarin Red S (IO, IS). Conditions for carrying out the measurements must be carefully controlled, and many substances may interfere with either the development of the color or the detection of the end point. A number of amperometric methods (2, 6, 7 ) use the dropping mercury electrode, but in general these are not conveniently adaptable to the determination of very low concentrations of fluoride. Recently Shoemaker (9) described a polarographic method for the determination of fluoride using the iron fluoride complexes. These complexes are
