Additional Acid-Base Indicators in Glacial Acetic Acid ORLAND W. KOLLING and THOMAS 1. STEVENS Deparfmenf o f Chemisfry, Soofhwesfern College, Winfield, Kan.
b Selected azine, acridine, thiazine, and thiazole dyes were evaluated for suitability as acid-base indicators in glacial acetic acid as the solvent. Safranine 0, neutral red, and thioflavine T are satisfactory visual indicators for the perchloric acid titration o f bases having pKb values less than 6.1. In addition to these dyes, phosphine i s acceptable for titrations in which the equivalence point i s determined photometrically. Indicator half-neutralization numbers were obtained for eighi dyes, and these were applied to an empirical acidity function measurement. The pCHc,O, range from 2.20 to 4.50 i s determinable, using dyes from the four families investigated.
AZINES Safranine 0 (Color Index 841)
have been proposed and used as acid-base indicators for titrating bases in glacial acetic acid media with perchloric acid (6, l a ) , and a few of these have been applied to the photometric detection of the equivalence point (3, 6). For most dyes, sufficiently reliable indicator constants are not available to permit their evnluation for empirical acidity function measurements in glacial acetic acid. The previous study by Kolling and Smith (11) on the triphenylmethane dyes demonstrated the suitability of selected members of a single dye family for all three applications-photometric indicators, visual indicators, and acidity function determinations. The work reported 1 ..in is an extension surveying EVERAL DYES
'
other related classes of amino-nitrogen dyes, including members of the azine, acridine, thiazine, and thiazole families. The notation consistent with the Lewis theory that has been proposed (9) was utilized for calculating the indicator half-neutraliaation numbers (LIl2 values) The empirical acidity function L, defined by the [ Z B ] / [ Z A ] ratio ( I I ) , is equivalent to the (HO).PP. function of Bruckenstein ( I ) . The pC~clo,range of 2.25 to 5.10 can be measured with triphenylmethane indicator pairs. The dyes evaluated below extend this range only slightly toward greater acidity. The following amino derivatives of the azine, acridine, thiazine, and thiazole families were selectedjor study. I
Acridine orange (Color Index 788)
Acridine yellow (Color Index 785) r H l+
J
L
S
Neutral red (Color Index 825)
THIAZINES Methylene blue (Color Index 922) Induline 6B (acetin blue) (Color Index 860) H
r
1'
1' New methylene blue N (Color Index 927)
ACRIDINES Phosphine (Color Index 793) H
L
-I
THIAZOLE Thioflavine T (Color Index 815)
c1-
1384
ANALYTICAL CHEMISTRY
r
l+
EXPERIMENTAL
Apparatus. Absorbance measurements and photometric titrations were made with a Bausch & Lomb Spectronic 20 spectrophotometer. The potential difference of the glasscalomel electrode pair was determined with the Beckman Model H2 p H meter. Reagents. The glacial acetic acid (Mallinckrodt A. R.) contained less than 0.001% water by Karl Fischer titration of samples from each bottle of the two cases used in this work. Consequently, no additional purification was necessary. Solutions of 0.09M perchloric acid and 0.1M sodium acetate, o-chloroaniline, potassium acid phthalate, sodium salicylate, and urea were prepared and standardized as reported previously (8, 11).
The dyes were obtained from Hartman-Lcddon Co. and were recrystallized from bcnzene-methanol mixtures. The compositions of the mixed solvent used for each dye were:
Dye Acetin blue Phos hine Acriine orange Acridine yellow New methylene blue Methylcne blue Thioflavine T Safranine 0 Neutral red
Ratio CeHs/ CHaOH, (v./v.) 3: 1 1:l 1:l 1:l 3: 1 3:l 1:l 1:l 1:l
Stock solut,ions of the last four indicators in acetic acid were 0.001M; however, because of the low solubility of the first five indicators in glacial acetic acid, 0.0001Ai stock solutions of these were prepared. Procedures. The basic procedures used were those reported in the initial study on the triphenylmethane dyes (11). Since each of the indicators listed in Table I shows only one color change in the visible region of the spectrum, the electromotive force (e.m.f.) of the glasscalomel electrode pair for that change was determined with respect to the 0.100M sodium salicylate reference solution. Samples of bases titrated with perchloric acid were in the range 0.25 to 0.5 mmole. Because of the large overlap of the absorption bands for the acid and base forms for phosphine, acridine orange, acridine yellow, methylene blue, and new methylene blue, Beer's law plots were made for wave lengths differing from the absorption maxima, selected so that the overlap was a t a minimum. The indicator half-neutralization numbers for these five dyes and for thioflavine T required the calculation of the color ratio using Equation 1,
where the concentration of the base form Ce was determined from the measured
Visual Color Changes of Indicators in Titrations of Bases with Perchloric Acid 70Titration E r r o r KHPhthal- o-Chlorw Dye Color Change E - E g , Mv.0 ate aniline Pink t o violet 250 f 4 0.11 0.10 safranine 0 Red to blue-violet 247 f 3 0 11 0.74 Neuttal red Pale yellow t o colorless 306 f 3 2.7 ... Phosphine Pale yellow t o colorless 282 f 3 2.1 ... Acridine orange 298 f 1 . 5 2.7 ... Acridine yellow Yellow t o pale yellow 254 f 3 0.25 1 .o Thioflavine T Yellow to colorless
Table 1.
0 E is the measured e.m.f. for the solution containing the indicator and the reference solution.
Eg
is that for
Table II. Determination of Indicator Half-Neutralization Numbers Wave Length, Mean Used, Mp Acid form Base form Log I~ll.4 ~ C H C I O ~ PLUZ Dye 0.873 4.15 -3.30 Safranine 0 580 520 0.648 3.97 (A0. O l ) Neutral red
615
535
Phosphine
375
460
Acridine orange
515
485
Acridine yellow
...
450
Methylene blue
...
605
New methylene blue
.. .
625
Thioflavine T
...
420
0.549 0.456 0.424 0.659 0.256 -0.154 0.979 0.644 0.158 0,043 0.804 0.125 0.062 -0.063 0.502 0.125 0.015 -0.043 0.537 0.158 0.043
3.85 3.75 3.97 4 . I5 3.75 3.27 4.15 3.85 3.37 3.27 4.15 3.50 3.37 3.27 3.85 3.50 3.37 3.27 3 85 3.50 3.37
0.720 0.288 0.298 0.149 -0.791 -0.074 -0.242 -0.511
3.85 3.50 3.37 3.27 3.85 4.45 4.15 3.97
0 . m
absorbance, and CT was the total concentration of the indicator base (in the range 10-6 t o 10-M). In the photometric titrations the change in absorbance of the base form of the dye was recorded, and the end point was determined as before (11). For safranine 0, neutral red, and acridine yellow, titrations were made using both the acid and base form wave lengths. RESULTS AND DISCUSSION
Visual Indicators. Of the nine dyes studied, six had sufficiently distinct and reproducible color changes t o be detectable visually. These are listed in Table I. The potential differences for the glasscalomel electrode pair, corresponding to the color changes, are greater than the values for the first color change of any of the triphenylmethane dyes. From the quadruplicate titrations of
-3.48 (f0.03) -3.20 (f0.02) -3.35
(
=to.02)
-3.31 (f0.06) -3.31 (f0.02')
3 27
-3.13 (k0.03) -4.51 (f0.07)
0.5-mmole samples of the bases, one would conclude that safranine 0, neutral red, and thioflavine T are suitable for indicating the equivalence point in the perchloric acid titration of strong bases (potassium acid phthalate) having pK, values of 6.1 or smaller in glacial acetic acid. Thioflavine T is the least desirable of the three because of its yellow to colorless transition. In the titration of bases having strengths comparable to o-chloroaniline (pKb of 8 to 9 in HOAc), only safranine 0 gave results of acceptable accuracy. Phosphine and the acridines gave obscure end points in the titration of the weaker base. Indicator Constants. To determine values for indicator constants that are sufficiently precise to be ufiable in acidity function measurements, the influences exerted by any added salts upon the color ratio, [ z B l / [ I A l , VOL. 33,
NO. 10, SEPTEMBER 1961
1385
must be considered carefully (10). Changing ionic environment accompanying changes in concentration of base-salt mixtures (as a result of ionpair dissociation) may completely overshadow the function of a buffer solute, as was observed in the studies of Conant and Werner (2), and Higuchi, Feldman, and Rehm (6). The previously adopted experimental expedient (11), in which the solutions used to determine the indicator half-neutralization numbers were saturated (at 25' C.) with potassium perchlorate, was again employed. The concomitant ion-pair dissociation equilibria in Equation 2 maintain a constant ionic environment and, presumably, ZH +ClodK+C10,-
& ZH +
+ ClOr-
e KfC104-
SK +
+ C104-
_-
constant activity coefficients for the colored species of the indicator. From values obtained for the triphenylmethane dyes as well as those listed in Table 11, it appears that this system is a t least equal in precision to the more recent method of Higuchi and Connors (4). The less precise values for acridine yellow and thioflavine T result from the extensive overlapping of the absorption bands used to determine the [ I B ] / [ I A ] ratio. Changes in the spectra of induline 6B in its acid and base forms were so slight that no meaningful quantitative calculations could be made. The representative data in Table I1 include values selected from twenty individual measurements on each dye; however, the mean pLliz values given were computed from all of the individual values for each dye. From comparisons of pLliz numbers for dyes in the same family, the usual trend is followed in which base strength increases in going from a primary to secondary aromatic amine. There are no significant differences between basicities of the acridine and thiazine analogs in glacial acetic acid as a solvent. Thioflavine T is comparable to brilliant green in base strength and is one of the stronger indicator bases in this solvent. Acidity
Function Measurements.
Only the most suitable acidity function indicators are included in Figure
Table 111.
1. 2. 3.
Thioflavine T Neutral red Acridine orange
1. The expected linear relationship between L and the log [ I B ] / [ I A ] was obtained, and the slope is 1.00 for each dye used to determine the negative logarithm of the analytical concentration of perchloric acid. Thioflavine T resembles the triphenylmethane dyes in having its longer range on the acid side ( [ I B ] / [ I A ]less than 1.0) and shows considerable scattering from linearity on the basic color. However, each representative of the azine, acridine, and thiazine families has a symmetrical range of linearity from approximately 1.0 to -1.0 for log [ I B ] / [ I A ] . Both new methylene blue and thioflavine T show too great a scattering of points to recommend their separate application to acidity function measurements. The most suitable dyes and their individual ranges are: neutral red, 2.85 to 4.40; safranine 0, 2$.20 to 4.30; acridine orange, 2.45 to 4.35; and acridine yellow, 2.30 to 4.35. The range of measurable acidity is not extended by these dyes beyond that determinable
Photometric Titrations of Bases with Perchloric Acid
Safranine 0 Keutral red Phos hme Acritne orange Thioflavine T
a
b
-0.8 -3.0 -0.7
-0.5
..
-1.4
-2.3 0.5 -3.4 -0.3
D
-18.1 -21.0
..
-13.8
End point determined by the method of Hummelstedt and Hume ( 7 ) End point obtained from Type I1 plots ( 6 ) . 1386
ANALYTICAL CHEMISTRY
2
Figure 1. Empirical acidity function measurements in glacial acetic acid, using various indicator bases
Relative Error, % Sodium Acetate o-Chloroaniline Dye
3
4
(2)
b
- 7.2 -10.0 - 9.3 -12 5 - 6.9
4. 5. 6.
Acridine yellow Safranine 0 New methylene blue
with the triphenylmethanes; however, nearly the same range can be covered using a single indicator rather than an overlapping pair of indicators, as is required with the triphenylmethanes. The pair, acridine yellow-thioflavine T can be used to measure the range 2.30 to 4.50, if desired. Photometric Titrations. Both the general method of Hummelstedt and Hume (7) and the Type I1 plot (6) were used to determine the equivalence point in the photometric titrations, and these results are given in Table 111. The latter method is to be preferred when the course of the titration is being followed by the absorbance of the acid or base forms of an indicator. On the other hand, the direct determination of the titration end point from the ultraviolet absorption peak of o-chloroaniline, reported by Hummelstedt and Hume ( 7 ) , gives more accurate results than those given for any of the dyes in Table 111. For the titration of 0.5-mmole samples of strong bases with 0.09M perchloric acid, the dyes, safranine 0, phosphine, and thioflavine T, are acceptable. These three dyes would be expected to be suitable indicators for the photometric titration of such bases as guanidine, tribenzylamine, diethylaniline, and potassium acetate, having pKb values from 5.4 to 6.3 in acetic acid. (The pK6 for sodium acetate is 6.6 in HOAc.) However, none of the dyes gave quantitative results in the titration of o-chloroaniline, a moderately weak base. The results in Table I11 include only the better indicators of the eight dyes
t.ested, and were obtained €rom a minimum of four determinations on each base. Urea was titrated photometrically also; however, the values were from 23 to 48% below quantitative recovery and were not of sufficient meaning to be included in Table 111. ACKNOWLEDGMENT
The authors express their appreciation to Research Corporation for financial support of this investigation.
LITERATURE CITED
(1) Bruckenstein, S., J . Am. Chem. SOC. 82,307 (1960). (2) Conant. J.. Werner., T.., Ibid.., 52., 4436 (1930). ' (3) Connors, K., Higuchi, T., A X A L . CHFM.32,93 (1960). (4) Higuchi, T., Connors, K., J. Phys. Chem. 64, 179 (1960). (5) Higuchi, T., Feldman, J., Rehm, C., ANAL.CHEM.28, 1120 (1956). (6) Higuchi, T., Rehm, C., Barnstein, C., Ibzd., 28,1509 (1956). (7) Hummelstedt, L., Hume, D., Zbid., 32,576 (1960).
.,
(8) Kolling, 0. W., J . Am. Chem. SOC. 79,2717 (1957). (9) Kolling, 0. W., J. Chem. Educ. 35, 452 (1958). (10) Kolling, 0. W., Trans. Kans. Acud. Sci. 63, 67 (1960). (11) Kolling, O., Smith, M., ANAL. CHEM.31,1876 (1959). (12) Stock, J., Purdy, W., Chemist Analyst 48,22, 50,55 (1959).
RECEIVED for review April 17, 1961. Accepted July 3, 1961.
-
Organophosphites The Effect of Ionizing Electrons on the Relative Abundance of Their Ion Species H. R. HARLESS Research Department, Union Carbide Chemicals Co., South Charleston, W . Va.
b The mass spectra of a series of dialkyl phosphites and the effects of ionizing electrons upon the relative abundance of their ion species have been studied. Anomalous rearrangement phenomena were observed in the ionization-dissociation schemes of dimethyl, diethyl, di-n-propyl, diisopropyl, diallyl, and di-n-butyl phosphites. Severe alkyl chain scission of phosphites occurs in the mass spectrometer with a resultant extreme degree of migration of hydrogen atoms from the alkyl moiety to the electrophilic PO1 entity. Relative intensities of ionized fragments of the various phosphites are correlated with structural features of the parent compounds. This work has led to the assertion that phosphorus(V) is a prevalent state of the element and that the quadricovalent state is not always achieved b y ionizing electrons.
T
BEHAVIOR of organophosphates in the mass spectrometer has been. reported by Quayle (6) in a study of trialkyl, triaryl, and alkyl/aryl phosphoric esters. The triaryl phosphates dissociate and ionize in a classical manner of ordinary bond scission observed in the mass spectra of hydrocarbons (1). Trialkyl and alkyl/ aryl phosphates, however, exhibit a singular mode of rearrangement when bombarded by ionizing electrons in that niany of their ions are formed by the shifting of one or two hydrogen atoms from substituent groups to the strongly electrophilic phosphate central body. The multiple rearrangements of hydrogen atoms in triethyl phosphate have been noted by McLaf-
HE ANOMALOUS
ferty (3), who also studied replacement rearrangements (4, 6) for many types of compounds with electronegative substituents such as ketones, amides, esters, nitriles, acids, phosphates, and sulfites. Since organophosphates exhibit a degree of rearrangement or atomic migration unparalleled by previously reported classes of compounds, it was expected that this mode of ionization would hold true for other types of phosphorus compounds. However, organophosphites display an even more severe mode of rearrangement under the effect of ionizing electrons. To explain the nomenclature for dinlkyl phosphites, the quinquevalent esters of phosphorous acid, our choice of structure is that used by Van Wazer (7). The dinlkyl phosphites are those compounds in which a hydrogen is bonded directly to the phosphorus atom and the molecules do not contain hydroxyls: (RO)zPH(0). This, therefore, is the series of compounds formerly designated as dialkyl hydrogen phosphonates, a term still frequently used by the British. APPARATUS A N D MATERIALS
The mass spectrometer was a General Electric Model G-5 (60' sector, &inch radius instrument, with magnetic scanning). Samples were introduced into the mass spectrometer by glass pipets through a mercury orifice (8). Sample pressures (microns of mercury) were' measured by the CEC Model 23-105 micromanometer (Consolidated Electrodynamics Corp.). The alkyl phosphites were obtained from Hooker Chemical Corp., Niagara Falls, N. Y., and Virginia-Carolina Chemical Corp., 401 E. Main St., Richmond, Va.
DISCUSSION
The mass spectra of simple dialkyl hydrogen phosphites were studied and anomalous rearrangement phenomena were observed. The spectra of dimethyl, diethyl, dipropyl, diisopropyl, diallyl, and dibutyl hydrogen phosphites are shown in Figure 1. Ions of insignificant intensities are not shown. A detailed discussion of the various spectra are given and intercomparisons are made for analogous and for incongruous ionization-dissociations. Dimethyl Phosphite. Dimethyl phosphite is characterized by a classical type of bond fission reported for various hydrocarbons and other classes of organic molecules. Excluding a mass-49 ion and a mass-65 ion, its mode of fragmentation is one of the rupturing of single bonds of carbonoxygen or phosphorus-oxygen, All of the expected ions (CH3+, C&O+, PO+, HPO+, POz+, HP03+, etc.) are observed. A single hydrogm shift from an alkyl group to the phosphorus electrophilic entity is achieved in two cases. A mass49 ion is formed by the migration of a hydrogen from a methyl group to the H P 4 nucleus and a mass-65 ion is arranged by the shifting of one hydrogen to the 0-P(H)=O central group. The full molecule (parent peak) is an abundant ion in the spectrum of dimethyl phosphite; all other homologs exhibit a small to almost insignificant parent peak because of the high probability of bond rupture, a condition which often occurs in large molecules. Diethyl Phosphite. I n addition to the expected simple bond scissions, the diethyl ester spectrum displays VOL. 33, NO. 10, SEPTEMBER 1961
1387
