the largest error in R and CY in the region of CY = a / 2 - 8. R is taken as the observed difference to sum ratio. I t follows that from Equation 8
R=-
sin28sin2a
1
+ cos28cos2a + E,
(9)
One may a t this point use a calibrating substance to evaluate E,. However, close examination of the Kahn data indicates that the contribution to E,, caused by polaroid imperfections varies with the angle (8 C Y ) . And, of course, the dark current will vary with the degree of amplification. With the exception of depolarization caused by scattering which is absent in a clear solution, the above sources of error can be evaluated and eliminated to a considerable extent. The quantity of radiation that passes a crossed polarizer and analyzer can be determined. For angles other than go’, deviations from Malus’ law may be used for this purpose. We have found polaroids reasonably satisfactory for values of (a 0) as high as 85’; however, complete extinction does not seem to be possible with polaroids. Thus a t (CY 8) = ~ / 2 the , useful signal may be swamped by the unpolarized
radiation For mork of the higheit accuiac) ! prisms that can > ield coniplete extinction zhould be uzed The approlriate uie of a shutter should enable one to eliminate the noise component caused by dark current To obtain an estimate of the qignal to noise ratio for polaroid- in the region where the rotation O( = T 2 - 0, J+e note according to Equation 9 that the maximum T alue for R is
+
+
+
ai; the reference energy, this being then three to four times the noise level. I n the light of these findings it wvould seem that the best precision would be obtained if the value of the fixed angle, 8, were adjusted to the noise level of the polarimeter. Calibration with known substances would still be advantageous for reasons that have been considered previously ( 2 ) 3 ) . LITERATURE CITED
(1) Carroll, B., “Methods in Carhohydrate Chemistry,” Whistler ed., Val. IV, Chap. 33, Academic Press, S e w
from which
E,
=
(l/R,nax- l)sin2(20)
(11)
Csing the Kahn value>, 0 = 84.72 and = 0.870, in Equation 11 yields
R,,,
E,
=
5.0 X
or 0.5%.
Ascribing the noise entirely to imperfections in the polaroid elements, the per cent of equivalent light passing through crossed polarizer-analyzer system will be E , 2 or 0.25Yc,, a rather modest value. However when 8 is 84.72’, even though the rotation, CY, were vanishingly small we have, according to Malus’ lawv, only cos2 (84.72’) or 0.85% of the initial radiation
York, 1964.
( 2 ) Carroll, B., Tillem, H. B., Freeman, E. S., ha^. CHEW.30, 1099 (19583. (3) Kahn, L. I)., Calhoun, R . L., Jr., Witnauer, L. P., J . d p p l . Polymer Sei.
8, 439 (1964). (4) Ilouy, A. L., Carroll, B., A s a ~ . CHEM.33, 594 (1961). ( 5 ) I b i d . , in press. (6) Ilouy, A. L., Carroll, B., Quigley, T. J., Ibid., 35, 627 (1963). ACGVSTEL. ROUY BENJAMIN CARROLL
Rutgers, The State University Newark 2 , N . J. WORKsupported by the Corn Industries Research Foundation.
Dimethylsulfoxide as Solvent for Acidimetric Determination of Ammonium a n d Substituted Ammonium Ions SIR: Although the salts of aromatic amines are sufficiently strong acids to be titrable in aqueous solution, aliphatic amines are not. Useful titrations are obtained only in nonaqueous systems. Titrimetric determinations of amine salts may involve either the anion or the cation. Examples of the former are alkalimetric titrations of nitrates and sulfates and of halides, after reaction with mercuric acetate. For such a determination, it is ncessary that the conjugate acid of the anion be significantly weaker than the titrant acid. This restriction excludes the perchlorate salts. In addition, this type of determination makes no distinction between amine salts and metal salts, because the anion is being determined. Determinations have been described which involve acidimetric titration of ammonium or substituted ammonium ions in various solvents: dimethylformamide (DMF), ethylenediamine (EDA), acetonitrile (MeCX), acetone, butylamine, and pyridine ( 2 ) . Price and Whiting have described the use of dimethylsulfoxide (DMSO) with the sodium salt of DMSO for titration of extremely weak acids, such as cyclo2502
0
ANALYTICAL CHEMISTRY
pentadiene ( 2 ) . We find that the use of DMSO as solvent in conjunction with the conventional basic titrants, potassium methoxide and tetrabutylammonium hydroxide (TEilH), offers quite considerable advantages, compared with the other solvent systems mentioned, for the titration of moderately weak acids. Solvent characteristics of pttrticular importance for this determination are basicity; dielectric constant; solvent power; suitability for conventional end point detection, sensitivity to contamination, especially from the atmosphere; stability; toxicity, and odor. I n this case it is desirable to use a slightly basic, negligibly acidic solvent for obtaining sharp end points while retaining the ability to differentiate between aliphatic amine salts on the one hand and mineral acids, carboxylic acids, and strongly acidic aromatic amine salts on the other. DMSO does permit this differentiation to be made. This is also true of RleCN, acetone, and D M F . EDA, butylamine, and pyridine are too basic to allow this differentiation. A large dielectric constant is desirable to facilitate solute ionization
and stable potentiometric observations. The dielectric constants of D M F , M e C S , and DMSO are around 40. Those for acetone, pyridine, and ED.1 fall between 10 and 20. That for but,ylamine is less than five. DMSO has definite advantages over the other solvents in that it is apparently not at all toxic and in that the reagent grade material has only a very slight odor. EXPERIMENTAL
Reagents and Apparatus. Tetrabutylammonium hydroxide ( T R A H ) titrant was prepared by reaction of silver oxide with the iodide. Sodium and potassium methoxide solutions were prepared by dissolving the metal in methanol and diluting with benzene. Titrants were standardized against reagent grade benzoic acid. Reagent grade D X S O was used as received. The best available grade3 of n-butylammoniuin chloride, hydrazine sulfat,e, ammonium perchlorate, and ammonium chloride were uaed as received. The hydrochlorides of dibutylamine, hexadecylamine, tripropylamine, and nonylamine were prepared by precipitation b y gaseous HCl from 3-methylpentane solutions of the free bases. n-Butylammonium perchlorate was prepared by dissolving t,he chloride
Table I.
I 1
Salt’ n-Butylammonium perchlorate
Titrant Tetrabutylammonium hydroxide Potassium methoxide
n-Butylammonium chloride
Sodium methoxide
Alanine Tripropyl: ammonium chloride Hexadecy!ammonium chloride
TBAH TBAH
30.98 99.22 64,26
TBAH
51.18
50.52
Potassium methoxide Sodium methoxide
RESULTS AND DISCUSSION
A number of ammonium and amine salts were titrated. Typical titration curves may be seen in Figure 1. A summary of results is presented in Table I. Two-step curves were obtained for hydrazine sulfate and for hexamethylenetetrammonium chloride. On the basis of the properties of the solvents mentioned above, it is evident that DMSO, D M F , and MeCN .would be expected to give better results than acetone, EDA, butylamine, and pyridine when moderately weak acids are to be titrated. In practice, DMSO may be preferred when compared to D M F and MeCN. DMSO is an extremely good solvent for amines and amine salts, as is D M F . All amine chlorides and perchlorates tried-as well as ammonium chloride and nitrate-and hydrazine sulfate are readily soluble. Among the compounds examined. ammonium bicarbonate and
Ammonium perchlorate
Potassium methoxide TBAH
TBAH TBAH
31.67 35.69 32.51 33.21 29.45 51,14 34.53 98.17 28.94 15.98 19.44 22.98 17.42 27.64 13.06 15.30 26,23 37.64 33.98 78.96 91.90 70.74 48.13 81.63 62.67 22.74 22.88
phosphate and alanine were difficult to dissolve. These compounds can be titrated if they are first taken up in a little water and then added to DMSO. When compared with DMSO and D M F , MeCK is a much less useful solvent. Titrations in DMSO, unlike D M F , are not subject to interference from small amounts of water. We find no significant changes in the titration curves when determining n-butylammonium chloride with as much as 11% water present. Similarly sodium ion concentrations up to 1.5F do not interfere with potentiometric end point detection. The glass-calomel electrode system permits very satisfactory end point determination in DMSO. This is true also for D M F , acetone, and MeCN. Measurements are not as stable in pyridine; E D A and butylamine are not well suited. Reddy (3) found that DMSO undergoes decomposition when concentrated aqueous hydroxide is added. We have found no evidence of decomposition when potassium methoxide or TBAH in methanolic benzene solution is used. We have, however, occasionally observed distortions in titration curves
Recovery,
72
mg.
TBAH
VOLUME OF TITRANT IMLI
in 0.1F sodium perchlorate-MeCK solution. Precipitated KaC1 was filtered, and aliquots of this solution were used for titration. Thymolphthalein, phenolphthalein, and thymol blue indicators were used as 0.1% solutions in methanol. Titrations were performed with a Beckman Zeromatic p H meter, using a glass-calomel electrode system. The sleeve-type calomel electrode contained saturated KC1 in methanol. Titrations were carried out in a closed vessel under a stream of nitrogen. A blank was titrated; then the ammonium salt was added, either as a solid or in solution. The end point was determined both potentiometrically and with an indicator.
Found,
Dibutylammonium chloride Xonylammonium chloride Hydrazine sulfate
Ammonium chloride
TBAH
TBAH
Figure 1. n-Butylammonium chloride vs. (a) TBAH; ( b ) sodium methoxide; (c) potassium methoxide. TBAH vs. (d) aniline-dibutylammonium chloride mixture
Added, mg. 69.19 3~0.34
69.17 68.93 68.44 69.60 69.39 69.60 31.95 36.31 33,25 32.91 29.94 51.09 35.73 99.19 28.91 15.82 19.56 22.36 16.74 27.16 13.34 15.54 25.21 37.04 33.10 79.48 89.56 70.21 47.75 81.81 62.21 22.38 27.02 22.10 25.65 31.39 100.03 64.85
y -
3006
Titration Data
99.97 99.63 98.91 100.59 100.29 100.59 100.89 101.74 102.28 99.10 101.67 99.90 103.48 101.04 99.90 98.91 100.61 97.30 96.10 98.26 102.14 101.56 96.11 98,41 97.41 100.65 97,46 99.26 99.21 100.22 99.27 98.42 118.82 96.59 112.11 101.33 100.82 100.92 98.71
beyond the end point with D M F solutions. With reagent grade solvents, as received, entirely satisfactory blanks are obtained for DMSO and for acetone, EDA, and MeCN. Under similar conditions, blanks obtained with D M F , pyridine, and butylamine were so large that they would require distillation prior to use. LITERATURE CITED
(l).Fri>z, J. S., Hammond, G. S., “Quantitative Organic Analysis,” chap. 3, Wilev. Sew York. 1957. (2) Prrce, G. G., Whiting, M. C., Chem. Ind. 1963, p. 775. (3) Reddy, T. B., Dissert. Abslr. 2 1 , 1781 (1961). INVESTIGATION supported by research grant GM-10064 from the Public Health Service of the U. S. Department of Health, Education, and Welfare. K. I(.B. thanks the National Science Foundation for support in the form of a Graduate Fellowship for the 1963-64 academic year. KAREXK. BARNES K. M A N N CHARLES Department of Chemistry Florida State University Tallahassee, Fla. 32306 VOL. 36, NO. 13, DECEMBER 1964
2503
