regarding the acidic behavior of nonphenolic, noncarboxylic compounds is that all show enhancement of acidity relative t o the carboxylic acids when titrated in pyridine, and may show such enhancement in any low dielectric solvent, Two anomalous compounds were biuret and trichloro-N-methylacetamide. Biuret gave a good titration curve, but calculates as only 50% of theory. Trichloro-N-mcthylacetamide does not have any acidic hydrogens, but can be titrated to about 90% of the theoretical. A rapid dehalogenation of the compound by the reagent is suggested. The reaction may be general for trihalogen d&-. rivatives. MIXTURES
Mixtures of compounds were titrated which showed a probability of resolution from AHNP data (Table 111). Resolu-
tion of mixtures of cyanamide and dicyandiamide is excellent. However, if the sample contains appreciable quantities of biuret, no clear resolution of end points is possible. No difficulty was encountered in resolving mixtures of chloro- or dihydroxyphenols. Resolution of mixtures such as cresols or various monochlorophenol derivatives is impossible. ACKNOWLEDGMENT
The author thanks Seymour Sandler, who performed a great many of the titrations. LITERATURE CITED
(1) Boyd, D. R., J . Chem. SOC.1915, 1538. (2) Bruss, D. B., Harlow, G. A., ANAL. CHEM.30,1836 (1958). (3) Davis, M. M., Hetzer, H. B., J . Research Natl. Bur. Standards 60. 569 (1958). (4) Fieser, L. F., Fieser, M., "Organic Chemistry," 3rd ed., Heath, Boston, 1956.
(5) Fletcher, W. H., J . Am. Chem. SOC. 68,2726 (1946). (6) Fritz, J. S., Yamamura, S. S., ANAL. CHEM.29, 1079 (1957). (7) Gawron, Oscar, Duggan, Marjorie, Grelecki, C. J., Ibid., 24, 969 (1952). (8) Guerillot, C. R., Compt. rend. 240, 1107 (1955). (9) Halban, H., Seiler, M., Helv. Chim. Acta 21,385 (1938). (10) Harlow, G. A., Bruss, D. B., ANAL. CHEM.30,1833 (1958). (11) Judson, C. M., Kilpatrick, J., J. Am. Chem. SOC.71,3110 (1949). (12) Kameyan, J., Chem. Ind. (Japan)24, 1263 (1921). (13) Kertes, S., J . Chem. SOC.1955, 1263. (14) Lauer, K., Ber. 70B, 1127 (1937). (15) Schoenstein, E., Perko, G. hI., Monatsh. 85, 580 (1954). (16) Streuli, C. A., ANAL. CHEM.31, 1652 (1959). (17) Streuli, C. A,, hliron, R. R., I b i d . , 30, 1978 (1958). (18) Turnball, D., J . Am. Chem. SOC. 65. 212 11943). (19) 'Walton, H. F., Schelt, A. .4.,I b i d . , 74, 4995 (1952). RECEIVED for review September 29, 1959. Accepted December 7, 1959.
Evaluation of Standard Model D Keston Polarimetric Attachment for the Beckman DU Spectrophotometer KNUD G. POULSEN' Department o f Chemistry, University of Culifornia, 10s Angeles 24, Calif.
b The Standard Model D Keston polarimetric attachment for the Beckman DU spectrophotometer has been tested, to evaluate the unit for measurements of optical rotatory dispersion, especially in the vicinity of an optically active absorption band. The optical rotation of aqueous sucrose solutions at 20" C. can b e determined to an accuracy of about 2% over the approximate range 0.3" to 2' and 480 to 590 mp. The error increases rapidly outside these limits. The unit can b e used as a convenient analytical tool for measurements in the normal part of a rotatory dispersion curve, especially for highly colored solutions. A complete set of equations relating optical rotation to relative transmittance or relative absorbance has been derived for specified conditions.
I
work on chromium complexes, for which optical rotatory dispersion measurements were required, the Standard Model D Keston polariN CURRENT
1 On leave of absence from the Technical University of Denmark, Copenhagen.
410
ANALYTICAL CHEMISTRY
metric attachment (5, 6) for the Beckman DU spectrophotometer was tested. The attachment can be used with highly colored solutions and has the advantage of relatively low cost compared to conventional spectropolarimeters. No evaluation of the instrument appears to have been published and the principles on which the instrument is based may, under certain conditions, require the use of other equations for calculating the optical rotation than the one given in the manufacturer's manuals ( 8 ) or by Gallop (1). According to Gallop, the absorbance measured with the Keston unit is theoretically linear with rotation for sufficiently small angles. The data available to date were obtained (according to the original purpose of the instrument) in the range of only a few tenths of a degree and with optically active substances which do not show anomalous dispersion. We report here the derivation of a complete set of equations relating optical rotation to relative absorbance valid under various specified conditions, and findings on the possible extension of the Keston unit to larger rotational angles.
PRINCIPLE OF KESTON ATTACHMENT
When a sample with an optical rotation, a,is placed between the two polaroids of the attachment, the amplitude vector of the light is oriented after passing through polarizer and sample. I n Figure 1 the length of that vector is called S. The projection of S on the plane of polarization of the analyzer changes from A d to A , when the analyzer handle is moved from down to up, rotating the analyzer Polaroid through the fixed angle 20. The intensity of light reaching the multiplier phototube of the spectrophotometer is proportional to A: or A t , from which it is evident that (Y can be determined from readings of the transmittance, T, or absorbance, A (equivalent to optical density, D, D = A = -log T ) corresponding to A i and A i . It is convenient to adjust the spectrophotometer transmittance reading in one of the two positions to 1.00, such that the other position will give the smaller transmittance reading (as the Beckman spectrophotometer can read transmittance only between 0 and 1.10). Then, by definition T = A."/A: (la)
T = A,2/A.2 (1b) (whichever gives 0 G T G 1)
FIXED PLANE OF POLARIZER
I
Calculation of the rotation a from a reading of relative transmittance, T (or relative absorbance, A ) , is illustrated below for CY between 0' and 0' (first quadrant).
+
A, =
s COS (90"
A~ =
s COS (900 - e - a) =
-e
(e -
S sin
(e + a )
+ a)
d~ = tan 0 - tan tan
0
CY
a =
(3)
(5)
el.lS2A/2
- e-l.152A/2
e1.15ZA/2
+
e-l.162RIZ
tan e tanh (0.5764)
tan e
(7)
When CY is not between 0" and eo (first quadrant), an analysis of the problem shows that other equations must be used. Table I gives the proper equation to use for given ranges of optical rotation and indicates the deduction of the sign of the optical rotation from the operation of the Keston-unit analyzer handle. It is evident from Table I that there exists no single-valued relation between the direction the handle is operated and the sign of the optical rotation. Moreover, the magnitude of the rotation of an unknown sample cannot be calculated even approximately from a single measurement of T . For sufficiently dilute solutions measured a t wave lengths not in the "anomalous" part of the rotatory dispersion curve the rotations encountered will easily lie within the range -6 G Q G 0 (0-0.1 radian-6' in the Keston attachment), and t h e simple treatment outlined in the manufacturer's manual may be valid.
Table 1.
Figure 1. Principle of Keston polarimetric attachment
However, when a is determined a t a wave length within an optically active absorption band (Cotton effect)-e.g., for certain complexes of transition metals in the wave length range of visible light-a rotation of f180' or more may be encountered with solutions as dilute as 10-3Jl. (When the optical rotations of solids are measured, high values may also be encountered.) Therefore, when measuring systems where the rotation is not known even approximately, one should dilute the sample-e.g., 5-fold-several times and measure each of these new samples. Alternatively, thin optically active plates -e.g., quartz-of known rotation may be placed successively with the sample between the polaroids, or the approximate rotation may be established with a conventional polarimeter if a light source of the proper wave length is available and if the solution is not too highly colored. With either procedure the new transmittance readings and possible new operation of the analyzer handle will reveal to which range the original CY belongs. APPROXIMATE EQUATIONS
By introducing into Equation 4 the approximation sin2 (e f a) E (e + CY)*,
e(o.576~)
(9)
It is not easy, however, to compute from the squared-sine relation (Equation 4) the error involved in the use of Equation 8. This is better done from the exact Equation A of Table I, from which Equation 8 may be derived by use of the two approximations tan a s a and tan e E 0. The former approximation introduces errors of 0.6, 0.3, and 0.1% for angular rotations of 0.130 (7.45'), 0.100 (5.73"), and 0.0500 radian (2.86')' respectively. Since 0-5"' the tan 0 E 0 approximation results in an error of -0.3'%, which may be eliminated by omitting this approximation and using the equation in the form (tan e) tnnh (0.5764)
=
(10)
because actually it is tan e which is directly derived from measurements on samples of known rotation when the instrument is calibrated. At low absorbances Equation 10 reduces to the approximate relation a =
0.576A(tan e) = CA
(11)
where C = 0.576 tan 8, and the angular rotation is directly proportional to the measured absorbance if C is truly a fixed instrument constant. From tables of tanh one finds that to keep the error < 0.5% when using Equation 11, A must be < 0.2, which excludes the use of part of the range where the spectrophotometer is more accurate (minimum photometric error occurs when the absorbance is 0.43, corresponding to 37y0 transmittance). The manufacturer states (8)that a "quite linear" relationship between a (e and the function (100 -TR) 7 is obtained when the unit is used in the
+
Selection of Equation and Deduction of Sign of Optical Rotation
Range of Rotation = Any Integer)
Equation tan 1 a I/tan e
(n
1 -.\/T 1 + dT 1 + v?x
F
dT+1d F - 1 -e-
dF+l
Operation of Handle
=
tanh(0.576A)
(A)
Down Up
1
tanh(0.576A)
-1 = tanh(0.576A)
T - e + ~ ~ ~ < i k~+ l n
through the polaroids when samples of high opticai activity are used. EFFECT
OF SLIT
WIDTH
The data in the uqable range were obtained I Q rising ii spectrophotometer slit n idth of 0 1 to 0.2 mm. and with full sensitivity of the multiplier phototube. Outsick that range larger slit widths nere in most cases necessary. This inc rmses the spectral band width and a b ~ ii~~fluences tho reading-e.g., when 3 iiiciose sample was measured at 625 nid r( t d i n g ~of 1002' P(IUd1 t o 90.2, 90.4, 90.': 90 A and 90.1 n e r e obtained for a z i i t n iiltli oC 0.6>0 5 0 4 0.3, and 0.2 r i m , rwpectivelj TI114 effect may be aiiot1lt.r ~rqtriction on the accuracy of i e m t - ohrninable CONCLUSIONS r)E
wive length and
of tile paiaroid disks by u::it a i i j presumably also ~.mausi:of 100% polarization protiui'tti iq-thch Sic01 prisms. The increaseti ic.ngth oi the unit due to such ,i
1
.
a change should not involve serious technical problems, but because a large aperture is required it may be costly. The higher transmittance of Nicol prisms would permit the use of smaller slit widths. The slit width of 0.2 mm. a t 500 mp necessary with the current unit corresponds to a half-intensity band width of 4 mp for the light entering the sample. The rotatory dispersion curve for the sucrose solution can be considered only approximately linear within that band width. So calibration of the instrument-e.g., with sucrose-will be accurate only to a degree which depends in a complicated way on the monochromaticity of the light used. As for ordinary optical rotation measurements, the instrument may be considered a valuable analytical tool. It is especially useful when working with colored substances, where visual polarimetry causes considerable strain on the eye or may even be impossible. The precision of approximately 2% is considered goo& for small rotations (-0.3") but the visual polarimeters are superior a t iarger values of rotation when the ~n1u:ic~nsare not too highly colored. It is r;nfa-tunate that technical problcis appear t o impose some tne use of the ingenious limitations 0'7
principle upon which the Keston at tachment is based. ACKNOWLEDGMENT
The author thanks C. S. Garner for his interest in this investigation and for valuable discussions. Financial support from the U. S. Atomic Energy Commission under Contract AT( 11-1)-34, Project 12, is gratefully acknowledged. LITERATURE CITED
(1) Gallop, P. M., Rev. Sci. Instr, 28
209 (1957).
(2) Gibbs, R J., Agricultural Research Service, U. S. Department of Agricul-
ture, Beltsville, Md., private communication, October 28, 1959. (3) Gibbs, R..J., Fryar, A. J., Abstracts. 132nd Meeting. ACS. New York, X . Y., September 19g?, p. 32C. (4) Grabau, & J. I. Opt. , SOC.Am. 27, 420 (1937). (5) Keston, A. S., Abstracts, 12ith hfeeting, ACS, Cincinnati, Ohio, April 1953, p. 18C. (G),Keston, A. S., Lospalluto, J , Federat z o n Proc. 12, 229 (1953). (7) Lowry, T. M., Richards, E. 31. J . Chem. SOC.125,2523 (1924). (8) Standard Polarimeter Co., Kew Tork, S . Y.,,,BuIl. 2 and "Calibration Procedure. RECEIVEDfor review July 15, 1989. Accepted December 31, 1959.
Determination of Sulfur in Petroleum Products by Hydrogenation E. C. SCHLUTER, Jr., E. P. PARRY, and GEORGE MATSUYAMA' Ilnior! Oil Co. of California, Union Research Center, Breo, Calif. F A i a p i d method for the determination of sulfur throughout a wide concentratior range i s described. The sample i s heated in a hydrogen stream ana passed over a nickel catalyst at 1 2 W C Sulfur is converted to hydrogen sulfide and absorbed in a dilute sodium hydroxide solution. The absorbed hydrogen sulfide i s titrated ainperametrically with standard mercuric chloride solution if the sulfur conBelow centrotior i s above 0.1%. 0.1 ";b sulfur the methylene blue method I S usea to determine as little as 5 p.p.m. of sulfur in a 0.2-gram sample. as littie as p . ~ . r n OX sulfur may be aetermined because no Siank is detectabie unaer normal operating conditions. The accuracy for iow sulfur concentrations is equal to or better than the lamp method below 1OC p.p.m. Average recovery OR a variety of sulfur types was 99%. With petroleum samples, comparisons with referee methods showed recoveries of
97 to 100%.
With the sample introduction system described, recovery of sulfur decreases as the amount of heavy suifur-containing material in the sample increases. Halogens, nitrogen, phosphorus, and arsenic d o not interfere at concentrations normally found in petroleum. The method requires a high temperature furnace, but otherwise the equipment is simple. Depending on the character of the sample, av analysis requires 30 to 45 minutes.
BRIOUR combustion iiicthods are used ior the determination of total sulfur ir. petroleum products. The choice of method depends on the nature of thP samplc, Put in all, the sulfur is oxidized to either sulfur dioxide or sulfur trioxide, which is then deter1 Present address, Beckman Instruments, Inc., Fulierton, Calif.
niined titrimetrically, gravimetrically or colorimetrically. The combustion approach gives good results but has some limitations. Halogens, nitrogen, and phosphorus interfere by forming acids in the lamp, induction furnace, and alkalimetric finish methods (1, 2 ) . Gravimetric methods are not subject to this interference, but are slow and insensitive. Gaseous and volatile samples cannot be analyzed by the bomb or induction furnace methods; these methods are also limited to small amounts of sample. Sample size limitation is overcome a t the expense of long Combustion time in the iamp method, but materials n hich burn with a smoky fiame or contain elemental sulfur give unsatisfactory results
