ANALYTICAL CHEMISTRY
618 in carbon tetrachloride based on metal complex formation, the average for which is (20.29 * 0.82) X 103. Eleven determinations by direct 1Teighing of purified dithizone gave (20.2 =t0 2) X l o 3 for this value. The values for ED%460 and of E for the metal dithizonates at their wave length of maximum absorption (Tables I and 11) are to be considered as less accurately known than ED. 620, because the thiocarbadiazone, from oxidation of dithizone, is nonreactive to metal and absorbs in this region of wave length. For ED^ 450 the presence of traces of metal dithizonates would produce abnormally large absorption at 450 mp. The values of the molecular extinction coefficients of dithizone in chloroform are reported with no claim as to accuracy, as they were determined on weighed “purified” dithizone. Because dithizone is actually purified in solution ( 1 ) and its molar extinction coefficient a t 620 mp in carbon tetrachloride is knon.n, or can be ascertained, it becomes unnecessary t o obtain the solid material in a highly purified condition in order to prepare dithizone solutions of known concentration. A stock solution of dithizone in carbon tetrachloride or chloroform can be purified by stripping with dilute aqueous ammonia, phase separation made, and the dithizone reverted t o fresh carbon tetrachloride. The optical density a t 620 mp for this purified dithizone can be measured and its concentation ascertained by use of 34.6 X 108 for its molecular extinction coefficient. Molecular extinction coefficients for the metal dithizonates can now be determined from the known value for dithizone. SUMMARY
The preparation of dithizone solutions of known concentration from “purified” solid dithizone is not possible because the dry product is not pure. Dithizone can be prepared pure in solution and its concentration can be determined from its optical density and molar extinction coefficient. By complexing purified dithizone in carbon tetrachloride with lead, zinc, silver, and mercury(I1) the molar extinction coefficients of dithizone a t its primary and secondary absorption maxima,
620 and 450 mp, are found to be (34.60 =t 0.84) x l o 3 and (20.30 * 0.82) X 103, respectively. Coefficients for dithizonates a t the wave length of their maximum absorption in carbon tetrachloride are found to be: (68.6 * 1.9) X lo3 a t 520 mp for PbDz2; (92.6 * 2.8) X l o 3at 535 mp for ZnDz,; (27.2 * 1.8) X lo3 at 462 my for AgDz; and 70 X lo3 a t 490 my for HgDz,. The coefficient for dithizone a t 620 mp is most easily determined by measuring the change in optical density of a purified dithizone in carbon tetrachloride when shaken with an equal volume of aqueous Ag+ whose concentration is known, [Ag+],but which is in deficient quantity to complex all the dithizone. The coefficient is given by
e620 =
AD020 __
IAg’I‘
LITERATURE CITED
(1) Assoc. Offic. Agr. Chemists, “Official and Tentative hlethods of
Analysis,” 1945. (2) Biddle, D. -4., ISD.ENG.CHEM.,ANAL. ED.,8, 99 (1936). (3) Biefeld, L. P., and Patrick, T . M., Zbid., 14, 275 (1942). (4) Clifford, P. A . , J . Assoc. Oj%. Agr. Chemists, 26, 26 (1943). (5) Clifford, P. A., and Wichmann, H. J., Ibid., 19, 130 (1936). (6) Fischer, H., 2.angew. Chem., 47, 685 (1934). (7) Hibbits, J., thesis, St. Louis University, 1950 (8) Greenleaf, C. A., J. Assoc. Ofic. AQT.Chemists, 24, 337 (1941). (9) Irving, H., Andrew, G., and Risdon, E. J., J . Chem. soc., 1949, 545.
(10) Irving, H., Risdon, E. J., and Andrew, G., Ibid., 1949, 537. (11) Kolthoff, I. hI., and Sandell, E. B., J . Am. Chem. SOC., 63, 1906 (1941).
(12) Liebhafsky, H. A., and Winslow, E. H., Ibid., 59, 1966 (1937). (13) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” Chap. IV, See. I, New York, Interscience Publishers, 1944. (14) Tipton, G. hI., S.J., unpublished thesis, St. Louis University, 1949 (to be published later). (15) Welcher, F. J., “Organic Analytical Reagents,” Vol. 111, New York, D. Van Nostrand Co., 1947. (16) Wichmann, H. J., IND. EXG.CHEM.,ANAL.ED., 11, 66 (1939). RECEIVED July 21, 1950. Abstracted from the thesis presented by Sister Mary Louise Sullivan, S.C.L., to the faculty of the Graduate School of St. Louis University in partial fulfillment of the requirements for the degree of doctor of philosophy.
Densities and Refractive Indexes for Propylene Glycol-Water Solutions GORDON
M A C B E T H AND A. RALPH THOMPSON University of Pennsylvania, Philadelphia, P a .
A
SALYTICAL data which may be used for determining the compositions of aqueous propylene glycol ( lJ2-propanediol) solutions are presented in this paper. These were obtained by making careful measurements on a number of solutions for density a t 35 O C. and for refractive index a t 25 O C. Values for the pure organic material have been reported in the literature (1- 5 , 5 ) , but only a graphical representation of the specific gravity for solutions of propylene glycol in water could be found (1), PREPARATION OF SOLUTIONS
Commercial grade, 99%+, propylene glycol was fractionated a t approximately IO-mm. absolute pressure in an 18-inch glass column, 0.5 inch (1.25 cm.) in diameter,, packed with 3/16-in~h glass Fenske rings, using a high reflux ratio (about 30 to 1). To protect this hygroscopic material, the distillation was carried out in an entirely closed system and the distillate receivers were vented through drying tubes containing anhydrous calcium sulfate. Only the middle third of the constant boiling point distillate \vas used, the balance being discarded.
The refractive index, ~ Z D ,of the glycol at9 received was 1.4314 a t 25 O C. and that of the purified material was found to be 1.4316, which agrees exactly with that determined by Schierholtz and Staples (6). The specific gravity, compared t o water in a separate determination a t 20” C., was found to be 1.0381. This is in precise agreement with two previously reported values ( 1 , 8 ) and very close to others (5, 5 ) . The water content of the purified propylene glycol was determined by means of the Karl Fischer reagent t o be 0,04y0 by weight. Boiled distilled water from a laboratory Barnstead still waa used in making up the solutions. Nine solutions of varying glycol concentration from 10 to 90 weight % were prepared by pipetting first the water and then the glycol into 50-ml. ground-glassstoppered Erlenmeyer flasks. The actual amounts of each constituent added were determined by weighing t o 0.1 mg. on an analytical balance. DENSITY AND REFRACTIVE INDEX DETERMINATIONS
Density measurements were made using 10-ml. Weld specific gravity bottles and a constant temperature bath maintained a t
V O L U M E 23, NO. 4, A P R I L 1 9 5 1
619
As a means of determining the compositions of aqueous propylene glycol (1,2-propanediol) solutions, over the entire composition range, density data at 35" C. and refractive index values at 25' C. were obtained for prepared solutions of known concentration. Experimental points and smoothed values at intervals of 10 weight YQ propylene glycol are presented. The refractive index data should be useful in determining the propylene glycol content within about +O.lq~over the entire composition range. As a result of a maximum in the density-composition curve, the value of density as a measure of the composition varies with the glycol content, but may be used in determining the glycol content to within +0.04% when the concentration is below approximately 50% glycol.
composition change of 10.04%. Because of the maximum in the density curve, as seen in Figure 1, the change in density with composition is relatively slight in the region centering on approximately 70% glycol concentration, and hence analysis by density in this region is not very satisfactory. Beyond approximately 80% glycol, a change in composition of 10.1% corresponds to a change in density of +0.00003 gram per nil.
5 0
1.41 0
f W
2
$ 4
1.37
a w LL
a
35.00" * 0.02" C. as determined by a calibrated thermometer. Duplicate determinations were made on each solution. Refractive indexes were measured by means of a Bausch and Lomb Abbe-type refractometer, with compensating prism, and an incandescent light source. Using a constant temperature bath :nd circulating pump, the temperature was maintained a t 25.00 * 0.02"
1.33 40 60 80 WEIGHT X PROPYLENE GLYCOL
0
20
Figure 2.
100
Refractive Indexes of Aqueous Propylene Glycol Solutions at 25" C.
c.
In summary, the refractive index data, at 2.5' C., should be useful in determining the propylene glycol content within about + O . l % over the entire composition range, while the density data a t 35' C. may be used in determining the glycol content to within *0.04% when the composition is less than ahout 50% glycol.
E
3 1.02
i B i
z w 0
~~~~
Table I.
1.00 0
Figure 1.
20
60 80 WEIGHT % PROPYLENE GLYCOL 40
too
Solution
Propylene Glycol, Weight %
Experimental Data Refractive Index, no a t 25.00'' C.
Absolute Density. G . / l I l . a t 35.00° C.
3323 3431 3346 3660 3772 3879 1,3973 1 4071 1.4160 1 4240 1 4316
0,99406 ( 4 ) 1.00086 * 0.00004 1.00863 == 0.00004 1.01628 == 0.00001 1.02304 * 0.00002 1.02822 * 0.00003 1.03138 * 0.00002 1.03297 * 0.00003 1.03245 * 0.00006 1.02987 * 0.00003 1.02510 * 0,00001
1 1 1 1 1 1
Absolute Densities of Aqueous Propylene Glycol Solutions at 35" C.
All weighings were reduced to values in vacuo and the absolute densities a t 35" C. were calculated as grams per milliliter. Expressed in these units, the density is numerically equal to the specific gravity a t 35" C. compared to water a t its maximum density (approximately 4" C.). Experimental data are listed in Table I. Large scale plots of the data for both density and refractive index, similar to Figures 1 and 2, respectively, were prepared and values obtained a t even cornposition increments, from smooth curves through the points. These smoothed values are presented in Table 11. LIMITATIONS OF DATA
The compositions of the solutions, based on the weight of the smallest amount of material weighed (about 5 grams), are known to approximately 1 part in 30,000. The duplicate density determinations agreed on the average within better than *0.00003 gram per ml., the maximum deviation being +0.00006 gram per ml. for the 80% sample. Based on manufacturer's claims for the instrument used, refractive index values are believed to be correct within about +0.0001. For purposes of analyzing aqueous solutions of propylene glycol, the refractive index data are limited to about * O . l % by weight composition change corresponding to *0.0001 for the value of the refractive index. The value of the density measurements for analytical purposes varies with the glycol concentration. Below approximately 50 weight % glycol content, the maximum variation is *0.00004 gram per ml., corresponding to a
Table 11. Propylene Glycol, Weight % 0 10.00
20,oo 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00
Smoothed Values
Refractive Index, %D at 25.00' C. 1.3326 1.3432 1 3344 1 36.58 1 3770 1.3878 1.3980 1.4073 1.4162 1.4241 1.4316
Absolute Density,
G./ZII. a t 35.00° C. 0,99406 ( 4 ) 1.00089 1,00852 1.01615 1.02295 1.02818 1.03154 1,03299 1.03240 1,02982 1.02610
LITERATURE CITED
(1) Carbide and Carbon Chemicals Corp., 30 East 42nd St., New r o c k 17, N. Y., "Glycols," 1947. (2) Paragon Testing Laboratories, Orange, N. J., "Fine Organic Chemicals, List No. 5," 1946. (3) Pukirev. A. G., Trans. Inst. Pure Chem. Reagents (Moscow), No. 15, 45-50 (1937). (4) Reilly, J., and Rae, W. N., "Physioo-Chemical Methods," 3rd ed., Vol. I, Kew York. D. Van Xostrand Co., 1939. (5) Schierholta, 0. J., and Staples, M. C., J . Am. Chem. SOC.,57, 2709-11 (1935). RECEIVEDJuly 18, 1950. Presented at t h e Meeting-in-Miniature, Philadelphia Section AXERICAN CHEMICAL SOCIETY,January 18, 1951.
