ANALYTICAL EDITION
March. 1944 fnwtive index-dry substance table. structed by the following procedure :
Such a table was con-
The refractive index was obtained at two temperatures. 20.00' and 45.00" C., for the dry substance corresponding to the commercial Baum6s 40 t o 47 according to the accepted values for this relationship ( 5 ) . The three master graphs, 42.00, 55.00, :tnd 89.00 D.E., were employed. These data were plotted on millimeter paper employing refractive index as the ordinate, each millimeter equaling 0.0002 refractive index, and as abscissa, t.he dextrose equivalent, each millimeter equaling 0.10 D.E. Thus each set of points represented one Baume and astraight' line was found to be the best curve through these points. The deviation of points from the straight line was not greater than 0.0001 refractive index-Le., within the limits of the refract.ometer and the graph paper. The curve was extrapolated to 30.00 and 92.00 D.E. The extrapolated value* for 32.8 D.E. were either equal to or not greater than 0.0001 refractive index from the ex erimental values for the sirup, as were also the interpolated v & m for the 83.4 D.E. sugar sirup. From the graph of these values, tables of refractive index-commercial Baume for a range of dextrose equivalents have been calculated (Table IV). A column of values is also given for the corn sugar sirup-90.7 D.l,;.,1.22% ash-which, because of its ash content, is not covered by the above method of graphing but, is of interest because such a corn sugar sirup is typical of that. required for an ''80'' sugar. SUMMARY
The refractive index of starch conversion products decreases witjh increasing dextrose equivalent. The effect of ash (sodium clilori(le) is to increase the refractive index. The decrease of
Determination A. S. MlCELl
AND
OF
165
refractive index with increasing dextrose equivalent is proportional to the increase in dextrose equivalent if the ash content of t,he products is also proportional to the dextrose equivalent. The temperature-refractive index relationship is linear within the range 18' to 45' C. when the concentration exceeds 14% dry substance and within the limits o f precision of a four-place instrument. Tables are presented covering the refractive index-dry subJtance relationship for starch conversion products typical of vommerce, and refractive index-commercial Baume for typical c*nmrnerc.ialproduck of varying dextrose equivalents. LITERATURE CITED
Official Agr. Chem., Official and Tentative Methoda of Analysis, 5th ed., Sec. XXXIV, 486-7 (1940). (2) Bishop, W. B., and Young, Neil, IND. EKG.CHEM.,24, 1171 (1)
\WJC.
(1932). (3) Cleland, J. E., and Fetzer, R. R . , IND.ENG.CHEM.,ANAL.ED., 14, 27, 124 (1942). (4) Evans, J. W., and Fetzer, W. R., Ibid., 13, 855 (1941). ( 5 ) Faueer, E. E., Cleland, J. E., Evans, J. W., and Fetzer, W. R., Ibid., 15, 193 (1943). (6) Kroner, W., Reischel, W., and Hoppner, W., 2. arid. Chem., 122, 321 (1941); Chem. Zentr., 1942, I , 277 (1942). (7) Tilton, L. W., J . Optical SOC.Am., 32,371-81 (1942). (8) Tilton, L. W., J . Research Nail. Bur. Standards, 30, 311-28 ( 1943). (9) Tolman, L. M . , and Smith, W. B., J . Am. Chem. Soo., 28, 1476 (1906).
Zinc in Cyanide Brass-Plating Baths
I. 0. LARSON, Motor Products Development Laboratory, U. S. Rubber Company, Detroit, Mich.
T
HE large number of papers on the determination of zinc which have appeared during the past 8 years attest to the widespread need for rapid and reliable analytical procedures for this determination. Of the many methods available for the determination of zinc, only two appear to meet the demands placed on control methods. A control method does not long survive unless it is inherently reliable, rapid, and inexpensive. The two methods found to possess these qualities are: (1) the ferrocyanide titration using diphenylbenzidine as internal indicator id with ferric ion held in solution as the pyrophosphate complex ( I ) , and (2) electrodeposition (6). Where electrolytic equipment is largely tied up by copper determinat'ions, the determination of zinc by ferrocyanide titration is designat,ed. The application of this method to cyanide brass-plating solut,ions is discussed beIOU.. The application of the electrolytic determination of zinc will be discussed in a later paper. Recently (4) the polarograph has been successfully employed for determination of the copperzinc ratio of electrodeposited brass. The authors do not as yet report ita use in the determination of copper and zinc in the Iir;w-pIating bath. PROCEDURE
C'entrifuge until clear a portion of the cyanide brass-plating q(11utionto be analyzed, pipet 10 ml. into a 180-ml. electrolytic beaker, add dropwise 10 ml. of concentrated hydrochloric acid. :ind heat until clear, If the solution is not heated until clear, tile ferrocyanides initially precipitated by the acid will not completely redissolve in the next step. Cool and carefully add 7 ml. ( J f a mixture consisting of 3 volumes of concentrated nitric acid and 2 volumes of concentrated sulfuric acid. Evaporate to sulfur trioxide fumes, finally heating over an open flame t o cause copious fuming. This step appears essential to assure com l e k destruction of ferrocyanides and of organic matter. Coofthe residual moist salts, wash the beaker sides, add 1 ml. of 1t o 1 nitric acid, and remove copper by electrodeposition. After the removal of copper, add 3 drops of 5% ammonium
ersulfate solution to oxidize any ferrous iron to the ferric state. $vaporate to 25 ml. to destroy excess persulfate. Cool, rinse beaker walls, add 2 grams of sodium pyrophosphate to form the ferric iron complex, and finally, 5 ml. of concentrated ammonia. A t this point the solution should be alkaline. If not, make just alkaline to phenolphthalein. Neutralize the solution with 1 to 1 sulfuric acid and then add 6 ml. in excess. Warm the solution to 40" to 45" C. At this stage commence vigorous mechanical agitation of the solution. This is essential for the rapid response of the indicator to ferrocyanide additions. Add 3 drops of 1% diphenylbenzidine in sirupy phosphoric acid and 3 drops of 0.2% potassium ferricyanide. The latter addition causes the formation of the violet oxidation product of diphenylbenzidine. Titrate to a permanent end point, using a 0.025 molar solution of potassium ferrocyanide containing 0.3 gram of potassium ferricyanide per liter. The ferricyanide in the titrating solution is essential for obtaining reproducible results and for maintaining good indicator reactivity. L I M I T A T I O N S AND ERRORS
The successful functioning of the internal indicator requires careful control of the solution composition. However, the determination is not subject to all the limitations cited in the literature. Kitrates can be present to the extent of 0.5 ml. of concentrated nitric acid per 50 ml. of solution. This makes readily possible the use of small amounts of nitric acid as cathode depolarizer in the electrodeposition of copper. The indicator functions properly in a solution containing 4.5 grams of ammonium sulfate, 6 ml. of 1 to 1 sulfuric acid, and a t least 20 mg. of zinc ion per 50 ml. of solution. These proportions must be maintained in order to presewe the sensitivity of the indicator. Samples containing less than 20 mg. of zinc ion may require the addition of known amounts of standard zinc chloride solution prepared from metallic zinc. To determine the effect of extraneous substances in concentrations higher than those encountered in the analysis of a typical plating solution, a standard zinc chloride solution was used.
.
166 Table 1.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Effect of Extraneous Substances on Diphenylbenzidine End Point k l . of 0.025 M Potassium Effect on Indicator
Eubstance (Plus Blank) Blank (21.6 mg. of Z n + + in s p roximately 50 ml. of
Ferrocyanide 8.85
Color Change Yellow-green end point color change
* O h
10 mg. 10 mg. 10 mg. 10 mg. 10 mg.
of P b + + of A s + + + of S b + + + of 8 n + + of A1 + + +
20 mg. of F e + + + (as pyrophosphate complex) 40 mg. of F e + + + (as pyrophos hate complex) 60 mg. of F e + +$? (as pyrophosphate complex) 10 ma. of N i + + 10 mz. of T h +
10 mg. of Magnus cleaner b 10 mg. of gelatin 10 mg. of thioglycol 10 mg. of acid pickling inhibitore 1 2 3
‘
0
b e
8.85
No effect N o effect No effect No effect Slightly slower color change Blue-green end point
8.85
Blue-green end point
8.85
Blue-green end point
8.85 8.85 8.85 8.85 8.85
15.05 8.85 8.85 8.85 8.85 8.85 9.15 8.90 8.85 8.85 8.80 8.85
... ... ... ...
(Ni precipitated as the ferrocyanide) N o effect N o effect N n nffwt. . .. .. .
No effect Color change slow No change No change No change Color change less sharp Color change less sharp No effect No color or end point
No color or end point No color or end po/nt No color or end point
An alkaline cleaner with about 10% silicate and 0.501 phosphate.
Similar t o Durodex b u t containing a small amount otsoap. The three types of inhibitors used are thought to be: 1, a pi eridine derivative. 2,a suyfonated primary or secondary aromatic amine. 3, an aldehydearomatic amine reaction product.
Each titration sample was prepared by pipetting 10 ml. of standard zinc chloride solution into a 180-ml. electrolytic beaker, adding 4 grams of ammonium sulfate, 6 ml. of 1 to 1 sulfuric acid, and finally the extraneous material to be tested. The resulting solution was diluted to 50 ml., heated to 45’ C., and titrated with the ferrocyanide solution. A common source of indicator trouble arises from the presence of small amounts of surface-active organic matter in the solution. Reference to Table I shows that such material is capable of preventing completely the formation of the colored form of the indicator and must as a consequence be destroyed before proceeding to the zinc titration. Certain common inorganic ions in concentrations as high as 0.2 mg. per ml. of solution do not interfere significantly with the titration. The end point is unaffected and
Determination
Vol. 16, No. 3
the color change remains sharp, though slight changes in hue may result. A silicate-ion concentration above 0.5 mg. per ml. renders the color change slow and less distinct, In the absence of iron the end point color is yellow-green; in the presence of iron, bluegreen. A few trials with solutions of known zinc content will familiarize the analyst with the various stages of the color changes before and a t the end point. However, a few hints will aid ( 3 ) . With the initial addition of 3 drops of ferricyanide, a viplet color should develop. Lack of color a t this point indicates serious divergence from the suggested procedure. A new sample is indicated. With the addition of 0.025 molar potassium ferrocyanide (containing 0.3 gram of potassium ferricyanide per liter), the initial violet changes to blue. As the titration proceeds, the blue color fades to a light shade of blue. At a few milliliters from the end point, this light blue will change through blue-green, then yellow-green, and finally attains a light violet color. The color transitions obtained following the light blue stage depend on the rate a t which ferrocyanide is added, on the temperature, and on the rate of stirring. Some stages in the color change may not appear. The color a t about 2 ml. from the end point should, however, be violet. Enough time must be allowed for the development of the violet complex, which a t 40’ to 45’ C. is a very aensitive and mobile indicator.
If sufficient time is allowed for its development after each increment of ferrocyanide, the end point can be approached with certainty and precision. The titration requires 5 to 10 minutes. Starting with clear bath solution, the determination of both copper and zinc requires an average of 1.5 hours. A distinct advantage of the suggested procedure is that no transfers are required, the analysis being started and finished in the same vessel. ACKNOWLEDGMENTS
The labor of sifting, by laboratory trials, the present method from among the many methods reported in the literature fell also upon other members of the Motor Products Development Laboratory staff. The authors wish to acknowledge the help of V. F. Felicetta, C. A. Ihrcke, R. E. Mosher, and J. H. Sinclair. They wish also to thank the United States Rubber Company for permission to publish this work. LITERATURE CITED
(1) Aruina, A. S., Zavodahya Lab., 8,565 (1939). (2) Cone, W. H., and Cady, L. C., J. Am. Chem. SOC., 49, 356-60 (1927). (3) Oesper, R. E., “Newer Methods of Volumetric Analysk”, pp. 176-8, New York, D.Van Nostrand Co., 1938. (4) Tyler, W. P., and BIown, W. E., IXD.ENG.CHEM.,ANAL. ED., 15, 520 (1943). (5) Weiner, R.,and Kaiser, F., Z. Electrochem., 41, 153-8 (1935).
OF
Sesamin
MARTIN JACOBSON, FRED ACREE, JR., AND H. L. HALLER U. S. Department of Agriculture, Bureau of Entomology and Plant Quarantine, Beltsville, Md.
A method for the quantitative determination of resamin in sesame oil is based upon measurement of the greenish-yellow color produced b y sesamin when it is allowed to react with a mixture of perchloric acid and hydrogen peroxide.
S
ESAME oil, a vegetable oil extensively used in Europe and Asia for culinary purposes, has commanded but little attention in this country, and the number of published American investigations (7) on it is small. I n a large measure this is probably because it is an imported oil and is not an important article of our commerce. In 1940 Eagleson (1, 3) showed that the toxicity to houseflies of a kerosene solution of pyrethrins was considerably increased by the addition of a small amount of sesame oil. The oil
alone in kerosene was without effect and was the only one of 42 animal and vegetable oils tested (g) that produced synergism. By fractional distillations of sesame oil Haller et al. ( 5 ) showed that the principle responsible for this synergistic effect is sesamin, one of the components of the nonsaponifiable fraction and a characteristic constituent of the oil. Sesamin is a substituted bicyclodihydrofuran and is not very reactive chemically. It can also be removed from the oil by extraction with 90% acetic acid, or by adsorption (6) on charcoal or clay, from which it can be removed by elution with suitable solvents. Besides sesamin, sesame oil contains sesamolin, which on treatment with mineral acid yields sesamol, a phenol. This compound is responsible for several of the color tests (8, 9) used to identify sesame oil. Its value as a synergist with pyrethrum is not known.
