V O L U M E 2 6 , NO. 9, S E P T E M B E R 1 9 5 4 Herasymenko, P., 2. Elektrochem., 34, 7 4 (1928). (12) Heyrovsk9, J., and IlkoriC, D., Collection Czechosloz. C hem.
(11)
(1.3) (14) (15)
(16)
Communs., 7, 198 (1935). Kolthoff, I. 11..and Lingane, J . J., "Polarography," pp. 374378, Kew T o r k , Interscience Publishers, 1941. Koutecky, J., and Rrdicka, R., Collection Czechosloa. Chem. Commztns.. 12. 337 (19471. Itosenthal, I., Albripht, C. H., and Elving, P. J., J . Electrochem. Soc., 99, 227 (1952). liosenthal, I., and Elving, P. J., J . A m . Chem. Soc., 73, 1880 (1951).
(17) Iloaenthal, I., Tang. C. S., and Elving, P. J., Ibid., 74, 6112 (1852).
1459 (18) Saito, E., Bull. soc. c h i m . Fiance, 1948, 404. (19) Silverman, I.., Chemist-Analyst, 36, 57 (1947). (20) Vopicka, E.. Collection Czechoslov. Chem. Communs., 8 , 349 (1936). (21)
Warqhowsky, R.. Elving, P. J., and JZandel, J., A s ~ L CHEM., . 19, 161 (1947).
RECEIVED for review July 22, 1962. Accepted June 26. 1954. Detailed tables of data covering the polarographic behavior of nialcic and fumaric ai,idr are available from the senior author. Abstracted from a thesis submitted by Isndore Rosenthal as part of the requirement for the P h . D . degree, The Pennsylvania State University, 1931.
Polarographic Determination of Zinc in Gold SAMUEL B. DEAL Tube Division, Radio Corp. o f America, Lancaster, P a .
The purpose of this investigation was the development of a method for the quantitative analysis of small amounts of zinc in gold. No spectrographic standards were available, and the solution technique of spectrographic analysis resulted in inconsistent results. Colorimetric methods available were nonspecific and too time-consuming. By the use of the polarographic technique, accurate results in the range of 0.001 to 1% zinc were obtained. This method of analysis can be used for the quantitative determination of zinc in gold as a routine procedure. 4 minimum of reagents and equipment is required. The niethod is simple and easil? carried out.
T
HIS paper describes the use of the polarographic method of analysis for the determination of small concentrations of zinc, ranging from 0,001 to 1%, in a zinc-gold alloy. I n this analysis, the first step is the separation of gold from the alloy by precipitation Tq-ith an aqueous solution of sulfur dioxide. IVhen the gold has been removed, the polarographic method can he used to determine the zinc concentration with a high degree of accuracy. TIlEORETIC4 L COKSIDERATIONS
Gold-Zinc Alloy. I n t8heanalysis of a gold-zinc alloy. preliminary separation of gold is necessary because t'he high diffusiqn current of gold would mask the diffusion current of the zinc. One of the simplest and most complete methods (8)for this preliminary separation is the precipitation of gold wit,h an aqueous s jlution of sulfur dioxide. This method of prec-ipitatim results in t,he complete removal of gold and eliminat,rs the necessity for a reyrecipitatim. EXPERIMENTAL PROCEDURE
Reagents. Four molar ammonium hydroxide plus 131 animonium chloride solution ( I ) containing 5 ml. per liter of a solution of methyl red and bromocresol green. Methyl red-bromocresol green solution composed of three parts of a 0.2% alcoholic solution of methyl red and two parts of a 0.2% alcoholic solution of bromocresol green. Saturated aqueous sulfur dioxide solution. Removal of Gold. A 1-gram sample of a gold-zinc alloy n-as dissolved in a freshly prepared mixture of 5 ml. of concentrated nitric acid and 15 ml. of concentrated hydrochloric acid. The sample was heated in the acid mixture on t,he hot plate until solution was complete; then it \\-as evaporated to a low volume. The residue was transferred to the steam bath and evaporated to dryness. The resulting salts were dissolved in wirm water and diluted to approximately 60 ml. One milliliter of concentrated hydrochloric acid \vas stirred
into thc solution and 25 nil. of a saturated aqueous solution of sulfur dioxide were then added. The solution was stirred well, and left on the steam bath for approximately 1 hour to allow precipitation of gold. After 1 hour, additional 10 ml. of sulfur dioside were added, and the solution was left on the steam bath for 5 minutes longer. Y)
t
3 6C > 4 a
E 50 m
f 40 z
iI
5 30 8
F 20 w J
Figure 1. Current-Voltage Relationship of Sample Solution Prepared from Gold-Zinc Alloy
The solution and precipitate of metallic gold were transferrcd to a 100-mI. volumetric flask and diluted to the mark xvith distilled water. After thorough mixing, the gold was allowed t o Fettle. -150-ml. aliquot of the supernatant solution was transferred to a second 100-ml. volumetric flask. This aliquot in the second 100-ml. volumetric flask was diluted to the mark with 4.U ammonium hydroxide-ammonium chloride base solution rontaining methyl red-bromocresol green as maximum suppressor. Sample solution and base solution were thoroughly mixed and a portion was withdrawn for analysis. Analysis of the Sample. I n the analyeip, a capillary tuhe, having a bore of 0.05 mm., was used for the mercury-dropping electrode. Sitrogen was used for the removal of oxygen from the solution to be analyzed, and was also passed over the surface of the solution during analysis to prevent the entrance of oxygen. Galvanonirter readings were taken a t 0.05-volt intervals over a range from 0.90 to 1.66 volts. Potential values were not corrected to standard potentials. The sensitivity control of the Fisher electrodropode used in ohtaining polarographic measuremrnts ir-as set a t 2X during the entire series of analyseP. The current-voltage curve obtained by plotting the experimental values determined for the sample solution is given in Figure 1. Preparation of Calibration Curve. Three standard solutions containing 0.5, 1.0, and 1.5 mg. of zinc, respectively, were analyzed polarographically to obtain data for the preparation of a calibration curve. The current-voltage curves obtained for the three solutions are showii in Figure 2 . T h e residual current of a solution blank, containing all components with the exception of zinc, was used as a base line for graphical measurement of diffusion currents.
1460
ANALYTICAL CHEMISTRY
Table I.
Analysis of Synthetic Gold-Zinc Samples Zn Added, Zn Recovered,
Sample No.
n'i g .
hlg.
I
>
I
I
I
I
I
I
I
I
5E 50 m
f 40 2
L
5 30 20
The diffusion current of zinc was then plotted as a function of zinc concentration in the three standard solutions to obtain a calibration curve. ACCURACY OF METHOD
Synthetic samples of spectrographically pure gold, to which zinc chloride solution was added, were analyzed polarographically in order to determine the accuracy of the method described. The procedure used in the analysis of the synthetic samples was the same as that used for the analysis of the gold-zinc alloy. The results of the analyses are given in Table I.
J w
8
10
1
w
? 5 U
0
c.5
IO ELECTROLYSIS- C E L L
15
VOLTS
20
Figure 2. Current-Voltage Relationship of Three Synthetic Samples Containing 0.5, 1.0, and 1.5 Mg. of Zinc, Respectively
gestions given by L. C. Copeland and F. S. Griffith of The Kew Jersey Zinc Co.
CONCLUSION
The polarographic method for the determination of zinc in gold affords an accurate method for the determination of small concentrations of zinc in the range from 0.001 to 1%. ACKNOWLEDGMENT
The author wishes to express appreciation for the helpful sug-
LITERATURE CITED
JI.,and Lingane, J. J.. "Polarography," Kew York, Interscience Publishers, Inc., 1941. (2) Treadwell, F. P., and Hall, W. T., "Analytical Chemistry," 9th ed., Vol. 11, p. 128, Kew York, John Wiley & Sons, Inc., 1946. (1) Kolthoff, I.
RECEIVED for review J u n e 27, 1953. Accepted .July 1, 1954.
Alumina-Adsorption Analysis of Petroleum Aromatics in 420" to 600" F. Range C. M. McKlNNEY and R. Bureau
L. HOPKINS
of M i n e s Petroleum Experiment Station, Bartlesville, Okla.
A method for t h e separation of the aromatic portion of the 420" to 600' F. fraction of distillate fuels was needed. 4 high-dilution alumina-adsorption fractionation provided compositional data and samples to study burning quality. Smoke points were thus correlated with the presence of different aromatic types. The method w-as applied to aromatic concentrates prepared from fuels from 11 crude oils, selected to provide fuels with considerable variation i n composition. Density and refractive index data for some aromatic hydrocarbons t h a t might be expected i n t h e boiling range studied are given for comparison with data determined on fractions from the alumina-adsorption analysis. Accuracy of the polycyclic aromatic determination for distillate low i n sulfur content is believed to be within *3% of the percentage polycyclic aromatics in t h e aromatic concentrate. Smoke point determinations on blends of the monocyclic and polycyclic aromatics with distillates from which they were separated indicate the polycyclic aromatics are poorer i n burning quality. The alumina-adsorption method provides a means not preFiously available of partial analysis for aromatic types in distillates to 600" F.
T
HE composition of the aromatic portion of petroleum dis-
tillates in the jet-engine (JP-4) fuel (18) boiling range was investigated to provide information that may be related to burning characteristics of the distillates. h primary objective of the investigation x a s to determine the percentage of polycyclic aromatics in straight-run distillates from several sources. To accomplish this, a method was developed for separating the aromatic portion into a predominantly monocyclic aromatic portion and a predominantly polycyclic portion. Application of the method was limited to the 420' to 600' F. range because distillate boiling between these temperatures contains most of the polycyclic aromatic hydrocarbons in JP-4 fuel. The current military specification of 550" F. maximum end point by ASTA'I distillation limits the maximum temperature of the true boiling point to someTThat less than 600" F., and the separation of aromatics boiling below 420' F., essentially monocyclic, either into classes or individual constituents, was neither an objective of this study nor believed to be feasible by the method used. The aromatic portion n as analyzed by blending approximately 20 ml. of aromatic concentrate, previously separated from the paraffin-naphthene portion by a silica gel-adsorption procedure, with 200 ml. of mixed amylenes and introducing the diluted sample into a vertical glass column 38 mm. in inside diameter and 125 cm. long, packed with a proximately 500 grams of activated alumina. The top section o r t h e column was expanded to form
