A Differential Method for Accurate Quantitative Infrared Analyses Assay for Cyclohexane i n P e t r o l e u m Concentrates C . F. HAMMER
AhD
H. R. ROE. E. I . du Pont de Nemours & Co.,Inc., Wilmington, Del.
for improving the accuracy and precision of quantitaanalyses is described. The method employs tive infrared . a differential technique for measuring infrared absorption, using a double-beam spectrometer (Perkin-Elmer Model 21 ). The procedure was applied to the analysis for 80 to 90 a t . 9 cyclohexane in petroleum concentrates with an accuracy and precison of 3 ~ 0 . 1 %absolute. This development amounts to an evaluation of the instruments under optimum conditions and provides an estimate of the practical precision limit which can be expected for similar applications. Previous authors (1, 2) have reported on the optimum absorbancy for obtaining maximum precision in quantitative infrared analyses. These values together with data on practical limits in instrumentation and running time can be used to estimate the precision limit on analytical results. This limit is about i l % relative and often depends upon the readability of the chart paper. Robinson ( 5 )reported a differential technique for quantitative analyses which appeared capable of reducing this limit. However, the practical precision limits have not been ascertained. Work in this laboratory using Perkin-Elmer Model 21 doublebeam spectrometer has established the operating precision as z t O . l % absolute. In principle, this differential method depends upon the use of a double-beam instrument to record the difference in infrared absorbance values for a particular band between the unknown sample and a reference solution. The latter solution contains a known concentration of the same absorber. Of particular interest to this laboratory was the assay for 80 to 90% cyclohexane in petroleum concentrates. These concentrates were mixtures of C 6 and C, aliphatic and alicyclic hydrocarbons. Khile the work was confined to this particular problem, it is felt that this development amounts to an evaluation of the instrument under optimum conditions and provides an estimate of the practical limit of precision which can be expected for similar applications.
PREPARATION OF WORKING CURVE
\mrHoD
A.
Table I.
Analytical Data on Petroleum Cyclohexane Control Samples
Cyclohexane, Wt. % 85.00 84.81 84.85 84.78 89.90 XO. 93 89.91 89.98
Found
Error
85.05 84.95 84.80 84.85 89.85 89.85 89.85 90.05
+0.5 4-0.14 -0.5
First a stock solution was prepared xhich contained the major petroleum cyclohexane impurities in the relative concentrations normally encountered. This stock solution was mixed with pure cyclohevane in order to prepare five control samples in the range 80 to 90 wt. % of the latter component. The differential absorbancies a t 11.6 microns were measured by the method described, and the resulting data were used to prepare a quantitative working curve shown in Figure 1. EXPERIMENT4L RESULTS
In order to determine the accuracy of the method, eight synthetic control samples of known cyclohexane content were analyzed. Results appear in Table I. * The precision of the method is less than 0.1% absolute as determined from the scatter of these point. along the working curve. INTERFERENCES
The concentrations and infrared absorptions of the compounds generally present as impurities in crude petroleum cyclohexane are known. Interferences from these compounds in their usual concentrations can be evaluated and eliminated by the empirical calibration described. For example, the first sample in Table I contained 1.74% methylcyclopentane, 7.08% 2,4-dimethyIpentane, 3.037, 2,2-dimethylpentane, 0.61% 2,3-dimethylpentane, 0.30% 3,3-dimethylpentane, 1.53% 2-methylhexane, 0.114, n-hexane, and 0.61% 1,l-dimethylcyclopentane. At these concentrations none of the compounds appear to interfere with the analysis. The reader should recognize that even in similar applications these compounds, or the presence of others which absorb near 11.6 microns, will produce interference and reduce the accuracy of the method. DISCUSSION
The key to high precision is the proper choice of cell thickness to provide suitable absorbancy. The absorbancy desired is that which, for single-beam operation, provides a large number of absorbance units for small changes in concentration, but which still allows enough energy to pass through the cell to drive the recording system of the instrument. An operating single-beam absorbancy was chosen between 1.0 and 1.3 units. -4suitable
+0.07 -0.05 -0.08 -0 06 +0.07
90,
PROCEDURE
The strong absorption band of cyclohexane a t 11.6 microns was selected for analytical measurement because the interfering compounds normally encountered in petroleum cyclohexane have low absorption in this region. Measurements were made on a PerkinElmer Model 21 double-beam spectrometer equipped with a sodium chloride prism. The following instrument settings were found to be the optimum for accuracy and reproducibility of results: slit, 0.700 mm.; response, 2 ; speed, 0.4 micron per minute; gain, 6 (11 to 11.5 microns), 8 (11.5 t o 11.7 microns), and 6 (11.7 to 12.0 microns). The petroleum concentrate was loaded into a 0.116-mm. cell and placed in the sample beam of the spectrometer. A 0.092-mm. cell containing pure cyclohexane was placed in the reference beam. Then the spectrum was recorded from 11.0 to 12.0 microns. This scan was repeated five times. The differential absorbancies a t 11.6 microns were measured by the base line method, and the average value was applied to a quantitative working curve (differential absorbance u s . wt. % cyclohexane).
668
80
-.os
Figure 1.
1
I
I
I
I
.08 DIFFERENTIAL ABSORBANCE A T 11.6 M I C R O N S 0
Standard Curve for Cyclohexane in Petroleum Concentrates
V O L U M E 2 5 , NO. 4, A P R I L 1 9 5 3 amount of the same absorber was placed in the reference beam so that the pen was recording in the 0.01 to 0.10 absorbance range. Thus the spectrum is traced on the portion of the chart paper which permits accurate measurement of small absorbance differences. A practical drawback lies in the servo response limitations of the instrument which decrease the response time of the pen during the most important portion of the spectrum-while the pen is traversing the peak of the band. To overcome this dificulty, the gain is increased at a reasonable point along the slope of the absorption band. This permits more rapid pen response
669 near the absorption peak. Once past the absorption peak the gain is reduced in order to maintain stable operation of the servo system. B wide slit width is used to permit the passage of sufficient energy for optimum amplifier gain and loop response conditions throughout the scan. LITERATURE C I T E D
(1) Hiskey, C . I'..; i s i ~CHEM., . 21, 1440 (1949)
(2) Robinson, D. Z.,Zbid.. 23, 273 (1951). (3) Zbid., 24, 619, (1952). RECEIVED for review September 6. 1962.
Accepted Derernber 2. 1952.
The Determination of Zinc in Brass by Controlled Potential Electrolysis DUNCAN G. FOSTER Swarthmore College, Swarthmore, Pa. purpose of this investigation x a s to find an electrolytic would be more reliable than existing methods and a t least as rapid. Existing methods are mostly constant amperage methods, and are slow and often disappointing. illthough in the analysis of brasses, zinc is the last element to be determined, and there is consequently no problem of separation involved, the writer felt that the application of cathode potential control might make the duplication of conditions easier and result in a saving of time. This turned out to be the case, and a method was developed which is accurate and reproducible and which requires an electrolysis time of less than 1.5 hours. Specifically, the problem involved the deposition of the zinc from solutions from which copper had been deposited by the method of Torrance (4),and lead and tin by the method of Lingane (3). These solutions contained, in addition to the zinc, 14.3 grams of disodium tartrate, 1 gram of succinic acid, 2 grams of urea, 21.1 grams of sodium chloride (from the neutralization of 30 ml. of concentrated hydrochloric acid with sodium hydroxide), and 3 grams of hydrazine dihydrochloride, all in a volume of about 500 ml. In order to diminish the hydrogen ion concentration and lessen the possibility of hydrogen evolution, ammonia solutions R ere used according to one method of standard practice, but for better control the solutions were buffered with ammonium chloride. Both were made 1 molar for simplicity. The zinc in such solutions would be present chiefly as ammonia complex ion, which has the additional advantage of lowering its reduction potential. The electrolyte from the previous procedures might also be expected to affect the redurtion potential. This was determined by a polarographic examination of solutions made up to duplicate the composition of this electrolyte, both with and without zinc present, a t the rotating platinum electrode ( 3 X 0.6 mm., 600 r.p.m.). The reduction potential was estimated by extending the nearly vertical portion of the polarographic wave to its intersertion with the extension of the nearly horizontal portion (residual current). Solutions were made u p as described above. Enough solid ammonium chloride and 0.1 % gelatine solution were added to make the final concentrations 1 J E in ammonium ion and 0.01% in gelatine. They were then adjusted to pH 9 on a pH meter M ith concentrated ammonia, and the polarogram was recorded. Without zinc present a hydrogen wave appeared a t -0.91 v. 2s the saturated calomel electrode (S.C.E.). Upon the addition of enough zinc chloride to make the solution 0.02 molar the wave appeared a t -1.20 v., but there was no distinct plateau. Increasing the zinc concentration to about 0.05 molar produced a definite maximum on the lower portion of the wave, indicating that the zinc was being reduced first, the hydrogen overvoltage being higher on zinc than on platinum. r THE
1method for the analysis of zinc in brass which
Calculation of the reduction potential of zinc in 0.02 molar solution, using the value -1.03 v. for the molar potential in ammonia solution (t?)gave - 1.32 v. us. the S.C.E., which is somewhat more negative than the observed value, indicating that the residual electrolyte did affect the potential. Calculation of the potential of a 10-6 molar zinc solution (the concentration corresponding to satisfactory analytical removal), using the observed value of - 1.20 v., and assuming that it is simple zinc ion which is reduced, gave the value - 1.34 v. us. the S.C.E. This, within the limits of experimental accuracy, should be the reduction potential of zinc from these solutions a t 10-6 molar concentrat ion. Electrolyses a t - 1.4 v. were successful from the start, except that it was found necessary to increase the potential for the final 20 minutes or so to -1.5 v., to ensure complete removal of the zinc. A series of runs %-as made with solutions containing zinc in amounts varying from 100 to 350 mg. Runs were made also in the presence of iron, aluminum, and manganese separately, and since iron was found to interfere, of aluminum and manganese together. The results of these experiments are shown in Table I, Six analyses of a brass of known composition were then carried out using a single sample of 20 grams dissolved and made up to 1 liter. A 50-ml. pipetful was used for each run. These results are set forth in Table 11. Table I. Taken, M g . 100 0 150 6 200 0 250 0 300 2 350 2 100 4 200 4 200 2 200.2 200 1
Table 11.
Electrolytic Deposition of Zinc in Buffered Tartrate Solutions Found, Llg. 100.2 160.4 200.0 250.4 300.2 360.1 134.7 200.0 200.6 200.6 201.0
Difference M g . 0.2 - 0.2 0.0 f 0.4 0.0 - 0.1 f34.3
+ -
0.4
f 0.4 ++ 0.4 0.9
Other metals present, JIg Sone Sone Sone None None None Fe, 40 Al,100 4 h l n , 10 hln, 100 d l , 100; h l n , 100
~-
.4nalysis of a Typical Brass, Thorn Smith's Sample No. 320 7~Cu
% P b f Sn 78.24 9.7aa 78.12 9,95a 78.I 1 9.995 78,23 10.03b 78.23 10.04b 78.23 10.05b Mean 78.19 9.97 AI-. dev. 0.06 0.07 Pts./1000 0.8 1.4 Llanufacturer's valueC 78.01 9.93 a P b and Sn determined separately. b P b and Sn determined simultaneously. C Analyzed entirely b y gravimetric methods, without precipitation.
% Zn
Total
11.83 11.88 11.82 11 86 11.85 11.86 11.86 0.01 0.9 11.98
99.87 99.95 99.92 100.12 100.12 100,14 100.01 99.92
corrections for co-
