801
V O L U M E 20, NO. 9, S E P T E M B E R 1 9 4 8 1 mercuiy indicator electrode cannot be used in solution5 whose oxldatiori potential is greater (more oxidizing) than that of the mercury-mercurous couple, as otherwise the mercury itself will be oxidized by the oxidant being titrated and the observed potential 15 ill be that of the mercury-mercurous couple rather than the true potential of the oxidant-reductant syqtcm In iulfuric acid media the pcxrtinent meicury couple is HgSO,
and
in
+ 2e - 2Hg + SO;-;
E” = +0.373 volt
IS.
S.C.E,
chloride media it is
HgiC1,
+ 2e = 2Hg + 2Cl-,
E” = +0.026 volt
IS.
6.C.K.
In practice the upper limit for the use of the mercuiy electrod? is about +0.2 volt in sulfuiic acid solutions, and about -0.1 volt in chloride solutions. As the standard potential of the ferric-ferrous couple is +0.53 volt us. saturated calomel electrode titration of iron-titanium mixtures must be performed by using s, platinum electrode until the end point for the reduction of the ferric ion has been reached, or passed, and then introducing a mercury indicator electrode for the titration of the titanium. Curve 1 in Figure 5 is a typical automatic recording of the titration of a solution containing 10 ml. each of 0.09 M ferric sulfate and 0.1057 M titanic sulfate in 100 ml. of 4 S sulfuric acid a t room temperature. h platinum indicator electrode was used until the iron end point Kas slightly passed. B t the point indicated by the arrow the titrator R as stopped, the mercury electrode was placed in the solution, and the titration aac: then allowed to continue through the titanium end point. The electrodes were held in loose holes in the rubber stopper of the titration cell, so that they could be immersed or withdrawn from the solution without dismantling the cell. Curve 2 in Figure 5 was obtained with the same mixture as curve 1. The recorder switch was first set a t t 0 . 2 0 volt for the titration of the ferric ion. After the titrator had stopped, and the counter reading had been noted, the mercury electrode was introduced (break in the curve), the recorder switch was resct t o
-0.30 volt, and the titration of the titanium was allowed to proceed. The titration of the iron lequired (3.00 ml. of the 0.1000 M chromous solution, in exact agreement with theory, and the titration of the titanium required 10.70 ml. compared to the theoretical 10.57 ml. Because the sensitivity of the recorder w&9 decreased to obtain both parts of the titration curve on the same voltage scale, with consequent impairment of the sensitivity of the recorder switch a t the relatively small titanium end point, this result does not represent the best obtainable precision in the titanium stage of the titration. A more precise titration of the titanium can be achieved by increasing the sensitivity of the recorder after the iron has been titrated. It is evident that a small amount of iron can be determined easily in the presence of large amounts of titanium. For the determination of a small amount of titanium in the presence of large quantities of iron, the ferric ion should first be reduced to the ferrous state by sulfur dioxide, and after the excess of the latter has been boiled out, the Ti(1V) limy be titrated. The electronic trigger circuit described by Muller and Linganr (9) may be used in these and other automatic titrations in place of the more expensive recording potentiometer, after the characteristics of a titration curve have been established. LITERATURE CITED
Brintainger, H., and Schieferdeckel,W., Z . anal. C h m . , 76, 277 (1929).
Diethelm, B., and Foer5ter, F.,Z . phgsik. Chem., 62, 129 (1908). Forbes, G. S., and Richter, H. W., J . Am. Chem. Soc., 34, 1140 (1917).
Hillebrand, W. F., and Lundell, G. E. F., “-4pplied Inorganic Analysis,”p. 459, New York, John Wiley & Sons, 1929. Kolthoff, I. M., Rec. trau. chim., 43, 768 (1924). Latimer, W. M.,“Oxidation Potentials,” p. 250, New York. Prentice-Hall, 1938. Lingane, J. J., ANAL.CHEM.,20, 285 (1948). Lingane, J. J., and Pecsok, R.L., Ibid., 20,425 (1948). MLItlller, R. H., and Lingane, J. J., I b i d . , in press. Perky, G. A, Trans. Electrochem. SOC.Preprint 92-15 (1947). RECEIVED January 22, 1948.
Determination of Trace Impurities in Reference Fuel Grade Iso-octane By Infrared Absorption Spectroscopy JAlIES A. ASDERSON, JR., Humble Oil & Refining Company, Baytown, Tex. .i procedure has been developed for using infrared absorption spectroscopj to determine trace quantities of impurities present in reference fuel grade isooctane of 99.570 purity prepared commercially by continuous fractional distillation of butylene alkylate. The method, which has a time requirement of 2 hours, is useful for fractionating tower control purposes in plant operation for the determination of the relatiFe content of the less volatile and more volatile impurities present with the iso-octane product. The method is accurate to *O.lq~for the total concentrations of low- boiling impurities, high boiling irnpurities, and total impurities.
T
HE increasing commercial production of pure compounds from petroleum has accentuated the need for precise, yet rapid, control analyses of the finished product. In most cases the product must conform by test to rigorous specifications designed to ensure the quality and purity of the product placed on the commercial market. Examples of products of this type are butadiene for the manufacture of B u m rubber, isobutylene for the manufacture of Butyl rubber, and nitration grade toluene
for the manufacture of T I T . 111 the usual case, these high purity products contain impurities in concentrations of only 0.1 to 2.075, yet analytical methods must be available for constant check on the concentration of these undesired components. h problem of this type is encountered in the commercial production of iso-octane of reference fuel grade from commercial butylene alkylate by continuous fractional distillation. 111 this operation, a freezing point specification is usually imposed upon
ANALYTICAL CHEMISTRY
802 tank oar shipments of the product.
During 1946, the specific: tion was set a t a freezing point of - 107.49" C., which, based 011 a freezing point of -107.365" C. for pure 2,2,4-trimethylpcTritane, corresponds to a purity of 99.50%. ( A value o! - 107.31O C. has also been quoted in the literature for the freezing point of iso-octane; on this basis a freezing point of - 107.49' C. cwiespoiids to a purity of 99.25% for iso-octane.) The ferd itock to the fractional distillation operations contains approximately 20 romponents, and the concentration of 2,2,4-trinieth>1pentane contained thcrcin is of the order of only 18 to 25% In the separation of a refrrence fuel grade iso-octane of minimum 09.50% puiity, it is necessary t o use a minimum of t n o distillation q t q s to separate low boiling and high boiling component\ oi the feed from the desiicd 2,2,4trimethylpentane. For efficient fiactionating column control, it is therefore necessary to know thc concentration of low boiling and high boiling impurities in the fii a1 product. The specification freezing point test, in addition to being unsatisfactory for control analyses because of the excessive time requirements, can be used only for the determination of total impurities. For this reason, an infrared procedure was developed for the rapid control analysis of reference fuel product icr small amounts of low and high boiling impurities. Duiing the past several years, there have been frequent references in the literature (1, 2, 4,7-10) to methods and techniques employed for the quantitative multicomponent analysis of liquid mixtures by the application of infrared spectroscopy. These methods are nearly always applied, however, to the analysis of mixtures wherein many of the components are present in concentrations of several per cent or greater, and an individual component present in concentrations of 0.2 to 2.0% is considered a minoi component in the sample analyzed directly. There are, of course, many references to the use of infrared spectroscopy for the determination of trace impuritieq, and the advantages of the infrared method for this purpose are well known; however, in these cases, the mixture analyzed has usually not contained more than two or three components for which analysis is desired (14). Commercial reference fuel grade iso-octane may contain as many as eight impurity components, totaling 0.50% or less, and determination of these individual components by application of infi ared spectroscopy presents a somewhat more difficult problem. Mixtures of this type have been analyzed successfully by a combination of recognized techniques in infrared spectroscopy, together with careful control of instrumentation. The accuracy of the method is indicated to be of the order of ==O.l% on the high boiling and low boiling impurities, and on the total impurities. Time requirements for thc analysis, including calculation., are about 2 hour..
Source stability of this order was approached by the use of a 500-watt Sola voltage regulator, equipped with a 300-watt ballast load, with 200 watts of energy supplied to the Globar. Each measured thermocouple signal a t a given spectral position was followed directly by a reference measurement on a rock salt plate with a minimum elapsed time of 5 to 10 seconds. In this manner, all intensity measurements were directly measured against reference intensity. and errors due to fluctuation in the source, or in the measuring, amplification, and recording system were minimized. Each instrument n aq housed in an air-conditioned room, ai111 temperature control was maintained t o + l o F. Under thew conditions, drift vias not a problem. Short-period noises arising in vibrational effects in the galvanometer assembly were variable but were usually of the order of +O.l7'of full scale. Fortunate1 the vibrational effect recorded was of a regular periodic charactc and it was possible, therefore, to average the fluctuations n-ith an estimated error from this source of somewhat less than * O . l q c of full scale. Conditions of amplification were adjusted so that the therniocouple signal required for full scale on the 0-10 millivolt Brown Electronik recorder was of the order of 0.5 microvolt. At two sDectra1 positions where measurements of relative optical densities were somewhat more critical, increased accuracy was obtained by using wider slit widths for the measurements of radiant intensity ( I ) transmitted by the sample and the reference standard than were used for measurements of the incident intensity (Io),transmitted by the rock salt plate. At 9.87 and 9.95 microns, the slit width used for I measurements was increased for both the reference standard and the sample, so that the Z value was increased to approximatelv three times the value that would have been obtained without change of slit width.. In order to calculaqe the I values that would have been obtained wlth normal slit width, the observed I values were divided by the ratio of the increased I signal to the normal I signal obtained for the reference standard. This ratio for each of the two wave lengths was obtained from average of a number of determinations on the reference standard. Sample cells employed for the work were of conventional design, and consisted simply of rock salt plates separated by lead gaskets 1 mm. thick. As all measurements were made in the same sample cell, and calculations were based on differential absorption, a slight amount of fogging of the sample cell windows was not a problem. Fluctuations of wave-length calibration arising from * 1 F. temperature changes, although detectable, were not a limiting factor in the analysis. During certain period3 of the day when outside temperature changed rapidly, a small but noticeable effect on wave-length calibration was observed. Because the major effect of such fluctuations was to change the interfering effect of the 2,2,4-trimethylpentane, this effect was minimized by making reference measurements against a standard sample of reference fuel g_ade iso-octane a t the wave-length drum position a t which the given intc nsity was measured.
Table I.
Impurities in Commercial Alkylate Bo i 1in g Point,
APPARATUS AND EXPERIMEKTAL TECHNIQUES
The work described in this article has been carried out on two different infrared instruments. One of these was a Perkin-Elmer Model 12-A equipped with a sodium chloride prism, adapted for the scanning of spectra with a wave-length drive powered with a constant speed motor and equipped with a photoelectric galvanometer amplifier of the type described by McAlister, Matheson, and Sweeney (9). This instrument was used for the development of the analysis and most of the work described herein is based on the use of this instrument. Upon reduction of the analysis to routine practice, the control analytical work was performed in the service laboratories using a Beckman infrared spectrophotometer (Model IR 108), manufactured by the Sational Technical Laboratories, also equipped with a sodium chloride prism, and operated manually with galvanometric measurement of thermocouple signals. I t was found in the early phases of the investigation that control of the source intensity was one of the most important features in the analysis. Short-period requirements for source stability were necessarily of the order of 0.1% of full scale deflection in order to assure accuracy of relative optical density measurements of *0.001 to *0.002.
Compound 2,3-Dimethylpentane 2-Methvlhexane
F.
193.6 194.2 197.6 210.6 228.4 228.9 229.6 236.3 238.6
% of
Total Alkylate 2.6 0.4 0.3 18.3 3.4 3.7 1.2 11.0 9.7
Impurity,
"lo of
Component Listed 8.1 1.2 0.9
..
10.5 11.5 3.7 34.1 30.0
DEVELOPWEhT OF THE METHOD
At the outset of the work, it was apparent that base-line mcthods (such as those described by Wright, 14 and others, 7 ) of detection of individual impurities would not be satisfactory for detection of these minor components because of the weakness of the bands due to these impurities. In fact, scanning of representative samples of reference fuel grade iso-octane shows no absorption bands a t the n ave-length position corresponding to absorption bands of the impurities, so that qualitative analysis by identification of bands is a virtual impossibility. Furthermore, attempts to increase the intensity of these bands hv increasing
803
V O L U M E 20, NO. 9, S E P T E M B E R 1 9 4 8 hample cell thickness increase background absorption due to t hr 2,2,+trimethylpentane a t a faster rate than it accentuates the hand due to the impurity, so that complete extinction is reached briore the absorption band can be detected. There is, however, at each band position a small but definite increment of absorption which cannot be explained by 2,2,4-trimethylpentane. The multicomponent analysis described herein is based on such difft,rriices, measured between the actual sample and pure 2,2,4trimethylpentane. A41-mm. sample cell was chosen for optimum halance between magnitude of partial optical density due to the impurities and extent of absorption by 2,2,4-trimethylpentane. Because the impurity components present in the reference fuel grade iso-octane could not be detected by qualitative scanning of the spectrum of the sample for location of bands due to the impurities, an alternative procedure \yas chosen. rlnalyses of ~~ommercial butylene alkylate have been available in this laboraI ory for some time and the major impurity components expected i n i-o-octane prepared from butylene alkylate can therefore be t 4 m a t r d . Data on commercial alkylate composition have ticrii published recently by Heigl, Bell, and TThite ( 7 ) . For p ~ r p of o illustration, ~ ~ Table I shon-s the relative abundance of various impurity components of commercial alkylate boiling as low as 193.6" F. (2,3-dimethylpentane), and as high as 238.6" F. (2,3,3-trimethylpentane). -1lthough the distribution of impurities given in Table I is not representative of the distribution in actual samples of ret'cwnct. fuel grade iso-octane, it is apparent that determination of all or nearly all these impurity components is necessary in order that the distribution between high and low boiling impurities can be determined by analysis. In this case, no absorption bands for impurities were available where absorption characteristic.< w r e similar, and the absorption effects could be combined for R total impurity analysis as describrd by Brady ( 3 ) and Seyfrietl and ing as a basis the approximate estimated distribution of impuritirs available from analyses of commercial alkylate and the total concentration of these impurities as set by specification, spectral positions were selected for the analysis with primary emphasis upon minimum absorption by 2,2,4-trimethylpentane, and srrondary emphasis upon interference effects of other impurities. Unfortunately, no spectral position that could conceivably be used with desired accuracy for the determination of 2,5-dimethylhexane was found. This component was therefore not included ill the analytical scheme; however, the effect ot omission of this component has not proved to be serious, and has been indicated to be within the accuracy of the method. Table I1 lists the spectral positions selected for the analysis of each impurity. Onc of the most accurate ways of determining the composition and total amount of impurities in samples consists of determining the differential absorption between pure 2,2,4-trimethylpentane and the unknown sample a t each of the spectral positions shown in Table 11, substituting these differential optical density values into simultaneous equations written for the absorption of the impurities a t the given spectral positions, and calculating the distribution of impurities according to conventional methods of solution of simultaneous equations. Although this is the essence of the method actually employed, certain modifications werts necessayy for accurate pract>icable application of the technique. Insufficient 2,2,4-trimethylpentane of high purity was available for use for direct comparison in each analysis. For this reason, a set of reference standards was adopted. The p:imary reference %-asNational Bureau of Standards 2,2,4-trimethplpentane (freezing point -107.39" C., calculated purity 99.88y0). The secondary reference employed was Rohm & Haas commercial iso-octane, Batch 36 (freezing point - 107.47' C., calculated purity 99.58Y0). The optical density differences betxeen the primary and secondary references were determined, and the secondary reference was used in all subsequent analytical work: corrections
Table 11.
Component 2,3-Dimethylpentane 2-Methylhexane 3-Methylhexane 2,4-Dimethylhexane 2,2.3-Trimethylpentanr 2,3,4-Trimethglpentane 2,3,3-Trimethylpentane
Spectral Positions Boiline Rance DesiLnation (Relative to Iso-octane) Low boiling Low boiling Low boiling High boiling High boiling High boiling High boiling
Wave Length, Microns 9.87 13.70 13.53 13.01 9.23 9.63 9.95
for the difference in optical denjitirs of the primary and secondary standards were made for all san- 12s. Because of the rather extreme requirements of the accuracy of determination of differential optical densities a t each spectral position, and the necessity for carrying out all measurements on sample and reference in the same cell (to avoid errors due to cell differences that would vary with time), it was necessary that a reference measurement be made on a rock salt plate a t a given spectral position within 5 to 10 seconds after measurement on the liquid-filled sample cell. CALIBRATION AND CALCULATION PROCEDURES
The instrument employed for analysis was calibrated by measurement of the optical density differential between 5% blends of cxach compound and 2,2,4-trimethylpcntane, and the results were cwniputed to a 100% basis for thc impurity, assuming Beer's Ian. to apply. Incident intensity or IO measurements were basrd o n reference rneasuremcnts with a rock salt plate. Corrrction was made for the differenrc in 2,2,4-trimethylpentane concentrations in the two samples, because, to a slight degree, the measured differential is dependent on the 2,2,4-trimethylpent,ane cvncentration. For actual samples, measured optical density differentials ht+\l-eent'he secondary reference and the sample were computed, using measurements made on the rock salt plate for reference intensity, or lo a t each spectral position. These optical density differentials were corrected for difference between the primary and secondary references, and then substituted in simultaneous cquations w i t t e n for the differential absorption at each spectral position. To a first approximation, the optical density difference a t any qpectral position is of the conventional form AD1 =
CiK11
+ CZK21 4
. . . . . .C,K71
(1)
where C1, CZ,etc., refer to the concentration of impurities, Kll is the calibration coefficient for component 1 at spectral position 1, etc., and ADl refers to the differential optical density of the sample a t spectral position 1. These equations may be arranged for solving by the method of successive approximations so that, (2) In actual practice, equations of form 2 were employed. After substitution of the optical density in the equations of form 2, these equations were solved by successive approximation to yield values for the concentrations of the several impurities. The sum of the impurities was then subtracted from 1.0000 (or 100.00, on a percentage basis), the volume per cent isooctane determined by difference, correction applied for the minor difference in 2,2,4-trimethylpentane concentration in the actual sample, and the concentration of individual impurities recalculated. In practice, this second step was not necessary for most cases, as the iso-octane concentration never varied more than a few tenths of 1%, and correction for this effect was made 1x1 the initial calculation.
A N A L Y T I C A L CHEMISTRY
804
culated for slt~riplevis dependent upon the value emploved. A value of - 107.365' C. was employed ~" Synthetic 1 Synthetic 2 __ in this study, as calculation of the purity of NaCornpoaition of Synthetic Synthesis Found A Synthesis Found A tional Bureau of Standards 2.2.4-trimethvl~entane , , +0.01 2,3-Dimethylpentane 0.31 0.32 0.09 0.00 0 . 0 9 3 was based on this value. This has little significance 0.29 -0.07 0.22 0.08 0.05 2-hiethylhexane 0.32 i0.03 3-Methylhexane 0.36 0.08 0.16 +0.08 n-ith regard to the accuracy of the infrared 2.2.4-Trimethylpentane 98.00 97.94 99.41 -0.01 -0.06 99.42 +0.01 i rnethvl hexane method, however, because this method is based 0.28 0.35 +0.07 0.08 0.09 0.28 0 24 -0.04 0.05 0.08 -0.03 on measurement of the difference in purity between 0 38 +0.12 0.11 +0.02 0.09 0.26 0.20 -0.06 0.13 0.08 +0.05 Sational Bureau of Standards 2,2,4-trimethyl-0.04 0.92 0.89 -0.03 0.25 0.21 f0.05 pentane and the actual sample, corrected for 0.38 1.08 1.17 i-0.09 High boiling impurities 0.33 2.00 2.06 L0.06 0.59 Total .impuritieR 0.58 +O.Ol the calculated impurities in the Xational Bureau of Standards 2,2,4-trimeth>lpentane. Similarly, computed purity values based on freezing points Table 1V. Purity Values are liaatd on t,he value of -107.366" C. for the freezing point Calculated Purity Freezine Infrared P;int. Freezing eamplr of 2,2,4-trimethylpentane. If a t some later date it should method A C. point NO. he shown that the value of the freezing point of 2,2,4-trimethyl- 107,43 99.74 99,65 -0.09 1 pentane is different from that used in this study, the fact should - 107.53 99.34 +0.06 99.40 2 - 107.49 99.50 99.44 -0.06 3 in no serious way invalidate the results shown herein for total -107 51 99.43 99.50 4 +0.07 - i n 7 48 99.54 99.54 ~. 0.00 5 impurities, as all values are based on the same reference. 91- 107.474 -0.26 99.56 99.30 6 though a minor distortion of calculated impurity distribution in -n nq - 107.475 99.56 99,47 7 - 107,474 -0.01 99.56 99.55 8 actual samples is expected, inasmuch as the distribution of im- 107.501 A 0 04 99.46 99.50 9 - 107.492 -0.01 99.49 99.48 LO purities in Sational Bureau of Standards 2,2,4-trimethylpentane - 107.47 -0.02 9 9 . Ii8 99.56 11 is not known, this effect is not considered serious in view of the - 107,473 99,57 -0 10 99.47 12 -0 16 99.43 - 107,51 99.27 13 small amount of total impurities present in this spectroscopic 99.60 - 107.467 - 0 01 99.59 14 standard. Average deviation = 0.07 (all d a t a included) Table 111.
Distribution of Impurities
"
l
ACKNOWLEDGMENT
EVALUATION OF T H E BlETHOD 111 order to check on the accurac) of tlic mrthod for deteiniination of the distribution of high and lo\\ boiling impuiities in refelenre fuel iso-octane, t n o synthetic samples were anal hy t h c procedurc described above (Table 111). Although the results obtained on these synthetic samples are not considered to be the best obtainable with the method, they illustrate the accuracy obtained under ioutine conditions. The most serious discrepancies noted are for thr 2,3-dimethylpentane content of Synthetic 2, and the 2,3,4-trimethylpentane content of Synthetic 1. That these are probably random errors, hoFqevcr, is indicated by the fact that in each case the coiresporiding analyii5 for the same component in the other synthetic is satisfactory. Further data on the accuracy ot the method were obtained by meching the purity data by the infrared method with puritv data by the freezing point mcthod on actual samples from plant production. Values shonn in Table IT7 for purity data by the infrared method are corrected for the calculated 0.12% impurities present in the National Bureau of Standards primary refeience 2,2,4-trimethylpentane. Two values for the freezing point of pure 2,2,4-trimethylpentane have been published ( 5 , 6), and the actual purity ral-
The work discussed in this article was conducted in the laboratory of Humhle Oil &- Refining Company, and the author wishes to csprrss thanks for permission t o report the data obtained. The contributions of various individuals in the company who hr~lprvlin thc drvclopmrnt of thip method are acknonledged. LITERATURE CITED (1) di-ery,W.H., J . Optical Soc. Am., 31, 633 (1941). (2) Barnes, I
