1659
V O L U M E 23, NO. 11, N O V E M B E R 1 9 5 1 These data were obtained by extracting three specimens of X-452 GR-S in each of two flasks on 15 different days. T h e refractive index of each specimen was determined and corrected to 25” C. by the temperature coefficient, given above. The data are presenkd in Table I. I n Figure 1 the ordinate is the average of the six measurements made on each day. T h e solid line represents the mean of the ninety determinations made during the 15 days, and the dotted lines represent the maximum range t o be expected if there is no day-to-day variation. All but two of the measurements fall well within these lines;.
determination (95 out of 100 random observations may be espected to fall within a range of 2sl around the mean value); and ( 5 ) s,,,, the etandard deviation of the mean of all 90 determinations appearing in Table I. T h e latter standard deviation was obtained by the following formula: sm = (st2/90
+ si2/2 + ~ 2 / 1 5 ) ’ / ~
where the divisors 90, 2, and 15 refer, respectively, to the 90 individulrl determinationq, the 2 flasks, and the 15 days involved in the entire experiment. LITERATURE CITED
(1j .Irnold, Aurelia, and Wood, L. A , ”Rate of Change of Refractive
Y)
0 X
$ 0
c
a
I
2
3 4 5 6 7 8 9 IO II 12 13 1415 DAY
Figure 2.
Effect of Flask-to-Flask Variability on Reproducibility of Test
Figure 2 shows the differences: between the two flask averages T h e solid line in the center represents their mean. It’can readily be seen that there are no appreciable differences between estractions. An analysis of variance was made on these data (9)and from it five standard deviations w x e calculated: (1) si, the standard deviationcorresponding t o intrinsic variability (the error reflected by the variability of replicate determinations made in the same flask by the same operator a t approsimately the same time): ( 2 ) SJ, t,he standard deviation corresponding t o flask-to-flask variability (the additional variability introduced when replicate deterniinations are made in different flasks); (3) a d , the additional standard deviation corresponding t o day-t,o-day variability: ( 4 ) sr, the standard deviation which might he expected for R single random
Index of GR-S with Temperature,” unpublished report to Office of Rubber Reserve, Feb. 2 8 , 1949. (2) .irnoid, Aurelia, and Wood, L. A , “Simplified Procedure for the Determination of Bound Styrene in GR-S by Measurement of Refractive Index,” unpublished report to Office of Rubber Reserve, March 4, 1949. (3) Custer, Patricia, “Effect of Aging on the Refractive Index of GR-9,” unpublished report to Office of Rubber Reserve, Dec. 10, 1946. (4) Dryden, H. L., Jr.. “Extraction of Alum-Coagulated GR-S for Refractive Index Determinations,” unpublished report to Office of Rubber Reserve, ,July 9, 1945. (5) Fanning, R. J.. and Bekkedahl, Korman, .%NAI.. CHEM.,23, 1653 (1951). (6) McPherson, h. T., and Cumniings, A. D., .I. Research .VatE. R ~ L TStandards, . 14, 553 (1935). RP786; Ruhbar Chem. and Technol., 8 , 421 (1935). ( i )Madorsky, Irving, and Wood, L. “Measurement of Refractive Index and Determination of the Styrene Content of GR-S Copolymers,” unpublished report to Office of Rubber Reserve, Sept. 13, 1944. (8) Madorsky, Irving, and Wood, L. A , , “Procedure for the Measurement of the Refractive Index of Specification GR-S,” unpublished report to Office of Rubber Reserve, N o r . 30, 1944. (9) Jnedecor, G. W., “Statistical Methods,” 4th ed., p. 238, Tables 10-16, ..imes, Iowa, Iowa State College Press, 1946. (10) IViley, R. H., Brauer, G. XI., and Bennett, .i. R., .I. Polymer Sci.. 5 , 609 (1950). (11) 11-ood, L. .I.,unpublished work a t Sational Bureau of Htandards. Aiiril 5 , 1931. I’rewnted before t h e 58th meeting of t h e Division of Rubber Chemi$try. . ~ \ I E R I C A S C H E V I C A SOCIETY, L Washington, D. C., M a r c h 2 . 19S1. RECEIVED
Estimation of Sulfur in Petroleum Products Electrical Device f o r Use in Determining Sulfur b y the Lamp Method C . W. BROWN Research und Development D e p a r t m e n t , Socony- V a c u u m Laboratories, Paulsboro, N. J .
S
L-LFUH. in gasoline and related light products is usually dr-
termined by burning a sample in a wick-fed lamp, absorbing the combustion gases in a peroxide solution and measuring the resultant sulfate ion in the absorbing solution either volumetrically ( 1 , 3 ) or gravimetrically ( 2 ) . The volumetric procedure has heen more generally used because it is simpler and Icss timc-milPuming although it may be slightly less accuratr. SIZE O F SAMPLE
A major problem that has always confronted operators of the lamp test for sulfur has been t h e ever-present uncertainty of knotring how much sample to burn. It has been customary, berause of this uncertainty, to burn :i relatively large sample in order to produce sufficient acid for :t suitable titration with the conventional 0.0624 .J7 sodium hydroxide solution. This procedure is fairly satisfactory when a fuel of low sulfur content is being
test,ed, but when a sample contains relatively high sulfur, the sulfuric acid formed in the absorbing solution is high and this necessitates a long painstaking titration, Because much time could be saved if the “end point” of thc titration c,ould be estimatcti closely, it seemed highly de~ir:ihle to find a w i y to pre:lict Lvithin rcasonable tolerance the amouiit of sulfuric acid formed in the ahsorbing solution ( t hi$ presupposes fuels containing only carbon, hydrogen, and sulfurj. SOattempt was made a t this time t o provide a substitute for the conventional volumetric titration procedure, but merely to institute a means for checking the acid content of the absorbing solution a t any time during the sample-burning period. Of three possibilities for accomplishing this end result, the third was selected as offering the most promise and was developed for incorporating into the routine procedure.
ANALYTICAL CHEMISTRY
1660
Unnecessary time and samples have been consumed for many determinations of sulfur content by the lamp method because the minimum size of sample is fixed to accommodate the low-sulfur samples. Thus for the same volume burned, a high-sulfur sample will produce a superfluous amount of sulfuric acid for subsequent titration. .i conductivity cell was constructed with the area and spacing of the platinum electrodes designed to give, a t a potential of 1.0 volt direct current, a meter reading of approximately 1ma. for each 0.0624 equivalent of sulfuric acid pro-
1. Addition of a n indicator to the absorbing solution, whose color change could be calibrated t o the increasing acid content and would be plainly visible to the operator throughout a test This possibility was discarded because no indicator was found which was stable in the peroxide absorbing solution for the duration of a test. 2. Taking small ali uots from the absorbing solution periodically t o check the a c i j content. This possibility was rejected because of the multiple titrations and calculations involved. 3. A simple measurement of the electrical conductivity of the absorbing solution seemed t o be a logical and practical approach because sulfuric acid is a good electrolyte. The measurement of conductivity between two platinum plates seemed reasonable because the danger due to polarization would be minimized bv the very nature of the absorbing solution-Le., 3.0% hydrogen peroxide. Moreover, conversion of conductivitr in terms of concentration of sulfuric acaid appeared possible by simple calibration.
Table I.
2 8 3 9
Add 50.0 ml. of 3.0% hydrogen peroxide solution to each of two absorbers. These are “blanks.” Add 50.0-ml. portions of 3.0y0hydrogen peroxide solution containing increasing amounts of 0.0624 9 sulfuric acid to each of other pairs of absorbers. Draw a mixture of carbon dioxide and oxygen through these solutions as in actual test, and record the milliamperes of cwrrent flow. These data are listed in Table I and also are used to form the bases for curve 1 of Figure 3. These data indicate that results are reprodurihle floni one ap-
Relation of Conductivity and Titer
Electrode S o
1 7
duced in the absorbing solution. The latter is equivalent to 1.0 ml. of 0.0624 N sodium hydroxide (1ml. = 1mg. of sulfur), the usual titrating solution used in routine testing. Thus the meter reading provides a convenient means for approximating t h e acidity of the solution. The device has proved satisfactory in routine use. The method provides a control on amount of sample to be burned, allows for quick estimation of sulfur content, and gives an independent check upon conventional titration results.
Absorbing SolLitions (50.0-MI. Portion.;) .\Ia.Usb 0.0624 S HsSOi, LIl. 0 4 0 28 (blank) 0.4 0 28 (blanh) 1 6 1 6
1 30 1 30
2 9
2 42
2 42 4.36 4 4,s 4.36 10 4.7 8.4 8.51 11 5 8.4 8.61 6 14.1 14 51 12 14.1 14.51 a Voltage IS always adjusted t o 1 volt direct current when reading current flow (milliammeter). Polarity-reversing switch is thrown several times t o constant reading and to minimize polarization effects. b Platinum electrodes must be thoroughly clean. Immersing in clean, strong chromic acid is necessary before each test. An occasional immersion i n aqua regia has helped when regular chromic acid treatment seemed inadequate. 2 8
In order to check the feasibility of this latter idea, several esperimental assemblies were set up and equipped with platinum electrodes in the absorber. Figures 1and 2 show changes from the A S T N apparatus currently employed for lamp determination of sulfur. As a first step it was decided to determine if a relationship could be established between conductivity expressed as milliamperes and titer in terms of 0.0624 normality of various solutions of standard sulfuric acid in 3.0% hydrogen peroxide solution. The acid content of the solutions was prepared t o cover the range usually encountered in routine test work, Conductivity was measured in all cases when a potential of 1 volt direct current was applied t o the electrodes. It became apparent early in the work that the electrodes could be made of such area and spacing that a 1-volt direct current potential would produce milliampere readings approximately numerically equal t o milliliters of 0.0624 N sodium hydroxide solution (the regular titrating solution, 1.0 nil. of which is equivalent t o 1.0 nig. of sulfur).
2-LEAD CONDUIT
H
u
Figure 1. Diagram of Combustion a i d \lijor.pt i o r i .ipparatus and Electrical Devices .4. Graduated lamp 5 . Copper flame adjuster C. Borosilicate glass chimney D . Absorption tube with fritted disk E. Platinum electrode, 0.0 X 1.2 cm. 0 8 cm. apart F. Bakelite adaptor, drilled for wires’frdru electrodes to panel contacts G. Borosilicate glass spray trap H . Cork etopper J . Rubber stopper K. Stopper and borosilicate glass tube vent, 60 mrn. X 1.0 1 3 1.5 urn. inside diameter
1661
V O L U M E 2 3 , N O . 11, N O V E M B E R 1 9 5 1 paratus to another, provided the area and spacing of the platinum electrodes are controlled within reasonahle tolerance. After the fact had been established t,hat a relationship existed i)et~ e e nconductivity and titer of the ahsorbing solutions made u p with standard sulfuric acid in 3.0yohydrogen peroxide, it was ncxt considered necessary t o find out if the same relationship exists when the sulfuric acid content of the 3.0% hydrogen peroxide absorbing solution is produced from the sulfur of a fuel under test c*ontlitions. The points shown on Figure 3 were plotted from data obtained from m&h actual routine test work and curve 2 was drawn as the best average of these points. During this work nieasurahle increases in niilliamperes m r e noted for sniall increments oi sample hurned, and the order of magnitude of the readings obtained between iuels of various sulfur levels for any given quantity burned was sufficiently large to detect reasonable differences of sulfur content. The slopes of curves 1 and 2 in Figure 3 show marked similarity, but curve 2, drawn from actual test data, indicates a slightly higher numher of milliliters of 0.0624 N sodium hydroxide for any gircn milliampere reading. This difference is irrelevant and no attempt has hcen m : d e to deteriiiine the reason.
Figure 2.
may be cheched at any time by a simple conductometric procedure. Burning of the sample can he discontinued a t any predetermined acid concentration and thus make possible a suitable titer. This eliminates the usual tedious and painstaking titration step that is imposed in testing any sample of unknown sulfur content. If the milliammeter records 12 to 14 ma. relatively quickly, a fuel of high sulfur content is indicated and hurning is discontinued; if the increase in milliamperes is very slow, a fuel of low sulfur content is indicatcd and hurning is continucd until 3 t o 4 ml. of fuel are conqumrd 20
c
Electrical System
Milliammeter, range 0 t o 1.5 ma. V. Voltmeter, range 0 t o 2 volts BAT. Two 1.5-volt dry cells in parallel SW. Polarity-reversing switch PE. 12 conductivity cells with platinum electrodes, 12.0 X 6.0 mm., 6 mm. apart M.4.
I~:x:tiiii~~ation oi' the dupe and position of each curve s h o ~ t,hat s a 1 to 1 ratio hetwecn milliamperes and milliliters of 0.0624 aV sodium hydroxide is not adhered to over the entire range; however, the divergence is not too great to prevent fair approximation for "spot check" purposes. Curve 2 is the reference line from which milliliters of 0.0624 N sodium hydroxide are determined from the millianipere readings of the absorhing solution for spot check purposes. Spot checks for per cent sulfur can be made a t any time during and after the burning of a test sample by noting simultaneously the current flow in milliamperes of the absorbing solution, and the volume and temperature of the sample burned. Curve 2, Figure 3, indicates the milliliters of 0.0624 N sodium hydroxide corresponding to the milliampere current flow of the absorbing solution: sample volume is converted to weight and the per cent sulfur is calculated in the conventional manner. If the established relationship hetwcen milliamperes and niilliliters of 0.0624 AT sodium h>-tlroxide-i.e., 1 t o I-is noticeably out of line--and this condition \vas encountered during the csperimental a-ork-the per cent sulfur as determined by usual titration should be questioned. This disruption of the 1 t o 1 relationship may he due to the fact that a fuel inadvertently contains maproduces during the terial-such as nitrogen or fluorine-that burning period an acid other than sulfuric. Table I1 shows the comparative conductivity in milliamperes of 50.0-ml. portions of :%.Oyo hydrogen peroxide containing equal quantitie? of 0.0621 N sulfuric, nitric, and hydrofluoric acid sol~tioiis.
0
3
u
e
E.
7
8
9
IO
II
1213
1 ~ 1 6 1 6
MILLIAMPERES
Figure 3. Relationship between hLilliampere Reading and Milliliters of 0.06% ,V Sulfuric Acid 1.0 ml. = 1 mg. of sulfur Solid line. Curve based on titration of sulfuric acid formed in 3.0% hydrogen peroxide solution during routine lainp sulfur tests Broken line. Curve based on titration of standard 0.06244 N sulfuric acid solution in 3.0% hydrogen peroxide solution
The per cent sulfur content m ~ j kw - wtimated at any time after the sample has burned for approximately 15 minutes. This feature is of particular advantage in certain types of control work. The final reading provides a n indeprnderit means for checking the actual per cent sulfur content which is determined voluinetrically in the conventional manner. The occurrence of a marlied numerical difference between the milliampere (conductivity) reading of the absorbing .solution and the milliliters of 0.0624 'V sodium hydroxide t o titrate thP absorbing solution (1 to 1 ratio) indicates that the presenw of niatcrial
Table 11. M1. of 0.0624 N Acid per 50.0-MI. Portion of 3.0% HzOz E%lank( 0 . 2 8 ) 2.0 4 0 6.0
CONCLUSIONS
Comparative Conductivity 3 0% H202
0.4
Condiictix it1 Milliampere-_ _~ 0 0624 .\ 0 0624 N 0,06243 H2SO4 HNOp IIF
..
..
..
2.2 4.6
..
..
s:5 . 41
8.0
On the basis of the experimental work and esperience with rout,ine application of the technique involving more than five thouRand samples over a 3-year period it has been established that the progressive formatmionof sulfuric acid in the absorbing solution
2
I
. 10.9 -
12.u
lU.8
..
1' '2 .. 3.4
0 0624 N HzSOIis only acid to maintain 1 to 1 ratio (milliliters 0.0624 N milliamperes).
1's.
ANALYTICAL CHEMISTRY
1662 other than sulfur is contributing t o the acidity of the absorbing solution. An examination of the points shown on Figure 3, forming the bases for curve 2, indicates that use of the milliammeter reading as the sole index of sulfur content is not entirely feasible if the limits of accuracy, as established by ASThl methods, are t o be maintained. Any extension of the technique in this direction would have involved complicatione and possible delay that would defeat the main purpose of this investigation-viz., to predict within reasonable tolerance the amount of sulfuric acid formed in the absorbing solution. ACKNOWLEDGMENT
The author appreciates the assistance of C. S. Tegge of the l’h>,\ics Sertion on electrical problems, technicians G . L. Coolidge,
T. S. Scott, E. Szurek, and W. A. Tettemer of the Inspection Section, and I;,. T. Scafe, supervisor of the Inspection Section, in the editing of this pager. LITERATURE CITED
(1) Ani. SOC.Testing hfaterials, Committee D-2, “Proposed Method of Test for Sulfur in Petroleum Products by the Carbon
Dioxide-Oxygen
Lamp Method,” -4ppendix VI, November
1949.
Am. SOC.Testing 1Iatei.ials. “Sulfur in Petroleum Products by the Lamp Gravimetric Method (Tentative).” (3) Edgar, Graham, and Calingaert, George, IND. ENG. CHEM., AN.4L. ED.,2, 104 (1930).
(2)
R E C E K E DA P R I L12. 1931.
Sensitive End-Point Procedure for Coulometric Titrations W. DOKiLD COOKE’, C. N. REILLEY,
~ N D N. HOWELL FURMAN Princeton Cniversity, Princeton, N . J .
The development of techniques in the field of coulometric titrations has provided a method for the addition of extremely small quantities of titrating agents. As minute electrical currents can be accurately measured, these methods allow the addition of quantities as small as 10-’2, and possibly even lo-’’ equivalent of titrating agent. The usefulness of the titration is, however, limited by the sensitivity of the end-point detection. A modified amperometric procedure extends the range of coulometric titrations. The method has been applied to the titration of ferrous ion with electrically generated ceric sulfate. A sensitivity of 0.001 microgram of ferrous ion per milliliter of solution has been realized and titrations have been carried out at concentrations as low as 0.01 microgram per ml. The effect of interfering substances in the reigents is minimized.
I
S IIECEKT years coulometric procedures have become an
hcreasingly important method of analysis. The so-called coulometric titrations have been applied to a wide variety of oxidimetric and acidimetric systems, in both the macro and micro range. Although the methods can be applied t o accurate determinations of macro samples ( I ) , the greatest ad- ’ vantage over conventional procedures seems to be in the field of microtitrimetry. Amounts of titrating agents can be added coulometrically which would be difficult, if not impossible, by conventional methods. T h e addition of 10-12 equivalent has been accomplished by the procedures outlined here. It appears that quantities of titrating agent of the order of IO-’’ could be generated if the necessity arose. However, the limitation of this type of analysis lies not in the addition of reagent but in finding an end-point procedure of the desired sensitivity. In fact, it has heen stated that the determination of the end point is one of the major problems in microtitrimetry ( 7 ) . A search was made for a n end-point procedure capable of high sensitivity, so that the range of coulometric analysis could be extended. It seemed that a method employing the measurement of diffusion currents would be most applicable t o this problem. Amperometric titrations employing the measurement of such currents have been applied t o oxidation-reduction titrations ( 2 , s ) . With systems in which both the oxidant and reductant yield a diffusion current, the titration curve obtained is of the type shown in Figure 4. I n such systems as the titration of ferric ion with titanous solution, a voltage is impressed upon the indicator electrode, so that the diffusion current for the reduction of ferric ion is set up a t the start of the titration. As the ferric ion concentration is decreased linearly, the current decreases in a similar fashion and reaches zero a t the end point. When the end point is passed, a diffusion current caused by the oxidation of the titanous 1
PI‘.
Present address, Department of Chemistry, Cornel1 University, Ithaca,
Y,
ion is set up. A change in slope caused by the diffeience in diffusion coefficients is usually evident as the lines cross the zero axis. The circuit used with this type of procedure is shown in Figure 1. Reference cells of the correct voltage are sometimes substituted for the potentiometer, and no applied potential is necessary ( 4 ) . The choice of the potential to impress upon the indicator electrode is usually made from a study of the individual polarograms. I n Figure 2 are shown the polarograms of the ferric-ferrous and titanic-titanous procedures. For the voltage t o be impressed upon the indicator electrode, a value is chosen that will include the ferric ion and titanous ion plateaus. An analogous process was devised for the titration of ferrous sulfate with electrically generated ceric sulfate. The normal type of titration curve was obtained, but the results were not satisfactory at extreme dilutions. .4 precision of 5% wab ob-
REFERENCE ELECTRODE
Figure 1. Indicator Circuit
