July, 1922
T H E JOURNAL OF INDTJSTRIAL AATD ENGINEERING CHEMISTRY
589
Detonation Characteristics of Blends of Aromatic and Paraffin Hydrocarbons’ By Thomas Midgley, Jr.,2 and T. A . Boyd2 GENERAI, MOTORS RESEARCH CORPORATION, DAYTON,0x10
The following paper presents the results obtained in a careful measurement of the effects of uarious concentrafions of benzene, toluene, or xylene upon the detonation tendency of parafin fuels in badly carbonized or high compression engines. Most of the determinations were made on blends with kerosene, on account of its greater tendency to detonate. As a standard of comparison, use was made of parafin fuels confaining smallamounts of xylidine, which, in common with other aromatic amines. exerts a powerful suppressing action on detonation. A simple basis for determining the amount of benzene necessary to add to a parafin fuel in order to obtain a given effect is as follows: Up to a concentration of 70 per cent by molecules, the effecfiveness of benzene for suppressing detonation varies directly as the square of its molecular concentration (Fig. 3 ) . H E “knocking” that is so familiar in automobile engines when laboring on hills with wide open throttle has commonly been attributed to preignition. This conception, however, has been proved erroneous. The “knock” under such conditions is probably due to a detonation of part of the fuel charge, and the tendency of a fuel to detonate is a function of its chemical composition. Furthermore, the detonating tendency of a fuel of given composition and structure varies with the compression to which it is subjected, with the effectiveness with which the engine is cooled, and with the degree of carbonization in the cylinders of the engine. The tendency of a fuel to knock or detonate in a given engine is influenced also by some elements of design, such as, for example, the spark plug location. It has been known for some time that the addition of benzene and certain other aromatic hydrocarbons to paraffinbase gasolines greatly reduces the tendency of the fuel to detonate when used in automobile engines. The extension of the by-product coking industry in the United States during the recent war generated a capacity for the production of light oil which could only be absorbed by the use of a part of the material as a motor fuel. Since 1918, therefore, mixtures of benzene (or “motor benzene”) and blending naphtha have been rather widely sold as motor fuels in this country, and a t about the Same price as gasoline. Very few accurate quantitative data have been published on the effect of the blending of aromatic hydrocarbons with paraffin-base gasolines upon the tendency of the latter to knock or detonate. The primary reason why more of these fundamental data have not been obtained and made generally available is the previous lack of a means for measuring the detonating tendency of fuels with sufficient accuracy. A device has recently been developed with which the tendency of a fuel to detonate can be determined with a high degree of accuracy and reliability.3 The purpose of this paper is to present some results obtained in the careful measure-
T
1 Presented before the Petroleum Section at the 63rd Meeting of the American Chemical Society, Birmingham, Ala., April 3 to 7, 1922. * Chief and Assistant Chief of the Fuel Section of the General Motors Research Corporation, Dayton, Ohio. For a discussion of the relative merits of the various means that have been used far observing and measuring detonation in engines see Midgley and Boyd, “Methods of Measuring Detonation in Engines,” J . SOC.Automotive Eng., 10 (1922),7.
*
I
ment of the effects of various concentrations of different aromatic hydrocarbons upon the detonation tendency of paraffin fuels in badly carbonized or high compression engines. I n order that the effects of aromatic hydrocarbons in as wide a range of concentrations as practicable might be measured, they were blended with kerosene for making the majority of the determinations reported in this paper. The greater tendency of kerosene than lighter paraffin hydrocarbons to detonate made i t possible to determine the detonation characteristics of blends up to a concentration of 80 per cent benzene without introducing the difficulties incident to excessively high engine compressions.
~o!-;:
FIG. APPARATUS FOR MEASUREMENT O F DETONATION BY THE BOUNCING PIN METHOD
On account of variations in motor conditions it is evident that data obtained from any particular engine are applicable in a quantitative way only to that one motor design and set of conditions. But, although widely different behavior may characterize the combustion -of a given fuel in two engines, the relative behaviors of two given fuels will be comparative in whatever type of motor they may be run. Hence, in measuring the detonating tendency of any fuel it is essential that some standard fuel be used as a basis of comparison. I n the tests reported herein small percentages of xylidine, in the same paraffin fuel as was used for blending with the aromatic hydrocarbons, were employed as a standard. Xylidine has the property, common to aromatic amines and considerably more marked in a number of other materials, of exerting a powerful suppressing action on detonation, when present in R fuel in percentages that are
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
590
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Vol. 14. No. 7
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TABLEI-PHYSICALDATAO N FUELSUSEDIN TESTS -DISTILLATION TEMPERATURES, C.1 Absorption in Cold HzSOP First HYDROCARBON Percent Droo 10% 30% 40% 50% 60% 70% 80% 90% 95% 20% Dry Kerosene. .................... 0.816 7 186 201 207 212 217.5 222 227.5 233.5 241 253.5 268 291 Commercial .~~~ gasoline. 0.734 5 40 65 83.5 99 111.5 125 140 226 157.5 177 200 219 “High Test” gasoline.. 0.704 3 44 59 68.5 76 82.7 89.3 96 103 114 178 128 157 Benzene (90’ benzene). 0.878 74 77.5 78.7 79.2 79.8 80.1 80.5 81.1 82 85 92.5 . . . Toluene. 0.860 (about) 107 108 108.5 108.6 108.7 108.8 108.8 108.8 108.9 109 109.2 ... Xylene.. 0.860 (about) 135 136 136.2 136.5 136.7 136.9 137.1 137.3 137.5 137.8 138.1 . . . 1 The apparatus used conformed approximately to the standard of the Committee for the Standardization of Petroleum Specifications as given in their Bulletin 8 , published by the U. S. Bureau of Mines. Sp. Gr. 15“ C.
~
.......... ......... ........ ..................... .....................
.. .. ..
relatively very small. Thus, it may be seen from Fig. 2 that 1 per cent of xylidine in kerosene is equivalent for the elimination of detonation to about 15 per cent of benzene in the same fuel. This property of the material makes i t possible to convert kerosene into a fuel that will withstand very high compressions without knocking, and this with the addition of such a small percentage of xylidine that the combustion characteristics of the fuel, other than its tendency to detonate, are not materially changed.
in every way, except that the mixing valve was bored to a larger inside diameter, that a means was provided for adjusting the spark timing, and that the compression was increased by stages from the normal ratio of 3.47 : 1 to a ratio of 5.36 : 1, by means of a series of cylinder heads which had been cut down by differentamounts so as to reduce the clearance volume by corresponding stages. The device that was employed for measuring the relative intensities of different detonations, and called the “bouncing pin” apparatus, is shown diagrammatically in Fig. 1. An PROPERTIES OF MATERIALS USEDAS FUELS essential part of this device is a combination of a bouncing The materials used as ingredients of the various fuels pin with the standard pressure element of the Midgley that were either examined or employed as standards in the Indicator.4 This pressure element is designed to screw examinations were “high test” gasoline, commercial gasoline, directly into the combustion chamber and to come flush kerosene, xylidine, benzene (90 O benzene), toluene, and with the inner surface of the cylinder head. The lower end xylene. The xylidine employed was a commercial material, of the element embodies a small piston, the upward movecomposed of the mixed xylidines. Some physical properties ment of which is resisted by a heavy spring. I n the original element the movement of the piston resulting from pressure of the other materials are tabulated in Table I. changes in the combustion chamber is transmitted through APPARATUS a very light rod to an arm,which operates to change the vertiA */,-kw. Delco-Light engine was used for making all the cal angle of a small mirror, supported on a horizontal axis determinations. This is a single cylinder, air-cooled engine, at the top of the element, in proportion to the pressure on directly connected to a 32-volt d. c. generator, and having the piston. In the bouncing pin apparatus the mirror and a 2.5-in. bore and a 5-in. stroke. The engine was standard the shaft connecting it to the piston have been replaced by a heavy pin which rests on the piston simply by gravity. During normal combustion this pin moves up and down very slightly (only a few thousandths of an inch), owing to its following the movement of the piston resulting from normal pressure changes in the cylinder. But when detonation occurs in the combustion chamber the rod jumps upward a noticeable distance and entirely clear of the piston. The amplitude of movement of the pin varies with the intensity of detonation. It has been observed to jump as high as 1.5 in. above its normal position. Since two successive explosions in a cylinder are rarely exactly alike, the desired degree of accuracy could not be obtained without an integration of the fluctuations of the pin over a period of time. The rest of the apparatus illustrated in Fig. 1 was arranged to accomplish this result. Contact points were held in position in an electric circuit by springs immediately above the upper end of the bouncing pin, in such a way as to be closed when the pin was thrown free of the piston. An integration of the length of time the contact points were closed was then obtained by measuring the volume of gas evolved during a 1-min. period by an electrolytic cell placed in series with the points. T o the cell, which contained a 10 per cent solution of sulfuric acid in distilled water, 110 volts direct current were applied through $3 a resistance unit, this high voltage being used to reduce polarization. Platinum electrodes were used in the cell which, as is shown in Fig. 1, was designed so that the evolved gas was automatically collected in a slender graduated tube Y l a t the top where its volume could be easily noted. Since 6! 0 o IO PO 90 40 JQ SQ PO 80 GO I~OAROM/ITICthe amount of gas evolved during a given period by one fuel was always compared with that evolved by a fuel of similar loo 80 80 70 60 5 0 80 jK, 20 10 0 &WAFFIN detonation characteristics that was run immediately before PERCENP BY VOLUME and after it, no correction of the volume of gas was made for
F, P
s2
Fro. &GRAPHICALREPRESENTATIONOF DATAOBTAINEDIN DETERMINADETONATION CIlARACTERISTICS OF BLENDSOF AROMATIC AND PARAFFIN HYDROCARBONS (Plotted from data in Columns 4, 5, and 7; Tables 11, 111, IV, and V)
TlON OP
1 Foi description of this pressure element and its function see “High Speed Indicators,” by Thomas Midgley, Jr., Trans. Sac. Aulomoh’W En& 15 (1920),317.
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
July, 1922
591
DETERMINING DETONATION CHARACTERISTICS OF VARIOUS BLENDSO F BENZENE AND KEROSENE BENZENE-KEROSENE BLEND DETERMINED EQUIVALENT XYLIDINE DETERMINATION Compression Spark, Degrees Benzene Kerosene IN KEROSENE, PER CBNT BY VOLUME NUMBER Ratio before T. D . C. Per cent by Vol. Per cent by Vol. Individual Average TABLE 11-DATA
OBTAINED IN
3.47 3.47 3.47 3.47 3.47 3.47 3.87 3.87 3.87 4.59 4.59 5.36 5.36 5.36 5.36 5.36 5.36 5.36
12 20 21 13 15 16 11 22 23 28 29 10 37 38 39 40 41 42
43 43 43 43 43 43
..
43 43 32 32
..
25 25 25 25 25 25
1.0
15 15 15 25 25 =25 40 45 45 55
1.0 1.0 2.05 1.95 2.25 4.3 4.7 4.7 7.05 7.05 10.65 11.3 11.5 11.4 15.8 15.85 15.85
55
70 70 70 70 80 80 80
1.0 2.1 4.3 4.7 7.05
11.2 15.8
AND KEROSENE DETONATION CHARACTERISTICS OF VARIOUS BLENDSOF TOLUENE TABLE111-DATA OBTAINED IN DETERMINING DETERMINED EQUIVALENT XYLIDINE TOLUENE-KEROSENE BLEND IN KEROSENE, PER CENT BY VOLUME Spark, Degrees Toluene Kerosene DETERMINATION Compression Individual Average Per cent by Vol. Per cent by Vol. before T. D . C. NUMBER Ratio 20 80 1.8 1.8 43 17 3.47
14 24 25 30 31 32 45 46 43
3.47 3.87 3.87 4.59 4.59 4.59 5.36 5.36 5.36
43 43 43 32 32 32 25 25 25
25 45 45 55 55 55 70 70 75
75 55 55 45 45 45 30 30 25
2.55 5.25 5.35 7.4 7.15 7.15 11.75 11.9 14.4
2.55 5.3
7.25 11.85 14.4
TABLETV-DATA OBTAINED I N DETERMINING DETONATION CHARACTERISTICS OF VARIOUS BLENDSOF XYLENE AND KEROSENE XYLENE-KEROSENE BLEND DETERMINED EQUIVALBNT XYLIDINE DETERMINATION Compression Spark, Degrees Xylene Kerosene IN KEROSENE, PER CENT B Y VOLUME NUMBER Ratio before T. D. C. Per cent by Vol. Per cent by Vol. Individual , Average
3.47 3.47 3.87 3.87 4.59 4.59 4.59 5.36 5.36
43 43 43 43 32 33 32 25 25
20 20 45 45 55 55 55 70 70
63
64 61 62
1.8 2.05 6.05 6.05 7.75 7.80 7.85 12.2 12.7
1.95 6.05 7.8 12.45
TABLEV-DATA OBTAINED IN DETERMINING DETONATION CHARACTERISTICS O F BLENDSO F BENZENE AND GASOLINE BENZENE-GASOLINE BLEND DETERMINED EQUIVALENT XYLIDINE IM Spark, Degrees Benzene IGasoline GASOLINE, PER CENT BY VOLUME Individual Average before T.D. C. Per cent by Vol. Per cent by Vol. Designation 50 Com. gasoline 5.0 50 5.36 5.1 5.05 50 Corn. gasoline 50 5.36 1.7 75 Corn. gasoline 25 4.59 1.85 75 Com. gasoline 1.8 25 4.59 75 H. T. gasoline 1.65 25 5.36 1.65 1.65 75 H. T . gasoline 5.36 25
DETERMINATION Compression NUMBER Ratio
59 60
80 80 55 55 45 45 45 30 30
temperature or for the slight liquid pressure resulting from the design of the apparatus. A rapid and complete change of the fuel entering the engine when desired was facilitated by the use of a special fuel system. This device consisted of two carburetor float bowls, so arranged as to be adjustable for vertical position, and two vertical glass tubes placed side by side, each connected at its bottom to one of the float bowls. Each of these tubes was connected in turn to a 3-way stopcock from which a single fuel line of small diameter ran to the engine. This arrangement permitted rapid switching from one fuel to another maintained a t the proper level to give the correct amount of fuel to the engine. A mixing valve, which was standard, except that it was bored to a larger internal diameter, was used for metering the fuel to the engine; so that in changing fuels no complication was introduced from the residual fuel that remained in the float bowl of a carburetor. A means was provided for the complete draining of the part of the fuel system not in use, so as to permit the ready substitution of different fuels.
METHOD The method of making the determinations can best be explained by giving a specific example, for which the com-
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parison of a 46 per cent benzene-56 per cent kerosene blend with fuels composed of small percentages of xylidine in kerosene will be employed. A compression ratio of 3.87 was used, so that some detonation would occur, but which was not so violent as to cut down seriously the power of the engine or to cause it to operate in an erratic manner. The fuel under examination was put into one side of the fuel system, and the mixing valve on the engine was adjusted so as to give a maximum of detonation, which is an adjustment that lies close to the leanest possible mixture for maximum power. By trial it was found that 5 per cent of xylidine in kerosene had a very slightly lesser detonating tendency than the benzene-kerosene blend under examination. This fuel was then placed in the other side of the fuel system, and its level was adjusted so as to give the point of maximum detonation. The setting of the mixing valve was left undisturbed throughout the determination so that the compression pressure of the engine would be unchanged. A number of alternate 1-min. runs were then made with the 5 per cent xylidine in kerosene and the 45 per cent benzene55 per cent kerosene blend, and the amount of gas evolved in the electrolytic cell during each period was recorded. The output of the generator on the plant in volts and amperes was also kept, simply as a matter of record. After from
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 14, No. 7
TABLE VI-AVERAGE MOLECULAR WEIGHTS OF PARAFFIN FUELS,AND DATAON WHICHTHEYAREBASED Average FIRSTNORMAL PARARFIN FIRSTNORMAL PARAFFIN FUEL “High Test” gasoline.. , Commercial gasoline. Kerosene..
.. . . . . . . . . . .. .. ....
Sp. Gr.
15” c.
Roiling Point
0.704 0.734
0.816
O
c.
95 126
HYDROCARBON BELOW AVERAGE BOILINGPOINT Formula B. P. 69
226
three to six runs had been made on each fuel, the 5 per cent xylidine in kerosene was replaced with 4 per cent xylidine in kerosene, and a second series of runs was made in the same manner. The amounts of gas evolved during the 1-min. runs were then averaged, and the values thus obtained for the xylidine-kerosene fuels were plotted on a coordinate chart, having as its vertical axis the amount of gas evolved per minute and as its horizontal axis the percentage of xylidine in kerosene. These two points were joined by a straight line. From the point a t which this line crossed the horizontal line corresponding to the volume of gas evolved by the 45 per cent benzene-55 per cent kerosene fuel a vertical projection was made to the horizontal scale a t the bottom of the chart giving per cent of xylidine in kerosene. The intersection of this projected line with the bottom scale then gave directly the percentage of xylidine in kerosene which was equivalent in its effect for the suppression of detonation to 45 per cent of benzene in kerosene.
RESULTS The data obtained in the tests are tabulated in Tables 11, 111, IV, and V. Fig. 2 gives a graphical presentation of these results. The consistency of the results obtained with different concentrations of the same fuel and the close checks that were made in different determinations of the detonating characteristics of a given blend indicate that the values obtained have a high degree of accuracy. The general shape of the curves in Fig. 2 is much like what might be expected. when the aromatic concentration of an aromaticparaffin blend is comparatively high, say 40 per cent or more, the percentage of the paraffin fuel in the blend falls off rapidly as the percentage of the aromatic constituent is increased; and so an increase in the percentage of aromatic content that is relatively small on the basis of the whole blend exerts a large influence on the detonating tendency of thk fuel. Although the amount of xylidine used as antiknock material in the kerosene in some of the tests was over 15 per cent, it was never high enough to change materiaIly the heating value of the fuel. Practically the only effect of the addition of xylidine to the paraffin fuels was the elimination of detonation that resulted from its use. While this effect is probably not exactly a straight-line function, a t least up to the higher percentages of xylidine, the amounts of the antiknock material used were never large enough to cause a great variation on account of the diluting action of the xylidine on the fuel.
125.6 214.5
Approximate HYDROCARBON ABOVE AVERAGE Average MolecuBOILING POINT lar Weight of Fuel Formula B. P. 98.5 149.5 234
98.5 114.3 178.5
I t will be observed from Fig. 2 that toluene, on the basis of volume, is more effective than benzene for eliminating detonation conditions, and that xylene is, in turn, still more effective than toluene for this purpose. As may be seen from the shape of the curves in Fig. 2, the addition of only a small percentage of aromatic hydrocarbons to a paraffin fuel has but a slight effect in suppressing detonation. The curves illustrate to good advantage the reason for the observed fact that the blending of less than 20 per cent of benzene with a commercial gasoline or a naphtha exerts only a small influence toward causing the engine to give smoother operation while running on it. When benzene is blended with paraffin fuels in percentages above 20, however, its effect increases rapidly as its concentration is raised relative to the paraffin fuel. Attention has previously been called to the fact that this is due in part a t least to the greater percentage reduction in the amount of the paraffin constituent present as the aromatic content, is increased. The vertical scale a t the right of Fig. 2 shows approximately the increments in compression presmre of the engine that are made possible by the addition to a paraffin fuel of the corresponding percentages of xylidine given on the vertical scale to the left. From the two scales on the chart i t will be observed that the addition of 1 per cent of xylidine to a fuel that just gives incipient detonat,ion in a given engine makes it possible to raise the compression of the engine about 10 lbs., and this without any greater detonation being obtained than with the untreated fuel at the original and lower compression. The increment in compression that each per cent of xylidine makes possible can only be approximated, but the value given is based upon a number of observations made under practical operating conditions, on motors ranging from the single cylinder Delco-Light to the twelve cylinder Liberty, and over a compression range of from 50 to 160 lbs. By referring the curves of aromatic-paraffin fuels to the scale a t the right an approximation may be obtained t’o the relative composition necessary to give smooth operation a t a corresponding increase above the normal limiting or critical compression of the paraffin fuel alone. Table VI1 and Fig. 3 give the relations between percentages of xylidine by volume afid benzene b y molecules required to impart like detonation characteristics to two representative paraffin fuels. The composition of the benzene-paraffin blends on the volume basis corresponding to the percentages of xylidine in the paraffin fuel as given in Table VI1 was taken from the curves in Fig. 2. The values for the average molecular weights of the paraffin fuels used in converting
T H E JOURNAL OF INDUSTRIAL S N D ENGINEERING CHEMISTRY
July, 1922
percentages by volume to the molecular basis were obtained by the method of Wilson and Barnardn5 I n computing the percentage composition of a blend on the molecular basis from its composition by volume, the specific gravity and the average molecular weight of each of the ingredients were
593
amount than is given in Fig. 2 . Fig. 3 shows thas on the basis of molecular concentration a somewhat lower percentage of ben7,ene is required to produce a given effect on the critical compression of a benzene-paraffin blend, as the average molecular weight of the paraffin fuel becomes smaller. But, since a relatively large amount of benzene is required to produce a given effect on the detonation factor, the actual percentage of reduction in the amount necessary with decreasing molecular weight of the paraffin fuel is not large. A simple basis for determining the amount of benzene i t is neceasary to add a paraffin fuel in order to obtain a given effect is as follows: Up to a concentration of 70 per cent by molecules, the effectiveness of benzene for suppressing detonation varies directly as the square of the molecular concentration (Fig. 3).
New Qualitative Test for Uranium’ By Harold D. Buell SYRACUSE
I
k5 14
3 4
& $ 2
P I
&
UNIVERSITY,
0
0 IO 20 100 90 8 0
30 40 50 60 70 80 SO 7’0 60 50 40 30 20 10
j
0
P E R C EBY ~ MOLECULES FIG. 3-RELATIONS
BETWEEN PERCENTAGES O F XYLIDJNE B Y VOLUMF BENZENE BY MOLECULES REQUIRSDTO I M P A R T TO PARAFFIN F u E L s LIKE COMBUSTION CHARACTERISTICS FROM STANDPOINT O F DETONATION (Plotted from data in Table VII)
AND
employed. In view of the somewhat wide distillation range of the benzene used in the tests (Table I) a molecular weight of 79 instead of 78 was taken for benzene. The values obtained in these computations are given in Table VII.6 The curves of Fig. 3 give a more definite basis for estimating the percentage of benzene that must be added to a paraffin fuel in order to change its detonation tendency a certain 6 R . E. Wilson and D. C. Barnard, 4th, “Condensation Temperatures of Gasoline- and Kerosene-Air Mixtures,” THISJOURNAL, 13 (1921), 906. For this purpose the distillation data of the fuel (Table I) were arranged in the usual type of curve in which temperature is plotted on the vertical axis against per cent distilled on the horizontal axis. From this curve the percentages of the fuel distilling in each interval of 10’ were obtained, and these values were plotted on a chart on which the scale of the vertical axis was in terms of per cent distilled and that of the horizontal axis was in terms of temperature. The average boiling point of the fuel was taken as the point a t which a perpendicular passed through the center of gravity of the area enclosed under this differential distillation curve cut the horizontal or temperature axis. The values obtained in this way are given in Column 3 of Table VI. T h e approximate molecular weight of each of the paraffin Iuels was computed so as to bear the same proportionate relation t o the molecular weights of the hydrocarbons next above and below i t in t h e paraffin series as the average boiling point of the fuel bore to the boiling points of the normal paraffin hydrocarbons occupying like positions with respect t o it. The data used in the calculations and the value obtained for the average molecular weight of each fuel are tabulated in Table VI. 6 It is recognized that these values are only close approximations; but, in view of the wide variations between different samples of commercial gasolines, the degree of their accuracy is as great as can have any significance when dealing with such materials. Because it is such a small factor, no account was taken in making these calculations of the slight increase in volume that occurs when aromatic and paraffin hydrocarbons are blended.
SYR ICUSE, N E W
Y ORK
N testing slags and ores containing or supposed to contain uranium, it was found that when uranium was present zinc, added to a nitric acid solution of the material, gave a yellow deposit on the zinc. This test has never been recorded in the literature. It is very simple in manipulation and requires no special caution in regard to acid strength and temperature. A nitric acid solution of the sample is prepared. A large excess of acid is to be avoided because the reaction may become so violent as to boil out of the test tube and an unnecessary amount of zinc will be used up. An excess of granulated zinc is added to the solution and the reaction is allowed to proceed until the acid is spent, when a yellow deposit appears on the zinc. If the reaction is too violent the acid may be diluted; if too slow, more acid may be added. The yellow color develops more rapidly as the concentration of uranium is increased, but always appears when the reaction completely stops. The same yellow deposit was obtained from an aqueous solution of pure uranyl nitrate crystals, with no free acid present. The color did not develop, however, for two days, and the aqueous solution was acid to litmus as a result of hydrolysis. I n a solution of pure uranyl nitrate crystals with enough free nitric acid to start reaction with the zinc, it was possible to detect 0.88 mg. of uranium per cc. of solution. By concentration of the solution a more vivid color was obtained. Gold, platinum, thorium, lead, tungsten, titanium, chromium, mercury, and copper do not interfere with the test. Iron and vanadium interfere only when present in large quantities. I n the latter case, the spent liquid is removed as soon as action has ceased, and the zinc and the deposit are treated with enough nitric acid to start reaction. The deposit dissolves, but reappears when the acid is again exhausted, and vanadium and iron remain in solution. The test is not applicable in the presence of sulfuric or hydrochloric acids, when a black deposit is obtained. The yellow deposit appears only in an oxidizing solution of nitric acid, for uranyl salts are i-eadily reduced to uranous salts by nascent hydrogen. This should, however, serve as a preliminary test to indicate the presence of uranium a t the beginning of an analysis, rather than as part of a systematic scheme of qualitative separation. From the literature, it appears that the deposit is UOa.2Hz0. This is the only oxide which corresponds in color to the deposit obtained. 1
Received February 10, 1922.
