Modified High Frequency Apparatus for the Determination of Moisture in Solids Determination of Moisture in Sodium Chloride and Ammonium Nitrate F. W. JENSEN, M. J. KELLY,
and
M. 6. BURTON, JR.
Chemistry Department, Agricultural and Mechanical College of Texas, College Station, r e x .
A high frequency method for the determination of moisture in sodium chloride and ammonium nitrate is based on the change in properties of a mixed solvent sgstem due to the equilibrium extraction of the moisture and salt from the solid. d n apparatus and procedure are described that are believed to be applicable as a method for the determination of moisture in other compounds. The method was found to have a precision of 0.0270 moisture in the solid sample for the salts examined. The method is rapid.
M
ANY high frequency analytical methods have been ap-
pearing since the development of the high frequency titrator by Jensen and Parrack ( 5 ) and independently by Blake ( 2 ) . The interest in such devices stems primarily from their relative accuracy, speed, simplicity, and reproducibility. Because of the wide versatility of these instruments, it was felt that high frequency techniques could be applied to the determination of moisture. Methods for the quantitative determination of moisture are numerous ( 2 , 3, 6, 8-f3). Many of the methods are specific for certain materials, while others are of a more general nature. Most of the methods which are not limited by the variety of materials upon which the determination is valid appear to be complicated with delicate techniques or cumbersome equipment. RIany others are simply time-consuming. Few methods have been devised which can be classed as readily adaptable general methods of analysis. Because of the limited number of general methods, an investigation was initiated into the possibilities of adapting high frequency techniques to develop a general method for determining moisture in solids. R7est et al. ( 1 3 ) have published a high frequency method for determining water in alcohols. The method presented here may possibly be extended to liquid systems. APPARATUS
The apparatus used in this project was essentially that developed by Kelly ( 7 ) , with certain necessary modifications. This particular apparatus Tvas selected because of its simplicity of operation and its escellent time stability. The apparatus had a conductivity cell Jvith no contact between the plates and the solution, t,hereby removing many errors inherent in the usual conductivity cell. The apparatus as shown in Figure 1 consisted of a modified Clapp oscillator operating a t 9.45 megacycles Kith a sampling column placed in series with a diode rectifier and a direct current, microammet,er. The apparatus was equipped 11-ith a device whereby the direct current potential across the meterdiode combination v a s balanced out with a n opposing voltage from a battery source, B1. The screen potential could be adjusted by potentiometer R2 to compensate for changes in temperature. Readings were made on the microammeter, which could be balanced to an arbitrary zero reading by adjustment of the coarse and fine resistors, R4 and R5. I t Tvas found desirable t,o control the sensitivity of the inetrumerit by varj-ing the value of the shunt resist,or, R3, across the diode rectifier. Sensitivity could further be controlled by adjusting the pori-er supply voltage, and a 200-volt input from the power supply w t s found to give sufficient sensitivity. The capacit,or, C'B, beloir- the rectifier filtered off most of the RF component of the signal, so that the nieter received only pulsating direct current.
The sample tube was of thin-walled borosilicate glass with aluminum sleeves fastened flush t o the outside wall of the tube. Since the response was sensitive to change in position of the sample tube, it was mounted on the case of the apparatus. I n order to facilitate cleaning, the tube was mounted by two rigid banana jacks which Rere plugged into the plastic panel. EXPERI l I E Y TAL
I n attacking this problem, two salts FTere used. Throughout the development of the method, sodium chloride was used because of its stable nature with regard to ease of moisture determination by oven drying and because of the ease with which it could be handled. As an added test for the method, ammonium nitrate v a s introduced because of its relatively thermally unstable nature with regard to moisture determination by oven drying and also because of the lack of a completely satisfactory rapid method for the determination of moisture in this particular salt, with the exception of the Karl Fischer reagent technique. Efforts to determine moisture upon the solid sample itself were discarded because it was not possible to obtain reproducibility in filling the sample tube. Instead, a more desirable method was to extract the samples with an appropriate solvent. While not all of the moisture would be extracted from the moist salt by the solvent, the salt-solvent system would come to an equilibrium condition. With salt samples of higher moisture content, the loading of the oscillator would be greater because of the higher content of moisture and salt in the solvent at equilibrium conditions. Because of higher solubilities of water and salt when polar solvents were used, the loading effect tended to be great, thereby causing low response to changes in moisture content, although total response was high. On the other hand, when a nonpolar solvent was used, only a small fraction of the moisture and salt dissolved and total response was low and insufficient. Therefore, in order to arrive a t a solvent that would utilize the normally linear response of the instrument, the characteristics of a number of pure and mixed solvents, both polar and nonpolar, were investigated in an effort to obtain a solvent system which would dissolve amounts of water and salt to give optimum loading. The ideal solvent appeared to be one that would dissolve water well and yet dissolve the salt only enough to provide a solution with an appreciable but not an excessive conductivity. Methanol dissolves water well, but i t dissolves so much of the COAXIAL CABLE
SAYPLE TUBE
RECULATED
Figure 1.
1716
Circuit Diagram
1717
V O L U M E 26, N O . 1 1 , N O V E M B E R 1 9 5 4
aluminum friction bearing, held in a rubber stopper of an appropriate size for the flask. The stirring rotor itself had three circular propellers which were of a diameter just small enough to clear the neck of the flask. These propellers were spaced in such a fashion that there was complete stirring throughout the mixture. The whole apparatus was clamped rigidly to an upright stand in such a manner that changing the flask was not inconvenient. I n each extraction, an effort was made t o control the speed of the stirrer to give approximately equal stirring to all samples.
200
150
bl tl a
8
K
.-
z
.-
200
-
I50
-
100
-
50
-
100
c
d
50
(I)
t
e
E
.H
3 Percent
Moisture in NH4N03
Figure 2. Determination of Optimum Concentrations of 1,4 -Dioxane and Methanol for Ammonium Nitrate Figures indicate percentages of 1,4-dioxane
salt that the oscillator was overloaded. Water is readily soluble in 1,4-dioxane, but salts are generally only slightly soluble and consequently response was low. Variohs mixtures of methanol and 1,4dioxane were investigated with the results shown in Figures 2 and 3. Examination of the curves in Figure 2 with ammonium nitrate as the salt reveals that a mixture containing 69% by volume of l,&dioxane gave linear results. From Figure 3 it can be seen that a mixture containing 3501, by volume of 1,4dioxane produced the beet results for sodium chloride. The fact that the optimum solvent composition is not the same for each salt is due to the follon-ing considerations. I n methanol the solubility of ammonium nitrate is higher than the solubility of sodium chloride, whereas in 1,4-dioxane the reverse is true. The solubility of each salt individually is lower in 1,4-dioxane than in methanol. Therefore, in order to produce a solvent mixture which mill give optimum response in each case, i t is necessary to add more methanol to the mixture when sodium chloride is used. As optimum response results from the correct loading of the oscillator, the parameters of the measuring system and the sample tube are also governing factors. I n order t o keep at a minimum the time required for the determination i t was decided that the solvent and salt mixture should be agitated vigorously for whatever length of time was required for the supernatant liquid to give a constant current flow through the column. This time was determined by preparing a group of identiral ramplee t o be stirred for varying time intervals and introducing them to the column. The extraction was found to reach equilibrium consistently after 4 minutes of stirring. The stirring time for this work was set at 5 minutes in order to give a margin of safety. The stirring was performed in 100-ml. wide-mouthed roundbottomed flasks with a stainless steel stirring rotor attached to a high speed laboratory stirring motor with rheostat control. Evaporation and contamination by moisture from the air were held to a minimum by passing the stirring rod through a machined
/ 01.0
I
I
2.0
3.0
Percent Moisture in NaCl
Figure 3. Determination of Optimum Concentrations of 1,4 Dioxane and Methanol for Sodium Chloride
-
Figures indicate percentages of 1,4-dioxane
The size of the sample tube nas deteiinined experimentally t)y trial and error until a tube was found that gave good response
for very low percentages of water content. The optimum value of R3 was determined by simulating the qalt-saturated solventfilled column with a 500-ppf. condenser in serirs with the appropriate size carbon resistor. This value was determined b? aelecting one which gave the same current reading on the meter as the filled column. The best value of R3 was found t o be between 5 and 10% of the total resistance of the column. Actually, the value of this resistor is governed by the combined relationship between the solvent system, the salt, and the parameters of the instrument and sample tube. Using a borosilicate glass sample column of 15-mm. inside diameter, 1-mm. wall thickness, and 63 mm. long between sleeves 40 mm. wide, the value of R3 was 100 ohms for the sodium chloride determinations and 37.5 ohms for the ammonium nitrate determinations. Temperature compensation was based on the premise that the predominant effect of a change of temperature would be due to the change in solubility of the salt and to the change in condurtance of the ions in the solution Thus, the change in loading effect caused by the solvent itself ovei a given temperature range would be minor in comparison with the change caused by the same solvent when it was saturated with the d r r salt. By adjusting the screen voltage to produce the same gain for the solvent qaturated with dry salt over the solvent itself, temperature
ANALYTICAL CHEMISTRY
1718 control could be achieved. Alternatively, in cases where difficulty is encountered in producing a dry salt in bulk quantity, a salt of known moisture content may be substituted for the dry salt for establishing the initial gain. I n illustration of this technique, the initial gain for the ammonium nitrate determination was established with a salt containing 0.36% moisture. The magnitude of the initial gain was governed largely by the parameters of the instrument, the solubility of the salt, the conductance of the ions, and the desire to keep the readings within the scale of the microammeter over the range of moisture in the samples. For the sodium chloride determinations the gain was chosen as 100 pa., and for the ammonium nitrate samples the gain was chosen as 60 pa. After the temperature compensator was set, and with the 0% moisture sample still in the tube, the microammeter was set back to zero or to any arbitrarily chosen value, by adjustment of R4 and R5. Thereupon the instrument was ready for use with no further adjustment.
2a
The ammonium nitrate used for this investigation was in very large irregular crystals. I n order to reduce these crystals to a small consistent size, the ammonium nitrate was recrystallized from absolute methanol. A large amount of this solid was dried in a vacuum desiccator until the desiccator would hold a constant pressure of 11 cm. of mercury a t 35' C. over a 24-hour period. The residual moisture in this material was found to be 0.36q;b by the method of Guichard (4). I n this method small samples are dried a t 114" C. for 2 or 3 days, being weighed a t intervals. The weight shows a rapid decrease for a short period of time as the sample loses intercrystalline a-ater. Thereafter, the decrease is much less rapid as decomposition occurs After the losses in lveight are plotted against the corresponding elapsed times, the loss in moisture can be obtained by extrapolation hack to zero time. The moist ammonium nitrate samples were prepared from the material containing 0.36% moisture by adding a predetermined amount of water from a microburet to amounts of the sample so t h a t the total weight was 40.00 grams. These samples were well shaken and stirred for 5 minutes with 60 ml. of the solvent containing 69y0 by volume of 1,4-dioxane. Since the suspended ammonium nitrate settled slowly, the samples were firmly stoppered and centrifuged. This procedure was not necessary in the case of the sodium chloride samples, as settling took place rapidly. Determinations were made on the supernatant liquids from the ammonium nitrate samples in the same manner as in the case of the sodium chloride samples. RESULTS
Figure 4 shows the results from prepared sodium chloride samples a t two temperatures. Run 1 was made a t 28.8" C. and run 2 was made a t 24.7' C. Reproducibility was very good and the maximum deviation from linearity was 1pa. for the two temperatures. The results from the ammonium nitrate determinations are shown in Figure 5. I n run 1 a t 30.8" C. no effort was made t o adjust the screen potential for temperature compensation. However, in runs 2, 3, and 4, a t 23.0", 29.5', and 28.0" C., the screen potential was adjusted to an initial gain of 60 pa. for the salt containing 0.36% moisture. I n runs 1, 2, and 3 the moisture 20c
+ - F6m I, 28.8T. temp. comp o-
~ r 2.n 24.7.C. temp. comp. I50
I
I I .o
I
I
2 .o
L
Porcent Moiatun in W
Figure 4.
Determination of AIoisture in Sodium Chloride
yl
s
e .u 100 z
.-c c
c
The sodium chloride was first ground in a ball mill to small, consistent sized crystals which were dried in a n oven a t 110" C. for a 48-hour period. This salt was considered to be a 0% moisture sample. Ten salt samples of varying percentage moisture were prepared by adding random amounts of water t o approximately 100 grams of the dry salt in such amounts t h a t the approximate percentage moisture was known. The samples were vigorously agitated periodically for 3 days to ensure consistent distribution. Two small samples were then removed from each large sample and weighed. These small samples were dried for 48 hours in the oven a t 110" C. to determine the actual per cent moisture. A number of 50.00-gram samples, including one 0% sample, were extracted using 50 ml. of the solvent containing 35% by volume of 1,4-dioxane and a stirring time of 5 minutes. The supernatant liquids from these samples were placed in the column individually and the changes in current flow were recorded. Between samples the column was thoroughly washed first wjth methanol, followed with ether, and then finally dried by passing warm dry air through the column.
2 V a
0 - Run I, 30.8OC. not temp. comp. +- Run 2, 23.O0C. temp.comp.
A - Run 3,
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X-
I
I
I
I
29.5.C.
temp. camp.
Run 4, 28.0°C. temp. comp.
I
I
I
Percent Moisture in NH6NOs
Figure 5.
Determination of Moisture in Ammonium Nitrate
I
V O L U M E 26, NO. 1 1 , N O V E M B E R 1 9 5 4 was determined from the known amount of water added with correction for the amount of water present in the original recrystallized sample. The samples for run 4 were made by adding random amounts of water to samples of the recrystallized salt weighing over 40.00 grams, after shaking frequently over a period of 48 hours. Small samples were then removed and dried in a desiccator until the pressure remained constant a t 11 cm. of mercury for 24 hours. Forty-gram samples were used from the remainders for extraction. This variation was added as a check on the vacuum drying method. I t is apparent that temperature compensation is not only possihle but very good. The maximum deviation was in the order of 1 pa., rrhich Tas within the mechanical reproducibility of the microammeter. This deviation represents an error of about 0.027, moisture in either of the two salts. The apparatus had excellent time stability characteristics and results could be readily reproduced over extended periods of time. Determinations could be made rapidly. With the initial gain already set, a single determination required less than 10 minutes, including the time for stirring the salt and the solvent. ACKNOWLEDGMENT
The authors wish to express appreciation to The Texas Co., n-hich supported this work.
1719 LITERATURE CITED
Blake, G. G., J. Sci. Instr., 22, 174 (1945). Boeke, J., Philips Tech. Re*., 9 ( l ) ,13 (1947). Dean, E. W., and Stark, D. D., J . Ind. Eng. Chem., 12, 486 (1920).
Guichard, Marcel, Compt. rend., 215,20 (1942). Jensen, F. W., and Parrack, -4.L., Texas A. & M. College Eng. Expt. Sta., Bull. 92, 1946; IND. ENG.CHEM.,ANAL.ED., 18, 595 (1946).
Jolson, L. AI., 2. anal. Chem., 108,321 (1937). Kelly, M. J., “A Linear, Temperature Compensated, High Frequency Salinity Neasuring Device,” thesis, A. &. AI. College of Texas, College Station, Tex., 1951. Kieselbach, R., ISD.ESG. CHEW,ANAL.ED., 18, 726 (1946). llitchell, John, Jr., and Smith, D. &I., “dquametry,” Kew York, Interscience Publishers, 1948. Muller, G. R. (to Allgemeine Elektricitats Gesellschaft), Ger. Patent 696,056 (Aug. 8, 1940). Sand, H. J. S., “Electrochemistry and Electrocheniical.-lnalysis,” 1-01. 111, p. 84, Brooklyn, Chemical Publishing Co., 1942. Suter, H. R., BNAL.CHEM.,19, 326 (1947). West, P. W., Senise, P., and Burkhalter, T. S.,ANAL. CHEM, 24, 1260 (1952). RECEIVED for review March 15, 1954 Accepted August 18, 1954 PreSOCIETY, Xew Orsented at the Regional Conclave, .k\rzRIcas CHEMICAL leans, L a , December 10,1953.
Identification of Compound Types in a Heavy Petroleum Gas Oil H. E. LUMPKIN and B. H. J O H N S O N Humble
Oil and
Refining Co., Baytown, Tex.
.Inalytical methods for petroleum products have been extended from gas through gasoline, kerosine, and heating oil in recent years. One of the last major challenges in composition studies of petroleum lies in the gas oil and lubricating oil ranges. A combination of separation techniques and ultraviolet and mass spectrometric data on the separated materials has been applied to the identification of hydrocarbon and sulfur compound types in the aromatic portion of a heavy gas oil. The majority of the sulfur compounds in the sample of fairly high sulfur content investigated is shown to be of the condensed aromatic-thiophene type. Figures are given showing the compound type identifications and the order of removal of the types from alumina gel.
I
S .iSY business venture involving a raw material and a fin-
ished product, knowledge of the composition of the feed stock and of the materials derived therefrom is highly desirable. -4nalytical methods for petroleum products have been extended from gas through gasoline, kerosine, and heating oil in recent years. A single mass spectrometer or infrared scan yields all the data necessary for the analysis of many gaseous samples (20, 21). Simple distillation with the application of some instrumental method to a few fractions is sufficient for component analyses in the lower gasoline boiling range ( I , 7 , I?‘). -4s the boiling range is increased through the heavy gasoline to the kerosine and heating oil ranges, component analyses give way to compound type analyses ( 3 , 4 , 15) and the procedures are still not overly complex or time-consuming. In the gas oil and lubricating oil ranges, the literature has been concerned mostly with structural group analysis employing refractive indices, density, mean molecular weight, bromine number, specific dispersion, and ultimate analyEes for carbon and hydrogen content (6, I S , 14). Van S e s and
van Westen (18) have reduced the time requirements of their n-d-M method of structural group analysis to 1to 2 hours and have shown many applications to refining operations. It is the authors’ belief, however, that a true compound type analysis can have much more meaning in evaluating crudes, adjusting refinery operations, and elucidating the nature of the mechanisms involved in many refining processes. This paper describes experimental work and spectral interpretations which have led to the identification of the major hydrocarbon and nonhydrocarbon compound type present in the aromatic portion of a heavy (600’ to 1000° F.) gas oil. The oil contained CIS to C35 compounds with maximum concentrations in the C27 to Cpg region. Chromatographic separations, ultraviolet and mahs spectral data, and sulfur analyses have been employed in this investigation. EQUIPMENT
A Consolidated Model 21-103 analytical mass spectrometer mas modified by the installation of a narrow exit slit analyzer tube and a high field magnet, in order that ions up to about m/e 600 could be resolved. Ap ropriate changes in the scanning circuit to decrease the rate ofscan to a value commensurate with the amplifier speed were made. A high temperature inlet system, differing principally from that of O’Seal et al. (19) in the design of the valve, was fabricated. A diagram of this heated inlet system is shown in Figure 1. The 2-liter borosilicate glass reservoir, valve, frit, and leak line are heated to 575” F. with 1/2-inch flexible heating tape. Each section of tape is controlled with a Variac and thermocouple. The glass apparatus is insulated with various layers of asbestos tape and heavy aluminum foil in order to conserve the input heat and to prevent the presence of undesirable cold spots. The gallium valve has a 1-inch-diameter port and is designed to conserve the relatively expensive gallium. About 7 ml. of gallium was used in the valve shown. The leak line was heated through the cover plate to within about 6 mm. of the ionization chamber. Ultraviolet spectra were obtained on a Cary h4odel 11 doublebeam recording spectrometer.
