12.5 mole % of n-hexadecane. The constancy of the temperature of this phase change throughout the concentration range studied suggested a eutectoid between the crystal I phase, the waxy phase, and the liquid-sol phase. The phase boundary marking the transition from waxy phase to the liquid-crystal mesomorphic phase was lowered gradually by increasing percentages of nhexadecane up to about 60 mole 70. At lower soap concentrations the boundary was constant at 190" C. The temperature of final solution of lithium stearate in n-hexadecane was lowered in a gradual manner by increasing concentration of the hydrocarbon throughout the range studied. The phase diagram for lithium stearate-n-hesadecane reported by Vold and Vold (9) showed a phase island called nonaqueous middle soap, by analogy to a similar phase reported for the system sodium stearate-n-hexadecane (I) and to aqueous, sodium-soap systems (4). The middle soap for lithium stearate was described as liquidcrystalline and was said to be completely bounded by the waxy and isotropic solution phases. The esperimental observations of the present study clearly indicated that the liquidcrystal phase had the same character as the Vold and Vold middle soap, but that the phase was continuous with the liquid-crystal phase of the dry soap and persisted to as low a soap concentration as measured. There was no evidence for any phase island. The differential thermal analysis thermogram of the 3.9 mole yo soap sample still clearly showed four peaks marking all the major phase boundaries. Visual observations of samples of low soap content in the hot stage was soinen hat ambiguous because birefringence in the liquid-crystal phase
becomes vanishingly dim a t these low soap contents. Holvever, there was a sharp transition between the jelly structure of the liquid-crystal phase and the fluid mobility of the solution. The visually observed jelly-sol transition in the low soap samples occurred at the same temperature as the top peak in thermograms. The present disagreement concerning the phase behavior of lithium stearate in n-hexadecane may be compared to a similar case involving the system sodium stearate-n-hexadecane. Strosb and Abrams (6) reportpd that this system contains no phase islands, all phases being continuous with ones occurring in the pure soap. Their report, which was based mainly on differential thermal analysis data, stands in contrast to those of Doscher and Vold ( 1 ) and of Smith and McBain (b), who reported phase islands in sodium soap-hydrocarbon systems. These latter investigators based their phase diagrams on visual observation alone. Stross and Abrams stated that a possible reason for the discrepancy between their findings and those of Doscher and Vold might lie in the failure of the latter to exclude all traces of water from the system. The major differences between results of the present lithium stearate system and those of Vold and Vold stem from the differing number of phases found in the pure soap and the different shape found for the solution boundary. These differences, in turn, seem to be explained by the greater accuracy and sensitivity inherent in the differential thermal analysis apparatus described above. SUMMARY
An apparatus for differential thermal
analysib of lubricating greases operates automatically in controlling heating rates and recording all pertinent temperatures and differential temperatures. Five different determinations may be run simultaneously, allowing rapid collection of data. The cell and thermopile design permits high sensitivity and accuracy in locating phase changes. Differential thermal analysis of lithium stearate revealed a previously unreported mesomorphic phase occurring b e h e e n 225" and 229" C. A differential thermal analysis study of the system lithium stearate-n-hesadecane yielded a phase diagram n-hich appcars to be more accurate than one previowly reported. LITERATURE CITED
PI.,Vold, It. D., J . Colloid Sci. 1, 299 (1946). (2) Doscher, T. &I..Vold. R. D., J . Phiis. & Colioid Chem. 52, 97 (1948). 13) Evans. D.. Hutton. J. F.. Matthrns. J. B'., J : A p p l . Chem. ?London) 2 ,
(1) Doscher, T.
\
,
252 (1952).
(4) McBain, J. W., Vold, R. D., Vold, hl. J., J . Am. Chem. SOC.60, 1866
(1938). (5) Smith, G. H., RlcBain, J. K., J .
Phys. & Colloid Chem. 51, 1180
( 1947). (6) Stross, F. H., Abrams, S. T., J . .I m. Chem. SOC.73,2825 (1951). (7) Vold, ?vl. J., ANAL. CHEM.21, 683 (1949). (8) Vold, 11. J., Doscher, T. M., IUD. ESG. CHEX, ANAL.ED. 18, I54 (1946). (9) Vold, 11. J., Vold, R. D., J . Colloid Sci. 5 , 1 (1950). (10) Vold, R. D., Vold, &I.J., J . Phys. Chem. 49, 32 (1945). RECEIVEDfor revim July 20, 1956 Accepted January 12, 1057. Division of Analytical Chemistry, lleeting-in-Miniature, ACS, New York, N. Y., March 1956. Divisions of ilnalytical Chemistry and Physical and Inorganic Chemistry, 129th LIerting, ACS, Dallas, Tex., April 1956.
Thoron-meso-Tartaric Acid System for Determination of Thorium MARY H. FLETCHER, F. S. GRIMALDI, and LILLIE B. JENKINS
U. S. Geological Survey,
Washington 25,
b In the spectrophotometric determination of thorium with thoron, mesotartaric acid is used as a masking reagent for zirconium. The effects of different experimental variables such as the concentrations of the reagents, time, and temperature, and the behavior of 35 ions which might b e present in thorium ores are dis-
D. C.
cussed. A dilution procedure is given for the direct determination of thorium in zircon (ZrSiOd) that is also generally applicable to other materials.
T
HE determination of thorium in mineral separates such as zircon is important in Geological Survey pro-
grams on geochronology. However, the determination is usually lengthy, because zirconium is a serious interference in most methods for the determination of thorium. An earlier paper (I) stated that meso-tartaric acid should be superior to d-tartaric acid as a masking reagent for zirconium in the determination of thorium with thoron. VOL. 29, NO. 6 , JUNE 1957
963
This paper confirms that statement and shows that no other major change results from the substitution of mesotartaric acid for d-tartaric acid and t h a t meso-tartaric acid is so effective in masking zirconium that a direct spectrophotometric method can be used to determine thorium in zircon. However, it is advantageous to make a potassium hydroxide separation which reduces the amount of several undesirable ions such as tin and fluorine. The dilution procedure t h a t follows may be used wherever applicable, but was designed primarily for zircon samples that contain 0.1% or more of thorium dioxide. I n the range below 0.1% of thorium dioxide, the results are reliable to only one significant figure. APPARATUS A N D REAGENTS
Spectrophotometer. A Beckman DU spectrophotometer with a housing for 5-em. cells and with the lamp housing cooled by circulating water thermostatically controlled a t room temperature was used. Sand bath. The temperature of the sand (just below the surface) m s 170" to 190" c. All chemicals used were reagent grade. Potassium hydroxide (precipitating solution), 50% by weight. Potassium hydroxide (wash solution). Dilute 2 ml. of the 50% solution to 500 ml. with mater. Hydroxylamine hydrochloride, 10% WJV. aqueous solution. Cupric chloride (catalyst solution), 1 ml. equivalent to 1 mg. of CuO. Dissolve 0.538 gram of cupric chloride dihydrate in 250 ml. of water. Thoron [disodium salt of 2-(2-hydroxy - 3,6 - disulfo - 1 - naphthy1azo)benzenearsonic acid], 0.1% w./v. aqueous solution. meso-Tartaric acid, 10% w./v. aqueous solution. Filter if necessary. The reagent is available from several suppliers such as The Jasonols Chemical Corp., Delta Chemical Co., and Bios Chemical Co. PROCEDURE
Afix 50 mg. of a representative finely ground sample with 4 to 5 grams of fresh sodium peroxide in a small crucible and cover the crucible. Sinter and leach (2). Then place the covered beaker on a steam bath and digest the solution to remove hydrogen peroxide. After 30 minutes on the steam bath, acidify the solution with (1 plus 1) hydrochloric acid and add about 2-ml. excess. Remove the crucible and lid, police, and rinse them with water. If the sample is not completely decomposed, add a little paper pulp to the solution, filter off the residue through a slow 7-cm. filter paper, and wash well with water. Reserve the filtrate. Ignite the residue, add sodium peroxide, and repeat the decomposition procedure. 964
ANALYTICAL CHEMISTRY
"
0
2 00 MG.
Figure 1.
400 OF
600
MESO-TARTARIC
000
1000
ACID
Effect of mesa-tartaric acid
++
A. 60.36 y of Thoz 3 mg. of thoron B. 20.12 of Tho2 3 mg. of thoron C. 3 ing. of thoron D. 1 mg. of ZrOz 3 mg. of thoron E. 5 mg. of ZrOz 3 mg. of thoron
++
Add the solution from the second sinter to the original filtrate. Neutralize the combined solutions with 50% potassium hydroxide and add 5-ml. excess for each 50 ml. of solution. Digest the solution on a steam bath for 15 to 20 minutes, then filter it through a fast 7-cm. filter paper, and wash with dilute potassium hydroxide wash solution. Drain the precipitate and funnel stem by placing the palm of the hand over the funnel and pressing down. Dissolve the precipitate from the paper with hot (1 plus 1) hydrochloric acid (total volume about 20 ml.) and heat the solution on a steam bath for several minutes to ensure complete solution. Cool the solution, transfer it to a 25-ml. volumetric flask, and make to volume with (1 plus 1) hydrochloric acid. Transfer a 3- to 5-ml. aliquot of this sample solution to a 100-ml. beaker and add 1 ml. of perchloric acid. Evaporate the solution on a steam bath until all of the hydrochloric acid has been removed and fumes of perchloric acid appear. Transfer the beaker to a sand bath and evaporate until all of the perchloric acid has been removed. Careful heating a t sandbath temperatures is necessary here because heating on a hot plate or fuming over a burner often converts zirconium to an insoluble form that occludes thorium. Cool the beaker and add 1 ml. of (1 plus 1) hydrochloric acid. Rotate and tip the beaker so that the acid comes in contact with all parts of the residue. ,4110~ to stand 1 t o 2 minutes, then add 4 ml. of water. Cover the beaker and place on a steam bath. After 5 minutes, add 1 ml. of hydroxylamine hydrochloride solution and 0.2 ml. of cupric chloride catalyst solution. Recover the beaker and heat on a steam bath for 5 more minutes. Cool to room temperature. If the solution is clear and there is no residue, transfer it directly to a 25-ml. volumetric flask. Otherwise, add a little paper pulp and
filter the solution into the flask through a very small slow filter paper and wash. Rinse the neck of the flask with a little water. Add 4 ml. of meso-tartaric acid and mix. Add 3 ml. of thoron solution, rinse the ground-glass part of the neck of the flask, and add water almost to volume. Allow 1 to 2 minutes for drainage in the neck of the flask, then make the solution t o volume. Afeasure the absorbance of the solution a t 545 mp against a reference blank that contains 1 ml. of (1 plus 1) hydrochloric acid, 1 ml. of hydroxylamine hydrochloride, 4 ml. of meso-tartaric acid, and 3 ml. of thoron per 25 ml. of solution. Make the measurement within 5 to 10 minutes after adding the mesotartaric acid if a 10-mg. sample aliquot was used. Use 5-cm. matched or Calibrated cells for the solutions. If the measurements are made with a Beckman spectrophotometer, use a slit width of 0.15 mm. or smaller. Determine the amount of thorium present in the aliquot by reference to a standard curve or by calculation. Data for a standard curve are obtained from measurements of solutions containing known amounts of thorium chloride. EXPERIMENTAL
Establishment of Operating Conditions. From the data obtained earlier ( I ) , it was decided to develop a thoron-meso-tartaric acid system based on 3 mg. of thoron and 1 ml. of (1 plus 1) hydrochloric acid per 25 ml. of solution. With this concentration of thoron there is a sufficient excess to permit the determination of as much as 150 y of thorium dioxide and tolerances to rare earth and negative ions should be somewhere between those of the two systems using d-tartaric acid (I) The effect ,of the concentration of meso-tartaric acid is shown in Figure 1 ,
where the difference in the absorbances due to meso-tartaric acid of solutions containing thoron alone (3 mg.), thoron with 20.12 and 60.36 y of thorium dioxide, and thoron with 1 and 5 mg. of zirconium dioxide are plotted as functions of the concentration of mesotartaric acid. The reference solution for these measurements contained 3
mg. of thoron but no meso-tartaric acid. These data indicate that 300 to 400 mg. of meso-tartaric acid will mask the effect of 5 mg. of zirconium dioxide. I n the experiments with 5 mg. of zirconium dioxide, a precipitate of zirconium meso-tartrate formed with time in sharp contrast to the high solubility of zirconium d-tartrate. The effect of
I ~
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-
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E
.OB-
-
-
r' 0
-
I
-06-
n
-
W 0
-
z
.04-
-
a 0
-
Q
-
10
5
15 TIME IN MINUTES
ea
20
Figure 2. Effect of concentration of meso-tartaric acid on the reaction of 5 mg. of zirconium dioxide
A . 200 mg. of meso-tartaric acid B. 400 mg. of meso-tartaric acid C. 500 mg. of meso-tartaric acid
c
A
B
C
E
D
i
F
06-1
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i
i
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ul 0
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the concentration of meso-tartaric acid was, therefore, examined further. Absorbance measurements were made periodically on three solutions all of which contained 5 mg. of zirconium dioxide and 3 mg. of thoron and all of which were identical except for their meso-tartaric acid contents, which were 200,400, and 500 mg. The absorbances of these solutions were measured against corresponding reference solutions containing the same amount of mesotartaric acid and other reagents but no zirconium. The data are presented in Figure 2, where the absorbance differences due to zirconium are plotted as a function of the period of time between the addition of meso-tartaric acid and measurement of absorbance. Practically no changes in the absorbances of the solutions occur during the first of 5 to 11 minutes following the addition of meso-tartaric acid. I n this region of the curve, 400 mg. of meso-tartaric acid are effective in preventing the formation of enough zirconium-thoron complex to interfere. The break a t the end of the regions of almost constant absorbance in each of the curves in Figure 2 corresponds to the time required for precipitation of zirconium meso-tartrate to begin. As the break in each of the various curves occurs after the same period of time, the precipitation time is essentially independent of the concentration of meso-tartaric acid a t these concentration levels. The precipitation was also followed by observation of the Tyndall effect which is evident a t the start of the precipitation. The same period of time was required for the appearance of a Tyndall beam in solutions containing 5 mg. of zirconium dioxide and 400 mg. of meso-tartaric acid with and without thoron. The precipitation time for zirconium meso-tartrate is thus practically independent of the thoron concentration. On the basis of these tests 400 mg. of meso-tartaric acid was selected for the final procedure. Standard Curve. The standard curve relating absorbance and concentration of thorium is linear from 3 t o 150 y of thorium dioxide per 25 ml. of solution. I n this range of concentration a n absorbance difference of 0.010 is equivalent to 1 y of thorium dioxide. Below this range the standard curve has a slight heel if the phototube normally supplied with the Beckman instrument is used The substitution of a photomultiplier tube allows the use of very narrow slit widths (0.015 to 0.045 mm.) and results in a standard curve that is linear over its entire length. Sources of Error. VARIABLEVOLUMES. For greatest accuracy, the sources of possible error should be recognized and precautions taken to minimize the errors. The reference
5
20
10 TIME
IN
50
100
200
MINUTES
Figure 3. Effect of time on interference from zirconium dioxide A . 8 mg. of ZrOz E. 3 mg. of B. 6 mg. of ZrOI F. 2 mg. of C. 5 mg. of ZrOz G . 1 mg. of D. 4 mg. of ZrO,
different amounts of
ZrOz ZrOI Zr02
VOL. 29, NO. 6 , JUNE 1957
965
Figure 4. Time-zirconium areas for noninterference of zirconium
A . Time for start of precipitation B. Time for an increase of 0.010 in absorbance
blank has a n absorbance of 1.33 with respect t o water. If a n absorbance difference of 0.002 between sample and reference solution is t o be meaningful, great care must be exercised in preparing each; the volume of thoron added to the reference and sample solutions must be reproduced to 1.5 parts per 1000; the volume of meso-tartaric acid added, to 2.5 parts per 1000; and the final volume of the solution, to 1.5 parts per 1000. Because the precision in the reproduction of the measurements of these volumes is the significant factor, the same pipet should be used to add a given reagent to both sample and reference solutions. TEhlPERATURE. If the reference Sohtion is hotter than the sample solution, the sample solution will read too high; if the reference solution is cooler, the sample solution will read too lorn-. The relationship between the difference in temperature of the tn-o solutions and the difference in their absorbances is linear. A difference of zkQ.8" C. between the two solutions results in a n absorbance difference of zk 0.002. Behavior of Various Ions Under Operating Conditions. The data on t h e behavior of various amounts of zirconium are summarized in Figure 3, where the absorbances are plotted against the time between the addition of nzeso-tartaric acid and measurement of the absorbance. The small variations between the absorbances in the horizontal parts of the different curves are probably not significant, as insufficient time mas allowed for proper drainage in the necks of the flasks. I n Figure 4, A , the time after the addition of meso-tartaric acid a t which the break occurs in each of the curves in Figure 3 is plotted as a function of the amount of zirconium present. It it safe to measure the absorbance of a solution containing a given amount of zirconium a t a n y time along that part of its ordinate that lies beloir- curve A in Figure 4. Figure 4, B , represents the total time required for a n absorbance increase of 0.010 greater than any point in A . The relationship between time and the beginning of precipitation of zirconium meso-tartrate shown by the curves in Figures 3 and 4 holds only if the zirconium used is present as a simple ionic species. The standard zirconium solutions used in these studies n-ere prepared 966
ANALYTICAL CHEMISTRY
I
Table I.
Milligrams of Various Elements (as Oxides) Equivalent to an Absorbance Difference of 0.0 10
Element Tested
(Slit width = 0.15 mm.) M g . Equivalent to ilbsorbance Difference of 0.010 0 y ThOp 50.3 y ThOn 150 9 y ThOt > 2.0 0.11 0,120 2.6 2.6 6.56 0.70 >10.0 0.45 0.49 1.12 0.37 1.3 0.43 0.83 0.76 1.0 0.68 0.56 0.60 3.6 1.25 1.25 4.5 1.85 i.4 1.45 2.2 ... ... 3.8 8.5 > 10 1.2 0,019 0,009 8 8 8 0.075 0.22 0.20 0.035 > 1.0 0.045 1.2 2.0 > 10 0.55 0.42 0.36 0.47 0,056 0.10 > 10 > 10 >10 > 5.0 > 5.0 > 5.0 > 30 > 30 >30 5.i 21 25.2 0,055 0.062 0.035 11.0 20.0 i.0 11.5 12.0 12.5 0.42 > 10 0.12 1.25 0.65 > 10 0,001 4.I 0.003 > 30.0 > 30.0 >30.0 5.9 8.5 6.8 42 88 40 >500 29 68 >100 30 23 13 39 10 ... 10 > 40 4.0 > 5.0 > 5
a h
using cupric chloride catalyst .
by boiling zirconj-1 chloride in (1 plus 1) hydrochloric acid and bv maintaining this high acidity whenever weaker zirconium solutions were made by dilution. I n more dilute acid solutions of zirconium, unless freshly diluted from strong acid solutions, zirconium is often present in polymerized form. As little as 1.5 mg. of zirconium in this form give a precipitate almost immediately with meso-tartaric acid. Provision is made in the detailed procedure to guard against polymerization of zirconium. Using this procedure, 5 t o 8 mg. of Zirconium dioxide may be present in the spectrophotometric determination of thorium if absorbance measurements are made within the time shown to be safe by A , Figure 4. The behavior of other ions when 0, 50.3, or 150.9 y of thorium dioxide also are present is summarized in Table I. This table shows the amount of the element, calculated to the equivalent amount of oxide, that causes a change in the absorbance of 0,010 a t three concentration levels of thorium. Fluorine, titanium, tantalum, tin, and niobium are serious interferences. Iron is not included in this group of seriously interfering elements because,
Table II.
Test of New Procedure for Determination of Thorium in Zircon
Sample Zircon 2 Zircon 1 Zircon 3 0
Thoriiini Dioxide Found, --____ % Other methods S e w method 0.15 0.15 0,065
0 05gn
0.07 0.07 0 06 O.OT 0 06
Sample in Final Aliquot, Mg. 1 8 8 8 8 10
Method described by Grimaldi and others ( 2 ) .
as is done in the final procedure, it is readily reduced by hydroxylamine hydrochloride in the presence Of cupric chloride which serves as a catalyst for the reduction. When reduced to the ferrous state, as much as 10 mg. of equivalent ferric oxide may be present. Test of Procedure. The procedure was used to determine thorium in several zircon samples t h a t had been analyzed carefully by other methods. The Percentages of thorium dioxide determined bv independent means
ACKNOWLEDGMENT
The authors are indebted to A. Sherwood, E. 8. Geological Survey, for some of the thorium determinations. LITERATURE CITED
( 1 ) Grimaldi, F. S., Fletcher, M. H., ANAL.CHEX 28, 812 (1956).
( 2 ) Grimaldi, F. S., Jenkins, Lillie, Fletcher, hl. H., Ibid., 29, 848
(1957). RECEIVEDfor reviem- August 15, 1956. AcceDted December 15, 1956. Publica-
Determination of Carbon in Bromine MAURICE CODELL and GEORGE NORWITZ Pitman-Dunn laboratories, Frankford Arsenal, Philadelphia, Pa.
,Carbon in bromine can b e determined by passing the sample with oxygen through a tube a t 1000" C. to convert the carbon compounds to carbon dioxide. The bromine is frozen out in dry ice traps, and the carbon dioxide is purified by passing it through copper sulfate, manganese dioxide, and Anhydrone.
A
was needed for the determination of carbon in bromine, because bromine frequently contains halogenated hydrocarbons such as chloroform, bromodichloromethane, and bromoform (6, 7 ) . The effect of carbon compounds in C.P. bromine in rausing a very high blank in the bromination-carbon reduction method for determining oxygen in metals has recently been discussed ( 9 ) . 90accurate method is known for the determination of carbon or carbon compounds in bromine. However, a method has been used since a t least 1896 (1, N ACCURATE METHOD
6-8, 10) for the detection of organic compounds in bromine. The method consists of adding 1 ml. of bromine to 25 ml. of 10% sodium hydroxide solution and allowing it to stand overnight. The presence of organic material is indicated by oily drops or a film of oil. This method is of questionable validity. Khen it was applied to bromine that contained lyOby volume of chloroform, a clear solution was obtained. This is not surprising because chloroform is easily oxidized to carbonyl chloride or carbon dioxide (4). An alkaline solution of bromine is a strong oxidizing agent and would be expected to oxidize many organic compounds to watersoluble substances. I n the method proposed here, the sample is treated with oxygen in a tube a t 1000" C. to oxidize the carbon compounds to carbon dioxide. The bromine is frozen out in dry ice traps, and the gases are passed through copper sulfate (on asbestos), manganese dioxide, and Anhydrone, and then into a tared bulb
containing Ascarite. The copper sulfate absorbs traces of broniineandhydrogen bromide. Previously, copper sulfate was recommended for theabsorption of chlorine and hFdrogen chloride in the determination of carbon ( 3 ) . The effectiveness of copper sulfate in absorbing bromine is readily demonstrated by passing bromine into a tube packed with the copper sulfate reagent. No bromine will issue from the exit end of the tube. ;\lost of the hydrogen bromide in the bromine would he oxidized to water and bromine in the combustion tube. The purpose of the manganese dioxide is to absorb sulfur dioxide. Manganese dioxide has been used for removing sulfur dioxide in the determination of carbon in steels (9, I t ) . Only high purity manganese dioxide of large surface area should be used. [The following were satisfactory: manganese dioxide, Special, Fisher Scientific Co., or manganese dioxide prepared as described by Pigott (9) or U. s. Steel Corp. ( 1 1 ) ] . Any sulfur trioxide (freeaVOL. 29, NO. 6, JUNE 1957
a
967
