alcohol-ether mixture was shaken for 30 minutes with a solution 1N in hydrochloric acid and 0 . 6 s in sodium iodide. Considerable iodine formed during the process. The solution layer ivas drained off, after which the solvent mixture was shaken with dilute sodium thiosulfate to reduce the iodine. This in turn was drained off. The amyl alcohol-ether was then washed several times with water and redistilled. This reagent s h o m d no improvement over untreated solvent. Samples for a standard curve were prepared by measuring 5, 10, and 20 y of tellurium from a solution containing 10 y per ml. directly into 10-ml. volumetric flasks. Hydrochloric acid (3 ml.),water ( 2 ml.), and stannous chloride (1 ml.) were then added exactly as in the urine analyses. After dilution to volume and mixing, per cent transmittance was read a t 440 mp: Values w r e > respectively, 92.2, 83.7. and 68.5. The gentle slope of the linear plot of these results is indicative of a rather Ion- sensitivity for the tellurium sol method. Duplicate standards prepared with less than 5 y of tellurium per 10 ml. gave considerable variation in transmittance readings. The portion of the curve above the 5-7 level was, on the other hand, more precisely reproducible. Rather than to attempt to use the lower reaches of the curve, it was decided that adding a t least 10 y of trllurium to each sample might
give better precision. Measurements could then be made in the reproducible range and the difference between the tellurium added and that recovered would be considered the quantity originally present. It would appear logical, considering these undesirable features of the tellurium sol method, to measure the iodotellurite complex spectrophotometrically. This was considered, but results in previous attempts to use this method on very small amounts of tellurium were disappointing, presumably because of intereference from the oxygen-iodide side reaction (1, 7 ) . I n the application specifically to urine analysis, interfering elements would be more likely to be encountered if the iodotellurite method were used. Most elements which form complex iodides under the experimental conditions are extracted to some extent by the n-amyl alcohol: bismuth, lead, mercury, gold, platinum, palladium, rhodium, and iridium. Bismuth is sometimes present in urine and lead more often. Bismuth would definitely interfere in the iodotellurite method. I n this investigation there was some evidence that lead would also interfere, and its presence is considered harmful ( 7 ) . Only the elrments which precipitate or girc colored solutions with stannous chloride interfere in the tellurium determination as presented. Thus the method is inadequate only when mer-
cury, gold, palladium, platinum, rhodium, or iridium is present. Because of their comparative rarity, however, it may be very frequently applied. ACKNOWLEDGMENT
The m i t e r wishes to express appreciation for suggestions offered by John R. Lewis, Department of lletallurgy, by Randall E. Hamm, Department of Chemistry, University of Utah, and by D . E. Rushing, chief chemist, U. S. Public Health Service, Occupational Health Field Service, Salt Lake City, 'C'tah. LITERATURE CITED
(1) Brown, E. G., Analyst
(1954).
>LA.,
79, 50-4
I""".
ger, K,, Durst, A. M.,2. anal. Chem. 135, 11-14 (1952) Johnson, R. A,, Andersen, B. R., ~ ~ K B L CHEU. . 27, 120 (1955). Johnson, R. A , , Kwan, F. P., Ibzd , 23, 651 (1951). Johnson, R. A,, Kwan, F P , \Testlake, D. IT.,Ibid., 25, 1017 (1953). Steinberg. H. H.. 1Iassari. S. C.. MineryA. C., Fink, R., J . Ind. Hyy: Tozicol. 24, 183 (1942). RECEIVED for review April 6, 1956. .kccepted April 10, 1957.
Sensitive Quartz Beam Microbalance A. W. CZANDERNA and J. M. HONIG Department of Chtmisfry, Purdue University, lafayette, Ind.
,A sensitive quartz beam microbalance is described which operates on the principle of a normal gas density balance. The instrument is capable of detecting mass changes to a precision of 5 X l o p 8 gram and to gram; with an accuracy of proper temperature control, it is stable over long intervals of time. The device is very rugged in relation to its sensitivity; it can be constructed, maintained, and operated without requiring unusual experimental skill.
&lop7
microbalances, capable of detecting mass changes of the order of lo-' gram or less, have been described in the literature (1-7, 9, 10, 12-15. 17, 18). These balances can be classified as belonging to one of N u m m OF
1205 *
ANALYTICAL CHEMISTRY
three categories--.piral types (helices), torsional suspensions, and adaptations of gas density balances (pivotal types). Instruments belonging to the first two categories can be made very sensitive, but their installation, operation, and proper maintenance often require a high degree of evperiniental skill and patience. I n particular, quartz helices are quite susceptible to breakage; their operation in high yacuum is complicated by oscillations which result from incomplete shock mounting. Torsion balances of conventional design likewise are sensitive to shocks and require frequent recalibrations. However, a new model has recently appeared, in which these undesirable characteristics are eliminated (16). The design for the balance described below represents an adaptation of the
pivotal model described previously (9, 17). I n both cases the authors were primarily interested in measuring gas densities; however, no clear-cut indication was provided concerning the optimum sensitivity, reproducibility, and long range stability which could be achieved. It thus seemed desirable to report the results of extensive tests on the balance described below-. This instrument is capable of yielding a sensitivity comparable to that of other models; moreover, it is extremely rugged in relation to its sensitivity because of the rigidity of all parts of the assembly. The balance also exhibits satisfactory long term stability. Reproducible results may be obtained much more easily with a balance of this type than with other models. Finally, the instrument offers the ad-
H 7
mm.
Figure 1.
Quartz beam microbalance and supporting frame
under 10 X magnification showed that some tips vere split. (Longitudinal splitting of the wire occurs during cutting; this condition was removed b y further electropolishing.) Others exhibited a concave curvat,ure, a condition which was removed by applying lower voltages to the circuit. Still others displayed a slightly convex shape; these tips were used in t h r balance. Initially the t'ips converge to a very sharp point (not exceeding 1 micron in diameter), but after use in the balance the tungsten and t'he supporting quartz cups adjust to each other because of the occurrence of plastic flow; undrxr the conditions stated, the elastic limit for tungsten ( 2 1 ) (roughly 4 x 1012 dynes per sq. em.) and the tmsile , strength for vitreous silica (8) ( 3 x lo9 dynes per sq. cm.) arc' (weeded. For highest sensit'ivities it may he necessary to change the points periodically. For the purpose for which the balance is now being used, it has not been necessary to change the points in nearly 1 year. The tungsten points rest in cups (G) that have a depression resembling a parabolic cone (insrt of Figure I). The cups m r e niade according to the procedure described by Johnston and S a s h (9). This operation can readily be performed by a glassblo~ver. An indicat,or hook, J , and a reference hook, K , made from 1-ycor glass, were attached to beam A and to frame C, The relat,ive position of t h resptct to the frame was determiiicd by measuring the distance between the indicator hook and rcference hook with a travelling niicroscope to + 1 micron. illternatively, the niicroscope served as a sight on the hooks when the balance was used as a null iiistrunient. Straight tungsttn wire of 2-mil dianithat had been bent to contain eter (H), a curved depression, was fastened across each 1'-shape. All tungsten-to-quartz contacts (such as H t o A ) and Vycor-to-quartz contacts (such as J to A) were crnieiited with fused silrcr chloride. The system was assembled in such a m y that the center of gravity \?-as immediately below the point of support (bottom of the tungsten t'ips). The balance was placed in a glass housing which n-as connectrd to a vacuum line (Figure 2). A sealed buoyancy bulb (for calibration purposes) and an open quartz sample hulh w r e suspended from one side using T'ycor fibers: a matching .oprii quartz compexi~ationbulb and a 3-inch l(,ngt'li of iiiagnrtixed Alnico n-ire cnclosod in a glass tube n-rre suspended from the other sidr. Platinum wire rvns used to countcr~wigtit thc h l a n c e to its null position and -4piczon \\* wax w ~ s cniployed to seal thc ground-glass joint,. t o the housing. Thr asscinbly n z s placed a t rye lr1-el in 311 insiilatcd wmden wclosurp: the tcmprraturt was iiiaintained t o =k@,l O C. by circulating air over a heater \vir(, or a cooling coil, utilizing a small f:in. A nicwury contact relay ivas uscd as tho vnsing device. Transmission of shocks was
I i
GROUNO-
GLASS JOINTS
-
Figure 2. Microbalance system
-VYCOR FIBER 24" LONG
Scale:
ALNICO
1/4
inch = 1 inch
COMPENSATION WIRE
' R A T I N U M WEIGHTS BUOYANCY BULB
.QUARTZ SAMPLE TUBE
QUARTZ COMPENSATIOP TUBE
QUARTZ SAMPLE BULB
QUARTZ COMPENSATION BULB
.
vantage that much larger mass changes may be compensated than is possible with other types of equipment of comparalile sensitivity. DESCRIPTION
Quartz naq chosen ds the liaw building material because of its strength, small temperature coefficient of expansion, and chemical inertness. The final design of the balance is qhonn in Figure 1. The materials used in the construction of the balance nere as follon s: Tlir s>ninietrical beam of the halance, A , nas constructed from l-nim diameter quartz rod.
The vertical supports, B , the base frame, C, and the four "feet," E : wrre formed from 2-mm. diameter cjuart'z rod. Beam arrests, D , constructed from the same size rod, were rrccted on cither side of the pivot points to limit t'he beam deflection. Tungsten wires, 0.005 inch in diameter, \yere subjected to an elrctropolishing technique and used as balance supports ( F ) in quartz cups (G). The point's m r e made by fast'eiiing 1 inch of the n-ire perpendicular to one end of a liorizont,al copper 1erc.r. The nire \Tas dipprd in and out of a 20% sodium hydroxide solution bg a rotating cccciitric acting on the other end of the lrvcr. \ \ h n 10 volts w r r applied from a S'ariac to complete a n r,lcctropolishing circuit, the &e was "burned" to a sharp point. Eramination of the points
vot.
29, NO.
a,
AUGUST 1957
1207
minimized by placing the balancesupport inside the box on foam rubber pads, supported b y a 1.5-inch thickness of paper, The box itself was also placed on foam rubber pads. When the length of the suspension fibers exceeded 15 to 18 inches, it was found that the balance housing could be connected directly t o the vacuum line; otherwise, it was advisable to interpose a borosilicate glass helix to minimize transmission of shocks from the vacuum line to the housing. For optimum performance the following mecautions should be observed: Great cleanliness, especially in the region of the quartz cups and a t the cross wires, H . Minimization of friction in these same positions. I n particular, the radius of curvature of the T'ycor glass hook suspended a t H should not be too small. Alignment; the tungsten tips must rest precisely a t the minima of the parabolic quartz cones. When the balance is in a null position, both tungsten supports must be exactly in a vertical position. The plane of the Y's must then coincide exactly with the horizontal plane.
Figure 3. Microbalance calibration slopes in nitrogen using two different buoyancy volumes
" I
0
too
200
330
PRESSURE mm. Hg
0PERATION
A number of experiments were performed in m-hich the deflection of the balance was measured directly as a function of its load. However, it is evident that the measurement of mass changes by this method is limited to the free w i n g of the beam. Hence, it is more convenient to use the balance as a null instrument. This r a s accomplished by allowing a magnetized Alnico wire to hang into a magnetic field of force of a solenoid. Thus, as the load is varied, the balance is kept in the null position b y altering the current through the solenoid. The current was provided from several 6-volt batteries connected in parallel and was changed by adjusting a series resistance box in the circuit. I n order to determine the current passing through the solenoid, the e.m.f. developed across a standard resistor in series with it was measured, using a Rubicon-type potentiometer. With currents of the order of 200 ma. or less, no difficulties were encountered from possible nonlinearities in the circuit or from the hysteresis properties of the illnico n-ire. CALIBRATION
The balance was calibrated by admitting a gas a t a series of pressures into the vacuum line and the balance housing. With changes in pressure, the force acting on the sealed buoyancy bulb varies according to Archimedes' principle, thus causing a deflection of the beam. The balance was returned to its null position by adjusting the compensation current. I n general, equilibrium readings were obtained within a few minutes; thereafter, no further
1208
ANALYTICAL CHEMISTRY
Figure 4. data volume VI
of
Differential plot obtained using See Figure 3
AP mm. Hg change in balance position was observed as long as the pressure and temperature remained constant. With nitrogen gas a calibration curve was first obtained with the buoyancy bulb of volume VI. The results are shown in Figure 3 as a plot of e.m.f. across the standard resistor a's. pressure in the vacuum line. The buoyancy bulb \vas then replaced by an equivalent mass of platinum of volume V,, and a second calibration was carried out. If the slopes of these two straight-line graphs are now subtracted, the unknown buoyancy effects due to asymmetries in the balance beam, suspension fibers, counterweights, compensation bulbs,
sample bulb, and ;Ilnico wire envelope are eliminated. The balance sensitivity can be computed from the relation
where S = balance seiisitivity in micrograms prr microvolt M = molecular weight of nitrogen (28.016 grams per mole) VI = volume of buoyancy bulb (cubic cn1.j V , = volume of platinum counterxeights (cubic em.) SI = slope obtained using the buoyancy bulb (microvolts per mm.) Sz= slope obtained using the
Figure 5. High temperature dip in calibration curve due to thermomolecular flow of gases
being the resolution of the null position by the cathetometer (motion of the indicator hook of less than 1 micron is not observable). Some of the important characteristics exhibited by the balance during a series of calibration runs are:
1. The same sensitivity is obtained on different days. 2. Correct response to the use of different types of gases is observed. The ratio of slones for calibration curves using o s y g h and nitrogen is 32 to 28. 3. The sensitivity is not altered by moderate jarring. The only result is a shift in the equilibrium position. 4. The same sensitivity is obtained after removing the buoyancy bulb and resuspending it in the same position. 5 . Calibration curves are strictly linear a t room temperature (see below). 6. The balance exhibits long term stability; no dftectable drift in zero readings 15-asobserved over ti70 separate 5-day periods !Then a constant pressure \vas maintained in the balance housing. 7 . The range of detection is largeLe., changes of 10,000 y may be observed reproducibly to +0.1% or better. 8. A precision of 5 X lo-' gram has been achieved to date. 9. With a net buoyancy volume of approximately 0.5 cubic cm., a drift in the null point of the balance with room temperature amount'ed to approximately 1 y per "C. K i t h a net buoyancy volume of approximately 0.02 cubic cm., no temperature drift of the null point 17as observed over a 5" change in t'emperature. Since a drift of the order of 0.2 y might, have been anticipated on the basis of changes in gas density, it appears that a part of this effect is due t80 the desorption of vapors from the glass surfaces as the temperature is increased. 10. The balance is sensitive t'o sudden shocks. The simple shock mounting, descxibed earlier, has made the balance insensitive to steady vibrations. However, if the balance housing is accidentally bumped or if gas is admitted to the housing too rapidly, causing the balance to bump against the arrest, a shift in the equilibrium position results. If reasonable care is exercised b y the operator, shocking of the balance can be completely avoided. ~~
I5 mm Hg
&003-0
0
15 PRESSURE mm. Hg
w m
4000-
CK
$ 3000n
a
-
50
PRESSURE mm.Hg
Figure 6. Physical adsorption of oxygen on titanium dioxide a t 78" K.
- --
ANATASE
I5O
90.5
-
Figure 7. Adsorption of oxygen on titanium dioxide a t 504" C.
RUTILE MP 960-2
A
A
T
0
10
mg. of
I
-
RUTILE MP 980-5
2 0
I
3 0
PRESSURE mm.Hg platinum weights (microvolts per mm.) P = 760 mm. T' = 22,400 cubic em. 2 = 1.0915 as obtained from the virial equation to correct for the expansion of nitrogen from 273" to 298" K. and for the nonideality of nitrogen
Volume VI TTas determined by weighing the buoyancy bulb in air and in \Tater; Vp was obtained from the density of platinum. The differential plot of A mv. us. 4 P (Figure 4) s h o w that a reproducibility of z t O . 1 y is obtained. This compares to a minimum detectable mass change of 1 0 . 0 5 y. the limiting factor
Concerning linearity of the calibration curve if the sample bulb is heated to high temperatures, the calibration curve is no longer linear a t low pressures but contains a "dip" of the general shape shon-n in Figure 5 , where the distance, A , has corresponded to as much as 1000 y. This dip may be essentially eliminated by careful adjustment of the furnace positions around the sample and counterweight sides. For this purpose it appears essential to make both sides as symmetrical as possible n-ith respect to the temperature gradient which is set u p and to the
vot.
29, NO.
a,
AUGUST 1957
1209
surface areas exposed to heating. The above effect may be due either to the adsorption and desorption of vapors or to the presence of thermomolecular forces. I n any event, no undue difficulty was encountered in reducing the dip size to the equivalent of 10 to 40 y ; with great care, the remaining dip may be suppressed. APPLICATIONS
The above balance has been utilized in gas adsorption studies for nearly 2 years. At all times it has exhibited reproducible behavior, and has required recalibration only when the magnetic compensation solenoid has been removed and replaced. The balance has been used to study physical adsorption of oxygen and nitrogen on various titanium dioxide powders a t 78” K. A typical ouygen adsorption isotherm on rutile is shown in Figure 6. After 5200 y of oxygen had been adsorbed, the pressure was 1oTYered; this resulted in the desorption of oxygen. The balance ultimately returned t o its original null position within experimental error. Typical isotherms relating to oxygen sorption on commercial titanium dioxide samples a t 504’ C. are presented in Figure 7 . The dots and triangles for the anatase curve were obtained using two aliquot samples from the
same lot. The maximum total uptake on these samples was 23.0 and 61.3 y, respectively; inspection shows that the two runs agree within experimental error. The total uptake for sample melting point 980-5 was 13.8 y. Currently the balance is being utilized to study the composition of praseodymium oxide as a function of temperature and oxygen pressure. KO attempt has been made thus far to increase the sensitivity of the instrument to its limit by raising the center of gravity.
(5) Cini, R., Sacconi, L., J . Sci. Znstr. 31, 56 (1954). (6) Cunningham, B. B., Nucleonics 5 ,
62 (1949). ( 7 ) Dar. A. G.. J . Sci. Znstr. 30, 200 (‘lb53). ‘ Johnston, E. W.
(12) (13)
ACKNOWLEDGMENT
The authors wish to express their appreciation to A. Y. Gerritsen and D. H. Damon, who are undertaking parallel investigations a t the Department of Physics a t Purdue University, for many helpful discussions and suggestions. The authors are likewise indebted to Edward Fagen of the Physics Department for help in the preparation of the tungsten tips. LITERATURE CITED
(1) Barrett, H. 11., Biinie, A. IT., Cohen. 11.. J . .4m. Chem. SOC. 62, 2859 ( 1 h O ) . (2) Beams, J. IT.,Rez). Sci. Znstr. 21, 182 (1950). (3) Bradley, R. S., J . Sci. Znsfr. 30, 84 (1953 ). (4) Carmichael. H.,C a n . J . P h y s . 30, 324 (1952).
(14)
(15)
(16) (17) (18)
krd Ed.. o.-271. Reinhold: New York, 19iK Lowr,B. W., Richards, F. &I.,Yuture 170, 412 (1952). McBain, J. IT.,Bakr, iZ. W., J . Am. Chem. SOC. 48,690 (1920). McBain, J. IT., Tanner, H. Y., Proc. Roy. Soc. (London) A 125, 579 (1929). Partington, J. R., “Advanced Treatise of Physical Chemistry,” Yol. 1, pp. 753-754, Longmans, Sew Tork, 1949. Rodder, J., Microtech Services Co., Berkelev, Calif., personal communication. Simons, J. H., Scheiier, G. L., Jr., Ritter, H. L , Rev. Sei. Znstr. 24, 36 (1953). R’eissberger, A , , ed , “Physical 1Iethods of Organic Chemistrv,” 2nd ed., Vol, 1, pp. 279-280, Interscience, S e w York, 1949.
RECEIVED for review October 11, 1956. Accepted February 23, 1957. Division of Analytical Chemistry, 130th meeting, ACS, Atlantic City, N. J., September 1056. This work was supported by a grant from the Research Corp.
Determination of Low Concentrations Radioactive Cesium in Water BERND KAHN’, DAVID K. SMITH, and CONRAD
P.
STRAUB2
Oak Ridge National laborafory, Union Carbide Nuclear Co., Oak Ridge, Tenn.
b Three methods for determining small amounts of radioactive cesium in water are presented. Radioactive cesium, with milligram quantities of cesium chloride carrier, i s precipitated as cesium ammonium phosphomolybdate, coprecipitated with sodium potassium cobaltinitrite, or concentrated on a cation exchange resin from liter volumes of water, and then purified b y established radiochemical procedures. Studies with radioactive tracers indicate satisfactory chemical yield, tracer recovery, and decontamination from long-lived fission products. After development, the methods were used to determine radioactive cesium in river water in concentrations between 1 O+ and 1 OP8 microcurie per ml. 12 10
ANALYTICAL CHEMISTRY
XXBER of procedures .for determining low concentrations of radio-. active c&um in water have been studied to permit determination of the path of long-livcd cesium-134 and -13i which are discharged into a stream from the Oak Ridgc Sat’ional Laborat,ory. The proccdurrs w r e ncedrd to obtain more accurak analyscs in the concentration range of 10-4 to 10-8 microcurie per i d . than arc possiMc with prcsent methods (S!11. 1 5 ) . Thc nerisitiT-ity of the present methods--\\-hicli are not intended to be uscd for lo^ levcl analys~s-is limited by t8hr small (1- to 10-ml.) sample volumes irnposcd by the relatiwly large solubility of the cc>siumprecipitates used for srparation, and by the small weight of cesium carrier required for efficient beta counting.
The methods studied were suggested by those used for determining other radionuclides in small concentrations, such as absorption on ion exchange resin combined with continuous counting of the resin (?), coprecipitation of insoluble hydrouides with aluminum hydroxide (I). and coprecipitation of barium and strontium with calcium carbonate ( I O ) from large volumes of mater. TT’hile concentration by evaporation (6) is common, including the use of ingenious pipetting devices xhich permit evaporation of large volumes on a 1 U. S. Public Health Service, assigned to Oak Ridge National Laboratory. Present address, Robert A. Taft Sanitary Engineering Center, C . S. Public Health Service, Cincinnati, Ohio.
