contained only pyridine nitrogen and no nitrile or amide. The presence of even 2 or 3% of unhydrolyzed acrylonitrile would have increased the nitrogen content of the samplw enough to reduce the apparent pyridine absorptivities by 20% or more. It was concluded that the hydrolysis procedure as used in this method for the determination of acrylonitrile is valid and quantitatively converts nitrile nitrogen to ammonia. Methylvinylpyridine Determination. T h e reason for the high back-
ground absorbance encountered in the ultraviolct spectrum of unhydrolyzed copolymer samples is not known. A possible esplanation is that the unhydrolyzed samples form sus1;ensions instead of true solutions and scatter some of the shorter wave length radiation t o produce a n apparent absorbance background. Partially hydrolyzing the samples in sulfuric acid solution increases their solubility and removes the background absorbance caused by the suspended material. The difference in the molar absorptivities of poly(methylviny1pyridine) and methylethylpyridine (Table I) is of interest, in that it shows that the ultraviolet absorbance of the pyridine is affected by its environment. However, this change in absorptivity appears to be relatively small in going from methylethylpyridine to copolymers containing
Table 111.
?recision of Determinations
Standard DeviationTotal
Determination Acrylonitrile In unhydrolyzed samples In partially hydrolyzed sample 2-Methyl-5-vinylpyridine
S o . of ?io. of Samples Detns. 4 5
9
20 weight % or less of pyridinc. Consequently, purified methylethylpyridine can be used as a calibration standard for the determination of the pyridine content of methylvinylpyridine-acrylonitrile copolymers containing 20 weight % or less of pyridine. Precision
of
1:i 17 31
?’resent,
”;
of
‘10 7
absolutc
amount prcsciit
81-89 17-19 2.a15
0.27 0.3‘3 0 .OGG
0 3G 2 1 1.1
lute precisions figures agree reasonalily well, and if the random error is considered to be essentially indcpendcstit of acrylonitrile concentration, the same absolute precision figurc ill scrve for both types of samples.
Determinations.
Standard deviations of the determinations, calculated using the deviation factor as described by Dean and Dihon (B), and pooled t o give aggregate standard deviations by the procedure given by Youden (37, are given in Table 111. The acrylonitrile determinations in hydrolyzed and in unhydrolyzed samples are reported scparately because the relative precision figures differ greatl!. in the two types of samplcs. The hpdrolyzed and unhydrolyzed niatrrials h a w very dissimilar physical propertirs and it is possible that the nitrile dctcrniination is less precise on the hydrolyzed sample. On the othrr hand, thc abso-
s’,
ACKNOWLEDGMENT
The authors express appreciation to Phillips Petroleum Co. for permission to publish this paper. LITERATURE CITED
( 1 ) Burleigh, J. E., LIcKinney, 0. F., Barker, 11. C., ASAI.. CIIEJI.31, 1684 (1959). ( 2 ) Dean, R. B., Disori, \T. J., Zbid.,23, 36 (1953). (3) Youden, W. J., “Statistical Alcthods for Chemists,” Chnp. 2> IVilei,, Sc\r York, 1951.
RECEIVED for review \larch 30, 195‘3. Accepted July 20, 1959.
Simple Recording Thermobalance for Vacuum and Pressure Studies J. G.
RABATIN and C. S. CARD‘
Advance Development laboratory, Chemical Products Plant, General Electric Co., Cleveland, Ohio
b A new recording thermobalance is dekribed for operations in vacuum, a t 1 atm. and up to 600 p.s.i. pressure of such gases as carbon dioxide, nitrogen, hydrogen sulfide, carbon monoxide, oxygen, and ammonia. Its principal features are a torsion mechanism and a photocell-light transducer system which permit easy operation a t high pressures in a mediumsized pressure chamber. The new thermobaiance was compared with the Chevenard thermobalance. Maximum sensitivity is about f0.2 mg. for weight changes of 100 to 200 rng.
S
work of Honda (4) and Guichard (2) many articles on thermobalancts have been written. INCE the early
Among the more recent innolrations are a recording quartz spring balance ( 3 ) utilizing a linear variable difftrcntial transformer in the dctccting circuit ant1 a thermobnlaiice using a strain gage transducer ( I ) . Conimcrcinlly available are thc Clirvenarri, St:intoi:, Sartorius-Werkc, and Arni7ic.c tlimnobalanccs. A rcrent rcgcrt (8) tlrscribcd the niotlificntions of thc Chcvcnartl thermobalance to niukr possible simultaneous thcrnioernvin~t.tric anti difierentiai thernxil analysw. il wlativcly incxpcnsive T hwmoldnncc~ his also becn dcscribrd ( S ) . None of thesc thcrmobalanccs is simple or compact enough to be readily enciosed in a pressure system for studies in various gascs a t pressures up to 40 atm., aithough a tiierxobnlance usable
:n n y x u u i n has bccn rciiortcd (5). The thormoi)alance drscribvd hcrr WIIsists of a niodificd torsion balanci.. with a photocrll-light transtluccr s y ~ . - tcm which w n he u s d in n xicuuni, ilp t o IO-atni. prcssure or as n usual type ci thrrrnot)n!ancc. Such un instrumcnt l.i desirable vI;~vi:iily for kinetic stutlics involving the -wctio;l of ccxrtain inorganic compounds under unusuai atmospht,ric mntlitions. PRINCIPLE OF OPERATION
During the operat,ion of the thernio1 Present address, Heavy RIilitary Electronics Department, General Electric CO., Syracuse, N. Y.
VOL. 31, HO. 10, OCmBER 1959
1689
It
ji
I
9
I
-2
I-
0
Figure 2. system
I
Main features of weight change detecting
-7 FROUr
Figure 1 . assemb Iy
VIEW
SlirE
VIEW
Schematic diagram of torsion thermobalance
balance, if a given n eight change occurs as the temperature is increased, the torsion balance arms will move proiiortionally to new equilibrium positions. This linear change in position will cause a n opaque flag t o move, thus intercepting more or less parallel light, depending on whether weight loss or gain occurs. A proportional change in photocell current will result and can be registered by means of a recorder. When properly calibrated, the recorder tracings give the actual weight changes that occur during the therniobalance run. A similar photocell-light transducer system has been used (7) for a sedimentation balance.
size may vary from as littlt. as 50 mg. to about 5 grams, depending on the experimental requirenients. An important feature of the torsion balance is that it allows the sample to be located above the balance mechanism, 'thus greatly minimizing convection heating problems. Because oil dampers eliminate practicallv all oscillations, the resulting recorder trace is a smooth line. Magnetic clanipiiig may also be used. Light
and
Photocell
Circuitry.
Shown in Figure 2 are the main features of the weight changedetecting system which consists of light from a source, 1, being collimated by a lens, 2, with a G E photovoltaic cell, No. DESCRIPTION OF APPARATUS 88 X 565, 3, detecting the amount of light not intercepted by the opaque Torsion Balance Mechanisms. flag, 4. The light source is a &volt The torsion balance (2 1/4 )< 2 X 6 inches) (Figure 1) is similar t o the General Electric 88 lanip operated one manufactured by the Torsion through a constant voltage transBalance Co. former, 5. I n close proximity t o the photocell assembly is a slit arrangeThe torsion rlements, 1, consist of \\-atch main springs, S o . 2114 (Sandment, 6, which can beadjusted toallow steel Springs.Division, Randie Sterling, the desired amount of light t o fall on Yew York, Y.Y.): width 1.00 mni.. the photocell. A 2500-ohm voltage thickness 0.12 a m . , and length 25 cni. divider, 7, allows different amounts of Each spring holder is c o n s t i ~ ~ c t e d the photocell output to be applied t o a v i t h two setscrews, 2, so t h z t the main Speedomax recorder, Type G, with a .spring may be clamped a t each end, 10-mv. full scale sensitivity. By .\-ound wound the holder, and then these two controls, the proper setting :)ut under tension t o act as the torsion may be obtained t o hare, for instance, c.!ement. 4 x r a m i c sample bolder 100 nig. show a full scale deflection of :,od, 3 , is clamped t o cjne arm, 4, of the 10 inches on the recorder paper. The 'naiancP. Y o the cither arm is stability of the system after about 1:ittadieti m opaque flag, 6 . with anhour warmup is such t h a t a base line (:ther ceramic rod, 6, .shicI? acts as a drift of about 0.3 mg. will occur in 24 t.oUntrr\VPigi>t. The opaque flag inhours 3f continuous operation. As tcrcepts psra?iel light. Located a t most runs take less thak 3 hours, this rhe hnttcim of each arm are perfobase line drift is negligible. :attd brass oil dampers, 7 , attached t o A linear variable differential transthe halance by fine spring wire. Vacformer transducer system was also ~ ~p iurm:p oil of high viscosity is used X S C Q in place of the photocell-light a s the clamping medium. The counassembly. The core of the transtrrweights, !ocated on the balance former was mounted in place of the warns, are for :_rross.d , and fine adopaque flag, 4. The rest of the difsr,mrnts, 3. Zounterweights may ferential transformer was attached -0 be added t o t h e 4-ml. counterrigidly over the movable core. Thus, .\eight tmcibie. i5. The sample is as a weight change occurred, the torp l a d 111 the +mi. crucible, 11, losion balance moved t o a new equia. : i t e e c,n t h e other arm. The sample librium position. The core moved 16%
e
ANALYTICAL CHEMISTRY
Figure 3. Cross section of thermobalance pressure chamber
simultaneously, resulting in a voltage change proportional t o the distance moved. This voltage change was registercd by a recorder through appropriate circuits. This method was not utilized for the closed pressure system because it required more circuitry and was difficult to adjust after pressure build-up. Pressure Chamber. Shown in Figure 3 are the essential features of the pressure chamber used to enclose the therniobalance, together with the furnace and associated apparatus. The chamber is constructed of two sections of 12-inch extra-heavy steel pipe which have flanges (Crane Co. 854E), 1 , n elded to one end, and end caps (Crane Co. 4573), 2, welded t o the other end. Also provided are Crane Co. Weldolet type threaded openings for entry through Conax sealants (Conax Corp., Buffalo, N.Y.) of heating wire, 4, control thermocouple, 5, sample temperature thermocouple, 6, and a gas inlet-outlet opening, 7 . For heat baffles, two section separators, 9, are used arodnd the furnace, 3, which is wound with Kanthal A-1 wire and can be operated t o 1200' C. These help t o prevent convective currents from developing, especially at high pressures. A tube arrangement,
10, plus insulators around tlrc sample holder stem prevents heating of the torsion elements by radiant heat at temperatures above 800" C. Tempered glass windows, 8, '/,-inch thick sandwiched between two rubber gaskets and bolted by heavy plates, are provided for the optical system, which aiso includes a double convex lens, 11, of 10-inch focal length. The torsion balance, 12, is iocated at the bottom of the pressure chamber. The pressure chamber is 12 inches in diameter by 2*,'4 feet (over-all). A morc compact furnace assembly and a different arrangement of the lower chamber will give a smaller chamber. In operation. the top section containing the furnace is first lowered to the bottom flange by a winch mechanism and guide rods. For ordinary runs the thermobalance apparatus may be used thus. For vacuuni or Iwrssure studies, the flanges are bolted together and the appropriatc gas is introduced after the chambw is cvarua tctf .
Thr present setup is sufficiently strong for pressures up to 600 p s i . Gases such as nitrogen, oxygen, hydrG pen, carbon monoxide, hydrogen sulfide, carbon dioyide. and ammonia may br used, sonic at only modrraw prcssures hcausc. of thcir reactivity a t high trniperatures. Temperature Programmer. The trmperaturr controller and programmer assembly consists of a West, JSG, stepless programmer and a Kest 1.5kva. reactor, with a driver unit. A continuously varying power input is maintained, such t h a t very small furnace temperature fluctuations occur. The programmer cam may be cut t o give almost any desired sequrnre of temperatwe changes. For the standard runs the change is 7" C . per niinUte.
PERFORMANCE O F THERMOBALANCE
Operation. Once the instrument is properly constructed and aligned, operation of the thermobalancc is simple.
After a weighed amount of sample is placed in the 4-ml. sample crucible, the torsion balance is brought t o the proper level by means of counter weights. S e x t the transducer system is adjusted t o give the desired sensitivity, such as 100 mg. full scale on the recorder. T h e slit is adjusted for the proper chart pen position, full scale left, full scale right, or sonic intermediate position. After a warmup time of about 1 hour necessary for the stable (nondrift) operation of the photocell, the thermobalance is calibrated by simply adding the appropria t e fractional analytical weight, such as 100 mg. The number of chart divisions corresponding t o the change
IO
':i c
I3 A t m .
10
YnCO,
200
400
600
*C.
T E YPER ATUR E
Figure 4. Thermograms for decomposition of manganous carbonate in air and in 13 atm. carbon dioxide
in recorder position due t o the change in the torsion balance equilibrium is recorded on the chart. For subsrquent analyses, the value (say 0.66 chart division per mg.) is used to convert the recorder curve t o a weight per cent curve. After calibration, the top (furnace) section is lowered over the sample. If pressures are to be used, the two flanges are bolted together, the chamber is evacuated, and the proper pressure of gas (such as carbon dioxide) is built up. The temperature programmer is turned on. The thermobalance is now in operation and will record w i g h t change as a function of time, temperature, etc. At the end of the run, the sample is weighed on the analytical balance to check the total w i g h t changr. Calibration and Sensitivity. The thermobalancc is calibrated in a simple direct manner by the use of fractional analytical weights. By this means the total response of the balance to a weight change is determined. This includes the change in torsion balance equilibrium position and the linearity of the photocell-light setup. In actual practicr, the linear response falls between about 20 and 100 divisions of the recorder chart. It is possible to estimate the chart to about +O.l division. Becausr of this limitation of reading the chart paper. tlie maximum sensitivity is approxiniately +0.2 mg., which is obtainable for a full scale setting of 100 mg. The parallel light-photocell
Table 1.
Resistance
Weight Change Ranges and Sensitivities
Maximum Torsion Weight Balance Selector Chart Division Change, Movement,, PosiMg. Mm. tion per Mg. 700 14.0 1 0.130 300 6.0 2 0.267 200 4.0 0.400 3 2.2 0.736 110 4
systan \ \ i l l nirasurcs awurat(biy :I 1iiio:ir rhangt~of about 0.02 nrn:. As the therniobalancc is now sc-t up. thr rangcs of weight change which niaj' be rwortled and the corresponding sensitivities are listed in Table I. Comparison with Chevenard Thermobalance. A comparison iwtwcben thc torsion thermobalancc~anti tlie C'hrvrnard thermobalanct~. d s r \ ayailable a t this laboratory, showed that for most runs the rrsults were almost identical. Howevrr. in thc pyrolysis of certain polymers, major difftwncw were noted in the thermobalancr curvrs. These tliffcrrnces were tracw! to t h c x slower response of the Chevenard t.hwmobalance, which takrs 2 minutrs to register a full scalr w i g h t changc ( v - n though t.hc rhanges \vert' almost iiist:intanrous; the torsion thermobalancr takrs 2 seconds. Thus, thr prrsrnt therniobalancc is more suitablt, for following t.hc kinetics of rapid reactions. Pressure Applications. In using the thermobalance a t high prrssurcs, it is important to rralize that gas buoFancy becomes a major factor.. Thus, it is ncressarj- to determine tht* rstrnt to which buoyancy contributes to tlw observed w i g h t changr. Thc volunw differenw betwrrn the tH-o arms of thv thermobalance was determined by nicaswing t.lir position of thr base lino a t several pressures. By appjying simple gas law relations, the volume differrncr \vas c.alrulatrc1 to bc 0.45 rc. for thr present thermobalancr. A t 14 atm. of nitrogen this difference amounted t n 6.6 nig. I n actual practice this initial changc a t room temperature is not iniportant becaausc the thermobalance is reset aftrr the pressure is built up. T h r changv i n buoyancy as the sample is heatcd is vasily corrcctcd by nuking a. blank run to check the base linc drift. For esamplr, a t 180 p.s.i. carbon dioxide gas, the drift corresponds to 2.3 mg. rip to 400" c. The application of thc prcssurr therniobalancr apparatus is shown in Figurc 4 for manganese carbonate heated in air a,t 1 atm. and in carbon dioxidc a t 13 atm. The differences are due primarily to the repression of the decomposition of manganese carbonate in carbon dioxide ant1 tlic mechanism of the subscquent oxidation of manganese oside in carbon dioxide.
DISCUSSIOI.;
I n the construction and operation of a thermobalance usable a t high pressures certain features are highly desirablcsensitivity, ruggedness, high temperature and pressure stability, ease of operation, and compactness. The automatic recording torsion thermobalancr to a large extent incorporates these features. For pressure applications, VOL. 31, NO. IO, OCTOBER
1959
1691
spring-type thermobalances were not suitable. A t high pressures, the arrangement of a spring above a heating zone is very difficult to shield from conductive-convective heating. As no counterweight is possible, very large changes in apparent weight occur simply .because of the change in buoyancy on heating. Also, the weighkletecting system must be enclosed in the pressure system, thus preventing easy adjustments. The automatic torsion thermobalance, because of its compactness and special weightdetecting feature, is easily enclosed in a medium-sized pressure chamber. None of the elements are readily attacked by various gases. Befause of the counterweight feature, the buoyancy correction is small. For
applications to fast reactions, the present thermobalance is more suitable than the commercially available Chevenard thermobalance; otherwise, per€ormance is about the same, the Chevenard being slightly more sensitive. The thermobalance was not designed specifically for vacuum work, but has been used successfully to less than 1 mm. of mercury. For high vacuum work, large difhsion pumps and long p u m p a u t times are needed because of the difficulty of removing adsorbed water and gases from the insulating fibers and other parts of the chamber, although a furnace using radiation shields could avoid some degassing problems. The commercially available Aminco thermobalance performs well as
a high vacuum thermobalance, ‘becauseit is enclosed in a small glass chamber. LITERATURE CITED
(1) Bartlett, E. S., Williams, D. N., Rev. Sn’. Znstr. 28,919 (1957). (2) Guichard, M., Bull. SOC. him. 37, 62 (1925). (3) Holley, J. G., Can. J . Chem. 35, 374 (1957). ( 4 ) Honda, K., Sn’. Repts. Tohoku L’niv. Imp. 4, 97 (1915). (5) Pope, M. I., J. sei. Zmtr. 34, 229 (1957). (6) Powell, D. A., Ibid., 34, 225 (1955). (7) Rabatin, J. G., Gale, R. H., -ASAL. CHEM.28, 1314 (1956). ( 8 ) Wendlandt, W. W., Zbid., 30, 56 (1958). RECEIVED for review February 26, 1959. Accepted June 8, 1959.
Determination of Iron in Urine Using 4,7-DiphenylI ,I 0-phenanthroline PETER COLLINS and HARVEY DIEHL Department of Chemistry, Iowa State University, Ames, Iowa
b The determination of iron in urine can be made rapidly and accurately by wet ashing with nitric and perchloric acids and extracting the red color of iron(l1) with 4,7-diphenyl- 1 ,I O-phenanthroline (bathophenanthroline) into nitrobenzene.
T
demand for increased speed and improved accuracy brought about by the growing u8e being made in medical diagnosis of determinations of iron in blood s e d m and urine is attested by the growing body of literature on the subject. Not only has the clinical chemist been of help to the physician in applying the findings of biochemical rr-*arch, but he has been quick to utilize mort’ sensitive reagents as they have been offered and to increase his output by rmpioying modern instrumentation. The twnd is illustrated by two recent papers by Petcrson (2, 3) which make clw of the superlative iron reagent bathophenanthroline introduced by Case, Smith, %Curdy, and Diehl HE
(1, 4 ) .
A procedure which further increases the speed and accuracy of the determination of iron in urine with 4,745phtnyl - 1,10- phenanthroline (bathophenanthroline) takes advantage of the speed with which large samples of urine can be wet-ashed with nitric and perchloric acid. The d a c u l t y which 1692
0
A N A L Y l l W CHEMISTRY
plagued earlier applications of bathophenanthroline in the presence of perchlorates, the formation of a turbidity in the isoamyl alcohol extract of the ferrous-bathophenanthroline ion, is obviated completely by using nitrobenzene as the extracting liquid. The molar extinction coefficient of the ferrousbathophenanthroline perchlorate is somewhat larger in nitrobenzene than in water or isoamyl alcohol and repeated extractions can be made with ease if necessary, inasmuch as the nitrobenzene is the lower phase. Extraction Fvith nitrobenzene proceeds rapidly and the method preserves the advantages of the original isoamyl alcohol extraction procedure in affording a large concentration factor as well as a method of freeing the various reagents of iron and thus of reducing thc blank to essentially zero. For the concentrations of iron found in normal urine, tenths of a microgram per 50 ml., the method gives consistent results to -within 0.1 7 ; for the larger amounts of iron accompanying disease, the relative error is proportionally less. RECOMMENDED PROCEDURE
Bathophenanthroline, 0.001M. Dissolve 33.2 mg. of 4,7diphenyl-l,l0-phenanthroline (bathophenanthroline) (available from the G. Frederick Smith Chemical Co., Columbus, Ohio) in 100 ml. of ethanol.
Reagents.
Hydroxylammonium Chloride, 10%. Remove iron from the solution by adding 10 ml. of 0.001M bathophenanthmline and extracting with 15 ml. of nitrobenzene. Sodium Acetate-Acetic Acid Buffer. Prepare a solution 2M in acetic acid and 2M in sodium acetate. Remove iron by adding 10 mi. of 10% hydroxylammonium chloride and 10 ml. of 0.001M bathophenanthroline and extracting -kith 15 ml. of nitrobenzene. Nitric Acid. Redistill reagent grade nitric acid from an all-glass still and store in a borosilicate glass container. Perchloric Acid, 70%. Use doubly distilled perchloric acid (available from the G. Frederick Smith Chemical Co., Columbus, Ohio). Ammonium Hydroxide. Use reagent grade ammonium hydroxide, or to reduce the blank tQ the ve.ry minimum, distill anhydrous ammonia into water. Deionized Water. Pass distilled water through a column of mixed cationanion exchange resins. Standard Iron Solution, .approximately 0.7 7 of iron per ml. Weigh accurately approximately 0.55 gram of electrolytic iron, dissolve in dilute hydrochloric acid, transfer to a 1-liter volumetric flask, and dilute to volume. Pipet 50.0 ml. of this solution into a 1liter volumetric flask, add 10 ml. of hydrochloric acid, and dilute to volume. From this solution pipet a 25.0-ml. aliquot into a 1-liter volumetric flask, add 10 ml. of hydrochloric acid, and dilute to volume. The concentration
