Levitating a magnet using a superconductive material

PROTOCOLE 624
Levitating a Magnet Using a Superconductive Material
Frederick H. Juemem.'
.~
H. Dieckrnann? and Ronald I. Perklm'.3
- . Arthur 6. E l l i ~Gunther
University of Wisconsin-Madison, Madison, WI 53706
The levitation of a magnet above a superconducting material (see Fig. 1) is known as the Meissuer effect. We have
developed a technique, using an overhead projector, for
demonstrating this effect to a large group of students.
Materials
Overhead nroiector. olaced on its hack. with a mirror arraneed to
moiect the imaee'dnto
a vertical nroi&tion
"
. . screen (Fie.2):
Plastic foam (coffee)cup. The base of such cups usually has a rim so
that, when inverted, a small amount of liquid can be retained
within the base.
Pellet of superconducting material (yttrium barium copper oxide,
YBazCusO7-,), about 13mm in diameter by 3 mm thick. (Details
on preparation are given below.)
50-mL beaker
Test-tube holder
Approximately 0.2-L liquid nitrogen
Insulated rubber-coated gloves
Magnet about 1 mm X 1 mm X 3 mm. A samsrium-eohalt magnet
(Edmund Scientific Co.) causes greater levitation than a chip
obtained by breaking or core-drilling a typical ceramic-based
magnet, because of its higher field strength.
Plastic tweezers
.
A
Figure 1. The Meissner effect: cobalt-samarium magnet levitated above
superconducting ynrium barium copper oxide pellet.
Preparation of the "1-2-3" Superconductlng Pellet (1)
When cool, re-grind the dry powder in the mortar and
pestle (do not use acetone). Return the powder to the boat,
and heat i t in the tube furnace for a 5-h period a t 950 O C in
air. Again, cool it to room temperature. Grind the now black
powder for a third time in t h e mortar and oestle.. this time
using acetone. Allow the acetone to evaporate completely.
L i e the pellet press to form pellets at about 30.000 Iblin.'
pressure. The pellets a t this point are typically'somewhat
fragile. Our pellets are -13 mm in diameter and twicallv-3
mm thick, iorresponding to the use of -2 g o f b e mixed
oxide powder. Three pellets may be formed using the
amount of material specified.
Place the pellets in the alumina boat, and place the boat
inside the quartz tube. Place the assembly in the tube furnace, and heat a t 950 "C for l h in order to sinter (heating
just below the melting point to increase streneth and densitv
~~-~
i n d to promote interirinular bonding) thepilets. Allow the
tube lurnare to cool to 500--600O C for the rrucial "sensitization" step, and pass pure oxygen gas through the quartz
tuhe, over the pellets, a t a r a t e of about 10mLlmiu for 3 h. At
the conclusion of the 3-h period, turn off the furnace, and
allow i t to cool to room temperature while maintaining the
flow of pure oxygen over the pellets.
Spray coat the pellets with an acrylic resin such as K ~ y l o n
No. 1303. This will protect the pellets from chemical decomposition, which can occur with prolonged exposure to water
or water vapor.
Materials
Dust mask and safety goggles
Mortar and pestle, agate (e.g., VWR Scientific)
50 mL acetone
1.00 g yttrium oxide, Yz03(Alfa,99.99%)
2.11 g cuprie oxide wire, CuO (e.g., Baker, reagent grade)
3.50 g barium carbonate, BaC03 (e.g.,Baker, reagent grade)
Alumina boat, 90 X 17 X 11.5 mm (e.g.,Thomas Scientific)
Utility tongs
Heat-proof gloves
Ilent-prooi pad
Tuhe furnnc~.1 in. diameter (Lindherg 54032 or equi\,nlent)
Ppllet mess. of the kind used fur makme KBr el lets
~uartz'tuhe(24 mm o.d.), equipped withair-tight connections made
at one end for attachment toan oxygen tank and at the other end
for attachment to an oil-filled bubbler. The tuhe should extend
about 20 cm from either end of the furnace.
Tank of oxygen, with regulator and tubing to connect to quartz tube
Krylon No. 1303, acrylic resin spray
~~
~
Procedure for Pellet Preparation
Wear a dust mask and safety goggles a t all times, and
cohduct grinding operations in a fume hood. In the mortar
and pestle, grind together the yttrium oxide, cupric oxide,
and barium carbonate with enough acetone t o make a thick
slurry. Allow the acetone to evaporate completely in air; a t
this point the mixed powder will flow freely. Place the powder in the alumina boat, and heat it in the tube furnace a t
950 O C in air for 1 h. Wearing heat-proof gloves and using
tongs, remove the boat containing the mixed oxides from the
tube furnace and place it on the heat-proof pad. Allow the
mixture to cool to room temperature. It will be black or
green-black.
~.
The Demonstrallon
Preparation
Support the overhead projector on its "back", so that the
the staee
staee is vertical. and arranee a mirror to -oroiect
"
image onto the projection screen. (We use a special device
formerlv sunnlied bv the 3M Comoanv that lets the overhead prijectdr rest in the approprke~position,provides a
-
' Institute of Chemlcal Education.
Deparlment of Chemistry.
Greenwich High School, Greenwich CT 06830
~~~
1
Volume 64
Number 10 October 1987
851
PROTOCOLE 624
Figure 2. Overhead projector set up to display the Meissner effect shown In
Figure 1.
stage to support items to he projected, and supports the
~irror.A
) photograph of the device is given in Figure 2. A
similar support could k$ constructed for other overhead
projectors. A TOPS projector could also he used (2). Invert
the foam cup and place i t adjacent to the stage of the overhead projector. Place the mixed-oxide pellet on the cup's
hase. Turn on the projector and focus the image of the pellet
onto the screen.
Presentation
Fill the 50-mL heaker with liquid nitrogen, and using the
test tube holder to grasp the heaker pour liquid nitrogen
over the oxide pellet into the shallow depression formed by
the hase of the cup, filling it. Use care to avoid hitting the
glass of the projector stage with liquid nitrogen; the glass
could shatter. As the liquid nitrogen evaporates, replenish it
in the ahove manner. After about a minute, the oxide pellet
~houldbe nxded helow its supwconducting temperature.
Pick uo the magnet with the plastic tweezers, and place it
about 2 Am abovethe center ofthe oxide pellet. ~ e l i a s the
e
maenet. It will he levitated approximately 3 mm ahove the
peliet, and its image will be projected onto the screen. The
magnet will remain suspended until the pellet warms to
ahove its critical superconducting temperature, a t which
time the magnet will no longer he levitated above the pellet;
it may either settle to the pellet's surface or "jump" away
from the pellet.
Hazards
Liquid nitrogen is extremely cold, having a boiling point of
-196 OC. Skin contact with liquid nitrogen or with an object
chilled by liquid nitrogen can result in severe frostbite. Protective gloves that will not absorb the cold liquid should be
worn whenever liquid nitrogen is used (3).Excess liquid
nitrogen should he disposed of promptly since, upon
standing in an open container, i t can condense oxygen from
the atmosphere, slowly enriching the liquid with liquid oxygen. This mixture of liquid oxygen and liquid nitrogen is a
powerful oxidizer and may react violently with easily oxidizable substances. The toxicity of the pellet and magnet
have not been fully characterized. They should not be ingested, and they should be kept away from small children.
Care should he taken to avoid inhalation of powder particles
should they he accidentally created. The levitation experiment should not he performed near flames, sparks, or flammable materials since powder particles from the rare-earthbased magnet can burn in air and create sparks.
Disposal
The only material to he disposed of is excess liquid nitrogen. I t may he allowed to evaporate from its container, or
slowly poured onto the floor. Dispose of excess liquid nitrogen promptly; see the cautionary note ahove.
2
852
Journal of Chemical Education
Dlscusslon
From experiments with magnets and iron filings, evidence
can be seen of lines of force surrounding magnets. These
lines of force compose a magnetic "field", the strength of
which varies a t different locations relative to the magnet.
Whenever an electricallv conductive material oasses
through the magnetic field, a current is induced in the conductor. The maenitude of the current is affected hv the
electrical resistance inherent in the conductor. ~ h i s ' p h e nomenon, announced in 1832 by both Joseph Henry in
America and Michael Faraday in England, is now called
electromaenetic induction. Hans Oersted in 1820 demonstrated t h i t a wire carrying an electrical current would deflect a compass needle placed nearby. The compass needle is
of course 'magnet, and this showithat an electric current
induces a magnetic field in the space surrounding the conductor. Commercial electric power generation exploits these
effects by the use of conventional conductive materials such
as copper wire. However, even these materials have some
resistance to electrical current flow.
For manv. vears
. i t has been known that certain materials
(such as mercury) lose all resistance to the flow of direct
rlectrical current when rooled 1)elow a certain critical temperature. This phenomenon, together with the Meissner effect, is called superconducrivity. A manifestation of the
hleissner effect is the Ievitnrion crf n magnet above u superwnducting material. M w h g a magnet near the superconductor induces n supercurrent (i.c., a current [hat does not
decay) -xithin the superconducu)r. This supercurrent generates its own maenetic field such that the total magnetic
field
"
inside the superconductor disappears. The two opposing
fields, one from the magnet, and the one induced by the
superconductor, cause the magnet to he repelled by the
sunerconductor. iust as two like maenetic poles repel each
otker. If the rep;lsive force at the sirface of the sGperconductor is greater than the force of gravity on the magnet, the
magnet will he levitated at a distance from the superconductor such that the gravitational force downward is counterbalanced by the re&ive force at that distance.
Until recently the critical temperature for all materials
that exhibit superconductivity was extremely low, near the
boiling point of liquid helium, which is 4 K. As a result,
because of the expense of maintaining such low temperatures, applications of superconductivity were limited, althoueh the ~ o t e n t i a lbenefits of the phenomenon were a
subject of wihe speculation. Rapid advinces have been made
receutlv. in svnthesizine
materials that exhibit the phenome.
non of superconductiv%y a t much higher temperatures (4).
I t is now possible to achieve superconductivity at the temperature of liquid nitrogen (77 K, -196 OC), a coolant that is
much less expensive to prepare and store. This demonstration uses one of these materials, the "1-2-3" pellet first
reported earlier this year ( I ) . We also use a samarium-cobalt
magnet, which has a particularly high field strength. This
provides a more effective demonstration, since the samarium-cobalt magnet will create more repulsive force and cause
greater levitation from the oxide pellet than will a similarsized conventional magnet with lower field strength.
-
Note
If facilities for preparing the "1-23" superconducting
pellets are unavailable ( 5 ) ,the Institute for Chemical Education of the University of Wisconsin-Madison has a limited
supply of pellets and magnets ("levitation kits") and will be
happy to supply them at cost. Details may he obtained by
writing to the Institute.
Acknowledgments
We wish to thank Heidi Grant and B. Z. Shakhashiri for
providing the samarium-cobalt magnet mentioned above.
We are grateful to E. Hellstrom, J. Nordman, and D. Larhalestier for helpful comments, to T. Harkins for experimental
PROTOCOLE 624
~ . ~ ~ .
~
~~~~
reference 5.
~ i v ior~chemical
i ~ ~ E ~ u c ~E ~~ ~
S ~o O
PA,
~~ :,1965.
3. Shakhashiri, B. 2. Chemical Demonstrations; University of Wiseonain: Madison, WI,
1985: Vol 2, p 26.
Literature CHed
4. For an oveniew of the recent sdvsnees and the significance of c u m n t aupermnductivity reaparch, SPO Dagani,R. Cham.Eng. News 1987,ffi (May 11). 7.
1. Our synthesis is hasod on those reported in: (a) Wu, M. K.:kshbum, J. R.;Tomg,C. J.;
5. Adeseriptionof a s y n f h ~ i s o1-2-3
f
pelletausing faeilitieathatmsy heavailablein h'ih
Hor,P.H.:Meng,R.L.:G~,L.;Huang.Z.J.:Wsng,Y,Q.;Chu,C.W.Phys.Reu.Letf.
schmls may be found in Grant, P. M. New Sci. 1987,115 (July 301.36.
19R7.58.908; ibl Cava. R. J.; Batlogg, B.: uanDouer,R. B.; Murphy,D. W.;Sunahinc,
3
Volume 64
Number 10
October 1987
853
PROTOCOLE 624
A Simple Demonstration of High TcSuperconductive Pow* ..
Roger Baker
1303 Bentwood, Austin, TX 76722
James C. Thompson
Department of Physics, University of Texas, Austin, TX 78712
The photos were obtained with a magnet constructed of 16
small ceramic magnets glued between two steel sheets with
silicone rubber c i m e n t r ~2 X 40-mm gap was formed by
cutting one sheet in the center. The bottom of a soft drink
can was used to contain the powder. The liquid level need he
only a few mm above the bottom of the can. The can bottom
is the circular obiect in each ~ h o t oIn
. Fieure 1.the diaeonal
white stripe is t i e high-fieldAregionand ;he grky smudie on
either side is the powder. In Figure 2, the two irregular dark
areas are formed from mixed powder after swirling the Dewar for several seconds;. again
- the highest field is between
them.
Expulsion of a magnetic field (Meissner effect) and the
conseauent diamaenetism is one manifestation of superconducti\,'ity. Since the new copper oxide-tlased superfondurtorsare produrrd in the formof powders it turnsout that the
.Mris~nereffect is much more easily demonstrated than is
h i ~ hrundurtivity. This note descrit~esa simple demonstration that also provides a way to determine if a given sample
contains even a small frartion of superconductinp material.
The rmulsion ofthe oowder from a maenetic
field is inrlica"
tive of superconductivity.
The procedure follows the standard elementary demonstration used to map out a magnetic field. In the usual
demonstration, one sprinkles iron filings onto a nonmagnetic surface over a magnet. The filings are magnetized and
oriented by the field. The pattern of filings makes the regions of high and low field visible. In the present case, one
sprinkles the superconductor powder into a small Dewar
flask containing liquid nitrogen and a permanent magnet. In
contrast to iron, the superconducting powder avoids high
field regions leaving a bare spot close to the magnet. For a
minimum display, it is possible to use a piece of white paper
on top of an ordinary "refrigerator" ceramic magnet. Only a
little refinement yields a cleaner result.
Refinements
First, i t is better to observe the powder pattern on a metal
surface to avoid electrostatic charges. Second, i t is better to
use a concave surface so as to concentrate the powder over
the magnet. This also allows the suspension in liquid nitrogen to he gently rocked from side to side, much as gold is
panned. Two percent of superconducting powder ground
together with 98% of inert copper oxide can be detected in
this way. Finally, using an arrangement yielding a gap hetween two poles increases the field.
Powdered material is snrinkled into the dish from ahove
the liquid surface. If the powder is totally superconductive,
then it will arrange cleanlv on o.~.o o s i t esides of the aaD
.. . (Fig.
1). I f the powderk .illghtl; contaminated with foreign material. then the nmsu~erconductiveort ti on will fall inside the
gapwith the remaiider clearly delineated a t thesides. If the
powder contains only a small percentage of superconducting
material (2% in the photos in Fig. 2) no separation is
achieved. but the Dewar mag then be rocked from side to
side, causing the powder toshbw aslight tendency toarrange
itself in two parallel lines to either side of the gap.
Figure 1. The superconductor demonstration performed using the bonom of
a soda can as a Dewar.
This work was supported in part by the US NSF through MRG Gram
No. DMR8418086. The photos were taken by K. I. Trappe. H. Steinfink
provided the superconductive powder.
Figure 2.
The demonstration after swirling the container.
4
Volume 64
Number 10 October 1987
853
PROTOCOLE 624
Preparation, lodom tric Analysis, and Classroom
Demonstration of uperconductivity in YBa2C~308-x
2
Daniel C. Harris, Marian E. Hills, and Tenell A. Hewston
Chemistry Division, Research Department, Naval Weapons Center. China Lake, CA 93555
After the discovery in 1911that mercury loses itselectrical
resistivity when cooled below 4 K, superconductivity was not
seen above 23 K in other materials until 1986. In that year
the floodgates of research were opened by a report that an
oxide of barium, lanthanum, and copper was superconducting up to 35 K ( I , 2). The transition temperature of this
compound, Ba,La2-,CuOa (z a 0.15) (3-9, was exceeded in
1987 by that of another oxide, YBa2Cu30s-, ( x I),which is
superconducting up to 100 K (6, 7). This article describes a
student preparation of YBanCusOs-,, a classroom demonstration of its superconductivity, and an analytical chemistry experiment dealing with the oxidation state of copper in
the material.
30
-
25
-
.
.
.
.'
5 20 o
ui
0
f "'
z 15 -
sL"
..
:10 -
0)
Superconductors
As the name implies, one outstanding property of a superconductor is zero electrical resistivity when it is cooled to a
sufficiently low temperature (Fig. 1).Once started, electrical
current in a superconducting ring will continue forever unless a force is applied to change the current. How long is
forever? An experiment (8) with the superconductor Nh3Zr
showed that the current decayed less than one part per
billion per hour, implying a resistivity of less than
ohmm.
A second outstanding feature of superconduetors is that
the magnetic field, B, inside a bulk specimen is zero. This is
called the Meissner effect. When a field is applied to a
specimen, currents flow in the outer skin of the material such
thatthe applied field is exactly opposed by the induced field,
and the net field inside is zero.' A magnetic field that decays
exponentially as i t enters the bulk is in the skin of the
supercouductor. The depth at which the internal field decreases to l/e times the external value is called the penetration depth and is of the order of 10-100 nm at temperatures
well below the superconducting transition temperature.
A paramagnetic substance is attracted into a magnetic
field and a diamagnetic substance is repelled by a magnetic
field. Figure 2 shows the magnetic susceptibility2 of a sample
as a function of temperature. The temperaof YBa~Cu30~.65
ture near 100 K at which the susceptibility begins to curve
downward is considered to be the onset superconducting
transition temperature, T,. Above T, this sample is paramagnetic, but it becomes strongly diamagnetic as the temperature is lowered.
If the applied magnetic field exceeds a critical value, superconductivity is lost. In a TypeImaterial (Fig. 3), the field
5
-
I
o
100
150
TEMPERATURE, K
200
Figure 1.Direct current electrical resistance of a pellet of YBaPu.0.~.
9
t;
z
o
2
+0.005
.
/cL---TRANSITION
0
TEMPERATURE
IT,)
k
-0.015
Y)
3
0)
2 -0.020
z
2 -0.025
I
?
.
' The magnetic field B is also called the magnetic induction and is
measured n SI units of tesla (T).The field inside a solid is equal to the
app1:ed feld B. plus the inducedfield,mM: B = B.- 7 ."OM,
- where no is
the permeabili~vof free ~ ~ aand
c e~ itheimaanetization induced in
the solid. The value of B is zero inside a suoe&onductor. so "AM =
-6, Magner~zaton is eqdai to the magnebc doole moment & unlt
vo .me an0 nas the unlrs amperes per meter For a current t tlowmg
around a loop of area a, the magnetic dipole moment has the magnitude 1 . a, with units of amperes. meters2.
Magnetic susceptibility is measured by observing the force exerted on a solid by a magnetic field gradient. its measurement and units
are described in references Sand 10.
I
i
I
50
I
I
80
100
120
140
I
I
160
I
I
180
I
I
200
TEMPERATURE. K
Figure 2. Magnetic susce~tibilityof YBa.Cu.0.
ss Cer
.. mole of Cu) corrected
far the diamagnetic contributions of Me elements. The molar magnetic s u s
ceptibllity in the SI units of this figureis equal to 4r X lo@ times the molar
susceptibility in cgs units.
~
5
Volume 64 Number 10 October 1987
847
PROTOCOLE 624
v
I
Bc
APPLIED FIELD
SUPER-
( C O N O U C T I N G FVORTEX
~
STATE
TNoRMAL-
TYPE II
Figure 3. Behavior of superconductorsin a magnetic field.The direction of the
induced field opposes the applied field.
induced in the superconductor is equal in magnitude (hut
o ~ ~ o s iin
t edirection) to the aDDlied
field.. UD
..
. to the critical
firid. B,. At this point, the material luars its superconducri\.its.The valucoftherritical field isa functionoftem~erature
and decreases as the temperature increases, hecoking zero
a t T,.YBa2Cu30s-, is a Type I1 material in which the applied magnetic field is exactly cancelled by the induced field
up to a lower critical field, BC1(Fig. 3). Between BC1and Bc2
the applied field is not completely balanced by the induced
field, and the material is said to be in a vortex state. The
sample loses superconductivity above Bc2.A cylinder of material in the vortex state contains filaments in the superconducting state and filaments in the normal state. As lone as
superrunducting filaments are present, there is a superronductinr uath from one end to the other. and the resistivltv is
e
field is increased, the fraction of n o r i a l
zero. ~ i i h applied
material increases and the fraction of su~erconductinr!material decreases.
0 COPPER
Structure of YBaZCunOa-,
Figure 4 shows the unit cell of YBazCun07,which is derived from a defect perovskite structure. Defect structure
means that vacancies (unoccupied atomic sites) are present.
There are ~ositionsfor nine atoms of 0 in the unit cell. hut
only seven are occupied.3
Why are there not nine 0 atoms in the unit cell? The
normal oxidation states of Y and Ba are +3 and +2, respectively. If all Cu were +2, the formula of the compound would
~ ~ 6.5 02-needed to balance the cation
he Y B a z C ~ 3 0with
charges. If all Cu were Cu3+,which is an unusual oxidation
0 OXYGEN
(2OXYGEN
VACANCY
state for Cu, the formulawould he YBa2Cu30s.The observed
composition is variable, hut hovers near YBa2Cu307,which
implies that one third of the Cu is Cu3+and the remainder is
C o z - .This assignment of oxidation states is purelv formal.
populaThe material is metallic ahore 7;. so it must have a .
.
tion of mobile electrons.
The structure in Figure 4 is an idealization emphasizing
the location of oxygen vacancies and the approximately
When counting atoms In the unit cell in Figure 4, remember that
most atoms in the diagram are shared by more than one unit cell.
Atoms on a face of the cell are shared by two cells, so the number of
these atoms must be divided by two. Atoms on edges are shared by
four unit cells, and those at vertices are shared by eight unit cells.
848
Journal of Chemical Education
6
PROTOCOLE 624
LIQUID NITROGEN
LEVITATING MAGNET
SOLID SUPERCONDUCTOR
STYROFOAM
\
OVERHEAD
PROJECTOR
Figure 6. Projected image produced by apparatus in Figure 5.
net pruduce a magnetic firld that repels the magnet.
Tc, show to an entire clash, the lwitarion experiment is set
upon thelitayeof an overhead projector, asshown in Figures
5 and 6. PIXI: the magnet on top of the solid superconductor
in a low-cut expanded polystyrene cup. TWO-mirrorssupported at 45' angles direct the projector light across the
superconductor. T o bring the image into focus, move the cup
along the horizontal line between the two mirrors. Liquid
nitroaen is ooured into the shallow cun.
. . and the sunerconducrmr hegins I O cool. When i r cools sufficiently, themagnet
~lmrnaricallsrises and remains susoended to the delieht of
all observers.
Figure 5. Demonstrating magnetic levitation with an overhead projector.
square planar coordination of Cu by 0. All 0 positions in the
horizontal plane containing Y are vacant. The other vacancies are located in the top and bottom Cu-0 planes. Each Cu
atom is surrounded by four 0 atoms with a Cu-0 distance of
approximately 194 pm. The coordination of the eight Cu
atoms in the planes ahove and below Y could he considered
as square pyramidal if the next-nearest 0 atoms at 238 pm
are included. The eight 0 atoms surrounding the Y atom are
not coplanar with the Cu atoms as drawn hut are puckered
out of the Cu planes toward the Y atom. An idealized description of the Cu-0 network is that there are horizontal
sheets of vertex-sharing Cu04 squares. These alternate with
perpendicular CuO4 squares sharing two opposites vertices
to form infinite strings.
lodometric Analysis of Copper Oxldation States In
YBa2Cu308-,
In Experiment A Y B ~ ~ C U is~ dissolved
O ~ - ~ in dilute HCI,
in which Cu3+is rapidly reduced to Cu2+(11)
Preparation of the Superconductor
Place in a mortar 0.750 g of Yz03, 2.622 g of BaC03, and
1.581 g of CuO (atomic ratio Y:Ba:Cu = 1 2 3 ) . (Ordinary
reagent-grade chemicals are suitable and the scale can he
varied over a wide range.) Grind the mixture well with a
pestle for 20 min, and transfer the powder to a porcelain
crucihle or boat. Heat in the air in a furnace at 920-930 "C
for 12 h or longer. Turn off the furnace, and allow the sample
to cool slowly in the furnace. This slow coolingallows atmosphericoxygen to he taken up by the sample and produces the
desired oxygen content. The crucihle may he removed when
the temperature is below 100 'C. The hlack solid mass
can he gently dislodged from the crucihle for the demonstration described below. Alternatively, the solid mass may he
ground for 20 min and reheated to produce higher quality
material. If the powder is green instead of hlack, raise the
temperature of the furauce by 20 "C and reheat.
The total Cu content can then he measured by treatment
with iodide
Cu2+(aq)+ 21-(aq)
-
CuI(s) + '&(aq)
(2)
and titration of the liberated iodine with standard thiosulfate
Each mole of Cu in YBa2Cu308-, is equivalent to one mole of
S ~ 0 3 in
~ -Experiment A.
In Experiment B YBa2Cu308-, is dissolved in HCI solution containing I-. In this case, the C I P selectively oxidizes
two moles of I' (and precipitates with a third mole)
Classroom Levitation Demonstration
The solid hlack mass of YBazCuaOs-, taken directly from
the crucihle is used for this demonstration. Alternatively,
the hlack powder obtained by grinding the mass can he
gently pressed into a crucihle (or pressed in a pellet press)
and reheated. To demonstrate superconductivity to a small
group, cool the solid product in liquid nitrogen and place a
small magnet such as a 1.5- X 8-mm Teflon-coated stirring
bar over the superconductor. If positioned carefully with a
plastic tweezer, the magnet will levitate in the air ahove the
superconductor. Alternatively, place the magnet on the solid
before cooling, and the magnet will rise when the solid is
cooled. Currents induced in the superconductor by the mag-
The moles of S ~ 0 3 required
~to titrate the liberated I2 are
equivalent to Cu2+ 2Cu3+in Experiment B.
Experiment A gives the total Cu content of Y B ~ ~ C U ~ O ~ - ~ ,
and the difference hetween the results of Experiments A and
B gives the Cu3+ content. With these two pieces of information, it is possible to calculate the value of x in the formula
Y B ~ & U ~ ODifferent
~ - ~ preparations give values of x that
hover about the value x = 1.It is instructive to carry out each
titration three times. Using the standard deviations as a
measure of the uncertainty of each result, carry out a propagation of uncertainty analysis to find the uncertainty in the
value of x .
+
7
Volume 64
Number 10
October 1987
849
PROTOCOLE 624
.
.
endpcnt is ha'ider to distinguisli. The Na2S203should he standardized three or four times and the average molarity computed.
T h e results of iodomktric & l y s i s are in good agreement
with those calculated from the mass lost b y YBazCusOs-,
when i t is heated to 1000 OC under Hz (11)
Experiment A
Accurately weigh 150-200 mg of YBazCuaOs-, and dissolve it in
10 mL of 1.0 M HCI in a titration beaker in a fume hood. Boil gently
far 10 min to ensure destruction of Cu3'. Cool to room temperature
and place the cork with gas inlet and buret on the beaker and begin
gas flow. Quickly add 10 mL of water containing 1.0-1.5 g KI and
titrate as above with magnetic stirring.
0.03MNa2Sp03 Solution. ~issol've3.7 g of NazS203.5H20 plus
0.05 g of Na2C08 in 500 mL of freshly boiled distilled water. Add 3
drops of chloroform and store in a capped amber bottle. The
NaZCO3and chloroform act as preservatives. The solution is stable
for a period of days or weeks but should be restandardized after
several weeks.
Starch Indicator. A slurry containing 1gof soluble starch plus 1
mg of HgIp (a preservative) in 10 mL of distilled water is poured into
90 mL of boiling distilled water to give a nearlv clear solution that
should he stablefor several weeks in a closed bottle.
Standard Cu. Weigh accurately 0.5-0.6 g of reagent Cu wire into
a 100-mL volumetric flask. In a fume hood, add 6 mL of distilled
water and 3 mL of 70%nitric acid and hoil gently on a hot plate until
the solid has dissolved. Add 10 mL of distilled water and boil gently.
Add 1.0 g of urea or 0.5 g of sulfamic acid and hoil for 1 min to
destroy HNOz and oxides of nitrogen that interfere with the iodometric titration. Cool to room temperature and dilute to 100 mL
with 1.0 M HC1.
Standardization of Nn2S203With Cu. The ease of I- oxidation
by O2 in acid requires that the titration he done as rapidly as
possible under a brisk flow of N2or AI. The titration vessel is a 180mL tall-form beaker (or a 150-mL standard beaker) with a loosely
fitting two-hole cork a t the top. One hole serves as the inert gas inlet
and the other is for the buret. Pipet 10.00 mL of standard Cu into
the beaker and flush with inert gas. Remove the cork just long
enough to pour in 10 mL of distilled water containing 1.0-1.5 g of KI
(freshlv dissolved) and heein mametie stirrine. Titrate with
~ a. A~0. .solution
,
in a 50-mi huret.addine 2 droos of starch iust
bri.,rr the last trace of l2 cdor disappears. If starch is added rolr
soon,there ran be irrwersible attarhment uf 12mthestarchand the
Emeriment B
Arcurattdy weigh 1.3-20U rng of YRnrCu O,., into the trtratbn
vesscl, and hegin mert ya.i flou. Add 10 mL uf 1.0 11 IICI O 7 M KI
and rrir mncnrtically fur 1 min. Add 10 ml. of distilled water and
complete the titretion as above
Acknowledgment
W e are grateful t o K. T. Higa a n d W. A. Weimer for
helping devise t h e levitation demonstration. D a t a for Figures 1a n d 2 were kindly provided b y B. Chamberland, University of Connecticut. T h i s work was supported b y t h e
Office of Naval Research.
Literature Cited
"
-
850
Journal of Chemical Education
."".,."",
8. Pile, J.: Mil1s.R. G.Phya. R e ~ L e f f 1363.10.93.
.
9 . Lindoy, L. F.; KataviC: Buseh, D. H. J . Chsm. Educ. 1972,19,117.
10. Pars, G.;Suteliffe, H. J.Chem.Educ. l971,48,180.
11. Harris, D. C.; Hewt0n.T. A. il Solid State Chem. 1987,in prwd
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