master’s thesis m-698 law, catherine ann. the thallium
TRANSCRIPT
M ASTER’S THESIS M -698
LAW, Catherine Ann. TH E TH A LLIU M -TH A L.LO US SU LFA TE M ERCURY-MERCUROUS SU LFA TE C ELL.
The Am erican University, M, S ., 1964 Chemistry, Physical
University M icrofilm s, Inc., A nn Arbor, M ichigan
THE THALLIUM-THALLOUS SULFATE MBRCURY-MBRCUROUS SULFATE CELL
byCatherine Ann Law
Submitted to the Faculty of the College of Arts and Sciences
of the American University
in Partial Fulfillment of the Requirements for the Degree
ofMaster of Science
Dean oè €iie CollegeDate:
Signatures of Committee:
Date:
1964
The American University Washington, D.C.
AMERICAN UNIVERSITY LIBRARY
AUG 311964WASHINGTON. DC
#= 503 7
ii
TABLE OF CONTENTSSECTION PAGE
I, INTRODUCTION 1
a. Historical I
Clark cell 2
Weston cell 2
Limitations of both 3Efforts to eliminate limitations 3
b. Proposal of thallium cell 5
Calculation of emf. 6TlgSO^ as electrolyte 9
Thallium amalgams 11
Calculation of temperature coefficient 17
II, EXPERIMENTAL
a. Preparation of Materials. ' 20
Mercurous sulfate 20Thallous sulfate 21
Thallium amalgams 28
Cell blanks 30
Assembly of cells 30
b. Electromotive force measurements. 32III. CALCULATION OF RESULTS 37
a. Calculation of temperature coefficient 37
b. Calculation of thermodynamic functions 37
Calculation of AG 37
Calculation of AH
tii
TABLE OF CONTENTS (CONTD’S)
IV. Conclusion 40
a. Summary of results 40
b. Work to be done 40
IV
LIST OF TABLES
TABLE PAGE
I. Thermodynamic Data on Thallium and Mercury 7
II. Solubility of Thallous Sulfate in Water 12
III, Spectrochemical Analysis of Mercurous Sulfate 23
IV, Spectrochemical Analysis of Thallous Sulfateand Thallium 25
V. Spectrochemical Analysis of Thallous SulfatePrepared from Thallium 29
VI, Weights of Mercury and Thallium in 55Percent Thallium Amalgams 31
VII. Electromotive Forces of Thallium Cells 34
VIII, Temperature Coefficient of Thallium Cells 58
LIST OF FIGURES
FIGURE PAGE
1, Diagram of Weston of Cadmium Sulfate Cell 4
2, Electromotive Forces of Amalgam Cells 15
3, Electromotive Forces of Cells with DifferentPercents of Cadmium Amalgams 14
4, Phase Diagram for Mercury-Thallium 15
5, Phase Diagram for Mercury- Cadmium 16
6, Circuit Diagram and Apparatus for Preparationof Mercurous Sulfate 22
7, Diagram for Electromotive Force Measurement 35
VI
ACKNOWLEDGMENT
The author wishes to express her gratitude to
Dr. Bernard Miller and Dr. Walter J, Hamer for their helpful
suggestions and advice during the course of this work.
I. INTRODUCTION
a. Historical. The Weston standard cell is used for the maintenance
of the volt In the United States, and as a standard in primary stand
ards laboratories throughout the country. Since cells which are of
the same type may increase or decrease in emf at the same rate, it
would be desirable to have an alternative type of cell as a standard.
For it would be improbable that another type of cell would change at
the same rate at the same time and in the same direction as the Weston
type cell. The ratio of the two types of cells over a period of
years would give information concerning the stability of the unit of electromotive force( emf).
Among the fundamentally important points in the consideration of any system as a possibility for a standard cell are 1.) its
constancy, 2.) its reproducibility, 3.) the value of its emf, 4.) its
temperature coefficient, and 5.) the possibility of hysteresis. This
study has the purpose of proposing a new cell, making the cell,
measuring the emf, determining the temperature coefficient and also
to determine if hysteresis is present. The long term areas of research,
that is, the stability and the reproducibility will be discussed later.
Some of the cells which have been proposed as standards of
electromotive force are^ the DanidD l cell (1836), the Clark cell (1872),
the De la Rue cell (1878), the Helmholtz cell (1882), the Weston-Clark
cell (1884), the Gouy cell (1888), the Carhart-Clark cell (1889), and
George W, Vinal, "Primary Batteries",John Wiley& Sons, iHc. New York, New York, 1950, p. 165
the Weston cell (1893), The only ones to be used for any period of
time are the Clark and the Weston cell.The Clark cell. The Clark cell
(-)Zn(Hg)|ZnSO^« 7 ZnSO^ sat. sol.| H g g S O ^ | ^ + ) (1)
was adopted as the cell to be used by international agreement in
1893 by the International Congress which met in Chicago. The Clark
cell has a zinc amalgam anode which consists of the liquid amalgam
and solid zinc. The cathode is a mercury, mercurous sulfate electrode. The electrolyte is a saturated solution of zinc sulfate. In saturated
solutions zinc sulfate hydrolyzes to give a soltition containing
0.004 N sulfuric acid. This concentration of acid is sufficient to
prevent the hydrolysis of the mercurous sulfate at the positive
electrode.The Weston cell. In 1893 Edward Weston proposed the weston
or cadmium cell. The cell was adopted in 1911 as the standard for
maintenance of the International volt and in 1948 its emf was defined
in ttfrms of absolute volts. The cell is being used at the present
time by all countries maintaining voltage standards. International
comparisons of the volt made every two or three years by the Interna
tional Bureau of Weights and Measures are accompl iAmH us ing Weston
cells as the means of comparing voltage standards of the various
participating countries.^ The Weston cell, which is constructed in
2Francis B. Silsbee, "Establishment and Maintenance of theElectrical Units,"National Bureau of Standards, Circular 475.
M. Romanowski, Travaux des Poids et Mesures, 21.43(1952)
M. Romanowski, Travaux et Mémoires du Bureau International
an H - Shaped container (Figure 1. ), has a cadmium amalgam anode
and a mercury - mercurous sulfate cathode. The electrolyte is a
saturated solution of cadmium sulfate,(.)Cd(Hg)(2p)j CdSO^'B/SH^O^g) |cd80^ sat. aol. f Hg^SO^^^ Hg^ j(+) (2)
Limitations of both cells. The Clark cell has several
disadvantages. 1, The temperature coefficient of the cell is about
thirty times that of the Weston cell. 2. A transition point,
ZnSO^ . 7 H^O to ZnSO^ • 6 H^O, occurs at 39°C. 3. In the Clark cell
gas, probably hydrogen, evolves over the surfate of the amalgam,
pushing the crystals of zinc sulfate up until electrical contact is
“broken. 4. Zinc alloys with platinum so that the platinum wire
through the glass ( Figure 1.) breaks.The Weston cell also has several disadvantages. I. The
mercurous sulfaté is hydrolyzed by cadmium sulfate solutions to form
mercurous oxide. Tests show that the stability of the cells is
lowered by formation of the latter compound. 2, A transition point,
CdSO^* 8/3 H^O to CdSO^ • H^O, occurs at 43.6°C . 3. The Weston cell
has a temperature coefficient of 53.9 microvolts per degree at 28°C.
Although with proper temperature control this is relatively small,
it would be desirable to have a cell with a smaller temperature coefficient.
Efforts to eliminate limitations. In the Clark cell the disadvantages of breaking leads can be overcome by using a different
L.H.Brlckwedde, J. Research Natl. Bur, Standards, 36,377(1946)
I < , /'y y- -- :
I f f e i ^
c
C
b
type of seal. However, the unfavorable temperature coefficient and the transition point still would be disadvantages.
In the Weston cell the hydrolysis of mercurous sulfate
in cadmium sulfate solutions has been overcome by the addition of
dilute ( 0.023 to 0,050 N ) sulfuric acid to the electrolyte.
Attempts have been made to reduce the temperature coefficient by
the addition of salts to the electrolyte or by using three component
amalgams^»®. These cells do not have the stability of the regular
Weston ot cadmium sulfate cell. Another modification was made by
substituting D^O for the HjO in the cell^. This cell has an emf
about 350 microvolts lower than the regular cadmium sulfate cell.
Cells containing "heavy water" have almost the same temperature coefficient as the cells with normal water. However, the hysteresis
effect is less in the "heavy water" cells. ( Hysteresis as defined in standard cell.<»usage is the temporary deviation from the correct emf value which follows an abrupt change in temperature. It is
usually greater when the temperature is decreased than when the
temperature is increased.) Even if the hydrolysis of the mercurous
sulfate can be eliminated and the temperature coefficient can be
reduced, the transition point would be somewhat of a disadvantage.
b. Proposal of the thallium cell. With the preceding background
-, ^W.C.Vosburgh, M.Guagerty, W.J.Clayton, J.Am. Chem. Soc.,59 , 1256(1937)
^W.C.Vosburgh, and H.C.Parks, J.Am. Chem. Soc.6l}652(1939)7L.H.Brickwedde and G.W.Vinal, J, Research Natl. Bur.
Standards. 20,599(1938)
in mind the following cell is proposed for investigation as an emf standard:
(-)Tl(Hg)(2p)|Tl2S0^(g) sat. aol. jng^SO^^^j |Hg^j j(+0 (3)
The cell would have a thallium amalgam anode and a metcury - mercurous sulfate cathode; the electrolyte would be a saturated solution of
thallous sulfate.
Calculation of the emf. The emf of the cell can be calcu-plated by using the appropriate data from Table I. Since all the
necessary potentlàl values are not available, the free energy of
formation has been used to calculate the potential of the cell.
The reaction for the cell is
HggSO^ + 2 T1 --^ TlgSO^ ♦ 2 Hg (4)
The free energy of the reaction, A G° , is the sum of the free energies
of formation of the products of t he reaction, less the sum of t he
free energies of formation of the reacting substances.
(5)AG® * r (HgjSO ) (Tl)_Substituting the values from Table I, in equation (5),we have the following :
AG® V 1 - 196.8 + 0 ] - [-149.12 + o j (6)and
0A G — —47.7 kcal. (7)Since the potential or emf is related to the free energy by the equation
8These values are from Wendell M. Latimer, "Oxidation
Potentials," Prentice-Hall, Inc. Englewood Cliffs, New Jersey, 1952, pp.164 and 176.
TABLE I,THERMODYNAMIC DATA ON THALLIUM AND MERCURY
(Heat and free energy of formation in kaal. Entropy of substancein caX/deg.)
Formula State A G° A S®
Hg liq 0.0 0.0 18.5HgjSO^ c -177,34 -149.12 47.98
T1 c 0,0 0.0 15.4
TljSO^ c -221.7 -196.8 (52.8)*
^estimated value
99A G ■ - n E J (8)
the emf of the cell can be calculated. Substituting in equation (8),
we have-47.7 - 2 X 96^487 ( E )
4.1840
E = 1.034 volts
The potential of a cell is given by the equation
(9)
(10)
E = E - RT In n ? ®(Hg) * ^(TlgSO^)
,, (Hg^SO^)X a ( Tl)
(11)
where E is the potential of the cell and E the potential when all
the activities are unity, and where R is the gas constant, n and F
have the significance given in footnote 9, and a is the activity of
the substance designated by the subscript. The activities of the
thallium and t he mercury are equal to one for the pure metal.
However, in the proposed cell the thallium is an amalgam, and the
activity is not the same as forthe pure metal.
Richards and Daniels^^ in their work on thallium amalgams
give the potential of a two-phase amalgam ( in this case 49.48%)
versus pure thallium at 20°c to be 2,5 millivolts and at 30®C to be
In this equation n = the number of equivalents per mole which for this reaction is two and JT = the Faraday, which is equal to 96,487. This value of the Faraday is based on the C scale of atomic weights. In the original paper, D.N.Craig, J.I.Hoffman,C.A.Lawy and W.J.Hamer, J.Research Natl. Bur. Standards,64A, 381(1960), the values based on the physical and chemical scales of atomic weights 96,516.5 and 96,490.0, respectively, are given.
10(1919)
T.W.Richards and F.Daniels, J.Am. (Jhem. Soc. , 41,1732
fi
2,7 millivolts with the note that these are only approximate
figures. Assuming they are correct and interpolating the results,
we have the potential of 2.6 millivolts at 25®C, This would give
B = 1.034 - 0.0026 (12)E = 1.031 (15)
Tl^SO^ as electrolyte. The proposed cell would have thal
lous sulfate as the electrolyte. Since this salt is not a hydrate,
it would not be supposed that a transition point would occur as in
the case of ZnSO^* 7 H2O and CdSO^ • 8/3 HgO. Most salts exhibiting
transition points ate hydrated and lose one or more waters of hy
dration or the salts have two crystalline forms, such as BaClg from
the monoclinic to the cubic form at 925®C. Another problem is thedecomposition of the salt in aqueous or acid solutions, especially at higher temperatures. No mention of a transition point or #f
decomposition in aqueous or acid solations was found in the literature. Because thallium has two valence states, another problem
was considered. If thallous sulfate crystals were placed over the mercurous sulfate, it would be possible that the thallous sulfate
could be oxidized to thallic sulfate by the mercurous sulfate according to the following reaction
Tl^SO^ + 2 Hg^SO^ + 7 H^O — >TlJSO^)y 7 H^O +4Hg (14)
For this reaction to take place AG would have to be negative. However,
/\G could not be calculated for the reaction, since the free energy
of formation of thallic sulfate is not given in Latim^or in NBS
Latimer, cit., p.164
10
12'Circular 500 . In order to determine if the reaction will occur,
white mercurous sulfate ( that is mercurous sulfate without free
mercury present ) was prepared chemically by the addition of
sulfuric acid to an aqueous solution of mercurous nitrate. The
white mercurous sulfate was washed with water several times and then
with dilute sulfutic acid and the solutions decantéd after each
washing. A saturated solution of thallous sulfate was then added
and the solution allowed to stand over the mercurous sulfate with
occasional shaking in a darkened room (see later). If mercury was
formed, it would cause darkening of the white ipsrcurous sulfate or
visible mercury droplejcs. After ten days no evidence of mercury
was observed. This would indicate that crystals of thallous sulfate
can be placed directly over, and in contact with the mercurous
sulfate.
Since it seems likely that crystals of thallous sulfate
can be placed over the mercurous sulfate, as well as over the amalgam,
then the composition of the solution (saturation) over each electrode
after a change in temperature should be very nearly the same and the :
length of time for equilibrium of the electrolyte would be reduced.
Ifccyystals could be placed only over the amalgam electrode, the
equilibrium of the electrolyte would have to depend on diffusion
after a temperature change.
In order to tecrystallize the thallous sulfate and to
£2 Frederick D.Rossini, Donald D.Wagman, William H.Evans, Irving Jaffe and Samuel Levine, "Selected Values of Chemical Thermodynamic Properties", Natl. Bur. Standards Circular 500,(1952).
11
prepare saturated solutions, the solubility of the thallous sulfate13should be known. The solubility of the salt is given in Table II,
14Thallium amalgams, Richards and Daniels investigatedthallium amalgams and found that the amalgams give sharp and constant
vklües for their potentials in aqueous solutions of their salts.
Theyalso found that when the saturation point of the liquid amalgam
has been reached, an excess of thallium is without effect on the
potential. The saif curves break into a horizontal straight line ata point giving the concentration at which solid and liquid are in
equilibrium. (Figure 2. ) This is the aâae type of behavior observed15with cadmium amalgams . (Figure 3. )
Since a two-phase amalgam (equilibrium of solid and liquid) is desirable, (Figure 2,), the best percentage of amalgams can be ascertained from the phase diagram for Hg-Tl from the International
Critical Tables^^*^^(Figure 4.). The percent of amalgam usually used in the Weston or cadmium cell is 10 or 12 1/2% cadmium, de
pending on the temperature range over whiuh the cell is expected to
be used. ( These percentages give a two-phase amalgam for the usual
working range of temperatures for the cadmium sulfate cell.) (Figure S.)
^^Chatles D.Hodgman(ed. in chief). Handbook of Chemistry and Physics. 31® ed.( Chemieai Rubber Publishing Co., 1949, Cleveland) p.1420
14Richards and Daniels, op. sit., p.1732
l^Sir Frank Smith, Proc. Phya.Soc., (London) 22,11(1910)l^Edward W. Washburn(ed.in chief). International Critieal
Tables of Numerical Data, Physics Chemistry and Technology (New York: McGraw-Hill Book Co.)lI pp.429 and 436 <1927)
^^M. Hansen, "Der Aufbau der Xaeistofflegierungen, "Edwqrds Brothers Inc., Ann Arbor Mich., 1943,pp.422 And 816.
12
TABLE II.SOLUBILITY OF THALLOUS SULFATE IN WATER
Temperature Grams per 100®C grams of water
0 2.70
10 3.70
20 4.87
30 6.1640 ___
50 9.2160 10.92
70 12.74
80 14.61
90 16.55
100 18.45
13
200
180 4 0
160 2 0
140 30
100
60
40
20Percent T h a l l iu m I
Electromotive Forces ofAmalgam Cel Is
; FIGURE 2, i
.0245
.0235CP
0225
1.0215
.0205
ô t.0195
1.0185Solid and liquid
PhaseslOi
0175
olb
1.0165CP
•o1.0155
01450 4 8 12 16 20
J.4
. _ Percentage of cadmium __ELECTROMOTIVE FORGES OF CELLS WITH DIFFERENT PERCENTS OF CADMIUM AMALGAMS
FIGURE --------------- ----- -----
A '.A , J n ' . | : I ' i f J v -
Jiï
PI
U !
I•
g §2OQWCO$a,
a j n ; o j a d u i a j .
16
<2L
m
0»
“O ro
<\jro
E2E"OoO
a
gcCD 5
g
Qg 9jrn.DJadujsj_
19.
The percent of cadmium and thallium given in the diagrams is weight
percent and the temperature is in degrees Celsius.
The phase diagram for Hg-Tl ( Figure 4. ) shows a-two-
phase amalgam is present above 0°C for compositions of amalgam
containing more than 40percent and less than 84 percent thallium, and,
the<^+ liquid state is present at 82 percent from 0°C to 303°C,18The diagram Is basâd on the work of G.D.Roos . From his paper we
see that the most favorable percentage of amalgam is probably in the
51 to 63 percent range. At 50,68 atomic percent the temperature of
crystallization is 75.5°C and at 62.65 atomic percent the temperatureoof crystallization is 158.0 C. The temperature range which is consid
ered for thia cell would be 0°C to 100°C. If an amalgam that becomes
completely liquid at temperatures well above 100°C is used, the prob
lem arises of introducing the amalgam into the cell. After a consid
eration of the preceding data, the amalgam ^ich seems best would
then be one of about 55 percent.
Calculation of temperature coefficient. The temperature
coefficient can be calculated using the values for H from Table I,19the calculated E and the Gibbs-Helmholtz equation . The equation
is used in the following form:
ZIH = - n f E - T fiE (15)
18G.D.Roos, Zeit. fur Anorgan.Chem.. 94,358(1916)19Samuel Glasstone, "An Introduction to Electrochemistry",
D.Van Nostrand Company, Inc. New York, 1942,pp.194-5
18
where ziH Is the change of heat content for the cell reaction,E is t he reversible cell emf calculated above, T is the absolute
temperature ( °K ), n and ^ have the significance given in footnote
9, and / ^ j is the temperature coefficient at constant pressure.
(16)
p
Substituting in equation (15), we have- 44360 = -2 X 96,487 | 1.031 - 298.16 f ]L
/ IE I “ ^ 10 volts per degree (17)\ P
However, this value is for a cell with pure thallium for the anode
rather than for a two-phase thallium amalgam.
II. EXPERIMENTAL
a» Preparation of Materials, The materials used in electrochemical
cells have to be of high purity. All of the materials needed in
construction of this cell can be prepared from four starting materi
als, that is the only materials needed are mercury, sulfuric acid,«
water, and thallium. Each of these starting materials can be pre
pared or obtained commercially in a pure state. The preparation of
the other necessary materials for the construction of the cell is
described in the following sections.
Mercurous sulfate, Mercurous sulfate was prepared by the 20d c or Hulett method . In this method pure mercurous sulfate can
be prepared from mercury, sulfuric acid, and water. The mercurous
sulfate was prepared by placing 900.5 grams of mercury in an inner
shallow dish supported on a tripod in a deeper and larger dish.
( See Figure 6.) The mercury was Fisher reagent with the maximum
limits of base metals of 0*0001 percent and t he maximum limits of gold and silver of 0.0005 percent. 173.0 grams of sulfutic acid,
redistilled in an all Pyrex still, was shaken with mercurous sulfate
and mercury of pure grade to precipitate any ions which were less
soluble than the mercurous or sulfate ions. The redistilled sulfuric
acid was added to 544.0 grams of distilled water and added to the
vessel containing the dish of mercury. The mercury was made the anode and a piece of platinum foil was made the cathode and placed near the
top of the sulfuric acid solution. The stirrer was set in motion at all times while the current was on, A 20 volt source from lead
20 H.S.Carhart and G .A.Hulett, Trans. Am. Electrochem* Soc,, 5, 59(1004) '
21
acid storage batteries was used. The current was 0.9 amp. and the
current density was 0.2 amp. per square centimeter. The circuit for
the preparation ofthe mercurous sulfate is given in Figure 6.The mercury anode was stirred ( 76 to 80 rpm) so that the
mercurous sulfate was swept offithe anode, so that the reaction can continue at the anode, and so that mercuric sulfate will not be formed.
As the stirrer was in motion, finely-divided mercury was also swept
into the outer dish and was mixed with the mercurous sulfate, yield
ing a gray product. The electrolysis was continued in a darkened room for 14 hours and 55 minutes. At the end of the electrolysis the
current mas turned off, and the remaining mercury was emptied into
the mercurous sulfate, and the dish which had contained the mercury
and the tripod were removed. The mercury and the mercurous sulfate
mere stirred for 45 minutes. The mercurous sulfate was removed with
a platinum spatula to a clean, dry flask with a ground-glass stopper.
The mercurous sulfate was stored in a dark place under some of the
solution remaining after the electrolysis. ( Mercurous sulfate is
a light sensitive materiel and should be kept in the dark. The emfs
of cells made with the light-struck (brownish) material do not have
the stability that cells made with the gray mercurous sulfate.) The
mercurous sulfate was analyzed by spectrochemical analysis. The results are given in Table III.
Preparation of thallous sulfate. The first attempt to prepare thallous sulfate was by recrystallization of the commercial
salt. The thallous sulfate ( Fischer’s CP salt. Lot# 705056 ) was
dissolved in distilled water and allowed to sit covered with filter
paper at room temperature. The crystals separated out as the water
<s>22
2 0 ] ^
V —
52SLA V j/W ^
3 0 -aA / N /
H SOo u t e r
Pt cothodc^
C o v e r
St i r r a r
Inn e r d i s h
A n o d e con ne ct ion
Tr ipod
D I A G ^ AND APPARATUS FOR PREPARATION OF MERCUROUS SULFATEfigure 6.
23
TABLE III.
SPECTROCHEMICAL ANALYSIS OF MERCUROUS SULFATE*
Element Concentrât ion
Ag FT
Cu FT
Mg FT
Pt FT
Si T
In general the following concentration ranges are indicated for
the qualitative examination: T, 0.001-0.0001%; FT, less than 0.0001%. * This analysis was made by the Spectrochemistry Section of the
Analytical and Inorganic Chemistry Division of the National Bureau of Standards.
24évapora ted, and the solution became saturated. The last bit of m mother liquor was decanted, and the crystals rinsed with distilled
water and dried at room temperature. A spectrochemical analysis was
run on the starting material, as well as the recrystallized thallous
sulfate ( Table IV. ).
As is evident from Table IV., the recrystallized thallous
sulfate contained about one percent indium. Therefor^ other
methods of purifying thallous sulfate were considered. Ion exchange
methods could probably be used to remove the indium from the thallous
sulfate. A study would have to be made of factors,such as the proper absorbant, effluent, impurities imparted to the salt during
the ion exchange and other factors. Another method would be to dis
solve thallium in sulfuric acid. A sample of thallium was analyzed
spectrochemically at the same time as the thallous sulfate, and the
results are given in Table IV.
Since the impurities in the thallium metal are low, it was
decided to try the latter method given above. One disadvantage of
dissolving thallium in sulfuric acid is that thallic sulfate may be
formed instead of thallous sulfate or both of the compounds may be
formed as represented by the following equations :
2 T1 + HgSO^ ---> TlgSO^ ♦ Hg (18)
2 T1 + 3 H^SO^ + 7 H^O -- > *112(80^)3 ' W HgO + 3 Hg (19)
Since there is a possibility of forming thallic ion even in the presence of thallium, a method for determining thallic ion
in the pcesence of thallous ion was needed. A number of analytical
methods for determining thallium are available. Several of these
25
TABLE IV.SPECTROCHEMICAL ANALYSIS OF THALLOUS SULFATE*
AND THALLIUM
Element
Sample 1Concentration
2 3 4
Ag FT T VW VWA1 T FT T FTCa W VW W TCd VW T T -?Cr FT T T FTCu T VW VW WFe T FT? T FTIn S M W T
Mg VW T VW TPb VW T T VWSi VW T T T
In general, the following concentration ranges are indicated for
the qualitative examinations: S, 1-10%; M, 0,1-1%; W, 0.01-0,1%;
Vw, 0.001-0.01%; T, 0.0001-0.001%; Ft, less than 0.0001%.
Sample 1 is sample as received from Fischer and used in the re
crystallization; sample 2 is the recrystallized thallous sulfate; sample 3 is another sample from Fischer; and sample 4 isthe thallium metal.
*This analysis was made by the Spectrochemistry Section of the
Analytical and Inorganic Chemistry Division of the National Bureau of Standards.
26
21 22 23depend on thallium being in the thallic state * * • Therefore,
If thallic ion were present, it could probably be detected, Shaw^^
describes a colorimetric method in which thallium was determined
cclorimctrically by oxidizing the thallous ion with bromine water
and then adding the solution to an acid solution of potassium iodide.
The color èfkthetliberated iodine was an indication of the amount of
thallium in the original solution. Shaw used the thallium in the
chloride form,and the amount of iodine liberated is given by the following reaction:
T1C1_ + 2 KI --> TlCl + 2 KCl + 2 I (20)3The iodine was extracted by carbon disulfideC CSg), It was decided
to try this method for the presence of thallic ion in the solution
in which thallium was dissolved in sulfuric acid. The reaction
here, if thallic ion were present, would be:
Tl2(S04)3 ♦ 4 KI ^ TlgSO^ + 2 KgSO^ + 41 (21)
The disadvantage of this colorimetric analysis is that thallous iodide Is less soluble than thallous sulfate and precipitates, masking
the iodine color. However, if more than twice as much CSg is used
than acid solution then the iodine color can be distinguished.
In order to test this method for determining thallic ion in the presence of sulfate ion rather than chloride ion, a small
piece of thallium was dissolved in sulfuric acid. Then 10 ml of a
21L.A.Haddock, Analyst,60,594(1935)22C.W.Sill and H.E.Peterson, Analytical Chem., 21,1268(1949)
23P.A.Shaw, Ind.Eng.Chem.Anal.-Ed..5.93(1935)^^Ibid.
27
fresh solution made by mixing 200 grams of water, 32 grams of concentrated sulfuric acid, and 1 gram of potassium iodide was added to each of five flasks containing 20 ml of catobon disulfide. The first
flask was used as a blank to determine if any iodine would be liberated by the oxygen in the air. To the second flask was added a
small sample of the solution containing the dissolved thallium. To
the third flask was added a sample of the thallium solution which had
been treated with sodium sulfIteCNaHSO^). A yellow precipitate was
formed in the second and third flasks, but no color was developed 1 n the carbon disulfide.layer. To the fourth flAsk was added a sample
of the thallium solution which had been boiled with bromine water
until colorless to oxidize the thallous ion to thallic ion. This
solution gave a yellow precipitate in the acid layer and a purple
iodine color in the eaçbom disulfide layer. To the fifth flask was added an aqueous solution to which bromide water had been added and
then the solution boiled until colorless. This gave a colorless solution as did the blank..
Since a method was then available to determine the presence
of thallic ion in the solution,and this method showed that thallic
ion was not formed if thallous sulfate was prepared in the presenceof thallium metal, it was decided to prepare the thallous sulfate by
25dissolving thallium in sulfuric acid. Since there is some indication
that thallous sulfate is more soluble in solutions containing sulfate ion and in order to prevent formation of thallic sulfate, it was
desirable to have a small piece df thallium in the solution at all
25J.E.Ricci and J.Fischer, J.Am. Chem.Soc.. 74 , 1607(1952)
2$times. Therefore, an excess of thallium metal was used for the prep
aration of the thallous sulfate.A 28 gram stick of thallium was placed in each of two
flasks,and an amount of concentrated sulfuric acid was added so that
the thallium would be in excess. Distilled water was then addëd, and
the solutions heated on a hot plate at cctemperature between 80°C and
100°C. Water had to be added from time to time because of evaporation.
As this process took place at a very slow rate, the contents of each
flask were transferred to clean dry platinum dishes and the heating
continued. The reaction seems to go faster in platinum than in
Pyrex. The crystals were removed from the platinum crucibles when
there was still some thallium present. The heating of the flasks
and the crucibles continued over a two-week period. The crystals,
which were removed from the crucibles after allowing them to cool, were dissolved in distilled water and recrystallized. The recrystal
lized thallous sulfate was then analyzed spectrochemically, and the
results are given in Table V.Thallium amalgams. The thallium amalgam was prepared for
each cell individually. The thal1ium was cut with a hack saw which
had been wiped clean and then used to cut a piece of scrap thallium into several pieces. The pieces of purA thallium were then placed
in a solution of sulfuric acid to remove any dirt and oxide film.
Each piece was then removed, washed with distilled water, weighed,
and then placed in a casserole with the proper weight of mercury
and covered with a solution of dilute sulfuric acid and heated on a
hot plate until it became liquid. The sulfuric acid was decanted, the amalgam washed with distilled water, dried with filler paper and
29
TABLE V.SPECTROCHEMICAL ANALYSIS OF THALLOUS SULEAN
PREPARED FROM THALLIUM*
Element Concentrât ion
Sample 1 2 3
Ag FT FT FTA1 T T VWCa FT FT T
Cu FT FT FT
Fe T -? T
Mg FT FT TPb FT T TSi VW VW VW
In general, VW, 0.001-0.01%; T, 0.0001-0.001%; FT, less than 0.0001%. Samj le 1 is recrystal lized from flask 2; sample 2 is re crystallized
from flask 1 ;and sample 3 is from flask 1 ( not recrystallized).* This analysis was made by the Spectrochemistry Section of t he .
Analytical and Inorganic Chemistry Division of the National Bureau of Standards.
30
then introduced through a funnel into the cell. The amalgams were immediately covered with a saturated solution of thallous sulfate.
The weights of thallium and mercury used in each cell are given in
Table VI.Cell blanks. The cell blanks, the guide tubes, the delivery
tubes for filling the cells, the casseroles, the apparatus for the
preparation of the mercurous sulfate, and the other glassware were
soaked from four hours to overnight in 1:1 nitric acid, rinsed several times with distilled water and dried overnight in an oven
set at 105°C.
The cell blanks were leached for two weeks with distilled
water, the water emptied, the blanks rinsed, then the water was allowed to stand in them for three days. They were then emptied
again, rinsed and the wate* allowed to stand in them for one day.
The leads of thé blanks were then checked for continuity, numbers placed on the blanks and cemented on with collodion, and then the
blanks were steamed with distilled water for ten minutes. The blanks were then allowed to dry and were then capped with small beakers and
stored in a cabinet overnight.
Assembly of the cells. The amalgam was introduced into
the cells as previously described. Then enough mercury was introduced by pipet into the positive limb to cover the platinum lead. The mercurous sulfate was placed in a casserole and washed with 0.6 N
sulfuric acid three times, with 0.06 N sulfuric acid three times, and
finally with a saturated solution of thallous sulfate three times.
Each time the solution was decanted between washings. Before being
introduced into the cell the mercurous sulfate was mixed with thallous
31
TABLE VI.WEIGHTS OF MERCURY AND THALLIUM
IN 55PERCENT THALLIUM AMALGAMS
Cell No, Weight of thallium Weight of mercurygrans grama
1 6.5 5.32 6.75 5.5
3 7.5 6.1
4 7.0 5.7
5 7.0 5.7
6 7.5 6.1
32
sulfate crystals and enough thallous sulfate solution to form a
soft paste, making it easier to introduce into the cell. A small -
amount of saturated thallous sulfate solution was then introduced
over the mercurous sulfate.The next step in filling the cells was po introduce crystals
over the amalgam and over the mercurous sulfate. The crystals pre
pared from the thallium metal and the sulfuric acid were introduced
in cells numbers 1,3,and 5. The recrystallised thallous sulfate containing one percent indium was used in cells numbers 2,4,and 6. These latter crystals were used to ascertain if the indium has an
effect on the emfs of the cells. The saturated solution was then
added to each cell until the level of the solution was just slightly above the crossarm of the cell. The saturated solution used in the
assembly of the cells was made by dissolving recrystallized thallous
sulfate in the proper amount of distilled water according to Table II.
The saturated solution was titrated with standardized sodium hydroxide
and the acidity adjusted by the addition of 0.06 N sulfuric acid, so
that the solution was 0.017 N. The solution was allowed to evaporate
until crystals formed. After each operation of the filling, the caps were replaced on each cell to prevent dust and dirt from getting
into the cell. After the cells were filled, they were hermetically sealed.
Electromotive force measurements. The electromotive
forces (emfs) of the six cells were measured at 22®C, 25®C and 28®C.
The cells were measured using a saturated standard cell of the Vteston
or cadmium sulfate type, made at the National Bureau of Standards.
The emf of the reference cell was known to better than 0.6 microvolts
33
in termsoE the National Reference Group of Standard Cells, the group
of cells used to maintain the volt for the United States. The emf• V
of each cell of the six thallium cells was measured by connecting
the cell in series but in opposition to the reference cell, that is
the negatives of the reference cell and the unknown cell (thallium
cell) were in common , and the positive lead of each of the cells
was connected to the K 3 potentiometer made by Leeds and Northrop,
as shown in Figure 6.The emfs of the six cells were read asedifferences from
the reference cell, and the emf calculated by adding the difference
of the unknown cell from the reference cell to the emf of t he ref
erence cell. The values of the cells arc given to one microvolt in Table VII. (The error on this range of the K 3 potentiometer is
not more than - 2 microvolts.)The emfs of the cells were measured while the cells were
in oil baths which are used for the measurement of standard cells
at the National Bureau of Standards. The baths were within a few thousands of a degree of the nominal temperature during the measure
ments, that is for the measurement at 28^0, the temperature of the
bath was controlling between 27.998 and 28.002®C. The variations of
the temperature of the bath containing the reference cell and the
temperature of the bath in which the six thallium cells were maintain
ed were - O.OOl^C during the measurements. The temperatures were measured using a platinum resistance thermometer in each bath,and
a Mueller Temperature Bridge ( G2) which is temperature controlled.
( Both the thermometers and the bridge were calibrated at NBS.) Ice
points ( R^) were calculated for the thermometers using the resistan-
34
TABLE VII,ELECTROMOTIVE FORCES OF THALLIUM CELLS
Mean temperature 22,004°C
Cell Number Electromotive Forc& *s.d.a.volts V.
1 1.0565M 1.52 1.05647* 0.93 1.056556 1.64 1,056526 2.85 1.056549 1.56 1.056520 3.0
Mean 1.056529
Mean temperature 25,000°C
Cell Number 2Electromotive Force s.d.afvolts
1 1.057514 1.62 1.057506 0.33 1.057516 1.24 1.057505 0.65 1.057515 1.26 1.057510 1.7
Me#n 1.057511
Mean temperature 27,9995°C
Cell Number Electromotive Force^ , *s «volts A\.v.
1 1,058467 0.42 1.058455 0.93 1.058469 0.54 1.058464 0.45 1.058467 Ow46 1.058467 0.3Mean 1.058465* standard deviation of the mean1 Mean of 3 readings ; 2 Mean of 9 readings; 3 Mean of 5 readings.
35
X "C e II f. C e l l f— -JH I----1
P o t e h t I om ete r
t FOR ELECTROMOTIVE FORCE measurement
FIGURE 7, ,
36
ces measured at the triple point of water. The differences in thet
calculated R s were less than the detectable limit of the bridge,
that is the change in the ice points were less than 0.000l.x\_ «The internal resistances of the thallium cells were measured
using a ten megohm resistor. The resistances at 28* C ranged between
390-n- to 430_r*_- . The internal resistance of a comparable cadmium
sulfate cell is between 650j\^nd 750_A— .
37
III CALCULATION OF RESULTS Calculation of temperature coefficient. The emfs of the cells were measured at three different temperatures. The change of emf
for a change in temperature can be calculated and the temperature
coefficient can also be calculated. The temperature coefficient
of each of t he six cells is given in Table VIII, From the tempera -
ture coefficient, the'entropy change of the reaction for the cell
can be calculated using the following equation:
n y = A s° (22)
Substituting the mean or average temperature coefficient for the
six cells in equation 22, we haveA 8° = 32,0 X 10“^ volts per deg, (23)
A S° = 14.759 cal. per deg. (24)Calculation of thermodynamiic functions. The values for the other
thermodynamic functions, a 6, the change of free energy for the reaction,and A H, the change of heat content for the reaction, { - aH
represents the quantity of energy released in thermal form) can be
calculated using the above emfs and the temperature coefficient.
Calculation of A G, The change of free energy for the
reaction can be calculated by the following equation:
A G — — n E y (25)
Substituting in equation 25 the mean emf value of the six cellsand the values for n and y given in footnote 9 on page 6 , we have
A G = -2 X 96.487 x 1.057511 (26)4,1840
and
A G = - 48.775 kcal. (27)
TEMPERATURE
TABLE VIII.
COEFFICIENT OF THALLIUM CELLS
38
Cell Number (ff.v )28-25 C 28-22°C
1 953 19192 949 19813 953 19134 959 19385 952 19186 957 1947
Mean 953.8 1936.0
Volts per degree 31.79 X lO”^ 32.26 to-5
39
Calculation of Al H. The change in heat content for the reaction can be calculated by the following equation:
A H = Ad + TA 8 (28)Substituting in equation28 the values for a G and A 8 at 25°C,
we have
A H = - 48.774 + 298.16 (14.759) (299
and
a h = - 44.374 kcal. (30)
40
IV CONCLUSIONa. Summary of the work. This study has shown that the potentials of the thallium cells exhibit very good short-term stability.
The values for A G and a H and A S do not agree with the values
given in the thermodynamic tables in the literature. The largest
discrepancy is in a . G, a difference of one kcal. and,therefore, there is also a discrepancy in the calculated potential of the cell.
The temperature coefficient was also larger than the calculated
value. The use of the crystals containing one percent indium show
that no significant difference in short term stability is due to
the indium.
b. Work to be done. If the thallium cell is to be considered as a
cell for use as a standard, a method of lowering the temperature coefficient should be studied. Also to be studied is the long-term
stability of the cells, that is the constancy of the emfs over
SSWSM&'' months and years. The change, if any, of the potential
with a change in the percent of thallium in the amalgam needs to
be studied. Also to be studied is the hysteresis of the emfs of the cells and the temperature coefficient over a wider raUge.
41
BIBLIOGRAPHY
A. BOOKS
GlAsstone, Samuel, An Introduction to Electrochemiatry. New York:D.Van Nostrand, Inc*, 1942
Lewis, G.N* and BAndall, M. Thennodynamies. New York; Mcgraw-Hill Book Company, Inc. ,1923
Latimer, Wendell M* Oxidation Potentials* Englewood Cliffs, New Jersey; ' Prentice-Hal 1, Inc. , 1952
Vinal, George W. Primary Batteries. New York; John Wiley & Sons,Inc., 1950
B. PUBLICATIONS OF THE GOVERNMENT, LEARNED SOCITIES,AND OTHER ORGANIZATIONS
Hodgman. Charles D,(ed. in chief). Handbook of Chemistry and Physics. 31 ed it ion. Cleveland : Chemical Rubber Publishing Co. , Inc., 1949.
Rossini, Frederick D.,Wagman, Donald D., Evans, William H., LevineSamuel, and Jaffa, Irving, Selected Values of Chemical Thermodynamic Properties. National Bureau of Standards, United States Department of Commerce, Circular 500. Washington:Government Printing Office, 1952.
SilsbeA,Francis B., Establishment and Maintenance of the Electrical Units. National Bureau of Standards, United States Department of Commerce, Circular 475. Washington : Government Printing Office,1948.
Washburn, Edward W. (ed. in chief). International Critical Tables of Numerical Data, Physics, Chemistry, and Technology. New York: Mcgraw- Hill Book Co., 1927.
Weston, Edward, German patent 73,194(January 5, 1892); British patent 22,482(February 6, 1892); United States patent 494,827(April 4, 1893).
C. PERIODICALS Agar, J.N. and Breck, W.G., Trans. Faraday Soc., 53 , 167(1957)
Beattie, James A., j.Am.Chem.Soc., 46^, 2211(1924)
42
Carhart, Henry S, and Hulett, George A., Trans. Am. Electrochem.Soc., 5,59(1904).
Coperthwatte, I.A., La Mer, V.E., and Barksdale, J. , J.Am. Chem.Soc.,56,544(1934).
Haddock, L.A. Analyst, 60,394(1935).
Hulett, Q.A.Phys.Phys. Rev.. 32^,257(1911).Lewis, G.N. and Randall, M.,J.jjh. Chem. Soc., 43,233(1921).
Wwis, G.N. and von Ende, C.L., J. Am. Chem. Soc., 32,732(1910).Mellon,M.G. and Henderson, W.E., J.Am. Chem. Soc., 42,676(1920).
Richards, T.W, and Lewis, G.N., Pfoc. Am. Acad., 34,87(1898).
Richards, T.W. and Daniels, F., Trans. Am. Electrochem. Soc., 22. 343(1912).
Ricci, John E., and Fischer, Jack, J.Am. Chem. Soc., 74^. 1607(1952).
Roos, G.D., Zeit. fur anorgan. Chem., 94,358(1916).Sill, Claude W. and Peterson, Herber, Analytical Chemistry, 21, 1268
(1949).
Romanovski, M., Travaux et Mémoires du Bureau International des Poids et Mesures, 21 45(l95ji).
Shew, P.A., Ind. Eng. Chem. Anal.-Ed.,5,93(1933).
Smith, Sir Frank, Proc. Phys. Soc., (London), 22,11(1910).
Richards, T.W. and Daniels, F., J»Am. Chem. Soc», 41, 1732(1919),
Craig, D.N., Hoffman, J.I., Law, C.A., Hamer, W.J., J. Research.Natl. Bur. Standards. 64A. 381(1960).
Brickwedde, L.H., and Vinal 6.W., j. Reasearch Natl. Bur. Standards, 20,599(1938).
Vosburgh, W.C., Goagerty, Mqry, dayton, William J., J.Am. Chem. Soc., 592. 1256(1937).
Vosburgh W.C, and Parks, Helen C», f .Am. Chem. Soc.. 61^, 652(1939).
Vinal, G.W., and Brickwedde, T TT JjrXliilitW1||t1 Bur. Standards. 26,455(1941).
Wolff, Frank A. , Jr., Trans. Am. Electrochem. Soc., 5,49(1904).