DIFFTSIOX CURREST O F YTTERBIUM
683
and >Ira Ern-in S. Smith in preparation of samples and equipment for these experiments, and RIr. Wayne Tenolia for determination of the ammonia isotherm of charcoal. REFERENCES (1) BRUNAUER, DEMING,DEMISG,A N D TELLER: J. Am. Chem. SOC.62, 1723 (1940). A X D BRUNAUER: J. Am. Chem. SOC. 69, 1553 (1937). (2) EMMETT A N D D E I T Z :J. Research Natl. Bur. Standards 36, 285 (1945). (3) GLEYSTPEN (4) HARKIXS ASD E W I N G :J. Am. Chem. SOC.43, 1787 (1921). (5) KLOTZ:Chem. Revs. 39, 211 (1946). (6) LAMBAND COOLIDGE: J. Am. Chem. SOC.42, 1146 (1920). (7) MCBAIS:Sorption of Gases and V a p o u r s by Solids. Geo. Rutledge and Sons, Ltd.. London (1932). (8) PEARCE A S D RhED: J. Phys. Chem. 39,293 (1935). (9) PIERCE A X D SXITH:J . Phys Colloid Chem. 62, 1111 (1948). (10) SMITH A N D PIERCE:J. Phys. Colloid Chem. 62, 1115 (1948). Phil. Mag. [4] 42, 448 (1871). (11) THO~UPSOX: (12) WILKINS:Proc. Roy. SOC.(London) A164, 496 (1938).
D E P E S D E X C E O F THE DIFFUSION CL-RRENT OF YTTERBIUM ON SI!PPORTI?;G ELECTROLYTE =IND ~€1’ GERALD B. B.4RTOK
AR’D
J . D. KURRATOV
Department of C h e m i s t r y , The Ohio Stat< 1-niversity, Columbus 10, Ohio
ISTRODU CTIOS
It has heen ,ihunn by Xoddacl; and Bruckl (6) and by Laitinen and T a ~ b e l
(4)that YII-~exhibits a polarographic wive with a half-rave potential of approximately - 1.4 v. against a saturated calomel electrode (S.C.E.). Europium is the only rare earth shon-ing a lower (more positive) reduction potential. Laitinen (3) and TTalters and Pearce (S) by measurement of the YbA3--Ybi2 couple found that the wave at - 1.4 v. is due t o the reduction of Tb-t3 to I-h+? Laitinen and Taebel (4) have shon-n that the IlkoviE relation is obeyed at concentrations from 0.623 to 3.110 millimoles per liter of ytterbium in 0.1 -1ammonium chloride at presumably constant pH. The present n-ork covers the concentration range from 0.032 to 1.41 m1111mole. per liter and the pH range from 3.5 to 7.5. The purpose of this study was to find some indication of the aggregation of ions ;it hydroxyl-ion concentrations bclov, that at n-hich coagulation occurs and to observe thc influence of tliffercnt wpportiiig electrolytes on the difluqion current and cmgulation. 1 Presented before the Division of Physical and Inorganic Chemistry a t the 113th National Meeting of t h e American Chemical Societv, Chicago, I l l i n n i
684
GERALD B. BARTOX AKD J. D. KURBATOV MSTERIALS
Spectroscopically pure Yb203 obtained from Johnson Matheny and Go. Ltd. (Laboratory Xo. 1071) was used for the various solutions. Solution KO. 1 was prepared by dissolving 10 mg. of ytterbium oxide in nitric acid, evaporating to dryness repeatedly (four to five times) with the addition of hydrochloric acid, and finally dissolving the residue in 100 ml. of doubledistilled water. Sufficient lithium chloride (0.42 g.) was added to make the solution 0.1 N in lithium chloride. Solution S o . 2 was prepared by dissolving 110.0 mg. of ytterbium oxide in hydrochloric acid, evaporating to dryness twice, and redissolving the residue in 25 ml. of 0.1 N hydrochloric acid. This solution was then diluted to 250 ml. with double-distilled water. PROCEDUHE
Series 1 , constant p H , uariable concentration: The predetermined quantity of solution KO. 1 mas pipetted into a beaker and sufficient 0.1 iV lithium chloride added to make a total volume of 15 ml. The p H was then adjusted to 4.3 by addition of 0.01 AT hydrochloric acid and the solution was transferred to the cell. h'itrogen (purified by being passed through ammoniacal cuprous chloride in ammonium chloride, potassium hydroxide solution, sulfuric acid, and a spray trap) was bubbled through the solution for 10 min. to remove dissolved oxygen. The final pH was measured after running the polarograms. Remaining series, variable p H : A definite quantity of solution KO. 2 was pipetted into a beaker containing 5 ml. of 0.3 N lithium chloride or ammonium chloride and sufficient 0.01 N hydrochloric acid added to make the total volume 15 ml. The p€I was adjusted with 0.1 N lithium hydroxide or ammonium hydroxide. The volumes were recorded and used to calculate the final concentration of ytterbium. A saturated calomel anode was used with a Leeds & Northrup Electrochemograph. The temperature was controlled to 25°C. =t0.5" for all except the first series, which was at room temperature, 25°C. & 1". I n all cases two polarograms were run on each solution. The deviation in half-wave potential which might result from the time lag of the automatic recorder was evaluated on separate samples of ytterbium solution. The polarograms in these cases were obtained by setting the voltage manually and allowing the recorder to reach equilibrium. From the current obtained a t the various voltages by this procedure the curve and half-wave potential n-ere compared with those secured by the automatic process. Identical solutions of ytterbium a t the same pH and t,emperature were used for the calibrntion. DISCUSSION OF RESCLTS
The data obtained in the range of concentrations oi ytterbium from 0.032 t o 1.41 millimoles per liter with a p H of approximately 1.4 (table 1) and a p H of approximately 6.5 (table 2) with lithium chloride as supporting electrolyte show a direct proportionality of current to concentration, in agreement x i t h
685
DIFFUSION CCRREXT O F YTTERBIUM
the IlkoviE equation (figures 1 and 2). The close agreement of slopes obtained at pH 4.3 and 6.4 with the above concentrations indicates very little if any change in the state of ionic aggregation or dispersion below pH 6.4. Subtraction of the correct amount for the blank is a matter of uncertainty. This is sh0Tv-n by the difference in curves obtained by direct subtraction of the superimposed sample and the blank polarograms (L‘exact method”) and the curve obtained by subtraction of the extrapolated wave heights (curves I and I1 of figure 1). TABLE 1 Dependence of d i f l u s i o n current o n ytterbium-ion concentratjon Electrolyte, 0.1 N lithium chloride; temperature, 25OC. _ _
-
~~
~
( a) CVhCLhTR4TIOS
.
-
:oZh
pa.
pa.
w.
4.9 4.3 4.49 4.39 4.32 4.29 4.32 4.31
1.50 1.49 1.51 1.49 1.50 1.50 1.52 1.55
1.31 1.18 1.03 0.84 0.71 0.53 0.35 0.12
1.29 1.18 1.03 0.786 0.675 0.540 0.309 0.081
2.72 2.89 2.97 2.98 3.23 3.40 3.68 3.75
__
(EXTRAPOLATION)
1
,
1
’
1
PH
Eijr
volts
w.
6.40 6.41 6.40 6.28 6.70 5.30 5.79
1.488 1.485 1.482 1.483 1.4i3 1,470 1,464
4.07 3.39 2.50 1.67 0.825 0.853 0.432
miiiimoles/t,
-
id/c
El/?
,
(b)
idle a/millimle/t.
2.69 2.89 2.97 2.79 3.07 3.46 3.25 2.53 _____
TABLE 2 Dependence of d i f f u s i o n current o n ytterbium-ion concentration Electrolyte, 0.1 -V lithium chloride; temperature, 25OC.
COSCESTRATION
1.41 1.12 0.842 0.562 0.281 0,281 0.140
(a)
( d [EXACT)
PH
millimoles/!.
0.480 0.408 0.346 0.282 0.220 0.156 0.095 0.032
1
(b)
id
I ~
id /c pa./millimnolc/i,
2.89 3.03 2.97 2.07 2.94 3.04 3.09
..
‘The values of the difiusion current constant, G/C, calculated for the extrapolation data show a continuous increase corresponding to the failure of the line to pass through the origin. The numerical average of id/C from the ‘‘exact method” agrees n i t h the slope of the curve as drawn. The data which follow are from the measurements by the “exact method” only. The data obtained at experimentally constant concentrations of ytterbium, 0.56 and 0.28 millimole per liter, and variable p H show that up to p H 6.0 the relation between diffusion current and pH is nearly constant and is not affected by the nature of the supporting electrolyte (figure 3).
686
GERALD B. BARTON AND J. D. KURBATOV
With lithium chloride as supporting electrolyte the final pH was found to vary from the initial value, particularly at higher pH. It was noted that the
Slope 2.95
conc. milli m o l e / L . FIG.1. Dependence of diffusion current on concentration of Yb+3. 0.1 iV lithium chloride; pH, 4.3-4.5; 25'C. Curve I , current by direct difference between blank and sample; curve 11, current by extrapolation method.
id PO
z
4.-
-
/
FIG.2. Dependence of diffusion current on concentration of YbT3. 0.1 N lithium chloride; pH, 5.8-6.4; 25°C. Value of current obtained by difference between wave heights of blank and sample measured by extrapolation method.
ytterbium chloride exerted some buffering action-mainly a tendency t o bring the p H to the precipitation value. Ammonium chloride solutions maintained a more nearly constant pH.
687
DIFFCSIOS C T R R E S T OF YTTERBIUM
It \vas obsei,ved that in the solution containing ammonium chloride a drop in current for a given ytterbium concentration, indicating aggregation of t'he Y W ions, did not occur at as low p H as it did in lithium chloride solutions ( f i g i i ~3 ) . This extension of the soluhilit,y range was checked by visual obwrvation of the solut'ions. A visible precipit'ate was found in the solution containing lithium chloride, adjusted t'o p H 7.0. S o precipitate rvas observed in
P I ( ; ,3. Dependelice of dif-fusion current o n pH a t two fixed concentrations of Yb4 0.1 .V lithiunl chloride. Current by direct differences between polarograms of blank and s:imple. 0 0 . 1 .Y lithium chloride Yb-3 = 0.56 millimole per liter v 0 . 1 S ammonium chloride Yb+3 = 0.56 millimole per liter 0 0 . 1 S ammonium chloride = 0.277 millimole per liter nuti i n
TABLE 3 Dependence o j ytteibaurn digusion C U I rent on p H Electrolyte, 0.1 S ammonium chloride; temperature, 25°C. .
~
-
CU?XEXTXATIOS
Id
millimolesll.
0.279 0.278 0.277 0.277 0.278 0.278 0.277 0.276 0.2it
3.47 -1.26 4.88 5.09 5.42 5.89 6.42 6.92 7 40
aoils
pa.
1.585
0.71 0.78 0.92 0.828 0.832 0.852 0.772 0.720 0 240
1.464 1.464
1.479 1.476 1.476 1.179 1.179 1.416
_ l
_
~
id IC --
pa.jmillimolc/l
2.54 2.81 3.32 2.99 2.99 3.06 2.79 2.61 0.89
the ammonium chloi,ide solutions at p€I '7.5 even though there was a marked drop in t h e current. 31o(~llei,and Iii~e111t~i~s (5) have reported precipitation p H values of 6.30, 6.1S, anti 6.50 foi. ytterbium nitrate, sulfate, and acetat'e, respectively, using sodium hydroxide as the precipitating base. lTnder t'he present experimental cunditionb aggi,egation appeared to begin a t p H 6.5 in lithium chloride and at pEI (j.!b7.O in ammonium chloride.
688
GERALD B. BARTON AXD J. D. KURBATOV
The results obtained indicate that Yb+3 can be determined polarographically with ammonium chloride or lithium chloride as supporting electrolyte a t pH values from 3.5 to 6.4 when proper correction is made for the accompanying hydrogen-ion discharge. The optimum pH appears to be about 5-6, since there is little or no current due to hydrogen-ion discharge in this region. It was found that the half-wave potential for ytterbium obtained with lithium chloride as supporting electrolyte varied about 0.06 v. with a change in pH from 4.1 t o 6.7. I n table 4 it can be seen that there is a trend toward decrease TABLE 4 Dependence of ytterbium diffusion current o n p H Electrolyte, 0.1 N lithium chloride; temperature, 25°C. .-
DH ~
milLimles/l.
0.565 0.562 0.556 0.562 0.561 0.561 0.561 0.553
--
I
I
~-
-
3.51 4.08 5.80 5.59 5.90 6.60 6.67 6.88
-
id
€3112
I
1
-
__-
id/C -.
Paile
Ira.
&a /mdlimle’i.
1.564 1.503 1.470 1.479 1 .A70 1.464 1,464 1.436
1.66 1.72 1.66 1.73 1.61 1 62 1.30 0.31
2.94 2.86 2.90 3.08 2.87 2.71 2.32 0.56
TABLE 5 Dependence of y t t e r b i w n dig’usion current on p H Electrolyte, 0.1 N ammonium chloride; temperature, 2 5 T . ~
COSCENTRATION
~
_
_
_- -
PH
__
mrllinoles/l
0.558 0 557 0 556 0.553 0.546
5 60 6 03 6 60 6 99 7.39
___
-
~.
Ei/i
Id
-____
-
Polls
&ti:d (.,!oiifct.eiiri. UII Surface tteactioi dcieiititic P'uper So,1379 f r o i n thr \Vcstinghous(~Ilrseiircli I,:tboi~:itories. b u r g h , 1'enrisylv:tiii'u. I
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