350
JACK SCHUBERT B S D J. W. RICHTER
REFEREXCES (1) BOYD,G. E , BROSI,A . R., CONS, E . , LESLIE,W.,.\ND SCHCBERT, J . : Unpublished work. (2) BOYD,G. E., SCHUBERT, J., ANI) hn.msos. .I.W.:J . -4m. Chem. Soc. 69,2818 (1947). (3) CANNAS,R . K., . ~ N DKIBRICK,A . : J. Am. Chem. Soc. 60,2314 (1938). (4) GLASSTONE, S.: Physacal Chemistry, p . 955. D . Van Sostrand Company, Inc., S e w York (1940). (5) HASTINGS, A . B., ~ I C L E C F. S , C., EICHELBERGER, L . , HALL,J. L., ASU DACOSTA, E.: J. Biol. Chem. 107, 351 (1931). (6) JENKY, H . : J . Phys. Chem. 36, 2217 11932). (7) MYERS,R. J.: ‘(Synthetic Resin Ion Exchangers,” in Adoances in Colloid Science, T’ol. I. Interscience Publishers, Inc., S e w T o r k (1942). (8) SELSON, R . , ASU WALTOS,H. F.: J. Phys. Chem. 48, 406 (1911). L.: T h e Nature of the Chemzcal B o n d , 2nd edition, p. 3. Cornel1 Cniversity (9) PAULING, Press, Ithaca, Kew Y ork (1940). (10) SCHUBERT, J.: Unpublished work. (11) SCHUBERT, J., A N D REVINSON, D.: Unpublished work. (12) SCHCBERT, J., AND RICHTER,J . IT.: J. Phys. Colloid Chem. 62, 350 (1948). S., ASD ~ I I S D L E R A ,. B . : Chem. Ind. 16, 789 (1945). (13) SUSSXAN, (14) VANSELOW, A . P . : Soil Sci. 33, 95 (1932). A . P.: J. .4m. Chem. Sac. 64, 1307 (1932). (15) YANSELOW,
THE USE OF I O S EXCHASGERS FOR THE LlETER3IISXTIOS OF PHYSICAL-CHEhlIChL PROPERTIES OF SUBSTAXCES, PARTICULARLY R.IDIOTRhCERS, I S SOLCTIOS. I1
THEDISSOCIATIO?; CONSTASTS
O F STROXTIUM CITRATE AND STRONTIUM TARTRSTE’
JACK SCHCBERT2 A K D J. W. RICHTER3 Clinton Laboratories, Oak Ridge, Tennessee Receiced r l i ~ g u s 20, t 1947 INTRODCCTION
Cations of the alkaline earths readily form complex ions with the anions of carboxylic acids. The most commonly used methods for measuring the dissociation constants of these comples ions are the determination of solubility (7) and electrometric procedures (4,6). The dissociation constants of the citrate complexes of calcium, strontium, and magnesium have also been studied, using a 1 The specific material discussed here is derived from part of the studies reported in May 1945 by the authors and is based on work performed under Manhattan District Contract Eo. W-7405-Eng-39 a t the Clinton Laboratories, Oak Ridge, Tennessee. * Present address: Department of Physiological Chemistry, Cniversity of Minnesota, Minneapolis 14, Minnesota. 3 Present address: Department of Chemistry, University of Minnesota, Minneapolis 14, Minnesota.
10s EXCHAKGERS IK DETERJZISATIOS OF PHYSICAL PROPERTIES.
11
351
frog heart preparation ( 5 ) . These techniques, except the ion-eschange method, require that all of the components of the complex ion be present in macroscopic concentration. Paper I of this series included a discussion of the theory and a development of the equations employed when using a cation exchanger for the determination of the dissociation constants of certain soluble complex ions (9). A unique feature of the ion-eschange method is the fact that the dissociation constant of an organometallic complex ion is measured when the metal component is present in radiochemical concentrations, i.e., about lo-” mole per liter. I n this paper 11-e shall present the experimental procedures and the results obtained from an ion-eschange study of the dissociation constants of the complex ions of strontium citrate and strontium tartrate. The measurements involved the determination of the percentage of radiostrontium which was adsorbed by the cation eschanger in the presence and absence of a known amount of citric acid or tartaric acid and in a solution adjusted to constant ionic strength with ammonium chloride. EXPERIMEXTXL
The adsorbent I n all cases the adsorbent used was the cation exchanger Amberlite IR-l4 classified into particles of 40-60 mesh. The resin was converted to the ammonium form, KHIR, by treating the hydrogen form, HR,of the resin with excess ammonium chloride a t a pH of 7 . 6 . The resin was washed free of excess salts with distilled water and was air dried. The ammonium resin contained 1.92 millimoles of exchangeable ammonium ion, XHt, per gram of the air-dried material. The moisture content of the resin as determined by drying a sample t o constant weight at 110°C. was found to be 18 per cent.
Solutions All the chemicals used were chemically pure. Stock solutions of citric acid and tartaric acid were made up just before theywere used. Spectrographic analyses of the solutions after equilibration with the resin revealed no detectable quantities of foreign cations or of carrier strontium.
Kadiotraccr The tracer used was carrier-free Sr” which has a half-life of 53 days, PSrSy--+
53 d
Y8’ (stable). -4stock solution of ammonium chloride was “spiked”
with the tracers. This solution was used to prepare all the solutions used in the experiments reported here. On the arerage the experimental solutions contained about 80 counts of &*’per second at 8 per cent geometry, i.e., a minimum of IO-” mole per liter. 4 Manufactured by the Rohin and Haas Conipany, Resinous Products Division, Philadelphia, Pennsylvania.
352
JACK SCHUBERT AND J. W. RICHTER
-4nalylical procedures Concentrations of ammonium ion in the equilibrium solutions were determined by a micro Kjeldahl procedure (10). The radiations from SrE9were measured directly in a Geiger-Muller counter after evaporation of a given aliquot. Factors such as the geometry and the self-absorption characteristics were kept as uniform as possible. Inasmuch as the experiment consisted in measuring relative beta activities, no corrections for decay, self-absorption, and the like were found necessary.
Equilibrium procedure
A given weight of NH4R and a given volume of solution were shaken for 3-hr. periods, after which the mixture was centrifuged and the centrifugant analyzed for the content of radiostrontium. It would have been advisable to determine the radiostrontium in the resin as well, had not conditions been chosen so as to yield results of optimum accuracy without necessitating a direct analysis on the resin. Rate studies have shown that 3 hr. is sufficient time for the adsorption of the tracer to reach a reproducibly constant value (2). RESULTS AKD DISCUSSIOS
The stoichiometric dissociation constant, K,, for the comples ions of types like strontium citrate and strontium tartrate is given by the relation ( 3 , 3 , 8, 7) :
where ,4 = the anionic part of the comples ion (SrA4)*-'and z = the charge on the anion. The equation which espresses the value of K , in terms of cation-exchange eyuilibria follows:
where a = the per cent of the cation which is adsorbed by the exchanger a t equilibrium when the complex-forming anion, A', is present, s = the per cent of the cation which remains in solution when hzis present (i,e,, s = 100 - a ) , A = the molar concentration of the comples-forming anion, and A 0 = the ratio a / s when -4is absent from the solution, i.e., the slope of the adsorption isotherm.
Eflect of p H on adsorption of strontium All pairs of solutions must be compared at the same pH because the capacity of an ion exchanger, and hence Xo, usually increases (8) with increasing pH. Measurements of the effect of pH on the adsorption of radiostrontium were made
10s EXCHAKGERS I S DETERMIWSTIOS OF PHYSICAL PROPERTIES.
I1
353
a t constant ionic strength ( p = 0.2) and are tabulated in table 1. It is seen that as the pH increased from 5 to 8 the variation of XO with pH is linear within experimental error (figure 1).
S r + + ADSORBED AT EQUILIBRIUM (CALCULATED)
Sr++
pH OF
IN SOLUTION AT EQUILIBRIUM
SOLUTION AT EQUILIBRIUM
An
per cent
5.1 5.2 5.4 5.7 6.0 6.6 7.4
I
1
~
24.0 21.2 19.2 16.3 15.3 11.9 9.4
76.0 78.8 80.8 83.7 84.7 88.1 90.6
I ~
3.16 3.72 4.21 5.13 5.54 7.40 9.64
* 0.5 g. of air-dried NH4R was shaken with 50 ml. of solution for 3 hr. The p H of the solutions was adjusted with ammonium hydroside. t The original solutions contained 4900 counts of SrsY per minute at a geometry of 8 per cent.
pH
FIG.1. Effect of pH on the adsorption isotherm, XO, of radiotracer Sr* from 50 nil. of 0.20 M animonium chloride by 0.5 g. (0.96 mole) of ammonium resin, NHaR. t = 25°C.
Dissociation corzstaiat of strontium tartrate Measurements were made under conditions similar to those employed in studying the variation of the adsorption of radiostrontium as a function of pH. The ammonium-ion concentration was kept constant a t 0.2 mole per liter.
354
JACK SCHUBERT AND J. W. RICHTER
The total ionization of tartaric acid, HzC4HzOs,is practically complete a t the pH and ionic strengths employed in our experiments (3). It has been shown that (4) the dissociation constant, K,, of strontium tartrate is given by the relation :
K,
=
(Sr") (Tar- -) (Sr Tar)
(3)
~
where Tar-- = the ionized anionic part of the tartaric acid molecule. I n table 2 are given the data from n-hich the dissociation constant was calculated. Each point represents the average of at least two runs. The value of Xo was obtained from figure 1. Equation 2 was used to calculate K c . The average value found, 2.02 X lo-* or, pk', = 1.69, is in good agreement with the value of Cannan and Kibrick (-I), n-ho found pZ(, = 1.65 from a study of hydrogen electrode titration curves of potassium chloride-strontium chloride mixtures at a constant ionic strength of 0.2. While the agreement with their T-ABLE 2* The dissociation constant of strontium tartrate a s calculated from ion-exchange data at 25" + 1°C.
'
'
I
aolesllrter
0.10 0.05
11
5.46 7.11
I
,
per cenl
41.1 71.9
1 I
I
per cent
58.9
4.20
28.1
8.84
Average., . . . . . . . . , . . , . . . . . . . . . . . . . . . . . . . . . . . . . , . ,
1' 1
.I
1.99
x
10-2 10-2
I
2 . 0 2 X 10-2
1,
2.01
x
1.69
* 0.5 g. of air-dried ammonium resin was shaken for 3 hr. with 50 ml. of the solution which contained a total of 0.2 mole per liter of ammonium ion. result is very good, it is possible to ascribe the direction of the deviation to the difference in the bulk electrolytes which were employed. There is evidence to indicate that the activity coefficient of a tracer substance is greatly influenced by the bulk electrolyte (2). The activity coefficient of potassium chloride a t an ionic strength of 0.2 is greater than that of ammonium chloride at the same ionic strength. It is expected, therefore, that the activity coefficient, 7, of radiostrontium would be slightly greater (at least for mixtures of halide salts) in solutions of potassium chloride than in solutions of ammonium chloride. From the relation
where K , is the thermodynamic dissociation constant, it is seen that the dissociation constant of strontium tartrate dissolved in ammonium chloride would tend to be less than in potassium chloride solutions.
IOK EXCHASGERS I S DETERMIS.1TIOS O F PHYSICAL PROPERTIES.
355
I1
Dissociatioiz constant of strontium citrate From the known ionization constants of citric acid (1) it can be shown that under the conditions of our experiments, citric acid is completely ionized to the tertiary citrate ion, Cit- - -, where Cit- - - represents the anionic part, CFH607. TABLE 3 Dissociation constant of strontium citrate, Sr Cit- , calculated from ion-exchange data at 26" i 1°C. I
,oLu~ I CONCENSAMPLE NO.
WEIOHT )F NHaR
'",s,"r1
PHOF
)LCTIp'i T EQtI .IBRIUM
TRATION O F CITRIC ACID
1
A, . . . . . . A' . . . . , 1, , .... . 1'. . . . , . 2, , . . . , . 3, ,., .. .
grams
ml.
0.50 0.5 0.5 0.5 0.5 0.5
50.0 50
E:
,
i ::::
50 j0
0 0
0.01
I
1
1 8.10 8.10 I 7.76 1 7.76 7.64 i .10
TOTAL RACIOSTRONT I U X ADSORBED
p e r cent
94.6 94.5 54.5 54.7 67.1 i3.4
CONCENTRATIOh' O F SHc
SLOPE, AD ?OR GIVE1
I
KO
CONDITIONS'
1
I moles/ I
PKC
liter
0.165 0.165 0.165 0.165 I 0 165 0.165
h
17.3 15.8 15.8 15.4 13.1
0.0°5 Average
1.63 x 1.66 x 1.53 x 1.33 x
10-3 10-3
10-3 10-3
1.54 X
2.81
--___
4......
4).. . . , , 5..... 5'. . . . . .
5.0 5.0 10.0 10.0
50 50 50
50
' ~
0
0.05 0 0.05
6.80 6.80 6.72 6.70
67.8 20.6 80 30.4
1.05 1 .05 1.05 1.05
3.1 2.1
7.03
x
10-3
4.0
4.0
6.12 X
Average
6.58 X
2.18
* The values of XO for samples .4'-3 were estimated from equation 6 as described in the text.
The dissociation constant for strontium citrate has been shown (5, 6) to be given by the relation: l
