incbmplete bromide precipitation is expected. If the cxcess is too high, the chloride co;?recipitation increases. This means that the general Iange of the halides in the sample must be known; otherwise a double precipitation would be necessary to establish the purity of the bromide and chloride precipitates. R,esults given in Table IV show that chloride is quantitatively precipitated below piE 9.3, and that solubility losses are slight. Iodide can be determined in the presence of an equal molar c o n c e n h tion of bromide and chloride (Table V). Bromide and chloride can also be determined reasonably well on the same sample. The range of errors is of the same order of magnitude as in the potentiometric method (4) for analyzing mixed halides. However, the potentiometric method requires a set of correction equations t o achieve this accuracy. It can be applied to a wider range of halide mixtures than the method described here. A survey of passible anion inter-
Table V.
Precipitation of Silver Halides
(Average of four determinations) Taken, Found, Mg. Mg. iodide 25.4 25.2 Q 0 . 2 Bromide 16.0 15.9 Q 0 . 5 Chloride 7.1 7 . 3 f 0.5" Corrected for reagent contamination 5
of 0.5 mg.
ferences showed that anions forming silver salts more soluble than silver bromide will not interfere in the determination of iodide, and that anions forming silver salts more soluble than silver chloride will not interfere in the determination of bromide. Anions such as cyanide and thiocyanate, which form silver salts with a solubility close to silver bromide, would be expected to interfere in the determination of bromide. Anions such as phosphate and carbonate, which form silver salts with a solubility close to silver chloride,
would be expected to interfere in the determination of chloride. Experimental results using the described procedure show that equal molar concentrations of cyanide and thiocyanate did not interfere in the determination of iodide. Equal molar concentrations of arsenate, carbonate, iodate, oxalate, and phosphate did not interfere in the determination of either iodide or bromide. Equal molar concentrations of sulfate did not interfere in the determination of iodide, bromide, or chloride. LITERATURE CITED
(I) Britton, H. T. S., AnaEysl 50, 601 (1925). (2) Gordon, L., Peterson, J. I., B u t t , B. P., ANAL.CHEM.27,1770 (1955). (3) Hayek, E., Hohenlohe-Profanter, Hohenlohe-Profanter M., Marcic, IB., Beetz, E., Angew. &hem. 70.307 fimx). 70,307 (1958). (4) Martin, A. J., ANAL. CHEM.30, 233 (1958). RECEIVEDfor review April 11, 1960. Accepted August 1, 1960. Presented in part before the Division of Anal ical
Chemistry, 137th Meeting, ACS, 8eveland, Ohio, April 1960.
rialkyl Selective
xtractants for Silver and Mercury
T. H. HANDLEY Analyfical Chemisfry Division, Oak Ridge Nafional Laboratory, Oak Ridge, Tenn. JOHN A. DEAN Department o f Chemistry, University of Tennessee, Knoxville, Tenn.
fx- Triiso-octyl thiaphosphate and tsi-nbutyl thiophosphate, neutral esters of monothiophosphoric acid which contain an isolated P=S group, are Isighiy selective extractants for silver and mercury(l1) ions in nitric acid medium from the 35 elements tested. The extracted species are formed rapidly and the partition is easily reversed. From aqueous solutions 6M in nitric acid, and for a 0.67M solution of the reagent in carbon tetrachloride, the partition coefficient at room temperature for silver exceeds 100; for mercury it ie 90. The influence of a wide range of nitric acid, reagent, and silver concentrations, and the e of temperature, diverse acids, and salting agents upon the partition equilibrium have bean investigated. Struciwres are postdated for the soivaree formed between rke reagent molecules and nitric acid und the metai nitrates. e
ANALYTECAL CHEMISTRY
contains numerous references to nonionic phosphoryiated reagents, such as tributyl phospkate, as extractants for metallic salts (3, 5, 7, 9 ) . Their application t o the purification of uranium and other metals by solvent extraction has been well established. The extraction properties of the phosphorylated reagents are influenced by the nature of the substituents attached to the phosphoryl group, P=O (a, 4). However, it was not known in what manner the properties of nonionic phosphorylated reagents would be influenced by the replacement of the isolated P=O group by a P=S group. The semipolar sulfur atom is the sole structural difference between the phosphates and the monothiophosphates. The employment of triiso-octyl thiophosphate or of tri-nbutyl thiophosphate, neutral esters of monothiophosphoric acid, as the extracting reagent has revealed a high HE LITERATURE
degree of selectivity for silver and mercury(I1) from a nitric acid medium. Earlier Pishchimuka (8) had found few solid salts capable of forming complexes with trialkyl thiophosphates. EQUILIBRIUM DISTRIBUTION STUDIES
Materials. TWOthiophosphates were tested: tri-n-butyl thiophosphate (TBPS) briefly, and triiso-octyl thiophosphate (TOTP) extensively. Both were obtained from Peninsular ChemResearch, Inc., Gainesville, Fla. The butyl compound was used without further purification. However, the octyl derivative was found t o contain approximately 0.00095N amount of an acidic component, pTesumably a dialkyl monohydrogen thiophosphate. Consequently, the reagent was purified by passing a solution of the reagent in carbon tetrachloride through a column of Fisher chromatographicgrade alumina. No acidic component was detected after purification; the
I
O1
io
20
TOTAL HkO, , M
c 01
L
L-
-
.
31 h?OLEZ?liAR "3,
L
L
10 ,M
A
A
10
Figure 1. Partition coeffcient of Ag f r o m aqueous HNOs solutions at various concentrations of TOTP and a single concentration of TBPS Ag concn., 0.093M, initially/ temp.,
alumina column became noticeably warm during the passage of the reagent solution. Purified TOTP has a density of 0.934 gram per ml., and is 2.23ilI in strength. Method. To a given amount of carrier (stock) solution, the radiotracer and enough nitric acid were added t o make a solution 2 molar in acid and y molar in silver. Exactly 5 ml. of salt solution were mixed with an equal volume of organic phase which contained a known dilution of thiophosphate in carbon tetrachloride. The phases were stirred vigorously, or shaken manually, for 2 minutes. Subsequent studies showed that a 1-minute shalcing period was adequate. The two phases were centrifuged for 2 minutes a t 2000 r.p.m. and then an aliquot of each phase was measured for silver-110 or mercury-203 by counting in a gamma scintillation counter having a sodium iodide crystal (thallium-activated). An alternative method for dead solutions involved the titration of suitable aliquots from each phase for acid content by titration with standard base and using phenolphthalein indicator, and for silver (or mercury) content by titration with thiocyanate in the presence of ferric ion as indicator. The analysis of the organic phase was handled by an extractive titration. While surveying the extractibility of other elements, flame photometric
25' C.
30 4 0 510 6 b iM, Y I T R l C A C I T ( t o ' o l )
__-
--L__I--
01 M HluO,
0 32
90 T O ' -
-_LL--
'0
-
1
I-1L
10
!undissociated
Figure 2. Partition coefficient of Hg(ll) f r o m aqueous "03 solutions at various concentrations o f TOTP H g concn., 0.050M, inltiallyj temp., 25' C.
methods were employed for elements for which suitable radioisotopes were not available. Extraction from Nitric Acid Solution. Because silver and mercury salts are often contained in a nitric acid solution, the extraction of silver and mercury(I1) from various nitric acid solutions mas investigated extensively. An aqueous nitric acid solution, 0.1 t o 12M, and 0.093M in silver or 0.0500M in mercury, was extracted with an equal volume, b u t varying in concentration, of TOTP in carbon tetrachloride: 0.111, 0 223, 0.869, 1.32, and 2.23M (5, 10, 30, 60, and 100% w./v., respectively). The silver (or mercury) content in each phase was measured. The results are shown in Figures 1 and 2, in which the logarithm of the partition coefficient is plotted against the logarithm of the undissociated (molecular) concentration of nitric acid. No linear segments were obtained if the abscissa were simply the logarithm of the total acid concentration. At nitric acid concentrations less than about 1.8M, no significant amount of undissociated acid exists in the aqueous phase (6). An aqueous phase G M in nitric acid and an organic phase 0.660JI (30?& I$./\-.)
in TOTP, or 15% w./v. in TBPS, provides optimal working conditions and a large partition coefficient for the extraction of silver. Lower acid concentrations demand a larger quantity of reagent. Acid concentrations greater than 9iM are unpleasant to handle and give rise to side reactions with the reagent on standing (8). Phase inversion also occurs a t the higher acid and reagent concentrations. Under the recommended conditions, as little as 0.04 pg. of silver has been completely removed in one extraction from 20 ml. of aqueous phase with only 5 ml. of organic phase. Figure 3 is a logarithmic plot of silver concentration in the organic phase against the aqueous silver concentration a t equilibrium. Curves of constant original silver concentration and constant original acidity are shown. It is possible to estimate from this figure how a solution with a given original silver and acid concentration will distribute itself between the phases. Indirectly, this figure also shows the effect on the distribution coefficient of varying the volume of aqueous sohtion used. Nitric acid is extracted by the reagent solution in the absence of any silver VOL. 32, NO. 13, DECEMBER 1960
o
1879
10007
ICO, L
z
fz
C.04
L
> 0
0 < a
.-
2
L
Y)
"z
w
a
0.304 w
2
I 0000100d2
I
0005
'Ab,
1
062
1
-
CCO93 MAgNO3
l l l l l
C'
035
1
SI-JES CONCEUTPATION (141 I\ 0 R G A h C 'HAS€
Figure 3. Distribution of Ag between an organic phase 0.223M in TOTP and an aqueous phase at several acid concentrations
or mercury salts (see Figure 7). However, the amount of acid extracted at high aqueous acid concentrations does not approximate that required to form an organic phase that is saturated with a 1 to 1 TOTP-HSO3 complex as happens with tri-%-butyl phosphate ( I ) . Considerably more nitric acid accompanies the extraction of silver into the organic phase and competes with the silver nitrate for the formation of a complex Kith the reagent. Undoubtedly this competitive effect is the cause for the curves in Figures 1 and 2 to flatten out at high concentrations of nitric acid, and even to bring about a decrease in the partition coefficient of the silver and the mercury (Figure 6). No relationship between the amounts of silver nitrate and nitric acid that were extracted could be discerned. The approximate ratios of acid to silver in the organic phase at each of three aqueous nitric acid concentrations were: 0.5 for 2 6 X , 1.0 for 5.1X, and 1.5 for 7 . 7 M . However, the ratios increased markedly for silver nitrate concentrations 0.0186M and less. With a 0.67Jf solution of T O T P the extraction of mercury(I1) from a 6M solution of nitric acid is 97% complete after two equilibrations. However, as will be shown later, the presence of a salting agent largely eliminates the need for a second equilibration. The extraction of mercury is more complete when using as extractant a 1.06H (30 volume %) solution of tri-nbutyl thiophosphate (TBPS) in carbon tetrachloride. With this extractant a larger reagent concentration is possible with the same volume per cent reagent solution. 1880
ANALYTICAL CHEMISTRY
--a-p-_i
3 020
Figure 4.
0 10c 100 T P I - ISCOGTYL THIOPHOSPHA'E.
6 00 Id
Partition coefficient of Ag as a func-
tion of reagent concentration for various con-
centrations of "OB Ag concn., 0.093M, initially
On standing for several days, the silver and mercury complexes gradually darken. Pishchimuka (8) also observed this discoloration and found that elimination of a n alkyl group OCcurred concurrently. Initially, the metal complex with silver is brownish yellow; the molar absorptivity is not large and the absorbance extends over a considerable portion of the visible wave lengths without eny distinctive features. Reagent Concentration. The effect of t h e concentration of T O T P in carbon tetrachloride on the extraction of silver and mercury is shown as the parameter in Figures 1 and 2 and independently in Figures 4 and 5 , A 0.669M reagent solution is recommended. It forms t h e denser phase and does not offer emulsion troubles t h a t often are encountered with higher reagent concentrations. The interaction of the multiple forces involved in the distribution of silver is revealed by plotting the logarithm of the partition coefficient against the logarithm of the uncombined reagent concentration-that is, the total reagent concentration diminished by the amount combined with the silver and with the nitric acid present in the organic phase.
The results are shown in Figure 6 for two fixed reagent concentrations, a t varying concentrations of total silrer nitrate, and for a series of fixed concentrations of nitric acid in the aqueous phase. The partition coefficient tended to converge in all cases. When one assumes that 1to 1complexes of TOTP, $gx03, and TOTP-HKOs form, the convergence value equals the initial reagent concentration in the organic phase. The concentration of silver present initially in an aqueous phase for optimal extractibility appears to be approximately 0.019111 (2.0 mg. per ml,). The partition coefficient decreased for larger or snialler silver concentrations at each of the nitric acid concentrations studied. Effect of Other Anions and Acids. The extraction of silver from several concentrations of other anions, added a3 their sodium salts, is shown in Table I. T h e interference noted for the individual anions correlates with the free silver ion concentration in equilibrium with the slightly soluble or insoluble silver salts, or with the soluble anion complexes. The extraction of mercury(I1) from various acid solutions and from nitric acid solution in the presence of other
HhO,,
100r I
M
I
12
c
-1
1
a" E
i
c
/
r I
1
0.1 0.02
I1
1
I I 1
0.05 0.1 0.2 0.5 4.0 TRI- ISOOCTYL THIOPkiOSPHATE, M
I 1 1 1 20 3 0 6 0 40.0 450 30.0 T O T P IN C C l 4 ( % ) 1
1C
2.0
I
5.0
1
j0074
i
I
I
60.0 400.0
Figure 5. Partition coefficient of Hg(ll) as a function of reagent concentration for various concentrations of "Os
c 23
C3'
I
cc5
3cz C T O T F-
\ LAqkC,,,,, . i -
1
i
,l
C'
I
3Fi
1
c5
02
+ [hNC.
1
F?EE
10
YTF , M
3L4,
Figure 6. Partition coefficient of Ag as a function of uncombined (free) reagent concentration
Hg concn., 0.050M, initially
anions, added as their sodium salts, is shown in Table 11. The partition coefficient of mercury is largest in acetic acid. probably due to the formation of mercury(I1) acetate, which is slightly ionized.
Table 1.
Effect of Anions on Extraction of Silver
[Initial aqueous phase, 0.0928M in silver nitrate and 6Af in nitric acid, was contacted with 10 volume % (0.22M) triiso-octyl thiophosphate in carbon tetrachloride] Anion Concn., Partition Tested M Coeff. Xone , . . 2.11 i 0.070 Acetate 1.0 1.90 0,001 2.08 Chloride Fluoride Phosphate
0.01 0,Ol
0.1
1.0 0 01 0.1
1 0
Sulfate
0,001 0.01
b
2.00 2.10 1.90 2 07 1.87
1.95 2.19 2.15 2 21
0.1 1 0 1.91 a Standard deviation. Precipitate of silver chloride formed in'aqueous phase.
Stripping Silver from Organic Phase. While the analytical radiochemist may often measure directly t h e radioactivity of the organic phase in the determination of silver (or mercury), at times i t is desirable to re-extract (strip) t h e metal into a n aqueous solution. This is particularly desirable if one wishes to perform a colorimetric analysis for the metal or to perform subsequent analytical steps. Organic phases containing two concentrations of reagent were stripped with successive equal volumes ( 5 ml.) of distilled water. The results for silver are shown in Table 111. Nore effective stripping is accomplished with dilute solutions of sodium hydroxide (or aqueous ammonia), which removes the nitric acid that accompanies the silver nitrate into the organic phase and thus reverses the trends shown in Figures 1, 3, and 6. Since the partition coefficient for silver nitrate in the absence of free nitric acid is 0.07, each stripping with dilute alkali or ammonia (equal volume of two phases) will remove 93% of the silver from the organic phase. Salting Agents. T h e addition of a second nitrate to t h e extraction system results qualitatively in the formation of a higher proportion of ion association solute niolecules (AgKOa,
TOTP) through the common ion effect, and hence, in enhanced extraction (Figure 7). On plotting the logarithm of the partition coefficient
Table II. Effect of Anions and Acids on Extraction of Mercury
Concn.,
Sample Tested
..
1M
Partition Coeff. of Hg
ANIONP
Acetate Chloride (as acid) Fluoride Perchlorate (as acid) Sulfate (as acid)
...
44
1.o
100; 32b
2.0
0.001b 1.7C
1.0
1.0 1.0
250
4.5b 9.05
6.0
kXDSd
Acetic Hydrofluoric Iiitric
1.0 1.o 6.0 9 0
80
1.3c 90 100
50 Perchloric 1.0 a 30 volume yo (0.67M) TOTP in CCL contacting an aqueous phase 0.50OM' in mercury(I1) nitrate and 6 M in nitric acid. Extraction made with 0.223M TOTP; with 6 M nitric acid alone partition coefficient is 17. c Precipitate at interface. d 30 volume yo (1.06M) TBPS in CClr contacting an aqueous phase 0.0500M in mercury(I1) nitrate.
VOL. 32, NO. 13, DECEMBER 1960
a
1881
Table 111.
Stripping of Silver Nitrate rganic Phase with Water (Initial organic phase was 0.0928M in
silver nitrate and original aqueous was 9M in nitric acid. Equal vo&%$ of two phases were used) KO.of Partition Coeff. Water Strip 1.34M TOTP 0 680 1 372 7 8
2 3
3 0
0.67ikI TOTP 630
0 1
510
1.0
2
I
of silver against the logaiithm of the total nitrate concentration, a constant slope of 0.30 is found when sodium nitrate is the salting agent. Addition of calcium nitrate exerts an increased salting action; the slope is 0.41. When the second nitrate is nitric acid, account must be taken of the competition for reagent molecules and of the equilibrium between the dissociated and undissociated forms of nitric acid (6). For concentrations of nitric acid 2.0M and less, only B normal salting effect is apparent. However, above acid concentrations of 2.0111 the partition coefficient exhibits a continual increase on the double log-plot. With mercury, the addition of sodium nitrate (1~11)increased the partition coefficient between the pure reagent, TOTP, and an aqueous solution 3 X in nitric acid from a value of 13.5 t o
34.5. Effect of Temperature.
The effect
of temperature upon t h e distribution
Table IV.
3.0'
~
'0
L p L _ _ - -J _ L 30 4.0 50 60 7.0
20 T C T A L MOLARITY AS NITRATE
Figure 7. Influence of salting agents upon partition coefficient of Ag Present were 0.223M TOTP and 0.093M Ag, initially
of silver between t h e aqueous solution and the organic phase for two reagent concentrations is shown in Figure 8. In the absence of free nitric acid, the partition coefficient for silver is doubled with every 25' decrease in temperature. Actual values of the partition coefficient with 0.67X TOTP as extractant are 1.03, 0.50, and 0.275 for 0", 2 5 O , and 50" C., respectively. By contrast, in the absence of silver the extraction of nitric acid into the organic phase increased about 33% for each 25" rise
in temperature over the same teniperature range. When silver is present, the amount of nitric acid that is coextracted is increased five fold but the extraction is no longer affected by a change in the temperature. However, 400
I
l
I
,
,
l
,
.
,
Partition Coefficients of Metal Nitrates between 0.67M Triiso-octyl Thiophosphate and 6M Aqueous Nitric Acid
Metal Nitrate Aluminum Antimony Arsenic(111) Brsenic(V) Barium Bismuth(II1) Boron Cadmium Calcium Cerium(II1) Cesium Chromium(1II) Cobalt
:Re'
I 1
Partition Coeff. 10 -4 3a 0 01 0 01
