Synthesis of a Reproducible and Chemically Stable Tantalum Antimonate Quantitative Separation of VO2+-AI3'-Ti4', V02'-Fe3'-Ti4', and UO,
2+-~i4+
Mohsin Qureshi, Jai Prakash Gupta,' and Veena Sharma Chemistry Department, Aligarh Muslim
University, Aligarh, (U.P.)lndia
A new ion exchanger, Tantalum antimonate (Sb/Ta = 1.3), has been synthesized by mixing 0.1M tantalum and antimony pentachloride solution in the ratio 1 : 2 at room temperature. The pH of the mixture was adjusted to 1 with ammonia. The precipitate obtained was refluxed with mother liquor for 16 hr. This procedure gave a reproducible ion exchanger. Molar distribution coefficients for 27 metal ions have been determined on this gel at pH 0, 1, 2, 3, 4, and 5. The following mixtures have been separated: (1) V02+-Fe3+-Ti4+, (2) VO2+-AI3+-Ti4+, (3) UOZ2+-Ti4+. UOZ2+ and V 0 2 + were removed with O.1M nitric acid, Fe3+ with 1 .OM nitric acid, AI3+ with 1 .OM hydrochloric acid, and Ti4+ with 2N sulfuric acid plus 3% H202 ( 9 : l ) .
The analytical importance of synthetic inorganic ion exchangers is now firmly established due to their high selectivity, thermal stability, and resistance to radiations. There are, however, two fundamental problems which still lack a solution: the ion exchangers, unless crystalline, do not have reproducible properties, and they are not chemically stable. Of the various ion exchangers studied zirconium antimonate ( I ) was found by Kraus and Philips to show an interesting reversal of properties. We have, therefore, been interested in the synthesis and ion-exchange properties of the various antimonates. We found that stannic (2) and titanium ( 3 ) antimonates show interesting ion-exchange properties while thorium and ferric antimonate (2) samples show only negligible ion exchange. Of the various compounds studied, the tantalum compounds have received scant attention. Tantalum phosphate ( 4 ) was examined by Kraus who found that this substance is severely hydrolyzed in alkaline solution and its capacity (0.6 mequiv/g in acid solution) is lost on drying a t 200 "C. Tantalum pentoxide ( 5 ) was investigated by Chidley e t al. who found t h a t its ion-exchange capacity for NH4+ is lower than for K + and lies between 0.66 and 0.84 mequiv/g. A search of the literature showed that no studies have been reported on tantalum antimonate (TaSb). We have, therefore, synthesized this exchanger and we have explored the conditions under which it shows a reproducible behavior even in the amorphous state. The importance of this synthesis has been demon-
Present address, Post Doctoral Fellow, Institute of General & Inorganic Chemistry, University of Perugia, Perugia, Italy. H. 0. Phillips and K . A . Kraus, J. Amer. Chem. SOC.,84, 2267 (1962). M. Oureshi, V. Kumar. and N. Zehra, J. Chromatogr., 67, 351 (1972). M. Oureshi and V. Kumar, J. Chem. SOC.,1970, 1488. H. 0. Phillips and K . A. Kraus, Oak Ridge Report ORNL, p 50, 2983 (1960). B. E. Chidley, F. L. Parker, and E. A. Talbot, U. K. At. Energy Auth, Res. Group. Rep.. AERER 5220, 10 (1966).
strated by achieving some difficult quantitative separations on its columns.
EXPERIMENTAL Reagents. Tantalum pentoxide (BARC, India) and antimony pentachloride (density 2.3 g/ml) (B.D.H., England) were used. Antimony pentachloride solution was diluted with 4M HCI to obtain the desired concentration. All other reagents were of AnalaR grade. Tantalum pentoxide (22.10 g) was heated with 100 ml of concentrated sulfuric acid containing 200 g of ammonium sulfate. The clear solution was diluted to 500 ml to obtain a solution which was 0.1M with respect t o tantalum. Apparatus. An electric temperature controlled S I C 0 shaker, Philips X-ray unit, and Bausch and Lomb Spectronic 20 colorimeter were used for shaking, X-ray studies, and spectrophotometric measurements, respectively. A Perkin-Elmer Model 137 spectrophotometer was used for IR studies. Synthesis. (TaSb). This was prepared by mixing acidic solutions of 0.1M tantalum and antimony pentachloride (1:2) at room temperature. Ammonia solution was then added dropwise with constant shaking until the p H was 1. The precipitate obtained was divided into 2 parts, one part was allowed to stand for 24 hr at room temperature (samples 1-4) and the other part was refluxed with the mother liquor for 16 hr (samples 5-6). After a t taining room temperature, samples were washed by decantation with demineralized water (DMW) and filtered by suction. They were then dried a t 40 "C. Finally ion-exchange samples were converted into the H f form with dilute nitric acid as usual. Synthesis of Hydrated Ta205. A concentrated solution of ammonium hydroxide was added dropwise to the 0.1M tantalum solution until the solution p H was l. The precipitate obtained was refluxed as for samples 5-6. Synthesis of Hydrated Sbz05. This sample was precipitated as tantalum pentoxide with the only difference that the SbClS solution was initially taken instead of the tantalum solution. Ion Exchange Capacity. The ion-exchange capacity of the various samples of TaSb was determined by column operation. The ion exchanger in the hydrogen form was placed in the column with glass wool support. Potassium chloride solution (1.OM) was used as the eluent. The effluent was collected. The hydrogen ion eluted from the column was titrimetrically determined with standard NaOH using phenolphthalein as the indicator. The results are summarized in Table I. The ion-exchange capacity of hydrated tantalum and antimony pentoxides was also determined as above. The results are given in Table 11. Composition. One-hundred milligrams of the well powdered material was dissolved in 10 ml of hot concentrated sulfuric acid. Antimony was estimated volumetrically (6) while tantalum was determined colorimetrically into pyrogallol (7). Dissolution of T a n t a l u m Antimonate. To determine the solubility of the phases, 0.5 g of the exchanger was shaken with 50 ml of the solution concerned at 30 f 1 "C. After removal of the undissolved material, tantalum and antimony were determined in the filtrate colorimetrically with pyrogallol (7) and rhodamin B (81, respectively. Either tantalum or antimony was absent in the filtrates of all the samples. Rhodamine B can detect 40 pg of antimony and pyrogallol up to 5 x 103 pg of tantalum. Molar Distribution Coefficient. Molar distribution coefficients for 27 metal ions in different concentrations of " 0 3 were determined. Their calculation was made using Equation 1. (6) N. H. Furman, Stand. Methods Chem. Anal., 1, 96 (1962). (7) E. B. Sandell, "Colorimetric Determination of Traces of Metals." Interscience, New York, N.Y., 1959, p 697. (8) Reference 7, p 262.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1901
-
~~
Table I. Synthesis and Some Properties of Tantalum Antimonate Molar distribution coefficient, X l o - ' Sample No.
Color
Ion-exchange capacity, mequiv/g
Ta:Sb ratio
Mg2+
AP+
Mn2+
1
Light gray Light gray Light gray Light gray Light pink Light pink Light pink Light pink
0.39 0.45 0.60 1 .o 0.99 0.96 0.97 0.98
1 :3.4 1 :2.4 1:7.1 1:1.5 1:1.3 1:1.3 1:1.3 1:1.3
3 7 4 3.5
3 3 3 3 3 3 3 3
2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3
2 3 4
5 6 7 8
0.8 0.8 0.8 0.8
-
Table II. Preparative Conditions and Ion-Exchange Capacity of Hydrated Tantalum, Antimony Pentoxides, and Tantalum Antimonate Ion lonTime of exchange pH of reflux- capacity, exchange material Reagents solution ing h r mequiv/g Ta205
0.1M tantalum
Sb205 "20
TaSb
1.0
16
0.36
1.0
16
0.22
1.0
16
0.99
soh
"20
0.1M antimony pentachloride soln 0.1 M tantalum soln 0.1M antimony pen tachloride
+
5 r
Y 3+
r
SOlfl
r
L
AI3'
CC?'
I , , , , , , l O 0
1
UO?'
o
2 . 3 4 5 6
q
a
Figure l b . Plot of log X vs. pH: (-.-o-)TaSb, SnSb, (-0-0-)TiSb
3
4
5
~
(-X-X-)
PH
Figure l a . Plot of log X vs. pH: (-e-.-)
TaSb, ( - X - X - )
SnSb, ( - 0 - 0 - ) T i S b
A, = c,/c, (1) C L = the molar concentration of cation i present in the liquid
c,
after equilibrium, and = the molar concentration of cation i present in the ion exchanger after equilibrium. The total volume of the solution was 50 ml and the amount of the exchanger used was 0.50 g. Uranyl (9) and titanium (10) were determined spectrophotometrically. All other cations were determined by titration with 0.002M EDTA. The results are given in Figure 1. X-Ray Studies. All dry samples of tantalum antimonate (samples 1-8) were shown to be noncrystalline from powder diffraction (9) Reference 7, p 915 (10) Reference 7, p 874 1902
L , ,L.,I..,. 0
d
i
a
3
4
I
,
5
6
0
1
a
3
4
5
6
0
1
1
3
4
5
6
PH
Figure IC. Plot of log A vs. p H data. X-Ray diffraction spectra of hydrated tantalum pentoxide and antimony pentoxide were also taken. Tantalum pentoxide is amorphous while antimony pentoxide shows a number of lines. The d spacings are given as follows: 5.90 vs, 3.10 vs, 2.96 vs, 2.56 m , 2 . 3 5 m , 1 . 9 7 m , 1 . 8 2 ~ , 1 . 7 4 m1, . 5 6 ~ , 1 . 5 5 s1, . 4 8 w , 1.47m.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
- 11 -
-
g- o a -
2 5 3
-ct
OYC
06-
2 052 2 04-
T>Sb
SnSb TaSb
W
2
03-
$4
oa010
300
100
300
400
600
SO0
700
800
900
? O k
TEt-tw-uaruuE ( " c )
Figure 2. Ion-exchange capacity of various antimonates dried at different temperatures
A -
'
30
Figure 4c. Separation of U02*+-Ti4+ on tantalum antimonate
JMO
3CW
3000
2600
19W
22W
1700
1500
I300
1100
900
700
500
FIeqWnEy ( C r n - ' 1
Figure 3a. IR spectra of tantalum antimonate heated at various temperatures
l
,
3804
,
l
3CW
.
l
30W
l
.
l
2600
l
l
2200
l
1900
.. l 1700l . 1500 . . 13W. . IIWl . 900l . 700 l . 5 0 0l .
"i
-tc
Li*4
--
"4
-t-e
50
0a=+
No?
+-.-+p Rb'
1
Frequmcy (Cm-1 )
Figure 3b. I R spectra of tantalum antimonate heated at various temperatures
tI 0
,
t
, 3.
I
t
I
i
1
1
I
3
&
5
6
'I
'8
9
I
j
I
'
I
o
'
J
~
-TIME \q LM\NUTE
Figure 5. Plot of pH as a function of time
9
40
80 ELUATTE VOLUME
LY\)
Figure 4a. Separation of VOz+-Fe3+-Ti4+ on tantalum antimonate
Figure 4b. Separation of VOZ+-Al3+-Ti4+ on tantalum antimonate
Heat Treatment. Sample 5 was heated a t different temperatures in the muffle furnace for 1 hr and the ion-exchange capacity was 0.95 mequiv/g at 40 "C and 0.02 mequiv/g a t 1000 "C. Other results of ion-exchange capacity as a function of temperature are shown in Figure 2. Ion-exchange capacity was also determined by performing ion-exchange experiments at 95 f 5 "C. For this purpose, the ion-exchange column was surrounded by a jacket through which steam was continuously passed. The ion-exchange capacity was found to be the same, i.e. 0.95 mequiv/g, as a t room temperature. Measurement of IR Absorption Spectra. IR spectra of the samples of tantalum antimonate in the hydrogen form dried a t 40-1000 "C were taken with a standard KBr disk technique, Figure 3. Separation. For separation studies, a glass column was filled with T a S b in the H+ form on glass wool support. The diameter of the column was 0.69 cm. It was regenerated with dilute H N 0 3 and washed with DMW until the effluent was neutral. The rate of flow in all separations was 0.5 ml/min. The molar distribution coefficient of VO2+ is very small compared with those of other metal ions; vanadyl ions were therefore eluted with 0.1M "03. Other metal ions retained in the column were eluted by eluants listed in Table IV. Separation of VO2+Zr4+ was also tried. Zr4+ could not be eluted while V 0 2 f was stripped off with 0.1M HN03. A model separation is shown in Figure 4. Separation Limits. Ti4+ (20-90 pg) has been separated from a much larger quantity of UO22+ (ie., 210-4550 pg) on tantalum antimonate columns. Separation limits of V 0 2 + , Fe3+, and Ti4+ and V 0 2 + , A13+,and Ti4+ have also been studied.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1903
~
~
Table I l l . Change in pH after Equilibriuma Final pH after Cation equii (obsd) Final pH (calcd) Mg2+ 3.0 3.7 Ca'+ 2.9 3.7 Sr2+ 3.2 3.7 Ba2+ 3.1 3.7 Mn2+ 3.2 3.7 zn2+ 2.9 2.5 cu2+ 3.0 3.7 Cd'+ 2.9 3.4 Ni2+ 3.0 3.7 La3+ 3.0 3.7 Y3+ 2.9 3.4 Zr4+ 2.8 3.5 Hf4+ 2.2 2.2 Ce3+ 3.2 3.7 pr3+ 3.2 3.8 Nd3+ 2.9 3.3 Sm3+ 3.2 3.2 Ga3+ 3.3 3.7 in3+ 2.6 3.3 AI^+ 3.3 3.6 Fe3+ 3.0 3.3 Hg2+ 3.4 3.9 Pb'+ 3.4 3.8 VO'+ 3.4 3.7 CO'+ 3.4 3.7 a The initial p H before equilibrium is 5 . 4 . R a t e of Ion Exchange. For this purpose, we have taken 0.5 g of exchanger and 50 ml of 0.1M solution of some mono- and bivalent cations. The pH was measured at different intervals of time. The results are shown in Figure 5.
DISCUSSION It is clear from Table I that even if all the conditions of preparation are the same, the ion exchangers obtained a t room temperature (samples 1-4) have different composition and ion-exchange capacity. The ion-exchange capacity varies from 0.39 to 1.0 mequiv/g while the S b / T a ratio varies from 1.5 to 7.1. Synthetic tantalum antimonate prepared under identical conditions shows, however, negligible variation in ion-exchange capacity or composition if it is refluxed with the mother liquor for 16 hr (samples 5-8). It therefore shows that refluxing for 16 hr leads to improved reproducibility in a n ion-exchange material. This is probably due to the fact that initially a number of phases are present in the ion-exchange material and, on refluxing, this material becomes more homogeneous. All the eight samples were found to be chemically stable in water as well as in 6 N "03. T o find out whether the new product (TaSb) is a mixture of the two oxides or not, antimony and tantalum pentoxides were prepared under the same conditions as tantalum antimonate (sample 5 ) . Hydrated tantalum pentoxide was mechanically stable and showed a n ion-exchange capacity of 0.36 mequiv/g while hydrated antimony pentoxide dispersed in water showing a n ion-exchange capacity of 0.22 mequiv/g. However, tantalum antimonate is mechanically as well as chemically stable (ion-exchange capacity = 0.99 mequiv/g). T h e degree of crosslinking of the ionexchanger matrix has a n obvious effect on the physical and mechanical properties of the material. If the degree of crosslinking is low ( i . e . , the cationic content in the product is very low), it swells considerably in water and can be smeared or possibly even dissolved (11). This may be a (11)
1904
Inczedy. "Ana!ytical Applications of Ion Exchangers." Pergamon Press, New York, N . Y . , 1966, p 28.
J.
reason why hydrated antimony pentoxide dispersed in water. In the case of higher cationic percentage, the material has a better mechanical resistance. The X-ray spectra of tantalum antimonate and tantalum and antimony pentoxides were also taken. Tantalum antimonate and pentoxide are amorphous while a number of lines are observed in the X-ray spectrum of antimony pentoxide. The d spacings of SbzO5.nHZO resemble Sb205 (ASTM Card No. 11-690). Moreover, tantalum antimonate shows the presence of tantalum and antimony both qualitatively and quantitatively. Owing to a large difference in the properties of these materials, we can say that tantalum antimonate is a new phase and is not a mixture of the two oxides. The ion-exchange capacity of tantalum antimonate on drying in air decreases with a n increase in temperature. However, when the ion exchanger is heated to the same temperature ( i e . , 95 & 5 "C) in the presence of water, the loss in capacity is negligible. A comparison of the loss of ion-exchange capacity of tantalum and stannic and titanium antimonates with temperature is shown in Figure 2. Up to 500 "C, all three materials can be of U F P although the order of thermal stability is SnSb > T a S b > TiSb. The IR spectra of tantalum antimonate (sample 5 heated to higher temperatures) are shown in Figure 3. The bands with maxima at about 3400. 1610, 1400, and 700 cm-I are characteristic of the stretching vibration of in(HzO or OH)], the terstitial water and the O H group M-OH deformation vibration [62 (HzO)], the deformation vibration of interstitial water [a, (HzO)]. and of the stretching vibrations of M - 0 [Q (Ta-0, Sb-O)]. respectively. They also show that bands a t 1610 and 1400 cm-1 are heat sensitive. A comparison of the IR spectra of tantalum antimonate with zirconium phosphate (12) and antimonic acid (13) shows that all of them contain interstitial water molecules and replaceable hydrogen ions in the form of OH- groups and metal-oxygen bonds. The T a : S b ratio was found to be 1:1.3 in the tantalum antimonate (Table I). It is evident from Figure 5 that the ion exchange is fast initially followed by a slow step. This is a Ftrong indication that diffusion of ions within the exchanger constitutes the rate-controlling step (14). In the determination of the molar distribution coefficient, the change in hydrogen ion concentration due to the exchange process is summarized in Table 111. The observed p H is slightly less than the calculated p H if there is complete ion exchange. This slight difference may be due to the hydrolysis of the ion exchanger. Our experience has shown that the adsorption of an ion is different on different ion-exchange materiais, i. e . , inorganic ion exchangers are selective for certain ions. High adsorption was observed in the case of antimonates. Titanium ( 3 ) and stannic (2) antimonates are selective for alkaline earths while zirconium antimonate ( I ) is selective for alkali metals. The order of adsorption on zirconiurn phosphate and organic cation-exchange resins for some uni-, bi-, and trivalent metal ions follows the Hofmeister's lyotropic row. [
I
J
~
1. Li < H < Na < NH4 < K < Rb < Cs < A g < T1 2. M n < Mg < Zn < Co < Cu < Cd < Ni < Ca < Sr < Pb < B a 3. A1 < Sc < Y < E u < S m < Xd < Pr < Ce < La
The adsorption sequence for some metal ions o n tantalum antimonate was found t o be as follows (12) V. Vesely and V. Pekarek, J. Inorg. Nucl Chem . 25, 697 (1963) (13) M .AbeandT. lto, Bull. Chem. Soc. Jap.. 40, 1013 (1967). (14) S. J. Harvie and G . H. Nancollas. J Inorg. Nucl. Chem.. 30. 273 (1968).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
Table I V . Some Separations on Tantalum Antimonate (Sample 5) Columns Separation V02+-Fe3+-Ti4+
UOZ2+-Ti4+
Vol of effluent ml
Amt of cations loaded, fig
Arnt of cations
Eluants V02+-0.1M H N 0 3 Fe3+-i .OM H N 0 3 Ti4+-2NH2S04 -t 3 % H202 (9:l)
80 60 25
339 41 3 90
357 435 90
2.0
VO2+-0.1M " 0 3 ~ 1 3 + - 1.OM H C I Ti4f-2NH2S04 43% H2O2 (9: 1)
80 10 25
339 139 90
357 135 90
1 .o
U022+-1 .OM HNO3 Ti4+-2NH2S04 43% H202 (9:l)
20 25
840 90
880 90
1.o
recovered, pug
Amt of exchanger in the column
Table V. Limits of Separation Sample No.
Mixture
VO'+
1
Fe3+ Ti4+
Eluants
Vol of effluent, ml
Amt taken, pg
Amt found, pg
YO of error
0 . 1 M HNO3 1 .OM " 0 3 2NH2S04 4-
40-80 40-60 25
77-385 200-400 20-90
77-357 21 0-405 20-90
-3.5 4-3.1 0.0
40-80 10-40 25
77-385 46-234 20-90
77-357 46-233 20-90
-3.5
-0.25 0.0
10 25
210-4550 20-90
21 0-41 75 20-90
+3.7 0.0
3 % Hz02 (9:l)
2
VO'+ ~ 1 3 +
Ti4+
0.1M H N 0 3 1.OM HCI 2NH2S04 43 % H202 (9:l)
uo2+
3
Ti4+
1 .OM " 0 3 2NH2S04 3% H202 (9:l)
+
pH 5 . 4
pH 2 . 4
> Zn > Cu > N i > Co > Ba > Sr > M g > M n > Ca Nd > Sm > Y > La > AI > Ce > Pr
Cd > Zn > Cu > Ca > N i > Co > Sr > Ba > Mn > Mg
Cd
N d > Sm > Y AI > Ce > Pr
pi! 1 2 Ni Sr
> Co > Cu > Cd > > Zn > M n = Ba =
Ca = Mg Nd Ce
> Y > A I >La > > Pr > Sm
> La >
pH 0.2
Ba > Co > Cd > N i = Cu = Zn = M n = Sr = Ca = Mg
Pr > La Nd > Y
> Sm > > AI = Ce
The plots of Figure 1 allow a comparative study of the adsorption behavior of numerous metal ions on tantalum and stannic and titanium antimonates. In most cases, there is only a light change in X values as the p H is altered from 5 . 5 to 2 . 5 . At p H 2.5, the concentration of hydrogen ions is very small and the cations compete successfully for the exchange sites. When the p H is changed from 2.5 to 0 . 2 , there is a n increase in the hydrogen-ion concentration and a decrease in ionized exchange sites and, hence, the hydrogen ions compete successfully and there is a sudden decrease in X value. A similar trend was also
noticed on papers impregnated with titanium arsenate (15). Higher adsorption on tantalum antimonate compared with either stannic or titanium antimonates (Figure 1) may be due to the fact t h a t the ion exchange in tantalum antimonate (IEC = 1.0 mequiv/g) is higher than stannic (IEC = 0.68) or titanium (IEC = 0.71) antimonates. Figure 1 also shows the specificity of tantalum antimonate. There is a large difference in the molar distribution coefficient for Ti4+, UO22+, VO2+, Fe3+, and Al3+. Quantitative ternary and binary separations (Tables IV and V) of V02+-Fe3+, VOz+-A13+-Ti4+, and U02+-Ti4+ were successfully achieved on a very small column of tantalum antimonate. The separations were achieved on sample 5 and they were repeated on samples 6-8 to check the reproducibility. Because of a very low adsorption of vanadium a t pH 1.2, it was eluted with 0.1M H N 0 3 . Fe3+ and A13+ have zero X values a t p H 0.2 and, therefore, elution was carried out with 0.1M H N 0 3 or HC1. Titanium could not be eluted even with 6M mineral acids or the mixtures of mineral acids with salts (NH4C1, NH4N03). I t was stripped off the column by passing a mixture of 3% H202 and 2 N H2S04 (1:9) because now it forms a complex (15) M. Qureshi, J. P. Rawat, and V. Sharma, Talanta. in press
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1905
with titanium (Ti(Hz024+)). Similarly uranium (200-4550 pg) has been separated from titanium (20-90 pg) using a very simple aqueous system (Table IV). The distribution studies a t diverse p H values (Figure 1) also give the possibility of the separation of Pr3+ .and La3+ from Ce3+ and other rare earths. The adsorption of Ce3+ and other rare earth metals is negligible while Pr3+ and La3+ are considerably adsorbed. Thus, it is hoped that such a study may provide better understanding for the selection of the particular ion exchanger in the specific separations.
ACKNOWLEDGMENT The authors thank A. R. Kidwai and W. Rahman for research facilities and encouragements and J. P.Rawat for helpful suggestions. Received for review September 14, 1972. Accepted February 7, 1973. Thanks are due to the Council of Scientific and Industrial Research (India) and University Grants Commission (India) for the financial assistance to J. P. G. and V. S. respectively.
Role of Hydrindantin in the Determination of Amino Acids Using Ninhydrin Paul J. Lamothe and Patrick G. McCormickl Department of Chemistry, Marquette University, Milwaukee, Wis. 53233
The mechanism of the reaction for quantitative determination of amino acids with ninhydrin has been extended to explain the dependence of color formation on hydrindantin concentration. Voltammetric techniques have been used to study the behavior of several intermediate compounds in this reaction. Conventional kinetic methods and digital simulation techniques have been employed to obtain numerical estimates for individual rate constants involved in the reaction at 40 “C. The revised mechanism is used to explain various observations appearing in the literature regarding the determination of amino acids and other ninhydrin-positive species.
Ever since the discovery by Ruhemann ( I ) of the reaction of ninhydrin with amino acids, there has been conjecture over the role of hydrindantin (I) in the reaction
++ 0
0
OH
0
0 I mechanism. Ruhemann suggested that this compound reacted with ammonia in the final step of the reaction, resulting in the colored compound diketohydrindylidenediketohydrindamine, abbreviated DYDA (11), and now
0-
0 I1
commonly referred to as “Ruhemann’s Purple.” Since the original work of Ruhemann, many mechanisms have been suggested, some involving hydrindantin, directly or indi1 Author
to whom correspondence should be directed.
(1) S. Ruhemann, J. Chem. Soc., 97, 2025 (1910).
1906
rectly in color formation. The entire body of literature is presented and discussed in an excellent review by McCaldin (2). In his review, McCaldin also presented a mechanism for the ninhydrin reaction. His mechanism did not involve hydrindantin in the color-formation pathway, but did involve the compound in a side reaction. This would seem to agree with the remarks of Stein and Moore ( 3 ) whose work is the basis for the popular colorimetric methods for amino acid analysis. They stated t h a t hydrindantin was needed in the ninhydrin reagent to prevent a side reaction. In fact, other modifications of this procedure have noted the critical nature of the hydrindantin concentration in the reagent solution ( 4 , 5 ) . The most thorough study of the role of hydrindantin was that of MacFadyen and Fowler (6). They also postulated a scheme for the ninhydrin reaction, and included hydrindantin as an active participant. Though their work was discussed by McCaldin, he had some minor objections to their conclusions and did not accept their proposals in formulating his own mechanism. It is noteworthy that McCaldin’s mechanism bears certain resemblances to one formulated previously by Filippovich (7), whose work is not referenced in the review. McCaldin’s proposal, however, is more specific, and since its publication, most workers have accepted it in substance as correct. Two recent workers have continued the disagreement over the importance of hydrindantin. Friedman and Sigel ( B ) , though they incorporated the substance in their reagent solutions, ignored its concentration in their kinetic calculations, and so stated, thus implying that it was not involved in color formation. Wittman et al. (9),on the other hand, have studied the ninhydrin reaction indirectly by working with chemically similar systems. Their work (2) D. J. McCaldin. Chem. Rev., 60, 39 (1960). (3) S. Moore and W. H. Stein, J , Bioi. Chem., 176, 367 (1948). (4) H. Rosen, C. W. Bernard, and S. M. Levenson, Anal. Biochem., 4, 213 (1962). (5) S. Mooreand W. H. Stein, J. Biol. Chem., 211, 9Oi (1954). (6) D. A. MacFadyen and N. Fowler,J. Bioi. Chem., 186, 13 (1950). (7) Yu. B. Filippovich, Nauch. Dokl. Vyssh. Shk., Khirn. Khim. Tekhno/., 1959, 110; Chem. Abstr., 5 3 , 16982e (1959). (8) M. Friedman and C. W. Sigel, Biochemistry, 5, 478 (1966). (9) H. Wittman, A. K. Muller, and E. Ziegler, Monatsh. Chem., 100, 497 (1969).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1 1 , SEPTEMBER 1973
